The Characteristics of PM 2 . 5 and Its Chemical Compositions between Different Prevailing Wind Patterns in Guangzhou

An intensive campaign for atmospheric pollution was carried out from October to November 2004 in Pearl River Delta Region (PRDR) of China in order to better understand the dominative factors for controlling air quality and impacts of pollutants on the regional environment. The object of this study aimed to investigate the characters and sources of particulates through the study of chemical compositions in different wind pattern in Guangzhou city. The ambient concentrations of mass, soluble species, and carbonaceous matter in PM2.5 particulates with time resolutions of 15 minutes, 15 minutes, and 12 hours were measured by TEOM, in-situ IC system, and carbon analyzer, respectively. During the sampling time, high concentrations of PM2.5 mass and its chemical compositions were found, which was comparable or higher than other measurements in mainland China. The average SO4, NH4, and NO3 concentrations were 38.6, 13.6, and 8.8 μg/m, which accounted for 26.5%, 13.6%, and 5.7% of the total particulate mass, respectively. In the frontal system, biomass burning was the significant emission source and the great amount of SO4 and NH4 particulates were observed in the aged secondary particulates, together with the dry prevailing northerly wind. In the local system, low value of sulfur oxidation ratio and high correlation coefficient between sulfate and elemental carbon were also observed under the prevailing southeasterly wind. It could be explained that the heterogeneous reaction of sulfate on soot particles causes high levels of NOx in Guangzhou. Great amount of SO2 would further convert to secondary sulfate particles through photochemical reactions during transport to the inner areas, which will cause more serious air pollutions in the southern China.


INTRODUCTION
Atmospheric aerosols play many important roles in the environment, including visibility, Earth radiation budget and human health effects (National Research Council, 1998;Rodrigues-Silva et al., 2012).With rapid industrial growth in the past decade, the emissions of anthropogenic substance in China have been on the rise and causes serious air pollution especially in urban areas with particulate matter as the main air pollutants.The Pearl River Delta Region (PRDR) including Hong Kong and Macao is one of the fastest developing regions in China.It has a total land area of 42794 km 2 with population over 38.7 millions (Cao et al., 2003).The rapid growth of industrialization and urbanization in this region has resulted in an excessive increase in emissions of major air pollutants, which impact chemical composition of the ambient pollutants and bring not only regional air pollution but also global effects through the long-range transport.The measurements of air pollutants are needed for evaluating air quality and determining the possible control strategy.The PRDR is one of the highly urbanized and industrialized regions in China in the last score years.An investigation of air pollutants in PRDR can provide valuable and insightful information for other regions.
The concentration of major soluble ions for PM 10 showed a summer minimum and a winter maximum in a coastal region of Hong Kong and the possible sources were identified as fossil combustion and anthropogenic emission, sea-salt component, mineral dust, vehicle and biomass emissions (Cheng et al., 2000;Man and Shih, 2001).Cao et al. (2003) used the thermal optical reflectance method to analyze the organic carbon (OC) and elemental carbon (EC) of aerosol in PRDR.They concluded that the carbonaceous aerosol accounted for 40.2% of the PM 2.5 and the average OC/EC ratio was 2.4 with strong correlation, which indicated that the dominant source of OC and EC was a primary emission, likely motor vehicle emission.Pathak et al. (2003) estimated that ~40% of the (NH 4 ) 2 SO 4 in Hong Kong originated from regional transport of non-local emissions.Cheung et al. (2005) indicated that particulate organic matter and nonsea-salt sulfate were the major components of PM 2.5 in PRDR, which were strongly influenced by synoptic wind flow.Depending on a 4-year monitoring data in Hong Kong, Wai and Tanner (2005) demonstrated that the major non-local sources from the Mainland China associated with northerly and north-easterly surface wind, in the forms of dust, industrial and transportation emissions, biofuel combustions and agricultural emissions.On the contrary, the major local pollutant sources were the sea salt and vehicle exhausts.Louie et al. (2005) reported PM 2.5 mass concentrations showed seasonal variations with high mixing levels between fall and winter, which resulted mostly from the elevated organic matter and NH 4 NO 3 .The seasonal variations of (NH 4 ) 2 SO 4 and crustal material associated with the long-range transport could be explained by the variation of source strength and the transition of the monsoon pattern.However, EC was spatially inhomogeneous and the ambient carbonaceous material was highly dominated by local mobile source emissions (Kim et al., 2011;Xiao et al., 2011;Zhang et al., 2012).
Chemical composition of ambient particulate is the determinant factor on evaluating the contribution of pollutant sources and assessing the impact on environment.Soluble ions and carbonaceous material in aerosols could account for up to two-third of the total aerosol mass in highly industrialized and urbanized areas.The chemical compositions of ambient aerosols, affected by meteorological parameters, atmospheric reactions and pollutant emissions, change rapidly at different spatial and temporal scales.Unfortunately, previous chemical analyses of particulate composition studies in PRDR were based on the results of aerosol sampling of relatively long sampling times, which could dilute the temporal impact of pollutant sources and was unable to reveal rapidly changing aerosol compositions in spatial and temporal scale.
The information of regional chemical compositions in PM 2.5 plays an important role to develop an effective fine particulate management plan in the PRDR (Hagler et al., 2006).To understand the particulate pollution levels and their impact in PRDR, the continuous hourly measurements of PM 2.5 , water-soluble species and 12-hour concentrations of particulate carbonaceous matter were conducted in Guangzhou.In this work, the concentrations of PM 2.5 and its chemical species were compared with other measurements in mainland China and characterized by prevailing wind direction for understanding the pollutant sources and its impact on environment.

Sampling Site
PM 2.5 ambient aerosol samples were continuously analyzed in the center of Guangzhou which is located at the northern tip of PRDR.Guangzhou city is the largest city in Southern China with a population over 10 millions in a 7434 km 2 area.Many industrial factories are located in both the urban and suburban areas of the city.The sampling site is on the rooftop (ca.50 m height above the traffic road) of GuangDong Provincial Environmental Protection Monitoring Centre building approximately 10 m away from a heavy traffic road.The sampling site is a typical urban site located in a mixed commercial and residential area where many diesel bus routes converge with numerous restaurants in the vicinity.

Instruments
The mass concentrations of PM 2.5 were continuously measured by R&P TEOM (TEOM Series 1400a, Ambient Particulate Monitor, Rupprecht & Patashinik Co., Inc., Albany, NY, USA) monitor equipped with an inlet having a 2.5 μm cutpoint.The TEOM monitor was operated at a total flow rate of 16.7 L/min (1.0 L/min for the main flow and 15.7 L/min for the bypass flow).The in-situ IC system, attached to the bypass flow line of the TEOM monitor, semi-continuously measured the water-soluble inorganic ions of PM 2.5 at intervals of 15 mins.The in-situ IC system, shown in Fig. 1, was used to measure the concentrations of soluble inorganic ions hourly.The principle of measurement had been previously described in detail (Chang et al., 2007).Briefly, the in-situ IC system consisted of the following: gas removal denuders, aerosol collecting device and ion chromatograph (Model ICS-90, Dionex Corp., Sunnyvale, CA, USA).The ambient sample from the bypass flow line was drawn through the denuders to remove acidic and basic gases.After leaving the denuders, the bypass flow was split into a 5.7 L/min filter flow and a 10 L/min RCP (Rapid Capture Particle) flow.The filter flow, which passed through the filter pack, was controlled by a mass flow controller.The sample flow, moving to the RCP, was turbulently mixed with saturated water vapor.The temperature of the saturated water vapor was held at 120°C and controlled by the PID (Proportional Integral Differential device, Model 6100, West, England).The aerosol and water vapor were then passed through a growth chamber with a cool wall to form large droplets in a supersaturated condition.The growth chamber was actively air-cooled by fans.The air flow containing the droplets flowed through a single circular nozzle and impacted on a flat quartz plate, where the droplets were collected.After impacting the plate, the air was drawn from a port at the top of the impactor housing and the liquid was transported by carrier water from the top of the quartz plate to the bottom of the RCP.The highly purified and deionized carrier water was regulated by a peristaltic pump at a flow rate of 0.1 mL/min.The final liquid sample, containing the water-soluble ions of the collected aerosol and air bubbles, was pumped from the bottom of the RCP to the two ICs at a rate faster than the accumulating rate.Before being injected into each IC, the liquid sample was introduced into the debubbler to remove the air bubbles.In this work, each IC consists of a 500 μL sample loop.The IC for anions equipped with an AS-11 guard column, an AS-11-SC separator column, and a self-regenerating suppressor, , respectively.The IC for cations equipped with a CS-12 guard column, a CS-12-SC separator column, and a selfregenerating suppressor, which employs 18 mM CH 4 O 3 S (MSA) as eluent.The retention time is 4.0, 4.5, 5.7, and 11.2 min.for Na + , NH 4 + , K + , and Ca 2+ , respectively.Each soluble species can be clearly separated and analyzed.
The samples for carbonaceous analysis were collected on quartz fiber filters.Before sampling, all the filters were baked under 900°C for 3 hours to remove organic contaminates.The sampling period of each sample was 12 hours.The mass loadings of organic carbon (OC) and elemental carbon (EC) on the filter samples were analyzed using a DRI-2001A carbonaceous aerosol analyzer, following the IMPROVE thermo-optical reflectance (TOR) protocol (Chow et al., 2001).

Sampling Program
The observational campaign was carried out from 15 Oct. 15:00 to 5 Nov. 12:00, 2004.PM 2.5 mass and watersoluble ion concentrations were measured with 15-min sampling intervals during periods of no rain.The 8 watersoluble ions such as Cl -, NO 2 -, NO 3 -, SO 4 2-, Na + , NH 4 + , K + , and Ca 2+ were determined during the sampling period.The uncertainty, associated with these measurements, was estimated by combining the uncertainties in flow rates, the TEOM calibration and the IC calibrations.Each flowmeter was calibrated prior to the measurement being taken.Uncertainties in the flows were estimated at 5-10% for the main flow (1 L/min) and 1-5% for the by-pass flow (15.7 L/min).The TEOM was calibrated with a pre-weighed TEOM filter, installed in its mass transducer as a calibration weight.The percentage difference between the audit and the actual value was lower than 0.13% for a filter weight of 0.11429 mg.The analyses of cation and anion were simultaneously performed by two IC instruments.Concentrated standard solutions of four anions and four cations, purchased from Merck, were used to prepare twelve standard solutions with different concentrations to calibrate the in-situ IC system.The different concentrations of standard solutions were used to confirm the accuracy of measurements every day.The average drift of retention time was lower than 0.1 min.after running for an extended period.The detection limits, calculated as the three times standard deviations of seven replicate blank samples, for Cl -, NO 2 -, NO 3 -, SO 4 2-, Na + , NH 4 + , K + , and Ca 2+ are 0.1, 0.2, 0.2, 0.2, 0.1, 0.2, 0.2, and 0.2 μg/m 3 .The results of blank samples were measured from de-ionized water in the same way as normal samples.The system contamination in the blank sample can be ignored, because it is much less than the measurement of sample.

General Descriptions of the Measurements Meteorological Conditions
Figs. 2(a) and 2(b) showed the hourly measurements of meteorological parameters such as wind speed (WS), wind direction (WD), temperature (T), and relative humidity (RH) at the sampling site during the October campaign.The average WS, T, and RH were 1.8 ± 1.3 m/s, 25.5 ± 2.6 o C, and 59.1 ± 19.7%, respectively.The clearly diurnal cycle of temperature was observed with the daily maximum value in the daytime.In contrast, the maximum value of RH diurnal cycle was found in the nighttime.In the observational period, the northerly wind and southeasterly wind were the prevailing wind with average frequencies of 63% and 28% respectively.The prevailing northerly was observed from Oct. 18 to 27, except for Oct. 23 due to the effect of a typhoon approaching.The prevailing southeasterly occurred in the rest of the sampling time.The average RH of northerly (47.2%) was significantly lower than that of southeasterly (72.5%).However, the northerly with average WS of 2.7 m/s was higher than the southerly with average WS of 0.8 m/s.The average ambient temperatures between two prevailing winds were similar.

Temporal Variations of Particulate Measurements
All valid observations of hourly measurements, including the concentrations of PM 2.5 mass and its soluble ions, in Guangzhou city were exhibited in Figs.2(c) and 2(d).The hourly average concentration of PM 2.5 was varied from 54.5 to 395.1 μg/m 3 with an average and standard deviation of 153.9 ± 52.1 μg/m 3 .Sulfate, ammonium and nitrate are the most abundant inorganic species in PM 2.5 particle.The sulfate concentrations ranged from 15.1 to 90.9 μg/m 3 (average 38.6 μg/m 3 ), and account for 11.7-51.7%(average 26.5%) of the PM 2.5 mass concentration.The ammonium concentrations ranged from 3.5 to 33.1 μg/m 3 (average 13.6 μg/m 3 ), and were 3.3-15.6%(average 9.1%) of the PM 2.5 mass concentration.The nitrate concentrations ranged from 1.1 to 43.1 μg/m 3 (average 8.8 μg/m 3 ), and were 0.9-26.1% (average 5.7%) of the PM 2.5 mass concentration.The average concentrations of Cl -, NO 2 -, Na + and K + were 1.2, 2.9, 0.8, and 2.6 μg/m 3 , respectively.The total concentrations of sea salt aerosol components (Na + and Cl -), biomass burning particulate constituent (K + ), and other ion (NO 2 -) were only 2.0-15.3%(average 4.9%) of the PM 2.5 mass concentration and 4.6-22.2%(average 10.9%) of the total dissolved ionic concentration.On the average, three major soluble inorganic species (SO 4 2-, NH 4 + , and NO 3 -) accounted for 41.2 ± 9.7% of the PM 2.5 mass concentration and 89.1 ± 3.5% of the total dissolved ionic concentration.

Comparison with Literature Measurements
Table 1 showed the concentrations of PM 2.5 mass and its chemical compositions measured in different sites in PRDR and other cities in Mainland China.Compared with the previous measurements (Wu et al., 2003;Hagler et al., 2006) in PRDR, the mass concentration of PM 2.5 during the sampling period was higher than the seasonal average in Guangzhou and much higher than those measured in Shenzhen, Conghua, and Macao.The chemical compositions had similar mass fractions to that in other sites in PRDR, except for NO 3 -and EC.High mass fractions of NO 3 -and EC were observed in urban sites.The vehicle emission was one of major reasons to cause the spatial variations of NO 3 and EC between urban and background areas in PRDR.
The concentrations of PM 2.5 mass and it major species were also compared with those measured in other developing cities in Mainland China (Table 1).The average PM 2.5 mass concentration was 153.9 μg/m 3 , comparable with the measurements of 105.2 to 182.2 μg/m 3 measured during autumn and winter in Beijing (Sun et al., 2004;Wang et al., 2005), higher than that in Shanghai (Ye et al., 2003).Lower concentrations and mass fractions of particulate nitrate were observed in Guangzhou than that in Beijing and Shanghai, but higher concentrations and mass fractions of particulate sulfate were measured in Guangzhou.The particulate nitrate was easily volatilized in the warm atmosphere in Guangzhou.Sufficient solar radiation and humid ambient environment provide favorable photochemical conditions for the formation of secondary sulfate particles.

The Relation between the Characters of Pollutants and Wind Patterns The Classification of Atmospheric Systems
Wind direction is one of the important factors to determine the origin of the air mass.The origin of air mass could influence the characteristics of the fine particles because different air masses traveling through different regions bring aerosols with different chemical components.In this work, the measurements of PM 2.5 particulates were classified into two groups according to the predominant wind directions in order to investigate the characteristics of PM 2.5 particulates under different wind flows.Figs. 3 and 4 showed the normalized frequently distributions of wind direction for various ranges of meteorological parameters, gaseous pollutants, particulate masses, and particulate species.When the prevailing winds blew from the northwest to the northeast, the average WS and RH were 2.7 m/s and 47.2%.The atmospheric condition could be characterized as the frontal system predominance due to the influence of the northern winter monsoon.When the prevailing winds blew from the east to southeast, the average WS and RH were 0.8 m/s and 72.5%.It could be characterized as the local system predominance due to the stagnant atmosphere.The average ambient temperature was consistent between those two atmospheric systems.

The Pollutant Distributions between Different Atmospheric Systems
Under the frontal system, the average concentrations of CO, SO 2 , PM 2.5 , Na + , NH 4 + , K + , Cl -; NO 3 -, and SO 4 2-were 0.8 ppm, 10.3 ppb, 141.3 μg/m 3 , 0.8 μg/m 3 , 12.9 μg/m 3 , 3.2 μg/m 3 , 0.9 μg/m 3 , 4.5 μg/m 3 , and 38.5 μg/m 3 , respectively.Under the local system, the average concentrations of CO, SO 2 , PM 2.5 , Na + , NH 4 + , K + , Cl -; NO 3 -, and SO 4 2-were 1.4 ppm, 41.6 ppb, 167.1 μg/m 3 , 0.8 μg/m 3 , 14.2 μg/m 3 , 1.9 μg/m 3 , 1.5 μg/m 3 , 13.1 μg/m 3 , and 38 μg/m 3 , respectively.The ratios of the local system to the frontal system were 1.7 for CO, 4.0 for SO 2 , 1.2 for PM 2.5 , 1.0 for Na + , 1.1 for NH 4 + , 0.6 for K + , 1.7 for Cl -, 2.9 for NO 3 -, and 1.0 for SO 4 2-.The sulfate is a dominant component of PM 2.5 in Guangzhou.As shown in Fig. 4(c), the average concentrations of SO 4 2were consistent between two atmospheric systems.It is expected that sulfate has stable and high ambient concentrations that correlate with sources in Guangzhou.Higher concentrations of particulate potassium, related to biomass burning, appeared to be brought by the prevailing northerly wind (Fig. 4(f)).It indicated that the northern part of the PRDR was a significant source of biomass burning and impacted the air quality in Guangzhou under the frontal system.Watson et al. (2001) indicated that the OC/EC ratios for coal combustion, vehicle emission and biomass burning were 2.7, 1.1 and 9.0, respectively.Tao et al. (2012) indicated that the low OC/EC ratio of 1.4, could be characterized as vehicle emissions in Guangzhou.During the sampling period, the OC/EC ratio was about 4.8 (Table 1), which showed the vehicle emissions were not the primary pollutant sources.In addition, Zheng et al. (2009) indicated that SO 2 emissions are mainly distributed over the central and southern PRD areas where large power plants and industrial plants are heavily located.The estimates also displayed that roughly 50.7% of SO 2 emissions in the PRD region came from the power plant, 40.7% from industrial sources, and 8.6% from other sources.The ratios of the local system to the frontal system were 4.0 for SO 2 and 0.6 for K + .The low ratio of K + revealed the biomass burning was not the primary pollutant source under the local system.However, the high ratio of SO 2 exhibited that the air pollution in Guangzhou was related to the emissions of the large point sources, including power plants and industrial plants in PRD region.

Ionic Charge Balance
The ion balance expressed by the equivalent ratio of cation sum to anion sum was calculated from measurements of every soluble ion species.Table 2 showed that the mean equivalent ratios of cation sum to anion sum were 0.93 (correlation coefficient: 0.96) and 0.88 (correlation coefficient: 0.95) between the frontal system and the local system respectively.It was indicated that the aerosol was acidic in Guangzhou.The excess anions in the charge balance, high amount of sulfate and nitrate were not balanced by ammonium and another cation, was expected since H + was not measured.In the local system, the more negative deviation in the charge balance could be possibly explained that more fresh sulfuric acid and/or nitric acid could be absorbed on the surface of deliquescent particles in the humid and stagnant atmosphere.

Sources Discussion of the Particulate Sulfate
In Guangzhou, soluble sulfate and ammonium are the dominant anion and cation respectively in PM 2.5 .Sulfur dioxide, the largest contributor among the sulfur-containing precursors, can be oxidized to H 2 SO 4 by atmospheric reactions, followed by condensation or nucleation of H 2 SO 4 both onto pre-existing particles and into new particles.
Particulate NH 4 + found in the atmosphere originates from NH 3 by the neutralization between ammonia and acidic species.The major sources for NH 3 included animal farming, fertilizers and organic decomposition.In general, sulfate and ammonium are the most common components of secondary particles in the atmosphere.In order to determine the degrees of atmospheric conversion of SO 2 to SO 4 2-, the sulfur oxidation ratio (SOR) was employed and defined as follows: where nss-SO 4 2- [S] and SO 2 [S] are the non sea-salt SO 4 2-and gas-phase SO 2 concentration as sulfur, μg/m 3 , respectively.Table 2 showed the variations of SOR between the frontal system and the local system.Compared with the previous studies (Lin, 2002;Wang et al., 2006;Li et al., 2009;Tan et al., 2009) , 1998-Apr., 19990.25 Lin et al. (2002) the frontal system suggested that the photochemical oxidation would have occurred and more secondary aerosols could exist, followed by the formation of (NH 4 ) 2 SO 4 for rapid reaction with H 2 SO 4 and gas-phase NH 3 , before the air mass reached Guangzhou urban.In the local system, the SOR (0.03) was clearly lower than other measurements in Table 3 and the median correlation between particulate SO 4 2-and NH 4 + with correlation coefficient of 0.52 was observed.However, the relationship between SO 4 2-and EC was linear with correlation coefficient of 0.99.Poppe et al. (1992) indicated that NO x is an indirect contributor to new-particle formation rate because it affects the amount of OH available for the SO 2 → H 2 SO 4 gas-phase reaction.At high NO x levels (> 30 ppb), the reaction of NO 2 with OH is important, and OH may decrease with increasing NO x .For high levels of NO x in Guangzhou, the oxidized reaction of SO 2 by OH to form H 2 SO 4 in the gas phase might be inhibited, which decrease the sulfur oxidation ratio.However, the particulate SO 4 2also can be formed by the heterogeneous reaction.SO 2 is dissolved on the moist surface of pre-existing particles and oxidized to SO 4 2-by aqueous ozone (Zhou et al., 2012).Under the prevailing southeasterly wind, the emissions of primary ash particles by coal-fired power plants without effective particulate controls could provide a large amount of surface area.Humid sea wind increases the moisture of the particulate surface.High correlation coefficient between SO 4 2-and elemental carbon could be explained by the heterogeneous reaction of sulfate on soot particles.
During the observed period, the atmospheric conditions could be classified as the frontal system and the local system depending on prevailing northerly wind and southeasterly wind respectively.The concentration ratios of the local system to the frontal system were 1.7 for CO, 4.0 for SO 2 , 1.2 for PM 2.5 , 1.0 for Na + , 1.1 for NH 4 + , 0.6 for K + , 1.7 for Cl -, 2.9 for NO 3 -, and 1.0 for SO 4 2-.The concentrations of PM 2.5 , Cl -, NO 3 -, NH 4 + , CO and SO 2 were higher in the local system than those in the frontal system.It indicated that the air pollution in Guangzhou was more aggravated under the local system due to accumulating the pollutants around the PRDR in stagnant and humid atmosphere.
In order to identify the characters and sources of the aerosols, the correlation coefficients and ratios between pollutant species were investigated.In the frontal system, biomass burning was the significant emission source and the great amount of SO 4 2-and NH 4 + particulates were observed in the aged secondary particulates from the longrange transport.In the local system, more acidic ambient particles with more negative deviation in the ionic charge balance were observed under the prevailing southeasterly wind.The possible reason is that more fresh sulfuric acid and/or nitric acid could be absorbed on the surface of deliquescent particles in the humid and stagnant atmosphere.The degree of conversion of SO 2 to sulfate was low and the correlation between EC and SO 4 2-was better than the correlation between NH 4 + and SO 4 2-in the local system due to the heterogeneous reaction of sulfate on soot particles.Great amount of SO 2 would be further formed to secondary sulfate particles by photochemical reactions during transport to the inner areas, which should cause more serious air pollutions in the southern China.Overall, the sources of ambient particles in this work were identified as traffic exhaust, coal combustion, secondary aerosol, sea salt, and crustal aerosol, which were similar to the results of previous studies in PRDR (Wang et al., 2006;Tao et al., 2012).
To reduce the PM 2.5 concentration in PRDR, the pollution control strategies should been implemented to reduce primary particle exhausts, SO 2 and NO x emissions, secondary aerosol formation, and biomass burning.The vehicle emission is the major source of NO x in the city.Guangzhou is likely to continue its high rate of economic growth, and NO x reduction will thus be very challenging for the city in the future.At high NO x concentrations, reducing the emissions of sulfate aerosol precursors should be combined with reductions in the emission of elemental carbon.

Fig. 1 .
Fig. 1.Schematic diagram of the TEOM and the in-situ IC system.

Table 2 .
in Table3, the higher value of SOR (0.The ratio and correlation coefficients (r) between the frontal system and the local system for major soluble species, SO 2 , and EC.

Table 3 .
SOR in PM 2.5 at different sites.