Influence of convective transport on tropospheric ozone and its precursors in a chemistry-climate model
[摘要] The impact of convection on tropospheric O3 and its precursors has beenexamined in a coupled chemistry-climate model. There are two ways thatconvection affects O3. First, convection affects O3 by verticalmixing of O3 itself. Convection lifts lower tropospheric air to regionswhere the O3 lifetime is longer, whilst mass-balance subsidence mixesO3-rich upper tropospheric (UT) air downwards to regions where theO3 lifetime is shorter. This tends to decrease UT O3 and theoverall tropospheric column of O3. Secondly, convection affects O3by vertical mixing of O3 precursors. This affects O3 chemicalproduction and destruction. Convection transports isoprene and itsdegradation products to the UT where they interact with lightning NOxto produce PAN, at the expense of NOx. In our model, we find thatconvection reduces UT NOx through this mechanism; convectivedown-mixing also flattens our imposed profile of lightning emissions,further reducing UT NOx. Over tropical land, which has large lightningNOx emissions in the UT, we find convective lofting of NOx fromsurface sources appears relatively unimportant. Despite UT NOxdecreases, UT O3 production increases as a result of UT HOxincreases driven by isoprene oxidation chemistry. However, UT O3 tendsto decrease, as the effect of convective overturning of O3 itselfdominates over changes in O3 chemistry. Convective transport alsoreduces UT O3 in the mid-latitudes resulting in a 13% decrease inthe global tropospheric O3 burden. These results contrast with anearlier study that uses a model of similar chemical complexity. Differencesin convection schemes as well as chemistry schemes – in particularisoprene-driven changes are the most likely causes of such discrepancies.Further modelling studies are needed to constrain this uncertainty range.
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[效力级别] [学科分类] 大气科学
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