Hourly near-ground NO₂ concentration retrieval from geostationary satellite observations

Abstract. Nitrogen dioxide (NO₂) is an important contributor to the formation of acid rain, photochemical smog and aerosol particles, which seriously endangers public health. At present, remote sensing of polar-orbiting satellites is a conventional means to obtain large-scale NO₂ distribution, but it cannot capture the rapid change because of long revisit periods. The Advanced Himawari Imager (AHI) on the Himawari-8 geostationary satellite has the advantage of high time resolution, which makes it possible to realize near-real-time atmospheric monitoring. Here, based on the absorption characteristics of NO₂ in infrared radiation, hourly near-surface NO₂ concentrations are retrieved based on the brightness temperature from AHI and auxiliary information such as meteorology and aerosol. The results of 10-fold cross-validation show that NO₂ estimations from satellite are in good agreement with in-situ measurements, and their determination coefficient (R²) can reach 0.79. Due to different emission and atmospheric diffusion conditions at different time, the model performance presents a diurnal variation of high accuracy in the noon and afternoon but low accuracy in the morning. Based on the retrieval dataset, it is found that high NO₂ concentrations are mainly concentrated in the densely populated and industrial areas such as the North China area. In addition, NO₂ pollution mainly occurs in autumn and winter, and the average NO₂ concentration in winter is about 1.63 times that in summer in 2021. This study provides a new insight for satellite retrieval of NO₂, which is of great significance for real-time pollution monitoring and public health protection.