“2+26”城市一次多因素叠加重污染过程的特征分析

2018年11月23日-12月4日，京津冀及周边地区"2+26"城市出现了一次长时间、大范围、高强度的复合型大气重污染过程，为揭示区域性重污染过程中多因素的综合作用，利用气象资料、空气质量监测等多源数据以及区域污染特征雷达图，对京津冀及周边地区"2+26"城市此次重污染特征和成因进行分析.结果表明:根据PM_2.5/PM₁₀ρ（PM_2.5）/ρ（PM₁₀），下同]可将此次重污染过程划分为4个阶段.第一阶段（2018年11月23-26日）PM_2.5/PM₁₀在0.5~1.0内波动，"2+26"城市大气扩散条件转差，一次污染物局地积累及SO₂、NO_x、NH₃等气态污染物在高湿条件下二次转化是污染形成并发展的主要原因；第二阶段（11月27日）PM_2.5/PM₁₀突降至0.2左右，"2+26"城市北部受形成于蒙古国的沙尘影响，短时ρ（PM₁₀）快速升高（峰值为818 μg/m3），中南部受形成于内蒙古自治区阿拉善盟的沙尘及上风向PM_2.5污染的传输影响，ρ（PM_2.5）和ρ（PM₁₀）均较高，维持日均重度污染水平（参照GB 3095-2012《环境空气质量标准》和HJ 633-2012《环境空气质量指数（AQI）技术规定（试行）》）；第三阶段（11月28日-12月2日）PM_2.5/PM₁₀由0.3逐渐升至0.8，在静稳、高湿的不利气象条件下，一次污染物积累并二次转化，第二阶段残留沙尘中的矿物质对硫酸盐起到催化作用，导致ρ（PM_2.5）快速上升，"2+26"城市大部分达日均重度及以上污染；第四阶段（12月3-4日）与第二阶段类似，PM_2.5/PM₁₀突降至0.2，"2+26"城市再次受到沙尘天气和区域传输的共同影响，因冷空气持续时间较长，污染被有效清除.研究显示，此次污染过程是气象条件、污染物一次排放和二次转化、区域传输、沙尘天气等多因素综合作用的结果.当静稳、高湿等不利气象条件或沙尘天气出现时，区域应加强对各类污染物排放的管控力度，以降低污染物的一次排放、二次转化以及沙尘和区域传输的共同影响，进而削弱污染严重程度. 

Characteristics of a Multi-Factor Superimposing Haze Episode in '2+26' Cities

The '2+26' cities in the Beijing-Tianjin-Hebei and surrounding region was hit by a severe haze episode from Nov. 23rd to Dec. 4th, 2018, where multiple factors superimposed. To unveil the comprehensive interaction between those factors, the meteorological data, the air quality monitoring data and the regional air pollution characteristic radar chart were used to analyze the causes and characteristics of the episode. Our results show that the entire episode can be divided into four stages based on the mass concentration ratio of PM_2.5 to PM₁₀ (PM_2.5/PM₁₀). During Stage 1 (Nov. 23rd-26th), PM_2.5/PM₁₀ fluctuated between 0.5 and 1.0. Under stagnant and humid conditions, primarily emitted pollutants accumulated and gaseous precursors (e.g. SO₂, NO_x and NH₃) were converted to secondary PM_2.5 components (e.g. sulfate, nitrate and ammonium), leading to the formation and aggravation of the episode. For Stage 2 (Nov. 27th), PM_2.5/PM₁₀ dropped to 0.2. The northern part of region was heavily affected by a dust storm originating from Mongolia, resulting in temporarily surged PM₁₀ concentration (up to 818 μg/m3). The mid-southern part of the region was influenced by another dust storm originating from Inner Mongolia Autonomous Region and the regional transport of particulate pollution from upwind area, resulting in elevated PM_2.5 and PM₁₀ concentrations and reaching daily heavy or severe air pollution (Grade Ⅴ or Ⅵ according to the GB 3095-2012 Ambient Air Quality Standards and HJ 633-2012 Technical Regulation on Ambient Air Quality Index (on trial). During Stage 3 (Nov. 28th-Dec. 2nd), PM_2.5/PM₁₀ gradually increased from 0.3 to 0.8. The accumulation of primary pollutants, secondary conversion of gaseous precursors and the catalyzing effect of residual minerals from Stage 2 on sulfate formation resulted in a rapid increase in PM_2.5 concentration. The majority of '2+26' cities reached daily heavy or severe pollution (Grade Ⅴ or Ⅵ). Stage 4 (Dec. 3rd-4th) was similar to Stage 2 with PM_2.5/PM₁₀ rapidly decreased to 0.2, when the region was again heavily influenced by dust storm and regional transport of particulate pollution. The northwesterly wind lasted longer in Stage 4 than in Stage 2, which effectively cleaned up the pollutants. This work revealed that the entire episode in '2+26' cities was the result of comprehensive interaction of multiple factors, including meteorology, primary emission, secondary conversion of precursor gases, regional transport and dust storms. Therefore, regulation on the emission of various pollutants in the study region should be strengthened prior to adverse meteorological conditions or dust storm periods to alleviate the severity of air pollution.