Distribution And Source Apportionment Of Traffic Related Aerosols In Xi’an City

Xi’an is a key national central city in Northwest China.Recently,atmosphere aerosol pollution is an attached issue.So far,the government has taken extensive effective actions to limit the urban industrial discharge,but traffic related pollutants concentration has not declined notably,including black carbon and ultrafine particles.Traffic emission is an important source of black carbon and ultrafine particles in urban areas,and it may be tough to reduce for a long time in the future.Therefore,a necessary step for the discussion of black carbon and ultrafine particles sources and characteristics in depth,can be difficult.None of the published work so far have investigated this feature in earlier studies of China.In this study,a high-resolution Aethalometer（MA-200）and portable wide particle size aerosol spectrometer（MINI-WRAS 1371）were used to observe atmospheric aerosol in the urban community-level and traffic environment of Xi’an in 2019.Sources and characteristics traffic-related particulate matter were investigated firstly by Aethalometer model and aerosol number concentration source apportionment model.Further,PSCF and CWT analysis of Traj Stat were used to simulate regional potential sources contribution to ambient aerosol pollution in Xi’an city.The results show that the average BC concentration was 1914±1229 ng/m³（mean±standard deviation）and 3272±2384 ng/m³ for urban background and near-road sites,respectively.The source apportionment results of an Aethalometer model indicated that fossil fuel combustion(i.e.,traffic BC;BC_ff)from vehicle engines contributes up to approximately 80.0%of total BC aerosols at near-road sites,greater than the relative contribution of BC_ff（74.4%）at the urban background site.The diurnal variation in BC_ff concentration showed a major increase during morning and afternoon rush hours and diesel-vehicle non-restriction hours.Besides,the most favorable ventilation coefficient distinctly neutralizes,or even diminish the concentration levels of fresh BC_ff at 16:00-19:00 rush hours.The wind direction parameter provided evidence from BC sources that near-road BC emissions were mainly from upwind traffic sources,and the wind speed parameter revealed that biomass burning(BC_bb)particles drifted from distant regional sources.The tracer results proved that the homology among BC,PM2.5 and NO₂ were existed.The average size of the BC_ff aerosols core were identified to be in the 84.6-94.0 nm range by using empirical method at the Xi’an,which is in accumulation mode.Moreover,urban near-road ambient particles were discriminated into two components:the percentage of N1（45%:particles directly emitted from vehicles and nucleated immediately after emission）was smaller than the percentage of N2（55%:particles formed during the dilution and cooling of vehicle exhaust emissions and by in situ new particle formation）at the Xi’an site for all days during the study period.N2 exhibits a strong inverse correlation with wind speed（WS）and relative humidity（RH）,but elevated aerosol-loading conditions and traffic emissions could also enhance N2 concentration.The result of particle size distribution show that significant differences in particle number and mass concentration distribution,and the limitations in separately evaluate the effect of aerosol mass or number concentration.The result of back trajectory cluster analysis shows that air masses arriving at the study site generally originate from the local transport pattern,middle distant and long-range transport,and the average proportion of long-range transport is only 8.13%.Concentration weighted trajectory（CWT）analysis indicates that a significant seasonal characteristic in aerosol sources.In spring,the main potential sources were observed in northwest Xi’an,and contribution to time-average PM2.5 concentration is approximate 60μg/m³;Middle south Shaanxi contributing 100μg/m³ to time-average PM2.5 concentration in autumn.The larger CWT potential source extends to north Shaanxi,east Gansu,northeast Sichuan,north Chongqing,northwest Hubei and west Henan in winter.This finding has implications for establishing traffic aerosol pollution monitorable,evaluable strategies and vehicles emission-reduction policies,and cooperative control in the region of aerosol pollution aiming to improve air quality in urban environments.