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.