Abstract
Abstract. We have run a chemistry transport model (CTM) to systematically examine the drivers of interannual variability of tropospheric composition during 1996–2000. This period was characterised by anomalous meteorological conditions associated with the strong El Niño of 1997–1998 and intense wildfires, which produced a large amount of pollution. On a global scale, changing meteorology (winds, temperatures, humidity and clouds) is found to be the most important factor driving interannual variability of NO2 and ozone on the timescales considered. Changes in stratosphere-troposphere exchange, which are largely driven by meteorological variability, are found to play a particularly important role in driving ozone changes. The strong influence of emissions on NO2 and ozone interannual variability is largely confined to areas where intense biomass burning events occur. For CO, interannual variability is almost solely driven by emission changes, while for OH meteorology dominates, with the radiative influence of clouds being a very strong contributor. Through a simple attribution analysis for 1996–2000 we conclude that changing cloudiness drives 25% of the interannual variability of OH over Europe by affecting shortwave radiation. Over Indonesia this figure is as high as 71%. Changes in cloudiness contribute a small but non-negligible amount (up to 6%) to the interannual variability of ozone over Europe and Indonesia. This suggests that future assessments of trends in tropospheric oxidizing capacity should account for interannual variability in cloudiness, a factor neglected in many previous studies.
Highlights
Reactive tropospheric trace gases play an important role in the Earth system, influencing climate (methane (CH4), ozone) and determining air quality (ozone, nitrogen oxides (NOx), volatile organic compounds)
We have presented an assessment of how meteorology, emissions and clouds drive the interannual variability of important tropospheric tracers based on chemistry transport model (CTM) calculations for the changes in meteorolwogwywan.adtma opasr-tcihcuelmar-lpyhiymsp.noertta/n1t0c/2o4m9p1o/-2010/ nent of this influence is the radiative effect of the variability in cloudiness
We have presented an assessment of how meteorology, emissions and clouds drive the interannual variability of important www.atmos-chem-phys.net/10/2491/2010/
Summary
Reactive tropospheric trace gases play an important role in the Earth system, influencing climate (methane (CH4), ozone) and determining air quality (ozone, nitrogen oxides (NOx), volatile organic compounds). Transpacific transport can be very important, with Asian emissions impacting clean oceanic regions, or even North America, depending on the meteorology (Liu et al, 2005; Cooper et al, 2010) This influence is strong during El Nino years. Clouds can directly affect gas-phase tropospheric chemistry by decreasing shortwave radiation below them, and increasing it above them This leads to important effects on photolysis and, through J (O1D) modifications, on OH concentrations above and below clouds (especially in the boundary layer), with a variety of subsequent effects on ozone and its precursors (for more details see Tie et al (2003); Liu et al (2006); Voulgarakis et al (2009b)).
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