Abstract
Abstract. Satellite observations of tropospheric NO2 columns are extensively used to infer trends in anthropogenic emissions of nitrogen oxides (NOx≡NO+NO2), but this may be complicated by trends in NOx 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 NO2 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 NO2 columns from summer to winter, which we explain in GEOS-Chem as reflecting a longer NOx lifetime in winter than in summer (21 h versus 5.9 h in 2017). The 2005–2018 summer trends of OMI NO2 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 NOx lifetime in summer, supporting the emission trend reported by the MEIC. The winter trend of OMI NO2 is steeper than in summer over the entire period, which we attribute to a decrease in NOx lifetime at lower NOx emissions. Half of the NOx sink in winter is from N2O5 hydrolysis, which counterintuitively becomes more efficient as NOx emissions decrease due to less titration of ozone at night. The formation of organic nitrates also becomes an increasing sink of NOx as NOx emissions decrease but emissions of volatile organic compounds (VOCs) do not.
Highlights
Emissions of nitrogen oxides (NOx ≡ NO + NO2) from fossil fuel combustion in China have been changing fast in the past few decades due to rapid economic expansion on the one hand and strengthening environmental regulations on the other hand
We focus on the large polluted region of central–eastern China (30–41◦ N, 112–122◦ E; rectangles in Fig. 1), where we may expect tropospheric NO2 columns to be most sensitive to Chinese NOx emissions and where the relatively narrow latitude range leads to consistent seasonal variations
In central–eastern China, we find that in both seasons over 70 % of the GEOS-Chem tropospheric NO2 column as would be observed by Ozone Monitoring Instrument (OMI) is in the boundary layer below 2 km of altitude
Summary
Emissions of nitrogen oxides (NOx ≡ NO + NO2) from fossil fuel combustion in China have been changing fast in the past few decades due to rapid economic expansion on the one hand and strengthening environmental regulations on the other hand. Differences in the trends between NO2 columns and NOx emission inventories could reflect errors in the inventories (Saikawa et al, 2017) and satellite retrievals (Lin et al, 2014; Lorente et al, 2017) and trends in the lifetime of NOx against atmospheric oxidation This lifetime is of the order of hours and may change with the chemical environment, including the NOx concentration itself (Stavrakou et al, 2008; Lamsal et al, 2011; Valin et al, 2011; Lu and Streets, 2012; Duncan et al, 2013; Gu et al, 2016; Cooper et al, 2017; Laughner and Cohen, 2019). The results have important implications for the use of satellite NO2 retrievals to infer trends in NOx emissions
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