[Pollution Characteristics and Factors Influencing the Reduction in Ambient PM_2.5 in Beijing from 2018 to 2020]

Air quality data from 33 environment sites and five regional sites from 2018 to 2020, as well as meteorological data, were used to research PM_2.5 variation,spatial and temporal change, diurnal variation, and heavy pollutions in Beijing. The annual average mass concentrations of PM_2.5 in Beijing were 51, 42, and 38 μg·m^-3, which showed great progress in air quality improvement. However, the PM_2.5 concentration in 2020 was still 8.6% above the national limit value despite a 30.9% decline since 2017. The PM_2.5 south-north gradient in Beijing remained throughout the three years, but this pattern showed a less significant trend. The highest monthly mean PM_2.5 concentrations in Beijing tended to occur in January-March, with the lowest in August-September. NO, CO, and PM_2.5 concentrations were significantly higher in the heating season than in the non-heating season by 58.4%, 52.9%, and 27.5%, respectively. Diurnal variation showed that greater PM_2.5 concentrations were observed at nighttime during the heating season and, conversely, at noontime during the non-heating season. Sixteen pollution episodes occurred in Beijing over the last three years, resulting in 25 heavy pollution days distributed in autumn-winter of 2018-2020. The regional heavy pollution characteristics of PM_2.5 in Beijing were significant. Through analysis, a continuous pollution reduction was still the most important reason for the yearly decrease in PM_2.5. The concentrations of organic matter, elemental carbon, and crustal matter in the PM_2.5 in Beijing decreased by 43.3%, 53.2%, and 51.5% since 2017, respectively, and nitrate, sulfate, and ammonium decreased by 34.2%, 52.2%, and 43.7%.The results showed that the control effect of PM_2.5 in Beijing was obvious.