Transactions on Ecology and the Environment vol 28, © 1999 WIT Press, www.witpress.com, ISSN 1743-3541

Correlations between Total Ozone and Near-Surface Ozone at Mountain Sites in Switzerland in Late Winter Guest contribution Stefan Bronnimann, Urs Neu and Heinz Wanner Institute of Geography, University ofBern, Hallerstr. 12, CH-3012 Bern, Switzerland Introduction The impact of changing UV radiation on tropospheric chemistry has become an important issue in environmental research in recent years. Theoretical considerations suggest that with increased UV radiation ozone photolysis and thus the formation of OH radicals is enhanced. This leads to an increase or decrease of near-surface ozone formation, depending on the NOx concentrations. Different model calculations (Liu and Trainer, 1988; Kriiger et #/., 1997) agree that an increase is expected for a polluted and a decrease for a clean atmosphere. Liu and Trainer (1988) calculated a threshold of 0.17 ppb NO% at 40° N in summer. Bronnimann and Neu (1998) suggest that increased UV radiation (e.g. due to low total ozone) was one cause for the unusually high ozone concentrations at two Swiss sites in February 1993. At this time of the year the still weak photochemical ozone formation at the considered elevated (not heavily polluted) sites is probably controlled by the sudden rapidly increasing solar radiation rather than by chemical limitations, thus sensitive to UV radiation changes. Depending on NOx, this photochemical link between stratospheric and nearsurface ozone contributes to positive or negative correlations between the two. This is studied in this paper. On an interannual time scale, total ozone is controlled by volcanic or anthropogenic ozone depletion in the stratosphere and oscillations (QBO, solar cycle, ENSO). On a short scale it is dynamically coupled with the tropospheric circulation which itself heavily influences surface ozone concentrations in winter (Davies et al., 1992). When analysing correlations between total and surface ozone, the latter influence must also be considered. Low total ozone is often accompanied by anticyclones (Vaughan and Price, 1991) and high Proceedings ofEUROTRAC Symposium '98 Editors: P.M. Borrell and P. Borrell © 1999: WITPRESS, Southampton

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temperatures in the free troposphere (Wege and Claude, 1997). Near-surface ozone production is therefore enhanced with low total ozone due to more favourable meteorology, also without taking the higher UV radiation into account (contributing to a negative correlation at the surface). Very high total ozone coincides with cut-off lows, which can lead to intrusions of stratospheric ozone into the middle troposphere (Davies and Schiipbach, 1994), contributing to a positive correlation in the free troposphere. In this study we focus on the contribution of increased UV radiation. Correlations between total ozone and near-surface ozone Ozone concentrations at two sites of the Swiss NABEL network (Fig. 1), Chaumont (1140 m a.s.l., on aridge),and Jungfraujoch (3580 m a.s.l., high alpine) are compared to daily total ozone values from Arosa (Dobson #101) and 500 hPa pressure fields. In order to capture the 'onset' of spring photochemistry, i.e. the earliest possible days with significant ozone production, the 20-day period from 5 to 24 February is analysed using 1992 to 1997 data. Only conditions favourable for photochemistry on a regional scale are chosen. The mean global radiation from 11:00-15:00 CET must be > 450 W/m^ at both sites (this is impossible before 5 February using a zenith transmittance of 0.9), resulting in a sample with 27 days (9 single days and 4 episodes).

Fig. 1: Location of the measurement sites Chaumont, Jungfraujoch, and Arosa. Shaded areas are higher than 700 and 2000 m a.s.l., respectively.

Transactions on Ecology and the Environment vol 28, © 1999 WIT Press, www.witpress.com, ISSN 1743-3541

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Daytime mean ozone concentrations (11:00-18:30) as a function of total ozone are shown in Fig. 2. At Chaumont a negative correlation between surface and total ozone exists which is strongest in the range of low total ozone. A second order polynomial fit explains 61 % of the variability. A weaker positive correlation is observed at Jungfraujoch. For high total ozone, surface ozone concentrations are 15 ppb higher at Jungfraujoch than at Chaumont. For very low total ozone the opposite is the case which is very unusual at this time of the year. The two correlations are probably due to one or both influences: UV radiation and circulation. = 0.61 (2nd order) = 0.22 (1st order) n = 27

+ CHA O JUN

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320 360 400 440 total ozone [DU] Fig. 2: Daytime mean ozone values at Jungfraujoch and Chaumont as a function of total ozone for sunny February days. Lines and explained variances refer to least square fits (Bronnimann and Neu, 1993).

The contribution of the tropospheric correlations

circulation to the observed

To investigate the influence of the tropospheric circulation, mean 500 hPa pressure fields are calculated for (a) the 17 days with total ozone < 320 DU and (b) the 10 days with total ozone > 320 DU (Fig. 3). For (a), high-pressure systems dominate. The geopotential height above the central Alps is 5700 gpm with a standard deviation of only 30 to 40 gpm. Daily maximum temperatures at Chaumont and Jungfraujoch are high, i.e. 6.4 to 8.8 °C and -7.0 to -4.1 °C, respectively (quartiles). Wind speeds at Chaumont are very low. The meteorology is thus favourable for photochemistry while transport is clearly limited. Within (a), temperature and NOx concentrations show little variability and are not correlated with total ozone. The mean pressure field for (b) shows dominating northerly advection to the Swiss Alps. The mean geopotential height is 200 gpm lower (or 6 standard deviations of (a)) and wind speeds higher than for (a). Daily temperature

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maxima are very low, i.e. -7.4 to -0.2 °C at Chaumont and -17.2 to -11.8 °C at Jungfraujoch, respectively (quartiles). Ozone can not be efficiently built up under these conditions. The high ozone values at Jungfraujoch could be due to large-scale transport; an influence of upper-tropospheric or stratospheric air masses is possible. Circulation patterns can thus partly explain the surface ozone differences between (a) and (b).

40W

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0

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Fig. 3: Mean 500 hPa geopotential height (12 UTC, 5° x 5° grid from NCAR) for groups (a) and (b), respectively. The contribution of photochemistry to the observed correlations To estimate the influence of regional photochemistry, mean diurnal cycles of ozone and NO% concentrations for both sites are shown in Fig. 4. NO% measurements at Chaumont suffer from cross sensitivities, representing rather NOy than NO% Still, with 10-12 ppb during the day, NOx levels at Chaumont for (b) are high and ozone formation is chemically restricted. The afternoon peak in the diurnal ozone cycle could be due to either photochemistry or

Transactions on Ecology and the Environment vol 28, © 1999 WIT Press, www.witpress.com, ISSN 1743-3541

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transport. However, most of these afternoon peaks are still well below background values. Thus, although photochemistry could be involved, ozone concentrations at Chaumont are kept low by the transport of cold and NO% rich air masses, which are depleted in ozone.

-O3 (a) 50 -I

6 12 18 time of day (GET)

6 12 18 time of day (GET)

Fig. 4: Mean diurnal cycles of ozone and NO% concentrations for groups (a) and (b) at Chaumont and Jungfraujoch, respectively. NO% concentrations at Jungfraujoch are median values. For group (a), both sites show distinct diurnal ozone cycles. The concentration at Chaumont increases from 9:30 to 15:30 by 1.2 ppb/h. Ozone concentrations at Jungfraujoch are highest in the morning, decrease during the day and recover during the night. Both diurnal cycles can be explained photochemically when taking the different NO% concentrations into account. The NO% (NOy) level at Chaumont, with a slight peak in the afternoon, is around 5 ppb which is favourable for photochemical ozone production. Chaumont is then often sited in the clean air above a marked inversion but still slightly supplied with emissions from the nearby Swiss Plateau by weak slope winds. At Jungfraujoch the median NO% concentration is 0.05 ppb (some data are missing). In this concentration range net photochemical ozone destruction can occur during the day. This is a possible explanation for the observed diurnal ozone cycle. The diurnal ozone and NO% cycles indicate that in group (a) photochemistry influences the ozone concentration during the day and thus the daytime mean values used in the correlations. Discussion and conclusions What can be concluded for the relation between total ozone and near-surface ozone? When regarding the difference between the two groups (a) and (b),

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e.g. comparing 420 to 300 DU, the main contribution to the observed correlations probably comes from the circulation patterns. For high total ozone advection of cold, ozone poor, and NO% rich air masses restrict ozone formation while for low total ozone high-pressure systems, high air temperatures, and less polluted air masses are favourable for photochemical build-up of ozone. However, this is no explanation for the differences within the group (a), e.g. between 300 and 240 DU. The correlation of near-surface ozone at Chaumont with total ozone is especially strong within group (a). At the same time the standard deviation of the mean pressure field is small and temperature and NOx levels change very little. Thus, the correlation between total and surface ozone within (a) can not easily be attributed to different circulation patterns or air masses. A more plausible cause is photochemistry, which is involved and is amplified by increased UV radiation. With record low total ozone, record high near-surface ozone concentrations were observed at Chaumont in 1993. On several days in (a) the radiation was also enhanced due to backscattering at fog layers (van Weele and Duynkerke, 1993) over the Swiss Plateau, but which usually disappeared around noon. This influence can not easily be quantified. It is suggested that large changes of UV radiation, to which total ozone changes contribute substantially, are a major influence on surface ozone concentrations within the group of low total ozone. It is planned to extend the study to other months and other sites to estimate the relevance of this influence for ozone climatology in Switzerland and for numerical modelling. Acknowledgements The study is supported by the Swiss Agency for Environment, Forests and Landscape (BUWAL) under contract FE/BUWAL/810.98.7. We thank the Swiss Meteorological Institute, BUWAL, and NCAR for providing data and Frank Neidhofer for data processing. References Bronnimann, S., U. Neu; J. Atmos. Chem. submitted, (abstract in Ann. Geophysicae 16 (1998)Suppl. II,C754). Davies, T.D., P.M. Kelly, P.S. Low, C.E. Pierce; J. Geophys. Res. 97 (1992) 9819-9832. Davies, T.D., E. Schtipbach; Atmos. Environ. 28 (1994) 53-68. Liu, S.C.,M. Trainer;/ Atmos. Chem. 6 (1988)221-233. Kruger, B.C., A. Martilli, J. Ktibler; Ann. Geophysicae 15 (1997) Suppl. II, C545. Van Weele, M., P.G. Duynkerke; J. Atmos. Chem. 16 (1993) 231-255. Vaughan, G., J.D. Price; Q. J. R. Met. Soc. 117 (1991) 1281-1298. Wege, K., H. Claude; Meteor. Z., N. F. 6 (1997) 73-87.