| Heavy air pollution frequently occurs in autumn and winter in northern China due to the influence of coal burning for heating.With the implementation of atmospheric control measures,the concentration of primary pollutants decreases significantly,but the decline rate of secondary components in particulate matter is significantly lower than that of primary pollutants.Secondary ions SNA:nitrate（NO3-）,sulfate(SO42-)and ammonium salt（NH4+）,as important components of secondary inorganic aerosols,are important factors driving up PM2.5 concentration during the haze.Baoding,located between Beijing and Shijiazhuang,suffers from severe air pollution in autumn and winter.The formation mechanism and influencing factors of secondary ions in particulate matter are not clear yet,so it is of great significance to study the concentration of air pollutants and the cause of secondary ions in Baoding.In this study,the conventional pollutant data,online ion data and online NH3 data were collected from the national control site,provincial control site and ecological environment bureau of Baoding during the autumn and winter of 2017-2019.PM2.5samples and particulate matter samples with different particle sizes were collected from the main urban areas and districts of Baoding City in different time periods and the water-soluble components in the particulate matter were measured.The causes of secondary ions in particulate matter were investigated by means of mathematical analysis and thermodynamic equilibrium model（ISORROPIA-II）.The main conclusions were as follows:The average concentration of PM2.5,PM10,SO2,NO2,CO and O3 in autumn and winter of 2017（solstice on October 1st and March 31st of the next year）were 85μg/m3,135μg/m3,26μg/m3,53μg/m3,2.6 mg/m3,99μg/m3.During the autumn and winter of2020,they were 72μg/m3,112μg/m3,12μg/m3,47μg/m3,2.3 mg/m3,96μg/m3,which decreased by 15.3%,17.0%,53.9%,11.3%,11.5%,3.3%,respectively,compared with the autumn and winter of 2017.Among them,SO2 decreased the most,followed by PM10 and PM2.5.The decrease range of atmospheric pollutant concentration in autumn and winter in Baoding was higher than that in"2+26"cities.There were differences about the concentration of air pollutants between the urban and the county.The difference of pollutant concentration between the urban and country in the cleaning period（AQI≤100）was small.However,in the pollution period（AQI>100）,the concentration of pollutants in the urban increased significantly,and the average concentration of PM2.5was 1.3 times that of county.During the firing period of fireworks,only when the urban implements banning measures,the urban was affected by the transmission of the surrounding counties,and the concentration of pollutants was significantly delayed.The average concentrations of SO2,NO2 and NH3 in heavy pollution period（AQI>200）were 52μg/m3,10μg/m3 and 8.3μg/m3,respectively,which were 1.1,1.3 and 0.8 times higher than those in clean period.Nitrate（NO3-）,sulfate(SO42-)and ammonium salt（NH4+）were the main water-soluble ions in PM2.5 in autumn and winter of Baoding,and the concentration ranges were 3.57～36.72μg/m3,1.92～22.43μg/m3 and 2.7～28.52μg/m3,respectively.The sum of the three ions accounted for 70.2%～88.1%of the total water-soluble ions and8.1%～24.8%of PM2.5.The particle size distribution indicated that there were roughly three sources of water-soluble ions in particulate matter:The first group included NO3-,SO42-,NH4+,K+,mesylate,and oxalate acid,which were mainly from secondary transformation with a single-peak distribution concentrated in the accumulation mode.The second group was composed of Cl-,Na+,formic and acetate,and these components came from primary emission and secondary conversion and showed dimodal distribution.The third type of ions mainly concentrated in the coarse mode and came from dust,including Ca2+and Mg2+.The heterogeneous reaction rate of SO42-increased significantly in the pollution period.ISORROPIA-II simulation results showed that the p H of PM2.5 in Baoding city was 4.3～7.5,which indicated that the heterogeneous transformation path of SO42-was dominated by the oxidation of SO2 by NO2 in the liquid phase on the surface of particulate matter.The atmosphere of Baoding city was under ammonia-rich condition,and the formation of NO3-was mainly limited by HNO3.The homogeneous gas phase transformation of NH3 participating in the reaction cannot explain the higher nitrate concentration during the pollution period,so it was speculated that sulfate aerosol and AWC promoted the hydrolysis of N2O5.The formation and transformation of secondary ions in particulate matter were affected by many factors.Oxalate could promote the heterogeneous transformation of secondary ions;Fireworks particles contributed to the oxidation of SO42-but had little effect on NO3-;Both temperature and relative humidity had effects on the nitrogen oxidation rate（NOR）,while the sulfur oxidation rate（SOR）was greatly affected by humidity.The secondary transformation degree of the southern air mass from Hebei Province was high,which has a great impact on the secondary components of the particulate matter.It was an important measure to reduce the concentration of air pollutants to improve the clean heating reconstruction rate in the southern districts and counties of Baoding. |
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