Estimation of anthropogenic NO2 emissions over polluted regions using the LMDZ-INCA model and satellite observations from TROPOMI and OMI

Nitrogen dioxide (NO₂), one of the major pollutants, impacts air quality (especially in industrial and urban regions), climate change, etc. We utilize tropospheric vertical column NO₂ for 2019 and 2020 from a high-resolution nadir viewing spectrometer TROPOspheric Monitoring Instrument (TROPOMI) which is on board the Sentinel-5 Precurser (S5P) satellite and also from polar orbiting Ozone Monitoring Instrument (OMI) which is on-board the NASA Aura satellite. The high quality observations for TROPOMI are selected using qa_value >= 0.75 and for OMI using cloud radiation fraction < 0.5, zenith angle < 70°, etc. First, we present the simulations of atmospheric NO₂ tropospheric vertical columns from a global couple chemistry transport model LMDZ-INCA (Laboratoire de Météorologie Dynamique - INteraction with Chemistry and Aerosol) with a spatial resolution of 1.26×2.5×79 (lat×lon×hybrid σ-level) and temporal resolution of one hour. Further, we utilize monthly global anthropogenic emission inventories from open-source Community Emissions Data System (CDES) and evaluate the model abilities in simulating the observation. In the northern hemisphere, we observe high atmospheric NO₂ concentration during the winter season as compared to the summer season from TROPOMI and model due to a longer lifetime and increase in anthropogenic emission. The tropospheric vertical column NO₂ in the model is overestimated during winter and underestimated during summer seasons over East US, East Europe, and West Europe. The 50 percentile values between the model and observations are comparable over East US, East Europe, West Europe, and China. However, the model shows high concentration during winter over East and West Europe.

Furthermore, TROPOMI L2 retrievals of tropospheric vertical column NO₂ are employed to estimate the surface anthropogenic fluxes of NO_x (=NO+NO₂). A specific inversion system has been utilized for the estimation of global anthropogenic NO₂ emissions based on LMDZ-INCA and TROPOMI NO₂ tropospheric vertical columns for 2020 and 2021. It follows a scheme adapted from Zheng et al. (2020). The posterior fluxes are derived by calculating two components. First, we calculated the gridded local sensitivity of concentrations due to changes in the emission. Second, relative observation changes due to the meteorology of a year of interest (e.g. for 2020 or 2021) are calculated using the fluxes of 2019. The reduction of posterior tropospheric total column NO₂ as compared to prior during the COVID lockdown period of 2020 is noticed over all the regions. The posterior NO_x is decreased by ~30% over China during February-April 2020 compared to the same period of 2019. A relative reduction of anthropogenic NO_x fluxes in 2020 as compared to 2019 is observed over other regions (East US, East Europe, West Europe, India, etc.) as well. Our result shows a relative increase in anthropogenic NO_x fluxes for 2021 and 2022 as compared to 2019 and 2020 over our study regions.