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Characterization of dicarboxylic acids in PM2.5 in Hong Kong
ARTICLE IN PRESS AE International – Asia Atmospheric Environment 38 (2004) 963–970
Characterization of dicarboxylic acids in PM2.5 in Hong Kong Xiaohong Yaoa, Ming Fanga, Chak K. Chanb,*, K.F. Hoc, S.C. Leec a
Institute for Environment and Sustainable Development, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong b Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong c Department of Civil and Structural Engineering, Research Center for Urban Environmental Technology and Management, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Received 28 June 2003; received in revised form 13 October 2003; accepted 31 October 2003
Abstract Dicarboxylic acids in atmospheric aerosols have received much attention because of their potential roles in affecting the global climate. The composition and the sources of dicarboxylic acids in PM2.5 were studied at one remote and two urban sites in Hong Kong in the winter of 2000 and in the summer of 2001. Oxalate was the dominant dicarboxylic acid in all samples. The winter oxalate concentrations were high and spatially uniform, with an average value of 0.36 mg m3, but the summer oxalate concentrations were low and had a large spatial variation. The influence of meteorological factors on the concentrations of dicarboxylic acids was also studied. The ratio of malonate to succinate was used to distinguish primary sources from secondary sources of these acids. This ratio at all three sites was close to that from direct vehicular exhaust in the winter, but it was close to that of secondary reactions in the summer. Hence, the acids were attributed to vehicular emissions in the winter and secondary sources in the summer. This hypothesis is also supported by a good correlation of oxalate with sulfate in the summer but a poor one in the winter. The correlations of oxalate with malonate, succinate, sulfate and K+ were also studied in terms of the routes of secondary formation of these dicarboxylic acids. r 2003 Elsevier Ltd. All rights reserved. Keywords: Oxalic acid; Malonic acid; Succinic acid; PM2.5; Secondary water-soluble organic compounds
1. Introduction Dicarboxylic acids in particulate pollutants can play important roles in the atmosphere. For example, they can act as cloud condensation nuclei (CCN) and also reduce the surface tension of particles to form CCN (Facchini et al., 1999; Kerminen, 2001). The dominant dicarboxylic acids in atmospheric particles are oxalic acid (C2), followed by malonic acid (C3) and succinic acid (C4) (Kawamura and Kaplan, 1987; Kawamura and Ikushima, 1993; Saxena et al., 1995; Yao et al., 2002). The concentration ratios of these acids in *Corresponding author. Tel.: +852-2358-7124; fax: +852235-80054. E-mail address: [email protected] (C.K. Chan).
atmospheric particles, in particular the C3/C4 mass ratio, are useful to understanding their importance in the atmosphere. The C3/C4 ratio has been reported to be 0.3–0.5 from vehicular emissions (Kawamura and Kaplan, 1987). Relatively low C3/C4 ratios have been found to be associated with the overwhelming contributions from vehicular exhaust to these acids in some studies, e.g., in downtown and west Los Angeles, in winters in Tokyo, and in Nanjing, China (Kawamura and Kaplan, 1987; Kawamura and Ikushima, 1993; Wang et al., 2002). On the other hand, the mass ratio of C3/C4 in secondary atmospheric particles is much larger than unity (Kawamura and Ikushima, 1993; Kawamura et al., 1996; Kawamura and Sakaguchi, 1999; Yao et al., 2002). For example, Kawamura and Ikushima (1993) reported a maximum mass ratio of 3 in the summer in
1352-2310/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2003.10.048
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Tokyo. They found ratios larger than unity concurrent with elevated concentrations of oxidants and attributed the source of dicarboxylic acids to secondary atmospheric reactions. Kawamura and Sakaguchi (1999) observed a mass ratio of 3 in the Pacific Ocean, where dicarboxylic acids are expected to originate from secondary reactions. Hence, the ratio of C3/C4 in atmospheric particles is a useful indictor to differentiate primary (vehicular) sources from secondary sources. Biogenic contributions of dicarboxylic acids are common in non-urban areas. They can also be important in some urban areas, in addition to primary vehicular emissions and secondary sources (Kawamura and Kaplan, 1987; Dutton and Evans, 1996; Yao et al., 2003). The metabolic processes of fungi are the major route of the biogenic production of oxalic acid through the hydrolysis of oxaloacetate from citric acid and glyoxylate (Dutton and Evans, 1996). However, these processes do not produce malonic acid or succinic acid. Overall, fungi are expected to make a minor contribution to oxalic acid in PM2.5 because fungi principally exist in the coarse particle size (Dutton and Evans, 1996; Bauer et al., 2002; Yao et al., 2003). Hong Kong is located along the coast of the South China Sea and is connected to the Pearl River Delta of Guangdong Province in China. Hong Kong is a warm, humid and cloudy coastal city. It can be considered as a typical Asian temperate coastal metropolitan area. According to the Hong Kong Observatory, the annual average temperature, relative humidity and the percentage of cloud coverage was 24 C, 78% and 69%, respectively, in 2001. The coldest month was January (17.3 C), with the corresponding average relative humidity of 78% and a percentage of cloud coverage of 68%. The hottest month was August (29.2 C), with a corresponding average relative humidity of 80% and a percentage of cloud coverage of 70%. Moreover, the amount of the rainfall in the summer is about 10 times of that in the winter in Hong Kong, resulting in a decrease in the concentration of air pollutants through wet deposition in the summer. In the last two decades, most industrial factories have moved out of Hong Kong to the Pearl River Delta. The Hong Kong Environmental Protection Department reported that local vehicular and power plant emissions account for 43.8% and 29.9% of the total emission of respirable suspended particulate (RSP) in Hong Kong (Environmental Protection Department, 2002). Local air pollutants in Hong Kong therefore principally originate from vehicular emissions and power generation. High concentrations of RSP usually are observed in the winter because of the frequent development of stagnating highpressure systems and inversion layers in the lower atmosphere (Environmental Protection Department, 2002). However, higher air pollutant emissions are expected in the summer because of an increase in the
energy consumption for air conditioning. For example, the electricity sales of CLP Holdings Limited, one of major electricity vendors in Hong Kong, in June to September of 2001 were 1.4 times those in January to March of the same year (www.clpgroup.com). Yao et al. (2002) reported that the C3/C4 mass ratios from a suburban site and two urban sites in Hong Kong were generally larger than unity, suggesting that the primary vehicle emissions were not the major source of dicarboxylic acids in the atmospheric particles at these sites. Instead, secondary sources, such as in-cloud processes, were found to be a major route of formation of dicarboxylic acids, based on the similarity of the size distributions of these dicarboxylic acids and sulfate. The urban measurements reported by Yao et al. (2002) were made at sites 20–25 m above the ground level and not close to the heavy traffic, which may explain the lower contribution of primary vehicular emissions to dicarboxylic acids than when measured at the ground level close to the heavy traffic. Seasonal variations in the concentrations of particulate dicarboxylic acids were not studied in Yao et al. (2002) because of the small size of their samples and because most of their samples were collected in summer. However, seasonal variations in these acids would be helpful in understanding their origins. Ho et al. (2003) recently examined the chemical characterizations of PM2.5 and PM10 at three different sites in Hong Kong: Hong Kong Polytechnic University (HKPU), Kwun Tong (KT) and Hok Tsui (HT). HKPU and KT are urban sites and close to the heavy traffic while HT is a remote ‘‘background’’ site. Ho et al. (2003) found that the ratio of organic carbon to elemental carbon was higher at HT than at HKPU and KT. The organic carbon to elemental carbon ratio in winter was higher than that in summer. Gas-aerosol equilibrium, favoring the partitioning of semi-volatile organic species in the particulate phase under the lower temperatures in the winter, may be an explanation for the observed seasonal differences. The elevated ratio of organic carbon to elemental carbon at HT could be due to a number of possible factors, including a significant secondary source of organic carbon, a lower ambient temperature and a higher biological emission flux at HT. We examine the contribution of secondary chemical reactions to organic acids at HT using the C3/C4 mass ratio. In this paper, we first present the characteristics of oxalate, malonate and succinate in PM2.5 in the winter and in the summer in terms of the relative contributions from the primary emissions and secondary sources in Hong Kong. The impact of meteorological factors on the concentrations and the composition of these dicarboxylic acids are then discussed. Finally, the routes of the secondary formation of particulate dicarboxylic acids in Hong Kong are examined.
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2. Experimental methods Detailed information about the sampling experiments has been described by Ho et al. (2003). The site at HKPU is 6 m above ground level and is heavily influenced by vehicular emissions. KT is a mixed residential/commercial/industrial area and the sampling site is 25 m above ground level. This site is also influenced by heavy traffic. HT is located on the southeast side of Hong Kong Island and has been considered to be a background station in HK (Ho et al., 2003). It is approximately 15 km away from major human activities and heavy traffic. Altogether, 70 sets of 24 h PM2.5 and 46 sets of companion PM10 samples were collected from 23 November 2000 to 1 March 2001, and from 27 June to 9 August 2001 at the three sites. Collocated samplings of PM2.5 and PM10 were carried out using the Graseby–Anderson high-volume PM2.5 and PM10 samplers, respectively. Whatman quartz microfibre filters were pre-heated at 900 C for 24 h before use. All sampling was conducted by HKPU. A quarter of each filter was sent to the Hong Kong University of Science and Technology for ionic analysis. The ionic concentrations of the aqueous extracts were determined by ion chromatography (Dionex LC20) with an electrochemical detector (ED 40). An AS11 column (4 mm) with an AG11 guard column and an Anion Trap column (4 mm) was used for water-soluble organic anion detection. This method to determine dicarboxylic acids in water extracts of atmospheric particles was firstly developed by Jaffrezo et al. (1998). Recently, it has been widely used by a number of researchers (Kerminen et al., . 2000; Rohrl and Lammel, 2001; Yao et al., 2002; Wang et al., 2002). A CS12 column and an eluent of 20 mM MSA were used for cation detection. The detection limits in mg/ml were 0.01 for oxalate, 0.02 for malonate and succinate, 0.004 for SO2 and 0.01 for NO 4 3 and 0.004 for Cl in aqueous extracts. Overall, the uncertainties in the measurements of ambient concentra-
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tions were 710% for oxalate, 715% for malonate and succinate, 7%5 for sulfate and 76% for nitrate. Parts of the filters were digested using a microwave extraction system for element analysis. Major crustal elements (Al, Ca, Fe and Mg, etc.) were analyzed by ICP-AES (Perkin-Elmer, OPTIMA 3300DV) and trace metals (As, Cr and Cu, etc.) were analyzed by ICP-MS (HP4500). The element analysis was conducted at HKPU.
3. Results and discussion 3.1. Concentrations of dicarboxylic acids in PM2.5 The average concentrations of the dicarboxylic acids in PM2.5 at the three sites are listed in Table 1. The samples collected from 23 November 2000 to 1 March 2001 and from 27 June to 9 August 2001 are defined as the winter samples and the summer samples, respectively. Oxalate was the dominant dicarboxylic acid in both seasons. In the winter, the spatial distributions were generally uniform for oxalate and succinate but not for malonate as shown in Figs. 1(a)–(c). The average concentration of oxalate at all three sites was about 0.36 mg m3. In contrast, there were larger spatial variations in the acids in the summer, with the highest average concentration of 0.17 mg m3 at KT, followed by 0.09 mg m3 at HKPU and 0.04 mg m3 at HT. However, there were four sets of samples with similar oxalate concentrations at HT and HKPU. Overall, the concentrations of dicarboxylic acids in the winter were 2–10 times those in the summer. The low mixing heights, the formation of the inversion layers, and the low wet deposition are believed to be the major factors for the accumulation of air pollutants (Environmental Protection Department, 2002; Pathak et al., 2003) in the winter in Hong Kong. In this study, the impact of the mixing height on the concentrations of dicarboxylic acid in PM2.5 was
Table 1 Average concentrations of dicarboxylic acids in PM2.5 Sampling period
Location
C2 (mg m3)
C3 (mg m3)
C4 (mg m3)
C3/C4
Winter
HT HKPU KT
0.3770.17 0.3570.14 0.3770.16
0.0270.01 0.0270.01 0.0370.02
0.0670.03 0.0570.03 0.0770.03
0.370.1 0.570.2 0.670.3
Summer
HT HKPU KT
0.0470.02 0.0970.06 0.1770.10
BDL 0.01370.007 0.01270.006
BDL 0.00770.008 0.01170.014
BDL 2.771.5 1.571.5
HKPU KT
0.25 0.32
0.015 0.03
0.03 0.05
0.5 0.6
Episodes 20 August 2001 11 July 2001 BDL: Below detection limit.
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(a) Oxalate
(a) Oxalate
0.6
0.4
0.2
0.0
0.0 0.3
0.4
0.5
0.6
(b) Malonate
-3
60 40 20 0 0
0.12
20
40 -3 x10
60
80
(c) Succinate
80
(b) Malonate
60 -3
0.2
x10
80
0.1
Concentration (µg m-3)
0.0
x10
Concentrations of dicarboxylic acids at KT and HT (µg m-3)
0.4 0.2
40 20 0
0.12
0.08
(c) Succinate
0.08
0.04
0.04
0.00 0.00
0.04
0.08
0.12
0.00 600
800
1000
1200
1400
Concentrations of dicarboxylic acids at HKPU (µg m-3) Fig. 1. Comparisons of the concentrations of dicarboxylic acids. +: in winter at KT; W: in winter at HT; : in summer at KT; X: in summer at HT.
investigated. The daily average of the mixing height was calculated from the meteorological conditions at 0000 and 1200 UTC (Pathak et al., 2003). The mixing height varied from 580 to 1110 m (on average 810 m) in the winter and from 890 to 1285 m (on average 1110 m) in the summer, as shown in Figs. 2(a)–(c). The concentration of each dicarboxylic acid, in general, decreased with increasing mixing height.
3.2. The ratio of C3/C4 in PM2.5 As shown in Table 1, the winter samples have significantly lower C3/C4 mass ratios than the summer samples have. Since the concentrations of C3 and C4 were too low in HT, we focus our discussions of the ratios measured at HKPU and KT. In the winter, the samples collected at HKPU and KT had an average C3/ C4 ratio of 0.5 and 0.6, respectively. Primary vehicle exhaust was the principal source of the dicarboxylic acids in the winter at these two urban sites. Secondary formation of dicarboxylic acids was possible, leading to a slight increase of the ratio from time to time.
Mixing height (m) Fig. 2. Variations in the concentrations of dicarboxylic acids with mixing heights (+: in summer, J: in winter).
Nevertheless, these ratios were still much smaller than a ratio of 3 from secondary formation of dicarboxylic acids as reported in the literature (Kawamura and Ikushima, 1993; Kawamura and Sakaguchi, 1999). In the summer, the average C3/C4 ratio was 2.7 and 1.5 at HKPU and KT, respectively, suggesting a large contribution of secondary sources to particulate dicarboxylic acid formation. Similar seasonal changes in C3/ C4 ratios have been reported in the literature. For example, Kawamura and Ikushima (1993) observed a high C3/C4 ratio of about 3 in the summer in conjunction with elevated concentrations of oxidants but a low ratio of 0.5 in the winter in Tokyo. Kerminen et al. (2000) also observed a C3/C4 ratio higher than unity (1.1) in summer but only about 0.2 in winter in Helsinki, Finland. An air pollution episode occurred on 20 August 2001 at HKPU (PM2.5 samples were not collected at KT and HT on the same day) when the concentrations of oxalate, malonate and succinate were about 2.5 times their average concentrations and the C3/C4 ratio was 0.5. Primary vehicle exhaust was the dominant source of dicarboxylic acids during this episode. The mixing
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height was only 740 m on 20 August 2001, much lower than the summer average of 1110 m. In the other summer samples, the C3/C4 ratio was always larger than unity. At KT, there was another air pollution episode on 11 July 2001 when the C3/C4 ratio was only 0.6 while it was 1.1 at HKPU on the same day. On that day, the maximum hourly average NO2 concentration reached 100 mg m3 and the mixing height (1140 m) was close to the average value in summer, favoring dispersion of air pollutants. This episode at KT could have been due to incidental traffic congestion near the sampling site. However, the C3/C4 ratios in the other summer samples were generally higher than unity. It can be concluded that, overall, secondary formation of dicarboxylic acids in PM2.5 was generally more important in the summer, although there were occasional events when primary exhaust emission was the dominant source. Malonate and succinate concentrations were below the detection limits in HT in the summer. It is interesting to note that the C3/C4 ratio at HT in the winter was similar to that of vehicular exhaust of 0.3–0.5 (Kawamura and Kaplan, 1987). HT is a remote rural site with little local vehicular traffic. Oxalate, succinate, sulfate, Ni and V (but not malonate) showed small spatial variations (Ho et al., 2003), which could be explained by the transport of urban air pollutants to HT. However, the concentration of organic carbon and elemental carbon at HT was only half and one-fourth of that at HKPU and KT (Ho et al., 2003). Ni and V are characteristic species from oil burning. The RSP data from 11 Hong Kong Environment Protection Department air-monitoring stations in 2001 show that the highest concentrations of Ni (annual average 6.7 ng m3) and V (annual average 16.4 ng m3) were observed at Kwai Chung, where the container terminals in HK are located. The high Ni and V concentrations at HT suggest that vessel exhaust from marine traffic is an important source of pollutants at HT although the possibility of atmospheric transport from urban areas cannot be excluded.
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3.3. The partitioning of dicarboxylic acids in PM2.5 and PM10 Altogether, there are 46 data sets with data on both PM2.5 and PM10. As listed in Table 2, in both the winter and the summer, oxalate and succinate generally reside in PM2.5, but more than half of the malonate was present in the coarse particles. In general, oxalate, malonate, and succinate originating from both secondary reactions and primary vehicle exhaust were mainly present in submicron particles (Ondov and Wexler, 1998; Yao et al., 2002). However, malonic acid is a more volatile species at ambient temperatures (Peng et al., 2001; Choi and Chan, 2002; Bilde et al., 2003). Peng et al. (2001) observed the evaporation of malonic acid but did not observe similar evaporation of oxalic acid in hygroscopic measurements using an electrodynamic balance. It is possible that oxalic acid is less volatile than malonic acid in the concentrated solutions used in the single particle studies or in aerosols containing ammonium salts because of hydrogen bonding with water and with ammonium. In fact, ammonium oxalate in aqueous droplets is not volatile (Peng and Chan, 2001). The diurnal variation of the ambient relative humidity results in the variation of liquid water contents in atmospheric particles. Malonic acid can transfer from fine particles to coarse particles after a few cycles of evaporation from fine particles and condensation onto coarse ones. In fact, the preferential presence of malonic acid in coarse particles has been previously reported (Yao et al., 2002). It is interesting to note the low PM2.5 to PM10 ratio of oxalate at HT in the summer. Fungi, which reside in supermicron particles, can release oxalic acid as a result of metabolic processes. Bauer et al. (2002) estimated that fungi account for up to 9.9% of organic carbon in coarse particles. Yao et al. (2003) also found that oxalic acid is sometimes dominant in the supermicron particles in the spring in Beijing, China. Biogenic emissions of oxalic acid may be important in PM10.
Table 2 Mass ratios of dicarboxylic acids in PM2.5 to PM10 and the correlation coefficients of dicarboxylic acids in PM2.5 at HKPU and KT Date
Location
Ratio of PM2.5/PM10
Correlation coefficients
C2
C3
C4
C4:C2
C3:C2
C4:C3
Sulfate: C3
Sulfate: C4
Sulfate:C2
Winter
HT HKPU KT
0.970.1 0.970.1 0.870.2
0.770.2 0.370.1 0.670.3
0.970.1 0.770.1 0.870.2
— 0.86 0.84
— 0.76 0.56
— 0.64 0.55
— 0.64 0.33
— 0.69 0.77
— 0.77 0.91
Summer
HT HKPU KT
0.470.2 0.870.2 0.870.2
BDL 0.470.2 0.570.3
BDL BDL 0.870.3
— 0.82 0.81
— 0.48 0.47
— 0.66 0.59
— 0.33 0.54
— 0.67 0.63
— 0.62 0.88
BDL: Below detection limit.
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3.4. Correlations of malonate, succinate and sulfate with oxalate Since there are only four data sets of PM2.5 at HT in the winter and the summer, we focus our correlation analysis on the data from HKPU and KT. Figs. 3(a)–(d) show the correlations of succinate and sulfate with oxalate in PM2.5 at HKPU and KT. The correlation coefficients of malonate, succinate and sulfate with oxalate are listed in Table 2. In the winter, succinate is well correlated with oxalate with correlation coefficients (r) of 0.86 and 0.84 at HKPU and KT, respectively. Oxalate also principally originates from primary vehicle exhaust. Sulfate is not as well correlated with oxalate with a correlation coefficient of 0.77 and 0.62 at HKPU and KT, respectively. In fact, the correlation of sulfate with oxalate at higher concentrations is evidently poorer than that at lower
(a) HKPU in winter
15
0.08
10
0.04
5
0.00
0
x10
25 20 15 10 5 0
0.2
0.12
0.4
0.6
(b) HKPU in summer
12 8 4 0
0.00
0.05
0.10
0.15
0.20
0.25
16
(c) KT in winter
12
0.08
8 0.04
4
0.00
0 0.0
x10
-3
Succinate concentrations (µg m-3)
-3
0.0
50 40 30 20 10 0
0.2
0.4
0.6
Sulfate concentrations (µg m-3)
0.12
12
(d) KT in summer
8
concentrations. For example, in the samples with oxalate concentrations larger than 0.3 mg m3 at KT in the winter, the correlation coefficient of sulfate with oxalate is only 0.39. The weak correlation between sulfate and oxalate indicates that oxalate originated from sources and/or atmospheric processes different from those of sulfate. The same can be concluded for oxalate concentrations at HKPU. This conclusion is consistent with the observation that the C3/C4 ratios were close to that of direct vehicular exhaust and were evidently smaller than that from secondary sources at HKPU and KT in the winter. In the summer, sulfate is well correlated with oxalate with a correlation coefficient of 0.91 at HKPU and 0.86 at KT, suggesting that oxalate originated from processes similar to sulfate formation, i.e., from secondary formation. This is consistent with the observed C3/C4 ratios being much larger than that of direct vehicle emissions. There are two types of atmospheric reactions forming oxalic and/or malonic and succinic acids (Kawamura et al., 1996). One is the oxidation of unsaturated fatty acids, which usually occurs in marine atmospheres (Kawamura and Sakaguchi, 1999). However, since unsaturated fatty acids originating from cooking are ubiquitous in Hong Kong (Zheng et al., 2000), oxidation reactions are also possible in urban areas. In this route, succinic acid is the precursor of malonic acid and oxalic acid and therefore has a good correlation with oxalic acid (and malonic acid when evaporation is not a problem). The other is the oxidation of aromatic hydrocarbons such as benzene and toluene, which occurs predominantly in urban atmospheres (Kawamura and Ikushima, 1993). In this route, glyoxal and glyoxylic acid are the intermediates in the formation of oxalic acid but succinic and malonic acids are not produced. Recently, Warneck (2003) suggested that acetylene and ethane can react with OH radicals to form glyoxal, which then forms oxalic acid. A good correlation between succinate and oxalate in our measurements indicates that the former route involving fatty acids is more likely. Succinic acid could be a major precursor of oxalic acid in the summer in Hong Kong. The poor correlation of malonate with sulfate is possibly related to the volatility of malonic acid as discussed earlier.
4
3.5. Correlations of dicarboxylic acids with K+ 0 0.00
0.10
0.20
0.30
Oxalate concentrations (µg m-3) Fig. 3. Correlations of C4 and sulfate with C2 (+: C4; J: sulfate): (a) rðC4:C2Þ ¼ 0:86; rðsulfate:C2Þ ¼ 0:77; (b) rðC4:C2Þ ¼ 0:82; rðsulfate:C2Þ ¼ 0:91; (c) rðC4:C2Þ ¼ 0:84; rðsulfate:C2Þ ¼ 0:62; (d) rðC4:C2Þ ¼ 0:81; rðsulfate:C2Þ ¼ 0:86:
K+ is an indictor of biomass burning in atmospheric particles (Yamasoe et al., 2000; Ikegami et al., 2001) because K+ is one of essential nutrients for plants. Biomass burning can also generate dicarboxylic acids (Rogers et al., 1991; Yamasoe et al., 2000). Particles from biomass burning can also act as CCN (Kaufman and Fraser, 1997; Narukawa et al., 1999), which form
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the droplet mode of dicarboxylic acids through in-cloud processes (Yao et al., 2002). In the winter, the dicarboxylic acids were poorly correlated with K+ (ro0:3). Moreover, the average ratio of oxalate to K+ was 0.25, which is much larger than that reported for the flaming and smoldering phases in burning plumes (0.030.1) (Yamasoe et al., 2000). Hence, we conclude that biomass burning is not likely the major direct source of oxalate in PM2.5 in the winter. In the summer, a good correlation of oxalate with K+, a moderate correlation of succinate with K+ (ro0:7), and a poor correlation of malonate with K+ (ro0:3) were found in both the HKPU and KT samples. The linear regression equations of oxalate with K+ are:
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nated from vehicular emission. However, the ratio was close to the reported value for secondary reactions in the summer, suggesting the dominance of secondary sources. This hypothesis is also supported by a good correlation of oxalate with sulfate in the summer but a poor one in the winter. In the summer, a good correlation of succinate with oxalate indicates that succinate could be a major precursor of oxalate in Hong Kong. A good correlation of oxalate with K+ and oxalate and sulfate can be explained by in-cloud formation of oxalate and sulfate from CCN derived from biomass burning. The role of particles from biomass burning in the cloud formation and subsequent formation of the secondary organics in Hong Kong requires further investigation.
½oxalate ¼ 0:85½Kþ ðr ¼ 0:90 at HKPUÞ; ½oxalate ¼ 0:90½Kþ þ 0:04 ðr ¼ 0:81 at KTÞ: Similar to the observations in the winter, the mass ratio of oxalate to K+ was much larger than that reported for biomass burning and hence direct biomass burning was not an important source for oxalate. However, 80–100% of biomass burning particles can act as CCN (Rogers et al., 1991). Hence, the in-cloud formation of oxalate is possible, based on the good correlation between oxalate and sulfate in PM2.5 as discussed earlier (Yao et al., 2003). This proposed in-cloud pathway can explain both the observation of a good correlation of sulfate with oxalate and a good correlation of oxalate with K+ and is worthy of further investigation. The poor correlation of malonate with K+ is also likely related to the volatility of malonic acid as discussed above.
4. Conclusion Seventy samples of 24-h measurements of PM2.5 were analyzed to study the composition of dicarboxylic acids at three different sampling sites in the winter of 2000 and in the summer of 2001 in Hong Kong. Oxalate was the dominant species of the dicarboxylic acids in all samples. In the winter, the oxalate and succinate concentrations were spatially uniform but the malonate concentration was not. In the summer, there were large spatial variations and the highest concentration appeared at KT. The concentrations of dicarboxylic acids in the winter were 2–10 times those in the summer and the elevated concentrations in winter were attributed to the low mixing height. The ratio of malonate to succinate was used to distinguish the primary and secondary sources of these acids in the summer and in the winter. The ratio of malonate to succinate at all three sites in the winter was close to the ratio given for direct vehicular exhaust in the literature, indicating that they overwhelmingly origi-
Acknowledgements Financial support from the Research Grants Council Earmarked Research Grants of the Hong Kong Special Administrative Region, China (Project no. HKUST9071/99P) and HKUST Emerging High Impact Area Fund (HIA98/99. ATC01) is gratefully acknowledged.
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Peng, C., Chan, C.K., 2001. The water cycles of water soluble organic salts of atmospheric importance. Atmospheric Environment 35, 1183–1192. Peng, C., Chan, M.N., Chan, C.K., 2001. The hygroscopic properties of dicarboxylic and multifunctional acids: measurements and UNIFAC predictions. Environmental Science and Technology 35, 4495–4501. Rogers, C.F., Hudson, G.J., Zielinska, B., Tanner, R.L., Hallett, J., Watson, J.G., 1991. Global Biomass Burning: Atmospheric, Climatic and Biopheric Implications. MIT press, Cambridge, MA, pp. 431–438. . Rohrl, A., Lammel, G., 2001. Low-molecular weight dicarboxylic acids and glyoxylic acid: seasonal and air mass characteristics. Environmental Science and Technology 35, 95–101. Saxena, P., Hildemann, L.M., McMurry, P.H., Seinfeld, J.H., 1995. Organic alter hygroscopic behavior of atmospheric particles. Journal of Geophysical Research 100, 18755–18770. Wang, G., Niu, S., Liu, C., Wang, L., 2002. Identification of dicarboxylic acids and aldehydes of PM10 and PM2.5 aerosols in Nanjing, China. Atmospheric Environment 36, 1941–1950. Warneck, P., 2003. In-cloud chemistry opens pathway to the formation of oxalic acid in the marine atmosphere. Atmospheric Environment 37, 2423–2427. Yamasoe, M.A., Artaxo, P., Miguel, A.H., Allen, A.G., 2000. Chemical composition of aerosol particles from direct emissions of vegetation fires in the Amazon Basin: watersoluble species and trace elements. Atmospheric Environment 34, 1641–1653. Yao, X., Fang, M., Chan, C.K., 2002. Size distributions and formation of dicarboxylic acids in atmospheric particles. Atmospheric Environment 36, 2099–2107. Yao, X., Fang, M., Chan, C.K., Hu, M., 2003. Formation and size distribution characteristics of ionic species in atmospheric particulate matter in Beijing, China: (2) dicarboxylic acids. Atmospheric Environment 37, 3001–3007. Zheng, M., Fang, M., Wang, F., To, K.F., 2000. Characterization of the solvent extractable organic compound in PM2.5 aerosols in Hong Kong. Atmospheric Environment 34, 2691–2702.
Characterization of dicarboxylic acids in PM2.5 in Hong Kong Xiaohong Yaoa, Ming Fanga, Chak K. Chanb,*, K.F. Hoc, S.C. Leec a
Institute for Environment and Sustainable Development, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong b Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong c Department of Civil and Structural Engineering, Research Center for Urban Environmental Technology and Management, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Received 28 June 2003; received in revised form 13 October 2003; accepted 31 October 2003
Abstract Dicarboxylic acids in atmospheric aerosols have received much attention because of their potential roles in affecting the global climate. The composition and the sources of dicarboxylic acids in PM2.5 were studied at one remote and two urban sites in Hong Kong in the winter of 2000 and in the summer of 2001. Oxalate was the dominant dicarboxylic acid in all samples. The winter oxalate concentrations were high and spatially uniform, with an average value of 0.36 mg m3, but the summer oxalate concentrations were low and had a large spatial variation. The influence of meteorological factors on the concentrations of dicarboxylic acids was also studied. The ratio of malonate to succinate was used to distinguish primary sources from secondary sources of these acids. This ratio at all three sites was close to that from direct vehicular exhaust in the winter, but it was close to that of secondary reactions in the summer. Hence, the acids were attributed to vehicular emissions in the winter and secondary sources in the summer. This hypothesis is also supported by a good correlation of oxalate with sulfate in the summer but a poor one in the winter. The correlations of oxalate with malonate, succinate, sulfate and K+ were also studied in terms of the routes of secondary formation of these dicarboxylic acids. r 2003 Elsevier Ltd. All rights reserved. Keywords: Oxalic acid; Malonic acid; Succinic acid; PM2.5; Secondary water-soluble organic compounds
1. Introduction Dicarboxylic acids in particulate pollutants can play important roles in the atmosphere. For example, they can act as cloud condensation nuclei (CCN) and also reduce the surface tension of particles to form CCN (Facchini et al., 1999; Kerminen, 2001). The dominant dicarboxylic acids in atmospheric particles are oxalic acid (C2), followed by malonic acid (C3) and succinic acid (C4) (Kawamura and Kaplan, 1987; Kawamura and Ikushima, 1993; Saxena et al., 1995; Yao et al., 2002). The concentration ratios of these acids in *Corresponding author. Tel.: +852-2358-7124; fax: +852235-80054. E-mail address: [email protected] (C.K. Chan).
atmospheric particles, in particular the C3/C4 mass ratio, are useful to understanding their importance in the atmosphere. The C3/C4 ratio has been reported to be 0.3–0.5 from vehicular emissions (Kawamura and Kaplan, 1987). Relatively low C3/C4 ratios have been found to be associated with the overwhelming contributions from vehicular exhaust to these acids in some studies, e.g., in downtown and west Los Angeles, in winters in Tokyo, and in Nanjing, China (Kawamura and Kaplan, 1987; Kawamura and Ikushima, 1993; Wang et al., 2002). On the other hand, the mass ratio of C3/C4 in secondary atmospheric particles is much larger than unity (Kawamura and Ikushima, 1993; Kawamura et al., 1996; Kawamura and Sakaguchi, 1999; Yao et al., 2002). For example, Kawamura and Ikushima (1993) reported a maximum mass ratio of 3 in the summer in
1352-2310/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2003.10.048
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Tokyo. They found ratios larger than unity concurrent with elevated concentrations of oxidants and attributed the source of dicarboxylic acids to secondary atmospheric reactions. Kawamura and Sakaguchi (1999) observed a mass ratio of 3 in the Pacific Ocean, where dicarboxylic acids are expected to originate from secondary reactions. Hence, the ratio of C3/C4 in atmospheric particles is a useful indictor to differentiate primary (vehicular) sources from secondary sources. Biogenic contributions of dicarboxylic acids are common in non-urban areas. They can also be important in some urban areas, in addition to primary vehicular emissions and secondary sources (Kawamura and Kaplan, 1987; Dutton and Evans, 1996; Yao et al., 2003). The metabolic processes of fungi are the major route of the biogenic production of oxalic acid through the hydrolysis of oxaloacetate from citric acid and glyoxylate (Dutton and Evans, 1996). However, these processes do not produce malonic acid or succinic acid. Overall, fungi are expected to make a minor contribution to oxalic acid in PM2.5 because fungi principally exist in the coarse particle size (Dutton and Evans, 1996; Bauer et al., 2002; Yao et al., 2003). Hong Kong is located along the coast of the South China Sea and is connected to the Pearl River Delta of Guangdong Province in China. Hong Kong is a warm, humid and cloudy coastal city. It can be considered as a typical Asian temperate coastal metropolitan area. According to the Hong Kong Observatory, the annual average temperature, relative humidity and the percentage of cloud coverage was 24 C, 78% and 69%, respectively, in 2001. The coldest month was January (17.3 C), with the corresponding average relative humidity of 78% and a percentage of cloud coverage of 68%. The hottest month was August (29.2 C), with a corresponding average relative humidity of 80% and a percentage of cloud coverage of 70%. Moreover, the amount of the rainfall in the summer is about 10 times of that in the winter in Hong Kong, resulting in a decrease in the concentration of air pollutants through wet deposition in the summer. In the last two decades, most industrial factories have moved out of Hong Kong to the Pearl River Delta. The Hong Kong Environmental Protection Department reported that local vehicular and power plant emissions account for 43.8% and 29.9% of the total emission of respirable suspended particulate (RSP) in Hong Kong (Environmental Protection Department, 2002). Local air pollutants in Hong Kong therefore principally originate from vehicular emissions and power generation. High concentrations of RSP usually are observed in the winter because of the frequent development of stagnating highpressure systems and inversion layers in the lower atmosphere (Environmental Protection Department, 2002). However, higher air pollutant emissions are expected in the summer because of an increase in the
energy consumption for air conditioning. For example, the electricity sales of CLP Holdings Limited, one of major electricity vendors in Hong Kong, in June to September of 2001 were 1.4 times those in January to March of the same year (www.clpgroup.com). Yao et al. (2002) reported that the C3/C4 mass ratios from a suburban site and two urban sites in Hong Kong were generally larger than unity, suggesting that the primary vehicle emissions were not the major source of dicarboxylic acids in the atmospheric particles at these sites. Instead, secondary sources, such as in-cloud processes, were found to be a major route of formation of dicarboxylic acids, based on the similarity of the size distributions of these dicarboxylic acids and sulfate. The urban measurements reported by Yao et al. (2002) were made at sites 20–25 m above the ground level and not close to the heavy traffic, which may explain the lower contribution of primary vehicular emissions to dicarboxylic acids than when measured at the ground level close to the heavy traffic. Seasonal variations in the concentrations of particulate dicarboxylic acids were not studied in Yao et al. (2002) because of the small size of their samples and because most of their samples were collected in summer. However, seasonal variations in these acids would be helpful in understanding their origins. Ho et al. (2003) recently examined the chemical characterizations of PM2.5 and PM10 at three different sites in Hong Kong: Hong Kong Polytechnic University (HKPU), Kwun Tong (KT) and Hok Tsui (HT). HKPU and KT are urban sites and close to the heavy traffic while HT is a remote ‘‘background’’ site. Ho et al. (2003) found that the ratio of organic carbon to elemental carbon was higher at HT than at HKPU and KT. The organic carbon to elemental carbon ratio in winter was higher than that in summer. Gas-aerosol equilibrium, favoring the partitioning of semi-volatile organic species in the particulate phase under the lower temperatures in the winter, may be an explanation for the observed seasonal differences. The elevated ratio of organic carbon to elemental carbon at HT could be due to a number of possible factors, including a significant secondary source of organic carbon, a lower ambient temperature and a higher biological emission flux at HT. We examine the contribution of secondary chemical reactions to organic acids at HT using the C3/C4 mass ratio. In this paper, we first present the characteristics of oxalate, malonate and succinate in PM2.5 in the winter and in the summer in terms of the relative contributions from the primary emissions and secondary sources in Hong Kong. The impact of meteorological factors on the concentrations and the composition of these dicarboxylic acids are then discussed. Finally, the routes of the secondary formation of particulate dicarboxylic acids in Hong Kong are examined.
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2. Experimental methods Detailed information about the sampling experiments has been described by Ho et al. (2003). The site at HKPU is 6 m above ground level and is heavily influenced by vehicular emissions. KT is a mixed residential/commercial/industrial area and the sampling site is 25 m above ground level. This site is also influenced by heavy traffic. HT is located on the southeast side of Hong Kong Island and has been considered to be a background station in HK (Ho et al., 2003). It is approximately 15 km away from major human activities and heavy traffic. Altogether, 70 sets of 24 h PM2.5 and 46 sets of companion PM10 samples were collected from 23 November 2000 to 1 March 2001, and from 27 June to 9 August 2001 at the three sites. Collocated samplings of PM2.5 and PM10 were carried out using the Graseby–Anderson high-volume PM2.5 and PM10 samplers, respectively. Whatman quartz microfibre filters were pre-heated at 900 C for 24 h before use. All sampling was conducted by HKPU. A quarter of each filter was sent to the Hong Kong University of Science and Technology for ionic analysis. The ionic concentrations of the aqueous extracts were determined by ion chromatography (Dionex LC20) with an electrochemical detector (ED 40). An AS11 column (4 mm) with an AG11 guard column and an Anion Trap column (4 mm) was used for water-soluble organic anion detection. This method to determine dicarboxylic acids in water extracts of atmospheric particles was firstly developed by Jaffrezo et al. (1998). Recently, it has been widely used by a number of researchers (Kerminen et al., . 2000; Rohrl and Lammel, 2001; Yao et al., 2002; Wang et al., 2002). A CS12 column and an eluent of 20 mM MSA were used for cation detection. The detection limits in mg/ml were 0.01 for oxalate, 0.02 for malonate and succinate, 0.004 for SO2 and 0.01 for NO 4 3 and 0.004 for Cl in aqueous extracts. Overall, the uncertainties in the measurements of ambient concentra-
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tions were 710% for oxalate, 715% for malonate and succinate, 7%5 for sulfate and 76% for nitrate. Parts of the filters were digested using a microwave extraction system for element analysis. Major crustal elements (Al, Ca, Fe and Mg, etc.) were analyzed by ICP-AES (Perkin-Elmer, OPTIMA 3300DV) and trace metals (As, Cr and Cu, etc.) were analyzed by ICP-MS (HP4500). The element analysis was conducted at HKPU.
3. Results and discussion 3.1. Concentrations of dicarboxylic acids in PM2.5 The average concentrations of the dicarboxylic acids in PM2.5 at the three sites are listed in Table 1. The samples collected from 23 November 2000 to 1 March 2001 and from 27 June to 9 August 2001 are defined as the winter samples and the summer samples, respectively. Oxalate was the dominant dicarboxylic acid in both seasons. In the winter, the spatial distributions were generally uniform for oxalate and succinate but not for malonate as shown in Figs. 1(a)–(c). The average concentration of oxalate at all three sites was about 0.36 mg m3. In contrast, there were larger spatial variations in the acids in the summer, with the highest average concentration of 0.17 mg m3 at KT, followed by 0.09 mg m3 at HKPU and 0.04 mg m3 at HT. However, there were four sets of samples with similar oxalate concentrations at HT and HKPU. Overall, the concentrations of dicarboxylic acids in the winter were 2–10 times those in the summer. The low mixing heights, the formation of the inversion layers, and the low wet deposition are believed to be the major factors for the accumulation of air pollutants (Environmental Protection Department, 2002; Pathak et al., 2003) in the winter in Hong Kong. In this study, the impact of the mixing height on the concentrations of dicarboxylic acid in PM2.5 was
Table 1 Average concentrations of dicarboxylic acids in PM2.5 Sampling period
Location
C2 (mg m3)
C3 (mg m3)
C4 (mg m3)
C3/C4
Winter
HT HKPU KT
0.3770.17 0.3570.14 0.3770.16
0.0270.01 0.0270.01 0.0370.02
0.0670.03 0.0570.03 0.0770.03
0.370.1 0.570.2 0.670.3
Summer
HT HKPU KT
0.0470.02 0.0970.06 0.1770.10
BDL 0.01370.007 0.01270.006
BDL 0.00770.008 0.01170.014
BDL 2.771.5 1.571.5
HKPU KT
0.25 0.32
0.015 0.03
0.03 0.05
0.5 0.6
Episodes 20 August 2001 11 July 2001 BDL: Below detection limit.
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(a) Oxalate
(a) Oxalate
0.6
0.4
0.2
0.0
0.0 0.3
0.4
0.5
0.6
(b) Malonate
-3
60 40 20 0 0
0.12
20
40 -3 x10
60
80
(c) Succinate
80
(b) Malonate
60 -3
0.2
x10
80
0.1
Concentration (µg m-3)
0.0
x10
Concentrations of dicarboxylic acids at KT and HT (µg m-3)
0.4 0.2
40 20 0
0.12
0.08
(c) Succinate
0.08
0.04
0.04
0.00 0.00
0.04
0.08
0.12
0.00 600
800
1000
1200
1400
Concentrations of dicarboxylic acids at HKPU (µg m-3) Fig. 1. Comparisons of the concentrations of dicarboxylic acids. +: in winter at KT; W: in winter at HT; : in summer at KT; X: in summer at HT.
investigated. The daily average of the mixing height was calculated from the meteorological conditions at 0000 and 1200 UTC (Pathak et al., 2003). The mixing height varied from 580 to 1110 m (on average 810 m) in the winter and from 890 to 1285 m (on average 1110 m) in the summer, as shown in Figs. 2(a)–(c). The concentration of each dicarboxylic acid, in general, decreased with increasing mixing height.
3.2. The ratio of C3/C4 in PM2.5 As shown in Table 1, the winter samples have significantly lower C3/C4 mass ratios than the summer samples have. Since the concentrations of C3 and C4 were too low in HT, we focus our discussions of the ratios measured at HKPU and KT. In the winter, the samples collected at HKPU and KT had an average C3/ C4 ratio of 0.5 and 0.6, respectively. Primary vehicle exhaust was the principal source of the dicarboxylic acids in the winter at these two urban sites. Secondary formation of dicarboxylic acids was possible, leading to a slight increase of the ratio from time to time.
Mixing height (m) Fig. 2. Variations in the concentrations of dicarboxylic acids with mixing heights (+: in summer, J: in winter).
Nevertheless, these ratios were still much smaller than a ratio of 3 from secondary formation of dicarboxylic acids as reported in the literature (Kawamura and Ikushima, 1993; Kawamura and Sakaguchi, 1999). In the summer, the average C3/C4 ratio was 2.7 and 1.5 at HKPU and KT, respectively, suggesting a large contribution of secondary sources to particulate dicarboxylic acid formation. Similar seasonal changes in C3/ C4 ratios have been reported in the literature. For example, Kawamura and Ikushima (1993) observed a high C3/C4 ratio of about 3 in the summer in conjunction with elevated concentrations of oxidants but a low ratio of 0.5 in the winter in Tokyo. Kerminen et al. (2000) also observed a C3/C4 ratio higher than unity (1.1) in summer but only about 0.2 in winter in Helsinki, Finland. An air pollution episode occurred on 20 August 2001 at HKPU (PM2.5 samples were not collected at KT and HT on the same day) when the concentrations of oxalate, malonate and succinate were about 2.5 times their average concentrations and the C3/C4 ratio was 0.5. Primary vehicle exhaust was the dominant source of dicarboxylic acids during this episode. The mixing
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height was only 740 m on 20 August 2001, much lower than the summer average of 1110 m. In the other summer samples, the C3/C4 ratio was always larger than unity. At KT, there was another air pollution episode on 11 July 2001 when the C3/C4 ratio was only 0.6 while it was 1.1 at HKPU on the same day. On that day, the maximum hourly average NO2 concentration reached 100 mg m3 and the mixing height (1140 m) was close to the average value in summer, favoring dispersion of air pollutants. This episode at KT could have been due to incidental traffic congestion near the sampling site. However, the C3/C4 ratios in the other summer samples were generally higher than unity. It can be concluded that, overall, secondary formation of dicarboxylic acids in PM2.5 was generally more important in the summer, although there were occasional events when primary exhaust emission was the dominant source. Malonate and succinate concentrations were below the detection limits in HT in the summer. It is interesting to note that the C3/C4 ratio at HT in the winter was similar to that of vehicular exhaust of 0.3–0.5 (Kawamura and Kaplan, 1987). HT is a remote rural site with little local vehicular traffic. Oxalate, succinate, sulfate, Ni and V (but not malonate) showed small spatial variations (Ho et al., 2003), which could be explained by the transport of urban air pollutants to HT. However, the concentration of organic carbon and elemental carbon at HT was only half and one-fourth of that at HKPU and KT (Ho et al., 2003). Ni and V are characteristic species from oil burning. The RSP data from 11 Hong Kong Environment Protection Department air-monitoring stations in 2001 show that the highest concentrations of Ni (annual average 6.7 ng m3) and V (annual average 16.4 ng m3) were observed at Kwai Chung, where the container terminals in HK are located. The high Ni and V concentrations at HT suggest that vessel exhaust from marine traffic is an important source of pollutants at HT although the possibility of atmospheric transport from urban areas cannot be excluded.
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3.3. The partitioning of dicarboxylic acids in PM2.5 and PM10 Altogether, there are 46 data sets with data on both PM2.5 and PM10. As listed in Table 2, in both the winter and the summer, oxalate and succinate generally reside in PM2.5, but more than half of the malonate was present in the coarse particles. In general, oxalate, malonate, and succinate originating from both secondary reactions and primary vehicle exhaust were mainly present in submicron particles (Ondov and Wexler, 1998; Yao et al., 2002). However, malonic acid is a more volatile species at ambient temperatures (Peng et al., 2001; Choi and Chan, 2002; Bilde et al., 2003). Peng et al. (2001) observed the evaporation of malonic acid but did not observe similar evaporation of oxalic acid in hygroscopic measurements using an electrodynamic balance. It is possible that oxalic acid is less volatile than malonic acid in the concentrated solutions used in the single particle studies or in aerosols containing ammonium salts because of hydrogen bonding with water and with ammonium. In fact, ammonium oxalate in aqueous droplets is not volatile (Peng and Chan, 2001). The diurnal variation of the ambient relative humidity results in the variation of liquid water contents in atmospheric particles. Malonic acid can transfer from fine particles to coarse particles after a few cycles of evaporation from fine particles and condensation onto coarse ones. In fact, the preferential presence of malonic acid in coarse particles has been previously reported (Yao et al., 2002). It is interesting to note the low PM2.5 to PM10 ratio of oxalate at HT in the summer. Fungi, which reside in supermicron particles, can release oxalic acid as a result of metabolic processes. Bauer et al. (2002) estimated that fungi account for up to 9.9% of organic carbon in coarse particles. Yao et al. (2003) also found that oxalic acid is sometimes dominant in the supermicron particles in the spring in Beijing, China. Biogenic emissions of oxalic acid may be important in PM10.
Table 2 Mass ratios of dicarboxylic acids in PM2.5 to PM10 and the correlation coefficients of dicarboxylic acids in PM2.5 at HKPU and KT Date
Location
Ratio of PM2.5/PM10
Correlation coefficients
C2
C3
C4
C4:C2
C3:C2
C4:C3
Sulfate: C3
Sulfate: C4
Sulfate:C2
Winter
HT HKPU KT
0.970.1 0.970.1 0.870.2
0.770.2 0.370.1 0.670.3
0.970.1 0.770.1 0.870.2
— 0.86 0.84
— 0.76 0.56
— 0.64 0.55
— 0.64 0.33
— 0.69 0.77
— 0.77 0.91
Summer
HT HKPU KT
0.470.2 0.870.2 0.870.2
BDL 0.470.2 0.570.3
BDL BDL 0.870.3
— 0.82 0.81
— 0.48 0.47
— 0.66 0.59
— 0.33 0.54
— 0.67 0.63
— 0.62 0.88
BDL: Below detection limit.
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3.4. Correlations of malonate, succinate and sulfate with oxalate Since there are only four data sets of PM2.5 at HT in the winter and the summer, we focus our correlation analysis on the data from HKPU and KT. Figs. 3(a)–(d) show the correlations of succinate and sulfate with oxalate in PM2.5 at HKPU and KT. The correlation coefficients of malonate, succinate and sulfate with oxalate are listed in Table 2. In the winter, succinate is well correlated with oxalate with correlation coefficients (r) of 0.86 and 0.84 at HKPU and KT, respectively. Oxalate also principally originates from primary vehicle exhaust. Sulfate is not as well correlated with oxalate with a correlation coefficient of 0.77 and 0.62 at HKPU and KT, respectively. In fact, the correlation of sulfate with oxalate at higher concentrations is evidently poorer than that at lower
(a) HKPU in winter
15
0.08
10
0.04
5
0.00
0
x10
25 20 15 10 5 0
0.2
0.12
0.4
0.6
(b) HKPU in summer
12 8 4 0
0.00
0.05
0.10
0.15
0.20
0.25
16
(c) KT in winter
12
0.08
8 0.04
4
0.00
0 0.0
x10
-3
Succinate concentrations (µg m-3)
-3
0.0
50 40 30 20 10 0
0.2
0.4
0.6
Sulfate concentrations (µg m-3)
0.12
12
(d) KT in summer
8
concentrations. For example, in the samples with oxalate concentrations larger than 0.3 mg m3 at KT in the winter, the correlation coefficient of sulfate with oxalate is only 0.39. The weak correlation between sulfate and oxalate indicates that oxalate originated from sources and/or atmospheric processes different from those of sulfate. The same can be concluded for oxalate concentrations at HKPU. This conclusion is consistent with the observation that the C3/C4 ratios were close to that of direct vehicular exhaust and were evidently smaller than that from secondary sources at HKPU and KT in the winter. In the summer, sulfate is well correlated with oxalate with a correlation coefficient of 0.91 at HKPU and 0.86 at KT, suggesting that oxalate originated from processes similar to sulfate formation, i.e., from secondary formation. This is consistent with the observed C3/C4 ratios being much larger than that of direct vehicle emissions. There are two types of atmospheric reactions forming oxalic and/or malonic and succinic acids (Kawamura et al., 1996). One is the oxidation of unsaturated fatty acids, which usually occurs in marine atmospheres (Kawamura and Sakaguchi, 1999). However, since unsaturated fatty acids originating from cooking are ubiquitous in Hong Kong (Zheng et al., 2000), oxidation reactions are also possible in urban areas. In this route, succinic acid is the precursor of malonic acid and oxalic acid and therefore has a good correlation with oxalic acid (and malonic acid when evaporation is not a problem). The other is the oxidation of aromatic hydrocarbons such as benzene and toluene, which occurs predominantly in urban atmospheres (Kawamura and Ikushima, 1993). In this route, glyoxal and glyoxylic acid are the intermediates in the formation of oxalic acid but succinic and malonic acids are not produced. Recently, Warneck (2003) suggested that acetylene and ethane can react with OH radicals to form glyoxal, which then forms oxalic acid. A good correlation between succinate and oxalate in our measurements indicates that the former route involving fatty acids is more likely. Succinic acid could be a major precursor of oxalic acid in the summer in Hong Kong. The poor correlation of malonate with sulfate is possibly related to the volatility of malonic acid as discussed earlier.
4
3.5. Correlations of dicarboxylic acids with K+ 0 0.00
0.10
0.20
0.30
Oxalate concentrations (µg m-3) Fig. 3. Correlations of C4 and sulfate with C2 (+: C4; J: sulfate): (a) rðC4:C2Þ ¼ 0:86; rðsulfate:C2Þ ¼ 0:77; (b) rðC4:C2Þ ¼ 0:82; rðsulfate:C2Þ ¼ 0:91; (c) rðC4:C2Þ ¼ 0:84; rðsulfate:C2Þ ¼ 0:62; (d) rðC4:C2Þ ¼ 0:81; rðsulfate:C2Þ ¼ 0:86:
K+ is an indictor of biomass burning in atmospheric particles (Yamasoe et al., 2000; Ikegami et al., 2001) because K+ is one of essential nutrients for plants. Biomass burning can also generate dicarboxylic acids (Rogers et al., 1991; Yamasoe et al., 2000). Particles from biomass burning can also act as CCN (Kaufman and Fraser, 1997; Narukawa et al., 1999), which form
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the droplet mode of dicarboxylic acids through in-cloud processes (Yao et al., 2002). In the winter, the dicarboxylic acids were poorly correlated with K+ (ro0:3). Moreover, the average ratio of oxalate to K+ was 0.25, which is much larger than that reported for the flaming and smoldering phases in burning plumes (0.030.1) (Yamasoe et al., 2000). Hence, we conclude that biomass burning is not likely the major direct source of oxalate in PM2.5 in the winter. In the summer, a good correlation of oxalate with K+, a moderate correlation of succinate with K+ (ro0:7), and a poor correlation of malonate with K+ (ro0:3) were found in both the HKPU and KT samples. The linear regression equations of oxalate with K+ are:
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nated from vehicular emission. However, the ratio was close to the reported value for secondary reactions in the summer, suggesting the dominance of secondary sources. This hypothesis is also supported by a good correlation of oxalate with sulfate in the summer but a poor one in the winter. In the summer, a good correlation of succinate with oxalate indicates that succinate could be a major precursor of oxalate in Hong Kong. A good correlation of oxalate with K+ and oxalate and sulfate can be explained by in-cloud formation of oxalate and sulfate from CCN derived from biomass burning. The role of particles from biomass burning in the cloud formation and subsequent formation of the secondary organics in Hong Kong requires further investigation.
½oxalate ¼ 0:85½Kþ ðr ¼ 0:90 at HKPUÞ; ½oxalate ¼ 0:90½Kþ þ 0:04 ðr ¼ 0:81 at KTÞ: Similar to the observations in the winter, the mass ratio of oxalate to K+ was much larger than that reported for biomass burning and hence direct biomass burning was not an important source for oxalate. However, 80–100% of biomass burning particles can act as CCN (Rogers et al., 1991). Hence, the in-cloud formation of oxalate is possible, based on the good correlation between oxalate and sulfate in PM2.5 as discussed earlier (Yao et al., 2003). This proposed in-cloud pathway can explain both the observation of a good correlation of sulfate with oxalate and a good correlation of oxalate with K+ and is worthy of further investigation. The poor correlation of malonate with K+ is also likely related to the volatility of malonic acid as discussed above.
4. Conclusion Seventy samples of 24-h measurements of PM2.5 were analyzed to study the composition of dicarboxylic acids at three different sampling sites in the winter of 2000 and in the summer of 2001 in Hong Kong. Oxalate was the dominant species of the dicarboxylic acids in all samples. In the winter, the oxalate and succinate concentrations were spatially uniform but the malonate concentration was not. In the summer, there were large spatial variations and the highest concentration appeared at KT. The concentrations of dicarboxylic acids in the winter were 2–10 times those in the summer and the elevated concentrations in winter were attributed to the low mixing height. The ratio of malonate to succinate was used to distinguish the primary and secondary sources of these acids in the summer and in the winter. The ratio of malonate to succinate at all three sites in the winter was close to the ratio given for direct vehicular exhaust in the literature, indicating that they overwhelmingly origi-
Acknowledgements Financial support from the Research Grants Council Earmarked Research Grants of the Hong Kong Special Administrative Region, China (Project no. HKUST9071/99P) and HKUST Emerging High Impact Area Fund (HIA98/99. ATC01) is gratefully acknowledged.
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