THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLES Atmospheric oxidation is critical for removal of many pollutants, e.g. • methane (major greenhouse gas) • CO (toxic pollutant) • HCFCs (Clx sources in stratosphere) Oxidation Reduced gas

EARTH SURFACE

Oxidized gas/ aerosol

Uptake

Emission

Reduction

Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.

RADICAL REACTION CHAINS IN THE ATMOSPHERE non-radical available for photolysis

Preparation:

Initiation:

non-radical

Propagation: radical + non-radical

Termination:

radical + radical

radical + radical + M

Recycling:

non-radical

radical + radical

photolysis thermolysis oxidation by O(1D)

non-radical + radical bimolecular reactions

non-radical + non-radical radical reaction non-radical + M 3-body recombination

radical + radical

photolysis thermolysis oxidation by O(1D)

Main oxidant: OH •

Known since 1950s to be produced in the stratosphere • O3 + hν -> O2 + O(1D) R1 λ OH + OH R3

•

Known since 1970s to be produced also in the troposphere

•

POH = 2 k3 [O(1D)] [H2O]

Main oxidant: OH •

Main loss reactions • CO + OH -> CO2 + H • CH4 + OH -> CH3 + H2O

R4 R5

•

Life time typically 1 second, highly variable in space and time

•

No production during night (e.g. polar night), and zero concentrations

We need to know concentrations and budgets of CO and CH4 CO: 50 – 150 ppbv in remote areas CH4: increased from 800 to 1800 ppbv

THE TROPOSPHERE WAS VIEWED AS CHEMICALLY INERT UNTIL 1970 •

“The chemistry of the troposphere is mainly that of of a large number of atmospheric constituents and of their reactions with molecular oxygen…Methane and CO are chemically quite inert in the troposphere” [Cadle and Allen, Atmospheric Photochemistry, Science, 1970] • Lifetime of CO estimated at 2.7 years (removal by soil) leads to concern about global CO pollution from increasing car emissions [Robbins and Robbins, Sources, Abundance, and Fate of Gaseous Atmospheric Pollutants, SRI report, 1967] FIRST BREAKTHROUGH: • Measurements of cosmogenic 14CO place a constraint of ~ 0.1 yr on the tropospheric lifetime of CO [Weinstock, Science, 1969] SECOND BREAKTHROUGH:

• Tropospheric OH ~1x106 cm-3 predicted from O(1D)+H2O, results in tropospheric lifetimes of ~0.1 yr for CO and ~2 yr for CH4 [Levy, Science, 1971, J. Geophys. Res. 1973] THIRD BREAKTHROUGH: • Methylchloroform observations provide indirect evidence for OH at levels of 2-5x105 cm-3 [Singh, Geophys. Res. Lett. 1977] …but direct measurements of tropospheric OH had to wait until the 1990s

WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT? Production of O(1D) in troposphere takes place in narrow band [290-320 nm]

solar flux I ozone absorption cross-section s

fsI O(1D) quantum yield f

Isaksen, I.S.A. and P.J. Crutzen, 1977: Uncertainties in calculated hydroxyl radical densities in the troposphere and stratosphere. Geophysica Norvegica, 31, 4, 1-10.

Målinger av OH

Schlosser et al., ACPD, 2009

OH-trender basert på CH3CCl3. Montzka et al. Science, 2011

CO oxidation mechanism (low NOx) •

• •

Reaction chain • CO + OH (+O2) -> CO2 + HO2 • HO2 + O3 -> OH + 2O2

R4+R6 R13

Hvilke komponenter er katalysatorer her?

Net: CO + O3 -> CO2 + O2 OH&HO2 catalysts in loss of O3 in the troposphere in clean environments (low NOx)

CO oxidation mechanism (high NOx) •

• • •

Reaction chain • CO + OH (+O2) -> CO2 + HO2 • HO2 + NO -> OH + NO2 • NO2 + hν (+O2) -> NO + O3 Net: CO + 2 O2 + hν -> CO2 + O3

R4+R6 R10 R11

OH&HO2, NO&NO2 catalysts in production of O3 in the troposphere Termination • HO2 + HO2 -> H2O2 (soluble) + O2 R7

Hvilke komponenter er katalysatorer her?

CH4 oxidation mechanism •

Reaction chain starting with • CH4 + OH -> CH3 + H2O

R5

•

The chain proceeds through several hydrocarbons, in different pathways, to produce O3 and HOx

•

Maximum yield (high NOx) • CH4 + 10 O2 -> CO2 + H2O + 5 O3 + 2 OH Minimum yield (low NOx) • CH4 + 3 OH + 2 O2 -> CO2 + 3 H2O + HO2

•

RADICAL CYCLE CONTROLLING TROPOSPHERIC OH AND OZONE CONCENTRATIONS O2

hn

O3

STRATOSPHERE 8-18 km TROPOSPHERE

hn

O3

NO2

NO

OH

HO2

hn, H2O

Deposition

CO, CH4 SURFACE

H2O2

IPCC [2013]

Tropospheric ozone Is the third most important anthropogenic greenhouse gas

IPCC [2013]

IPCC [2013]

IPCC [2013]

Utslippsscenarier RCP, brukt av IPCC, 2013

CARBON MONOXIDE IN ATMOSPHERE Source: incomplete combustion Sink: oxidation by OH (lifetime of 2 months)

Estimert utvikling av utslipp av CO og VOC 1980-2010

Monks et al., ACP, 2015

Betydningen av teknologistandarder for biler Her hovedsakelig diesel

Monks et al., ACP, 2015

Utslipp av biogene VOC

Monks et al., ACP, 2015

Monks et al., ACP, 2015

SATELLITE OBSERVATION OF CARBON MONOXIDE 150-250 ppb

MOPITT CO columns (Mar-Apr 01)

50-70 ppb

CO i Arktis, Observert og modellert (Shindell et al., ACP, 2008)

GLOBAL METHANE SOURCES, Tg y-1 [IPCC, 2007]

WETLANDS 100-230

BIOMASS BURNING ANIMALS 80-90 10-90 LANDFILLS 40-70 GAS 50-70

TERMITES 20-30 RICE 30-110

COAL 30-50

GLOBAL DISTRIBUTION OF METHANE NOAA/CMDL surface air measurements Sink: oxidation by OH (lifetime of 10 years)

HISTORICAL TRENDS IN METHANE The last 30 years

The last 1000 years

IPCC [2007]

Recent changes in CO2,N2O, CFCs and CH4

http://www.esrl.noaa.gov/gmd/aggi/

Metan fra smeltende Permafrost i Arktis – Er det en stor trussel?

Økte utslipp fra skifergass?

Metanutslipp fra skifergass

Monks et al., ACP, 2015

IPCC [2001] Projections of Future CH4 Emissions (Tg CH4) to 2050

Scenarios 900

800

700

A1B A1T A1F1 A2 B1 B2 IS92a

600 2000

2020 Year

2040

NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE

LIGHTNING 5.8

STRATOSPHERE 0.2

SOILS 5.1

BIOMASS BURNING BIOFUEL 5.2 AIRCRAFT 2.2 0.5

FOSSIL FUEL 23.1

Global budget of NOx • •

Emitted mainly as NO Fast chain (null cycle) • NO + O3 -> NO2 + O2 • NO2 + hv (+O2) -> NO + O3

•

Main loss day: • NO2 + OH + M -> HNO3 + M

•

Main loss night: • NO2 + O3 -> NO3 + O2 • NO3 + NO2 + M -> N2O5 + M • N2O5 + H2O (on aerosols, if available) -> 2 HNO3

Fuktige aerosoler v/RH