[Pollution Characteristics and Factors Influencing the Reduction in Ambient PM2.5 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 PM2.5 variation,spatial and temporal change, diurnal variation, and heavy pollutions in Beijing. The annual average mass concentrations of PM2.5 in Beijing were 51, 42, and 38 µg·m-3, which showed great progress in air quality improvement. However, the PM2.5 concentration in 2020 was still 8.6% above the national limit value despite a 30.9% decline since 2017. The PM2.5 south-north gradient in Beijing remained throughout the three years, but this pattern showed a less significant trend. The highest monthly mean PM2.5 concentrations in Beijing tended to occur in January-March, with the lowest in August-September. NOx, CO, and PM2.5 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 PM2.5 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 PM2.5 in Beijing were significant. Through analysis, a continuous pollution reduction was still the most important reason for the yearly decrease in PM2.5. The concentrations of organic matter, elemental carbon, and crustal matter in the PM2.5 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 PM2.5 in Beijing was obvious.

[Different Air Pollution Situations of O3 and PM2.5 During Summer in Beijing].

Two different pollution situations of O3 and PM2.5 during summer in Beijing were analyzed from theperspective of synoptic situations, meteorological elements, precursors, atmospheric oxidation, back-trajectories of air mass and chemical compositions of PM2.5. The results showed that the synoptic situations in the pollution situation that O3 reached middle level pollution and PM2.5 maintained low concentrations (O3high-PM2.5 low) could be characterized as northwest gas flow in 500 hPa height and high-pressure rear in the ground. Whereas the synoptic situations in the pollution situation that O3 and PM2.5 both reached middle level pollution (O3-PM2.5 high) could be characterized as westerly gas flow in 500 hPa height and low pressure in the ground. Compared with the O3high-PM2.5 low situation, meteorological elements in O3-PM2.5 high situation could be characterized as stronger southerly winds and higher relative humidity. In the O3-PM2.5 high situation, initial concentrations of O3 and PM2.5 were higher and diurnal variations of PM2.5 were more significant, nevertheless, the average concentrations of O3 were lower than those in the O3high-PM2.5 low situation, respectively. The analysis of precursors, atmospheric oxidation and chemical compositions of PM2.5 showed that the accumulation and hygroscopic growth of PM2.5 under unfavorable meteorological conditions as well as the regional transport caused by strong southerly winds might be the main factors leading to high PM2.5 concentrations in O3-PM2.5 high situation.

[Air Quality Characteristics in Beijing During Spring Festival in 2015].

To analyze the impacts of emissions from fireworks on the air quality, monitoring data of PM2.5, PM10, SO2, NO2 chemical compositions of PM2.5 of automatic air quality stations in Beijing during Spring Festival(February 18th-24th) in 2015 were investigated. Moreover, we also estimated the fireworks on the New Year's Eve produced based on the ratio of PM.5 to CO. Analysis results showed that the concentrations of PM2.5, PM10, SO2, NO2 during 2015 Spring Festival was 116. 85, 184.71, 22. 14, and 36. 27 µg.m-3 respectively, which raised 52. 61%, 92. 41%, - 40. 15%, - 0.46% respectively compared to the same period in 2014; the concentration peaks of PM2.5, PM10, SO2, NO2 at 1 : 00 am on 19th was 412. 69, 541. 63, 152. 73, 51. 09 µg.m-3, respectively, which was increased 19. 02%, 14. 37%, 76. 57%, 11. 35% compared to that of 2014; the concentration peaks at dense population area were significantly higher than that in other districts; fireworks had great influence on the chemical compositions of PM2.5 especially on the concentrations of chloride ion, potassium ion, magnesian ion, which were 18. 85, 66. 72, and 70. 10 times than that in 2013-2014; fireworks resulted in severe air pollution in a short time and the estimated fireworks on the New Year's Eve was approximately 2. 13 x 10(5) kg of PM2.5. Reduction of pollutants during Spring Festival had a positive significant impact on air quality in Beijing.

[PM2.5 Background Concentration at Different Directions in Beijing in 2013].

PM2.5, background concentration at different directions in 2013 in Beijing was analyzed combining the techniques of mathematical statistics, physical identification and numerical simulation (CMAQ4.7.1) as well as using monitoring data of six PM2.5 auto-monitoring sites and five meteorological sites in 2013. Results showed that background concentrations of PM2.5 at northwest, northeast, eastern, southeast, southern and southwest boundary sites were between 40.3 and 85.3 µg · m⁻³ in Beijing. From the lowest to the highest, PMPM2.5 background concentrations at different sites were: Miyun reservoir, Badaling, Donggaocun, Yufa, Yongledian and Liulihe. Background concentration of PM2.5 was the lowest under north wind, then under west wind, and significantly higher under south and east wind. Calculated PM2.5 background average concentrations were 6.5-27.9, 22.4-73.4, 67.2-91.7, 40.7-116.1 µg · m⁻³ respectively in different wind directions. Simulated PM2.5 background concentration showed a clear north-south gradient distribution and the surrounding area had a notable effect on the spatial distribution of PM2.5 background concentration in 2013 in Beijing.

[Analysis About Spatial and Temporal Distribution of SO2 and An Ambient SO2 Pollution Process in Beijing During 2000-2014].

Spatial and temporal distribution of SO2 during 2000-2014 was all analyzed based on the SO2 monitoring data that Beijing Municipal Environmental Monitoring Center released and the formation mechanism of a typical air pollution episode in January 2014 was also investigated by combining numerical model CAM(x). Analysis results showed that mass concentration of ρ(SO2) in Beijing in 2014 decreased 69% compared to that in 2000 with an annual gradient from 2000 to 2014 of - 3.5 µg x (m3 x a)(-1). Monthly average concentration of SO2 changed in a U shape curve and from the lowest to the highest, and seasonal variations of SO2 concentrations were as follows: winter > spring > autumn > summer; concentration of SO2 in heating season was significantly higher than that in non heating season. Annual average concentration of SO2 was lower in northern and western regions while higher in six city area and southern area. Concentrations of SO2 at Shijingshan, Dongsi, Tongzhou monitoring sites were significantly decreased related to SO2 emission reduction measures. During a heavy air pollution process in January 14 - 18th 2014 there was obviously SO2 regional transportation and model simulation analysis based on PAST showed that the contribution of SO2 regional transport to Beijing was 83% with elevated power plants surrounding Beijing accounting for 21% and the four major Beijing power plants contributing about 3.5% to the SO2 concentration during this heavy air pollution process.

[Temporal and spatial distribution characteristics of ozone in Beijing].

Ozone concentrations obtained from 35 automatic air monitoring stations in Beijing were analyzed to investigate their temporal and spatial distribution characteristics. A process with high ozone concentration in summer was analyzed. The results showed that ozone maintained relatively high concentration from May to August while in other months, the ozone concentration was at a low level. Overall, the average concentrations of ozone in different stations in a descending order were comparison and regional station, suburban environment evaluation station, urban environmental assessment station and traffic pollution monitoring station. Ozone diurnal variation showed a single peak distribution, the peak of which appeared at 15:00 or 16:00. Ozone concentration showed obvious weekend effect, which meant ozone concentration in daytime of weekend was higher than that in weekday. Ozone concentration was lower in urban Beijing, higher in surrounding counties and the highest in northeast area with more vegetation. A high ozone concentration process occurred in Beijing on June 3, 2013. Under the effect of southwest wind in the afternoon, the concentration peaks of ozone in Yufa, Fengtaihuayuan, Olympic center and Huairou station occurred in order from south to north. Concentration peak of ozone in Huairou station occurred at 20:00 in the night. It could be concluded that significant ozone transmission characteristic was reflected in this process.