[Characteristics of PM_2.5 Components and Typical Pollution Episodes in the Urban Areas of Beijing]

Based on the measurements of atmospheric fine particulate matter （PM_2.5） and its components, the evolution and sources of PM_2.5 were studied at the Chegongzhuang site in the Beijing urban area in 2023. The primary component was crustal matter, accounting for 26.3% of PM_2.5, followed by nitrate （24.1%）. Secondary inorganic ions （SNA）, including nitrate, sulfate, and ammonium, collectively accounted for 43.5% of PM_2.5. The PM_2.5 composition was influenced by multiple factors, such as sandstorms and secondary pollution, in the view of its components. The proportions of SNA were 35.3%, 37.4%, 54.0%, and 45.7% in spring, summer, autumn, and winter, respectively, with the highest proportions in February and September （56.2% and 55.1%）. The proportion of crustal material was 37.1% in spring, with the highest proportion of 45.6% in April. Different diurnal variations were observed for PM_2.5 components in all four seasons, owing to the different emission sources, generation mechanisms, and variations in boundary layer height. Overall, with the increase in PM_2.5 concentration, the proportion of organic carbon （OC） and elemental carbon （EC） decreased, and the proportion of SNA increased. Both the nitrogen oxidation rate （NOR） and sulfur oxidation rate （SOR） were significantly higher than 0.1, and NOR increased with PM_2.5. Secondary organic carbon （SOC） accounted for 59.2%-78.0% of OC. The PMF model showed that the sources of PM_2.5 in Beijing in 2023 were： secondary nitrate, vehicle sources, dust sources, secondary sulfate, secondary organic matter, industrial sources, coal combustion sources, and fireworks, with the contributions of 37.4%, 16.1%, 13.5%, 12.7%, 8.6%, 4.6%, 3.8%, and 1.1%, respectively. Secondary nitrate was the main source in spring, summer, autumn, and winter, with the contributions of 37.5%, 22.2%, 44.5%, and 39.6%, respectively. In summer, secondary sulfate and secondary organic matter contributed significantly, accounting for 21.0% and 21.2%, respectively. Dust was the second-largest source in spring, with the contribution of 26.2%. Three typical pollution episodes （haze pollution in the winter heating period, fine particulate matter superposition dust pollution in spring, and PM_2.5 and O₃ combined pollution in autumn） were analyzed. Secondary accumulation was prominent, with the contribution rates of 77.3%, 53.4%, and 78.7% from secondary sources, respectively, for the three typical pollution episodes. Regional coal sources had a substantial effect, and the average contribution of combustion sources was 4.8% during the haze pollution in the winter heating period. For the episode in spring, the average contribution of dust sources was 29.8%. During the PM_2.5 and O₃ combined pollution in autumn, atmospheric oxidation strongly promoted secondary conversion, and secondary sulfate contributed 32.2%.