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随着经济的快速发展和城市化进程的加快,我国大气污染问题越发严重,大气细颗粒物(PM2.5,空气动力学直径小于等于2.5 μm的大气颗粒物)在秋冬季频繁爆表,京津冀、长三角、珠三角和汾渭平原等区域已成为大气污染频发的重点区域,污染天数越来越长,涉及区域面积越来越广[1-4]. 大气污染会影响人们正常的生产生活,危害人体健康[5-6]. 郭新彪等[7]认为,有针对性地控制大气PM2.5污染,对人体健康意义深远. 研究大气重污染特征和成因是科学防控的关键,全国多个城市先后进行了大气重污染特征和成因分析的研究[8-11]. 王永宏等[12]通过分析沧州市2009年7月—2011年7月的ρ(NOx)、ρ(O3)、ρ(SO2)和ρ(PM10)等污染物的数据变化特征,认为沧州市秋冬季污染物主要为NOx、SO2以及PM10,夏季污染物主要为O3. 徐美等[13]研究发现,沧州市细颗粒物(PM2.5)为大气环境的首要污染物,污染天数占比达68%,冬季沧州市本地排放贡献最大,本地排放和不利的气象条件是污染形成的主要原因. 张敬巧等[14]发现,机动车排放形成的二次无机转化是重污染过程形成的主要原因之一. Zheng等[15]研究认为机动车和燃煤排放生成的OA、SOA等物质是造成京津冀区域PM2.5污染的主因之一. Chowdhury等[16]研究认为生物质燃烧排放能明显提高空气中EC和K等物质的浓度,是重污染天气形成的主要污染源之一. 我国大气重污染过程是一个复合型污染,通常由气象条件、污染物排放、光化学反应和区域输送等因素引起,而低温、高湿的静稳天气条件和污染物的高排放量,最易形成大气重污染天气[17-19].
沧州市位于京津冀区域的东部,是京津冀区域与山东及东南地区大气污染物进行区域传输的重要通道,研究沧州市重污染过程中的污染特征和成因,对于研究京津冀区域大气污染具有重要意义.在沧州市辖区共布设4个采样点,包括沧州市市区、黄骅市、青县和泊头市,于2017年冬季一次大气重污染过程采集了全天PM2.5滤膜样品. 以此次重污染天气为研究对象,结合气象资料、细颗粒物浓度数据、颗粒物中水溶性离子、无机元素和碳组分含量,对此次沧州市冬季重污染过程中PM2.5的污染特征和成因进行了分析[20-22].
2017年冬季沧州市一次重污染过程PM2.5污染特征及成因
PM2.5 pollution characterization and cause analysis of a heavy pollution event in winter 2017, Cangzhou City
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摘要: 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PM2.5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(PM2.5)在污染前期、污染期和污染后期分别为53.4、178、45.4 μg·m−3,ρ(PM2.5)最大小时值为296 μg·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、236 μg·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、22.7 μg·m−3,其中ρ(NH4+)、ρ(NO3−)和ρ(SO42−)(二次无机盐)在全市PM2.5中的总占比为78.4%。污染期水溶性离子中ρ(NO3−)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、6.98 μg·m−3,ρ(NO3−)/ρ(SO42−)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、16.1 μg·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1 m·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。Abstract: Cangzhou located in the important atmospheric pollutants transport channel of Beijing-Tianjin-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. PM2.5 samples were collected at four sites (downtown area of Cangzhou city, Botou, Qingxian and Huanghua) during a heavy pollution process from November 29th to December 5th, 2017. The water-soluble ions, inorganic elements and carbonaceous components of PM2.5 were analyzed and discussed. According to the level of ρ(PM2.5), the whole observation period was divided into three phases: before pollution phase ( from November 29th to December 1th ), pollution phase (from December 2th to 3th) and after pollution phase (from December 4th to 5th). The results showed that the ρ(PM2.5) of Cangzhou City in three phases were 53.4, 178 and 45.4 μg·m−3, respectively. The maximum hourly ρ(PM2.5) is up to 296 μg·m−3, The maximum daily concentration appeared on December 2nd 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(NH4+), ρ(NO3−) and ρ(SO42−) (SNA) accounted for 78.4% of ρ(PM2.5). The proportion of ρ(NO3−) in water-soluble ions of pollution phase had a biggest increase, The ρ(NO3−) in three phases were 12.5, 43.5 and 6.98 μg·m−3, respectively. ρ(NO3−)/ρ(SO42−) 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.
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表 1 ρ(PM2.5)成分浓度(μg·m−3)
Table 1. Component concentrations of ρ(PM2.5)
日期Date NH4+ NO3− SO42− 其他离子Other ions 无机元素Inorganic elements OC EC 污染前期 6.42 12.5 7.66 8.98 7.65 14.5 4.16 污染期 19.3 43.5 19.6 13.1 8.93 21.1 6.75 污染后期 4.36 6.98 5.45 5.88 7.72 12.5 3.65 -
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