A new method is presented to estimate urban OH concentrations using the downwind decay of the TROPOMI derived NO2/CO ratio combined with Weather Research Forecast (WRF) simulations. Seasonal OH concentrations, 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 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 % indicating that the method is quite robust. 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 15.4±3.4% and doubling the CO emission.