Effect of changing NOx lifetime on the seasonality and long-term trends of satellite-observed tropospheric NO2 columns over China

Satellite observations of tropospheric NO 2 columns are extensively used to infer trends in anthropogenic emissions of nitrogen oxides ( NO x ≡ NO + NO 2 ), but this may be complicated by trends in NO x lifetime. Here we use 2004–2018 observations from the Ozone Monitoring Instrument (OMI) satellite-based instrument (QA4ECV and POMINO v2 retrievals) to examine the seasonality and trends of tropospheric NO 2 columns over central–eastern China, and we interpret the results with the GEOS-Chem chemical transport model. The observations show a factor of 3 increase in NO 2 columns from summer to winter, which we explain in GEOS-Chem as reflecting a longer NO x lifetime in winter than in summer (21 h versus 5.9 h in 2017). The 2005–2018 summer trends of OMI NO 2 closely follow the trends in the Multi-resolution Emission Inventory for China (MEIC), with a rise over the 2005–2011 period and a 25 % decrease since. We find in GEOS-Chem no significant trend of the NO x lifetime in summer, supporting the emission trend reported by the MEIC. The winter trend of OMI NO 2 is steeper than in summer over the entire period, which we attribute to a decrease in NO x lifetime at lower NO x emissions. Half of the NO x sink in winter is from N 2 O 5 hydrolysis, which counterintuitively becomes more efficient as NO x emissions decrease due to less titration of ozone at night. The formation of organic nitrates also becomes an increasing sink of NO x as NO x emissions decrease but emissions of volatile organic compounds (VOCs) do not.