NO₂ concentration differences under clear versus cloudy skies and implications for applications of satellite measurements

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. While the lack of high-quality column measurements during cloudy conditions precludes validation of the satellite data, in situ surface measurements and model simulations can provide insight on the quantitative understanding of NO₂ during cloudy conditions. Here, we intercompare surface observations, satellite measurements, and models during 2019 over the contiguous U.S. to quantify how NO₂ concentrations are different under clear and cloudy skies. We find that in situ surface NO₂ measurements are, on average, +17 % larger on all days compared to restricting 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 U.S. and smallest in the Southwest U.S. and near major roadways. WRF-Chem simulated surface NO₂ between cloudy and clear conditions is on average much larger than the observed differences: +59 % on cloudy days vs. clear days for the model. This suggests that NO₂ in WRF-Chem is more responsive to sunlight and associated photochemistry than in reality. 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, and introduces some corrections to account for these biases.