2015 and 2016 winter-time air pollution in China: SO2 emission changes derived from a WRF/Chem-EnKF coupled data assimilation system

Dan Chen, Zhiquan Liu, Junmei Ban, Min Chen
2019 Atmospheric Chemistry and Physics Discussions  
<p><strong>Abstract.</strong> Ambient pollutants in China changes significantly in recent years due to strict control strategies implemented by the government. The control strategies also bring uncertainties to both the <q>bottom-up</q> emission inventory and the model-ready gridded emission inputs especially in winter season. In this study, we updated the WRF/Chem-EnKF Data Assimilation system to quantitatively estimate the gridded hourly SO<sub>2</sub> emissions using hourly surface
more » ... y surface observations as constraints. Different from our previous study, in which meteorology and emission were both perturbed to obtain larger spread aiming to improve forecast skills; in this study, only emission was perturbed to ensure analyzed emission purely reflect necessary adjustments due to the emission uncertainties. In addition, direct emissions instead of emission scaling factors were used as analysis variable, which allowed for the detection of new emission sources. 2010 MEIC emission inventory (for January) was used as priori to generate 2015 and 2016 January analyzed emissions. The SO<sub>2</sub> emission changing trends for northern, western and southern China from 2010 to 2015 and that from 2015 to 2016 (for the month of January) were investigated. The January 2010&amp;ndash;2015 differences showed inhomogeneous change patterns in different regions: (1) significant emission reduction in southern China, (2) significant emission reduction in larger cities but widely increase in surrounding suburban and rural regions for northern China which may indicate the missing raw coal combustion for winter heating that not taken into account in the priori emission inventory; (3) significantly large emission increase in western China due to the energy expansion strategy. This not only reflected the changes during the five years, but also combined the uncertainties in the priori emissions. The January 2015&amp;ndash;2016 differences showed widely emission reduction from 2015 to 2016, indicating the stricter control strategy fully executed nationwide. These changes were corresponded to facts in reality, indicating that the updated DA system was capable to detect the emission deficiencies and optimize the emission. By generating the hourly analyzed emissions, the diurnal pattern of emissions (in terms of hourly factors) were also obtained. Forecast experiments showed the improvements by using analyzed emissions were much larger in southern China than that in northern and western China. For Sichuan Basin, Central China, Yangzi River Delta, and Pearl River Delta, BIAS and RMSE decreased by 61.8 %&amp;ndash;78.2 % and 27.9 %&amp;ndash;52.2 %, respectively, and correlation coefficients increased by 12.5 %&amp;ndash;47.1 %. However, the improvement in northern and western China were limited due to small spread. Another limitation of the study is that the analyzed emissions are still model dependent, as the ensembles are conducted through WRF/Chem model and thus the performances of ensembles are model dependent.</p>
doi:10.5194/acp-2018-1152 fatcat:zlwfgsvvknbxbnkpp3yd64kusm