Observationally Constrained NOₓ-Catalyzed Ozone Production in the Upper Troposphere During the Asian Summer Monsoon

Abstract

Intense and frequent convection occurring during the Asian Summer Monsoon (ASM) rapidly transports surface emissions to the upper troposphere (UT). Depending on their chemical reactivities, pollutants transported to the UT via this mechanism may either undergo chemistry in the UT region or be transported into the lower stratosphere. The Asian Summer Monsoon Chemical and CLimate Impact Project (ACCLIP) used high-altitude research aircraft to characterize chemistry in the tropopause region within the ASM anticyclone and outflows of monsoon convection during summer 2022. Here, we use measurements of trace gases from ACCLIP and results from a 0-D model constrained by airborne observations to calculate net ozone production rates in airmasses influenced by recent convection and in the summer monsoon background upper troposphere/lower stratospheric air. We find that ozone production inside the polluted air downstream of recent convection is up to one order of magnitude higher than that in the cleaner background upper troposphere. The ozone production is driven by NO + HO₂ produced from CO and OVOC oxidation, rather than from organic peroxy radicals produced from volatile organic compounds inside the highly polluted airmasses. Consistent with previous modeling work, we find efficient HOₓ (OH + HO₂) cycling is dominated by reactions with CO. Ozone production in this region is primarily NOₓ-limited and increases with higher NOx such as during convective events with lightning NOₓ production. Further, we find that the dominant impact of enhanced NOₓ here is through acceleration of HOₓ cycling, and thus an increase in ozone production.