TROPOMI-based near-surface NO2 concentration estimation and typical variations in provinces and cities during COVID-19 pandemic

Abstract

COVID-19 was a global public health crisis but, for atmospheric scientists, also an unprecedented opportunity. The lockdown measures unexpectedly created a near-perfect “controlled variable” experiment, enabling the development of a TROPOMI-based correction framework for near-surface concentrations of nitrogen dioxide (NSC-NO₂) and the construction of a ten-day-scale dataset of NSC-NO₂ across China, thereby facilitating precise quantification of human impacts on air pollution. The main findings are as follows: (1) the number of stations with a correlation coefficient greater than 0.6 between ground-based measured NO₂ (GBM-NO₂) and corrected column concentration of NO₂ (CC-NO₂) increased from 54 to 1200, indicating substantial quality improvement; (2) cross-validation results yielded an R² of 0.91 and an RMSE of 4.05 μg/m³ for the estimated NSC-NO₂, demonstrating strong spatiotemporal consistency with GBM-NO₂; and (3) using the estimated NSC-NO₂, China's NO₂ concentrations showed a marked decrease attributable to both the “holiday effect” and the COVID-19 lockdown. This research on NSC-NO₂ during the pandemic provides critical value for accurately quantifying human contributions to environmental change, validating correction models, informing stringent emission-reduction policies, and tracking progress toward the United Nations Sustainable Development Goals.