Inorganic Chemical Characterization of Aerosols in Four Asian Mega-Cities

This study is a systematic field observation campaign that examines the chemical characteristics of aerosols in four different mega-cities in Asia, namely Beijing (China), Hanoi (Vietnam), Kolkata (India), and Tokyo (Japan). The unity of the analytical method and the synchronicity of the sampling periods are quite important, especially when developing a comparative risk assessment in different places. Sampling was thus carried out in each city continuously for a one-year period between 2008 and 2010, as this enabled consideration of the seasonal variations that are associated with the Asian monsoon system that governs the climate of this region. The study found that the sum of the concentrations of measured elements decreased in the order of BJ > KK > HA > TY, while the sum of the concentrations of measured ions decreased in the order of BJ > HA > KK > TY. The concentration level of chemical species in aerosols in Beijing was one order of magnitude higher than that in Tokyo. The risks associated with carcinogenic metals in the four cities have also been calculated. We conclude that the calculated carcinogenic risks to humans by chromium were higher than the risks caused by nickel in all four cities.


INTRODUCTION
Many mega-cities around the world are adversely affected by air pollutants such as aerosols and hazardous chemical species.From an air quality perspective, the adverse health effects caused by aerosols are the biggest driver of policies to improve air quality, due to the large amount of epidemiological health effect evidence (Monks et al., 2009).Many Asian cities have undergone rapid development in recent years.It is necessary to investigate the trend towards higher concentrations of aerosols that is associated with the rapid development of these cities (Okuda et al., 2008).Long-term observations are very useful for elucidating how the air quality has changed during the rapid development of the target city (Okuda et al., 2008(Okuda et al., , 2011)).Alternatively, comparative studies using the same method in some different cities are also important for improving our understanding of the chemical characteristics of air quality in different cities.Every mega-city has unique aerosols pollution sources that are determined by their geography and cultural practices (Parrish et al., 2009).The Asian region is one of the most diverse areas in the world; therefore, it is important to understand the pollution characteristics that are associated with each city individually.Comparative studies of cities, such as this one, should be conducted during the same time period, while the cities being assessed are experiencing the same period of rapid development.Most previous studies have considered only a few cities or sites simultaneously.
The unity of the analytical method used and the synchronicity of the sampling periods are especially important when a comparative risk assessment is carried out in different places.Uncertainties that are caused by differences in methodology or sampling period should be avoided as much as possible.This study is a systematic field observation campaign for elucidating the chemical characteristics of aerosols in four different mega-cities in Asian region.Sampling was conducted in each city continuously for one year between 2008 and 2010.This one year time period allowed the study to consider seasonal variations that are associated with the Asian monsoon system that governs the climate in this region.The risks that are associated with airborne carcinogenic metals in the four cities have been calculated.

Aerosol Collection in Four Asian Cities
Total suspended particles (TSP) were collected in four different cities in Asian region (Fig. 1).Beijing (BJ): The sampling site was Tsinghua University, located 15 km in a northwesterly direction from the center of Beijing city, China.This site can be considered a good representative of the entire area of Beijing city in terms of the concentrations of particulate matter (Okuda et al., 2004).Sampling has been done for 1 year (n = 47), from October 2, 2008 to October 1, 2009.Hanoi (HA): The sampling site was a private house, located 4 km in a southwesterly direction from the center of Hanoi city, Vietnam.Sampling has been done for 1 year (n = 42), from September 28, 2009 to September 27, 2010.Kolkata (KK): The sampling site was a private house in a center of Kolkata city, India.Sampling has been done for 1 year (n = 48), from January 1 to December 29, 2010.Tokyo (TY): The sampling site was the University of Tokyo, located in a center of Tokyo metropolitan area, Japan.Sampling has been done for 1 year (n = 46), from January 5 to December 27, 2010.The sampling periods for HA, KK, and TY were almost the same, whereas that for BJ was earlier than the other sites.This was due to a mechanical trouble happened at BJ site.According to Beijing Municipal Environmental Protection Bureau (2010), the mean concentration of PM 10 in 2010 was almost the same as that in 2009.Therefore we considered the sampling periods for our four sites in this study were almost simultaneous.Populations of these cities in 2005 were, BJ: 12.4, HA: 2.8, KK: 15.6, and TY: 36.7 (in million, United Nations, 2010).Quartz fiber filters (QFF, Pallflex 2500QAT-UP) were used for collecting aerosol samples.High volume air samplers (Kimoto Model-120 for BJ; SIBATA HV-1000F for HA, KK, and TY) were operated for 24 h at an air flow rate of 800 L/min.We cut a 47-mm i.d.part of the QFF filter, and then the samples were subjected to the following analytical procedure.

Sample Analysis
Filter samples were analyzed by an energy-dispersive Xray fluorescence spectrometry (EDXRF) without any pretreatment using the EDXL300 spectrometer manufactured by Rigaku Corp., Japan.For emitting primary radiation, an X-ray tube (I max = 2 mA, V max = 50 kV) with a 50W Pd anode was used.EDXL300 has three-dimensional (Cartesian geometry) polarization optics and secondary targets that allow the researchers to optimize the excitation source for analytes of interest.In this study, we used three secondary targets.The secondary targets and duration time were, Mo: 400 s, Cu: 400 s, and RX9 (graphite crystal): 100 s.The quantification of each element in aerosol samples was performed using the fundamental parameter (FP) method called Rigaku Profile Fitting -Spectra Quant X (RPF-SQX).Thirteen elements (Al, S, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, and Pb) were measured in this study.Detection limits for each element were 1.7-28 ng/m 3 , and repeatability (precision, as described as relative standard deviation (RSD)) was smaller than 5% in most elements except for V (28%) and Cr (12%).The analytical results obtained by EDXRF agreed well with those obtained by ICP-MS (EDXRF/ICP-MS ratios for each element were 1.1 ± 0.3).Instrument calibration was performed daily using a Herzog glass pellet with known elemental composition.In order to check of the instrument condition, NIST SRM2783 (Air Particulate on Filter Media) was analyzed daily.Detailed procedure for this multi-elemental analysis has been described elsewhere (Okuda et al., submitted).

Meteorological Data
Meteorological data for each sampling site were obtained from the National Climatic Data Center, National Environmental Satellite, Data, and Information Service, National Oceanic and Atmospheric Administration (NCDC, 2011).Since data for Hanoi was not available on the NCDC system, we used alternative data for Nanning, where the distance from Hanoi was about 300 km.In this study, we defined two periods, winter/dry season (from November 15 to March 15) and summer/rain season (from May 15 to September 15), for considering the seasonal variation of concentrations of measured species due to the following reasons; (1) in Beijing, Coal combustion for residential heating is permitted from November 15 to March 15, (2) the differences in average temperature between the two seasons for Beijing and Tokyo were as large as 25°C and 18°C, respectively, and (3) the total rainfall amounts in the summer/rain season were 12-, 5-, and 23-times higher than those in the winter/dry season for Beijing, Hanoi, and Kolkata, respectively.

One-year Average Concentration of Each Component in Aerosols
Table 1 shows the concentrations of elements and watersoluble ionic species in aerosols in four Asian cities.The sum of the concentrations of measured elements were in the order of BJ > KK > HA > TY.The sum of the concentrations of measured ions were in the order of BJ > HA > KK > TY.The concentration level of chemical species in aerosol in Beijing was about one order of magnitude higher than that in Tokyo.This situation is similar to the pollution level of annual PM 10 in these cities (BJ: 121 μg/m 3 , HA: 89 μg/m 3 , KK: 148 μg/m 3 , and TY: 23 μg/m 3 , Hien et al., 2011;WHO, 2011).Although Tokyo is the most populated mega-city among target cities in this study, Tokyo has the best air quality in terms of the aerosol pollution.
Chemical characteristics of aerosols have been discussed using the enrichment factor, which was the ratio of chemical concentration of an element in aerosol to that in the average crustal rock with Fe as the reference element.Concentration levels of elements obtained in this study were also compared to several previous studies.In Beijing, all elements showed low EF (1.0-2.7)except for S, Cu, Zn, and Pb.This feature is similar to previous studies (Lu et al., 2007, Schleicher et al., 2011).EFs of all elements for Hanoi and Kolkata were less than 10 except for S, Zn, and Pb.Hopke et al. (2008) reported the elemental composition of PM 10 in Hanoi that was similar to this study.As far as we know, this study is the first report that shows comprehensive chemical composition of aerosols in Kolkata.EF for Tokyo showed similar pattern comparing to the other three cities, but several elements showed slightly higher EFs.The EF pattern in Tokyo in this study was similar to a previous study (Furuta et al., 2005).Overall, elemental compositions in aerosols as described using EF for these four cities were basically similar although the concentration level for each city was quite different.
Ionic balances that are defined as the ratio of total cations to anions in aerosols for each city were shown in Fig. 2. The ionic balances were cation-rich for Beijing, Hanoi, and Tokyo whereas it was almost neutral for Kolkata.Ca 2+ , which was provided as soil particle that contains calcium carbonate, in Kolkata was lower than those in Beijing and Hanoi.In Tokyo, sulfate concentration was low as well as Ca 2+ .These results suggested that the neutralization potential of aerosols against acidic chemical species in air in Kolkata was lower than the other three cities.

Correlations among the Chemical Species in Aerosols
The time trend of meteorological data and concentrations of selected chemical species were shown in Fig. 3. Correlation matrices for each component were also shown in Table 2.In Beijing, Al, K, Ti, V, Mn, and Fe showed higher correlation (r > 0.79) each other.These elements are considered having soil or crustal origin.On the other hand, Pb, NH 4 + , K + , NO 3 -, and SO 4 2-showed higher correlation (r> 0.75) each other.Pb was also highly correlated with Zn (r = 0.84).These elements are considered having anthropogenic origin.
Hanoi aerosols had a similar feature of the correlation matrix when compared to Beijing.Al, K, Ti, and Fe showed higher correlation (r > 0.81) each other in Hanoi.Ca also showed higher correlation with these four elements (r > 0.72).Pb was highly correlated with Zn (r = 0.79).NH 4 + , K + , and SO 4 2-showed higher correlation (r > 0.75) each other.The correlations were high between Ca 2+ and NO 3 -, (r = 0.87), and between NH 4 + and SO 4 2-(r = 0.81).This was different from Beijing case where NH 4 + had high correlation with both NO 3 -and SO 4 2-but Ca 2+ didn't.It seems that very high concentration of ammonia easily neutralized nitric acid in Beijing whereas alkaline particles that have calcium in   it played an important role in neutralizing nitric acid in Hanoi.
The correlation matrix for Kolkata was quite different from the other three cities.Almost all of the measured species showed higher correlation (r > 0.5) each other except for Na + .Chemical species in Kolkata showed a clear seasonal trend (Fig. 3).Obviously, the concentrations were high in the dry season and low in the rain season.
K, Ca, Ti, Mn, and Fe showed higher correlation (r > 0.74) each other in Tokyo.The correlations were high among Cu, Zn and Pb (0.69 < r < 0.78).The correlation was high (r = 0.77) between V and Ni that are often used as tracers for oil combustion (Okuda et al., 2007a, Wang et al., 2006).It should be noted that the correlation was low (r = 0.422) between NO 3 -and SO 4 2- , and this was quite different from the other three cities.The concentration of NO 3 -was high in winter in Tokyo since it would be dependent on ambient temperature that controlled its gas/particle partitioning.On the contrary, the concentration of was high in summer in Tokyo since it would be dependent on the intensity of solar radiation that would helped converting SO 2 to SO 4 2-.In Hanoi and Kolkata, the gas/particle partitioning of NO 3 would not be so important since the ambient temperature was not so low through a year.In Beijing, enormous amount of SO 2 input derived from coal combustion in winter could lead the high concentrations of SO 4 2-in winter.These are possible reasons why the correlation between NO 3 -and SO 4 2-in Tokyo was different from the other three cities.

Seasonal Variation of the Chemical Species in Aerosols
Correlations among each component for each season at each site were calculated.In most cases, the correlation matrices for each season were similar to those for all data at each site.According to this result, the major sources governing the concentrations of inorganic chemical species in each Asian city studied would be generally constant through a year.
The concentrations of measured species in the winter/dry season and summer/rain season were shown in Table 3.The winter/summer ratios for each component ranged 0.7-2.0 in Beijing except for Na + and Cl -.This means that the seasonal variation in the concentrations of chemical species in aerosols was not so large.The dry/rain ratios were 0.9-1.9 in Hanoi except for S, NH 4 + , K + , NO 3 -and SO 4 2- . One of the reasons of this seasonal variation in Hanoi would be biomass burnings around Hanoi city.The dry/rain ratios were 2.7-17.1 in Kolkata except for Na + .This is probably due to high concentrations of pollutants in the dry season because almost no wet scavenging of aerosols was expected during the dry season in Kolkata city.Additionally, high dry/rain ratios for Zn and Pb (9.7 and 6.3) suggest that more anthropogenic inputs of particulate pollutants into the air should be considered during the dry season in Kolkata.The winter/summer ratios vary from 0.3 to 1.8 in Tokyo.The minimum winter/summer ratio was shown for V.One of the possible reasons of this is due to increase of the use of oil fuel for power generation around Tokyo metropolitan area in summer season (Okuda et al., 2007b).These data could also serve as the basis for epidemiological studies if appropriate health effects data are also available.

Calculation of the Carcinogenic Risks to Humans Associated with Cr and Ni
Calculation of the carcinogenic risks to humans associated with carcinogenic metals has been done using our dataset presented in this study.We focused on Cr and Ni for this calculation because these two metals are classified into Group 1 (carcinogenic to humans) by the International Agency for Research on Cancer (IARC, 2012).The concentrations corresponding to an excess lifetime risk of 10 -5 have been estimated as 0.25 ng/m 3 for hexavalent Cr and 25 ng/m 3 for Ni (WHO, 2000).We calculated the ratio of the observed value to the concentrations corresponding to an excess lifetime risk of 10 -5 for Cr and Ni in each city.The results are shown in Table 4.Note that we measured not hexavalent Cr but total Cr.Previous studies have reported that the ratio of hexavalent Cr to total Cr varied from 2.6% to 50% (Świetlik et al., 2011l;Tirez et al., 2011).When we assume the ratio of hexavalent Cr to total Cr was 2%, the ratio of the observed value to the concentrations corresponding to an excess lifetime risk of 10 -5 for Cr would be 2.2 ± 1.0 for BJ, 0.7 ± 0.3 for HA, 2.5 ± 1.6 for KK, and 0.8 ± 0.6 for TY, respectively.These ratios are still higher than those for Ni.Besides, the higher ratio of hexavalent Cr to total Cr would cause much higher potential of carcinogenic risk.We conclude the following two points: (1) the calculated carcinogenic risks to humans by Cr were higher than those by Ni in wide area of Asian cities, and (2) chemical speciation that allows us to estimate the risks caused by Cr much precisely is needed for further study.

SUMMARY
This study is a systematic field observation campaign for elucidating the chemical characteristics of aerosols in four different mega-cities in Asian region, Beijing, Hanoi, Kolkata, and Tokyo.The concentration level of chemical species in aerosol in Beijing was the highest among four Asian cities.Chemical features of aerosols in the four cities have been characterized in this study.The calculated carcinogenic risks to humans by Cr were higher than those by Ni in wide area of Asian cities.Chemical speciation that allows us to estimate the risks caused by Cr much precisely is needed for further study.1.1 ± 0.4 0.5 ± 0.2 0.7 ± 0.4 0.4 ± 0.1 a The concentrations corresponding to an excess lifetime risk of 10 -5 are estimated as 0.25 ng/m 3 for hexavalent chromium and 25 ng/m 3 for nickel (WHO, 2000).b Assuming that the observed Cr is in the form of hexavalent chromium.

Fig. 1 .
Fig. 1.The maps showing four Asian cities being investigated in this study.

Fig. 2 .
Fig. 2. The concentrations of total cations and anions in aerosols for each Asian city.

Table 1 .
The concentrations of elements and water-soluble ionic species in aerosols in four Asian cities.
Mason and Moore, 1982.n of crust was cited fromMason and Moore, 1982.

Table 2a .
Correlation matrix for chemical species in Beijing.

Table 2b .
Correlation matrix for chemical species in Hanoi.

Table 2c .
Correlation matrix for chemical species in Kolkata.

Table 2d .
Correlation matrix for chemical species in Tokyo.

Table 4 .
The ratio of the observed value to the concentrations corresponding to an excess lifetime risk of 10 -5 for Cr and Ni a in aerosols in four Asian cities.