Observational study of aerosol hygroscopic growth factors over rural area near Beijing mega-city

Observational study of aerosol hygroscopic growth factors over rural area near Beijing mega-city X. L. Pan, P. Yan, J. Tang, J. Z. Ma, Z. F. Wang, and A. Gbaguidi Chinese Academy of Meteorological Science, China Meteorological Administration, Beijing, China Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Graduate University of Chinese Academy of Sciences, Beijing, China Received: 25 December 2008 – Accepted: 12 January 2009 – Published: 25 February 2009 Correspondence to: J. Tang (tangj@cams.cma.gov.cn) Published by Copernicus Publications on behalf of the European Geosciences Union.

Aerosol hygroscopic growth with increasing relative humidity (RH) may lead to dramatic changes in its mass concentration, size distribution and corresponding optical properties (scattering coefficient, single scattering albedo, asymmetry factor etc.), which could enhance the cooling effect of aerosols in the atmosphere by directly scattering more light radiation (Carrico et al., 1998;Kotchenruther and Hobbs, 1998;Carrico et al., 2000;Randles et al., 2004), or change cloud microphysical properties (Crumeyrolle et al., 2008) by serving as cloud condensation nuclei (CCN) (Houghton et al., 2001).To precisely evaluate the direct radiative forcing, light scattering coefficient (σ sp ) of aerosol particles and its dependency on relative humidity (RH), defined as f (RH)=σ sp (scanning RH)/σ sp (dry), have been investigated for decades through Integrating Nephelometers equipped with humidity control devices in clean background regions, marine boundary layers, rural and urban areas (Kotchenruther et al., 1999;Malm and Day, 2001;Malm et al., 2005;Carrico et al., 2003;Kim et al., 2006;Magi and Hobbs, 2003).Previous studies reported that different types of aerosol particles usually have distinct hygroscopic growth properties (Kim et al., 2006;Cruz and Pandis, 2000).Hand and Malm (2006) indicated that the scattering coefficients of (NH 4 ) 2 SO 4 and (NH 4 )HSO 4 aerosols could be enhanced by a factor of three when relative humidity is over 85%.Dust particles, dominant in coarse mode, are mostly insoluble with f (RH=80%) smaller than 1.1 (Li-Jones et al., 1998), but they could also be hygroscopic when coated by sulfate or other soluble inorganic aerosols during transportation (Shi et al., 2008;Perry et al., 2004).
During the period of Aerosol Characterization Experiment (ACE-Asia), dust particle's hygroscopic factor of f (RH=80%)=1.25 was observed in Ron Brown cruise (Carrico et al., 2003), and even as high as 2.0 in Korea (Kim et al., 2006).Combustion-emitted smoke organic matters and photochemically formed organic compounds, accounting for a significant proportion of particulate aerosols, could be hygroscopic or hydrophobic depending on the types of organics and their oxidation status in the atmosphere.
Studies (Carrico et al., 2005) conducted in the Yosemite National Park indicated an inverse relationship between the organic carbon mass fraction of PM 2.5 and the hygroscopicity of aerosols.Researches carried out by Malm et al. (2005) in the same location showed that f (85%<RH<90%) decreased from 2.0 to <1.2 as the ratio of organic carbon mass to (NH 4 ) 2 SO 4 increased from 0.57 to 11.15.Nevertheless, inorganic aerosols' hygroscopicity enhanced by organic acids were also observed in some studies (Choi and Chan, 2002;Cruz and Pandis, 2000).Therefore, following the great temporal and spatial variations of aerosols' hygroscopic growth properties, in situ and laboratory investigations are needed.
In the past two decades, the rapid economy growth triggered China's annual GDP reaching over 9%, growing number of megacities (population over 10 millions), and increasing number of private cars (http://www.stats.gov.cn/), and consequently, the coun-Introduction

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Full try become an important source of pollutants and aerosols production.Aerosols mass concentrations and their optical properties must have changed to some extent.In such condition, providing insights into aerosols optical properties and its dependence on relative humidity in some Chinese key regions seems to be an important scientific issue to better understand the relationships between aerosols and climate.Previously, studies of Yan et al. (2008) at Shangdianzi (SDZ), a baseline air pollution monitoring station of North China, showed that the f (RH=80%) was 1.16 in clean air conditions and 1.34 during pollution episodes in winter time.Kim et al. (2006) obtained higher value of 2.75 during the ACE-Asia period.Experiment carried out in the Pearl River Delta (PRD) region (Liu et al., 2007) also indicated the urban pollutants aerosols hygroscopic growth factor f (RH=80%) of about 2.04 for Guangzhou.The present analysis of the aerosol hygroscopic factors f (RH) over the rural area in near Beijing Mega-city is based on an observational experiment performed from 25th April to 15th May, 2006.The main purpose of this study is to examine the effect of aerosol chemical composition on the hygroscopic growth and its scattering properties.
The relationship between the aerosols hygroscopicity and the air mass back-trajectory pathway will be also discussed.

Site description and meteorology
Measurements of the hygroscopic properties of aerosol scattering coefficient and aerosol sampling were conducted at Xin'An weather operational station in Baodi county (referred simply as Xin'An site, 39 • 44 N, 117 • 17 E, altitude: 6 m) located in the rural areas of Jing-Jin-Tang region (Fig. 1).Baodi is a major agricultural district of northern China, where rice and corn are cultivated.The principal human economical activities are farming and tree growing.The site which is about 85 km, 70 km and 105 km far from Beijing, Tianjin and Tangshan respectively, seems to be strongly under megacities air Introduction

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Interactive Discussion pollution influence (seen Fig. 1).During the observation, meteorological condition was characterized by clear and sunshiny days, except for the floating dust event at approximately 1600LST (Local Standard Time) 30th April.Air temperature average of the experimental period was 16.1 • , and relative humidity about 52.9%.The surface wind direction at the Xin'An site was dominated by NW wind and E wind, the wind speed less than 5 m/s. .

Instrumentation and method
Aerosol scattering coefficient (σ sp ) was measured with M9003 integrating nephelometer at 525 nm wavelength with light integrating angle 10 • -170 • .The measurement range of this instrument is 0-2000 Mm −1 (1 Mm −1 =10 −6 m −1 ) with its lower detection limit of about 0.3 Mm −1 (lower than one tenth of Rayleigh scattering of air molecules).
The increase of scattering coefficients of aerosols as a function of relative humidity is represented by the ratio of aerosol scattering coefficient at conditioned relative humidity and that under the reference "dry" conditions (usually with RH less than 40%).This method was initially introduced by Covert et al. (1972).In measurement, relative humidity of the sampled air with reference nephelometer for "dry" measurement was controlled at RH below 40%, and the other one was operated at the RH scanning from 40% to 90% controlled with humidity control device (scanning humidifier).The structure of humidifier was similar with that used in NOAA/GMD (http://www.esrl.noaa.gov/gmd/aero/instrumentation/humid.html),It consisted in an internal water-vapor penetration membrane tube immerged in "deionized water bath" of the sheath pipe.The vapor was permeated to mix with sample air and the relative humidity of the air was modulated by controlling of the water temperature.The relative humidity and temperature of inflow air were measured with a build-in sensor in the nephelometer.Fig. 2 presents the structure of the humidifier.
The aerosol hygroscopic growth factor is determined by the formula: Obtained data are then fit into the following empirical equation.
During the observation period, the nephelometers were calibrated with the zero gas (dry filtered air) every 24 h, and with the standard calibration gas (R134a) every 7 days.The data were automatically recorded every 5 minutes.Additional test on humidity sensor and dry scattering measurements were performed.The comparison of humidity sensors of two nephelometers with HMP41/45 Visala humidity indicator showed in good agreement with the difference less than 3%.To correct the systematic signal bias of two nephelometers for aerosol scattering coefficient measurement, the normalized procedure reported by Day and Malm (2000) was conducted to make the ratio of scattering coefficients equal to one when RH was lower than 35%.

Aerosol sampling and chemical analysis
Aerosol particles were collected by using two sets of Anderson size-segregated impactors (KA2000), The cut-off size ranges are: >11 µm, 11-7 µm, 7-4.7 µm, 4.7-3.3µm, 3.3-2.1 µm, 2.1-1.1 µm, 1.1-0.65 µm, 0.65-0.43µm, and <0.43 µm, respectively.Teflon membranes were used as impaction substrates for the top eight stages, and Zeflour membranes (Pall Corporation) were for the backup filter (the bottom stage).Organic carbon matters were sampled with Quartz membranes (Whatman QMA).The mass flow controller was used to maintain a stable flow rate at 28.3 L/min during the sampling period, and the flow rate check was performed with a dry gas rotemeter before and after each sampling cycle.The sampling generally started at 08:30 LST, and sampling intervals were more than 22 h.The inlet height was 2.5 m above ground level.The membranes containing aerosol samples were refrigerated at a temperature below −4 Aerosol mass concentrations were determined by means of weighting filters with Satorius microbalance (precision: 10 µg) before and after sample collection in the glove box.The filters were balanced in the glove box at a stable relative humidity (40±2%) and temperature (20±1 • ) environment for about 72 h prior to weighting process.Each membrane was weighted three times in 2 days according to the standard procedure, and the difference of gross mass of the filter weighted each time was less than 50 µg.
Inorganic anions (F 4 ) was analyzed with Dionex 500 ion chromatography (IC), and cations (NH ) with HITACHI 180-70 flame atomic absorption spectrophotometer (FAAS) at the Key Laboratory of Atmospheric Chemistry, China Meteorological Administration (CMA).Organic carbon (OC) was analyzed using thermal/optical carbon analyzer (Sunset Lab) at the Center of Environment, Peking University, and the precision and detection limits (LDC) are 10% and 2 µg cm −2 , respectively.The Quartz filters was pre-treated at temperature of 600 • for 4 hours before sampling, and the sampled filters were stored in a refrigerator at −20 • at the site before there were sent to the laboratory for analysis.

Aerosol mass concentrations and chemical characteristics
During spring over Northern China, aerosol chemical properties are rather complex because of the mixture of different sources including transportation of heavy air pollution from urbanization and industrial activities, desert dust particles from frequent dust events, and local emitted plant organic compounds during vegetal "greening".Statistic summary of aerosol mass concentrations, water soluble inorganic ion species, organic carbon are illustrated in Table 1.The average concentrations of fine particles (PM 2.1 , particle matter which pass through a size selective inlet with a 50% efficiency cut-off at 2.  ) µg•m −3 in PM 2.5 was also observed (Duan et al., 2004) in the background site of Beijing surrounding areas, and the high OC concentration was mostly in connection with urban residential activities (Yang et al., 2005) and biomass burning (Duan et al., 2004).In PM 2.1 category, mass concentration of soluble SO All the f (RH) displayed a similar appearance in the general shape of the hygroscopic growth (monotonic increasing) in spite of significant scattering of data points and wide range of deliquescent RH, and temporal variation of f (RH=80%) (Fig. 3).In the plot, the high variations should be closely related to the unique dominant aerosols types at Xin'An site.We therefore categorized aerosols into four different groups according to air mass concentration and chemical compositions (Table 2).Also, 48-h air mass back-trajectories were used by means of the Hybrid Single-particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Hess, 1998).Meteorological fields with 6-hour time interval (formatted as FNL) used in the back-trajectory analysis were obtained from US National Centers for Environmental Prediction (NECP).The back trajectory starting geographical position was located at Xin'An site with starting altitude of 100 m.The dissociation of aerosols concentrations periodicity can be described as follows: 1 3. Consider the periods where particles (PM 2.1 ) are over 50 µg•m −3 as influenced by urban pollution, because the site is located in the center of three pollution source areas, Beijing, Tianjin and Tangshan, urban emitted anthropogenic pollutants might be easily transported to the observation site with sea-land breezes and northwest wind.As shown on Fig. 3, on 11th and 15th May, aerosol hygroscopic growth factors f (RH=80%) were apparently higher than that of other periods, we defined this phase as "mixed pollution type" discussed in Sect.3.2.5.Dissociated other periods were considered as "urban pollution" featured by relative high mean mass concentrations of SO 2− 4 , NO − 3 ions about 13.1 µg•m −3 and 7.2 µg•m −3 respectively.Back-trajectories analysis showed that air mass mostly come from pollutant areas of southern regions such as Tianjin, Shijiazhuang, and eastern Tangshan towards Xin'An site.Detailed analysis of the periods allows quantifying the variation of aerosols hygroscopic factors.

Clean period
During the clean days, aerosol hygroscopic growth features, f (RH=80%) showed large temporal variations (Fig. 3) with averaged measured of 1.31, corresponding to about 10% higher than (f (RH=80%)=1.16)indicated by Yan et al. (2008) Carrico et al. (2000) in the clean air conditions in Sagres (Portugal, ACE-2) respectively.The values above were derived from the fitting equations provided by authors.In Bondville site, local high sulfate burden and SO 2 concentration, reported by Charlson et al. (1991) and Harris and Kahl (1994) might be the possible contributions to relatively higher f (RH=80%), while Studies of Yan et al. were performed in the baseline air monitoring station of Northern China, where geographical environment within 30 km radius distance was characterized by rolling hills farmland, orchard and forests with lower pollutants concentrations.Besides, the differences of dominant aerosol composition in winter and in spring might also be the poisssible reason for this disparity.Note that the hygroscopic growth factor f (RH) depends on size range of measured inlet aerosol particles and wavelength of nephelometer optics.For the present study, we measured scattering coefficient and its hygroscopicity of total suspended aerosols, whereas the above authors obtained values of f (RH=80%) were determined under aerosol diameter less than 10µm condition.These factors may be an explanation of such discrepancy in the results.

Dust episode
Measured hygroscopic growth factor f (RH=80%) was 1.20 in dust episode.This value seemed to be close to the results obtained by Carrico et al. (2003), of which aerosol hygroscopic factors f (RH=82.5%)were 1.18 and 1.39 for particles size less than 10 µm and 1 µm respectively in East Asia (ACE-Asia) during dust dominant period.Observational experiment performed by Kim et al. (2006) in Kosan (South Korea) indicated aerosol f (RH=85%)=1.73-2.20 during dust episode.These differences might be resulted from the mixing effect of dust particles with other inorganic materials during transportation.This is supported by many previous studies on dust particles over marine (Yamato and Tanaka, 1994;Zhang et al., 2003) or urban areas (Wang et al., 2007;Zhang and Iwasaka, 1999) of which the authors demonstrated that the mixing 5097 Introduction

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Full of sulfate, nitrate, and coarse calcium in dust particles could substantially enhance the hygroscopicity.Coarse calcium dust particles with nitrate-containing could appears in aqueous phase even at 15% RH (Shi et al., 2008).

Urban pollution period
Under the urban anthropogenic pollutants dominance condition, humidograms showed little evidence of deliquescence but monotonic changes in scatting coefficient with increasing control RH (Fig. 4).Measured aerosol hygroscopic growth factor f (RH=80%) is 1.57, about 6% greater than the results (f (RH=80%)=1.48) for pollution episodes obtained by Yan et al. (2008) in the urban site of Beijing city.Mean mass concentration of PM 2.1 is 87.9 µg•m −3 two times higher than the value of "clean" periods, while averaged mass concentrations of SO 2− 4 , NO − 3 , NH + 4 ions in PM 2.1 are respectively 14.51 µg•m −3 , 8.06 µg•m −3 and 5.02 µg•m −3 , obviously higher than those obtained in clean periods (4.63 µg•m −3 , 1.78 µg•m −3 and 1.57 µg•m −3 ).Kim et al. (2006) reported approximately same f (RH=80%)=1.55-1.77for pollution (from Korea industrial area) episode in Gosan regional background site.ACE-2, experiments conducted by Carrico et al. (2000) in Sagres showed that f (RH=80%) value of 1.40 for anthropogenic pollutant (PM 10 ) from Europe, about 11% lower than measured results in the present study.This might be related to the differences of physico-chemical properties of regional scale aerosols (Allen et al., 1999).Many other studies (Table 3) indicated more hygroscopic properties of urban aerosols, 2.04±0.28 in PRD region (Liu et al., 2007), 1.7-2.0 in Yangtze delta region (Xu et al., 2002) and 1.81±0.37∼2.30±0.24 in the flight over east coast of United States (Kotchenruther et al., 1999).Among these studies, Liu et al. (2007) determined the urban pollution periods in terms of air mass back trajectories and Kotchenruther et al. (1999) classified the periods by airflow patterns, and they did not provide specific information of aerosol mass concentrations.It seems therefore difficult to provide wider insights into such results gap.Introduction

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Mixed Pollution episode
Under significant influence of urban pollution, we distinguished two main episodes (11th and 15th May respectively) of which aerosols hydrophilicity and its effects on scattering coefficient were relatively higher.Measured hygroscopic growth factor f (RH=80%) reached 2.33-2.48,close to the value of mixed aerosols (marine and urban aerosols) in the PRD region (Liu et al., 2007).Analysis of water-soluble inorganic ions highlighted drastic increment of Na + mass concentration in PM 2.1 from 11th to 15th May with mean value of about 3.78 µg•m −3 (standard deviation of 1.21), higher than that (averagely 1.10 µg•m −3 ) of unmixed pollution periods, suggesting evidence of marine aerosols impact (Chan et al., 1997;McInnes et al., 1996).Daily average mass concentration of SO 2− 4 at 11 and 15 May are 4.32 µg•m −3 and 9.26 µg•m −3 respectively.Analysis of back trajectory showed air masses direction oriented from Tianjin-Tangshan (Fig. 6).In addition, organic matters proportions in PM 2.1 increased of about 42.3% and 43.0% on 11th and 15th May respectively, 50% and 61% higher than mean values of unmixed urban pollution periods.Biomass burning may be the most probable source of suspended organic particles in the atmosphere (Duan et al., 2004), however, we do not find any signs of fire events around the Xin'An site from MODIS active fire data (provided by the MODIS Rapid Response System http://maps.geog.umd.edu/firms/)during that time.Then, the two episodes might be influenced by a complex "mixed aerosols" occurrence of involving urban pollution, marine and large portion of organic matters.

Aerosol hygroscopic growth function
Aerosols hygroscopic growth trends in dust, clean, urban pollution and mixed periods are shown in Fig. 4. Large variations of aerosols water absorbing ability appear among different dominant aerosols types.Aerosols hygroscopic growth functions are obtained from Eq. ( 1), and used for monotonic growth illustration (Carrico et al., 2003;Kotchenruther and Hobbs, 1998;Kotchenruther et al., 1999;Liu et al., 2007;Magi and Hobbs, 2003).The parameters of (a) and (b) are described in Table 4.Both (a) and (b) seem to Introduction

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Interactive Discussion be undergone a sharply increase from dust particles (a=0.64,b=5.17) to mixed aerosol types (a=7.68,b=7.62).

Influence of organic matters on aerosol hygroscopicity
Aerosol mass concentrations and proportions of SO 2− 4 , NO − 3 , NH + 4 , and organic carbon in PM 2.1 are summarized in Table 5.To better characterize the hygroscopic properties of aerosols, the ratio of organic carbon matters (OMC)/ammonium sulfate (AS) (Malm et al., 2005) (Malm and Day, 2001).Figure 5 describes the relationship between OMC/AS and f (RH=80%).Except the period of "Mixed pollution", the OMC/AS seems to be inversely correlated to f (RH=80%): an increase of the ratio increases (from 1.58 to 9.87) is associated with a decrease of f (RH=80%) .The proportions of SO 2− 4 , NO − 3 and NH + 4 in PM 2.1 also display same systematically trends (Fig. 5).This indicates that water soluble inorganic ions are key factors directly correlated to aerosols water absorbing abilities as reported by (Malm et al., 2005;Virkkula et al., 1999).Organic matters serve therefore as hydrophobic compounds in aerosol particles chemical structure.However, different trends are obtained in mixed pollution periods.Proportions of OC in PM 2.1 are 40.0%,43.0% for 11th and 15th May respectively, as much as two time of mean value (26.7%) of unmixed urban pollution periods (3rd , 4th, 12th May), while the aerosol hygroscopic growth factor increases by 46.0% associated with averaged f (RH=80%)=2.22.The existence of water-soluble organic compounds in the aerosols seems to be strongly probable in such condition as suggested in many previous studies (Cruz and Pandis, 2000;Choi and Chan, 2002).other pollution episodes, air masses are mostly from eastern China.Under the influence of microclimate and land vegetable coverage, the diversification of organic aerosol types from east (red dot shaded region in Fig. 6) and central part (blue dot shaded region in Fig. 6) of China might be an explanation of the discrepancies detected in hygroscopic properties.Furthermore, since aerosols physico-chemical properties and mixing type may be very likely undergone changes during transportation, meteorological parameters (solar radiation and mixing height) are specifically analyzed in pollution periods.Figure 7a shows the temporal and spatial variation of mixing height along the air masses pathways.The mixing height is about 3000 m at 14:00 LST on 15th May, and 1720 m on 11th May.This indicates strong vertical convection conditions in the boundary layer, favorable to mixing and chemical processes of local emitted and transported aerosols.The variations of solar radiation, susceptible to influence organic carbon photochemical reactions, are illustrated in the Fig. 7b.Three mostly strong solar radiation values: 871.5 Wm −2 , 812.1 Wm −2 and 655.7 Wm −2 are detected respectively on 15th, 3rd, and 11th May.Such condition may trigger the surface oxidation process of organic matters active function groups as well as corresponding water-absorbing ability (Chughtai et al., 1999;Charlson et al., 1992).Note that the f (RH=80%) on 3rd May is lower less than that of 11th and 15th May.This is probably related to the various sources of organic aerosols and the discrepancy in proportions of OC in PM 2.1 (daily average of 29.8%, 40.0% and 43.0% on 3rd, 11th and 15th May).However, further information on organic compounds still needed to insights into schematics of hygroscopicity and its influence on aerosols optical properties.

Conclusions
Aerosols hygroscopic growth experimental study over rural area (Jing-Jin-Tang) near Beijing mega-city through analysis of key meteorological parameters and aerosols chemical composition, results in smooth and monotonic growth features associated with increasing RH in most of observation periods.Experimental data allow catego-Introduction

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(3) During urban pollutant phase, measured aerosol hygroscopic growth factor f (RH=80%) is 1.57 , lower than values of 2.04±0.28 in southern coast area of China, 1.7-2.0 in Yangtze delta region and 1.81±0.37-2.30±0.24 in East coast of United States.(4) Marine and urban pollutant mixing aerosols present greater water soluble ability with averaged f (RH=85%) of 2.40, slightly higher than the value (f (RH=80%)=2.04) reported in marine dominant air mass period (Carrico et al., 2003), and that (f (RH=80%)=2.29±0.28) in PRD coastal region during the urbanmarine mixed period (Liu et al., 2007).Analysis of back trajectory, solar radiation and mixing height along the air masses pathways, indicates that organic aerosols types, sources and the mixing process during the transportation play key roles in determining particles hygroscopicity.The higher values of solar radiation also constitute an important favorable factor of water-soluble organic matters formation.Through present experimental study, organic matters seem to importantly contribute to uncertainties of aerosols hygroscopic properties as well as the variations of hygroscopic growth.Facing this complexity, further experimental long-term study associated with specific modeling analysis on the relationship between organic compounds and aerosols hygroscopicity will be done in near future to thoroughly understand such physico-chemical process.
1 aerodynamic diameter ) and coarse particles (PM 11 ) were 81.01 µg•m −3 and 214.32 µg•m −3 with standard deviations of 32.76 µg•m −3 and 125.34 µg•m −3 , respec- 11.5(7.6)µg•m −3 , followed by NO − 3 and NH + 4 ions with value of 6.05(4.52)µg•m −3 and 4.02(3.17)µg•m −3 .This indicated anthropogenic urban pollution influences during the observation period.In the PM 11 , soluble Ca 2+ ion makes up the second largest proportion of aerosol masses, which might be related to flowing dirt during the springtime generally featured by low ambient relative humidity and strong wind . Detect periods influenced by pronounced dust aerosols in accordance with observational records and 3 h meteorological information (Provided by China Meteorological Administration, CMA).The floating dust episode occurred in the afternoon of 30 April (approximately 17:00 LST) while the local visibility was less than 1 km.While mass concentration of coarse particles (PM 11 ) reached 693.25 µg•m −3 , the mass concentration of fine particles (PM 2.1 ) was about 128.22 µg•m −3 , accounting for only 18.5% of the total mass concentration.This is supported by observation performed by Wang et al. (2007) of which the mass concentration of TSP was about 419-879 µg•m −3 , with PM 2.5 of 156-286 µg•m −3 during the dust storm period in Beijing, 2004.Air mass 48 h back-trajectories also indicated that strong wind from northwestern China was oriented towards Mongolian Plateau and Hunshandake desert in north of Beijing.the periods where mass concentration of fine particles (PM 2.1 ) are less than 50 µg•m −3 as "clean periods", and use this value as delimitation of clean-topollution on the ground where the value is approximately equivalent to Air Pollution Index (http://www.mep.gov.cn/) of about 50 (indication of "good" air quality in China).On 5th, 10th and 13th and 14th May, the daily average mass concentration of PM 2.1 were 39.2 µg•m −3 , 42.0 µg•m −3 , 42.7 µg•m −3 and 49.8 µg•m −3 , those of PM 11 were 141.1 µg•m −3 , 144.8 µg•m −3 , 157.7 µg•m −3 and 145.6 µg•m −3 respectively, generally lower than the National Ambient Air Quality Secondary Standard of China (daily average 150 µg•m −3 ).

Figure 6
displays 48h Back-trajectory of air masses during pollution episodes (3rd, 4th, 11th, 12th and 15th May).It seems obvious that air masses on 11th May from remote areas of central Inner Mongolia province transiting over Tangshan and Tianjin as well as air masses on 15th May from Shanxi province, transiting over Taiyuan, Zhengzhou and Tianjin, may govern pollutants transport towards study site.During 5100 in four different cases (clean, dust, urban pollution and mixed pollution) in accordance with aerosols mass concentrations and chemical compositions variations.

Fig. 5
Fig. 5 Relationship between OMC/AS and f(RH=80%).With increasing ratio of OMC/AS, aerosol's hy decline to some extent, except for the mixing pollution periods (show in blue dot).

Fig. 5 .
Fig. 5. Relationship between OMC/AS and f (RH=80%).With increasing ratio of OMC/AS, aerosol's hygroscopicity decline to some extent, except for the mixing pollution periods (show in blue dot).
The concentration of PM 11 exceeded the National Ambient Air Quality Secondary Standard of China (daily average 150 µg•m −3 ), and the concentration of PM 2.1 is even over 2 times as high as the limit value of Primary Standard (daily average 35 µg•m−3 ) of US Environmental Protection Agency.The maximum of particle mass concentration took place in the dust episode (on 30th April, according to the meteorological records) with the daily average PM 11 concentration of 693.3 µg•m −3 and PM 2.1 of 128.2 µg•m −3 .Aerosol mass concentrations are more similar with the experiments carried out in the suburban area of Beijing (Sun et al., 2004; Wang et al., 2005).Na + , K + , Cl − ) and OC are listed in the Table 1.For the whole observation period, organic carbon accounts for the largest percentage of aerosol masses both in the PM 2.1 and PM 11 with mean values (standard deviation) of 29.84 (9.02) µg•m −3 and 61.44 He (2001)g•m −3 .Many previous observation studies showed an approximately same OC concentration level at the near study site (Jing-Jin-Tang).For instance,He (2001)reported that organic carbon was most abundant species of the total PM2.5 mass (annual mean value of 29.1 µg•m −3 ) in the Beijing urban site, ranging from 17.9% to 25.7%.OC annual average of 20.04(12.07 for an other rural site (100 km northeast far from Xin'An site) of north Beijing in winter, and about 11% 5096 Introduction is estimated, ammonium sulfate and organic carbon matters are derived from the equations: AS=0.944[NH

Table 1 .
Statistical summary of 24 h average aerosol species concentrations.

Table 3 .
Other observation results of hygroscopic growth factor of different types of aerosols.

Table 4 .
Curve-fitting parameters in different aerosol dominant episodes in terms of Eq. (2).

Table 5 .
Relationship between the hygroscopic growth factor with chemical composition in PM 2.1 .