Interactive comment on “ Observation of atmospheric aerosols at Mt . Hua and Mt . Tai in central and east China during spring 2009 – Part 1 : EC , OC and inorganic ions ”

This manuscript presents findings from measurements of aerosol particles at two mountain sites in central and east China. The quantification of both carbonaceous species (OC and EC) and water-soluble inorganic ions in the ambient particulate matter allowed for the identification of different types of source contributions, specifically the influence from automobile emissions and biomass burning, as well as mineral dust during an intensive dust storm episode. Higher concentrations of most species at the mountain site in east China (Mt. Tai) suggested a regional character of air pollution. Interestingly, nitrate was more abundant than sulfate ion at this site, reflecting increased emissions of NOx from vehicles and industrial processes in the urban areas along the east coast of China.


Introduction
Asian dust storms, which often occur in spring, are lofted up by strong winds from central Asia and Gobi deserts and can be transported into East Asia, North America (Arimoto et al., 2006;Heald et al., 2006;Leaitch et al., 2009; Published by Copernicus Publications on behalf of the European Geosciences Union.Parrington et al., 1983;VanCuren and Cahill, 2002;Wilkening et al., 2000), and even be transported more than one full circuit around the globe within two weeks (Uno et al., 2009).
During the long-range transport, dust particles react with a diversity of chemical species, coagulate with other particles, and/or provide reaction sites in the atmosphere, exerting a significant impact on the atmospheric environment (Huebert et al., 2003;Sun et al., 2010;Tobo et al., 2010;Trochkine et al., 2003) and human health (Chiu et al., 2008;Hong et al., 2010;Singh et al., 2009) of the downwind regions.
Numerous studies on changes in the chemical properties of Asian dust particles during long-range transport have been documented (Kanayama et al., 2002;Parrington et al., 1983;Zhang et al., 2003), but most of them were performed in urban (Huang et al., 2010b;Sun et al., 2004;Wang et al., 2002bWang et al., , 2003;;Yuan et al., 2008;Zhang et al., 2009), rural and marine areas (Decesari et al., 2005;Geng et al., 2009;Kanayama et al., 2002;Wang et al., 2009a) .In contrast, only a limited number of observations on aerosol chemistry were conducted at alpine sites, most of which are situated in the eastern part of China (Gao et al., 2005;Wang et al., 2009bWang et al., , 2009c)).Atmospheric environment in mountain area is unique because of lower temperature, higher relative humidity (RH), and stronger solar radiation.Tropospheric aerosols over mountain areas are derived mostly from longrange transport, and are thus representative of atmospheric characters in a larger scale.Recently Rosenfeld et al. (2007) found that the decreasing precipitation in mountain area of inland China is deeply linked with the increasing air pollution.Satellite observations (Richter et al., 2005;van Donkelaar et al., 2010) also pointed out that NO 2 and particle levels in east China are the highest in the world.These increasing anthropogenic pollutants, together with the frequent occurrence of dust storm, may have been changing the physiochemical properties of the downwind atmosphere (Huebert et al., 2003;IPCC, 2007;Liu and Diamond, 2005;Mori et al., 2003;Seinfeld et al., 2004).The purpose of this study is to recognize the difference in composition, concentration and size distribution of inorganic ions and elemental and organic carbon in airborne particles between Mt.Hua and Mt.Tai, two mountains located in central and east China, and investigate the impact of dust storm on aerosol chemistry of the mountain atmospheres.

Sample collection
Sampling sites of Mt.Hua (34.48 • N, 110.08 • E, 2060 m a.s.l.) and Mt.Tai (36.27 • N, 117.10 • E, 1545 m a.s.l.) are situated in mid and east China, respectively (Fig. 1).TSP and PM 10 samples were collected in a 24-h interval at an airflow rate of 100 l min −1 by using two mid-volume samplers (KC-120H, Qingdao Laoshan Company, China) fixed on a rooftop of the Meteorological Observation Station at the summits of Mt.Hua and Mt.Tai.Size-segregated particles were simultaneously collected at the same sites using a 9-stage sampler (Andersen, USA) at an airflow rate of 28.3 l min −1 with cutoff points as 0.4, 0.7, 1. 1, 2.1, 3.3, 4.7, 5.8, and 9.0 µm.Each set of the size-segregated samples was continuously collected for 4-6 days depending on aerosol loading.All the samples were collected onto pre-combusted (450 • C, 6 h) quartz filters and the air inlets were around 5-10 m above the ground.The Mt. Hua sampling was performed from 25 March to 29 April 2009, while the sampling at Mt. Tai was conducted from 27 March to 29 April 2009.On 24 April a massive dust storm originating from Gobi desert arrived in Mt.Hua and Mt.Tai simultaneously, during which TSP and PM 10 sampling time was changed into 3-6 h.Sampling at Mt. Tai for size-segregated particles was stopped during the event due to instrument problem of the 9-stage sampler, thus the sizeresolved data for the dust storm period are unavailable.Field blank was collected before and after the sampling by mounting a filter onto the sampler for about 10 min without sucking any air.After sampling, the sample and the blank filters were sealed in an aluminum foil, transported into the lab and stored in a freezer under −20 • C prior to analysis.Meteorological parameters at the two sites during the sampling periods are given in Table 1.

Inorganic ions
Detailed analytical method for inorganic ions has been reported elsewhere (Wang et al., 2002a(Wang et al., , 2010)).Here we only give a brief introduction.Filter aliquots from PM 10 and the size-segregated samples were cut in pieces and extracted with 5 ml pure water for 3 times each in 10 min by ultrasonication, respectively.Then the combined water-extracts were filtered through a PTFE filter to remove the particles and filter debris, and determined for pH using a pH meter (HANNA HI8424 pH meter, US) at an ambient temperature of 25 • C and inorganic ions using an ion chromatograph (Dionex 500, Dionex, US).Particle mass on the filter was gravimetrically measured using an electronic balance (Sartorius E5, Germany) under a relative humidity of 40 ± 5 % at ambient temperature.

Organic carbon (OC) and elemental Carbon (EC)
OC and EC in the TSP and PM 10 samples were analyzed using DRI Model 2001 Carbon Analyzer following the Interagency Monitoring of Protected Visual Environments (IM-PROVE) thermal/optical reflectance (TOR) protocol (Chow et al., 2004(Chow et al., , 2007)).Briefly, a size of 0.526 cm 2 sample filter was put in a quartz boat inside the analyzer and progressively heated to temperatures of 120   , and 550 • in a non-oxidizing helium (He) atmosphere, and 550 • , 700 • , and 800 • in an oxidizing atmosphere of 2 % oxygen in helium.
Inorganic ions, EC and OC in the field blanks were less than 10 % of those in real samples.Data reported here were all subtracted by the blanks.An intercomparison was made by comparing the species determined in PM 10 with those in the corresponding size-segregated samples.A good linear correlation was obtained for the two data sets (Fig. 2), demonstrating the consistency between the two samplers.

General description
Figure 3 shows the temporal variations of TSP and PM 10 during the campaign, while concentrations of chemical species in the samples are presented in Table 2. TSP and PM 10 during the non-dust storm periods were 103 ± 42 and 71 ± 28 µg m −3 at Mt. Hua, accounting for about 50 % of those at Mt. Tai.The massive dust storm event on 24 April significantly affected the atmospheres over both mountains with the highest 3-h TSP and PM 10 concentrations being 991 and 740 µg m −3 at Mt. Hua and 2280 and 1797 µg m −3 at Mt. Tai (Fig. 3), 6-8 times more abundant than those in the nonevent time.Although the lower altitude of Mt.Tai is more accessible to pollutants from lowland sources, the higher levels  of TSP and PM 10 at Mt. Tai in both the nonevent and event periods clearly demonstrate that aerosol pollution at Mt. Tai is more serious than at Mt. Hua, which is consistent with results observed by satellite and aircraft measurements for NO 2 (Richter et al., 2005) and PM 2.5 (van Donkelaar et al., 2010).

Non-dust storm period
As shown in Fig. 4 and Table 2, concentrations of SO 2− 4 and Na + of PM 10 during the non-dust storm period in the Mt.Tai atmosphere are comparable to those in the air of Mt.Hua, but PM 10 , EC, OC and other ions at Mt. Tai are 2-10 times more abundant than those at Mt. Hua especially for Cl − , NO − 3 , NH + 4 , and K + .Potassium ion has been considered a key tracer for biomass burning emissions (Engling and Gelencser, 2010;Li et al., 2003).Several studies on vegetation fire emissions from Africa savanna found that the smoke was enriched in Cl   during the fires was present in particle phase (Andreae et al., 1998;Pósfai et al., 2003).Thus, a strong linear correlation (r 2 = 0.79, Fig. 5b) found between Cl − and K + for the nondust PM 10 samples indicates that the high levels of Cl − and K + at Mt. Tai were mostly derived from biomass burning emissions in the North China Plain (NCP).However, such a significant correlation was not observed for the related samples at Mt. Hua (Fig. 5a).The equivalent ratio of Cl − /K + during the non-dust storm period was 0.5 ± 0.8 and 1.7 ± 0.7 at Mt. Hua and Mt.Tai, respectively, suggesting that KCl salt is not the only form by which Cl − and K + exist in the samples, and they may have other origins or experienced additional atmospheric processes during the transport of biomass burning smoke such as deposition and/or reaction with acidic gases.Chinese loess and dust from Gobi desert also contain certain amount of Cl − and K + (Cao et al., 2008).Arimoto et al. (2004) reported that springtime TSP aerosols at Zhenbeitai, a site located in inland China, contain some amount of Cl − (0.5 ± 0.5 µg m −3 ) and K + (0.3 ± 0.2 µg m −3 ) during non-dust storm periods.Size distribution of particles at both mountain sites showed that a significant amount of Cl − and K + was enriched in particles with a diameter larger than 3 µm, especially when dust was present (see detailed discussion in Sect.3.2), indicating an important contribution of soil/dust to the particulate Cl − and K + in the mountain atmospheres in addition to the contributions from biomass burning, dried salt lakes and sea salt, which results in the equivalent ratio of Cl − and K + being different from unity.Recent studies on biomass burning events have reported that KCl salt abundantly existing in young smoke can readily be converted into K 2 SO 4 and KNO 3 during the smoke aging process by reacting with acid gases HNO 3 and H 2 SO 4 (Hand et al., 2005;Ikegami et al., 2001;Li et al., 2003;Pósfai et al., 2003).HNO 3 and N 2 O 5 have been confirmed to be of ability to react with NaCl and release HCl, latter may further react with atmospheric alkaline particles like CaCO 3 (Finlayson-Pitts et al., 1989;Tobo et al., 2010).High levels of HNO 3 and N 2 O 5 in mega-cities in east coastal China have been documented recently (Pathak et al., 2009(Pathak et al., , 2010;;Wang et al., 2011), thus it is possible that KCl salt in the atmosphere of Mt.Tai may react with HNO 3 and N 2 O 5 in the same manner as does NaCl, which is another factor leading to the equivalent ratio of particulate Cl − and K + not be unity.
During the nonevent period more than 90 % of PM 10 samples collected at Mt. Hua showed that nitrate is lower than sulfate (Fig. 6a).However, around 80 % of PM 10 samples collected at Mt. Tai showed that nitrate is higher than sulfate (Fig. 6b).Coal is the major energy source in the country with more than 2.7 billion tons of coal being burned in 2003 (Aldhous, 2005).To improve the air quality Chinese government promulgated a strict law to reduce SO 2 emission in 2005.
Thus the increasing rate of SO 2 emission is expected to decrease.Concentrations of nitrogen oxides, on the other hand, have been increasing due to economic growth (Richter et al., 2005), leading to an enhanced nitrate aerosol in many urban areas especially in mega-cities such as Beijing and Shanghai with nitrate concentration being comparable and even higher than sulfate (Huang et al., 2010a, b;Pathak et al., 2009Pathak et al., , 2010;;Wang et al., 2006b).Compared to sulfate the more abundant nitrate in the Mt.Tai atmosphere are not only due to the high level of nitrogen oxides in the NCP region but also probably due to the unique atmosphere environment, because higher humidity and lower temperature of the mountaintop atmosphere are favorable for the transformation of HNO 3 from  gas to solid phase.However, the fact that concentration of NO − 3 exceeds over SO 2− 4 clearly demonstrates that the atmospheric environment in east China is significantly changing.
Figure 7 plots the 72-h backward trajectories of air masses reaching the two sampling sites.Air masses at both sites during the non-dust storm period were transported from the south and north directions, respectively (Fig. 7a and b).Thus the corresponding samples can be classified as two groups, i.e., southerly and northerly.At the Mt.Hua site concentrations of SO 2− 4 , Na + , NH + 4 and K + are higher in the southerly air masses than in the northerly (Table 3), while EC, OC and other major ions in both directions of air masses are comparable.In contrast, at Mt. Tai all species except for F − are equal or more abundant in the southerly air masses than in the northerly.Moreover, pH values showed that aerosols from south China are more acidic than those from north China especially in the Mt.Tai region (Table 3).EC is chemically stable, thus the normalized concentrations of species by EC can be useful for recognizing the changes in chemical compositions of aerosols during transport.As shown in Fig. 8a and b, relative abundance of OC and Ca 2+ to EC in the two mountain regions are similar.However, the ratios of NO − 3 , NH + 4 and K + to EC at Mt. Hua are lower than those at Mt. Tai, again suggesting the high levels of nitrogen oxides, ammonia and biomass burning emission in east China.In contrast, the ratio of SO 2− 4 to EC is higher at Mt. Hua than at Mt. Tai.The lower ratio of SO 2− 4 /EC at Mt. Tai (Fig. 8) can be explained by more vehicle exhausts in the east coastal region, which contain more EC and less SO 2 compared to coal burning emissions (Gaffney and Marley, 2009;Wang et al., 2007;Xie et al., 2010).Sulfate strongly reflects solar radiation, whereas EC strongly absorbs solar radiation.Thus the net radiative forcing is determined by the relative amounts of sulfate and EC (Ramana et al., 2010).The lower ratio of SO 2− 4 to EC suggests that climate-warming effect caused by aerosols may be more significant in the Mt.Tai area.A recent aircraft measurement (Xue et al., 2010) showed that boundary level SO 2 in east coastal China is about 10 times higher than that in the inland region, but the free tropospheric level (>1.5 km a.s.l) of SO 2 in the east and inland parts of China are comparable, which could be one of the reasons why SO 2− 4 presented a similar concentration in the two mountain areas.The ratios of NO − 3 , SO 2− 4 , NH + 4 and K + to EC at both sites are generally higher in the southerly air mass than in the northerly air mass, which is consistent with the observation for wintertime aerosols at Mt. Hua (Li et al., 2011).Figure 9  equivalent ratio of each species to the total of the three ions.The percentage of NH + 4 is higher at Mt. Tai than at Mt. Hua during the non-dust periods (Fig. 9a), indicating that in the Mt.Hua atmosphere more NO − 3 and SO 2− 4 are neutralized by soil dust derived alkaline ions such as Ca 2+ and Mg 2+ , in contrast to Mt. Tai, where more NO − 3 and SO 2− 4 are neutralized by NH + 4 .To further recognize the difference of aerosol composition between the two mountain atmospheres, an EC tracer method as follow was used to approximately estimate primary organic carbon (POC) and secondary organic carbon (SOC) in the PM 10 samples (Castro et al., 1999;Chu, 2005;Yu et al., 2009).

plots the
Where the (OC/EC) min is the minimum ratio of OC/EC for the samples at each site.Results based on the calculation showed that throughout the non-dust storm period SOC/POC ratio was 0.5 ± 0.4 at Mt. Hua and 0.9 ± 0.5 at Mt. Tai, while SOC/EC ratio at the sites was 1.5±1.3 and 2.4±1.3,respectively, suggesting that carbonaceous aerosols in east coastal China is more oxidized, which is consistent with the enhanced concentration of glyoxal, a photo-oxidation product of volatile organic compounds (VOCs), over the NCP region as reported by satellite observation (Wittrock et al., 2006) and is ascribed to the high levels of oxidants such as O 3   and NO x and VOCs in the region (Akimoto, 2003;Fu et al., 2008;Hatakeyama et al., 2005;Streets and Waldhoff, 2000;Takiguchi et al., 2008).The ratio of SOC/POC at Mt. Hua is also slightly higher than that (0.4 ± 0.4) in Baoji  Wang et al., 2010), a city nearby Mt.Hua, further indicating that mountain aerosols are more aged compared to those on ground surface.

Dust storm II on 24 April
As seen in Table 2, a moderate dust storm event, named as Dust storm I (DSI), occurred on 20 April at Mt. Hua with concentrations of 365 and 173 µg m −3 for TSP and PM 10 , respectively.Four days later an intensive dust storm event (Dust storm II, DSII), originating from Gobi desert, simultaneously reached Mt.Hua and Mt.Tai with averaged 3h concentrations of TSP and PM 10 being 689 ± 355 and 506 ± 303 µg m −3 at Mt. Hua and 1759 ± 542 and 1343 ± 450 µg m −3 at Mt. Tai, respectively.Here we only compare the composition of DSII samples to discuss difference in the impact of the event on the two alpine atmospheres.Compositions of chemicals in the highest loading of 3-h PM 10 samples collected during DSII are shown in Table 4. pH values showed that even in the dust period aerosols at Mt. Tai are still more acidic than those at Mt. Hua.EC in the 3-h sample at Mt. Hua was 1.1 µg m −3 but undetectable at Mt. Tai, whereas SO 2− 4 and Na + showed a similar concentration at both sites (Table 4).NO − 3 , NH + 4 , Ca 2+ and other ions in the DSII samples at Mt. Tai were 1-5 times more than those at Mt. Hua.Based on the characterization of single Asian dust particle, Sullivan et al. (2007) found that Fe-rich Asian dust particles were closely associated with secondary sulfate whereas Ca-rich particles could contain secondary sulfate and nitrate.Therefore, high concentration of Ca 2+ observed during the event at Mt. Tai was probably caused by enhanced heterogeneous reactions of H 2 SO 4 , HNO 3 and their precursors (e.g., NO 2 , N 2 O 5 , and SO 2 ) with dust particles.
Compared to those during the non-dust storm period, the average concentrations of particle mass, OC, Na + , K + , Mg 2+ and Ca 2+ of PM 10 at both sites increased by a factor of 1-9 during the DSII period with EC, NO − 3 and NH + 4 decreasing by 20-80 %.Interestingly, during both the event and the non-event time SO 2− 4 did not change significantly at the two alpine sites, further suggesting a homogeneous distribution of free tropospheric SO 2 in the DSII pathways.Due to an additional input of Ca 2+ the relative abundance of NH + 4 to the total equivalent of NH + 4 , NO − 3 and SO 2− 4 significantly decreased (Fig. 9c), compared with those during the non-dust period (Fig. 9a), and the abundance of Ca 2+ relative to the total of Ca 2+ , NH + 4 , and NO − 3 sharply increased, especially at Mt. Tai (Fig. 9b and d), indicating that the existence of Ca 2+ is unfavorable for the formation of NH + 4 , because mineral dust shifts ammonia from the particle to gas phase by changing the aerosol from a cation-to anion-limited state due to the presence of alkaline species (Song and Carmichael, 1999).

Comparison with urban aerosols
As shown in Table 5, the springtime concentrations of NO − 3 and SO 2− 4 in the Mt.Hua air are 2-5 times lower than those in Xi'an, Beijing and Shanghai, three mega-cities in China, while concentrations of NO − 3 and SO 2− 4 at Mt. Tai are similar to those in Shanghai and about 30-50 % lower than those in Xi'an and Beijing.Compared to the three cities NH + 4 is much lower at Mt. Hua but comparable and even higher at Mt. Tai.Concentration ratio of NO − 3 /SO 2− 4 is highest at Mt. Tai, followed by Beijing, Shanghai, Xi'an and Mt.Hua, being coincident with a higher level of nitrogen oxides in east China (Richter et al., 2005).The ratio of NO − 3 to SO 2− 4 at Mt. Hua is 50 % lower than that in Xi'an, a nearby city (see Fig. 1).In contrast, the ratio at Mt. Tai is 30 % higher than that in Beijing and Shanghai (Table 5), two mega-cities located in east coastal China.Such an opposite vertical pattern between the mountains and nearby cities suggests that the atmospheric environment in the Mt.Tai area is favorable to form nitrate aerosol, largely due to the high level of nitrogen oxides in the NCP region and the advantaged meteorological conditions of lower temperature and higher humidity of the mountain air.

Non-dust storm period
As seen in Fig. 10a and b, particle mass size distribution during the non-dust storm period at Mt. Hua was the same as that at Mt. Tai, showing a bimodal size distribution with two equivalent peaks in the fine (<2.1 µm) and coarse (≥2.1 µm)   ranges.Ammonium presented a similar unimodal pattern at both sites, dominating in the fine mode (Fig. 10c and d).K + showed a bimodal size distribution at the sites (Fig. 10e  and f), but coarse mode of potassium ion at Mt. Tai was much more pronounced.A similar pattern with K + can be seen for Cl − at Mt. Tai (Fig. 10n), together with the strong correlation of K + and Cl − in PM 10 mentioned above, again demonstrating biomass-burning emission as their ma-jor source.Mg 2+ , Ca 2+ and Na + at both sites dominated in coarse mode (Fig. 10g-l), because they are mostly derived from soil/dust.But the significant amount of Na + presenting in the fine size range indicates an importance of additional sources other than soil/dust (Fig. 10k and l).Cl − at Mt. Hua was almost entirely present in coarse particles, in contrast to the case at Mt. Tai, where Cl − largely occurs in the fine size range (Fig. 10m and n).Cl − at Mt. Hua, along with other cations such as Na + and Mg 2+ , probably originated in part from dried salt lakes in north and northwest China.Nitrate showed a bimodal distribution with two comparable peaks at Mt. Hua in the size ranges of 0.7-1.1 and >2.1 µm (Fig. 10o), and with a predominant peak in the fine mode and a small peak in the coarse mode at Mt. Tai (Fig. 10p).64 % of NO − 3 was present in the coarse particles at Mt. Hua, while only 39 % of NO − 3 was present in the coarse particles at Mt. Tai (Table 6).The increased coarse fraction of nitrate at Mt. Hua can be explained by an increased adsorption of gaseous HNO 3 onto alkaline particles due to more dust in the region (Takiguchi et al., 2008).As seen in Table 2, NH + 4 (12 ± 8.9 µg m −3 ) at Mt. Tai is 4 times higher than that (2.5±1.3 µg m −3 ) at Mt. Hua, which means NH 3 is also much more abundant at Mt. Tai than at Mt. Hua, because in China large amount of NH 3 is emitted through agricultural activities such as fertilizer application (Streets et al., 2003;Streets and Waldhoff, 2000) in addition to biomass burning emissions (Andreae et al., 1998).Particulate NO − 3 can be formed by a gas phase reaction of HNO 3 with NH 3 and enriched in fine particles (Seinfeld and Pandis, 1998), which is more significant at Mt. Tai due to the high level of NH 3 .However, due to the relatively lower level of NH 3 , gaseous HNO 3 in the Mt.Hua atmosphere has more chance to absorb onto alkaline dust.As a result, the coarse mode of NO − 3 at Mt. Hua is more abundant than that at Mt. Tai.Sulfate at the two alpine regions exhibited a similar bimodal pattern with a large peak in the fine mode and a small peak in the coarse mode (Fig. 10q and r).

Dust storm on 24 April
As shown in Fig. 10s-aa, the size distribution patterns of particles and all ions were altered during the dust storm event on 24 April with an increase in the coarse range, especially for PM, K + , Na + , NO − 3 and SO 2− 4 .Size distribution of PM and NO − 3 during the event changed from a bimodal pattern (Fig. 10a, and o) into a unimodal one in the coarse mode (Fig. 11s and z).Sulfate still exhibited a bimodal pattern, but its fine fraction significantly decreased compared to that in the non-dust storm period (Fig. 10q and aa).Such changes in size distribution pattern indicate that gas-to-particle conversion of NO − 3 and SO 2− 4 affect both sub-and supermicrometer aerosol modes by redirecting much of the deposition that would normally occur on the accumulation mode to the larger dust particles during the event (Seinfeld et al., 2004;Takiguchi et al., 2008).Other ions also showed an increase in the coarse mode when the dust was present, especially for K + , Na + and Cl − , with a significantly diminished and even disappeared mass peak in the fine range during the episode (see Fig. 10u, x and y).
Different compounds are of different solubility, for example, Ca(NO 3 ) 2 and CaCl 2 are highly soluble and much more hygroscopic than other insoluble or slightly soluble calcium salts like CaCO 3 , CaSO 4 , and CaC 2 O 4 (Sullivan et al., 2009; Tobo et al., 2010), affecting a particle's ability to activate as cloud condensation nuclei (Tobo et al., 2010).Thus, it is indispensable to investigate the chemical forms in which major ions exit in the particles.As shown in Table 6, during the nonevent and the event periods NO − 3 , SO 2− 4 and NH + 4 are the three major ions in the fine range (<2.1 µm), although only 22 % of nitrate centered within this mode during the dust storm period.In contrast, NO − 3 , SO 2− 4 and Ca 2+ are the three most abundant ions in the coarse size range (>2.1 µm) in both nonevent and event time.Here, therefore, we investigate the specific chemical forms in fine particles for NO − 3 , Mt. Tai than at Mt. Hua.Cl − at Mt. Tai showed a similar patter to K + , while Cl − at Mt. Hua was largely present in coarse mode, because Cl − at Mt. Hua is largely derived from dust or dried lakes in north and northwest China while K + and Cl − at Mt. Tai are mostly originated from biomass burning.Compared to that in Mt.Tai nitrate in Mt.Hua during the nonevent period was more enriched in coarse particles.When the dust storm was present, particles and all ions significantly shifted toward coarse particles, except for NH + 4 , with a diminished and even disappeared peak in the fine mode.Equivalent ratios indicate that during the whole campaign ammonium exists largely as NH 4 NO 3 and NH 4 HSO 4 at Mt. Hua and NH 4 NO 3 and (NH 4 ) 2 SO 4 at Mt. Tai, while calcium ion exists mostly as Ca(NO 3 ) 2 during the non-event and as CaSO 4 during the event.

Fig. 1 Fig. 1 .
Fig. 1 Sampling sites of Mt.Hua and Mt.Tai in East Asia

Fig. 2
Fig. 2 Comparison of data measured by the 9-stage Andersen sa and the PM10 sampler at the two mountain sites

Fig. 2 .
Fig. 2.An intercomparison of major components measured by the PM 10 and 9-stage samplers.
particle mass; b Nd: not detected; c pH = pH of water-extracts of the samples -pH of water-extracts of the filed blanks.
Fig. 3 TSP and PM10 concentrations in the atmospheres over Mt.Hua and Mt.Tai during spring 2009

Fig. 3
Fig. 3 TSP and PM10 concentrations in the atmospheres over Mt.Hua and Mt.Tai during spring 2009

Fig. 5 .
Fig. 5. Correlation of K + and Cl − in PM 10 at Mt. Hua and Mt.Tai during the non-dust storm period.

Fig. 6 Fig. 6 .8
Fig. 6 Frequence of PM10 samples containing nitrate higher than sulfate at (a) Mt.Hua and (b) Mt.Tai during non-duststorm days Fig. 6.Frequency of PM 10 samples containing nitrate with a concentration higher than that of sulfate.

Fig. 8 .
Fig. 8 Concentraions of species normalized by EC in PM10 during the non-dust storm

Fig. 9
Fig.9Triplots of ion equivalent concentraions in PM10 samples during the nondust storm and dust storm (a-b are the ratios of the species to the sum of NH4+,NO3-and SO42-, c-d are the ratios of the s to the sum of Ca2+, NH4+ and NO3-).All samples collected in rainy/snowy days are not included.

Fig. 9 .
Fig. 9. Ternary diagrams for the equivalent ratio of major ions in PM 10 samples during (a), (b) the non-dust and (c), (d) the dust storm II periods.
Fig. 10 Size distributions of major ions in PM10 during the non-dust period ((a-e) Mt.Hua and (f-j) Mt.Tai) and the dust period ((k-o) Mt.Hua)

Fig. 10 .
Fig. 10.Size distributions of particles and inorganic ions during (I) non-dust storm period and (II) dust storm period.
Fig. 11 Equivalent ratio of ammonium to (a) the sum of nitrate and sulfate and (b) the s in particles with a diameter less than 2.1 µm at Mt. Hua and Mt.Tai during the including the non-dust storm and dust storm periods

Fig. 11 .
Fig. 11.Equivalent ratios of ammonium to (a) the sum of nitrate and bisulfate and (b) the sum of nitrate and sulfate in particles with a diameter less than 2.1 µm at Mt. Hua and Mt.Tai during the whole sampling time including the non-dust storm and dust storm periods.

Table 1 .
Meteorological parameters during the sampling period.

Table 2 .
Concentrations of OC, EC and inorganic ions in aerosols from the atmospheres of Mt.Hua and Mt.Tai in China during the spring of 2009, µg m −3 .
Temporal variations of TSP and PM 10 concentrations in the atmospheres over Mt.Hua and Mt.Tai during spring 2009.

Table 3 .
Concentrations of species in PM 10 transported southerly and northerly at Mt. Hua and Mt.Tai during the non-dust storm period, µg m −3 .
a Ratio of average concentration of southerly to that of northerly; b pH = pH of water-extracts of the samples -pH of water-extracts of the filed blanks.

Table 4 .
Concentrations of species in the highest loading of 3-h PM 10 sample during the dust storm II event occurring on 24 April, µg m −3 .
a pH = pH of water-extracts of the samples -pH of water-extracts of the filed blanks.

Table 5 .
Concentrations of inorganic ions in the mountain and urban atmosphere over China, µg m −3 .