天津市夏秋季O_(3)-PM_(2.5)复合污染特征及气象成因分析

Analysis of characteristics and meteorological causes of O_(3)-PM_(2.5) compound pollution in summer and autumn over Tianjin

天津市PM_(2.5)-O_(3)复合污染主要发生在夏秋季,本研究筛选出2017年夏季一次和秋季两次典型PM_(2.5)-O_(3)复合污染过程,系统分析污染物变化特征,探究天气形势和主要气象因子的影响.结果表明,3次污染过程PM_(2.5)与O_(3)呈现不同程度的正相关性(日均值相关系数达到0.34~0.78),O_(3)与PM_(2.5)中硫酸盐和有机碳日均浓度存在较为一致的变化趋势.与非复合污染日相比,复合污染日无机盐占PM_(2.5)组分的比重增加,增长率为1.9%~7.3%;而碳组分占比减小.复合污染过程发生时,天津地区处于低压槽前或高压后部,近地面弱的偏南风辐合造成较差的大气扩散条件,导致大气污染物的累积.研究结果发现,夏季和秋季PM_(2.5)-O_(3)复合污染过程需要适宜的气象条件,温度阈值分别为25~35℃和20~30℃,相对湿度分别为40%~70%和55%~100%.夏秋季复合污染过程关键无机组分及其形成机制有所差异.夏季,日间O_(3)等强氧化剂对SO_(2)的气相氧化过程和夜间高湿条件(大约60%)下液相化学反应可能是主导的化学机制,硫酸盐增长率为8.2%.秋季高湿环境(≥80%)不仅促进SO_(2)向硫酸盐转化,秋季第一次复合污染过程硫酸盐增长4.7%,也促进夜间N2O5水解反应等,秋季第二次复合污染过程硝酸盐增长6.0%,两种机制成为秋季复合日PM_(2.5)显著增长的关键机制.本研究揭示了驱动天津市PM_(2.5)-O_(3)复合污染过程发生的适宜气象条件、PM_(2.5)关键化学组分及其化学过程,为复合污染的成因及协同控制提供了参考.

The PM_(2.5)-O_(3) compound pollution of Tianjin mainly occurred in summer and autumn. In this study, one typical process in summer(Case1) and two processes in autumn(Case2 and Case3) in 2017 were selected to analyze the characteristics of compound pollution and identify the influence of the meteorological conditions. A notably positive correlation between daily average concentration of PM_(2.5)(especially sulfate and organic carbon) and O_(3) was observed with the correlation coefficients of 0.34~0.78 in the three processes. Compared to non-compound days, the inorganic aerosols comprised an increasing fraction of chemical composition(1.9%~7.3%) in compound days, while the fraction of carbon components decreased. During the compound pollution processes, Tianjin was located in front of the low pressure trough or behind the high pressure. The convergence of weak southerly winds led to adverse atmospheric diffusion, which favored the accumulation of air pollutants. Our results showed that the temperature of about 25~35 ℃ and 20~30 ℃, and the relative humidity(RH) ranges of 40%~70% and 55%~100%, would be suitable conditions for the occurrence of compound pollution in summer and autumn, respectively. The key inorganic components and main chemical processes were different in the three cases. The gas-phase oxidation of SO_(2) by high oxidants concentration in daytime and the aqueous-phase chemistry at high RH of 60% at night were the key chemical mechanism of sulfate with an increase of 8.2%. The higher RH of ≥80% in autumn effectively promoted the aqueous formation of sulfate by 4.7% in Case2. Meanwhile, the condition also enhanced the nitrate formation by 6.0% such as the hydrolysis of N2O5 at night in Case3. The two formation mechanism became the key mechanism for the significant growth of PM_(2.5) on compound days in autumn. The study reveals suitable meteorological conditions and the possible chemical processes of the key PM_(2.5) components that drive the formation of compound pollution, which is benefit for synergic control of PM_(2.5) and O_(3) in Tianjin.