Chemical characterization of aerosols at the summit of Mountain Tai in the middle of central east China

Chemical characterization of aerosols at the summit of Mountain Tai in the middle of central east China C. Deng, G. Zhuang, K. Huang, J. Li, R. Zhang, Q. Wang, Y. Sun, Z. Guo, and Z. Wang The Center for Atmospheric Chemistry Study, Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China Department of Environmental Toxicology, University of California-Davis, Davis, California 95616, USA The Institute of Atmospheric Physics, Chinese Academy of Science, NZC/LAPC, Beijing 100029, China


Introduction
Aerosols have potential impact on the global atmospheric chemistry, cloud properties and precipitation development (Tegen et al., 1996;Arimoto, 2001;Rastogi and Sarin, 2005a;Rastogi and Sarin, 2006).Anthropogenic aerosols, including the primarily emissions and the secondary aerosols, are mainly in fine mode, which has much more adverse impact on climate and hydrologic cycling (Kaufman et al., 2002), visibility (Chan et al., 1999), and human health (Dockery et al., 1993).3×10 11 RMB in 2008 (http://finance.people.com.cn/), which resulted in the increasing emissions of SO 2 , NO 2 , and particulate matter, and, in turn, the severe acidic precipitation (Wang et al., 2008).Mountain Tai (200×50 square kilometers) with the highest altitude (1534 m high) in central-eastern China is located in Shandong and surrounded by Jiangsu, Anhui, Henan, and Hebei provinces.Aerosols, from the summit of MT could be the representative of the regional pollution.Previous study found that CO and O 3 at the top of MT exhibited summer high and winter low, which was attributed to the seasonal changes of meteorological conditions and seasonal variations of sources (Gao et al., 2005;Wang et al., 2001a).In addition, VOCs, O 3 , and CO were all higher than those observed at other rural mountainous sites (Suthawaree et al., 2010), which might be due to the strong sources surrounded.Average concentration of peroxides at MT was much lower than the measurements made at some rural mountain sites, suggesting that significant Introduction

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Full light and they are ideal tracers for anthropogenic pollutions (Novelli et al., 1994(Novelli et al., , 1998)).VOC, O 3 , CO, and peroxide are all related to the formations of secondary aerosols in ambient air, which suggested that characteristics of the aerosols at MT might be different from those at other sites due to the higher levels at MT. Mineral aerosols through long-range transport directly and indirectly affects on many properties of air mass by providing surfaces for many chemical and physical processes and serving as carriers of anthropogenic substances, which would affect on the global biogeochemical cycle and the global climate/environmental change (Guo et al., 2004;Dentener et al., 1996;Sun et al., 2004;Liu et al., 2002).Northwestern China is one of the main source areas of Asian dust, which can be transported to hundred and thousand miles away, passing through central and eastern China and even to the Pacific.The composition of mineral aerosols would subject to transform due to adsorbing gaseous species, surface reactions, and coagulation with anthropogenic aerosol on the pathway during transport.However, previous studies on MT have been seen only on trace gases, and little on aerosols.Also, the early study on aerosols in literatures mostly focused on the samples from ground level, and limited knowledge has been acquired about the aerosols at high elevation over the world.MT is just in the downwind of outflow Asian Dust from northwestern China to the northwest Pacific.Therefore, the summit of MT is an ideal site to examine the long-range transport of Asian dust from northwestern China and to observe the mixing of dust with anthropogenic aerosol.The experiments focused on aerosol chemistry in this region were carried out in order to understand the air quality at atmospheric boundary layer over central east China.This paper characterizes the aerosols and their composition at MT, and further analyses their sources and formation processes and the relation with the long-range transport.Introduction

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Sampling
TSP and PM 2.5 aerosol samples were simultaneously collected at the meteorological observation station located at the summit of MT (36.25 • N, 117.10 ).All of these filters were weighed before and after sampling with an analytical balance (Sartorius 2004MP, reading precision 10 µg) after stabilizing under constant temperature (20 • C) and humidity (40%) for over 24 h.The samples were put in polyethylene plastic bags right after sampling and reserved in a refrigerator.All the procedures were strictly quality-controlled to avoid any possible contamination of the samples.For comparison, aerosol samples were also collected at Tazhong, Urumqi, Tianchi in Xinjiang in northwestern China, Duolun and Yulin in northern China, and two urban sites in the megacities, Beijing and Shanghai, in spring 2007 (Fig. 1).The detailed analytical procedures were given elsewhere (Zhuang et al., 2001).

Ion analysis
One-fourth of each sample and blank filter was extracted ultrasonically by 10mL deionized water (18 MΩcm −1 ).After passing through microporous membranes (pore size, 0.45 µm; diameter, 25 mm; made by the affiliated plant of Beijing chemical school), the filtrates were determined for pH with a pH meter (model, Orion 818).Each filtrate was stored at 4 • C in a clean tube for IC analysis.12 anions (SO Introduction

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Element analysis
Half of each sample filter and blank filter was digested at 170 • C for 4 h in high-pressure Teflon digestion vessel with 3 mL concentrated HNO 3 , 1mL concentrated HCl, and 1mL concentrated HF.After cooling, the solutions were dried, and then added 0.1 mL concentrated HNO 3 , and diluted to 10 mL with deionized water (resistivity of 18 MΩcm −1 ).

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Full  with a heated catalytic scrubber for baseline determination, which was conducted every 2 h.The detection limit was 30 ppbv for a 2-min average, with a 2-s precision of about 1% for a level of 500 ppbv (2-min average).The overall uncertainty was estimated to be 10%.

Results and discussion
3.1 Overview of particle matters at summit of MT

Mass concentrations of PM and size distribution
Temporal variations of mass concentrations and the corresponding deviations of PM 2.5 and TSP at MT and other sampling sites are summarized in Fig. 2 and Tables 1-2 Sect.3.2.1).TSP from MT was comparable with those at other sampling sites, and there was no obvious decreasing trend at a height of 1534 m as being expected.It was seen that aerosols at MT was higher than those in Shanghai but lower than those in Beijing during the study periods.Both TSP and PM 2.5 at MT were much higher than those from Tianchi, which is also located at a high elevation (1900 m).Aerosol pollutions at MT were as severe as those at ground level or even worse, e.g.PM 2.5 at MT were much higher than those at other sites at ground level in summer, such as megacities, Beijing and Shanghai.The average ratio of PM 2.5 /TSP was 0.37 in spring and 0.91 in summer in 2006, indicating that fine particles dominated in summer while coarse particles in spring.The seasonal variation of fine particles should be firstly attributed to the seasonally different meteorology at MT (Table 3), which could have different impact on the air mass in boundary layer.Meteorological conditions at MT show typical seasonal variations with higher temperature, lower windy speed, lower atmospheric pressure and more solar radiation in summer than those in spring, all of which could strengthen the vertical convention of the atmosphere, so regional anthropogenic pollutants on the ground surface could easily transport upward, resulting in the increase of the height of the planetary boundary layer (PBL), even higher than the summit of MT.On the contrary, the height of the PBL layer in spring was so compressed that it used to be below the summit of MT, and the Mountain-valley breezes could not fully develop due to the weak solar radiation and the strong regional winds in ground level.Hence, the uptake of the regional pollutants to the summit was less frequent in spring than in summer, which resulted in lower concentrations of fine particles in spring.
Secondly, dust storms mostly occur in spring and MT is located on the very pathway of the crustal matters from northwestern China to the northern America of the long-range transport (Arimoto et al., 1996;Zhang et al., 1997;Sun et al., 2006).Frequently windy days of high speed in spring were accompanied with high concentrations of coarse particles, even at elevation of more than 1500 m high.Introduction

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Ionic and elemental composition of the particles at MT
Mass concentrations of ions in PM 2.5 and TSP at MT are listed in Table 4. Water soluble ions contributed 10.82% of TSP and 23.99% of PM 2.5 in mass concentration in spring, while 40.35% and 41.86% in summer, which revealed evidently that the secondary aerosol possessed much larger part in summer than in spring.The ratios of total ions to the total concentrations in PM 2.5 were higher than those in TSP both in summer and spring, suggesting that the pollution components prefer to be in fine mode.SO 2− 4 , NH + 4 , NO − 3 were the major water soluble ions in both spring and summer, accounting for 61.5% in spring and 73.8% in summer of the measured ions in PM 2.5 , while 69.2% and 70.8% in TSP, respectively (Table 5).Also note that higher concentration of K + , the tracer of the biomass source, in summer and Ca 2+ , likely more from the Asian dust source, in spring was observed, accounting for 8.4% and 22.0% of total ions in TSP, respectively.
Aerosols would exert many impacts on atmospheric processes, such as cloud formation, visibility, solar radiation, and play a major role in acidification of cloud, rain, fog, and even the formation of haze, and water-soluble ions were proved to play key roles in these processes because of their affinity with water (Tsai et al., 1999;Novakov et al., 1993;Matsumoto et al., 1997;Facchini et al., 2000).Haze days were likely to occur in stable, warm and moist air, and were characterized by the high levels of fine particles and high percentage of certain water soluble ions in the aerosols, especially for SO 2− 4 , NO − 3 , NH + 4 .The chemical composition of aerosols at MT and Beijing in dust, haze and clear days are summarized in Table 5.The results showed that C IC /C P , C (S+N+A) /C IC , SOR and NOR at MT in summer were very similar to those in haze days in Beijing, suggesting that haze could occur frequently in summer at the summit of MT, which could be proved by the lower visibility (9.0 km) on average at MT.
Nineteen elements and black carbon (BC) in aerosols at MT are listed in Table 6, which showed clearly that crustal element (Ca, Ma, Al, Mn, Ti, Sr, and Na) were higher in spring, while pollution elements (Pb, Cr, Cd, Zn, Ni, S, BC), except for As and Cu, Introduction

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Full were higher in summer.The elements could be classified into four groups according to their enrichment factors (EFs = (X/Al) aerosol /(X/Al) crust ) (Fig. 3): high enriched pollution (Pb and As), medium enriched pollution (S and Zn), slightly enriched pollution (Ni, Cu and Cr) and non-enriched crustal metals (Ca, Ma, Al, Mn, Ti, Sr and Na).EFs of all crustal elements were higher in TSP and lower in PM 2.5 , while pollution elements were higher in PM 2.5 and lower in TSP, respectively.EFs of As and Pb exhibited different seasonal variations with higher EF of Pb in summer and higher EF in spring of As.

Acidity and alkalinity of the aerosols at MT
Figure 4 shows the variations of pH of the filtrates of aerosols at MT, which revealed that pH decreased obviously from spring to summer.Mean pH values of the aqueous filtrates of aerosols at MT and other sampling sites are summed in Table 7.The aerosols at MT showed higher acidity for both PM 2.5 (pH=4.62)and TSP (pH=4.92) in summer in comparison to the weak acidity of PM 2.5 (pH=5.92)and slight alkalinity (pH=7.22) of TSP in spring.The pH of aerosols at MT in summer was the lowest among those sites at other locations, while TSP in spring showed slight alkalinity, which was similar to those from Tazhong (pH=7.39,original source of dust storm) and supper dust day in Beijing (pH=7.25,Wang et al., 2005).The higher acidity of aerosols at MT in summer was further supported by the fact that pH of rain samples in summer at MT was obviously lower than those in other seasons (Wang et al., 2006).The acidity or alkalinity of aerosols is basically restricted by the presence and the proportions of the cations and anions in aerosols.If all the cations and anions in the aerosol were measured, the ratio of equivalence concentrations of C/A (total cations/total anions) should be equal to 1.However, CO organic acids and the decrease of Ca 2+ , Mg 2+ in aerosols at MT in summer revealed that in summer much more aerosols at MT from secondary pollution ions, which would lead to the lower pH of the filtrates of the aerosols.As a comparison, aerosols from Shanghai in summer were of less acidity and less seasonal variation (pH of 5.27 and 5.29 in PM 2.5 and 6.48 and 6.37 in TSP in spring and summer, respectively) than those at MT, which suggested that the air pollution at MT in summer was even severer than that in urban area of Shanghai.

Influence of biomass burning
Biomass burning includes grassland, forest, and crop residue, with more than 60% attributed to crop residue burning in the world (Streets et al., 2003b).China has a large rural population whose major energy source was used to be biofuel.However, rapid economic development has increased rural access to commercial energy, and the use of biofuel is decreasing (EBCASY, 1992(EBCASY, -2001)).As a result, crop residue increasingly is being burned openly in the field after harvest.∼5.182×10 7 ton crop residues (accounted for 40.0% of the total crop residues in China) were directly burnt openly in harvest every year, especially in central eastern China, including Shandong, Jiangsu, Henan, Hebei provinces, among those Shandong, where MT is located, was in number 1 with the largest amount of crop residues (1.798×10 7 t) to be burnt openly (Cao et al., 2007).Biomass burning emissions are known to have contributed a considerable amount of PM and gaseous pollutants into the atmosphere, and crop residues contribute more fine particles and gaseous than other biomaterial residues, when they are burnt (Cao et al., 2005;Zhu et al., 2005).K + (water soluble potassium) is a good tracer for aerosols from biomass burning (Andreae, 1983).Source of total potassium ion (K + Total ) in aerosols might be attributed to soot from biomass burning, sea salt and crustal dust.It was reasonably assumed then that total potassium (K + Total ) is the sum of dust-derived potassium (K + Crust ), sea-20985 Introduction

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Full ), in which the concept is similar to that of Virkkula et al. (2006).It can be presented as follows: K

Elemental Al and water soluble Na
+ is markers of crustal dust and sea salt.The following composition were assumed: Na: 31%, K:1.1%, Al:0%, and Na: 2.6%, K:2.9%, Al:7.7% in sea water and crustal dust, respectively (Wedepohl, 1995), which revealed that the ratios of K + were much lower than Na + in sea salt and Al in crustal dust.That is to say, composition of Na + and Al ought to be increase more than K + in the aerosols if the aerosols were attributed to sea salt and crustal dust.Temporal concentrations of K + , Na + and Al in MT (Fig. 5) showed that K + in summer was significant high, and had a much more increase than Na + and Al, which suggested that the high concentrations of PM 2.5 in summer was mainly attributed to biomass burning rather than sea water and crustal dust.K + /Na + ratios of particles from sea salt is constant (0.037) (Chester, 1990), however, ratios of K + /Al of crustal dust were not the same between samples from different sites due to the variable background value and the influence of the human activities.
Assumed that minimum ratio of K + /Al and Na + /Al of 0.152 and 0.24 for PM 2.5 in the sample collected from MT on 26 April 2006, or 0.107 and 0.031 measured in the soil collected in MT served as background value of K + from crustal dust, then K + BB is given: Where K Na + Total is the concentration of Na + determined in PM 2.5 (µg/m 3 ) and Al Aerosol is elemental Al determined in PM 2.5 (µg/m 3 ).Very close concentrations of K + BB were found by the two methods (Fig. 6), and the averaged K + BB were 0.399 and 4.299 µg/m 3 in spring and summer for the former method, while 0.325 and 4.302 µg/m 3 for the later method.
The high level of K + BB in summer suggested the significant biomass burning emissions, Introduction

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Full which could also be confirmed by the fire spot data for the region in Fig. 7a-f.Fire spots near the site began to increase in May, and highly active fire disturbance appeared obviously in June.During 1-9 June (e), the fire spots spread mainly to the south of the site, and then extended to the northern places during 10-19 June(f), which was in the harvest season of wheat and rice from south to north over this region.(Table 9), also indicated evidently that biomass burning was one of major contributor of the aerosols pollution in summer over central eastern China.sulfate was produced in the aqueous-phase, humidity plays a key role in the formation of sulfate from SO 2 (Jill et al., 2001).At the summit of MT, total cloud days were 5.4 and 7.0 in spring and summer, respectively, and monthly average foggy days could reach 26, sometimes even 30 in summer, while only 10 in spring.The relatively high humidity at the summit in summer is in favor of the formation of sulfate.Secondary, oxidation of SO 2 occurs via three potentially important pathways: oxidation by hydrogen peroxide, ozone, and auto-oxidation catalyzed by Fe (III) and Mn (II), and the former two were proved to be the dominative oxidation processes under certain conditions (Jill et al., 2001).Averaged concentration of total peroxide was significantly higher in summer (mean: 0.55 ± 0.67 ppbv) than in spring (mean: 0.17 ± 0.26 ppbv) (Ren et al., 2009).

Secondary components, SO
The O 3 in 2003 (Gao et al., 2005) and 2006 (Table 3) showed that O 3 was also higher in summer than that in spring.High levels of O 3 and peroxide in ambient air in summer accelerated the conversion of SO 2 and NO 2 to be SO 2− 4 and NO − 3 .Lastly, the high effective oxidation of SO 2 at MT, might be related to the more abundance of hydrocarbon (RH) emitted by the abundant foliage (covering more than 90% of the surface of MT), as well as biomass burning emissions (Suthawaree, et al., 2010).O 3 accelerated the formation of SO 2− 4 by directly reacting with SO 2 and generating OH radical that further to peroxide via cooperating with RH under light radiation.RH can react with OH radical to produce HO 2 and RO 2 (OH + RH (Ariel et al., 2000), and HO 2 further react with SO 2 and NO 2 .Higher CO level in summer also contributed to the oxidation of SO 2 by generating peroxide with OH radical −→ HO 2 + CO 2 ).The reactive processes above would benefit the formation of organic acids: HO 2 +RO 2 → ROOH+O 2 .The high concentrations of diacids and the good correlations of diacids with SO summer than in spring.Also, higher temperature in summer could lead to the higher emission of atmospheric NH 3 from animal's excrements (Sacoby et al., 2007).

Mineral dust in spring time
Concentrations of mineral elements (such as Ca, Al, and Fe) are good indicators for crustal aerosols.Figure 12 showed that how the temporal variations of these mineral elements impact on the ambient air quality around the sampling sites while the dust transported from west to east and spreading in central China in spring 2007.A strong dust storm occurred during 30 March-2 April and resulted in sharp increase of three mineral elements at all the monitoring sites, especially at Taizhong, Yulin, Duolun, MT, and Beijing.Ca/Al ratio had proved to be a good tracer for different dust origin areas (Wang et al., 2005;Sun et al., 2005), for Ca/Al showed remarkable dependence of the source regions for both dust aerosol and soil samples.Ratios of Ca/Al in this study and the reported data are shown in Table 10, suggested clearly, that western desert sources (Tazhong: 1.55 ± 0.22) are characterized by high Ca, while northern sources (Duolun: 0.45 ± 0.12) by low Ca, and the high Ca/Al (1.37 ± 0.22) at MT might be associate with an air mass originated from western or northwestern high-dust sources.
The back trajectories further identified that the high mineral elements at MT in spring were mostly from the long-range transport of dust from northwestern China and passed through over the central/eastern China (Fig. 13).The facts mentioned above that the coarse particles dominated in spring (PM 2.5 /TSP of 0.37) and the alkalinity (pH=7.22) of the TSP in spring further confirmed that the dust from long-range transport was the major source of the aerosols in spring at MT.

The pollution elements As and Pb
The of Pb, while spring-high/summer-low of As.Statistic analysis (Table 11) revealed that Pb and As were high correlated to crustal elements in spring with the correlation coefficients of 0.701 for Pb to Al and 0.873 for Pb to Fe, while of 0.837 for As to Al and 0.778 for As to Fe.Furthermore, the correlation coefficient between Pb and As was as high as 0.949.These results indicated that both Pb and As were highly associated with the mineral components, which was long-range transported from northwestern China to MT in spring.Original dust from northwestern China would mix with As and Pb emitted from coal mining, widely distributing over northwestern China, and act as a carrier for As and Pb, which would gradually be enriched in the dust aerosol during its long-range transport.However, in summer the correlation coefficients of Pb to Al and Pb to Fe decreased to −0.184 and −0.194, while of As to Al and As to Fe to 0.469 and 0.456, and the correlation coefficient between Pb and As decreased to 0.494.These results indicated that in summer the sources of Pb and As were not from long-range transport, instead, they could be from those local/regional sources.It could be seen that Pb and As were both correlated well to Cr, Cu, and Zn in summer, suggesting that the sources of Pb and As could likely be from the local/reginal anthropogenic discharge in summer, and the strong convections of air mass would result in the pollutants at ground levels elevated to the summit of MT.The back trajectories shown in Fig. 14 indicated that the wind directions in summer at MT were mainly from south and east of MT, the central eastern China, where the economy was most developed with more anthropogenic pollution, which further demonstrated the local/regional sources of Pb and As in summer at MT.The local/regional anthropogenic source of Pb in summer could be much more than that in spring mainly from the long-range transport, which led to the seasonal variation of Pb with higher in the summer than spring.Introduction

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Full suffering from the invasion of long-range transported dust from northwestern China, the heavy local/regional anthropogenic emissions from the surrounding areas, and the severe secondary pollutions.Primary emissions of particles from biomass burning revealed to be a significant main source, and the aged air mass further deteriorated the air quality of this region.The high levels of peroxides and ozone, as well as the favourable meteorological conditions in the ambient air were in favour of the transformation of SO 2 and NO x to sulfate and nitrate, which resulted in the even severe secondary pollution in this region.The high enrichment of As and Pb in the aerosols over MT indicated that the long-range transport of air mass spread the heavy pollution from coal burning everywhere over China.Introduction

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Full  Full  Full  Full    Full  Full  Full  Full  Full  Full             ). ).
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . TSP were relatively seasonal stable (128.1 µg/m 3 , 143.8 µg/m 3 in spring and 135.0 µg/m 3 in summer), while PM 2.5 has highly seasonal variation with much higher concentration in summer (123.1 µg/m 3 ) than spring (46.6 µg/m 3 and 70.1 µg/m 3 ) at MT(PM 2.5 was relatively high in spring 2007 (26 March-18 May) was likely attributed to the different sampling periods compared to 2006 (14 March-6 May), which would be interpreted in Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | measured in this study due to the limitation of ion chromatography, hence, the difference of the total anions and cations could be used to roughly estimate the amount of CO 2− 3 or HCO − 3 unmeasured.C/A of aerosols in spring at MT reached 1.6, while only 1.1 in summer, suggesting that much more carbonate or bicarbonate presented in aerosols in spring.The obvious increase of SO Introduction Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | salt-derived potassium (K + SS ), and biosmoke potassium (K + BB Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | aerosols and their seasonal variations at different sites (MT, Urumqi, Beijing, and Shanghai) are summarized in Fig. 10.The results showed that these secondary ions in aerosols in summer at MT were more than five times of those in spring, while at Urumqi they were evidently greater in spring those that in summer, and in Shanghai and Beijing they were no big change between spring and summer.Discussion Paper | Discussion Paper | Discussion Paper | Fig. 11 further suggested the formation mechanism of the high concentration of secondary aerosol in summer at MT. NH + 4 increased significantly from 1.48 µg/m 3 in spring to 10.4 µg/m 3 in summer.This was likely due to the pesticide sprayed in late May at MT, the large area of farming fields, and the stock-raise over MT region, which could produce much more NH 3 Discussion Paper | Discussion Paper | Discussion Paper | pollution elements, As and Pb, were highly enriched in the aerosols with the EFs of 1541 and 679, in spring, while1470 and 1969 in summer in PM 2.5 at MT.The concentrations of As and Pb were of different seasonal variations with summer-high/spring-low Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 2 .
Fig. 2. Daily variations of TSP and PM 2.5 from 14 March to 29 June in 2006 at MT.

Fig. 9 .
Fig. 9. Distributions of variable source of PM 2.5 during campaign period at MT.
The recovery of each ion was in the range of 80-120%.The relative standard deviation of each ion was less than 5% for reproducibility test.The limits of detection (S/N =3) were less than 0.04 mg L −1 for anions and 0.006 mg L −1 for cations.The quality assur- ance was routinely carried out by using Standard Reference Materials (GBW 08606) produced by National Research Center for Certified Reference Materials, China.Blank values were subtracted from sample determinations.The details were given elsewhere

2.3 Meteorological datafire spot map and trace gases
Li et al. (2007)andCao et al. (2008)found very similar content of K + in PM 2.5 emitted from agriculture residues: 9.94 ± 11.8 and 9.56 ± 9.01(wt%) for wheat straw, and 11.38 ± 8.49(wt%) for rice straw.9.56(wt%) of K + in PM 2.5 in this study was used to assess the contributions of PM 2.5 from open burning of agriculture residues during the sampling campaign (K + BB /0.0956 for biomass burning and Al/0.08 for crust dust) (Figs.8-9), which revealed that fine particle from biomass burning accounted for 7.56% in spring, while 36.71% in summer, and even reached to 81.58% on the day of June 12.Burning of agriculture residues also resulted in the increase of other gaseous pollutants related to biomass burning, such as O 3 , CO and VOCs (see next section).The concentrations of K + at different sites are summarized in Table 8.K + at MT − , etc.

Table 1 .
Mass concentrations of particle matters (µg/m 3 ) at the summit of MT.

Table 2 .
Mass concentrations (µg/m 3 ) and size distributions of particles at the summit of MT and other sampling sites.

Table 4 .
Water-soluble ions in PM 2.5 and TSP and the corresponding ratios of summer/spring at MT.

Table 5 .
SOR, NOR of ambient air and main water-soluble ions in PM 2.5 and TSP at MT and Beijing.
C (S+N+A) : Aggregate mass concentrations of SO

Table 6 .
Concentrations (µg/m 3 ) of elements in PM 2.5 and TSP and the corresponding ratios of summer/spring at MT.
BDL: Below detection limits BC: Black carbon a Unit after BC: ng/m

Table 7 .
pH of aqueous filtrates of PM 2.5 and TSP aerosols at different sampling sites.

Table 8 .
Concentrations of K + in PM 2.5 at different sampling sites.

Table 9 .
Correlation coefficients between K + and other measured species in summer at MT.

Table 10 .
Ca/Al ratios for PM 2.5 during the dust case in 2007 and surface soil in Chinese desert regions.

Table 11 .
Correlation coefficients among the certain species in spring and summer at MT.