Characteristics of aerosol size distributions and new particle formation in the summer in Beijing

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114,, doi: /2008jd010894, 2009 Characteristics of aerosol size distributions and new particle formation in the summer in Beijing Dingli Yue, 1 Min Hu, 1 Zhijun Wu, 1 Zhibin Wang, 1 Song Guo, 1 Birgit Wehner, 2 Andreas Nowak, 2 Peggy Achtert, 2 Alfred Wiedensohler, 2 Jinsang Jung, 3 Young J. Kim, 3 and Shawchen Liu 4 Received 31 July 2008; revised 11 May 2009; accepted 20 May 2009; published 24 July [1] The Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006) were mainly focused on the influence of the regional aerosol on the air pollution in Beijing. The urban aerosol was characterized in detail. The particle size distributions were also compared to those measured at a regional site (Yufa) approximately 50 km south of the urban site at Peking University (PKU). At PKU, total particle number and volume concentrations were (1.8 ± 0.8) 10 4 cm 3 and 83.5 ± 57.9 mm 3 cm 3, respectively. Days in three consecutive summers of 2004, 2005, and 2006 were classified as polluted days with PM 10 over 150 mg m 3 and nonpolluted days with lower PM 10. On nonpolluted days, particle number size distributions showed a maximum at about 60 nm with Aitken mode particles dominating number concentration. On polluted days, the contribution of accumulation mode particles increased, shifting the maximum of the number size distribution to over 80 nm. On polluted days with stagnant meteorological conditions, secondary aerosol dominated, with SO 4 2,NO 3, and NH 4 + accounting for over 60% of accumulation mode particle mass. Particle number size distributions at both sites were similar. Number and volume concentrations of total particles at Yufa were 6% and 12% lower, respectively; those of accumulation mode particles were 2% and 15% lower. This means that air pollution in Beijing is mainly a regional problem. The regional accumulation mode particles are a metric for assessing the air quality since they influence most the visibility and total mass concentration. Their number and volume concentrations on polluted days were cm 3 and 30 mm 3 cm 3, respectively. Five new particle formation (NPF) events with continuous smooth growth were observed at both PKU and Yufa during CAREBeijing These NPF events are regional or semiregional. Growth rates at PKU ranged from 1.2 to 5.6 nm h 1, and formation rates ranged from 1.1 to 22.4 cm 3 s 1.SO 4 2,NH 4 +, and oxalate might be important contributors to NPF events. Citation: Yue, D., et al. (2009), Characteristics of aerosol size distributions and new particle formation in the summer in Beijing, J. Geophys. Res., 114,, doi: /2008jd Introduction [2] Aerosol particles influence the global climate by changing the radiative balance of the atmosphere, acting as cloud condensation nuclei (CCN), imposing a negative impact on human health, and degrading visibility [e.g., Sokolik and Toon, 1996; O Dowd, 2001; Jung and Kim, 1 State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China. 2 Leibniz Institut for Tropospheric Research, Leipzig, Germany. 3 Advanced Environmental Monitoring Research Center, Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, South Korea. 4 Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan. Copyright 2009 by the American Geophysical Union /09/2008JD ]. Absorption and scattering of the incoming radiation depends on the particle size and composition [Seinfeld and Pandis, 1998; Nishita et al., 2007]. The number concentration of accumulation mode particles explains the visibility degradation on hazy days [See et al., 2006]. Only particles within a certain size range have cloud-nucleating ability, and they can affect the microphysical and optical properties of CCN [Seinfeld and Pandis, 1998; Iorga and Stefan, 2005]. Dusek et al. [2006] reported that particle size matters more than chemistry in cloud formation. Aerosol particles can deposit in the respiration system and studies have indicated that ultrafine particles (UFP) can have a greater health impact than larger particles, because they are capable of penetrating deeper into the respiratory tract [Oberdörster, 2000; Nemmar et al., 2003]. Although UFP account for less than 5% of the submicrometer particle mass, they represent about 65% of the submicrometer particle number concentration [Morawska et al., 1998; Rodriguez et al., 2005]. 1of13

2 Toxic compounds in the particles will do more harm to the receptors [e.g., Guthrie, 1997; Yang and Ma, 1994]. [3] The size distributions of atmospheric aerosols, their composition, sources, and sinks are key elements to understand and manage their effects on health, visibility, and climate. All this shows that particle number concentration should get equal or even more attention than the mass concentration, and not only total particle number concentration is important, but also the number size distributions of particles. [4] In urban environments, air quality is strongly influenced by motor vehicle emissions, and number concentrations of aerosol particles usually exceed 10 4 cm 3 [Hussein et al., 2004]. In recent years, much research has been done on urban aerosol number size distributions, including longterm measurements such as in Helsinki, Finland [Hussein et al., 2004] and in Leipzig, Germany [Wehner and Wiedensohler, 2003], and short-term monitoring like in Atlanta, USA [Woo et al., 2001] and Birmingham, UK [Shi et al., 2001]. Most results show that 24-h average concentrations of particles in the nm range in urban areas are (5 25) 10 3 cm 3 [e.g., Woo et al., 2001; Kim et al., 2002]. Their diurnal variations are affected by the traffic emission, especially for the particles ranging from 25 to 100 nm [Wehner and Wiedensohler, 2003; Hussein et al., 2004]. Most of the research was done in the developed countries, only a few studies were carried out in the heavy polluted megacities of the newly industrialized countries, such as Mexico [Dunn et al., 2004], New Delhi [Mönkkönen et al., 2005], and Beijing [Wehner et al., 2008; Yu et al., 2005; Wu et al., 2007, 2008]. The characteristics of particle number size distributions in these megacities with much higher number concentrations are usually different from the cities in the developed countries, because of the high population density, fast industrialization, and rapid urbanization in these megacities. [5] Aerosol number size distributions in the urban area are mainly a function of the following factors: [6] 1. One factor is primary emissions by mobile sources, such as traffic emitting mainly UFP [Harrison et al., 1999; Thomas and Morawska, 2002], and stationary sources, including power plants, cooking, and heating, etc. [Shi et al., 2001]. [7] 2. Meteorological factors, such as wind, temperature (T), relative humidity (RH), pressure, precipitation, and radiation, affect the formation, growth, transformation, deposition, and transport process of the airborne particles. Changes in the boundary layer height, in wind speed and wind direction, and precipitation can lead to a decrease of particle number concentrations and change of particle size distributions in urban areas [Vakeva et al., 2000]. [8] 3. Long-range transport brings air masses from different sources of diverse regions and causes different number size distributions in the urban areas [Kulmala et al., 2000; Wehner and Wiedensohler, 2003; Wehner et al., 2008]. Regional aerosols are usually dominated by secondary aerosols formed during transport. [9] The urban aerosol number size distributions are the results of the contributions of all three factors, but their contributions may vary per location and/or season. [10] Beijing, the capital of China, is a megacity with more than 16 million inhabitants ( The high population density, rapid industrialization, and fast urbanization have led to a rapid increase of the primary emissions of pollutants. Under the usual summer conditions with high temperatures and RH, the combination of primary and secondary pollutants leads to high mass and number concentrations of particulate matter. The aerosol, especially particles in the size range below 1 mm, has become one of the main air pollution problems. PM 10 is always the most serious pollutant in the summer of Beijing ( zhb.gov.cn/). PM 1 accounts for 50 70% of PM 10, therefore these particles, consisting of nucleation mode, Aitken mode, and accumulation mode particles, have drawn much attention in Beijing. New particle formation (NPF) events are observed frequently [Wu et al., 2007] leading to more urgency in particle research in view of possible important health and climate effects. [11] In order to identify and understand their characteristics better, particle number size distributions from 3 nm to 10 mm during three consecutive summers of 2004, 2005, and 2006 were measured at the urban site in Peking University (PKU). The campaign from 10 August to 10 September 2006 was part of the Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006). The CAREBeijing-2006 results are the main focus, including the comparison of particle size distributions between PKU and a regional site, Yufa. Thus the objective of this paper is to summarize characteristics of NPF events and particulate pollution properties in terms of number size distributions and temporal variations. The major chemical component of the accumulation mode particles is also discussed to shed light on particle number size distributions under stagnant conditions with heavy pollution in the summer of Beijing. 2. Experimental Methods [12] A detailed description of the site and some instruments is given in the overview paper of T. Zhu et al. (manuscript in preparation, 2009). Our measurements were performed on the roof of a building (about 15 m above the ground level) in the campus of PKU, which is located in the northwestern urban area of Beijing, outside the fourth-ring road. An overview of the instrumentation is given in Table 1. [13] Dry particle number size distributions between 3 nm and 10 mm were measured with a system consisting of a Twin Differential Mobility Particle Sizer (TDMPS) [Birmili and Wiedensohler, 1997] and an Aerodynamic Particle Sizer (APS, TSI model 3321, TSI Inc., St. Paul, MN, USA) or Optical Particle Sizer (OPC). [14] The TDMPS measures the particle number size distributions from 3 to 800 nm. It is composed of two Hauke-type Differential Mobility Analyzers (DMA) and two Condensation Particle Counters (CPC, model 3010 and model 3025, TSI Inc., St. Paul, MN, USA) as described by Wu et al. [2007]. APS data of aerodynamic particle number size distributions between 800 nm and 10 mm were converted to Stokes diameters with an assumed particle density of 1.7 g cm 3, density of sulfate, the major component of aerosols in Beijing [Yao et al., 2002]. It is also similar to the apparent density of PM 10 of 1.68 g cm 3 as reported in this paper. RH within the whole system was kept below 30%. Size-dependent losses due to diffusion and 2of13

3 Table 1. Summary of the Instruments Instrument Parameters Time Resolution Time Period Reference TDMPS-APS/OPC particle number size distribution ( nm) 10 min (1 min for OPC) 4 to 25 Aug 2004 and 10 Aug to 10 Sep 2005 and 2006 Dust monitors and counters 265(Grimm) PM 10,PM 1 30 min CAREBeijing-2006 (10 Aug to 10 Sep 2006) Particle into Liquid Sampler (PILS) mass concentrations of SO 2 4,NO 3,NH + 4, 30 min CAREBeijing-2006 (10 Aug to 10 Sep 2006) Micro Orifice Uniform Deposit Impactor (MOUDI) LASTEM M7115 automatic meteorological station (LSI- LASTEM, Italy) and NO 2 in PM 10 size distributions of chemical compositions meteorological factors of T, RH, wind, and precipitation several hours CAREBeijing-2006 (10 Aug to 10 Sep 2006) 10 min CAREBeijing-2006 (10 Aug to 10 Sep 2006) Guo et al. (unpublished manuscript, 2009) Zhu et al. (manuscript in preparation, 2009) sedimentation within the inlet were corrected with empirical particle loss corrections as given in the textbook of Willeke and Baron [1993]. [15] When APS data were not available, OPC data were used instead. The particle size range of this OPC was 250 nm to 10 mm (optical diameter) and the size range from 800 nm to 10 mm was combined with TDMPS data. A comparison of the available average particle number size distributions by TDMPS-APS and OPC during CAREBeijing-2006 is shown in Figure 1. The agreement between TDMPS-APS and OPC data suggests that all these three instruments were reliable and captured reasonable data. [16] To keep the consistency, all the online data were hourly averaged, except for MOUDI sampler results. [17] As shown in Figure 2, particle size classes are defined as follows in this paper: nucleation mode (Nucl) particles from 3 to 20 nm, Aitken mode (Ait) from 20 to 100 nm, accumulation mode (Acc) from 100 to 1000 nm, coarse mode (Coa) from 1 to 10 mm, UFP from 3 to 100 nm, including nucleation and Aitken mode particles, and Total from 3 nm to 10 mm. Consequently, N- and V- mean number and volume concentrations of particles within a certain size range, respectively. [18] Number, surface area, and volume concentrations of size-resolved particles were calculated by integrating the particle number size distributions assuming spherical particles [Seinfeld and Pandis, 1998]. [19] V-Total, V-Acc, PM 10, and PM 1 showed clear correlations. Volume concentrations of particles within each mode can be multiplied with an assumed density to estimate the size-resolved particle mass concentrations [Pitz et al., 2003]. In a similar way, particle mass concentrations within a certain size range divided by corresponding particle volume concentrations can be used to calculate the average apparent density of these particles. V-Total and PM 10 during CAREBeijing-2006 were compared. The average ratio of PM 10 /V-Total was 1.68 (R 2 = 0.77), which means the average apparent density of PM 10 was about 1.68 g cm 3. Also the volume concentrations of particles from 3 to 1000 nm (V-UFP + V-Acc) and PM 1 were compared in a similar way. The average ratio of PM 1 /(V-UFP + V-Acc) was 1.43 (R 2 = 0.91), meaning the average apparent density of PM 1 in the summer of Beijing to be about 1.43 g cm 3. The lower density of PM 1 than PM 10 is explained by the fact that minerals with higher density are mainly present in the coarse mode particles ranging from 1 to 10 mm. [20] Particle composite densities can be estimated on the basis of the chemical composition. Here, the composition of the particles are sorted into sulfate, nitrate, and ammonium (SNA, SNA = SO 4 2 +NO 3 +NH 4 + ), organic matter (OM, OM = organic carbon 1.4), elemental carbon (EC), minerals, and others. Their densities in particles and percentages in PM 1 and PM 10 are given in Table 2. Sloane [1983] assumed the densities of the compounds in particles to be as follows: EC 1.0, OM 1.7, NH 4 NO , and (NH 4 ) 2 SO g cm 3 in his research on optical properties of aerosols. The estimated composite densities of PM 1 and PM 10 were 1.52 and 1.62 g cm 3, respectively. The differ- Figure 1. Average particle number size distributions measured by TDMPS-APS and OPC. The bars are the standard deviations. Figure 2. Size ranges of the defined particles. Coa, coarse mode; Acc, accumulation mode; Ait, Aitken mode; Nucl, nucleation mode; UFP, ultrafine particles; Total, total particle size range. 3of13

4 Table 2. Densities and Percentages of the Sorted Chemical Compostitions in Particles SNA OM EC Minerals Others Density (g cm 3 ) Percentage in PM Percentage in PM ence between the apparent and composite densities was 6% for PM 1 and 4% for PM 10. This result indicates that the apparent densities calculated with particle volume and mass concentrations are reliable; 1.43 and 1.68 g cm 3 are the apparent densities of PM 1 and PM 10 in the summer of Beijing, respectively. [21] The air mass backward trajectories were calculated using the NOAA HYSPLIT4 model ( gov/ready/hysplit4.html) [Draxler and Rolph, 2003]. The meteorological input data used by the model were obtained from the NOAA FNL archives. 3. Results and Discussion 3.1. Three Consecutive Summers Measurements of Particle Number Size Distributions Meteorological Conditions [22] Typically, the summer in Beijing is characterized by high temperature, high RH, and low wind speed compared to other seasons [Wu et al., 2008]. The average temperature in the measurement period of 2006 was about 26 C, higher than in 2004 and lower than in 2005 (Table 3). All the average RH in these 3 years were above 60%. About 80% of the measured wind speeds in the summer of these 3 years were lower than 2 m s 1 as shown in Figure 3. Thus, stagnant meteorological conditions favored accumulation and secondary transformation of particles, and usually contributed to pollution with daily average PM 10 exceeding the second grade of China s National air quality standard for PM 10 of 150 mg cm 3. Wind speed over 4 m s 1 mainly came from north or northwest (Figure 3), which is a relatively clean, rural area with mountains, less inhabitants, and less emission of particles and precursor gases, leading to lower particle concentrations than the urban area. If the solar radiation was strong, NPF events took place. [23] Precipitation in Beijing is mainly concentrated to summer months, accounting for 75% of the total annual precipitation, which normally ranges between 400 and 500 mm ( [Hu et al., 2005]. The total precipitation amount was 40 mm during the measurement period of 2004, 92 mm, highest in 2005, and 28 mm, lowest in The number of rainfall events was 6, 6, and 5 for the measurement periods in 2004, 2005 and 2006, respectively (Table 3). Precipitation scavenges particles in the atmosphere, reshapes the particle number size distributions, and ends pollution accumulation processes. Rain is normally accompanied by a change of air mass and strong northern wind (>4 m s 1 ), also contributing to clean air Overview of Particle Size Distributions [24] N-Total showed a decreasing trend during these 3 years, (3.0 ± 1.0) 10 4 cm 3 in 2004, (2.3 ± 0.6) 10 4 cm 3 in 2005, and (1.8 ± 0.8) 10 4 cm 3 in V-Total were 84.6 ± 54.6, 80.2 ± 52.6, and 83.5 ± 57.9 mm 3 cm 3 for 2004, 2005, and 2006, respectively, showing no significant trend. The frequency of periods with V-Total below 50 mm 3 cm 3 or over 150 mm 3 cm 3 was higher in 2006 than in 2004 and Low V-Total occurred on days with NPF (like 20 August 2006) events or precipitation (like 13 August 2006) with high-speed wind from the north. In contrast high V-Total occurred on the polluted days with high temperature, high RH, and low-speed wind, for instance on 24 August These findings correspond to the results as reported by Wehner et al. [2008] that clean air was usually connected with fast moving air masses from the north. [25] The average particle number and volume size distributions in the summer of 2004, 2005 and 2006 are shown in Figure 4. The shapes of the number size distributions for these 3 years were similar with the maximum at about 60 nm within the Aitken mode. The Aitken mode particles are the main contributor to N-Total, accounting for about 50%. UFP accounted for over 70% of N-Total. Accumulation mode particles dominate V-Total (over 60%) and coarse mode particles contribute about 25 30% of V-Total. The shift of the peak within coarse mode in 2006 might be related to an increase of constructions near the measurement site. Accumulation mode particles contribute over 80% of total particle surface area concentrations. Table 3. Meteorological Factors in the Summers of 2004, 2005, and 2006 a T ( C) RH (%) WS (m s 1 ) Precipitation b (mm) ± ± ± , c 59 d ± ± ± d ± ± ± d a Mean ± s. b Total precipitation amount (mm). c Six days out of twenty-two days from 4 to 25 August. d Six, six, and five days out of thirty-two days from 10 August to 10 September. Figure 3. Frequency distribution of wind direction and wind speed in the summers of 2004, 2005, and of13

5 air mass leads to lower turbulent depositional losses and less symmetrical dispersion, so the Aitken mode particles accumulate in the air. While on nonpolluted days, Aitken mode particles also have another important source, the growth of the nucleation mode particles. Figure 4. Average particle number and volume size distributions (3 nm to 10 mm) in the summers of 2004, 2005, and [26] Days with PM 10 exceeding the second grade of China s National PM 10 standard of 150 mg m 3 were classified as polluted days. PM 10 data were not available for 2004 and 2005, so a substitute V-Total standard of 90 mm 3 cm 3 was used, derived by dividing the PM 10 standard 150 mg cm 3 by the apparent density of PM 10,1.68gcm 3. In total, 7, 10, and 13 polluted days among the measurement days in 2004, 2005, and 2006 were identified, respectively. [27] Average particle number and volume size distributions for polluted and nonpolluted days in 2004, 2005 and 2006 are shown in Figure 5. Corresponding lognormal fit parameters of the average particle number size distributions are given in Table 4. The atmospheric aerosol number size distributions were fitted and described as the sum of multi lognormal distribution functions [Seinfeld and Pandis, 1998]. [28] The average particle number size distributions showed significant shifts to larger sizes on polluted days than nonpolluted days. On polluted days, the geometric mean diameters (GMD) of the nucleation, Aitken, and accumulation mode particles were larger under more stagnant conditions with shorter air mass backward trajectories from the south [Wehner et al., 2008], in combination with higher temperature, higher RH, and lower-speed wind. Such conditions favored stronger particle growth, as formed secondary materials also deposit on the surface of preexisting particles. The average N-Acc on polluted days in 2004 was about twice, in times, and in 2006 three times of that on nonpolluted days. The average N-Nucl on polluted days was lower, in 2006 only half of that on nonpolluted days. That is because strong coagulation scavenging is produced by the high concentration of accumulation mode particles [Mönkkönen et al., 2004]. The high concentration of accumulation mode particles provides a high particle surface area concentration and represents an effective sink for nucleation mode particles. With traffic emission as its main source, average N-Ait both on polluted and nonpolluted days were high. On polluted days, stagnant 3.2. Particle Number Size Distributions and Chemical Composition During Pollution Episodes [29] Several pollution episodes of 4 7 days can be identified from the time series of V-Total as well as PM 10 and PM 1 during CAREBeijing-2006, such as the one from 15 to 20 August Such pollution episodes were also observed in the summers of 2004 and 2005, but were not so obvious. During such a pollution episode, gradual increases of the aerosol mass and volume concentrations were observed until interrupted by conditions with highspeed wind and/or precipitation. However, this is not the case for N-Total during the pollution episode. On days with higher V-total and PM 10, more accumulation mode particles were observed. [30] The pollution episode (15 to 20 August 2006) was ended by high-speed wind from the north on 20 August. On 13 and 14 August, it rained, which scavenged a large fraction of the aerosols. The date of 15 August can be seen as a new start of the pollution episode, and it was not a polluted day. On 20 August, a typical NPF event was identified and classified as a clean type of NPF event according to the criteria established by Wu et al. [2007]. But all the other 4 days in between were classified as polluted days Sources of Particulate Pollution [31] Time series of particle number size distributions (3 800 nm) from 0000 local time on 15 August to 0000 local time on 21 August 2006 are shown in Figure 6. Obviously, Figure 5. Average number and volume size distributions on different days in the summers of 2004, 2005, and Polluted days are indicated by p, and nonpolluted days are indicated by n. 5of13

6 Table 4. Average Lognormal Fit Parameters of the Particle Number Size Distributions and Meteorological Factors on Polluted and Nonpolluted Days in the Summers of 2004, 2005, and 2006 a Nucleation Aitken Accumulation Ni GMD s Ni GMD s Ni GMD s T( C) RH (%) WS (m s 1 ) Nonpolluted, ± ± ± 0.7 Polluted, ± ± ± 0.6 Nonpolluted, ± ± ± 0.9 Polluted, ± ± ± 0.7 Nonpolluted, ± ± ± 1.4 Polluted, ± ± ± 0.9 a Particle number size distributions are nm. Ni, mode particle number concentration (10 3 cm 3 ); GMD, geometric mean diameter of the mode (nm); s, standard deviation of the mode. most particles were concentrated in the accumulation mode, with more particles of larger sizes (over 400 nm) than on nonpolluted days. The ratio of N-Acc/N-Ait for all the polluted days in the summer of 2006 was 0.9 ± 0.2, and that for all nonpolluted days was 0.4 ± 0.1. [32] During polluted days, increases of N-Acc, V-Acc are obvious. The same is true for V-Total, because accumulation mode particles dominate V-Total as shown in Figure 7. The maximum N-Acc was about cm 3 on 18 August. The maximum V-Acc and V-Total were 162 and 215 mm 3 cm 3, respectively, on 19 August. [33] Also obvious increases of PM 1,PM 10, and high mass concentrations of secondary ions in PM 10, including SO 4 2, NO 3, NH 4 +, and NO 2, were observed. PM 1 and PM 10 reached their maximums of 200 and 289 mg cm 3, respectively, also on 19 August. Two main reasons for this increase can be postulated: [34] 1. The first reason is accumulation. During the polluted days the mixing ratio of CO is much higher than on nonpolluted days, which means that the contribution of accumulation of all the pollutants should be very important. [35] 2. The second reason is secondary transformation. The mixing ratio of O 3 can be very high, over 100 ppb, indicating strong photochemical activity and conversion of precursor gases to particulate matter. The importance of secondary transformation is supported by the fact that the ratios of SNA/PM 10,SO 4 2 /PM 10, and the transformation rate of sulfur (equivalent ratio of SO 4 2 /(SO SO 2 )) during the polluted days increased compared to nonpolluted days. The increase can be as high as a factor of 2. SNA was the most important compound causing particulate pollution on the polluted days, accounting for about 50% of PM 10 on average. Seen from the aspect of particle size distributions, accumulation mode particles were the most important contributor. They accounted for about 80% of V-Total on average and had higher N-Acc values than N-Ait. The ratios of N-Acc/N-Total and V-Acc/V-Total also had similar variations as the ratio of SNA/PM 10, so SNA is likely to be the major compound in accumulation mode particles. [36] SO 4 2 can be produced through the oxidation of SO 2 by OH radicals, the latter are mainly produced by photochemical reactions [Seinfeld and Pandis, 1998], resulting in a correlation between O 3 and SO 4 2. Also, correlations between O 3,NO 3 and NH 4 + were observed, but the peak times of NO 3 and NH 4 + were earlier, because the dissociation constant of NH 4 NO 3 is a function of temperature and RH [Stelson and Seinfeld, 1982; Mozurkewich, 1993]. Low temperature and high RH increase the stability of NH 4 NO 3. Although high mixing ratios of HNO 3 (>0.8 ppb) were observed at the time with high O 3 mixing ratios (>70 ppb), the high temperature and low RH lead to dissociation of NH 4 NO 3. Higher NO 2 concentrations were also observed on the polluted days, but with different diurnal variations. NO 2 showed high concentrations at night and low values during the daytime just like its precursor, HONO. HONO can be produced heterogeneously on wet surfaces; it accumulates during the night, and is photolyzed to OH radicals during the daytime. Particles in the atmosphere supply Figure 6. Time series of particle number size distributions (3 800 nm) from 0000 local time on 15 August to 0000 local time on 21 August The number by the x ordinate is the time by hour of each day (ddhh). For the color bar, the number is dn/dlogdp (cm 3 ). 6of13

7 Figure 7. Time series of the related species during the pollution episode from 15 to 20 August much of the wet surfaces for HONO production, especially during nights with RH over 70% [Hu et al., 2008] Quantitative Assessment of the Contribution of SNA [37] An analysis of the formation rates of the two peaks on 18 August as illustrated in Figure 7 can be used to estimate the contribution and mechanism of secondary aerosol formation (similar phenomena were also observed on other polluted days, such as on 17 and 19 August). The increase rate is the slope of the concentration of a certain species versus time when it increases. During the first increase of aerosol concentrations, the increase rate of N-Acc was about 300 cm 3 h 1 on average, and of V-Acc was 4.6 mm 3 m 3 h 1 on average (corresponding data in Table 5). If the average density of accumulation mode particles is assumed to be the same as PM 1 of 1.43 g cm 3, the latter was equal to 6.7 mgm 3 h 1, lower than the increase rate of PM 1, 10.1 mgm 3 h 1. The increase rates of SO 4 2,NO 3, and NH 4 + were 1.9, 2.6, and 1.3 mg m 3 h 1, respectively. NO 3 increased faster than SO 4 2. Almost all SNA is in the form of fine particles, and most SNA is in accumulation mode particles. If we assume that 70% of SO 4 2 and NH 4 +, and 30% of NO 3 are in PM 1, or accumulation mode particles according to the measured chemical composition size distributions, SNA (actually 70% SO % NO % NH 4 + ) can explain about 50% of the accumulation mode particle mass increase and 30% of PM 1. The first peak appeared before 1000 local time (at about 0800 local time), when the radiation and O 3 concentration were relatively low. The photochemical activity of the atmosphere was quite low, so the influence of the photochemical reactions on the particulate pollution was negligible. At the same time, the nocturnal inversion caused a stable stratification of the lowest part of the boundary layer. This 7of13

8 Table 5. Increasing Rates of Major Species During the Pollution Episode Time Period N-Acc (cm 3 h 1 ) V-Acc (mm 3 cm 3 h 1 ) M-Acc PM 1 SO 4 2 NO 3 NH 4 + SNA SNA* a 18 Aug, I Aug, II Aug Aug b a SNA* = 0.7 (SO 4 2 +NH 4 + )+0.3 NO 3. b At Yufa. condition hindered the dispersion of pollutants. Furthermore, RH was quite high, which accelerated heterogeneous reactions of SO 2,NO x, and NH 3 to produce SO 2 4,NO 3, and NH + 4. [38] After 0800 local time, the fast development of the boundary layer led to the decrease of the particle concentrations. The second peak was characterized by an increase rate of N-Acc of about 270 cm 3 h 1 on average, and of V-Acc of 6.3 mm 3 cm 3 h 1 on average. Multiplied by the assumed density of 1.43 g cm 3, the latter was equal to 9.0 mg m 3 h 1, lower than the increase rate of PM 1, 9.8 mg m 3 h 1. The increase rates of SO 2 4,NO 3, and NH + 4 were 5.6, 2.3, and 2.4 mg m 3 h 1, respectively. SO 2 4 increased faster than NO 3. With the same assumptions as applied above, SNA can explain over 70% of the accumulation mode particle mass increase, and over 60% of PM 1. High radiation and O 3 concentration indicate that photochemical reactions had a more important role in producing SNA. With increased temperature and decreased RH, producing SNA through heterogeneous reaction was limited, and the high and well-mixed boundary layer also restricted the accumulation of SNA. [39] The increase rate of NO 3 during the second increase period was lower than that for the first one because of the shift in equilibrium of solid ammonium nitrate, ammonia, and nitric acid caused by higher temperatures and lower RH. The increase rate of SO 2 4 was higher than that for the 2 first. More SO 2 was probably transformed into SO 4 through photochemical reactions. [40] If the increase rates of the mentioned species from 0000 local time on 16 August to 0600 local time on 19 August were calculated in the same way as done on 18 August, the increase rate of N-Acc was about 40 cm 3 h 1 on average, and of V-Acc was 0.85 mm 3 cm 3 h 1, equal to 1.2 mg m 3 h 1, close to the increase rate of PM 1, 1.1 mgm 3 h 1. The increase rates of SO 2 4,NO + 3, and NH 4 were 0.57, 0.35, and 0.31 mg m 3 h 1, respectively. SNA can explain over 60% of the accumulation mode particle mass increase and of PM 1, showing clearly that SNA is the major composition of the accumulation mode particles and a significant contributor to particulate pollution in the summer of Beijing. The study of Maßling et al. [2009] showed that ammonium sulfate was the major inorganic composition of the urban submicrometer aerosol in Beijing and during heavily polluted times, submicrometer aerosols were highly influenced by aged air masses transported from the industrial regions around Beijing containing sulfate as a major compound Evolution of Average Particle Number and Chemical Composition Size Distributions [41] Twenty-four-hour average particle number size distributions of each day during this pollution episode are shown in Figure 8. From 15 to 18 August, GMD gradually shifted to larger sizes, with accumulation mode particles dominating N-Total on 16, 17, and 18 August. N-Nucl and its contribution to N-Total decreased during heavy polluted days as conditions with high particle surface area concentration and high RH do not favor the formation and existence of nucleation mode particles. [42] The average mass size distributions of SO 4 2,NO 3, NH 4 +, and oxalate are illustrated in Figure 9. About 70% of SO 4 2 and NH 4 +, and 30% of NO 3 were within the range of accumulation mode. Similar to the average particle number size distributions, there were obvious shifts to larger sizes as well as significant increases of maximum mass concentrations. NH 4 + and SO 4 2 shifted from mm on very clean days to mm on polluted days, and NO 3 Figure 8. Twenty-four-hour average particle number size distributions (3 800 nm) and air mass backward trajectories at 100 m for 72 h arriving at Beijing at 1200 local time ( each day during the pollution episode from 15 to 20 August The numbers in the labels and at the end of backward trajectory are the date (month-day). 8of13

9 formation and existence of NH 4 NO 3 mainly in the fine particles. Oxalate showed the same size distributions as SO 4 2, except that its concentration was lower, indicating that oxalate and SO 4 2 originated from similar atmospheric processes (S. Guo et al., unpublished manuscript, 2009). [43] Seventy-two-hour air mass backward trajectories with initial height of 100 m arriving at Beijing at 1200 local time of each day from 15 to 20 August are given in Figure 8. The length of backward trajectories was found to be shorter on the polluted days and mainly coming from the south, the major city center of Beijing. Long-term research of Wehner et al. [2008] has shown that the direction of air mass origin caused a difference in PM 1 or PM 10 mass concentration in Beijing. Clean air arrived from the northwest and more polluted cases were connected with air from the south or with air masses which had spent already a few days in the Beijing urban area. Figure 9. Average chemical composition size distributions during the daytime on 17, 18, 20, and 21 August. Dp is the mean aerodynamic cutoff diameter of two seriate stages of MOUDI. Numbers in the labels indicate the date (monthday); day indicates local time, AM indicates local time, and PM indicates local time. shifted from mm on very clean days to mm on polluted days. On clean days, more NO 3 is found in larger particles than NH 4 + and SO 4 2. However, on polluted days, all three ions peaked at mm. Different from the mass size distributions of SO 4 2 and NH 4 +,NO 3 also had another peak at mm, obviously increasing on polluted days and probably related to formation of NaNO 3 and Ca(NO 3 ) 2. On clean days, NO 3 was more abundant in coarse particles than in fine particles, as low RH is not favorable for the formation and existence of NH 4 NO 3. More HNO 3 was available to react with NaCl and CaCO 3, from soil and construction work, to form NaNO 3 and Ca(NO 3 ) 2. On polluted days with stagnant meteorological conditions, NO 3 accumulated both in fine and coarse particles; high photochemistry rates will lead to rapid formation of nitric acid and the high RH favors the 3.3. New Particle Formation Events [44] The criterion for discerning NPF events is the burst of the nucleation mode particle number concentration [Birmili and Wiedensohler, 2000]. And the specific criteria for discerning NPF events in Beijing used here are the same as those used by Wu et al. [2007]. His study shows that summer is the season with the lowest frequency of NPF events. [45] NPF events were found on 3 5 days during the measurement period of 2004, 2005, and 2006 (specific dates in Table 6) at PKU. Just before a NPF event, N-Acc and N-Ait decreased, which led to very low particle surface area concentration. During NPF events, N-Nucl increased very fast and reached the maximum value of over cm 3. Following the increase of nucleation mode particles, Aitken mode particles also increased significantly, and N-Ait was over three times higher than that before NPF. During NPF events with high-speed wind, accumulation mode particle concentrations were very low. [46] When NPF events occurred, high ozone concentrations (indicating photochemical activity) and intense solar radiation were observed. RH is usually lower compared to days without NPF events. NPF events usually happen with high-speed wind from the north, transporting clean air to the measurement site. Wu et al. [2007] summarized that NPF events need conditions with low RH, intense solar radiation, and low particle surface area concentration in Beijing. Sunny days in summer with low RH and high-speed wind (>4 m s 1 ) from the north or after precipitation can be expected to have NPF events during the day time. [47] Parameters describing NPF including growth rate (GR), formation rate (FR) of new particles, condensable vapor concentration (C_vap), source rate of the condensable vapor (Q), and condensational sink (CS) were calculated with the method described by Kulmala et al. [2001, 2005], Dal Maso et al. [2002, 2005], and Wu et al. [2007]. [48] FR is the flux of particles into the observable size range (3 nm in this paper). It can be calculated from the following equation: FR ¼ dn nuc dt þ F coag þ F growth ð1þ 9of13

10 Table 6. Parameters Describing NPF Events Date GR (nm h 1 ) FR (cm 3 s 1 ) C_vap (10 7 cm 3 ) Q (10 6 cm 3 s 1 ) CS (10 2 s 1 ) 4 Aug Aug Aug Aug Sep Sep Sep Aug Aug Sep Sep Sep Aug 2006 a Aug 2006 a Aug 2006 a Sep 2006 a Sep 2006 a Summer 2004 b Summer 2005 b a At Yufa. b Summer here is June, July, and August. where N nuc is the nucleated particle number concentration, F coag is the loss of particles due to coagulation, and F growth is flux of particles growing out of 25 nm. [49] GR describes the change in the mean diameter within a time period when a NPF event occurred. GR ¼ DD m Dt where D m is a mean geometric diameter of lognormal ultrafine particle mode, which has been fitted to the number size distribution [Heintzenberg, 1994]. [50] Assuming that particle growth is caused by condensation of compounds with low vapor pressure to the particle surface, the mass flux can be integrated to derive C_vap [Kulmala, 1988]. A simple approximate relation between GR in the nucleation size range and C_vap can be written as: C vap ¼ A GR where A is a constant, which has the value of h cm 3 nm 1 for a vapor with molecular properties of sulfuric acid. CS describes the ability of the preexisting particles to remove condensable vapors from the atmosphere. [51] If a steady state situation is assumed for the condensable vapor, the source rate of it can be calculated from equation [Dal Maso et al., 2002]: Q ¼ CS C vap [52] In 2006, GR ranged from 1.2 to 5.6 nm h 1 and FR from 5.0 to 22.4 cm 3 s 1. FR was mostly within those ranges in the summers of 2004 and 2005 (June, July and August), but usually lower than that in Atlanta cm 3 s 1 [Kulmala et al., 2004]. GR was in the range of values in the summer of 2004 and 2005 and corresponding with ð2þ ð3þ ð4þ typical particle growth rates (1 20 nm h 1 )[Kulmala et al., 2004]. Several hours after NPF events these newly formed particles would grow into Aitken and accumulation mode. C_vap, Q, and CS of NPF events during CARE- Beijing-2006 were mostly within their corresponding ranges in the summers of 2004 and 2005, which are usually higher than those in other seasons. The higher C_vap and Q are related to the enhancement of photochemical and biological activities during the summer months [Wu et al., 2007]. The higher CS might be one important reason for the lower frequency of NPF events in the summer. [53] On days with NPF events such as 20 and 21 August 2006, obvious peaks were found at smaller particle sizes in the mass distributions of SO 4 2,NH 4 +, and oxalate compared to days without NPF (Figure 9). Smith et al. [2005] applied Thermal Desorption Chemical Ionization Mass Spectrometer to investigate the composition of particles (6 15 nm) in Atlanta and reported that SO 4 2 and NH 4 + were the main species. Organic species can also be involved in NPF events and enhance the nucleation of H 2 SO 4 [Zhang et al., 2004]. Combining the growth process of the NPF events with the chemical composition distributions of SO 4 2, NH 4 +, and oxalate, it can be inferred that SO 4 2,NH 4 +, and oxalate might be important compounds of the newly formed particles. H 2 SO 4,NH 3, and oxalic acid might be important precursors for NPF and they might play a significant role in nucleation and growth of the very small particles in Beijing Comparison of Size Distributions Between Urban and Regional Sites [54] From 12 August to 8 September 2006, particle number size distributions from 3 nm to 10 mm were also simultaneously measured by a TDMPS-APS system at Yufa, a regional site about 50 km south of PKU. [55] Average particle number size distributions at PKU and Yufa during CAREBeijing-2006 were similar, as shown in Figure 10. Average number and volume concentrations of 10 of 13

11 Figure 10. Average particle number size distributions at PKU and Yufa in the summer of nucleation, Aitken, accumulation, coarse mode and total particles at PKU and Yufa are shown in Figure 11. These number and volume concentrations at both sites were also similar. Average N-Acc, N-Total, V-Acc, and V-Total were 2%, 6%, 15%, and 12% lower at Yufa, respectively. These results indicate that particulate pollution in the summer of Beijing is a regional phenomena and less a problem of local emission during pollution. The lower concentrations at Yufa than at PKU mean that the regional pollution dominated but pollution was also added within the city. [56] Pollution episodes were observed at both PKU and Yufa sites during CAREBeijing-2006, for instance the one from 15 to 20 August. The regional accumulation mode particles are a measure for the air quality since they can explain most the visibility and the total mass concentration. The increase rates of N-Acc and V-Acc at Yufa (0000 local time on 16 August to 0600 local time on 19 August) were calculated in the same way as done at PKU. They were 30 cm 3 h 1 and 0.45 mm 3 cm 3 h 1 on average, both smaller than those at PKU. The possible reasons might be that the percentages of SNA and water-soluble organic compounds (WSOC) were larger in particles at the urban site PKU than at Yufa [van Pinxteren et al., 2009], so particles grow more under the condition with high RH and temperature at PKU. When N-Acc versus time was compared for both sites, a background of about cm 3 was found for both sites; in the same way, a background of V-Acc at both sites was found to be about 30 mm 3 cm 3. These values can be seen as the regional background of the accumulation mode particles on polluted days in the summer of Beijing. From 16 to 19 August, the ratio of N-Acc/ N-Ait at Yufa was 1.4 ± 0.5, larger than that at PKU, 1.1 ± 0.3. This finding was mainly caused by the fact that N-Ait at Yufa was lower than that at PKU, because of the smaller vehicle density. The regional background of accumulation mode particles is the same at both sites, but larger increase rates of N-Acc and V-Acc, and more Aitken mode particles lead to more serious fine particulate pollution at PKU than at Yufa on polluted days. [57] At Yufa, five NPF events with continuous smooth growth were observed. The corresponding parameters are given in Table 6. NPF events happened at both PKU and Yufa on 21 August and 4 September. On 4 September, they occurred at about 0900 local time at both sites. GR and FR at PKU were larger than those at Yufa by 50% and 220%, respectively. Q, C_vap, and CS were also larger than those at Yufa by 40%, 40%, and 60%, respectively. Parameters describing NPF events on 21 August were similar at both sites, occurring at about 0800 local time at PKU and about 0900 local time at Yufa on that day. Northern wind prevailed from 0000 to about 1500 local time at both sites, and the air mass trajectories came from the north, indicating that the same clean air mass was found at PKU and Yufa. These two NPF events were regional. Chemical size distributions of SO 4 2,NH 4 +, and oxalate showed peaks in the size range of mm during this day. [58] When NPF events with smooth growth were observed at one site, there were also nucleation mode particle bursts for a short time at the other site, if no NPF events with smooth growth were identified. This indicates that NPF events are regional or semiregional in the summer of Beijing. 4. Conclusions [59] N-Total at PKU during CAREBeijing-2006 was (1.8 ± 0.8) 10 4 cm 3, lower than in the summers of 2004, (3.0 ± 1.0) 10 4 cm 3, and 2005, (2.3 ± 0.6) 10 4 cm 3. V-Total of 83.5 ± 57.9 mm 3 cm 3 was however comparable to 2004 and 2005, 84.6 ± 54.6 and 80.2 ± 52.6 mm 3 cm 3, respectively. On nonpolluted days, Aitken mode particles usually dominated N-Total with a ratio of 0.4 ± 0.1 for N- Acc/N-Ait. The maximum in number size distributions was at about 60 nm. On polluted days, the contribution of accumulation mode particles increased, shifting the maximum of the number size distribution to over 80 nm with N-Acc/N-Ait of 0.9 ± 0.2. [60] Pollution episodes with continuously increasing particle volume concentrations showed a periodicity of 4 7 days. High-speed wind from the north and precipitation ended these episodes. Stagnant meteorological conditions enhance the accumulation of particle mass, particles grow and age, and their concentrations reach high values. Secondary transformation dominated the particulate pollution on polluted days. SNA was the major composition of the particles. Seen from the size distribution aspect, accumulation mode particles were the main contributor to the Figure 11. Average number and volume concentrations of the size-resolved particles at PKU and Yufa in the summer of The bars are the standard deviations. 11 of 13

12 particulate pollution. SNA accounted for over 60% of accumulation mode particle mass in stagnant conditions. [61] NPF events occurred on 5 out of 32 days during CAREBeijing-2006 at PKU. GR ranged from 1.2 to 5.6 nm h 1 and FR from 1.1 to 22.4 cm 3 s 1, both within the ranges of the results obtained in the summers of 2004 and These new particles grow into Aitken and accumulation mode within a few hours. Sunny days with low RH and high-speed wind from the north (>4 m s 1 ) or after precipitation can lead to NPF events. SO 4 2, NH 4 +, and oxalate, with H 2 SO 4,NH 3, and oxalic acid as important precursors, might be important compounds of the newly formed particles. [62] Particle number size distributions were compared between the urban site PKU and the regional site Yufa. To assess the air quality, an intercomparison of the accumulation mode and total particles was done. The difference for N-Acc, V-Acc, N-Total, and V-Total was 2%, 15%, 6%, and 12% lower at Yufa, respectively. This means that the air pollution in Beijing is mainly a regional problem. The regional backgrounds for N-Acc and V-Acc on polluted days were cm 3 and 30 mm 3 cm 3, respectively. [63] At Yufa, five NPF events were also observed. NPF events occurred at both PKU and Yufa on 21 August and 4 September. These two events were regional NPF events. On other days, when NPF events with continuous smooth growth were observed at one site, there were also nucleation mode particle bursts at the other site. These findings indicate that NPF events are regional or semiregional in the summer of Beijing. [64] Acknowledgments. This work, as part of the Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing- 2006), is supported by the Beijing Council of Science and technology (HB ). It is also supported by the National Key Technologies R&D Program (2006BAI19B06), the National Natural Science Foundation of China ( ), and the Deutsche Forschungsgemeinschaft (DFG, WI 1449/9-1). 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