Estimation of OH in urban plume using TROPOMI inferred NO₂ / CO

Abstract. A new method is presented to estimate urban hydroxyl radical (OH) concentrations using the downwind decay of the Tropospheric Monitoring Instrument (TROPOMI) derived nitrogen dioxide (NO₂) / carbon monoxide (CO) ratio combined with Weather Research Forecast (WRF) simulations. Seasonal OH concentrations, nitrogen oxides (NOx) and CO emissions for summer (June to October, 2018) and winter (November, 2018 to March, 2019) are derived for Riyadh. WRF is able to spatially simulate NO₂ and CO urban plumes over Riyadh as observed by TROPOMI. However, WRF-simulated NO₂ plumes close to center of the city are overestimated by 25 % in summer and 40 to 50 % in winter compared to TROPOMI observations. WRF simulated CO plumes differ by 10 % with TROPOMI in both seasons. The differences between model and TROPOMI are used to optimize the OH concentration, NOx and CO emissions iteratively using a least squares method. For summer, both the NO₂ / CO ratio optimization and the XNO₂ optimization imply that the OH prior from the Copernicus Atmospheric Monitoring Service (CAMS) has to be increased by 32.03±4.0 % . The OH estimations from the NO₂ / CO ratio and the XNO₂ optimization differ by 10 %. Summer Emission Database for Global Atmospheric Research v4.3.2 (EDGAR) NOx and CO emissions over Riyadh need to be increased by 42.1±8.7 % and 100.8±9.5 %. For winter, the optimization method increases OH by ~52.0±5.3 %, while reducing NOx emission by 15.45± 3.4 % and doubling the CO emission. TROPOMI derived OH concentrations and pre-existing Exponentially Modified Gaussian function fit (EMG) method differ by 18 % in summer and 7.5 % in winter, confirming that urban OH concentrations can be reliably estimated using the TROPOMI-observed NO₂ / CO ratio.