2017年冬季沧州市一次重污染过程PM_(2.5)污染特征及成因

PM_(2.5) pollution characterization and cause analysis of a heavy pollution event in winter 2017,Cangzhou City

沧州市位于京津冀区域大气污染传输的重要通道上,研究沧州市大气重污染特征和成因对于研究传输通道上污染物的组分和通量具有重要意义.2017年11月29日—12月5日在沧州市区、泊头市、青县和黄骅市进行一次重污染过程的PM_(2.5)采样,对样品进行水溶性离子、无机元素(不可溶)和碳组分分析,并将整个观测期分为污染前期(11月29日—12月1日)、污染期(12月2—3日)和污染后期(12月4—5日)进行研究.结果表明,沧州市区ρ(PM_(2.5))在污染前期、污染期和污染后期分别为53.4、178、45.4μg·m^(-3),ρ(PM_(2.5))最大小时值为296μg·m^(-3).12月2日沧州市区、泊头市、青县和黄骅市ρ(PM2.5)的日均值均达到最大,4个点位分别为248、248、230、236μg·m^(-3),表明污染较重.沧州市在污染前期、污染期和污染后期的水溶性离子浓度分别为35.6、95.6、22.7μg·m^(-3),其中ρ(NH_(4)^(+))、ρ(NO_(3)^(-))和ρ(SO_(4)^(2-))(二次无机盐)在全市PM_(2.5)中的总占比为78.4%.污染期水溶性离子中ρ(NO_(3)^(-))占比变化最大,在污染前期、污染期和污染后期的浓度分别为12.5、43.5、6.98μg·m^(-3),ρ(NO_(3)^(-))/ρ(SO_(4)^(2-))由污染前期的1.63增加到污染期的2.22,说明机动车排放对此次污染形成的贡献较大.沧州市在污染前期、污染期和污染后期的ρ(OC)与ρ(EC)二者之和分别为18.6、27.9、16.1μg·m^(-3),ρ(OC)/ρ(EC)分别为3.49、3.13和3.43,说明3个观测期均存在二次有机气溶胶的生成,污染期一次排放的含碳物质略高于另外两个观测期.污染期无机元素明显增加,ρ(K)在无机元素中的占比增加了5%—10%,说明生物质燃烧是此次重污染过程的主要贡献源之一.此次污染过程中,污染期以较小的东北风为主,平均风速低于1m·s^(-1),最低小时温度达-4℃,最大相对湿度达99%,静稳天气条件不利于污染物的扩散和输送,静稳天气条件下机动车、燃煤和生物质燃烧等污染物的排放和不断积累是此次重污染形成的主要原因.

Cangzhou located in the important atmospheric pollutants transport channel of BeijingTianjin-Hebei region.The study on chemistry characteristics and causes of heavy atmospheric pollution in Cangzhou is important for researching the components and fluxes of pollutants in the transport channel.PM_(2.5) samples were collected at four sites(downtown area of Cangzhou city,Botou,Qingxian and Huanghua) during a heavy pollution process from November 29^(th) to December5^(th),2017.The water-soluble ions,inorganic elements and carbonaceous components of PM_(2.5) were analyzed and discussed.According to the level of ρ(PM_(2.5)),the whole observation period was divided into three phases:before pollution phase(from November 29^(th) to December 1^(th)),pollution phase(from December 2^(th) to 3^(th)) and after pollution phase(from December 4^(th) to 5^(th)).The results showed that the ρ(PM_(2.5)) of Cangzhou City in three phases were 53.4,178 and 45.4 μg·m^(-3),respectively.The maximum hourly ρ(PM_(2.5)) is up to 296 μg·m^(-3),The maximum daily concentration appeared on December 2^(nd) with 248,248,230 and 236 μg·m^(-3) in downtown of Cangzhou City,Botou,Qingxian and Huanghua,respectively,idicating that the pollution in Cangzhou was heavy.The total concentration of water-soluble ions in Cangzhou were 35.6,95.6 and 22.7 μg·m^(-3) in three phases,respectively.The sum of ρ(NH_(4)^(+)),ρ(NO_(3)^(-)) and ρ(SO_(4)^(2-))(SNA) accounted for 78.4% of ρ(PM^(2.5)).The proportion of ρ(NO_(3)^(-)) in water-soluble ions of pollution phase had a biggest increase,Theρ(NO_(3)^(-)) in three phases were 12.5,43.5 and 6.98 μg·m^(-3),respectively.ρ(NO_(3)^(-))/ρ(SO_(4)^(2-)) increased from 1.63(before pollution phase) to 2.22(pollution phase),indicating that the motor exhaust emission had more contribution to the heavy pollution event.The total concentrations of organic carbon(OC) and element carbon(EC) in three phases were 18.6,27.9 and 16.1 μg·m^(-3),respectively.The ρ(OC)/ρ(EC) were 3.49,3.13 and 3.43,respectively.It suggested that secondary organic aerosols(SOA) formed in all phases,and the contribution of primary emissions to carbonaceous compounds had a little increase in pollution phase.Inorganic elements increased significantly in pollution phase,The proportion of ρ(K) in inorganic elements increased by 5%—10%,showing that biomass burning was one of the major sources of the heavy pollution.During the pollution phase,it was dominated by weak northeast wind,wind speed less than 1 m·s^(-1),the minimum hourly temperature with-4℃ and the maximum relative humidity with 99 %.The stable meteorological conditions were not beneficial to the diffusion and transportation of pollutants,and the accumulation of pollutants emitted from motor vehicles,coal and biomass burning were the major causes of the heavy pollution.