Environmental Process and Convergence Belt of Atmospheric NO2 Pollution in North China

Both surface environmental monitoring and satellite remote sensing show that North China is one of the regions that are heavily polluted by NO2. Using the NO2 monitoring data from 18 major cities in the region, the tropospheric NO2 column density data from the Ozone Monitoring Instrument (OMI) on the Aura satellite, and the observations from the China Meteorological Administration network, this paper analyzes a regional NO2 pollution event in February 2007 over North China, examines the convergence of the pollutant, and identifies its correlation with the atmospheric background conditions. The results show that daily mean NO2 concentrations derived from surface observations are consistent with the mean values of the OMI measurements, with their correlation coefficient reaching 0.81. The correlations of NO2 concentration with general weather patterns and sequential changes of temperature structure from 925 hPa down to the surface indicate that the weather fronts, high pressure and low pressure systems in the atmosphere play a role in changing the temporal and spatial evolutions of NO2 through removing, accumulating or converging of the pollutant, respectively. It is also found that the eastern Taihang Mountains is most heavily polluted by NO2 in North China. Based on a model that correlates NO2 column density with surface wind vector, the relation of the NO2 concentrations in six major cities in North China to the surrounding wind field is analyzed. The results show that the maximum wind field is associated with the highest frequency of pollution events, and under certain large-scale atmospheric conditions together with the topographic effect, small- and meso-scale wind fields often act to transport and converge pollutants, and become a major factor in forming the heaviest NO2 pollution event in North China. Analysis of the causes for the severe NO2 pollution event in this study may shed light on understanding, forecasting, and mitigating occurrences of heavy NO2 pollution.