| Harbin-Changchun metropolitan（HCM）is the representative of the typical cold region in China.Although the average annual air quality has experienced an evolution process from continuous deterioration to gradual improvement in the past ten years,heavy air pollution events still occur.Hence,it has become research hotspots to reveal the causes of heavy pollution events and explore the formation mechanism in the low temperature environment.In this study,the real-time observation datasets of six air pollutants from 2015 to 2020 were used to in-depth analysis of the evolution characteristics of atmospheric PM 2.5 in the HCM.By using the WRF-CMAQ model,this study revealed the formation of PM2.5 pollution during the typical polluted periods in the winter and autumn in the HCM.This study also evaluated the contribution of emission change and meteorological conditions change to the multi-year variation of PM2.5concentration in autumn and winter.Based on the scenario analysis of pollution source reduction,scientific suggestions are provided for making pollution source reduction measures in the HCM.In order to explore the multi-year evolution characteristic of air pollution in cold urban agglomeration,the air pollution characteristics of HCM from 2015 to2020 are analyzed based on the real-time observation data of air quality monitoring station.The results demonstrated that the AQI value of HCM decreased by 29%from 2015 to 2020.From the perspective of spatial distribution,the high-value centers of AQI,PM10 and PM2.5 in HCM moved from Siping and Changchun to Harbin from 2018.The analysis of the multi-year variation characteristics of the SO2/NO2 value shows that the proportion of the f stationary combustion source is higher than that of the mobile in the source of PM2.5.in addition,the change of PM2.5/CO value during many years shows that the formation of secondary PM2.5 in the HCM is decreasing from 2015 to 2020.Convergent Cross-mapping combined with Pearson correlation method were used to quantitatively analyze the causality of PM 2.5 among cities in HCM.The result show that there is a strong bidirectional causality relationship of PM2.5pollution among cities in the HCM,and the PM2.5 concentration in the central city is more affected by the cities in the northwest than the cities in the southeast.The year of 2017 is the peak of the annual average values of AQI and PM 2.5concentration in the HCM from 2015 to 2020,and heavy pollution events often occured in autumn and winter in this year.Based on the high resolution open biomasa buring（OBB）emission inventoty established in this study,WRF-CMAQ model was used to quantify the contribution of OBB emissions and meteorological conditions changes to PM2.5 during typical autumn and winter pollution periods in cold urban agglomerations.Moreover,standardized multiple linear regression model（SMLR）was used to analyze the effects of seven meteorological factors on PM2.5 pollution formation in autumn and winter.The study results demonstrated that unfavorable meteorological conditions are the main reasons for the formation of heavey PM2.5 pollution.In addition,among the seven meteorological factors,relative humidity（RH）has the greatest influence on SOR,NOR and OC/EC in winter.With the increase of RH,the water content of particles accumulates,which promotes the formation of sulfate,nitrate and SOC by enhancing the liquid phase and heterogeneous reaction pathways.RH is the dominant meteorological factor that induces the explosive growth of PM2.5 during the heavy pollution periods in winter and autumn.Different from winter,the meteorological factor that has the greatest influence on NOR and OC/EC in autumn is solar radiation.The increase of solar radiation can promote the concentration of atmospheric oxida nt such as OH and O3,and then promote the gas phase oxidation reaction to produce secondary nitrate and SOC.Besides meteorological factors,the contribution of biomass combustion emissions to the formation of PM2.5 pollution in autumn heavy pollution period can be up to 61.9%.PM2.5 pollution from 2015～2020 in the HCM in autumn and winter showed different evolution characteristics.In this study selects 2017 as the based year and quantifies the contribution of meteorological conditions and pollutant emission changes to PM2.5 concentration changes in winter and autumn in this area from 2015 to 2020 based on the observation data and WRF-CMAQ numerical simulation results.The results demonstrated that meteorological conditions changes and emissions changes are the dominant driving factors for the interannual variation of PM2.5 concentration in winter and autumn,respectively.The continuous increase in pollutant emissions makes the winter PM2.5 concentration in 2017 in the region increase by 11.6μg/m3 compared with2015.The pollutant emission reduction after 2017 reduces the PM 2.5concentration by 19.0%from 2017 to 2021.Compared with 2017,the emission reduction measures in autumn led to a continuous decline in the concentration of PM2.5 in the autumn of 2015-2020,especially in the autumn of 2018,2019 and2020 when the PM2.5 concentration decreased by 86%,56%and 74%compared to 2017,respectively.The results of scenario analysis of pollutant emission reduction show that the key measures to improve air quality in autumn and winter in the cold urban agglomeration in the future are to completely prohibit open-air burning of straws and reduce emissions from residential source.The implementation of a comprehensive ban on open combustion of biomass can reduce the average daily concentration of PM2.5 in the HCM in the autumn of2017 by 27.5%.When the reduction of the residential source is more than 85%,the average concentration of PM2.5 in HCM in January 2017 can be reduced to less than 35μg/m3.Therefore,the HCM region should further strengthen the reduction of residential coal in winter,actively promote the use of clean energy to meet the strategic goals of"carbon peak"and"carbon neutralization",and further improve air quality. |
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