Nitrogen oxides (NOx) play a central role in tropospheric chemistry by regulating the atmospheric oxidation capacity. In the sun-lit atmosphere, NOx, together with volatile organic compounds (VOC), contributes to the accumulation of ozone, which has adverse effects on human health. In this study, the top-down NOx emissions over East Asia during May–June 2016 were estimated utilizing the satellite-observed nitrogen dioxide column densities and chemical transport model simulations. The estimated top-down emissions were 50–70% lower than the bottom-up emission inventory across most regions of China (reference year: 2010). This is associated with NOx emission reductions between 2010 and 2016, as well as uncertainties in bottom-up emission estimates for a given year. Applying the top-down emissions to the model simulations improved both the absolute values and spatial patterns of nitrogen dioxide (NO2), bringing them closer to the surface-level concentrations and satellite observations, although some biases remained in Seoul and other regions of South Korea. Surface ozone simulated using the estimated top-down NOx emissions increased, showing much better agreement with observations in Beijing and the North China Plains than the simulations based on the bottom-up emission inventory. Meanwhile, the model-simulated ozone levels using the top-down NOx emissions were overestimated in Shanghai (Yangtze River Delta, YRD) and Chengdu (Southwestern China, SWC). Additional sensitivity experiments indicated that the model also overestimated formaldehyde concentrations in Shanghai (YRD) and Chengdu (SWC), likely due to excessively high anthropogenic and biogenic VOC emissions, respectively. These results suggest that reducing anthropogenic VOC emissions can be effective in mitigating urban ozone pollution in the studied region.
Seo et al. (Mon,) studied this question.