Influence of convective transport on tropospheric ozone and its precursors in a chemistry-climate model

Abstract

Abstract. The impact of convection on tropospheric O 3 and its precursors has been examined in a coupled chemistryclimate model. There are two ways that convection affects O 3 . First, convection affects O 3 by vertical mixing of O 3 itself. Convection lifts lower tropospheric air to regions where the O 3 lifetime is longer, whilst mass-balance subsidence mixes O 3 -rich upper tropospheric (UT) air downwards to regions where the O 3 lifetime is shorter. This tends to decrease UT O 3 and the overall tropospheric column of O 3 . Secondly, convection affects O 3 by vertical mixing of O 3 precursors. This affects O 3 chemical production and destruction. Convection transports isoprene and its degradation products to the UT where they interact with lightning NO x to produce PAN, at the expense of NO x . In our model, we find that convection reduces UT NO x through this mechanism; convective down-mixing also flattens our imposed profile of lightning emissions, further reducing UT NO x . Over tropical land, which has large lightning NO x emissions in the UT, we find convective lofting of NO x from surface sources appears relatively unimportant. Despite UT NO x decreases, UT O 3 production increases as a result of UT HO x increases driven by isoprene oxidation chemistry. However, UT O 3 tends to decrease, as the effect of convective overturning of O 3 itself dominates over changes in O 3 chemistry. Convective transport also reduces UT O 3 in the mid-latitudes resulting in a 13% decrease in the global tropospheric O 3 burden. These results contrast with an earlier study that uses a model of similar chemical complexity. Differences in convection schemes as well as chemistry schemes -in particular isoprene-driven changes are the most likely causes of such discrepancies. Further modelling studies are needed to constrain this uncertainty range.

Representation of tropical deep convection in atmospheric models – Part 2: Tracer transport

“…Phys., 11, 8103-8131, 2011 www.atmos-chem-phys.net/11/8103/2011/ C. R. Hoyle et al: Modelling tropical deep convection 8105 convective transport. It is further pointed out by Doherty et al (2005) that their results contrast with the increase in global O 3 simulated by Lawrence et al (2003) with a similar model. They suggest that this could be due to model differences in convective mixing.…”

“…They attribute these differences not only to the strength and frequency of convective events, but also to processes such as wet scavenging, and chemistry, which are also affected by the transport of species to different regions. The chemistry-climate model STOCHEM-HadAM3 was used by Doherty et al (2005) to investigate the influence of convection on upper tropospheric O 3 . They find that the effect of convective overturning decreases the concentration of O 3 in the upper troposphere, despite a positive contribution from changes in O 3 chemistry associated with the Atmos.…”

Impact of climate change on tropospheric ozone and its global budgets

“…The climate change also comprises changes of convection, which play an important role in redistributing O 3 and its precursors in the troposphere (see Lawrence et al, 2003;Doherty et al, 2005). Figure 11 displays NO x for runs C-B; increases of NO x occur mainly in the tropical upper and middle troposphere, which are likely due to increased deep convection and the increased lightning activity respectively.…”

Tropical tropospheric ozone columns from nadir retrievals of GOME-1/ERS-2, SCIAMACHY/Envisat, and GOME-2/MetOp-A (1996–2012)

“…Lower tropospheric ozone is lifted up to the UT where ozone lifetime is longer, while, due to mass balance conservation, UT air rich in O 3 mixes and submerges into regions where ozone lifetime is shorter. As a result, the UT O 3 as well as the overall tropospheric O 3 column decreases (Doherty et al, 2005). Second, convective systems such as tropical cyclones can transport ozone precursors many kilometres away from their source, resulting in ozone production in remote areas where it builds up (Sauvage et al, 2006).…”