Evaluating NOx emission fluxes over East Asia by comparison between CMAQ-estimated and OMI-retrieved NO2 columns

Abstract:

To evaluate NO_x emission fluxes in East Asia, CMAQ-calculated NO₂ columns were compared with OMI-retrieved NO₂ columns. For a direct comparison between the two columns, the averaging kernels (AKs) retrieved from the Royal Netherlands Meteorological Institute (KNMI) algorithm were applied to the CMAQ model simulations. When the two NO₂ columns before and after the applications of AKs were compared over East Asia, it was found that, for example, the normalized mean errors (NMEs) between the CMAQ-estimated and OMI-retrieved NO₂ columns were reduced significantly, from ~103% to ~46%, from ~112% to ~45%, and from ~135% to ~40% during spring, fall, and winter, respectively. Also, the two tropospheric NO₂ columns were better correlated spatially in East Asia (correlation coefficients = 0.71-0.94) after the application of the AKs. From this study, it was found that the NO_x emission used were possibly underestimated in East Asia, although some overestimates were also found, partly over southern Central East China, the Sichuan Basin, and South Korea regions during the winter. However, these results can also be influenced by several uncertainty factors in the CMAQ model simulations, such as monthly variation and the strengths of the NO_x emissions. Thus, we also applied another monthly variation and different strengths of the NO_x emissions to the CMAQ model simulations over East Asia. The results showed strong impacts on the tropospheric NO₂ columns in East Asia, indicating that these two factors are also important. In this study, further sensitivity analysis was conducted with reaction probabilities of N₂O₅ onto atmospheric aerosols.