Evolution of global O3-NOx-VOCs sensitivity before and after the COVID-19 from the ratio of formaldehyde to NO2 from satellite observations

Abstract

Ozone production sensitivity is widely used to reveal the chemical dominant precursors of urban ozone rise. Here, we diagnose the impact of the decline in global human production activities level caused by the COVID-19 on ozone sensitivity through the ratio of formaldehyde (HCHO) and NO₂ (FNR = HCHO/NO₂) observations from the TROPOspheric Monitoring Instrument. We use a relative uncertainty threshold to clean the satellite FNR, and our satellite FNR present a good correlation (R = 0.6248) with U.S. Environmental Protection Agency observations. We found that the outbreak of the COVID-19 did not change the pattern of global ozone sensitivity, while the global regimes was transforming or strengthening to VOC-limited regimes due to the significant decline of human production activities levels. During the COVID-19, ozone sensitivity in Eastern China and East Africa continued to shift to VOC-limited regimes, while India, Western Europe and North America first moved to NO_x-limited regimes, and then changed to VOC-limited regimes with the resumption of production and the increase in travel. The clustering results tell that urban ozone sensitivity tends to shift towards NO_x-limited regimes as economic growing. The ozone formation in cities with lower FNR and per capita gross domestic product (GDP) are more sensitive to changes in VOCs, while cities with higher FNR and per capita GDP are more sensitive to variations in NO_x. Cities with intermediate FNR and GDP are good evidence of the existence of transitional regimes. Our study identifies the driving role of urban economics in orienting the evolution of ozone sensitivity regimes.