Source Appointment Of Fine Particulate Matter And Its Absorption Of A Typical Basin City In The Northwest Semi-arid Region

Various pollution sources in cities bring atmospheric particulate matter,especially fine particulate matter,through primary emissions and secondary generation processes,which both directly pollute the environment and affect the weather and climate through radiation processes,and have significant impacts on human health,economic development and other aspects,it is very important to quantitatively estimate the sources of pollutants.Along with social and economic development and changes in production and lifestyle,the sources of fine particulate matter have been constantly changing.Since the implementation of China’s air pollution prevention and control action plan in 2013,source appointment results from Chinese cities have shown a significant decrease in industrial emissions and an increase in vehicle emissions,and vehicle emissions have become an important cause of visibility reduction under unfavorable weather conditions in some cities.As a typical river valley basin city in the semi-arid region of northwest China,the source appointment results of fine particulate matter in Lanzhou are mainly based on the more polluted years of 2012—2013,and there are few studies on the source appointment of pollution under the reduced pollution scenarios after 2013,and the influence of unfavorable dispersion conditions caused by the closed topography of Lanzhou on the source appointment needs to be further clarified.Most of the existing studies are based on off-line analysis and daily variation information is not fully considered.At the same time,the rapid decrease of atmospheric single scattering reflectance with the increase of pollution in Lanzhou also puts new demands on the source appointment.To address the above-mentioned problems,this paper uses hourly resolution hourly online data of PM_2.5,organic carbon（OC）,elemental carbon（EC）,water-soluble ions and elements from December 2019 to November 2020,a positive matrix factorization model（PMF）and a multiple linear regression model,considering the characteristics of typical river basin cities in the northwest semi-arid region,including the conditions of unfavorable dispersion and source appointment,absorption enhancement and source appointment,etc.The source appointment of PM_2.5 and absorption coefficient were carried out separately,and the contribution of vehicle emission sources to fine particulate matter in typical basin cities in the northwest semi-arid region was given,and the main pollution sources affecting the absorption coefficient were obtained.The main results and conclusions are as follows.（1）The most serious pollution was found in Lanzhou in winter,with PM_2.5 mass concentration of 72.8μg·m^-3 and Organic Matter（OM）and Secondary Inorganic Aerosols（SIAs）as the most important components.The PM_2.5 mass concentrations were 39.2,24.3,and 43.6μg·m^-3 in spring,summer,and autumn,respectively,during the study period.The percentages of OM and SIAs decreased with increasing pollution in spring and summer,and the percentages of SIAs decreased with increasing pollution in winter,while OM was stable at about 40.0%.（2）The main anthropogenic pollution sources in Lanzhou are vehicle emissions,secondary formation and coal combustion,whose annual average contributions are37.0%,23.9%and 10.0%,respectively,among which the contribution of vehicle emissions is prominent,reaching 57.0%,36.0%,30.0%and 26.0%in autumn,winter,spring and summer,respectively.The contribution of vehicle emissions increases with increasing pollution in autumn and winter season.This is related to the accumulation of primary pollutants caused by the poor diffusion conditions in heavy polluted weather in autumn and winter seasons.The contribution of sand and dust sources is a natural source that cannot be neglected in Lanzhou,and its annual average contribution reaches21.0%,and increases with the increase of PM_2.5 in spring and summer seasons.（3）Vehicle emissions contribute most of the EC in Lanzhou,with an annual average contribution of 84.6%,in addition to coal combustion in winter,which contributes 23.0%of EC.The daily variation of the major sources indicates significant daily variation of anthropogenic sources,while the daily variation of mineral dust is not significant.The peak of vehicle emissions during the morning peak hours occurs earlier in spring and summer and later in autumn and winter,which is related to the timing of residents’work and rest,and both atmospheric dispersion conditions and human activities affect the daily variation of these anthropogenic sources.（4）Local emissions from vehicles are dominant in Lanzhou in autumn and winter,and when the mass concentration of vehicle emissions is high,they tend to correspond to unfavorable dispersion meteorological fields.Secondary formation and coal combustion in winter,spring and autumn all correspond to high concentrations of pollutants brought by strong westerly winds,which may be related to the distribution of factories and electric heating plants in Xigu.The analysis of the reasons for the prominent contribution of vehicle emission sources shows that the relative reduction of industrial emissions and the increase in the number of vehicles are the main reasons for the high contribution of vehicle emissions in Lanzhou and other Chinese cities.In addition to unfavorable dispersion conditions and semi-arid climate are also responsible for the high contribution of vehicle emissions,and this prominent contribution of vehicle emissions is also found in other cities with poor dispersion conditions around the world.（5）The results of both absorption coefficient source appointment models indicate that vehicle emissions are the most significant contributor to the absorption coefficient,and the PMF-based results show that its contribution reaches 67.5%,83.8%,77.4%and80.9%in winter,spring,summer and autumn,respectively,followed by coal combuation with an annual average contribution of 11.4%.The results of PMF-based absorption coefficient source appointment show a more obvious wavelength dependence of pollution sources,in which the contribution of vehicle emission sources increases with longer wavelength.The correlation coefficients of the absorption coefficients fitted by the two methods show that the PMF fitting results are very stable,while the multiple linear regression method fluctuates more,so the results of the absorption coefficient source appointment based on PMF are considered to be more informative.