[Pollution Characteristics of PM_2.5 Chemical Composition in Zhejiang Province]

To investigate the seasonal and regional pollution characteristics of PM_2.5 chemical composition in Zhejiang province, this study was based on manual sampling monitoring data from 11 sampling sites of four regions in Zhejiang province from October 1, 2019 to September 30, 2020. The results showed that during the observation period, the average ρ(PM_2.5) of the four regions ranged from 34.3 μg·m^-3 to 46.4 μg·m^-3. The PM_2.5 mass concentrations in the hinterland areas of western Zhejiang and northern Zhejiang were relatively high, 15.1% and 13.2% higher than the mean value, respectively. The PM_2.5 mass concentrations in the coastal areas of eastern Zhejiang and southern Zhejiang were relatively low, 8.4% and 14.9% lower than the average, respectively. The seasonal characteristics showed a higher concentration in autumn and winter and lowest concentration in summer. The seasonal variation in PM_2.5 mass concentration from autumn to spring was not obvious in southern Zhejiang, whereas in western Zhejiang, the PM_2.5 mass concentration followed a descending sequence of autumn>winter>spring>summer. In northern Zhejiang and eastern Zhejiang, the trend was winter>autumn>spring>summer. During the observation period in the inland area, the ρ(PM_2.5) of the scenic area, administrative area, residential area, and mixed area of commercial traffic and residents were (40.2±10.2), (46.3±9.6), (50.1±10.6), and (46.7±10.2) μg·m^-3, respectively. The highest value of ρ(PM_2.5) was in the residential area. During the sampling period in coastal areas, the ρ(PM_2.5) of the cultural and entertainment area and mixed area of commercial traffic and residents were (27.4±5.8) μg·m^-3and (37.2±5.6) μg·m^-3, respectively. The contribution rates of organic matter (OM), NO₃^-, SO₄^2-, NH₄⁺, trace elements, and crustal matter in PM_2.5were 26.4%, 15.4%, 12.4%, 9.0%, 7.1%, and 5.7%, respectively. The SNA, including SO₄^2-, NO₃^-, and NH₄⁺, contributed 36.8% in PM_2.5. In terms of seasons, the contribution of OM to PM_2.5 in autumn, spring, and summer was higher than that of other compositions, which accounted for 28.3%, 27.7%, and 26.3%, respectively. The contribution rate of NO₃^- in PM_2.5 was the largest in winter, reaching 24.3%. In terms of spatial distribution, SNA contributed the most to PM_2.5 in all regions, ranging from 32.8% to 39.7%, with the highest in northern Zhejiang and the lowest in southern Zhejiang. The SNA of all regions presented NO₃^->SO₄^2->NH₄⁺. Based on the backward trajectory clustering analysis, the main air sources of northern Zhejiang were the Yellow Sea-southern Jiangsu (autumn), northern Anhui (winter), East China Sea (spring), and western Jiangsu (summer) areas, with contribution rates of 38.11%, 35.28%, 37.46%, and 27.87%, respectively. The main air sources of western Zhejiang were the Yellow Sea-southern Jiangsu (autumn), southern Anhui (winter), eastern Zhejiang (spring), and northern Zhejiang (summer), with contribution rates of 38.11%, 37.50%, 46.55%, and 32.58%, respectively. The air of autumn, winter, spring, and summer in eastern Zhejiang were influenced by air masses from northern Hebei (36.07%), eastern Shandong (38.06%), East China Sea (30.17%), and southern Guangdong (34.43%), respectively. In autumn, winter, spring, and summer, southern Zhejiang was affected by air masses from the Yellow Sea (35.66%), northeast Anhui (34.44%), East China Sea (26.72%), and southern Fujian coast (35.00%), respectively. The regions in Zhejiang province showed large seasonal differences. The difference value between the maximum value of ρ(PM_2.5) in the northwest and the lowest value in the southeast was 21.0 μg·m^-3 and 20.5 μg·m^-3 in autumn and winter, respectively; the difference values in spring and summer were 10.4 μg·m^-3 and 6.1 μg·m^-3. Thus, the northern air mass had a certain exogenous contribution to PM_2.5 in autumn and winter in Zhejiang province. However, with the weakening of the northern air mass trajectory in spring and summer and the increasing contribution of the southern and east China Sea air mass to the air flow in Zhejiang province, PM_2.5 pollution showed a trend of improvement.