Fifteen-Year Trends (2005–2019) in the Satellite-Derived Ozone-Sensitive Regime in East Asia: A Gradual Shift from VOC-Sensitive to NO x -Sensitive.

To mitigate tropospheric ozone (O₃) pollution with proper and effective emission regulations, diagnostics for the O₃-sensitive regime are critical. In this study, we analyzed the satellite-measured formaldehyde (HCHO) and nitrogen dioxide (NO₂) column densities and derived the HCHO to NO₂ ratio (FNR) from 2005 to 2019. Over China, there was a clear increase in the NO₂ column during the first 5-year period and a subsequent decrease after 2010. Over the Republic of Korea and Japan, there was a continuous decline in the NO₂ column over 15 years. Over the entire East Asia, a substantial increase in the HCHO column was identified during 2015–2019. Therefore, FNR increased over almost all of East Asia, especially during 2015–2019. This increasing trend in FNR indicated the gradual shift from a volatile organic compound (VOC)-sensitive to a nitrogen oxide (NO_x)-sensitive regime. The long-term changes in HCHO and NO₂ columns generally corresponded to anthropogenic non-methane VOC (NMVOC) and NO_x emissions trends; however, anthropogenic sources did not explain the increasing HCHO column during 2015–2019. Because of the reduction in anthropogenic sources, the relative importance of biogenic NMVOC sources has been increasing and could have a larger impact on changing the O₃-sensitive regime over East Asia. [ABSTRACT FROM AUTHOR]