长三角背景点夏季大气PM_2.5中水溶性无机离子污染特征及来源解析

PM_2.5 samples were collected on a day/night basis on Chongming Island, a background site of the Yangtze River Delta from May 30th to August 15th, 2018, and measured for water-soluble ions in order to improve our understanding of the air pollution characteristics in the region. The PSCF (potential source contribution function) method was used to identify the spatial distribution of the pollutant emissions. The source contributions to PM_2.5 were quantified by using the PCA (principal component analysis) and PMF (positive matrix factorization) methods. Our results showed that: (1) The average mass concentration of PM_2.5 was (33±21) μg/m³ during the campaign with the highest value greater than 120 μg/m³, indicating that air quality on Chongming Island was generally better than the first grade standard value (35 μg/m³) of Ambient Air Quality Standards (GB 3095-2012). (2) The concentration of water-soluble inorganic ions in PM_2.5 during the campaign was (14±9.3) μg/m³, accounting for 42.4% of the PM_2.5 mass, and SNA was the dominant species, which accounted for 85.7% of the mass concentration of water-soluble inorganic ions. (3) A further analysis based on molar ratio of n(NH₄⁺) to n(SO₄^2-) showed that ammonium concentration was low in the periods with ρ(PM_2.5)<15 μg/m³ (the clean period). It was high in the periods with ρ(PM_2.5) between 15-35 μg/m³ (the transition period) and higher than 35 μg/m³ (the polluted period). Ammonium existed as NH₄HSO₄ and NH₄NO₃ during the transition periods and as (NH₄)₂SO₄ and NH₄NO₃ during the polluted periods. (4) PM_2.5 loadings and chemical composition results suggested that two types of pollution events occurred during the campaign, which were characterized by local biomass burning emissions (E1, from May 30 ^th to June 8 ^th) and regional transport (E2, from July 23^rd to August 1^st), respectively. Source apportionment further revealed that secondary formation was the largest contributor in the two events, accounting for 62.8% and 59.8% of the PM_2.5 masses, respectively, followed by biomass burning emissions, which contributed separately to 32.5% and 20.1% of the PM_2.5 during E1 and E2. Sea salt was also found to be an important source during the E2 period, contributing to 16.6% of the PM_2.5 mass and much larger than that (2.7%) during the E1 period. This work determined that PM_2.5 on Chongming Island is mostly derived from regional transport and the high level of PM_2.5 at the background site during the haze periods is largely caused by the accumulation of secondary particulate matter.