2020年2月南宁市一次大气PM2.5污染成因分析

2020年1月31—2月2日新冠肺炎疫情期间，广西发生的一次区域性大气PM_2.5污染引发社会关注.以南宁市为例，利用在线气体组分及气溶胶监测系统（MARGA）、颗粒物激光雷达，结合地面气态污染物和气象数据卫星火点和后向轨迹等分析本次PM_2.5污染成因.依据空气质量分指数，将观测过程划分为优、良和污染3个时段.结果表明，污染时段与生物质焚烧相关的K⁺、Cl^-明显升高，K⁺、Cl^-平均浓度分别为优时段的3.6和17.0倍.3个时段8种水溶性离子总浓度占PM_2.5均在30%左右，3种二次水溶性离子浓度之和占8种离子总浓度的比例为83.33%~89.18%，二次无机组分占比高，与秸秆焚烧促进二次转化有关.二次水溶性离子浓度变化趋势与占比不一致，除了与秸秆燃烧排放特征有关，还与不同二次无机离子形成的机制及主要影响因素不同有关.秸秆焚烧火点集中分布在良时段的南宁市及周边城市，良时段秸秆露天焚烧直接排放大量颗粒物、气态污染物等，在污染时段边界层高度明显下降，湿度增加、静风等不利气象条件下积累，是造成疫情期间南宁市大气PM_2.5污染的主因.污染时段南宁市无明显的颗粒物垂直传输过程，近地面主要受到广东、广西北部湾偏南气流影响，区域污染传输小.

Analysis on the causes of PM2.5 pollution in Nanning City in February 2020

During the novel coronavirus (COVID-19) lock down from 31^st January to 2^nd February, a regional atmospheric PM_2.5 pollution episode had arosed concerns of society in Guangxi. Based on Nanning as an sampling site, Monitor for Aerosols and Gases in Ambient Air (MARGA), particulate LiDAR, surface meteorological and environmental data, satellite remote sensing data and modeled HYSPLIT4 trajectory were used to analyze the cause of PM_2.5 pollution. According to the air quality sub-index, the observation durations were classified as the superior period, the fine period and the polluted period. The results indicated that K⁺ and Cl^- concentrations closely related to biomass burning (BB) increased significantly during the polluted period. The average concentrations of K⁺ and Cl^- in polluted period were 3.6 and 17.0 times higher than those corresponding figures of the superior period. The total concentration of eight water-soluble ions accounted for about 30% of PM_2.5 in all three periods. The total concentration of three secondary water-soluble ions accounted for 83.33%~89.18% of the total concentration of eight ions. The high proportion of secondary inorganic components was related to the secondary transformation promoted by BB. The variation trends of proportion and levels of secondary water-soluble ions were not consistent, which was not only attributed to the emission characteristics of straw combustion, but also related to the formation mechanism and main influencing factors of different secondary inorganic ions. Straw burning spots were centralized around Nanning city during the fine period. The accumulation of direct emissions of particulate and gaseous pollutants caused by open burning of straw during the fine period under decreased boundary layer height, increased humidity and low wind speed were the main causes for atmospheric PM_2.5 pollution in Nanning. During the polluted period, there was no obvious vertical transmission process of particulate matter in Nanning. The near ground was mainly affected by the southward airflow from the Beibu Gulf of Guangxi and Guangdong coastal, which indicated the regional pollution transmissions were insignificant.