Photoreactions of Chlorophyll at the Salt Water-Air Interface

Photoreactions of Chlorophyll at the Salt Water-Air Interface by Dorea Irma Reeser A thesis submitted in conformity with the requirements for the d...
Author: Dora Brown
2 downloads 0 Views 6MB Size
Photoreactions of Chlorophyll at the Salt Water-Air Interface

by

Dorea Irma Reeser

A thesis submitted in conformity with the requirements for the degree of Master of Science Graduate Department of Chemistry University of Toronto

© Copyright by Dorea Irma Reeser (2009)

Photoreactions of Chlorophyll at the Salt Water-Air Interface Dorea Irma Reeser Master of Science Graduate Department of Chemistry University of Toronto 2009

Abstract Glancing angle laser induced fluorescence was used to monitor the kinetics of the photodegradation of chlorophyll at the surface of various salt solutions. The loss was measured using varying wavelengths of actinic radiation in the presence and absence of gas phase ozone. The loss rate of illuminated chlorophyll was faster on salt water surfaces than fresh water surfaces, both in the presence and absence of ozone. On salt water surfaces, the dependence of the loss rate on [O3(g)] was different under illuminated conditions than in the dark. This was further investigated by measuring the excitation spectra and the dependence of chlorophyll loss on the concentration of salts at the salt water surface. The possible production of reactive halogen atoms is the likely reason for the observed enhancement. The following results provide evidence of photosensitized oxidation of halogen anions, in the UV-visible range of the spectrum, resulting in halogen atom release.

ii

Acknowledgments

Thanks to NSERC for funding this project. I owe a multitude of thanks to many individuals who have helped me throughout this project. First and foremost on this list is Professor James Donaldson. He is not only an excellent supervisor, but he has provided me with an abundance of academic and moral support. He has motivated me, listened to me and advised me when I most needed it. I could not have a better supervisor. I would like to thank Dr. Christian George for his contribution to our work and for helping me gain confidence at my first poster presentation. Thanks are due to Dr. Adla Jammoul for her experimental contributions to our work. I would like to thank my supervisory committee members, Professor Jennifer Murphy and Professor Jonathan Abbatt. I would also like to thank the late Professor Rob Haines, my BSc. Honours supervisor, who made everything entertaining and motivating. His life, charisma and energy are an inspiration. Thanks to Professor Brian Wagner for being an enthusiastic teacher and mentor and for teaching me my favorite chemistry subjects. He and Professor Haines made my experience in chemistry at UPEI invaluable. Thanks to Heike Hildebrand and Dr. Katrin Mackenzie for providing me with the opportunity to work with them at the Center for Environmental Research in Leipzig, Germany and for introducing me to environmental chemistry. I have made a number of friends since I arrived in Toronto; some I will never forget. I thank Tara Kahan, who introduced me to the city of Toronto and helped me through thick and thin. I thank Jenny Wong for her enthusiasm and spirit, for listening to me any time and for taking great care of my dogs. I must thank Sunny Nagra for inspiring me with his unwavering optimism and positivity, for providing me with new perspectives, for his understanding and for always making time for me. They are all amazing people and even better friends. The following is no exaggeration: I would not have made it through this past year without them. I would like to thank all of my lab mates, past and present. Thanks to Daniel Clifford for familiarizing me with the lab, to Dr. Diego Ardura for joining us and providing me with tidbits of assistance and more thanks to Tara Kahan for her limitless advice in the lab. I need to thank all of the current members of my lab for laughs and listening and for the occasional social outings. I would also like to extend my thanks to the entire discipline of Environmental Chemistry at the University of Toronto; an outstanding group of individuals. I thank my fellow executive members in the Chemistry Graduate Student’s Union, who have given me a reason to be as sociable as I can. I would specifically like to thank Pablo Tseng and Rob McWhinney, two more great friends. I thank Carolyn DiMaria, my roommate, for being easy to live with and for taking care of the dogs when I could not. Lastly, and not the least, I thank my dogs, Dessi and Dexter. They give me joy, comfort and an excuse for a break when I need it.

iii

Table of Contents   Acknowledgments.......................................................................................................................... iii  Table of Contents ........................................................................................................................... iv  List of Tables ................................................................................................................................. vi  List of Figures ............................................................................................................................... vii  1  Introduction ................................................................................................................................ 1  2  Experimental Methods ............................................................................................................. 12  2.1  Glancing Angle Laser-Induced Fluorescence ................................................................... 12  2.2  Laser Probes ...................................................................................................................... 17  2.3  Ozone Generator ............................................................................................................... 17  2.4  Chlorophyll Extraction...................................................................................................... 20  2.5  Chemicals .......................................................................................................................... 20  3  Photoenhanced Reaction Between Chlorophyll and Ozone at the Salt Water-Air Interface ... 21  3.1  Introduction ....................................................................................................................... 21  3.2  Experimental ..................................................................................................................... 23  3.3  Results ............................................................................................................................... 25  3.4  Discussion ......................................................................................................................... 31  4  Influence of Salts at the Salt Water-Air Interface .................................................................... 37  4.1  Introduction ....................................................................................................................... 37  4.2  Experimental ..................................................................................................................... 39  4.3  Results ............................................................................................................................... 40  4.3.1  Excitation Spectra of Chlorophyll at the Air-Aqueous Interface.......................... 40  4.3.2  Influence of Salts on the Photochemical Loss of Chlorophyll at the AirAqueous Interface ................................................................................................. 40  iv

4.4  Discussion ......................................................................................................................... 48  5  Summary and Implications ...................................................................................................... 52  References ..................................................................................................................................... 56 

v

List of Tables

Table 2.1: The wavelengths and pulse energies of the various lasers used……………………..17 Table 3.1: The relative decrease in slope of the dependence of the loss rate of chlorophyll versus ozone concentration, using various long-pass optical filters, compared with the relative output power of a 100 W Xe arc lamp…………………………………………………………………..31 Table 4.1. The averages loss rates of chlorophyll at various 1 M aqueous surfaces in the dark and under illumination………………………………………………………………………………..44 Table 4.2: Redox potentials of various salts…………………………………………………….51 Table 4.3: Activation energies (kJ mol-1) of halogens reacted with various organic molecules..51

vi

List of Figures

Figure 1.1: Photoreactions in sea surface waters………………………………………………....3 Figure 1.2: The structure of chlorophyll a………………………………………………………..8 Figure 1.3: Absorption spectra of chlorophyll a……………………………………………….....9 Figure 1.4: Average sea surface chlorophyll for the period January 1998 to December 2006 from NASA’s Sea WIFIS satellite……………………………………………………………………..11 Figure 2.1: Glancing Angle Laser Induced Fluorescence Diagram……………………………..14 Figure 2.2: Schematic of GALIF technique applied in these experiments……………………...14 Figure 2.3: Image of the resulting chlorophyll fluorescence via GALIF……………………….15 Figure 2.4: A real-time trace from a digital oscilloscope……………………………………….16 Figure 2.5: Diagram of ozone generation and detection used in ozone experiments…………...18 Figure 3.1: Halogen desk lamp output spectrum; courtesy of Daniel Clifford and James Donaldson………………………………………………………………………………………..26 Figure 3.2: Output irradiance spectra of various Oriel arc lamps………………………….……26 Figure 3.3: The loss of chlorophyll with varying ozone concentrations in the dark (triangles) and on pure water under illumination………………………….……………………………………..27 Figure 3.4. The dependence of the loss rate of chlorophyll at aqueous surfaces with respect to ozone concentration……………………...………………………………………………………28

vii

Figure 3.5 The decay of chlorophyll at the salt surface versus ozone concentration with various long pass filters……………………………………..……………………………………………30 Figure 3.6: Transient absorption spectrum in the region associated with chlorophyll triplet state and/or cation absorption………………………………………………………………………….33 Figure 3.7: Transient absorption spectrum in the region associated with Cl2-…………………..34 Figure 4.1: Excitation spectra of chlorophyll at the surface of an aqueous salt substrate, water substrate, and a reported chlorophyll a absorption spectrum…………………………………….41 Figure 4.2: The loss of chlorophyll over time at various aqueous interfaces…………………...42 Figure 4.3: Examples of the normalized relative losses of chlorophyll at the surface of various aqueous salt substrates…………………………………………………………………………...43 Figure 4.4: The dependence of chlorophyll loss on halide concentration………………………46 Figure 4.5: The observed rate of loss of chlorophyll versus nitrate and nitrite concentration….47

viii

1

1

Introduction

Approximately seventy one percent of the Earth’s surface is covered by salt water. Marine waters have been a major focus in research due to their diverse ecology, resources, physical properties and strongly influential climate effects. The ocean is also used for transportation, sport, and is the natural habitat for a variety of biological life. The chemistry of the ocean is particularly important to understand its influences on the environment in every phase, whether it be soil, sediment, fresh water or the atmosphere. Dimethyl sulfide is produced in the ocean, oxidized in the troposphere and can increase cloud condensation nuclei formation in the marine troposphere1. The relationship between marine waters and the atmosphere is of great interest because it is not well understood and reactions between the sea water-air interface may not reflect those purely in the gas or aqueous phase. For example, the lifetime against ozonation of anthracene adsorbed at the air-water interface is shorter than in the gas phase2. If reactions at the air-water interface proceed differently than in the bulk (in air or water), then the possibility of greater impacts needs to be explored. Aqueous surfaces provide a location for heterogeneous reactions, which can be particularly important for atmospheric processes. Heterogeneous reactions contribute to the depletion of stratospheric ozone, NOx deactivation, acid rain formation, growth of cloud condensation nuclei, and the formation of secondary organic aerosals3. A model study performed by Ardura et al shows that hydrolysis of hydrochloric acid and nitric acid occurs near the air-water interface4. Organic films at the aqueous-air interface can provide a site for the conversion of water-insoluble species to those that are water soluble via heterogeneous oxidation on aerosols or cloud droplets5. The uptake of anthracene and pyrene from the gas phase to the air-aqueous interface is enhanced on an aqueous

2

surface with an octanol monolayer, compared with pure water and, by a factor of up to 36. Monolayers of octanol present at the water-air interface have been reported by Clifford et al. to suppress the rate of hydrolysis at water surfaces for both nitric acid and ammonia7. Surface waters are an important site for chemical reactions because liquid, solid and airborne wastes, including petroleum seeps and spills, are deposited there 1. Surface waters are also exposed to solar radiation, and so chemicals, both natural and anthropogenic, can undergo photoreactions there as described in Figure 1.1. A compound can absorb light and react directly, or it can transfer that energy to another species that does not absorb within the solar spectrum, thus acting as a photosensitizer. Dissolved organic matter which is capable of absorbing a photon, particularly in UV-A(315-400 nm) and UV-B (280-320 nm) can transfer energy and produce highly reactive species such as superoxide (O2-), hydrogen peroxide (H2O2), peroxy radicals (RO2) and singlet oxygen (1O2)1, 8. The sea surface microlayer (SML) has varying definitions9 but is typically defined as the uppermost tens to hundreds of micrometers of the ocean surface, depending on the SML sampling technique used. It exhibits chemical and physical properties different from those of sub surface waters. The SML can contain anthropogenic chemicals from wet and dry deposition and petroleum seeps and spills9. Compared with subsurface water, the SML is enriched in dissolved and particulate organic carbon, organohalogens, PCBs, dimethyl sulfide derived species such as DMSP and DMSO, organically bound and particulate trace metals, carbonyl sulfide, ampiphiles derived from ocean biota (fatty acids, fatty alcohols, steriles, amines, amino acids, lipids, phenols, etc.), plant nutrients and microorganisms1, 10-14. Particulate species are the most enriched because they are stabilized at the air-sea interface through surface tension forces15.

3

Figurre 1.1: Photooreactions inn Sea Surfacce Waters

4

The chemistry of the SML is important to understand because all gases, liquids and particles must be able to pass through in order to be transferred to and from the sea and the atmosphere. The SML is the part of the ocean that is most available to solar radiation and an increase in photochemistry could increase chemical oxidation, which will increase reactions and processes that are not observed in bulk waters. Perhaps Blough explains the SML best, “the microlayer could well act as a highly efficient microreactor effectively sequestering and transforming select materials brought to the interface from the atmosphere and oceans by physical processes.”16 Possible effects of the SML on environmental processes include the transfer of chemicals and organisms through bubble bursting or aerosol formation, an influence on the air-sea gas exchange rates, potentially harmful effects on surface ocean biota, and the formation of slicks and foams15. The physical and optical properties at the SML are modified due to accumulation of organic species. This enrichment forms a film at the SML, which can have a large effect on the chemistry at the marine water-air boundary (some examples were discussed earlier in this chapter) such as inhibiting air-water gas transport17. The sources of natural organic matter include phytoplankton, terrestrial dry or wet deposition and or river inputs. The SML also contains a higher concentration of toxic materials, relative to bulk waters, that can accumulate in slicks and foams. Anthropogenic compounds can be introduced via industrial processes, agricultural runoffs, petroleum spills and wastes. Because of the decrease in stratospheric ozone, there is an increase of incoming UV-B (280 nm – 320 nm) radiation at the ocean surface 1, 18, and its effect on the ocean surface needs to be better understood. Unfortunately, almost nothing is known about the effects of an increase UV-B radiation at the SML. One study found negative impacts on neustons (organisms that live in the underside of the surface film of water such as spiders, protozoa and bacteria) from chemistry with toxic chemicals and increasing UV-B exposure19. Marine surface waters contain

5

many light absorbing species such as nitrate, nitrite, transition metal complexes, keto acids, riboflavin, pteridines, algal pigments (e.g. chlorophyll), cyanobalamine, thiamin, biotin and aromatic ketones1. Carbonyl sulfide (an abundant sulfur containing gas in the troposphere) is formed via the photooxidation of organosulfide compounds by chromophoric dissolved organic matter under UV-B radiation1. It has also been postulated that UV-B light could alter and possibly enhance the production of photosensitizers such as singlet oxygen, which can be formed via the energy transfer from triplet state DOM to ground state molecular oxygen, and is most effective with the absorption of UV-A (315-400 nm) and UV-B light1. Sea salts have been an area of considerable focus particularly due to their contribution to the catalytic destruction of tropospheric ozone3 and as a source of gas phase halogen compounds in the troposphere20, 21. Sea salts are a major source of halides in the atmosphere via the injection of sea aerosol particles generated by breaking waves; some of these are redeposited to the ocean while the rest (3 – 35 %) may be released from the aerosol as inorganic vapor8. The SML is expected to have high concentrations of reactive halogen species relative to sub surface waters1 and the concentrations of halides, at the air-water interface, increase with increasing size and polarizability: Cl- < Br- < I-1, 20. The primary reactions of bromide and chloride, in sea water, are the oxidation of organic substrates1. The possibility of organic films in the SML having an enhancement on the SML bromide concentration has been suggested but has not been shown experimentally22. There is much focus on the catalytic destruction of ozone via chlorine and bromine atoms, but some recent research concentrates on the oxidation of atmospheric mercury. Bromine (or chlorine) reacts with gaseous elemental mercury, Hg(0), transforming it to reactive gaseous mercury, Hg (II), causing Hg(II) deposits to the ground and/or ocean after conversion in the

6

marine boundary layer 23, 24. Chlorine and bromine atoms both react with Hg(0) and chlorine reacts faster with Hg(0), but bromine is reported to have a greater effect because the photolysis of Br2 is greater than that of Cl224. There are several other important halogen reactions at the sea water-air interface. Chlorine, released in the MBL has been observed to rapidly react with organic molecules leading to the formation of gas phase ozone in the presence of NOx species8. Experimental and modeling observations have shown that the reaction between ozone and sodium bromide particles forms molecular bromide by an order of magnitude faster than that produced using known gas and aqueous phase chemistry25. This implied that Br2 is being formed by unknown mechanisms at the air-aqueous interface, and was shown to be the case by Clifford et al who demonstrated that a surface reaction takes place between bromide and ozone on aqueous surfaces22. The production and reactivity of iodine, iodide and iodine-containing compounds have seen recent interest because of iodine’s importance in biological metabolic systems26 as well as its ability to catalytically destroy tropospheric ozone, increase gaseous chlorine and bromine release, oxidize dimethyl sulfide and remove NOx species27. Most iodine in the ocean is in its inorganic form, except in coastal waters. Iodide production has been observed as a product of iodate reduction26. Iodide, iodine and iodine-containing compounds are mainly produced or correlated with ocean biota; iodide has been observed to increase during phytoplankton blooms26, 27

as a product of algae exposed to ozone and/or light, and molecular iodine has been observed to

release from kelps28. Iodine-containing organic molecules (e.g. CH3I) are also produced by algal cultures21. Established sources of gas phase molecular iodine in the atmosphere are via photolysis of dissolved iodine in iodo-organic molecules as well as reaction with tropospheric

7

ozonee29. There is also evidennce of aqueouus phase moolecular iodinne in surfacee sea water, but b its 27 conceentration is unknown u .

Phytoplan nkton metabolic and deggradation prooducts are a considerable c e source of natural n organnic material in the SML. In this case, natural orgganic matter is either scaavenged by air a bubbles that rise through the water colum mn or arrivess in the SML L by flotationn9. Cloudiness over the occean has beeen associatedd with phytooplankton bloooms; this was w postulateed to be due to the produuction of cheemical precuursors for clooud nucleatinng particles by the phytooplankton30. Phytooplankton caan contain seeveral pigmeents includinng xanthophyylls and chloorophyll, andd the latterr is responsib ble for photoosynthesis, thhat is the proocess in whicch phytoplannkton obtainn the energgy needed to o survive. Chlorophy yll is a classs of large biooorganic mollecules that contain c the same s main macrocylcic ring, differing byy side chainss. The structtures of chloorophyll havee the same riing structture, differ by b side chainns and typicaally have maagnesium bouund to the ceenter of the ring. r The most abundant ch hlorophyll iss chlorophylll a, and the structure s is shown s in Figgure 1.2. Chllorophyll c ia, and is ressponsible forr initiating thhe reductionn of is fouund in plantss, algae and cyanobacter carboon dioxide to o carbohydraates:

by abbsorbing visiible light in the t red and blue b regions of the visible spectrum as seen in thhe ns of chloroophyll in surfface absorrption spectrrum of chlorophyll a in Figure F 1.3. Concentratio C waterrs spike duriing phytoplaankton bloom ms and are allso affected by b wind edddies, seasonaal mixing and storms. s Chlorophyll a is ubiquitous in i the ocean and has beenn quantitativvely measureed, at the

8

Fiigure 1.2: T The structuree of chlorophhyll a.

.

9

Figure 1.3: Absorrption spectrrum of chlorrophyll a31

10

ocean surface, since the late seventies when the Coastal Zone Color Scanner was introduced. This method led to modern satellite techniques that include the Sea-viewing Wide Field-of-view Sensor (SeaWiFS, US), Modern Resolution Imaging Spectroradiometers (MODIS, US) and the Medium Resolution Imaging Spectrometer (MERIS, ESA)32 and an example of a chlorophyll a SeaWiFS image in provided in Figure 1.4. Chlorophyll concentrations are at a maximum at the coast and a minimum furthest from coastal areas and are particularly easy to measure because chlorphyll absorbs visible light, with maxima at 440 nm and 680 nm as shown in Figure 1.3. Chlorophyll concentrations, in coastal regions, have been observed to increase up to 100 % posthurricane and can remain that way for up to two weeks following 33, 34. Chlorophyll is particularly abundant in the SML, where it has been reported to have up to twice the concentration of chlorophyll a than that of subsurface waters14, 35. The photoreactivity (and presence of double bonds) in chlorophyll makes it a likely site for photo-driven redox reactions in the SML. This possibility is the subject of our investigations in the following chapters. This work will examine the photochemistry of chlorophyll at the salt water-air interface as a proxy for the sea surface, the role of the nature and concentrations of halides on this chemistry at the aqueous surface and implications for halogen release by photosensitized oxidation of halide anions.

11

Figurre 1.4: Averrage sea surfface chlorophhyll for the period Januaary 1998 to December 20006 from 3 NASA A’s Sea WIF FIS satellite36 .

12

2

Experimental Methods

2.1 Glancing Angle Laser-Induced Fluorescence The technique used to probe the aqueous surface in the following research is Glancing Angle Laser Induced Fluorescence (GALIF). In general, it consists of a laser source directed at an angle of at least 75o to the surface normal so that it is reflected from the surface region of the sample as shown in Figure 2.1; the resulting fluorescence can be measured perpendicular to the sample surface. We cannot quantify the depth of the probe penetration but current and previous work in our group leads us to believe that it is a few molecular layers deep37.GALIF has been used in a variety of experiments on surfaces including aqueous surfaces2, 6, 7, 22, 38, 39,

40

ice41, 42

and organic films43. The typical GALIF scheme for the experiments described here is displayed in Figure 2.2 and more detailed schemes are provided in the following chapters. The aqueous solution (100 mL for aqueous salt solutions) being probed was contained in a 3-necked round bottom flask equipped with quartz side windows and 2 to 6 drops of chlorophyll were placed on the aqueous surface. The output of a pulsed laser (10 Hz) passes through a quartz side window and impinges on the solution surface at a glancing angle of at least 75o to the surface normal. The specific laser probe used depends on the experiment and will be discussed further in the corresponding chapters as well as in Section 2.2. The resulting chlorophyll fluorescence (image shown in Figure 2.3) was collected by a 7 mm diameter liquid light guide, held by a stopper, in the central neck of the flask approximately 0.5 cm directly above where the incident radiation struck the surface. The light from the light guide was filtered through an optical long-pass filter impeding light of wavelengths less than 560 nm, then collected through a 1/8 m monochromator. The light

13

intensity at the desired wavelength was detected by a red-enhanced photomultiplier tube. The resulting time-resolved fluorescence signal was averaged over 4 to 64 laser shots by a digital oscilloscope, and was read by a custom-made LabVIEW data acquisition program running on a PC and saved for future analysis. Figure 2.4 shows an example of a signal of the fluorescence intensity of chlorophyll at the surface of 0.5M NaCl aqueous solution. This signal corresponds to one data point in a chlorophyll decay curve. The relative fluorescence intensity used to analyze chlorophyll decay was an average of a “slice” (approximately 50 to 100 ns) of the intensity curve averaged as shown in the inset graph in Figure 2.4.

14

Figure F 2.1: Glancing G Angle Laser Induced Fluorrescence Diaagram

Figurre 2.2: Scheematic of GA ALIF techniqque applied in i these expeeriments.

15

Figure 2.3: Image of the resulting chlorophyll fluorescence via GALIF.

16

Figurre 2.4: A reeal-time tracce from a diigital oscillooscope colleected and savved using a custommadee LabVIEW program. The red box indicates i thee 50 to 100 ns “slice” of o the intensity curve that was w averaged d and used for fo data analyysis.

17

2.2 Laser Probes

Three different lasers were used as probes in these experiments; a nitrogen laser, a frequency-tripled Nd:YAG and a Nd:YAG-pumped tunable optical parametric oscillator (OPO). Their corresponding wavelengths and pulse energies are displayed in Table 2.1.

Table 2.1: The wavelengths and pulse energies of the various lasers used. Laser Probe

Wavelength (nm)

Energy/Pulse

Nitrogen laser

337.1

170 µJ

Tripled Nd:YAG

355

4 mJ

Tripled Nd:YAG pumped tunable 300 to 355

~1 to 2 mJ

OPO (UV) Tripled Nd:YAG pumped tunable 410 to 650

adjusted to ~1.4 mJ

OPO (Visible)

2.3 Ozone Generator

A Jelight Company, Inc. Variable Ozone Generator Model 600 was used in the experiments requiring gas phase ozone, and a schematic is shown in Figure 2.5. High purity (99.99%) oxygen  gas, supplied by BOC, was flowed through a flow meter and adjusted to a flow of approximately 1L/min. The O2 gas passes through a stainless steel chamber equipped with an ultraviolet ozone

18

Figure 2.5: Diagram m of ozone ggeneration annd detection used in ozonne experimeents.

19

generrating lamp, which photoolyses the molecular m oxyygen (2), andd the resultinng oxygen attoms can combbine with ano other molecuular oxygen to produce ozone o in a teermolecular reaction r (3)::

A quaartz sleeve is placed in the stainless steel cell annd can be useed to attenuaate some of thhe UV light and is adjusted dependinng on the deesired concenntration of ozzone. The O3/O2 mixturee passes throuugh a 10 cm long-path cuuvette so thee concentratioon of ozone present can be measuredd. A 254 nm mercury m pen lamp is directed throughh the cuvettee to a photoddiode detectoor, which gennerates a signaal that can bee measured using u a voltm meter. The cooncentrationn of ozone floowing can thhen be quanttified by com mparing the voltage wheen the ozone generator iss on to that when w it is offf. The Beer--Lambert Laaw can be ussed to calculaate the ozonee concentrattion using thhe following relatiionship: ln (I//Io) = -σabsNll

(4)

where I is the volltage measurred when thee ozone geneerator is, Io is the voltagee measured when w the ozonee generator is i turned offf, σabs is the absorption a crross section of ozone at 254 2 nm (1.22 x 10-17 -1 44 cm2 molecule m ) 4 . The ozonee then flows through the reaction flassk or it is veented into a

fumehood.

20

2.4 Chlorophyll Extraction

All experiments presented in this work involve chlorophyll extract, which was obtained via the following method. Approximately 100 mL of loosely packed and de-stemmed spinach leaves were placed in a beaker and the leaves were ground to a pulp either using a hand blender or a mortar and pestle. Acetone, approximately 1 mL at a time, was slowly added, using a Pasteur pipet, to the pulp, and the mixture was thoroughly blended. This was repeated until a smooth consistency was achieved. The mixture was then filtered by gravity to remove any residue, and about 5 mL of deep emerald green filtrate, which fluoresced in the red when exposed to visible light, was collected and stored in amber glass vials for use in experiments. This procedure was repeated as necessary (approximately once a month) in order to ensure reactions were performed using “fresh” chlorophyll extract.

2.5 Chemicals

A variety of salts, as well as oxygen, were used in the experiments described in this work. The oxygen used had a purity of 99.99% and was purchased from BOC. 18 MΩ deionized water was used in all the experiments as “pure” water or in solution. The salts used in aqueous solutions include sodium nitrite, purchased from Sigma-Aldrich, and all other salts used were from ACP; these include NaCl, NaBr, NaI, KNO3 and NaNO2. There were two pH adjusters used as aqueous solutions in the experiments; NaOH from ACP and 36.5-38% HCl from Baker Analyzed.

21

3

Photoenhanced Reaction Between Chlorophyll and Ozone at the Salt Water-Air Interface

The work discussed in this chapter has been published as: Reeser, D. I.; Jammoul, A.; Clifford, D.; Brigante, M.; D’Anna, B.; George, C.; Donaldson, D. J. The Journal of Physical Chemistry C. 2009, 113, xxxx.

3.1 Introduction The majority of the Earth is covered by the ocean (~ 71%), which plays an important role in atmospheric-water transfers and chemistry. This relationship is complex and has a major influence on the Earth’s climate and ecology and is important to understand in order to model and interpret it appropriately. The ocean surface can be a source of highly reactive species to the marine boundary layer (MBL) including superoxides, hydrogen peroxide1, 8, and halogen compounds20, 21, 27, 29. The sea surface microlayer (SML) has the greatest impact on the MBL as it is the uppermost micrometers of the sea surface, and the area of marine waters in closest contact with the atmosphere. The SML (described in greater detail in Chapter 1) is rich in dissolved organic matter (DOM), forming a surface film, and is the most available to photochemical reactions9. The DOM consists of natural and anthropogenic material and much of the natural organic matter is derived from marine water phytoplankton and little is understood about either. Understanding the DOM components and chemistry in the SML is important because many of these molecules can change the environment of the sea surface, and the MBL. An increase in anthropogenic

22

DOM could physically impede air-sea transfers17, modify the solubility of hydrophobic molecules5, 6 and change the chemistry at the air-sea interface7. Natural organic matter at the SML has received very little attention, and it also needs to be better understood, particularly due to the increase in UV-B (280-320 nm) radiation hitting the Earth’s surface due to stratospheric ozone depletion18. Much of the DOM in the ocean is photoactive, that is capable of absorbing light within the solar spectrum. Some of the DOM compounds can transfer the energy absorbed to other compounds, thus acting as photosensitizers and are therefore significant in understanding SML and MBL chemistry. The SML is not only a source of gas-phase compounds into the MBL, but it is an important and not well understood tropospheric sink for trace gases. Ozone deposition at the sea surface is a recognized sink for MBL ozone, and it has been thought that the loss mechanism depends on chemistry in the SML, particularly in coastal marine waters45. Heterogenous reactive loss of gas phase nitrogen dioxide and ozone has been demonstrated at surfaces containing photoactive compounds under illumination22, 39, 46, 47. It has also been observed that the deposition of ozone is approximately an order of magnitude greater than that on fresh water and is suggested to be due to organic films45. Clifford et al. performed work on the reaction between chlorophyll and ozone at the water surface39. The observed loss rate versus ozone concentration displayed a LangmuirHinshelwood kinetic mechanism. This suggests that the ozone is in rapid equilibrium between the gas and surface phases, and the reaction between ozone and chlorophyll occurs at the surface. This type of dependence of heterogeneous reaction rate on [O3(g)] has been observed in several other reactions of ozone with reagents present at the water surface, including polycyclic aromatic hydrocarbons40, bromide anions22, and fatty acid salts48. Oxidation of other substrates by ozone has been observed to have the same behavior on other surfaces49, 50. One such reaction is the

23

oxidation of iodide by ozone, especially in the presence of DOM27-29, which has recently been the focus of considerable interest. The following work discusses the photoenhanced reaction between chlorophyll and ozone under illumination at a salt water-air interface as well as its atmospheric implications.

3.2 Experimental

The experiments described below probe the fluorescence of chlorophyll deposited at the water surface. The chlorophyll used was extracted via the method described in Section 2.4. The experimental setup is a modified version of that in earlier studies in by our group39 as described in Section 2.1. It consists of a 250 mL Pyrex 3-neck round bottom flask equipped with quartz side windows. The flask was filled with 100 mL of either fresh water or an aqueous salt solution (adjusted to pH~8.3 using 2.5 x 10-2 M NaOH) and several drops of the concentrated spinach extract in acetone were gently placed on the aqueous surface using a Pasteur pipette. This extract was observed to spread across the surface over approximately 10 to 15 minutes, leaving a visible green layer at the interface. This surface layer persisted for over an hour - considerably longer than the time required for reaction - unless the sample was vigorously mixed. The output of a pulsed (10 Hz) nitrogen laser (337 nm; 170 µJ/pulse) was passed through a quartz window on the side of the flask and impinged upon the surface at a glancing angle, approximately 75o from the surface normal. Fluorescence excited by the probe laser was collected perpendicular to the incident radiation by a liquid light guide suspended in a stopper in the centre neck of the flask, approximately 0.5 cm above the surface. The collected radiation

24

was passed through a 1/8 m monochromator and detected by a red-enhanced photomultiplier tube. The resulting time-resolved fluorescence decay signal was averaged over 4 laser shots in a Tetronix TDS 220 real-time oscilloscope. Data were read from the oscilloscope by a custom LabVIEW program running on a PC, and a 40 ns time period around the peak of the fluorescence decay curve was averaged and saved for later analysis. Ozone was generated by flowing 1.0 L min-1 of high purity (99.99%) oxygen at atmospheric pressure over an ozone generating lamp. The resulting mixture (containing 1014 – 1016 molecules cm-3 ozone) flowed through a 10 cm long quartz-windowed absorption cell, then into a side neck of the reaction flask, and was exhausted through the opposite neck. The concentration of ozone entering the reaction cell was determined by measuring the attenuation of light from a 254 nm mercury pen-lamp passed through the absorption cell, using a photodiode detector. Each kinetics measurement run started by monitoring the fluorescence intensity at 674 nm with no ozone present to establish the initial chlorophyll surface concentration, then the ozone flow into the flask was started. A fluorescence intensity measurement was made every 3 seconds thereafter, until all of the chlorophyll had reacted, as inferred from the complete loss of fluorescence at 674 nm. After reaction, the color of the sample surface changed from green to clear. Experiments were done using a number of aqueous substrates. For most experiments, a salt solution composed of 1 M sodium chloride with an additional 1 mM sodium bromide was prepared by dissolving known amounts of the salts in 18 MΩ deionized water; the pH of the solution was raised to approximately 8.3 using approximately 2 x 10-2 M sodium hydroxide

25

solution. Some experiments were also performed using solutions to which base was not added; these had a pH in the range of 5.4-6.0. Initially, solutions were illuminated using the output of a 20 W halogen desk lamp (~310750 nm), but the majority of the results involved an Oriel 100 W Xe arc lamp (~200 nm to near IR), the latter was also passed through various long-pass optical filters, with short-wavelength cut-offs in the range from 350 nm to 715 nm, and directed through the Pyrex flask onto the surface of the solution. The emission spectra of the two lamps are shown in Figure 3.1 and 3.2, respectively.

3.3 Results The kinetics of the loss of chlorophyll due to reaction with ozone on pure or salt water in the dark and also on illuminated pure water correspond well with the results reported by Clifford et al. and these results can be seen in Figure 3.3. However, the loss of chlorophyll due to reaction with O3(g) at the salt water surface in the presence of illumination does not follow this Langmuir-Hinshelwood kinetic mechanism, but rather one which is linear with respect to gas phase ozone concentration. This change in kinetic mechanism was examined over the entire near-UV and visible spectrum and found to be independent of the illumination wavelength. As displayed in Figure 3.4, the reaction of ozone and chlorophyll on salt water surfaces is drastically enhanced when the solution is illuminated. Our apparatus was limited to studying ozone concentrations below approximately 1015 molecules cm-3 because the photodegradation of chlorophyll on salt solutions was too rapid to quantify accurately at higher ozone concentrations. The decay of chlorophyll is still first order, and is clearly linear up to an ozone concentration of

26

Figurre 3.1: Hallogen desk lamp outpuut spectrum m; courtesy of Daniel Clifford annd James Donaaldson.

Figurre 3.2: Outtput irradiannce spectra of various Oriel arc lamps. Thee one used in these experriments was a 100 W Xee arc lamp shhown as a daashed line51.

27

-1 kobs (s )

0.06

0.04

0.02

0.00 0

10

20

30

40

50

60

14 -3 [O3] (10 molec cm )

Figure 3.3: The loss of chlorophyll with varying ozone concentrations in the dark (triangles), and on pure water under illumination (red circles). Both exhibit a Langmuir-Hinshelwood kinetic mechanism. The error bars on each point represent the standard deviation of 1σ based on the average of 3 to 7 experiments.

28

Figurre 3.4. The dependencee of the loss rate of chloorophyll at aqueous a surffaces with respect to ozonee concentrattion. The deggradation ratte of chloropphyll on saltt solutions (bblue stars) apppears to increase linearly y with inccreasing ozzone concenntration unnlike the expected e LaangmuirHinshhelwood kin netic mechannism exhibitted by the looss of chlorrophyll on saalt water in the dark and on o pure wateer both illum minated and inn the dark (sshown by thee black circles).

29

less than 1015 molecules cm-3. Above these concentrations the loss rate of chlorophyll was greater than 0.10 s-1, which is the instrumental limitation given that the minimum data collection/transfer rate is 3 seconds and a minimum of three points during the decay are required to establish an observed rate of loss. The solid blue stars in Figure 3.4 show the results of experiments in which the reaction cell was illuminated using the full output of a Xe arc lamp, filtered by the Pyrex walls of the cell, providing radiation from approximately 300 nm to near-IR; comparable to the entire solar spectrum (Figure 3.2). A linear relationship between chlorophyll loss rate and the concentration of ozone in the gas phase is apparent; this linear relationship is also observed using a 20 W output halogen desk lamp to illuminate the cell. The output extends though the visible spectral region, from approximately 310 to 750 nm, which is similar to the solar spectrum. The same linear relationship was obtained using any one of several longpass optical filters which block light of wavelengths less than 350 nm; 430 nm; 660 nm and 715 nm, and the results are shown in Figure 3.5. The slopes of the dependence of the rate of loss of chlorophyll on ozone concentration, obtained using each filter, along with their respective uncertainties (based on 1σ of the average of 3 to 4 runs), are provided in Table 3.1. The slopes obtained using the filters are similar within their respective uncertainties. There is a slight decrease in the slopes of the results performed using filters compared to that using the full output of the arc lamp. This may be due to a decrease in the lamp power output as provided in Table 3.1. The results using these filters are shown as the solid diamonds in Figure 3.5. The kinetic mechanism of the chlorophyll decay reverts back to that seen in the dark when all the light at wavelengths below 800 nm is inhibited; this result is shown as purple squares in Figure 3.5.

30

d of chlorophyll at the t salt surfa face versus ozone o concenntration withh various Figurre 3.5 The decay long--pass filters. The blue stars s indicatte the loss while w underr full arc lam mp illuminaation, the yellow w circles arre that underr full desk lamp l illuminnation and the t diamondds are that under u the illum mination of liight of whichh wavelengtths less than 350, 430, 660 6 and 715 nm has beenn filtered out using u long paass color filtters. The purrple squares indicate thee results of thhe chlorophyyll decay with all light beelow 800 nm m filtered out. o The puurple dashedd line repressents the Laangmuirnetic mechannism observeed once all actinic a radiaation is filterred using thee 800 nm Hinshhelwood kin filter..

31

Filtered Light (nm)

Slope (cm3 molecule-1 s-1)

% Relative Lamp Output

None

0.017 ± 0.005

100

All Filters

0.010 ± 0.001

--

< 350

0.011 ± 0.003

84

< 430

0.013 ± 0.001

74

< 660

0.008 ± 0.001

61

< 715

0.009 ± 0.0009

61

Table 3.1: The relative decrease in slope of the dependence of the loss rate of chlorophyll versus ozone concentration, using various long-pass optical filters, compared with the relative output power of a 100 W Xe arc lamp.

3.4 Discussion The heterogeneous reaction between gas phase ozone and chlorophyll at illuminated aqueous salt surfaces seems to be much faster, and follow a different kinetic mechanism than that observed on pure water or salt solutions in the dark. The photoenhancement of the loss rate of an organic substrate in heterogeneous chemical reactions has not been reported before. This enhancement in the kinetics, and apparent change in mechanism under illumination, suggests that there may be other reactions taking place at the illuminated surface than occur in the dark and that these may also influencing the loss of chlorophyll.

32

The loss rate r of chloroophyll was dependent d onn the ozone concentratioon but also on the preseence of chlorride salts. Reecent work carried c out inn the Georgee group in Lyyon aimed too further invesstigate this reeaction; the results r were also reporteed in Reeser et al38. In thhat work, traansient absorrption spectrra were obtaiined following the photoo-excitation of chlorophyyll in aqueouus salt solutiions. Figure 3.6 displayss sample trannsient absorpption spectraa obtained inn that study. The Figurre displays both b the specctrum of a soolvated electrron and a sppectrum correesponding too the chlorrophyll cation (or possiblly a triplet sttate chlorophhyll). When the photoexxcitation of chlorrophyll was carried c out inn a NaCl sollution, the trransient absoorption specttrum of Cl2- (Figure ( 3.7.) was also observed. Thiss species cann only be prooduced in thee presence of both a chlooride anionn and a chlorrine atom, soo its observaation here is strongly s sugggestive that Cl atoms arre formed in illuuminated aqu ueous solutions containiing both chloorophyll andd chloride. The follow wing mechanism can explain all the experimentaal results 38:

33

nsient absorpption spectruum in the reggion associaated with chllorophyll tripplet state Figurre 3.6: Tran and/oor cation absorption, obbtained 1 µss following laser excitaation of a saalt water solution of chlorrophyll. The inset is a trransient absoorption specctrum in the region assoociated with hydrated electrrons, obtaineed 1 µs folloowing laser excitation e off a fresh wateer solution of o chlorophylll38.

34

nsient absorpption spectrrum in the region r assocciated with Cl C 2, obtainedd 0.2 s Figurre 3.7: Tran follow wing laser ex xcitation of a salt water solution of chlorophyll. c The line shoows a Gausssian fit to the daata, and is prresent only to t guide the eye.38

35

where Chl represents chlorophyll. Reaction (1) is production of triplet state chlorophyll via the absorption of a photon, which can rapidly lose an electron and form cationic chlorophyll. In solutions that are exposed to ambient air, particularly salt-free, molecular oxygen can form superoxide either by reacting directly with the triplet state of excited chlorophyll (as shown in reaction (2a)) or by capturing the released electron as shown in reaction (2b). The resulting superoxide anion could then react with the chlorophyll cation to reproduce the initial chlorophyll molecule (reaction (2c)). When chlorophyll is in a salt solution, the photoproduced chlorophyll cation can react with chloride anions, as shown in reaction (3), and this can produce chlorine atoms. A molecular chlorine anion is produced by combining a chloride anion and chlorine atom as in reaction (4). Thus the presence of such anions implies that there are indeed chlorine atoms being produced. Chlorine atoms have a high reactivity with large organic molecules and would be expected to react with the parent chlorophyll (reaction (5)), giving rise to chlorophyll loss in illuminated salt solutions (discussed in more detail in Chapter 4). Reaction (6) shows that in the presence of ozone, an additional chlorophyll loss is possible. Ozone may be reduced to O3-, which can be rapidly hydrolyzed to form the hydroxyl radical as in reaction (7). In reaction (8) the hydroxyl radical can further react with chloride anions producing more reactive chlorine atoms. Even further loss of ozone and chlorophyll is shown in reaction (9) in which ozone can react with Cl2- producing more chlorine atoms. Reactions (8) and (9) are likely to be the most important cause for the observed enhancement in the presence of ozone because the kinetic mechanism has only been observed in the presence of salts. The proposed reaction mechanism provides insight as to the reason for the photoenhanced loss of chlorophyll reacting with ozone at aqueous salt surfaces. This reaction occurs in addition to the loss of chlorophyll that would already occur in the presence of ozone in the dark. The enhancement is a consequence of the production of reactive species such as the

36

chlorine and hydroxyl radicals following absorption of light by chlorophyll. The production of Cl2- indicates that chlorine atoms are being produced and these can either react with other organic molecules, at the sea surface microlayer, or they could be released into the MBL. This could provide a source for chlorinated hydrocarbons and molecular chlorine in the MBL, resulting in the potential destruction of ozone. This has important implications to the chemistry of the marine boundary layer. Halogens are known to catalytically destroy ozone at the MBL and in the arctic stratosphere. Theoretical calculations, using known homogeneous chemistry, have been shown to underestimate the experimentally observed amounts of molecular chlorine formation from aqueous NaCl particles52. Perhaps heterogeneous chemistry needs to be better understood to fill this gap as suggested by other interfacial studies measuring the production of bromine53, chlorine54 and iodine in the MBL. The proposed mechanism could present an important source of chlorine to the MBL. The significance and possible influences of salts are explored further in Chapter 4.

37

4

Influence of Salts at the Salt Water-Air Interface The excitation spectra of chlorophyll at the air-aqueous interface have been previously

reported in Reeser, D. I.; Jammoul, A.; Clifford, D.; Brigante, M.; D’Anna, B.; George, C.; Donaldson, D. J. The Journal of Physical Chemistry C. 2009, 113, xxxx. The remainder of this chapter has not been published at this date.

4.1 Introduction The ocean is a vast component of the Earth’s surface, covering approximately seventy one percent of it. In addition to this, the ocean contains an extremely diverse amount of resources, biota and chemistry. The chemistry of the ocean is particularly important in understanding its influences on the environment as it can have an effect on the Earth in all spheres and acts as a source and sink to many species in the environment3, 9. One major area of interest is the transfer of species between the atmosphere and the ocean at the air-sea interface. The sea surface microlayer (SML) is the organic enriched layer at the air-sea interface and has been shown to have different constituents, concentrations and chemistry than that of subsurface waters35, 38. This is particularly important in understanding transfer processes between the SML and the marine boundary layer (MBL). A study in our group showed that the presence of a monolayer of octanol suppresses the hydrolysis of both ammonia and nitric acid at the water surface7. Other work has shown that the composition of surfactants at aqueous interfaces can have a significant effect on gaseous uptake55, 56. The SML is the part of the ocean most readily available to solar radiation and, given the abundance of organic matter, photochemistry occurring

38

here could be of great significance. A recent study performed by Christian George’s group in Lyon indicates that photoactive organic compounds could be a source for both halogen atoms and molecules due to photooxidation of aqueous halide salts57. Halides in sea-salt have been shown to be an important source of chlorine and bromine in the atmosphere21. The concentration of reactive halogen species is hypothesized to be higher in the SML relative to bulk waters, and the primary halogen reactions are likely the oxidation of organic substrates1. The only known mechanism for the release of gas phase molecular iodine is via the photolysis of iodine-containing organic molecules27, 29. Reaction with gas phase iodine is also a removal pathway for NOx species and ozone, leading to particle formation27, 28. Studies have also shown an increase in the release of molecular iodine from sea kelp in coastal regions28 and phytoplankton has been observed to produce iodide and iodine-containing molecules27. The main contribution to chlorophyll in the ocean surface is from phytoplankton, and our results imply that the reaction between chlorophyll and halides, at the salt water surface, may be a source of halogens and halogen containing compounds to the MBL. The purpose of the experiments discussed below was to provide further insight to the results presented in Chapter 3 for the reaction between chlorophyll and ozone at aqueous salt surfaces by (1) measuring the excitation spectra of chlorophyll at aqueous surfaces and (2) investigating the loss kinetics of chlorophyll at the salt water surface in the presence of light, but without ozone. An excitation spectrum of chlorophyll at the aqueous-air interface was acquired in order to determine whether there was some difference in the pathways available to excited chlorophyll when it is present on the salt water versus pure water surface. The loss rates of chlorophyll at the air-aqueous interface, as a function of the concentration and nature of the salts present in the substrate, were explored to test the mechanism presented in Chapter 3, particularly with respect to reactions (3a) and (3b).

39

4.2 Experimental

The experimental method used is similar to that described in Reeser et al.38 and Section 2.1. Stock solutions (1.0 M) of salts were made using the chemicals listed in Section 2.5 and aliquots were taken from this and diluted to 100 mL to achieve the desired concentrations (2 M and 1.5 M samples were prepared using a 2 M stock solution). The pH of the aliquots was adjusted to approximately 8.3 (as measured using an Orion Model 520A pH meter), just before use, using a few drops of either 2.5 x 10-2 M NaOH or 1 x 10-4 M HCl solutions. The solution was then placed in a 250 mL Pyrex 3-necked round bottom flask equipped with quartz side windows, and approximately three drops of chlorophyll extract were gently added to the surface using a Pasteur pipette. The solution was allowed to sit for at least fifteen minutes to let the extract spread as evenly as possible over the aqueous substrate. In experiments performed in the dark, the experimental set up was enclosed by black curtains to minimize possible illumination from fluorescent ceiling lights and outdoor light. In experiments which studied photoreactions, a 20 W desk lamp (emission is shown in Figure 3.1) was directed onto the surface of the solution. The concentration of chlorophyll at the water surface was monitored at 676 nm via GALIF using a tripled Nd/YAG laser at 355 nm as the surface probe (Section 2.2). A tunable Nd/YAGpumped optical parametric oscillator was used for the acquisition of excitation spectra. For some experiments which measured chlorophyll loss kinetics, this laser was also used as the probe, at an excitation wavelength of 430 nm (maximum chlorophyll a excitation). Otherwise, the experiments were carried out as described in Chapters 2 and 3.

40

4.3 Results

4.3.1

Excitation Spectra of Chlorophyll at the Air-Aqueous Interface

In an effort to understand the earlier kinetics results for the photoenhanced reaction between chlorophyll and ozone (presented in Chapter 3), we measured the excitation spectra (that is, the fluorescence intensity measured at 674 nm as a function of the wavelength of the excitation laser) of chlorophyll at pure water and salt water surfaces. The results are displayed as the black and red points, respectively, in Figure 4.1. It is clear that the excitation spectra track very well the reported absorption spectrum31 of chlorophyll a, shown as the dashed blue line in the Figure. There are only minor differences seen between the spectrum measured on a salt water surface and that on the pure water surface.

4.3.2

Influence of Salts on the Photochemical Loss of Chlorophyll at the Air-Aqueous Interface

The loss of chlorophyll at the surface of all solutions studied here follows first-order decay kinetics with respect to chlorophyll. The loss rate is drastically increased in the presence of salt anions as shown in Figure 4.2. The loss of chlorophyll in the dark, on both pure water and salt water, is significantly slower than that on illuminated pure water. Its loss rate on illuminated salt water is enhanced over that on illuminated pure water, or on dark salt solutions. Examples of the loss of chlorophyll with respect to time on these various salt substrates are shown in Figure 4.3. Under illumination from the desk lamp, the loss rates increased significantly over those

41

Figurre 4.1: Exciitation specttra of chloroophyll at thee surface of an aqueous salt substraate (black circlees), water substrate s (reed inverted triangles), and a repoorted chloroophyll a abbsorption specttrum (blue daashed line)311.

42

Figurre 4.2: The loss of chloorophyll over time at varrious aqueouus interfacess in the darkk on pure and salt s water solutions (inveerted green triangles, t bluue squares and a black diaamonds), illuuminated pure water (yello ow triangles and a red diam monds) and illuminated i s water (pink squares)). salt

43

Figurre 4.3: Exam mples of the pseudo-firrst order losss kinetics of chlorophyyll at the illuuminated surface of various aqueous saalt substratess.

44

measured without the lamp; by a factor of 10 for pure water and by a factor of 2 for NaCl solution, and these results are provided in Table 4.1.

Aqueous Substrate

Average Rate of Loss of

No. of Runs

-1

Chlorophyll (min ) Pure water, dark

0.04 ± 0.02

2

Pure water, light

0.5 ± 0.2

6

NaBr, light

1.8 ± 0.8

7

NaCl, light NaI, light

1.1 ± 0.3

0.9 ± 0.4

7 9

Table 4.1. The averages loss rates of chlorophyll at various 1 M aqueous surfaces in the dark and under illumination.

The magnitude of chlorophyll loss depends on both the identity and the concentration of the halide salt. The rate of loss of chlorophyll on 1M solutions of NaCl, NaBr, NaI are displayed in Table 4.1 and are calculated using a minimum of six runs and were 1.1 min-1, 1.8 min-1 and 0.9 min-1, respectively. All of these results exhibit first-order decay kinetics with respect to chlorophyll. The loss of chlorophyll, in the presence of actinic light, was found to be proportional to the concentration of halides in the aqueous substrates. The linear dependences can be seen in Figure 4.4, and the slopes for chlorophyll photodegradation on NaBr, NaCl and

45

NaI solutions were 1.3 min-1M-1, 0.7 min-1M-1 and 0.4 min-1M-1, respectively. The greatest photochemical loss rate of chlorophyll was seen on sodium bromide, then sodium chloride, followed by sodium iodide. The error bars (1σ), in Figure 4.4 are due to the averaging of 5 to 10 separate runs with variable results. Chlorophyll loss did not appear to be photoenhanced in the presence of either nitrite or nitrate. The average rate of chlorophyll photodegradation was approximately the same as that on pure water, 0.5 to 0.7 min-1. Because there was no photoenhancement, there was also no observed relationship between the salt concentration and the loss of chlorophyll as shown in Figure 4.5.

46

Figu ure 4.4: The dependencee of chlorophhyll loss on halide conceentration.

47

Figurre 4.5: The observed ratte of loss of chlorophyll versus nitratte and nitritee concentratiion.

48

4.4 Discussion

The increased loss of chlorophyll in the presence of actinic radiation implies that photodegradation of chlorophyll is taking place. The measured excitation spectra on both aqueous salt and pure water substrates follow that of the absorption spectrum of chlorophyll. This suggests that there is no change in the photoexcitation of chlorophyll on salt solutions compared with that on pure water. If there was a different excitation pathway for the reaction on aqueous salts, then there would be a change in the relative fluorescence intensities observed for excitation vs. the relative absorption strengths. That obtained on the salt solution substrate is somewhat more intense in the 400 nm region; this may be due to a ‘salting-out’ effect58 enhancing the chlorophyll concentration at the surface in this case. There are few techniques that are currently used to probe the air-aqueous interface because most methods are either influenced by the bulk solution or movement of the molecules in solution59. To date, there are two nonlinear optical techniques used; second-harmonic generation (SHG) and sum-frequency generation (SFG)60. The excitation spectra acquired in our results relies on placing the chlorophyll at the interface, the glancing-angle collection scheme and the subsequent fluorescence, which provides a new method for determining how a molecule absorbs light at aqueous surfaces. There is an increase in the loss of chlorophyll on pure water, in the presence of actinic radiation. In the presence of salts, the loss of chlorophyll increases drastically, by more than a magnitude, indicating that salts (halides in particular) have a strong influence on the reaction. It is proposed that these recent observations be slightly modified in the aforementioned reaction mechanism as the following:

49

where “Chl” indiicates chloroophyll and “X X” can repreesent Cl, Br or o I. The maj ajority of thiss mechhanism is disscussed in Chhapter 3; onlly reactions (3) and (4) will w be discuussed furtherr. In (4) X2- iss formed by the reaction between X and X- , andd this can occcur only if thhe X radical is produuced. This iss supported by b the Cl2- thhat was measured duringg the photochhemical losss of chlorrophyll in aq queous salt soolution menttioned in Chhapter 438. The photo odegradationn of chlorophhyll is clearlly enhanced at the surfacce of aqueouus salt substtrates, compaared to the looss rate on pure p water suurfaces, and depends on the nature of the solutiion. This dep pendence inddicates that the t rate deteermining stepp of the reacction must bee related to reaaction (3). Th he greatest loss l rate of chlorophyll c a the air-aquueous interfaace occurs inn the at preseence of Br-, then t Cl-, followed by I- and a little to no n loss was observed o in the t presencee of NO2and NO N 3-. This trrend can be explained e byy reactions (33a) and (3b). Reaction (33a) is expectted to proceeed rapidly in n solution; based b on the correspondiing gas phasse redox poteentials (Tablle 4.2) NO3- would reactt fastest, folllowed by I-, Cl-, Br- and lastly by NO O2-. This wouuld result in halogen produuction in (3aa) following the trend: I > Br > Cl. We W do not unnderstand whhat is occurriing betweeen chloroph hyll and nitrate or nitritee, if anythingg at all. It is possible p thatt one or bothh species

50

were oxidized in reaction (3a) but did not proceed through (3b). If such oxidation takes place, the corresponding production of NO3 or NO2 represents a novel source of reactive NOx species to the MBL. Examples of the activation energies of reactions between halogens and alkanes are provided in Table 4.3. Iodine would likely have a high activation energy for reaction with chlorophyll, so it is less likely to proceed through reaction (3b). Therefore both the redox potentials of the halogen anions, and the activation energies of reactions between halogen atoms and organic molecules can be used to explain the observed chlorophyll loss dependence and rate determining step. The excess iodine atoms released may be further reacting and forming molecular iodine and other iodo species. Therefore, this reaction may be a very important source of iodine atoms, molecular iodine and iodine-containing compounds as well as a source of other halogen atoms. The reason the photoenhancement of the loss of chlorophyll at the aqueous surface is observed is due to the production of these reactive halogen atoms via photosensitization by chlorophyll. If chlorophyll behaves this way in the SML then it is probable that other large, unsaturated, photoactive organic molecules in the SML may also act as photosensitizing agents. It was recently observed that a photoreaction between benzophenone and aqueous halides produced halogens and halogen atoms57. It is possible that this reaction follows a similar mechanism to that of chlorophyll described above. If this is the case, then this reaction mechanism could have greater implications as a major source of halogen atoms in the MBL.

51

Salt Anion 

Redox Potential (mV) 

I/I‐ 

1353 

Br/Br‐ 

1986 

Cl/Cl‐ 

2507 

NO2/NO2‐ 

970 

NO3/NO3‐ 

2216 

Table 4.2: Redox potentials of various salts61.

Atom 

CH4 

C2H6 

C3H8 

cyclo‐C5H10 



143 

114 

104 

104 

Br 

76 

56 

43 

39 

Cl 

13 

2.3 

2.9 

2.5 

Table 4.3: Activation energies (kJ mol-1) of halogens reacted with various organic molecules62.

52

5

Summary and Implications Experiments exploring the photochemical loss of chlorophyll at the air-aqueous interface

were performed using a glancing angle laser induced fluorescence technique. These studies are environmentally relevant due to the enhanced concentration of chlorophyll in the SML, and chlorophyll’s potential role as a photoredox substrate. Few studies have been performed which probe the photochemistry of natural organic matter at the air-aqueous interface, though this is the region of the ocean surface through which all chemical species must pass to transfer from sea water to air or vice versa. The reaction between chlorophyll and ozone at the salt water-air interface is drastically enhanced and follows a different kinetic mechanism under actinic radiation than that which is measured in the dark at the water-air interface, or at the air-pure water interface under illumination. In the dark, and on illuminated pure water, the reaction follows a LangmuirHinshelwood type kinetic mechanism. When chlorophyll at an illuminated salt water surface reacts with ozone the reaction kinetics are linear with respect to ozone concentration (within our experimental limitations). This implies that the reaction follows a different heterogeneous reaction mechanism under illumination than that in the dark. Using a range of long pass filters, it was determined that the photoenhancement of this reaction did not depend on the wavelength of light illuminating the reaction, as long as the wavelength of light was sufficient to excite chlorophyll. The excitation spectra of chlorophyll were measured at the air-salt water and air-pure water interfaces. To our knowledge, this is the first excitation spectrum at an aqueous surface which has been reported38. The observed spectra of fluorescence intensity versus excitation wavelength

53

on booth substratees followed the absorptioon spectrum well, indicatting that therre are no alteernate pathw ways for exccitation on aqqueous salt substrates. s More stud dies were performed to provide p furthher insight innto the photoochemistry of o chlorrophyll at thee air-aqueouus interface; specifically s we examineed the loss off chlorophylll in the preseence of vario ous types andd concentratiions of salts of environm mental imporrtance. The photoodegradation n of chlorophhyll at the aqqueous surfaace is dependdent on the tyype of salt as a well as salt concentration c n, and all of the loss kineetics displayy first-order decay. d The loss of chloroophyll underr illuminatio on was foundd to be propoortional to thhe concentraation of halidde present annd no enhanncement in the t presence of nitrate orr nitrite was observed. Bromide B induuced the moost rapid loss of o chlorophy yll, followed by chloride, then iodidee, indicating that the ratee determining step is directtly involved d with the hallide. Work performed p byy Christian George’s G grooup in Lyon had show wn that Cl2- was w producedd following the t excitatioon of chloropphyll in an aqqueous salt solutiion38. wing mechanism was prroposed to exxplain the abbove observaations: The follow

“ represennts Cl, Br or I. where “Chl” reprresents chlorrophyll and “X”

54

The importance in the above mechanism can be seen in reactions (3) through (8). Reactions (1) through (2c) involve the excitation of chlorophyll at the water surface under ambient air conditions and re-produce the parent chlorophyll. In the presence of halide salts, halogen atoms can be produced via a redox reaction between the chlorophyll cation and halide anion (3a). Following the halogen production, the chlorophyll may be degraded (3b). Under our illumination conditions, reactions (3a) and (3b) represent the rate determining step because the rate of chlorophyll loss depends first on how fast the halide reacts in a redox reaction (3a) to yield a halogen atom, and then on the reaction kinetics between this atom and chlorophyll (3b). The Cl2- observed by Christian George’s group is explained by (4)38. Reactions (5) through (8) show the photosensitized oxidation of ozone by chlorophyll, and the corresponding production of more halogen atoms. The photoenhanced rate of loss of chlorophyll at the salt water-air interface is due to the production of halogen atoms. These results, as well as the above reaction mechanism, have several important environmental implications in the chemistry in the SML and MBL. The lack of photoreactivity of chlorophyll with nitrate may indicate a possible source of reactive nitrogen oxide species. Halogens are known to catalytically destroy ozone, and the above mechanism may be an important source of halogens to the MBL; halogenated hydrocarbons, molecular halogens and halogen oxides. Iodine oxide, in the presence of ozone, has the potential for particle growth and could affect the Earth’s radiative budget27-29. Algae and phytoplankton blooms have been known sources of iodine species for several years but little of the chemistry is understood21, 26, 28. A reaction between chlorophyll and iodide could produce iodine in reaction (3a) and may be a major source of iodine in the MBL potentially forming molecular iodine, organoiodine compounds, or iodine oxides.

55

Our work provides important evidence of chlorophyll acting as a photosensitizer in an overall reaction between halides and ozone. If this photosensitization reaction occurs with chlorophyll, with or without ozone, then it may occur with other photoactive species in the SML and indicate an even greater source of reactive halogen species in the marine environment. In fact, halogen production has been observed during the photoreaction between benzophenone and aqueous salts, and it is possible that this reaction follows the above mechanism57. This work opens the door to a great deal of future research. It would be beneficial to measure the halogens and/or halides that are produced during the photoreaction of chlorophyll at the aqueous surface. It would also be interesting to perform similar research using other large, unsaturated, ubiquitous and photoactive dissolved organic matter. If this biological molecule is indeed a major source of halogens in the atmosphere then it is important that it be included in climate models.

56

References

(1) Plane, J. M. C.; Blough, N. V.; Ehrhardt, M. G.; Waters, K.; Zepp, R. G.; Zika, R. G. In Report Group 3- Photochemistry in the sea-surface microlayer; Liss, P. S., Duce, R. A., Eds.; The Sea Surface and Global Change; Cambridge University Press: Cambridge, 1997; pp 71. (2) Mmereki, B. T.; Donaldson, D. J.; Gilman, J. B.; Eliason, T. L.; Vaida, V. Atmos. Environ. 2004, 38, 6091-6103. (3) Finlayson-Pitts, B. J.; Pitts, J. N. In Global Tropospheric Chemistry and Climate Change; Chemistry of the Upper and Lower Atmosphere; Academic Press: San Diego, CA, 2000. (4) Ardura, D.; Donaldson, D. J. Phys. Chem. Chem. Phys. 2009, 11, 857-863. (5) Aumann, E.; Tabazadeh, A. J. Geophys. Res-Atmos. 2008, 113, D23205. (6) Mmereki, B. T.; Chaudhuri, S. R.; Donaldson, D. J. J. Phys. Chem. A. 2003, 107, 2264-2269. (7) Clifford, D.; Bartels-Rausch, T.; Donaldson, D. J. Phys. Chem. Chem. Phys. 2007, 9, 13621369. (8) Gianguzza, A.; Pelizzetti, E.; Sammartano, S., Eds.; In Chemistry of Marine Water and Sediments; Springer: Germany, 2002. (9) Liss, P. S.; Duce, R. A., Eds.; In The Sea Surface and Global Change; Cambridge University Press: Cambridge, 1997. (10) Chapman, P.; Liss, P. S. Limnol. Oceanogr. 1981, 26, 387-390. (11) Zemmelink, H. J.; Houghton, L.; Sievert, S. M.; Frew, N. M.; Dacey, J. W. H. Marine Ecology-Progress Series. 2005, 295, 33-42. (12) Yang, G. P.; Levasseur, M.; Michaud, S.; Scarratt, M. Mar. Chem. 2005, 96, 315-329. (13) Matrai, P. A.; Tranvik, L.; Leck, C.; Knulst, J. C. Mar. Chem. 2008, 108, 109-122. (14) Joux, F.; Agogue, H.; Obernosterer, I.; Dupuy, C.; Reinthaler, T.; Herndl, G. J.; Lebaron, P. Aquat. Microb. Ecol. 2006, 42, 91-104.

57

(15) Hunter, K. A. In Chemistry of the sea-surface microlayer; Liss, P. S., Duce, R. A., Eds.; The Sea Surface and Global Change; Cambridge University Press: Cambridge, 1997; pp 287. (16) Blough, N. V. In Photochemistry in the sea-surface microlayer; Liss, P. S., Ed.; The Sea Surface and Global Change; Cambridge University Press: Cambridge, 1997; pp 383. (17) Asher, W. In The sea-surface microlayer and its effects on global air-sea gas transfer; Liss, P. S., Ed.; The Sea Surface and Global Change; Cambridge University Press: Cambridge, 1997; pp 251. (18) Brasseur, G. P.; Orlando, J. J.; Tyndall, G. S., Eds.; In Atmospheric Chemistry and Global Change; Birks, J. W., Ed.; Topics in Environmental Chemistry; Oxford University Press: New York, 1999. (19) Hardy, J. T.; Hunter, K. A.; Calmet, D.; Cleary, J. J.; Duce, R. A.; Forbes, T. L.; Gladyshev, M. L.; Harding, G.; Shenker, J. M.; Tratnyek, P.; Zaitsev, Y. In Report Group 2- Biological effects of chemical and radiative change in the sea surface; Liss, P. S., Ed.; The Sea Surface and Global Change; Cambridge University Press: Cambridge, 1997; pp 35. (20) Ghosal, S.; Brown, M. A.; Bluhm, H.; Krisch, M. J.; Salmeron, M.; Jungwirth, P.; Hemminger, J. C. J. Phys. Chem. A. 2008, 112, 12378-12384. (21) Carpenter, L. J.; Liss, P. S.; Penkett, S. A. J. Geophys. Res-Atmos. 2003, 108, 4256. (22) Clifford, D.; Donaldson, D. J. J. Phys. Chem. A. 2007, 111, 9809-9814. (23) Skov, H.; Christensen, J. H.; Goodsite, M. E.; Heidam, N. Z.; Jensen, B.; Wahlin, P.; Geernaert, G. Environ. Sci. Technol. 2004, 38, 2373-2382. (24) Seigneur, C.; Lohman, K. J. Geophys. Res-Atmos. 2008, 113, D23309. (25) Hunt, S. W.; Roeselova, M.; Wang, W.; Wingen, L. M.; Knipping, E. M.; Tobias, D. J.; Dabdub, D.; Finlayson-Pitts, B. J. J. Phys. Chem. A. 2004, 108, 11559-11572. (26) Chance, R.; Malin, G.; Jickells, T.; Baker, A. R. Mar. Chem. 2007, 105, 169-180. (27) Carpenter, L. J. Chem. Rev. 2003, 103, 4953-4962. (28) McFiggans, G.; Coe, H.; Burgess, R.; Allan, J.; Cubison, M.; Alfarra, M. R.; Saunders, R.; Saiz-Lopez, A.; Plane, J. M. C.; Wevill, D. J.; Carpenter, L. J.; Rickard, A. R.; Monks, P. S. Atmos. Chem. Phys. 2004, 4, 701-713. (29) Martino, M.; Mills, G. P.; Woeltjen, J.; Liss, P. S. Geophys. Res. Lett. 2009, 36.

58

(30) Meskhidze, N.; Nenes, A. Science 2006, 314, 1419-1423. (31) Du, H.; Fuh, R. C. A.; Li, J. Z.; Corkan, L. A.; Lindsey, J. S. Photochem. Photobiol. 1998, 68, 141-142. (32) McClain, C. R. Annual Review of Marine Science 2009, 1, 19. (33) Babin, S. M.; Carton, J. A.; Dickey, T. D.; Wiggert, J. D. J. Geophys. Res.-Oceans. 2004, 109, C03043. (34) Banse, K.; English, D. C. J. Geophys. Res.-Oceans. 1994, 99, 7323-7345. (35) Zhang, H.; Yang, G.; Zhu, T. Cont. Shelf Res. 2008, 28, 2417-2427. (36) NASA SeaWiFS Project. http://oceancolor.gsfc.nasa.gov/SeaWiFS/. (37) Mmereki, B. T. Laser Induced Fluorescence Study of the Heterogeneous Interaction of Polycyclic Aromatic Hydrocarbons with Aqueous Interfaces: Adsorption and Reaction with Gas-Phase Ozone, University of Toronto, Toronto, 2005. (38) Reeser, D. I.; Jammoul, A.; Clifford, D.; Brigante, M.; D’Anna, B.; George, C.; Donaldson, D. J. J. Phys. Chem. C. 2009, 113, xxxx. (39) Clifford, D.; Donaldson, D. J.; Brigante, M.; D'Anna, B.; George, C. Environ. Sci. Technol. 2008, 42, 1138-1143. (40) Mmereki, B. T.; Donaldson, D. J. J. Phys. Chem. A. 2003, 107, 11038-11042. (41) Kahan, T. F.; Donaldson, D. J. J. Phys. Chem. A. 2007, 111, 1277-1285. (42) Kahan, T. F.; Reid, J. P.; Donaldson, D. J. J. Phys. Chem. A. 2007, 111, 11006-11012. (43) Kahan, T. F.; Kwamena, N. O. A.; Donaldson, D. J. Atmos. Environ. 2006, 40, 3448-3459. (44) Molina, L. T.; Molina, M. J. J. Geophys. Res-Atmos. 1986, 91, 14501-14508. (45) Gallagher, M. W.; Beswick, K. M.; Coe, H. Q. J. R. Meteorol. Soc. 2001, 127, 539-558. (46) Stemmler, K.; Ndour, M.; Elshorbany, Y.; Kleffmann, J.; D'Anna, B.; George, C.; Bohn, B.; Ammann, M. Atmos. Chem. Phys. 2007, 7, 4237-4248. (47) Stemmler, K.; Ammann, M.; Donders, C.; Kleffmann, J.; George, C. Nature 2006, 440, 195198. (48) McNeill, V. F.; Wolfe, G. M.; Thornton, J. A. J. Phys. Chem. A. 2007, 111, 1073-1083.

59

(49) Jammoul, A.; Gligorovski, S.; George, C.; D'Anna, B. J. Phys. Chem. A. 2008, 112, 12681276. (50) Kwamena, N. O. A.; Earp, M. E.; Young, C. J.; Abbatt, J. P. D. J. Phys. Chem. A. 2006, 110, 3638-3646. (51) Newport , Oriel Product Training: Spectral Irradiance, pp 24. (52) Knipping, E. M.; Dabdub, D. J. Geophys. Res-Atmos. 2002, 107, 4360. (53) Hunt, S. W.; Roeselova, M.; Wang, W.; Wingen, L. M.; Knipping, E. M.; Tobias, D. J.; Dabdub, D.; Finlayson-Pitts, B. J. J. Phys. Chem. A. 2004, 108, 11559-11572. (54) Knipping, E. M.; Lakin, M. J.; Foster, K. L.; Jungwirth, P.; Tobias, D. J.; Gerber, R. B.; Dabdub, D.; Finlayson-Pitts, B. J. Science 2000, 288, 301-306. (55) Cosman, L. M.; Bertram, A. K. J. Phys. Chem. A. 2008, 112, 4625-4635. (56) Stemmler, K.; Vlasenko, A.; Guimbaud, C.; Ammann, M. Atmos. Chem. Phys. 2008, 8, 5127-5141. (57) Jammoul, A.; Dumas, S.; D’Anna, B.; George, C. 2009, “to be submitted” (58) Demou, E.; Donaldson, D. J. J. Phys. Chem. A. 2002, 106, 982-987. (59) Shen, Y. R.; Ostroverkhov, V. Chem. Rev. 2006, 106, 1140-1154. (60) Korenowski G.M. In Applications of laser technology and laser spectroscopy in studies of the ocean microlayer; Liss, P. S., Duce, R. A., Eds.; The Sea Surface and Global Change; Cambridge University Press: Cambridge, 1997; pp 445. (61) NIST Chemistry WebBook. http://webbook.nist.gov/chemistry/. (62) Kerr, J. A.; Moss, S., J., Eds.; In CRC Handbook of Bimolecular and Termolecular Gas Reactions; CRC Press, Inc.: Boca Raton, Florida, 1981; Vol. I.

Suggest Documents