| Urban air pollution is currently one of the biggest environmental challenges at home and abroad,and fine particulate matter(PM2.5)and ozone(O3)in the atmosphere have important impacts on air quality,climate change and public health.In recent years,with the implementation of clean air policies in China,the overall fine particulate pollution in the atmosphere has improved significantly,while at the same time ground-level ozone pollution has increased rather than decreased.This issue has attracted widespread attention from the academic community,and many innovative results have been achieved in the synergistic control of pollution in the central and eastern regions of China.However,the scientific issues such as the pollution characteristics,formation mechanisms and interactions of urban fine particulate matter and ozone in the arid and semi-arid regions of northwest China,which have unique pollution source composition and special climatic conditions,are still unclear.Lanzhou,as a typical river basin industrial city in the northwest semi-arid region,used to be one of the most polluted cities in the world in terms of air pollution.Although significant results have been achieved in air pollution control in recent decades,fine particulate matter pollution in winter and ozone pollution in summer are still relatively serious.Therefore,it is important to study the pollution characteristics and formation mechanisms of fine particulate matter and ozone in the atmosphere of Lanzhou,and to investigate the interaction mechanism between fine particulate matter and ozone for scientific management of air pollution.This paper is based on the observations of Lanzhou Atmospheric Component Super Monitoring Station(LACMS)from December 1,2019to November 30,2020,including various high-resolution hourly online observations of particulate matter concentration,chemical composition,absorption and scattering coefficients,visibility,volatile organic compounds(VOCs)concentration,gaseous pollutant concentration and meteorological parameters,using radiative transfer(TUV),photochemical box(NCAR MM)and positive matrix factor decomposition method(PMF)models were used to study the pollution characteristics,formation mechanisms and interactions of fine particulate matter and ozone in Lanzhou.The main results and conclusions are as follows.(1)Particulate matter pollution is more prominent in Lanzhou in winter,with mean values of 45.7,70.4,114,and 136.3?g·m-3 for PM1,PM2.5,PM10,and total suspended particulate matter(TSP),respectively,in Lanzhou in winter 2019.Organic matter(OM)and secondary inorganic salts(SIAs)are the most dominant components of PM2.5 in Lanzhou.The oxidation ratios(SOR and NOR)of sulfur and nitrogen were low with mean values of 0.2 and 0.1,respectively.The combination of non-homogeneous reactions and weakened photochemical reactions led to the decrease of SOR at high PM2.5 concentrations.The opposite effect of decreasing O3 concentration and increasing ammonium salt concentration made NOR remain largely stable with increasing PM2.5 concentration.The main pathway for secondary particle generation is photochemical reactions.The contribution of coal combustion to PM2.5 decreases relatively with increasing PM2.5 concentrations,while the contribution of motor vehicle emissions increases.The results indicate that controlling the emissions of primary carbonaceous aerosols and motor vehicle exhaust is an effective measure to prevent and control PM2.5 pollution in winter in Lanzhou at present.(2)O3 pollution is more prominent in Lanzhou in summer,and the average daily maximum 8-hour average(MDA8 O3)concentration of O3 in summer 2020 is 133.8?g·m-3.Alkanes are the key component of atmospheric VOCs in Lanzhou(40.5%of the total).The contribution of aromatic hydrocarbons to the ozone generation potential(OFP)was the largest in spring and summer,and the contribution of olefins to OFP was the largest in autumn and winter.The contribution of aromatic hydrocarbons to the potential for secondary organic aerosol generation is the largest.NCAR MM model sensitivity tests show that Lanzhou is a sensitive area for VOCs,and the control of O3pollution should focus on reducing VOCs and controlling nitrogen oxides(NOx)on this basis,while the control of reactive VOCs such as olefins and aromatic hydrocarbons should be focused.(3)The correlation between PM2.5 and O3 in Lanzhou was positive in summer and negative in autumn and winter.SIAs and O3 were mainly positively correlated,while organic carbon(OC),elemental carbon(EC)and O3 were mainly negatively correlated.In summer,O3 concentrations were higher,single scattering albedo(SSA)was higher,UV radiation was higher,secondary production was enhanced,and the proportion of SIAs increased under high PM2.5 concentration conditions.In winter,O3 concentrations were lower,SSA was lower,UV radiation was lower,secondary production was lower,and the proportion of SIAs was lower under high PM2.5 concentrations.O3 promoted the production of SIAs,and there was a significant positive correlation between O3 and NOR and SOR in summer.(4)During the control phase of the new coronavirus(COVID-19)outbreak,PM2.5concentrations decreased by 19%and NOx concentrations decreased by 42%,while MDA8 O3 concentrations increased by 83%.PMF source analysis results showed that the contribution of motor vehicle emissions to PM2.5 decreased from 42%before control to 37%during the control phase,while the contribution of secondary generation increased from 26%to 30%.The scattering and absorption coefficients of aerosols decreased by 29%and 30%,respectively,leading to a 35%increase in visibility.The increase in O3 was caused by the combination of the increase in the ratio of VOCs to NOx,the decrease in PM2.5,and the increase in SSA.A positive feedback mechanism between O3,SIAs and SSA was revealed based on the observations. |
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