Clear-sky and cloudy-sky differences in NO ₂ concentrations over the United States: implications for satellite measurement applications

Abstract. Satellite measurements of tropospheric trace gases are often only used when there are few clouds, which screens out 20 %–70 % of the data, depending on geographic region. Although clouds cause satellite data gaps, in situ surface measurements and model simulations can provide insight on NO2 during cloudy conditions. Here, we intercompare surface observations, meteorological reanalysis (ERA5), satellite measurements (TROPOMI and TEMPO), and a model (WRF-Chem) during 2019 over the contiguous US to quantify how NO2 concentrations differ under clear and cloudy skies. We find that in situ surface NO2 measurements are, on average, +17 % larger on all days compared to clear-sky days and +36 % larger during cloudy days versus clear-sky days, with a wide distribution based on geographic region and roadway proximity: largest in the Northeast US and smallest in the Southwest US and near major roadways. WRF-Chem simulated surface NO2 between cloudy and clear conditions is larger than the observed differences: +59 % on cloudy days vs. clear-sky days for the model. We additionally find modeled jNO2 values are reasonable, suggesting this WRF-Chem NO2 bias could be arising from overly rapid OH removal of NO2 in sunlight, too slow NOz regeneration of NO2 in sunlight, or differing boundary layer depth biases under cloudy versus clear skies. Finally, using in situ NO2 matched to provisional TEMPO data, we find the NO2 differences between cloudy and clear conditions to be larger in the afternoon than morning. This study quantifies some of the biases in satellite measurements introduced by using only clear-sky data.