| With increase of vehicle population,vehicle emission has become one of the important sources of atmospheric fine particulate matter(PM2.5)in the Guanzhong area.Therefore,the study of the distribution and characteristics of PM2.5 concentrations emitted by road mobile sources in the Guanzhong has important guiding significance for the joint prevention and regulation of regional air pollution and the improvement of ambient air quality.The concentration characteristics of primary PM2.5 and secondary PM2.5(NO3-and SO42-)caused by vehicle emissions in the Guanzhong in winter(January)and summer(July)of 2017 was simulated,based on the Weather Research and Forecasting Model(WRF)and California Puff Model(CALPUFF).This paper studied the characteristics of spatial distribution and the variations of primary and secondary PM2.5 formed by exhaust gas precursors in the atmosphere.The main conclusions of this paper were as follows:(1)The atmospheric turbulence diffusion abilities in Guanzhong in January were mainly weaker,and it was easier to cause the accumulation of PM2.5 than that in July.The mixed layer heights(MLHs)in the Guanzhong were 315.18 m in January and 931.92 m in July,respectively.The atmospheric stability was mainly stable(F)in January,accounting for 45.1%,and unstable(B)in July,accounting for 24.9%.(2)The emissions of PM2.5,NOx and SO2 from vehicles in January were 343.1,6168.3 and 14.6 tons,and those in July were 380.1,6289.4 and 18.5 tons.The emissions of PM2.5,NOx and SO2(precursors of secondary PM2.5)from vehicles in the Guanzhong showed a small difference in January and in July.The spatial distribution characteristics of the emissions were consistent with the distributions of the regional road network density.(3)The total PM2.5(the sum of primary PM2.5 and secondary PM2.5)concentrations of vehicles in the Guanzhong were 12.65±9.38 and 3.75±1.76 μg/m3 in January and in July,respectively.Among the total PM2.5 concentrations emitted by motor vehicles in January,the proportions of primary PM2.5 and secondary PM2.5 were 65.32%and 34.68%,respectively.In the total PM2.5 concentrations emitted by motor vehicles in July,the primary PM2.5 and secondary PM2.5 accounted for 87.75%and 12.25%,respectively.In the secondary PM2.5 concentrations caused by vehicle emissions,NO3-and SO42-accounted for 96.06%and 3.94%,respectively.NO3-was the main component of the secondary PM2.5.(4)The calculation indicates that the spatial distribution of primary PM2.5 concentrations formed by direct motor vehicle emissions had no significant difference in January and in July.The higher values of primary PM2.5 concentrations mainly concentrated in expressways and those areas with densely road network.The spatial distribution of secondary PM2.5 concentrations had significant difference in January and in July.The secondary PM2.5 concentrations in January decreased gradually from the central area of Xi’an to surrounding areas,and the higher secondary PM2.5 concentrations in July were mainly concentrated in the main urban areas of Xi’an and Weinan in the Guanzhong city cluster.(5)The change in 24-hour concentrations of PM2.5 emitted by vehicles in the Guanzhong showed a bimodal distribution both in January and in July.The peak concentration appeared in the morning(8:00-9:00)and in the evening(18:00-19:00)of traffic flow,and the minimum values between the two peak concentrations appeared at 13:00 and at 17:00,which were related to the increase of the mixing layer height of the area.Compared with the primary PM2.5 concentrations caused by the vehicle emissions in the Guanzhong,the secondary PM2.5 concentrations presented a single peak distribution.The maximum secondary PM2.5 concentrations appeared between 9:00 and 10:00,which lagged behind the primary PM2.5 concentrations by about one hour.This was possibly related to the time needed turning NOx and SO2 from motor vehicle emissions into the secondary PM2.5 in atmospheric environment. |
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