1.National Meteorological Centre;2.Handan Meteorological Bureau of Hebei Province;3.Chinese Academy of Meteorological Sciences
2013年以来，北京市城区PM2.5质量浓度年均值呈逐年降低趋势，但PM2.5重污染事件仍旧频发，污染出现快速甚至爆发增长的成因和理化机制仍存在诸多不确定性。基于北京市城区2013—2020年常规气象要素、PM2.5及其化学组分观测资料，分析了PM2.5在缓慢、快速和爆发三种增长机制下的颗粒物浓度和组分的阈值及其与气象条件的相关关系。结果表明，2013—2020年，北京市城区PM2.5在增长时段（1-24h间隔）中平均累积速率呈逐年放缓的趋势，大气污染累积阶段中缓慢增长的比重逐年升高。在判别标准逐渐严苛的前提下，爆发增长的比重逐年变化不大（4%—7%）。2013—2016年爆发增长的PM2.5浓度阈值为62μg m-3，2017年后，该阈值严苛至45μg m-3。82 μg m-3为2018年以来极易出现PM2.5爆发增长的界限值，高于此值后爆发增长的概率将大幅提升。有机物（Org）在爆发增长中起到了至关重要的作用。同一时间间隔Org在亚微米气溶胶（PM1）增长浓度中的贡献缓慢增长<快速增长<爆发增长，其中一次有机气溶胶（POA）在快速和爆发增长中对Org增长浓度的贡献平均超过50%，高于研究时段40%的平均占比。无机组分中，SO42?在PM1增长浓度中的贡献爆发增长（13%）>快速增长（11.8%）>缓慢增长（11.1%），NO3?的贡献相反。二次气溶胶（SPM）在累积阶段的贡献高于一次气溶胶（PPM），但爆发增长中，PPM在污染增长中贡献（最高达45%）明显高于平均时段的33%，PPM在爆发增长中的贡献不可小觑。秋冬季的爆发增长开始后，温度有所降低（0.2—1.2℃），而湿度和气压明显升高。北京城区爆发增长中主要的气团来向为偏南向（三个高度占比分布为69%—82%），其次为偏东方向（12%—20%）。
Since 2013, the annual average mass concentration of PM2.5 in Beijing’s urban area has been decreasing year by year, but heavy PM2.5 pollution incidents have continued to occur frequently, and there are still many uncertainties in the causes and physical and chemical mechanisms of the rapid or even explosive growth of pollution. This study analyzes the thresholds of conventional meteorological elements, PM2.5 and its chemical components under three growth mechanisms of slow, rapid and explosive growth, as well as the correlation between the changes in meteorological elements and the increase in pollutant concentration from 2013 to 2020 in Beijing urban area. The results showed that from 2013 to 2020, the average accumulation rate of PM2.5 showed a trend of slowing down, and the proportion of slow growth in the accumulation phase of PM2.5 increased year by year in Beijing. Under the premise that the criterion is gradually strict, the proportion of explosive growth has not changed much year by year (4%—7%).The PM2.5 concentration threshold for an explosive increase from 2013 to 2016 was 62 μg m-3, and after 2017, the threshold was strict to 45 μg m-3. 82 μg m-3 is the threshold value that is extremely prone to explosive growth of PM2.5 since 2018. After this value, the probability of explosive growth will increase significantly. Organic aerosol (Org) played a vital role in the explosive growth. In the same time interval, the contribution of Org to the growth concentration of submicron aerosol species (PM1) is slow growth < fast growth < explosive growth, and the contribution of primary OA (POA) in rapid and burst growth to Org growth concentration on average exceeds 50%, which is higher than the average proportion of 40% during the study period. Among the inorganic components, the contribution of SO42? in increasing concentration of PM1 shows explosive growth (13%) > fast growth (11.8%) > slow growth (11.1%), while the contribution of NO3? is opposite. The contribution of secondary particulate matters (SPM) in the cumulative phase is higher than that of primary particulate matters (PPM), but in the explosive growth, the contribution of PPM to the pollution increase (up to 45%) is significantly higher than 33% in the average period, indicating that the contribution of PPM to the explosive growth cannot be underestimated. After the explosive growth began, the temperature decreased (0.2-1.2°C), while the humidity and pressure increased significantly in autumn and winter. The main air mass in the explosive growth is southward (the three heights account for 69%-82%), followed by the eastward direction (12%-20%) in Beijing urban area.