Upper-tropospheric CO and O₃ budget during the Asian summer monsoon

Abstract. During the Asian summer monsoon, the circulation in the upper troposphere/lower stratosphere (UTLS) is dominated by the Asian monsoon anticyclone (AMA). Pollutants convectively uplifted to the upper troposphere are trapped within this anticyclonic circulation that extends from the Pacific Ocean to the Eastern Mediterranean basin. Among the uplifted pollutants are ozone (O₃) and its precursors, such as carbon monoxide (CO) and nitrogen oxides (NO_x). Many studies based on global modeling and satellite data have documented the source regions and transport pathways of primary pollutants (CO, HCN) into the AMA. Here, we aim to quantify the O₃ budget by taking into consideration anthropogenic and natural sources. We first use CO and O₃ data from the MetOp-A/IASI sensor to document their tropospheric distributions over Asia, taking advantage of the useful information they provide on the vertical dimension. These satellite data are used together with MOZAIC tropospheric profiles recorded in India to validate the distributions simulated by the global GEOS-Chem chemistry transport model. Over the Asian region, UTLS monthly CO and O₃ distributions from IASI and GEOS-Chem display the same large-scale features. UTLS CO columns from GEOS-Chem are in agreement with IASI, with a low bias of 11 ± 9 % and a correlation coefficient of 0.70. For O₃, the model underestimates IASI UTLS columns over Asia by 14 ± 26 % but the correlation between both is high (0.94). GEOS-Chem is further used to quantify the CO and O₃ budget through sensitivity simulations. For CO, these simulations confirm that South Asian anthropogenic emissions have a more important impact on enhanced concentrations within the AMA (∼  25 ppbv) than East Asian emissions (∼  10 ppbv). The correlation between enhanced emissions over the Indo-Gangetic Plain and monsoon deep convection is responsible for this larger impact. Consistently, South Asian anthropogenic NO_x emissions also play a larger role in producing O₃ within the AMA (∼  8 ppbv) than East Asian emissions (∼  5 ppbv), but Asian lightning-produced NO_x is responsible for the largest O₃ production (10–14 ppbv). Stratosphere-to-troposphere exchanges are also important in transporting O₃ in the upper part of the AMA.