Tropopause‐Level NOx in the Asian Summer Monsoon.

Deep convection within the Asian summer monsoon (ASM) transports surface level air into the upper troposphere‐lower stratosphere (UTLS). This work aims to understand the distribution of NO2, NO, and NOx in the UTLS ASM anticyclone from satellite measurements. Observations of NO2 from the Optical Spectrograph and InfraRed Imager System, the Atmospheric Chemistry Experiment ‐ Fourier Transform Spectrometer (ACE‐FTS), and the Stratospheric Aerosol and Gas Experiment III on the International Space Station are considered. The PRATMO photochemical box model is used to quantify the NOx photochemistry, and to derive the NOx concentration using OSIRIS NO2 and O3 observations. The satellite data show a relative minimum in NO2 over the ASM in the summer months, while the corresponding NO and NOx anomalies are elevated, mainly due to low O3 and cold temperatures within the ASM. The observations within the ASM show reasonable agreement to simulations from the Whole Atmosphere Community Climate Model. Plain Language Summary: Motion within the Asian summer monsoon (ASM) transports air from the Earth's surface to the upper troposphere‐lower stratosphere (UTLS), an altitude of around 10–15 km. This provides a pathway for surface level pollutants to influence the chemistry of the UTLS. One such pollutant is NOx, which is the sum of NO2 and NO. Here we consider observations of NO2 from the Optical Spectrograph and InfraRed Imager System, the Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer (ACE‐FTS), and the Stratospheric Aerosol and Gas Experiment III on the International Space Station, along with NOx derived from OSIRIS NO2 and O3 observations. The satellite data show that a relative maximum in NOx within the UTLS ASM corresponds to a relative minimum in NO2. This is mainly due to low O3 and cold temperatures within the ASM that alter the NOx chemistry. The observations show good agreement with climate model simulations. These findings provide new information on UTLS NOx behavior and the chemistry that occurs within the ASM. Key Points: Satellite data show a relative minimum in the NO2 concentration within the Asian summer monsoon (ASM) in the upper troposphere‐lower stratosphere (UTLS)Photochemical box model calculations show corresponding maxima in UTLS NO and NOx, due to low O3 and temperature in the monsoonNO2 and NOx from satellite data and a climate model show good agreement within the ASM [ABSTRACT FROM AUTHOR]