Source identification and apportionment of ambient particulate matter in Beijing using an advanced computer-controlled scanning electron microscopy (CCSEM) system.

The use of electron microscopy to analyze the morphology, composition, and sources of atmospheric particles has been studied extensively worldwide. However, in China, there are few studies on single-particle source analysis based on computer-controlled scanning electron microscopy (CCSEM) technology for a large number of particles, and the related technical methods need to be established and improved. In this study, ambient particulate matter (PM) was collected simultaneously from urban, suburban, and background areas of Beijing in spring 2018 and subsequently characterized using the IntelliSEM-EPAS™ technology (an advanced CCSEM software). The deposition velocity model was used to deduce the size distribution and calculate the concentration of ambient PM. Based on the k-means algorithm and empirical rules, all particles investigated were quantitatively apportioned to nine major sources, including soil/road dust, carbonates-silicates, carbonates, irregular carbonaceous particles, irregular iron oxides, combustion/industry, calcium sulfate, secondary particles, and salt-related particles. The size-resolved contributions (mass and number) of different sources were calculated. For example, soil/road dust (65.1 %), carbonate-silicate (16.1 %), and carbonate (7.1 %) were the top three sources contributing to PM10 mass. This study was the first localized application of IntelliSEM-EPAS technology in China, demonstrating its great promise in PM source apportionment. For further accurate and refined source apportionment, it is essential to build localized individual particle source profiles.

Joint increase of aerosol scattering efficiency and aerosol hygroscopicity aggravate visibility impairment in the North China Plain.

China's "Blue Sky Action Plan" aimed at tremendous improvements in atmospheric visibility. While stringent emission control policies have substantially brought down PM2.5 mass concentrations, visibility improved much less than expected due to non-linear responses of visibility changes to PM2.5 reductions. In this study, we used long-term continuous humidified nephelometer system measurements of multi-wavelength aerosol scattering coefficients in both dry state and controlled relative humidity conditions in the North China Plain during spring and summer to attempt disentanlge the non-linear relationsips between visibility and PM2.5 mass.Aerosol scattering efficiency, optical hygroscopicity and air relative humidity are key factors for relating PM2.5 mass to visibility. It was found that aerosol volume scattering efficiencies (VSEs) were highly correlated (r > 0.8) with aerosol scattering coefficients. The continuous decrease of aerosol scattering Ångström exponent during pollution episodes revealed dominant contributions of secondary aerosol formation to aerosol size growth and mass accumulation, explaining aerosol VSE increases. Moreover, the optical hygroscopicity parameter κsca that describes the aerosol light scattering enhancement abilities through water uptake increased jointly with VSE and aggravated the visibility degradation during middle to final stages of pollution episodes. Thus, low visibility events (<3 km) only occurred when VSE and κsca were at their highest levels. The contribution of aerosol water to visibility degradation increased as visibility decreased, and contributed dominantly to visibility degradation under extremely low visibility conditions (<1 km). However, under hazy visibility conditions (3-10 km), which occurred most frequently, both aerosol water and scattering efficiency enhancement played significant roles. For setting up more efficient emission control strategies targeting on visibility improvement, our results highly encourage more future research on the linkages between secondary aerosol formation mechanisms and co-variations of aerosol scattering efficiency and aerosol hygroscopicity on the NCP.

Aerosol liquid water content of PM2.5 and its influencing factors in Beijing, China.

Aerosol liquid water content (ALWC) has important influences on atmospheric radiation and aerosol chemical processes. In this work, the changes in ALWC of PM2.5 were investigated over four seasons based on hourly monitoring of inorganic water-soluble ions and their gaseous precursors using the thermodynamic model ISORROPIA II. The results showed that the ALWC concentrations exhibited pronounced seasonal (autumn > summer > spring > winter) and diurnal variation characteristics. The sensitivity tests indicated that ALWC depended strongly on TSO4 (total sulfate (gas and aerosols) expressed as equivalent H2SO4), followed by TNO3 (total nitrate (gas and aerosols) expressed as equivalent HNO3). The relatively low concentration of TCl (total chloride (gas and aerosols) expressed as equivalent HCl) limit its importance in the atmosphere. ALWC showed exponential growth features as a function RH in all four seasons. RH became the most influential factor on the variation of ALWC when RH exceeded 80% in all seasons. The seasonal average data showed that the ALWC increased from 2.92 µg·m-3 to 75.83 µg·m-3 when ambient RH increased from 30% to 90%, the associated sulfate, nitrate, and ammonium (abbreviated as SNA) mass fraction in PM2.5 rose from 0.39 to 0.58 in the atmosphere. The ALWC facilitated the formation of SNA through gas-particle conversion and partitioning. The self-amplifying processes between ALWC and SNA enhanced aerosol formation. By modeling ALWC under different seasonal atmospheric scenarios, it was found that reductions in chemical species could reduce ALWC concentrations in different degrees. Based on the current emission conditions, controlling excess NH3 emission could effectively reduce ALWC to a maximum of 45.71% in summer, indicating that NH3 control was crucial for reducing ALWC and PM2.5 concentrations under high levels of SO42- and NO3-.

Insights into measurements of water-soluble ions in PM2.5 and their gaseous precursors in Beijing.

To better understand the characteristics and transformation mechanisms of secondary inorganic aerosols, hourly mass concentrations of water-soluble inorganic ions (WSIIs) in PM2.5 and their gaseous precursors were measured online from 2016 to 2018 at an urban site in Beijing. Seasonal and diurnal variations in water-soluble ions and gaseous precursors were discussed and their gas-particle conversion and partitioning were also examined, some related parameters were characterized. The (TNH3) Rich was also defined to describe the variations of the excess NH3 in different seasons. In addition, a sensitivity test was carried out by using ISORROPIA II to outline the driving factors of gas-particle partitioning. In Beijing, the relative contribution of nitrate to PM2.5 has increased markedly in recent years, especially under polluted conditions. In the four seasons, only a small portion of NO2 in the atmosphere was converted into total nitrate (TNO3), and more than 80% of TNO3 occurred in the form of nitrate due to the abundant ammonia. The concentration of total ammonia (TNH3) was much higher than that required to neutralize acid gases, and most of the TNH3 occurred as gaseous NH3. The nitrous acid (HONO) concentration was highly correlated with NH3 concentration and had increased significantly in Beijing compared with previous studies. The total chloride (TCl) was the highest in winter, and ε(Cl-) was more sensitive to variations in the ambient temperature (T) and relative humidity (RH) than ε(NO3-).

Impact of clean air action on the PM2.5 pollution in Beijing, China: Insights gained from two heating seasons measurements.

Comprehensive observations have been carried out in Beijing to investigate the impact of the Clean Air Action implemented in 2013 on changes in aerosol chemistry characteristics in heating seasons of 2016-2017 and 2017-2018. Results showed that PM2.5, SO2, NO2, NH3, O3 and CO concentrations decreased by 40.9%, 46.0%, 29.0%, 40.6%, 11.0% and 44.4%, respectively. Significant decreases were also observed for NO3- (32.5%), SO42- (52.9%), NH4+ (56.0%), Cl- (64.6%) and K+ (68.2%), on average. Enhanced PM2.5 pollution has changed from sulfate-driven to nitrate-driven. The decrease in SO2 was more significant than NO2 as a response to one reason of the larger decrease in SO42- concentration. The formation of sulfate was dominated by heterogeneous reactions in two heating seasons. Low pH could facilitate more efficient conversion of SO2 to sulfate. Photochemical reactions played a much more important role in the formation of nitrate in the second heating season, especially in the daytime. The major source regions for sulfate and nitrate were identified by back trajectories and the potential source function (PSCF). More nitrate was brought into Beijing when air masses coming from polluted regions in the southwest prevailed in 2017-2018 heating season. Thus, regional joint prevention and control are of great importance in the achievement of an effective reduction in PM2.5 pollution in the future.

Dust-Dominated Coarse Particles as a Medium for Rapid Secondary Organic and Inorganic Aerosol Formation in Highly Polluted Air.

Secondary aerosol (SA) frequently drives severe haze formation on the North China Plain. However, previous studies mostly focused on submicron SA formation, thus our understanding of SA formation on supermicron particles remains poor. In this study, PM2.5 chemical composition and PM10 number size distribution measurements revealed that the SA formation occurred in very distinct size ranges. In particular, SA formation on dust-dominated supermicron particles was surprisingly high and increased with relative humidity (RH). SA formed on supermicron aerosols reached comparable levels with that on submicron particles during evolutionary stages of haze episodes. These results suggested that dust particles served as a medium for rapid secondary organic and inorganic aerosol formation under favorable photochemical and RH conditions in a highly polluted environment. Further analysis indicated that SA formation pathways differed among distinct size ranges. Overall, our study highlights the importance of dust in SA formation during non-dust storm periods and the urgent need to perform size-resolved aerosol chemical and physical property measurements in future SA formation investigations that are extended to the coarse mode because the large amount of SA formed thereon might have significant impacts on ice nucleation, radiative forcing, and human health.

Cause of PM2.5 pollution during the 2016-2017 heating season in Beijing, Tianjin, and Langfang, China.

To investigate the cause of fine particulate matter (particles with an aerodynamic diameter less than 2.5 µm, PM2.5) pollution in the heating season in the North China Plain (specifically Beijing, Tianjin, and Langfang), water-soluble ions and carbonaceous components in PM2.5 were simultaneously measured by online instruments with 1-hr resolution, from November 15, 2016 to March 15, 2017. The results showed extreme severity of PM2.5 pollution on a regional scale. Secondary inorganic ions (SNA, i.e., NO3-+SO42+ NH4+) dominated the water-soluble ions, accounting for 30%-40% of PM2.5, while the total carbon (TC, i.e., OC + EC) contributed to 26.5%-30.1% of PM2.5 in the three cities. SNA were mainly responsible for the increasing PM2.5 pollution compared with organic matter (OM). NO3- was the most abundant species among water-soluble ions, but SO42- played a much more important role in driving the elevated PM2.5 concentrations. The relative humidity (RH) and its precursor SO2 were the key factors affecting the formation of sulfate. Homogeneous reactions dominated the formation of nitrate which was mainly limited by HNO3 in ammonia-rich conditions. Secondary formation and regional transport from the heavily polluted region promoted the growth of PM2.5 concentrations in the formation stage of PM2.5 pollution in Beijing and Langfang. Regional transport or local emissions, along with secondary formation, made great contributions to the PM2.5 pollution in the evolution stage of PM2.5 pollution in Beijing and Langfang. The favourable meteorological conditions and regional transport from a relatively clean region both favored the diffusion of pollutants in all three cities.

Pollution characteristics and potential sources of nitrous acid (HONO) in early autumn 2018 of Beijing.

Nitrous Acid (HONO) is an important precursor of hydroxyl radical (OH) and has significant impacts on the formation of Ozone (O3) and Secondary Organic Aerosol (SOA). The atmospheric concentrations of HONO were measured during early autumn in downtown, Beijing (China). This study investigated HONO pollution characteristics and potential sources during day and night. The maximum hourly HONO levels reached 5.16 ppb, with 1.23 ppb on average. HONO concentration exhibited typical diurnal variation characteristics, with maximum at nighttime and minimum at daytime. The potential sources mainly included vehicle emission, heterogeneous reaction of NO2 on aerosol surfaces (Photo-enhanced at the daytime) and photolysis of particulate nitrate (NO3-) in Beijing. Vehicle emission was an important HONO source, particular at the morning rush period and lower HONO concentration. The simulated results highlighted that the main contribution of HONO was NO2 heterogeneous reaction on aerosol surfaces. The photolysis of particulate NO3- was also an important daytime HONO source, particularly in the pollution period. The main loss routine was the photolysis of HONO and dry deposition at day and night, respectively.

Aerosol hygroscopicity based on size-resolved chemical compositions in Beijing.

Hygroscopicity is an important property of aerosols, which cannot be obtained for a wide range of particle sizes by online observation. A cascade impactor sampler, an essential method for obtaining the size-resolved chemical compositions of ambient aerosols, enables the acquisition of the size-resolved hygroscopicity for particles in multiple modes. A micro-orifice uniform deposit impactor (MOUDI-122, MSP) was used to collect the size-resolved aerosol samples during 2016 and 2017. The water-soluble components in these samples were analysed for different pollution levels in two periods. Then, the hygroscopicity parameter (κ) was calculated. The changing hygroscopicity in different size ranges was directly influenced by variations in the distribution of the water-soluble components. The contribution of sulfate to the κ was much higher in the summer period than that in the winter period due to the higher SO42- concentration in the summer. During the summer period, the contribution of nitrate to the κ of particle sizes above 0.56 µm was significantly higher than that of particles smaller than 0.56 µm, while in the winter period, the contribution of nitrate in finer particles with sizes below 1.8 µm was much higher than that in coarse particles. The contribution of chloride to the κ increased significantly in the winter period due to the influence of heating coal emissions. For particles below 1.0 µm, the contribution and fraction of water-soluble organic compounds (WSOCs) to the aerosol hygroscopicity increased with decreasing particle size. Compared with the aerosol hygroscopicity during the first stage of a pollution episode, the hygroscopicity of particles above 0.18 µm was significantly enhanced during the stages of pollution accumulation and formation. The results in this study were in good agreement with the results of other similar studies or data derived by other methods, indicating that the hygroscopicity based on size-resolved water-soluble components is reliable and can be used in the study of activation, radiation force, and heterogeneous reaction mechanism of particles with multiple sizes.

[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.

Size-resolved carbonaceous components and water-soluble ions measurements of ambient aerosol in Beijing.

A MOUDI-120 sampler was used in Beijing to collect multi-stage samples in the summer and winter of 2013 to 2015. Thirty-three sample sets were collected during the daytime, nighttime, and different pollution levels. The actual relative humidity in the impactors was calculated for the first time. The carbonaceous components (organic and elemental carbon, OC and EC, respectively) and water-soluble inorganic ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, and SO42-) were analyzed in each sample. The characteristics of the mass concentration distribution and charge balance were discussed. On the basis of relative humidity in the impactors, aerosols less than 1.0µm were sampled under relatively dry conditions in most cases. The concentration levels for the chemical species were higher in the winter than in the summer. Three modes (condensation mode, droplet mode, and coarse mode) could be identified from the distributions of NH4+, NO3-, SO42-, Cl-, K+, OC and EC. The distribution characteristics for the pollution dissipation process were different from the pollution accumulation process. NO3- and NO2- contributed most of the negative electric charges in the stage below 0.1µm. In the condensation mode, the cations were dominated by NH4+, which was sufficient to balance the anions. In the droplet mode of the heavily polluted samples, the ammonium was not sufficient to balance the anions. In the coarse mode, the positive electric charges were primarily composed of metal cations. The analyzed anions were not sufficient to neutralize the measured cations.

[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.

Exploring the nitrous acid (HONO) formation mechanism in winter Beijing: direct emissions and heterogeneous production in urban and suburban areas.

Continuous measurements of nitrous acid (HONO) were performed from December 12 to December 22, 2015 in both urban and suburban areas of Beijing to study the formation mechanism of HONO. The measurement campaign in both sites included a clean-haze-clean transformation process. HONO concentrations showed similar variations in the two sites, while they were always higher in the urban area. Moreover, correlations of HONO with NOx, NO2, NO, PM2.5 and relative humidity (RH) were studied to explore possible HONO formation pathways, and the contributions of direct emissions, heterogeneous reactions, and homogeneous reactions were also calculated. This showed that HONO in urban and suburban areas underwent totally different formation procedures, which were affected by meteorological conditions, PM2.5 concentrations, direct emissions, homogeneous reactions and heterogeneous reactions. PM2.5 concentrations and RH would influence the NO2 conversion efficiency. Heterogeneous reactions of NO2 were more efficient in suburban areas and in clean periods while direct emissions and homogeneous reactions contributed more in urban areas and in polluted periods when the concentrations of NOx and NO were at a high level.

[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.