[Size Distributions of Different Carbonaceous Components in Ambient Aerosols].

It is important to obtain the size distribution of carbonaceous components in aerosols for studying the formation and transformation mechanisms and radiation characteristics of regional aerosols. However, only a few studies on the size distribution of aerosol carbonaceous fractions have been conducted in Beijing. In this study, a Micro-Orifice Uniform Deposit Impactor (MOUDI)-120 sampler was used to collect size-resolved aerosol samples in three seasons in Beijing, and the concentrations of different types of carbonaceous fractions were analyzed. Furthermore, the size distribution, characteristics, sources, and interrelationship of each carbonaceous component in different seasons and under different pollution levels were systematically studied. The results show that the carbonaceous components were concentrated mainly in fine particles, and the proportion of carbonaceous components in fine particles in autumn and winter was higher than that in summer. The carbonaceous components are distributed in two main modes:accumulation mode and coarse mode. Organic carbon fraction 1 (OC1) and OC2 were distributed mainly in the accumulated mode, with a higher proportion in the range of 0.056-0.56 µm, and OC3+OC4 was more abundant in the coarse mode. The concentration of Soot-elemental carbon (EC) was low but was highest in the 0.10-0.18 µm size range, which indicates that the EC emitted by high temperature combustion was distributed mainly in the ultra-fine particle size range. The Char-EC concentration was much higher, accounting for the majority of EC. The distribution appearances of the main carbonaceous components were essentially the same in the daytime and at night. Summer and winter were more conducive to the formation of SOC, and the OC/EC ratio was significantly higher than that in autumn. The OC/EC values varied greatly in different particle sizes because the water-soluble organic compounds (WSOC) were distributed mainly in the range of 0.056-0.10 µm, with significantly higher OC/EC values than other particle sizes. Sunlight and high temperature were beneficial to the oxidation of gaseous organic matter to SOC, resulting in the OC/EC ratio in summer in daytime to be significantly higher than that at night. Among the carbonaceous components, EC1 and OC1 had the strongest interrelation. In addition, EC1 also had stronger interrelation with potassium.

[Size Distributions of Water-soluble Components in Ambient Aerosol of Beijing].

A micro-orifice uniform deposit impactor (MOUDI-122) was used to collect ambient aerosol at an urban site in Beijing in both winter and summer from 2016 to 2017. The water-soluble components, including ions and water-soluble organic carbon (WSOC) were analyzed. The characteristics of concentrations and size distributions for water-soluble components under different seasons and pollution conditions were determined. The results showed that NH4+, NO3-, SO42-, and K+ in both seasons and Cl- in winter mainly distributed in the accumulation mode, and Mg2+ and Ca2+ primarily distributed in the coarse mode. The secondary ions were still the main components of PM2.5 in Beijing. The concentrations of SO42- were higher in summer, whereas those of NO3-, K+, and Cl- were higher in winter. Mg2+ and Ca2+ had lower correlations with other main components of aerosols, indicating their independent sources. The average size distributions and concentration levels of NO3- and SO42- exhibited apparent differences between daytime and nighttime in summer. During polluted periods, the concentrations of secondary ions increased in both the accumulation and coarse modes but decreased in the Aitken mode. As pollution levels increased in winter, the mass median diameters of secondary ions in the droplet mode also increased. The WSOC concentration and particle size distribution under accumulation mode in summer were significantly larger than those in winter. The distribution peaks of WSOC in accumulation mode were higher in summer than those in winter. The WSOC in particles of 0.056-0.32 µm were relatively stable under different pollution levels. However, the WSOC concentration in particles larger than 0.32 µm during polluted periods was evidently higher than that during clean periods.

[Impact of Mountain-Valley Wind Circulation on Typical Cases of Air Pollution in Beijing].

The impact of mountain-valley wind circulation on the typical examples of pollution was analyzed through the selected pollution process, combining with the hourly PM2.5 concentrations and meteorological data in Haidian, Shangdianzi and Lishuiqiao in Autumn and Winter from 2013 to 2015, and also the data of Tower of atmospheric, wind profile of Haidian and automatic meteorological stations in the same period. The analysis showed that the average wind speed of valley wind was greater than that of the mountain wind, and they both would be "broken" during the conversion time in the mountain-valley wind days. In contrast with the mountain wind, the average duration of valley wind in autumn was longer than that in winter, and the start time of valley wind in autumn was earlier than the same wind in winter; influenced by the topography of Beijing area, the direction boundary of the transformation between mountain-valley wind was northeast-southwest. The frontier of mountain wind in autumn could fall down to the South Second Ring Road, and it could be pressed to the South Third Ring Road in winter; the average thickness of valley wind was greater than the mountain wind. Whether the moment was in autumn or winter, in the south, the average time when the PM2.5 concentration began to rise, was earlier than in the north in a day; the time when concentration of pollutants began to rise in the fall was earlier than in the winter, but the time when the concentration began to decline showed the opposite trend. The transition zone of different PM2.5 concentration in Beijing in autumn or winter located in South Second Ring Road (South Third Ring Road), and it would move to south over time. Duration autumn and winter seasons, this phenomenon lasted about 4 and 2 hours, respectively. Furthermore, the positive and negative feedback effects may exist between pollutant concentrations and mountain-valley wind.

[Characteristics of Number Concentration Size Distributions of Aerosols Under Processes in Beijing].

The aerosol number concentration size distributions were measured by a Wide-Range Particle Spectrometer (WPS-1000XP) at an urban site of Beijing from 2012 to 2014; and the characteristics of the size distributions in different seasons and weather conditions were discussed. The results showed that the daily average number concentration of Aitken mode aerosols was highest in the spring and lowest in the autumn; the daily average number concentration of accumulation mode aerosols was bigher in the spring and winter, while lowest in summer; and the average concentration of coarse mode was highest during the winter. The Aitken mode particles had the most significant diurnal variations resulted from the traffic sources and the summer photochemical reactions. In the spring, autumn and winter, the number concentrations of accumulation mode of the nighttime was higher than that of the daytime. The coarse mode particles did not have obvious diurnal variation. During the heavy pollution process, the accumulation mode aerosols played a decisive role in PM2.5 concentrations and was usually removed by the north wind. The precipitation could effectively eliminate the coarse mode particles, but it bad no obvious effect on the accumulation mode particles under small speed wind and zero speed wind. During the dust process, the concentrations of coarse mode particles increased significantly, while the accumulation mode aerosol concentration was obviously decreased.

[A Case Study on the Rapid Cleaned Away of PM2.5 Pollution in Beijing Related with BL Jet and Its Mechanism].

The concentration of PM2.5 decreased very rapidly from 18:00 to 23:00 on 17th Mar. 2015 in Beijing area. No cold air bringing strong north wind influenced Beijing. The reason leading to the clean away of PM2.5 was discussed. The results showed that a boundary layer jet played a key role. The ventilation in the boundary layer went up with the enhancement of southwesterly wind speed, which was favorable to the dilution of pollution. Besides, the development of jet also caused the increase of vertical wind shear. As a result, the turbulence in the boundary layer became more obvious and the mixing layer height rose. Furthermore, the geostrophic vorticity at the top of mixing layer was positive at 20:00 on 17th Mar. It means that the direction of Ekman-Pumping was upward. So, the pollution near the surface was brought to upper levels and transported downstream by the jet. The development of boundary layer jet attributed to inertial oscillation and atmospheric baroclinicity.

[Impact of Collision Removal of Rainfall on Aerosol Particles of Different Sizes].

The impact of collision removal of rainfall on aerosol particles of different sizes was analyzed through the calculation of Stokes number, combining with the hourly PM2.5 concentrations and meteorological data in Haidian from October 2012 to October 2014, and also the size distribution data in a selected rainfall process. The calculation results of Stokes number showed that the raindrops had little effect on direct collision removal of aerosol particles of smaller than 2 µm, and had more effect on aerosol particles of larger than 2 µm. Based on the statistical analysis of the observation data, the precipitation processes or the precipitation hours with significantly decreased PM2.5 were quite limited. However, PM2.5 concentrations were increased in 43.2% of the precipitation hours. By analyzing the size distribution data of aerosol particles during a typical precipitation process, we found that the precipitation had significant scavenging effect on Aitken mode particles (<0.1 µm) and coarse mode particles (>1.0 µm), except for the accumulation mode particles. Since the accumulation mode aerosols contributed most of the mass of PM2.5, the rainfall processes only had minor influence on the collision scavenging of PM2.5.

[Characteristics and Parameterization for Atmospheric Extinction Coefficient in Beijing].

In order to study the characteristics of atmospheric extinction coefficient in Beijing, systematic measurements had been carried out for atmospheric visibility, PM2.5 concentration, scattering coefficient, black carbon, reactive gases, and meteorological parameters from 2013 to 2014. Based on these data, we compared some published fitting schemes of aerosol light scattering enhancement factor [ f(RH)], and discussed the characteristics and the key influence factors for atmospheric extinction coefficient. Then a set of parameterization models of atmospheric extinction coefficient for different seasons and different polluted levels had been established. The results showed that aerosol scattering accounted for more than 94% of total light extinction. In the summer and autumn, the aerosol hygroscopic growth caused by high relative humidity had increased the aerosol scattering coefficient by 70 to 80 percent. The parameterization models could reflect the influencing mechanism of aerosol and relative humidity upon ambient light extinction, and describe the seasonal variations of aerosol light extinction ability.

[Atmospheric nitrogen deposition in urban area of Beijing].

Using ion exchange resin columns method, atmospheric nitrogen deposition was observed in the urban area of Beijing from March to September in 2009. The average value of atmospheric nitrate nitrogen deposition was 40.59 mg x m(-2) and that of atmospheric sulfite nitrogen deposition was 14.66 mg x m(-2) from March to June. The average value of atmospheric nitrate nitrogen deposition was 75.13 mg x m(-2) and that of atmospheric sulfite nitrogen deposition was 20.67 mg x m(-2) from June to September. Observational results show that atmospheric nitrate and sulfite nitrogen deposition had obvious local difference, that is to say, there was relatively large amount of deposition around traffic arteries and power plants, which shows the character of line/point source of atmospheric nitrate and sulfite nitrogen deposition. The average value of atmospheric ammonia nitrogen deposition was 12.19 mg x m(-2) from March to June, and 8.46 mg x m(-2) from June to September. Observational results show that the change of atmospheric ammonia nitrogen deposition among observation points was obvious smaller than atmospheric nitrate and sulfite nitrogen deposition, which shows the character of non-point source of atmospheric ammonia nitrogen deposition.