Remote sensing evidence of decadal changes in major tropospheric ozone precursors over East Asia

Recent regulatory policies in East Asia reduce ozone precursors, but these changes are spatially and temporally nonuniform. This study investigates variations in the long‐term trends of tropospheric NO 2 , HCHO, and HCHO/NO 2 ratios to diagnose ozone sensitivity to changes in NO x and volatile organic compound using the Ozone Monitoring Instrument (OMI). Using an adaptive‐degree polynomial filter, we identify extremums of time series of NO 2 to determine when and how NO 2 change. Due to the regulations in China, trends which were predominantly upward turned downward. The years undergoing these changes primarily happened in 2011 and 2012. OMI column densities, however, suggest that NO x sources in South Korea, the Pearl River Delta (PRD), Taiwan, and Japan have not consistently decreased. Specifically, as Chinese exports of NO 2 started subsiding, increasing trends in NO 2 columns over several Korean cities, including Seoul, become evident. To quantify the changes in NO x emissions from summertime 2010 to 2014, we conduct a 3D‐Var inverse modeling using a regional model with MIX‐Asia inventory and estimate NO x emissions (in 2010 and 2014) for the PRD (1.6 and 1.5 Gg/d), the Yangtze River Delta (3.9 and 3.0 Gg/d), north China (15.6 and 14.3 Gg/d), South Korea (1.6 and 1.5 Gg/d), and Japan (2.7 and 2.6 Gg/d). OMI HCHO shows upward trends in East Asia resulting from anthropogenic effects; however, the magnitudes are negative in the PRD, Japan, North Korea, and Taiwan. OMI HCHO/NO 2 ratios reveal that while South Korea, Japan, and the south of China have undergone toward more NO x ‐sensitive regime, areas around the Bohai Sea have become more NO x saturated.