大连市PM_(2.5)中水溶性离子污染特征及高酸度成因

Characteristics of Water-soluble Ion Pollution in PM_(2.5)and the Causes of High Acidity of PM_(2.5)in Dalian

为更加深入地了解大连市细颗粒物(PM_(2.5))及其水溶性离子(WSI)的污染状况及影响因素,进行霾、酸雨等污染事件的精准管控,于2021年6月至2022年5月在大连市进行PM_(2.5)样品的采集,分别采用重量法和离子色谱法测定了PM_(2.5)和WSI的浓度,分析其污染特征和来源,并探讨了春季PM_(2.5)高酸度的成因.结果表明,采样期间大连市ρ(PM_(2.5))及其ρ(WSI)年均值分别为(33.24±28.87)μg·m^(−3)和(18.66±20.52)μg·m^(−3),二次离子(SNA,即SO_(4)^(2−)、NO_(3)^(−)和NH_(4)^(+))在WSI中占比最高[(86.2±9.3)%].受到气象条件和秋末至春初集中采暖期燃煤排放的影响,PM_(2.5)及其WSI季节变化大小为:冬季>春季>秋季>夏季,SNA则为春季最高.相关性和主成分分析结果表明,PM_(2.5)中的WSI主要来自大气中SO_(2)和NO_(2)的二次转化、燃烧和扬尘混合源以及沙尘和海盐源.燃烧源在夏季主要为生物质燃烧,秋冬春则以燃煤为主;秋季到冬季风向的变化带来海盐源向土壤沙尘源的转变,与西北风有关的外来污染输送则导致春季WSI来源较为复杂.ISORROPIA-Ⅱ模型模拟得到NH_(4)NO_(3)是大连市PM_(2.5)中存在最多的固体气溶胶形态,其次是CaSO_(4)和(NH_(4))_(2)SO_(4);PM_(2.5)在夏秋冬三季pH值均接近中性,春季则明显呈酸性(2.03±3.18).春季的高酸度与低温高湿和高SNA浓度带来较高的气溶胶含水量及气-粒转化程度,最终形成贫氨环境.后向轨迹和PSCF结果表明,春季高酸度PM_(2.5)的外来输送主要来自西北(45.0%)和西南(40.8%)方向,前者主要与城市机动车和港口船舶排放有关,后者则受到相对较强的燃煤和工业源的影响.

To gain a deeper understanding of the pollution status and influencing factors of fine particles(PM_(2.5))and their water-soluble ions(WSI)in Dalian and to implement precise control of pollution events such as haze and acid rain,PM_(2.5)samples were collected in Dalian from June 2021 to May 2022.Then,the mass concentrations of PM_(2.5)and WSI were determined using the weight method and ion chromatography,respectively,and the pollution characteristics and sources were analyzed.Furthermore,the causes of the high acidity of PM_(2.5)in spring were discussed.The results showed that the annual average mass concentrations of PM_(2.5)and WSI in Dalian during the sampling period were(33.24±28.87)μg·m^(−3)and(18.66±20.52)μg·m^(−3),respectively,and the secondary ions(SNA,including SO_(4)^(2−),NO_(3)^(−),and NH_(4)^(+))accounted for the highest proportion of WSI[(86.2±9.3)%].The order of ion concentration levels from highest to lowest was:NO_(3)−>SO_(4)^(2−)>NH_(4)+>Cl^(−)>K^(+)>Ca^(2+)>Na^(+)>Mg^(2+)>F^(−).Due to the influence of meteorological conditions and coal combustion emissions during the concentrated heating period from late autumn to early spring,the seasonal variation in PM_(2.5)and WSI was winter>spring>autumn>summer,whereas SNA was the highest in spring and the lowest in summer.The results of correlation and principal component analysis showed that WSI in PM_(2.5)was mainly from the secondary transformation of atmospheric SO_(2) and NO_(2)(contributing to the majority of SNA),mixed sources of combustion and dust(characterized by K^(+),Mg^(2+),Cl^(−),and F^(−)),and sources of sand and sea salt(characterized by Na^(+),Ca^(2+),and Mg^(2+)).In summer,the main combustion source was biomass burning,whereas in autumn,winter,and spring,coal combustion emissions were predominant.The change in wind direction from autumn to winter brought by a shift from the source of sea salt to soil dust;additionally,the external pollution transported by northwest winds contributed to the complexity of the sources of WSI in PM_(2.5)during spring in Dalian.ISORROPIA-II model simulations suggested NH_(4)NO_(3) as the most present solid aerosol form in PM_(2.5)in Dalian,followed by CaSO_(4) and(NH_(4))_(2)SO_(4);compared to that in solid aerosols,more SNA existed in liquid aerosols.The annual average pH of PM_(2.5)in Dalian was 5.65±3.00,with pH values close to neutral in summer,autumn,and winter but significantly acidic in spring(2.03±3.18).The high acidity observed in spring was attributed to the combination of low temperature,high humidity,and high SNA concentrations.These conditions resulted in higher aerosol water content and increased gas-to-particle conversion rates,ultimately leading to an ammonia-deficient environment.The backward trajectory and PSCF results indicated that the external transport of high acidity PM_(2.5)in spring mainly came from the northwest(45.0%)and southwest(40.8%)directions.Mobile source emissions made the most significant contribution to the transportation of pollutants in the former,forming high-pollution source areas in the Beijing-Tianjin-Hebei Region,which may have been mainly related to urban motor vehicle and port vessel emissions;the latter was influenced by relatively strong stationary sources and showed higher SO_(2) emissions in the southern part of Henan Province and the central part of Jiangsu Province.