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Characteristics of organic and elemental carbon in atmospheric fine particles in Tianjin, China
Particuology 7 (2009) 432–437
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Characteristics of organic and elemental carbon in atmospheric fine particles in Tianjin, China Weifang Li a,b , Zhipeng Bai a,∗ a State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China b Institute of New Energy Material Chemistry, College of Chemistry, Nankai University, Tianjin, China
a r t i c l e
i n f o
Article history: Received 3 March 2009 Received in revised form 4 June 2009 Accepted 11 June 2009 Keywords: PM2.5 Organic carbon Elemental carbon Secondary organic carbon Tianjin
a b s t r a c t PM2.5 samples were collected at urban, industrial and coastal sites in Tianjin during winter, spring and summer in 2007. Concentrations of elemental carbon (EC) and organic carbon (OC) were analyzed using the IMPROVE thermal–optical reflectance (TOR) method. Both OC and EC exhibited a clear seasonal pattern with higher concentrations observed in the winter than in the spring and summer, due to cooperative effect of changes in emission rates and seasonal meteorology. The concentrations of carbonaceous species were also influenced by the local factors at different sampling sites, ranking in the order of industrial > urban > coastal during winter and spring. In the summer, the port emissions, enriched with EC, had a significant impact on carbonaceous aerosols at the coastal site. Total carbonaceous aerosol accounted for 40.0% in winter, 33.8% in spring and 31.4% in summer of PM2.5 mass. Good correlation (R = 0.84–0.93) between OC and EC indicated that they had common dominant sources of combustion such as coal burning and traffic emissions. The daily average OC/EC ratios ranged from 2.1 to 9.1, the elevated OC/EC ratios being found in the winter. The estimated secondary organic carbon (SOC) accounted for 46.9%, 35.3% and 40.2% of the total OC in the winter, spring and summer, respectively, indicating that SOC may be an important contributor to fine organic aerosol in Tianjin. © 2009 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Carbonaceous aerosol has attracted worldwide attention in the past two decades because of the role it plays as radiatively important species in climate and visibility reduction, as potential toxic agents to human health, and as surfaces that can catalyze atmospheric chemical reactions (Jacobson, 2002; Laden, Neas, Dockery, & Schwartz, 2000; Watson, 2002). China is a major global source of carbonaceous aerosol due to its high rates of usage of coal and biofuels (Cao, Zhang, & Zheng, 2006; Junker & Liousse, 2008). The high loading of carbonaceous aerosol in China has been identified as the important factor in regional climate change, urban haze formation, crop production, and adverse health effects (Chameides et al., 1999; Menon, Hansen, Nazarenko, & Luo, 2002; Ramanathan et al., 2001). Despite such importance, information concerning their concentrations, sources, spatial and temporal distributions in China is still limited.
∗ Corresponding author. Tel.: +86 22 23503397; fax: +86 22 23503397. E-mail address: [email protected] (Z. Bai).
Atmospheric particulate carbon is usually classified in two main fractions, organic carbon (OC) and elemental carbon (EC), with respect to their chemical properties. EC is essentially a primary pollutant, emitted directly during the incomplete combustion of carbon-containing fuels. OC, which is a matter of some concern because of possible mutagenic and carcinogenic effects, can be directly emitted from sources (primary OC), or produced from atmospheric reactions, involving gaseous organic precursors (secondary OC) (Turpin & Huntzicker, 1995). EC has a strong absorptivity of solar radiation and is considered to be the most important particulate component of global warming, whereas OC is mainly a scattering medium and exerts a negative climate forcing influence (Houghton et al., 2001). Up to now, the exact impacts of carbonaceous aerosol on climatic and environmental processes remain uncertain because of our poor understanding of its concentrations, chemical composition and formation mechanisms (Jacobson, Hansson, Noone, & Charlson, 2000). Tianjin is a typical industrial metropolis in northern China, with an area of 11,919 km2 and a population of approximately 10 million. It is located on the southeast of Beijing, at the lower reaches of Haihe River and adjacent to Bohai Sea. As one of the fastest growing regions in China, Tianjin also suffers from severe air pollu-
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doi:10.1016/j.partic.2009.06.010
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results in emissions of many organic compounds to the atmosphere. (3) A coastal site, 3 m above ground level, and located in the coastal Tanggu District, about 50 km southeast of downtown and adjacent to Bohai Gulf. Except Tianjin Port, there are few industrial factories around this site. Aerosol sampling was conducted during January, April and July in 2007 to represent different seasons of winter, spring and summer, respectively. Meteorological conditions of Tianjin city during the sampling periods are summarized in Table 1. The 24-h PM2.5 aerosols were collected on quartz membrane filters (∅ 90 mm, Pall Life Sciences, USA) using medium-volume impact samplers (TH-150A II, Wuhan Tianhong Intelligence Instrumentation Facility) operating at a flow rate of 100 L/min. The quartz filters were pre-heated in a muffle furnace at 800 ◦ C for 3 h before using to remove residual carbon. After stabilizing in a controlled desiccator for 48 h, all the filters were weighed before and after sampling by an electronic microbalance with 1 g sensitivity (Mettler Toledo MX-5). The exposed filters were stored in a refrigerator at about −18 ◦ C before chemical analysis to prevent evaporation of volatile components. OC and EC of the samples were analyzed by the IMPROVE thermal/optical reflectance (TOR) method with DRI Model 2001 Thermal/Optical Carbon Analyzer (Chow et al., 1993). Principle and details of the analytical procedure as well as QA/QC were given by Cao et al. (2005). Field blank filters were also collected and the sample results were corrected by the average of the blank concentrations. Fig. 1. The three sampling sites in Tianjin: urban, industrial and coastal.
3. Results and discussion
tion problems including visibility degradation, of which particulate matter (PM) has been observed as the principal pollutant. Fugitive dust, coal and biomass burning, motor vehicle and industrial emissions all contribute to the ambient PM (Bi, Feng, Wu, Wang, & Zhu, 2007; Tao et al., 2006). Prior PM studies in Tianjin have focused mainly on total suspended particulate with few focused on OC and EC in fine particles (Cao et al., 2007). This paper presents the results of three observation campaigns which were conducted during January, April and July 2007 at representative urban, suburban (industrial) and coastal sites of the city. The objectives of this study are (1) to examine spatial and seasonal variations of OC and EC concentrations, (2) to investigate the relationship between OC and EC, as well as secondary organic carbon (SOC) formation, and (3) to identify the possible sources and factors affecting carbonaceous species in Tianjin. 2. Sampling and analysis Three sampling sites were chosen (as shown in Fig. 1): (1) An urban site, located downtown and about 10 m above ground level. Surrounding the site are residences, restaurants, shops, and heavy traffic. (2) An industrial site, situated in the industrial complex of Dongli district, about 14 km northeast to the urban site, and 3 m above ground level. A large number of small- and medium-scale factories are located in the industrial complex including machinery, chemical manufacturing, automotive fitting factories, electronics, etc. Solvent use in organic chemical and manufacture processes
3.1. Concentrations of OC and EC The statistical summary of concentrations and mass percentages of PM2.5 OC and EC in Tianjin is shown in Table 2. The average concentrations of OC were 45.0 ± 32.8, 18.6 ± 7.6 and 13.0 ± 6.2 g/m3 , and EC were 7.9 ± 2.5, 6.1 ± 4.7 and 3.8 ± 3.1 g/m3 in January, April and July 2007, respectively. Both OC and EC exhibited higher levels in the winter than in the spring and summer. This seasonal variation can be attributed to the cooperative effects of changes in emission rates and seasonal meteorology. The high concentrations of carbonaceous species in winter could be attributed to the enhanced emissions from coal combustion heating and unfavorable atmospheric dispersion (e.g. low mixing layer height, frequent inversion, etc.). The wintertime OC increased by a factor of 2.5 and EC by a factor of 1.1, compared to the summertime averages. The higher increment of OC than EC suggested that coal combustion contributed more to OC than to EC. In the summer, rainfall is much more abundant (72% of the annual rainfall in the summer and a mere 2% in the winter), and carbonaceous aerosol can be efficiently removed by wet scavenging (Cao et al., 2005). Spring in Tianjin is windy and dry, favorable for the dispersion of pollution-derived fine particles, though at the same time the low humidity might not favor secondary particle formation. The seasonal variations of OC and EC in this study are similar to other studies conducted in China (Cao et al., 2007; Dan, Zhuang, Li, Tao, & Zhuang, 2004; Yang et al., 2005; Ye, Zhao, Jiang, Chen, & Meng, 2007). The daily variation of carbon concentrations within
Table 1 Meteorological conditions during each sampling period in Tianjin (ranges of daily variation with averages given in parentheses). Sampling period
Temperature (◦ C)
RH (%)
January 1–31, 2007 April 3–12, 2007 July 6–16, 2007
−7.0–2.4 (−2.6) 9.5–16.2 (12.3) 24.4–30.6 (27.3)
27–88 (50) 21–77 (43) 51–80 (67)
Wind speed (m/s) 0.5–5.3 (1.8) 1.2–3.6 (2.5) 1.3–2.7 (2.0)
Wind direction NW-N-SW E-NW-SE S-SE
Visibility (km) 1.8–20 (11) 9–20 (13) 8–15 (10)
Characteristics Cold and hazy Dry and windy Hot and humid
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Table 2 The average concentrations and mass percentages of OC, EC and SOC in PM2.5 during various seasons at the three sites of Tianjin. Period
Sampling site
Na
Concentration (g/m3 ) OC
Mass percentage (%) SOC
OC
January 2007
Urban Industrial Coastal Average
10 10 10
41.4 56.8 36.7 45.0
± ± ± ±
29.7 33.9 33.2 32.8
8.0 8.9 6.9 7.9
± ± ± ±
2.4 2.2 2.7 2.5
27.6 37.3 24.6 29.9
22.1 25.5 18.0 21.9
± ± ± ±
3.4 5.4 3.8 3.6
5.8 5.0 5.0 5.3
± ± ± ±
2.5 1.9 2.2 2.0
41.2 45.8 33.8 40.0
± ± ± ±
4.8 8.8 5.9 8.8
48.5 50.8 41.3 46.9
4.8 5.8 4.5 5.1
April 2007
Urban Industrial Coastal Average
10 10 10
19.2 25.8 14.7 18.6
± ± ± ±
6.6 10.0 5.5 7.6
5.6 7.6 6.0 6.1
± ± ± ±
2.1 4.3 2.8 4.7
6.3 8.3 5.1 6.6
17.3 17.7 17.9 17.6
± ± ± ±
5.0 3.2 2.6 3.6
5.1 5.6 6.2 5.6
± ± ± ±
1.9 1.6 4.0 2.7
32.7 33.9 34.9 33.8
± ± ± ±
9.6 6.8 5.3 8.0
32.6 35.4 38.0 35.3
3.6 3.6 3.9 3.6
July 2007
Urban Industrial Coastal Average
11 11 11
9.0 14.3 16.7 13.0
± ± ± ±
3.2 3.0 9.4 6.2
1.9 3.8 6.4 3.8
± ± ± ±
0.6 1.4 4.6 3.1
4.6 5.6 5.1 5.1
17.1 17.2 16.0 16.8
± ± ± ±
4.4 6.6 12.3 6.7
3.8 4.2 5.8 4.6
± ± ± ±
0.8 1.1 4.4 2.3
31.1 31.7 31.4 31.4
± ± ± ±
7.7 11.6 23.3 12.4
48.8 40.5 31.3 40.2
4.7 4.0 2.8 3.9
a b
EC
OC/EC b
EC
TCA
SOC/OC
Number of samples. Total carbonaceous aerosol = 1.6 × OC + EC.
each season was mostly dominated by meteorological conditions. Severe pollution episodes generally occurred under stable weather conditions characterized by weak wind, low mixing layer height and high RH. OC and EC concentrations among the three sites also varied, indicating that carbonaceous aerosols in Tianjin were influenced more or less by local factors at various types of sampling sites. The highest concentrations of OC and EC were observed at the industrial site during winter and spring, due to intensive emissions from the industrial complex. Meanwhile, relatively low carbonaceous concentrations were found at the coastal site, corresponding to lesser traffic and fewer factories as well as stronger wind. The average EC concentrations at the coastal site were 6.9, 6.0 and 6.4 g/m3 during winter, spring and summer, respectively, showing little seasonal differences. However, the wintertime EC at the urban and industrial sites was about two to four times the summertime values (as shown in Table 2). The coastal site is located in an open area without big stationary sources except Tianjin Port. Therefore, it is reasonable to assume a relatively stable contribution in different seasons from that port. In the summer, the concentrations of OC and EC at the coastal site were about 28% and 68%, respectively, higher than the 3-site averages. Influence of the port emissions enriched with EC is considered an important factor. In addition, the coastal area of Tianjin is distinctly impacted by the sea-land wind circulation during summer months, which can cause accumulation of inland and marine pollutants. Apparently, further investigation of aerosol source profiles of the port is especially needed for an in-depth understanding of their impacts on local air quality.
Comparison of OC and EC concentrations and OC/EC ratios at the urban site from this study with those in other cities is shown in Table 3, all based on the thermal–optical analytical method. The OC concentration is similar to those in Beijing, Guangzhou and Taiyuan—all heavily polluted cities, but much higher than those in overseas cities, pointing out the severe OC pollution in the urban atmosphere of China. The EC concentration in Tianjin is at a moderate level and comparable to those in most other cities, though higher than those in Los Angeles, Helsinki and Milan. The carbonaceous aerosol emissions in China are mainly from the burning of coal and biofuels (Cao et al., 2006), while transportation is the major emission source in those foreign cities. 3.2. Contributions of carbonaceous species to PM2.5 mass The sum of averaged OC and EC accounted for 27.2%, 23.2% and 21.4% of PM2.5 mass in winter, spring and summer, respectively. Evidently coal combustion heating leads to more carbonaceous species enriched in fine particles. OC was the predominant contributor to total carbon (TC), accounting for about 80% of TC that was lower than those measured in Taiyuan (85%) and Milan (86%), though higher than those (53–73%) in other listed cities. EC only accounted for about 5% of PM2.5 . The relative abundances of OC and EC determine the relative amounts of scattering and absorption. The high OC abundance implied that light scattering of carbonaceous aerosol should be one of the major factors causing visibility impairment in Tianjin. Total carbonaceous aerosol (TCA) was calculated by the sum of organic matter (OM = 1.6 × OC) and EC according to Turpin and Lim (2001). As Table 2 shows, TCA contributed an
Table 3 Comparison of OC and EC concentrations (g/m3 ), OC/EC ratio at Tianjin with results from other cities. Location
Sampling period
OC
EC
OC/EC
References
Tianjina , China Beijing, China Shanghai, China Guangzhou, China Taiyuanb , China Hong Kongc , China Chongju, Korea Seoul, Korea Los Angeles, USA Helsinki, Finlandb Milan, Italyb
January, April, July 2007 July 1999–June 2000 March 1999–March 2000 January–February 2002 December 2005–February 2006 1998–2001 October 1995–August 1996 March 2003–February 2005 January 1995–February 1996 July 2000–July 2001 August 2002–December 2003
22.7 23.9 14.6 22.6 28.9 8.9 5.0 10.2 7.7 3.0 9.2
5.1 8.8 6.1 8.3 4.8 4.7 4.4 4.1 3.8 1.2 1.4
4.4 2.7 2.4 2.7 7.0 1.9 1.1 2.5 2.0 2.5 6.5
This study He et al. (2001) Ye et al. (2003) Cao et al. (2003) Meng et al. (2007) Yu et al. (2004) Lee and Kang (2001) Kim, Huh, Hopke, Holsen, and Yi (2007) Kim, Teffera, and Zeldin (2000) Viidanoja et al. (2002) Lonati, Ozgen, and Giugliano (2007)
a b c
Measurement at the urban site. TOT method was used in these three studies, and TOR was used in others. Carbon content in PM10 .
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average 40.0% in winter, 33.8% in spring and 31.4% in summer to the total fine particle mass, indicating that carbonaceous aerosol is a significant contributor to PM2.5 pollution in Tianjin. The fine carbonaceous fractions in this study were similar to those (41.6% in winter, 30.6% in summer) measured at Tianjin by Cao et al. (2007), and also in agreement with what was reported (20–50%) for urban aerosols in the world (Duan et al., 2005). 3.3. Relationship between OC and EC The origin of OC and EC can be evaluated by the relationship between OC and EC (Chow et al., 1996; Turpin & Huntzicker, 1995). As shown in Fig. 2, good OC–EC correlations, differentiated according to seasons, were obtained with correlation coefficient (R) of 0.89, 0.93 and 0.84 for winter, spring and summer, respectively. These correlations indicated the presence of common dominant sources for OC and EC (e.g. coal combustion, motor vehicle exhaust)
435
because the relative rates of EC and OC emission would be proportional to each other. OC and EC had their strongest correlation in April 2007, implying that the majority of OC was primary and SOC formation might be minor. The different linear regression equations in various seasons could be related to the seasonal fluctuation of source emissions and SOC formation. The mass ratio of OC to EC (OC/EC) can be used to interpret the emission and transformation characteristics of carbonaceous aerosol. The daily OC/EC ratios of this study were in the range of 2.1–9.1, 2.3–5.3 and 2.3–6.0, with the averages of 5.1, 3.6 and 3.9 during winter, spring and summer, respectively. The elevated OC/EC ratios in winter could be attributed to several reasons. First, coal consumption for winter heating contributes more to OC than EC, and also increases the emission of volatile organic precursors. Second, low temperature leads to the adsorption and condensation of semi-volatile organic compounds onto existing solid particles. Third, the low mixing layer height in winter would enhance the SOC formation (Strader, Lurman, & Pandis, 1999). This seasonal pattern of higher wintertime OC/EC ratio was also observed in Beijing (Dan et al., 2004), Guangzhou and Hong Kong (Duan et al., 2007). The OC/EC ratios also displayed some spatial differences as a result of the influence of distinct local emissions. For example, the wintertime OC/EC ratio at the industrial site was higher than those at other two sites, which might be related to the emission pattern of higher OC and lower EC emission rates of local sources. The summertime OC/EC ratios at the coastal site were much lower compared with those at other sites because of higher EC concentrations, which could be attributed to the port emissions. Both marine container vessels and related land-based transportation activities at the port are mainly diesel-powered, and EC is the major exhaust of diesel-fueled vehicles (Watson et al., 1994). It is also found that the impact of the port emissions on local carbonaceous aerosol was stronger in the summer than other seasons. Many studies have related the OC/EC ratio to secondary organic particle formation. A primary OC/EC ratio of 2.2 or 2.0 was usually regarded as an indication of the presence of secondary organic particles. In other words, the additional OC that causes the OC/EC ratio exceeding 2.2 or 2.0 can be considered to be secondary in origin. According to these hypotheses, secondary organic carbon (SOC) might play an important role in carbonaceous pollution in Tianjin. However, it is difficult to conclude the presence of SOC from the absolute values of OC/EC alone because the relative abundance of OC and EC varies among sources and is also location- and meteorology-influenced (Cao et al., 2005; Chow et al., 2004). 3.4. Estimation of secondary organic carbon OC consists of a complicated mixture of species from both primary and secondary sources. The separation and quantification of primary and secondary OC is of great importance in understanding secondary aerosol formation as well as in controlling particulate carbon pollution. Since EC is predominantly emitted from primary combustion sources, it has often been used as a tracer of primary OC in evaluation of the SOC concentrations. The ratio of OC/EC in source emissions when compared to the same ratio in atmospheric samples will be indicative of the presence of secondary organic aerosol (SOA) formation. In the EC tracer method (Turpin & Huntzicker, 1995), SOC is estimated by means of the following equation:
OC
OCsec = OCtot − EC
Fig. 2. Correlation between OC and EC concentrations in PM2.5 in Tianjin during winter (a), spring (b) and summer (c).
EC
pri
,
(1)
where OCsec is the secondary OC, and OCtot the measured total OC. The primary organic carbon (POC) could be calculated from the formula EC(OC/EC)pri . However, it is challenging to know the exact contribution of different primary sources without performing
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a comprehensive source analysis. Castro, Pio, Harrison, and Smith (1999) suggested that the primary OC/EC ratio could be replaced by the minimum OC/EC ratio observed in atmospheric samples which contain exclusively primary carbonaceous compounds. Thus, SOC can be calculated as
OC
OCsec = OCtot − EC
EC
min
.
(2)
An estimation of SOC in Tianjin is made according to Eq. (2). It must be noted that this method can give only semi-quantitative information because of the high uncertainties associated with them. The observed values of (OC/EC) in this study (as Fig. 2 shows) were 2.1 in winter, 2.3 in spring and summer. These values were in the range of 1.1–2.4 reported for primary aerosols but larger than the minimum OC/EC ratios of 1.68 in Beijing (Yang et al., 2005), 1.3 in the PRDR (Cao et al., 2003), 1.5 at Kaohsiung City (Lin & Tai, 2001) and 1.1 in Birmingham (Castro et al., 1999). Given that small proportions of SOC may still exist in the samples with minimum OC/EC ratios, this OC/EC method provides a lower limit for SOC content. As listed in Table 2, the estimated SOC concentrations were 29.9, 6.6 and 5.1 g/m3 , accounting for 46.9%, 35.3% and 40.2% of the total OC in the winter, spring and summer, respectively. These results suggest that secondary organic aerosol may be a significant contributor to fine organic particles throughout the year in Tianjin. The seasonality of SOC was similar to that of the total OC with elevated concentrations during heating season, which could be attributed to the enhanced emission of organic precursors, as well as the low mixing layer height that results in the SOC precursors’ stagnation and SOC formation (Dan et al., 2004; Duan et al., 2005; Strader et al., 1999). 3.5. Distribution of eight carbon fractions According to the IMPROVE TOR protocol, the carbon content in aerosol samples can be separated into seven thermally defined carbon fractions (OC1, OC2, OC3, OC4, EC1, EC2, EC3), corresponding to the temperature plateaus through an analysis process, plus one fraction of optically detected pyrolyzed carbon (OP). The IMPROVE protocol defines OC as OC1 + OC2 + OC3 + OC4 + OPC and EC as EC1 + EC2 + EC3 − OPC. The distribution of eight carbon fractions provides some information on the sources of carbonaceous aerosol. For instance, in the particulate source profiles of Texas, USA (Chow et al., 2004), OC1 is enriched in the vegetative burning profile, OC3 and OC4 are enriched in the road dust profile. In the source samples collected in Xi’an, China (Cao et al., 2005), OC2 was the most abundant carbon fraction in coal combustion samples, and EC1 constituted a major fraction of TC in motor vehicle exhaust. Yu, Xu, and Yang (2002) observed a larger OP fraction for polar organic compounds extracted in water. The percentages of the eight carbon fractions averaged for each sampling period are shown in Fig. 3. The greatest variability among the three seasons is found for OC1 that represents semi-volatile
organic compounds. The percentage of OC1 reached 17% in winter and only about 7% and 3% in spring and summer, respectively. This result suggested that, apart from the enhanced emissions of biofuel burning for heating supply, more semi-volatile OCs tend to condense on existing aerosols under low temperature thus contributing to the high wintertime OC concentrations. The abundance of EC1, mainly from vehicle exhaust (Cao et al., 2005), was relatively stable in all the seasons. OC2 in summer is more than 5% higher than in other seasons, possibly due to photochemical SOC formation. The high OP fraction, ranging from 20.8% to 29.6%, implied that substantial water-soluble polar compounds may be present in Tianjin atmosphere. Increases of OC3 and OC4 in spring may be indicative of the impacts of road dust (Chow et al., 2004). Development of the source profiles of 8 carbon fractions in Tianjin is necessary for accurate source apportionment of carbonaceous aerosol. 4. Conclusions PM2.5 aerosol samples were simultaneously collected at an urban, an industrial and a coastal site of Tianjin in January, April and July of 2007 to investigate the characteristics of fine carbonaceous aerosols. Major findings are as follows: (1) The average OC concentrations in PM2.5 were 45.0 ± 32.8, 18.6 ± 7.6 and 13.0 ± 6.2 g/m3 , and EC were 7.9 ± 2.5, 6.1 ± 4.7 and 3.8 ± 3.1 g/m3 during winter, spring and summer, respectively. The seasonality of OC and EC is similar to other studies conducted in China. Elevated particulate carbon concentrations in the winter resulted from enhanced emissions from coal combustion heating coupled with poor atmospheric dispersion. Abundant rainfall in the summer can lead to efficient removal of carbonaceous aerosol by wet scavenging. (2) OC and EC concentrations for the three sites ranked in the order of industrial > urban > coastal during winter and spring. In summer, OC and EC at the coastal site were about 28% and 68%, respectively, higher than the 3-site average values due to influences of the port emissions. EC concentrations at the coastal site showed little seasonal variation indicating its relatively constant emission rate. (3) Carbonaceous aerosol was a significant contributor to fine particle pollution in Tianjin, accounting for 40.0% in winter, 33.8% in spring and 31.4% in summer of PM2.5 mass. About 80% carbonaceous species existed in the form of OC. (4) The average OC/EC ratios were 5.1, 3.6 and 3.9 during winter, spring and summer, respectively. SOC may be an important contributor to fine organic aerosol in Tianjin, whose estimated concentrations accounted for about 35–47% of the total OC with elevated contributions observed in the winter. (5) The distribution of eight carbon fractions suggested that semivolatile organic compounds tend to condense on existing aerosols under low temperature, contributing to the high wintertime OC concentrations. Acknowledgements This project is supported by The National Natural Science Foundation of China (Grant no. 20677030) and The Commonweal Project of National Environment Protection (Grant no. 200709013). References
Fig. 3. Average percentages of total carbon contributed by eight carbon fractions in different seasons.
Bi, X., Feng, Y., Wu, J., Wang, Y., & Zhu, T. (2007). Source apportionment of PM10 in six cities of northern China. Atmospheric Environment, 41, 903–912.
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Characteristics of organic and elemental carbon in atmospheric fine particles in Tianjin, China Weifang Li a,b , Zhipeng Bai a,∗ a State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China b Institute of New Energy Material Chemistry, College of Chemistry, Nankai University, Tianjin, China
a r t i c l e
i n f o
Article history: Received 3 March 2009 Received in revised form 4 June 2009 Accepted 11 June 2009 Keywords: PM2.5 Organic carbon Elemental carbon Secondary organic carbon Tianjin
a b s t r a c t PM2.5 samples were collected at urban, industrial and coastal sites in Tianjin during winter, spring and summer in 2007. Concentrations of elemental carbon (EC) and organic carbon (OC) were analyzed using the IMPROVE thermal–optical reflectance (TOR) method. Both OC and EC exhibited a clear seasonal pattern with higher concentrations observed in the winter than in the spring and summer, due to cooperative effect of changes in emission rates and seasonal meteorology. The concentrations of carbonaceous species were also influenced by the local factors at different sampling sites, ranking in the order of industrial > urban > coastal during winter and spring. In the summer, the port emissions, enriched with EC, had a significant impact on carbonaceous aerosols at the coastal site. Total carbonaceous aerosol accounted for 40.0% in winter, 33.8% in spring and 31.4% in summer of PM2.5 mass. Good correlation (R = 0.84–0.93) between OC and EC indicated that they had common dominant sources of combustion such as coal burning and traffic emissions. The daily average OC/EC ratios ranged from 2.1 to 9.1, the elevated OC/EC ratios being found in the winter. The estimated secondary organic carbon (SOC) accounted for 46.9%, 35.3% and 40.2% of the total OC in the winter, spring and summer, respectively, indicating that SOC may be an important contributor to fine organic aerosol in Tianjin. © 2009 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Carbonaceous aerosol has attracted worldwide attention in the past two decades because of the role it plays as radiatively important species in climate and visibility reduction, as potential toxic agents to human health, and as surfaces that can catalyze atmospheric chemical reactions (Jacobson, 2002; Laden, Neas, Dockery, & Schwartz, 2000; Watson, 2002). China is a major global source of carbonaceous aerosol due to its high rates of usage of coal and biofuels (Cao, Zhang, & Zheng, 2006; Junker & Liousse, 2008). The high loading of carbonaceous aerosol in China has been identified as the important factor in regional climate change, urban haze formation, crop production, and adverse health effects (Chameides et al., 1999; Menon, Hansen, Nazarenko, & Luo, 2002; Ramanathan et al., 2001). Despite such importance, information concerning their concentrations, sources, spatial and temporal distributions in China is still limited.
∗ Corresponding author. Tel.: +86 22 23503397; fax: +86 22 23503397. E-mail address: [email protected] (Z. Bai).
Atmospheric particulate carbon is usually classified in two main fractions, organic carbon (OC) and elemental carbon (EC), with respect to their chemical properties. EC is essentially a primary pollutant, emitted directly during the incomplete combustion of carbon-containing fuels. OC, which is a matter of some concern because of possible mutagenic and carcinogenic effects, can be directly emitted from sources (primary OC), or produced from atmospheric reactions, involving gaseous organic precursors (secondary OC) (Turpin & Huntzicker, 1995). EC has a strong absorptivity of solar radiation and is considered to be the most important particulate component of global warming, whereas OC is mainly a scattering medium and exerts a negative climate forcing influence (Houghton et al., 2001). Up to now, the exact impacts of carbonaceous aerosol on climatic and environmental processes remain uncertain because of our poor understanding of its concentrations, chemical composition and formation mechanisms (Jacobson, Hansson, Noone, & Charlson, 2000). Tianjin is a typical industrial metropolis in northern China, with an area of 11,919 km2 and a population of approximately 10 million. It is located on the southeast of Beijing, at the lower reaches of Haihe River and adjacent to Bohai Sea. As one of the fastest growing regions in China, Tianjin also suffers from severe air pollu-
1674-2001/$ – see front matter © 2009 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.partic.2009.06.010
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results in emissions of many organic compounds to the atmosphere. (3) A coastal site, 3 m above ground level, and located in the coastal Tanggu District, about 50 km southeast of downtown and adjacent to Bohai Gulf. Except Tianjin Port, there are few industrial factories around this site. Aerosol sampling was conducted during January, April and July in 2007 to represent different seasons of winter, spring and summer, respectively. Meteorological conditions of Tianjin city during the sampling periods are summarized in Table 1. The 24-h PM2.5 aerosols were collected on quartz membrane filters (∅ 90 mm, Pall Life Sciences, USA) using medium-volume impact samplers (TH-150A II, Wuhan Tianhong Intelligence Instrumentation Facility) operating at a flow rate of 100 L/min. The quartz filters were pre-heated in a muffle furnace at 800 ◦ C for 3 h before using to remove residual carbon. After stabilizing in a controlled desiccator for 48 h, all the filters were weighed before and after sampling by an electronic microbalance with 1 g sensitivity (Mettler Toledo MX-5). The exposed filters were stored in a refrigerator at about −18 ◦ C before chemical analysis to prevent evaporation of volatile components. OC and EC of the samples were analyzed by the IMPROVE thermal/optical reflectance (TOR) method with DRI Model 2001 Thermal/Optical Carbon Analyzer (Chow et al., 1993). Principle and details of the analytical procedure as well as QA/QC were given by Cao et al. (2005). Field blank filters were also collected and the sample results were corrected by the average of the blank concentrations. Fig. 1. The three sampling sites in Tianjin: urban, industrial and coastal.
3. Results and discussion
tion problems including visibility degradation, of which particulate matter (PM) has been observed as the principal pollutant. Fugitive dust, coal and biomass burning, motor vehicle and industrial emissions all contribute to the ambient PM (Bi, Feng, Wu, Wang, & Zhu, 2007; Tao et al., 2006). Prior PM studies in Tianjin have focused mainly on total suspended particulate with few focused on OC and EC in fine particles (Cao et al., 2007). This paper presents the results of three observation campaigns which were conducted during January, April and July 2007 at representative urban, suburban (industrial) and coastal sites of the city. The objectives of this study are (1) to examine spatial and seasonal variations of OC and EC concentrations, (2) to investigate the relationship between OC and EC, as well as secondary organic carbon (SOC) formation, and (3) to identify the possible sources and factors affecting carbonaceous species in Tianjin. 2. Sampling and analysis Three sampling sites were chosen (as shown in Fig. 1): (1) An urban site, located downtown and about 10 m above ground level. Surrounding the site are residences, restaurants, shops, and heavy traffic. (2) An industrial site, situated in the industrial complex of Dongli district, about 14 km northeast to the urban site, and 3 m above ground level. A large number of small- and medium-scale factories are located in the industrial complex including machinery, chemical manufacturing, automotive fitting factories, electronics, etc. Solvent use in organic chemical and manufacture processes
3.1. Concentrations of OC and EC The statistical summary of concentrations and mass percentages of PM2.5 OC and EC in Tianjin is shown in Table 2. The average concentrations of OC were 45.0 ± 32.8, 18.6 ± 7.6 and 13.0 ± 6.2 g/m3 , and EC were 7.9 ± 2.5, 6.1 ± 4.7 and 3.8 ± 3.1 g/m3 in January, April and July 2007, respectively. Both OC and EC exhibited higher levels in the winter than in the spring and summer. This seasonal variation can be attributed to the cooperative effects of changes in emission rates and seasonal meteorology. The high concentrations of carbonaceous species in winter could be attributed to the enhanced emissions from coal combustion heating and unfavorable atmospheric dispersion (e.g. low mixing layer height, frequent inversion, etc.). The wintertime OC increased by a factor of 2.5 and EC by a factor of 1.1, compared to the summertime averages. The higher increment of OC than EC suggested that coal combustion contributed more to OC than to EC. In the summer, rainfall is much more abundant (72% of the annual rainfall in the summer and a mere 2% in the winter), and carbonaceous aerosol can be efficiently removed by wet scavenging (Cao et al., 2005). Spring in Tianjin is windy and dry, favorable for the dispersion of pollution-derived fine particles, though at the same time the low humidity might not favor secondary particle formation. The seasonal variations of OC and EC in this study are similar to other studies conducted in China (Cao et al., 2007; Dan, Zhuang, Li, Tao, & Zhuang, 2004; Yang et al., 2005; Ye, Zhao, Jiang, Chen, & Meng, 2007). The daily variation of carbon concentrations within
Table 1 Meteorological conditions during each sampling period in Tianjin (ranges of daily variation with averages given in parentheses). Sampling period
Temperature (◦ C)
RH (%)
January 1–31, 2007 April 3–12, 2007 July 6–16, 2007
−7.0–2.4 (−2.6) 9.5–16.2 (12.3) 24.4–30.6 (27.3)
27–88 (50) 21–77 (43) 51–80 (67)
Wind speed (m/s) 0.5–5.3 (1.8) 1.2–3.6 (2.5) 1.3–2.7 (2.0)
Wind direction NW-N-SW E-NW-SE S-SE
Visibility (km) 1.8–20 (11) 9–20 (13) 8–15 (10)
Characteristics Cold and hazy Dry and windy Hot and humid
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Table 2 The average concentrations and mass percentages of OC, EC and SOC in PM2.5 during various seasons at the three sites of Tianjin. Period
Sampling site
Na
Concentration (g/m3 ) OC
Mass percentage (%) SOC
OC
January 2007
Urban Industrial Coastal Average
10 10 10
41.4 56.8 36.7 45.0
± ± ± ±
29.7 33.9 33.2 32.8
8.0 8.9 6.9 7.9
± ± ± ±
2.4 2.2 2.7 2.5
27.6 37.3 24.6 29.9
22.1 25.5 18.0 21.9
± ± ± ±
3.4 5.4 3.8 3.6
5.8 5.0 5.0 5.3
± ± ± ±
2.5 1.9 2.2 2.0
41.2 45.8 33.8 40.0
± ± ± ±
4.8 8.8 5.9 8.8
48.5 50.8 41.3 46.9
4.8 5.8 4.5 5.1
April 2007
Urban Industrial Coastal Average
10 10 10
19.2 25.8 14.7 18.6
± ± ± ±
6.6 10.0 5.5 7.6
5.6 7.6 6.0 6.1
± ± ± ±
2.1 4.3 2.8 4.7
6.3 8.3 5.1 6.6
17.3 17.7 17.9 17.6
± ± ± ±
5.0 3.2 2.6 3.6
5.1 5.6 6.2 5.6
± ± ± ±
1.9 1.6 4.0 2.7
32.7 33.9 34.9 33.8
± ± ± ±
9.6 6.8 5.3 8.0
32.6 35.4 38.0 35.3
3.6 3.6 3.9 3.6
July 2007
Urban Industrial Coastal Average
11 11 11
9.0 14.3 16.7 13.0
± ± ± ±
3.2 3.0 9.4 6.2
1.9 3.8 6.4 3.8
± ± ± ±
0.6 1.4 4.6 3.1
4.6 5.6 5.1 5.1
17.1 17.2 16.0 16.8
± ± ± ±
4.4 6.6 12.3 6.7
3.8 4.2 5.8 4.6
± ± ± ±
0.8 1.1 4.4 2.3
31.1 31.7 31.4 31.4
± ± ± ±
7.7 11.6 23.3 12.4
48.8 40.5 31.3 40.2
4.7 4.0 2.8 3.9
a b
EC
OC/EC b
EC
TCA
SOC/OC
Number of samples. Total carbonaceous aerosol = 1.6 × OC + EC.
each season was mostly dominated by meteorological conditions. Severe pollution episodes generally occurred under stable weather conditions characterized by weak wind, low mixing layer height and high RH. OC and EC concentrations among the three sites also varied, indicating that carbonaceous aerosols in Tianjin were influenced more or less by local factors at various types of sampling sites. The highest concentrations of OC and EC were observed at the industrial site during winter and spring, due to intensive emissions from the industrial complex. Meanwhile, relatively low carbonaceous concentrations were found at the coastal site, corresponding to lesser traffic and fewer factories as well as stronger wind. The average EC concentrations at the coastal site were 6.9, 6.0 and 6.4 g/m3 during winter, spring and summer, respectively, showing little seasonal differences. However, the wintertime EC at the urban and industrial sites was about two to four times the summertime values (as shown in Table 2). The coastal site is located in an open area without big stationary sources except Tianjin Port. Therefore, it is reasonable to assume a relatively stable contribution in different seasons from that port. In the summer, the concentrations of OC and EC at the coastal site were about 28% and 68%, respectively, higher than the 3-site averages. Influence of the port emissions enriched with EC is considered an important factor. In addition, the coastal area of Tianjin is distinctly impacted by the sea-land wind circulation during summer months, which can cause accumulation of inland and marine pollutants. Apparently, further investigation of aerosol source profiles of the port is especially needed for an in-depth understanding of their impacts on local air quality.
Comparison of OC and EC concentrations and OC/EC ratios at the urban site from this study with those in other cities is shown in Table 3, all based on the thermal–optical analytical method. The OC concentration is similar to those in Beijing, Guangzhou and Taiyuan—all heavily polluted cities, but much higher than those in overseas cities, pointing out the severe OC pollution in the urban atmosphere of China. The EC concentration in Tianjin is at a moderate level and comparable to those in most other cities, though higher than those in Los Angeles, Helsinki and Milan. The carbonaceous aerosol emissions in China are mainly from the burning of coal and biofuels (Cao et al., 2006), while transportation is the major emission source in those foreign cities. 3.2. Contributions of carbonaceous species to PM2.5 mass The sum of averaged OC and EC accounted for 27.2%, 23.2% and 21.4% of PM2.5 mass in winter, spring and summer, respectively. Evidently coal combustion heating leads to more carbonaceous species enriched in fine particles. OC was the predominant contributor to total carbon (TC), accounting for about 80% of TC that was lower than those measured in Taiyuan (85%) and Milan (86%), though higher than those (53–73%) in other listed cities. EC only accounted for about 5% of PM2.5 . The relative abundances of OC and EC determine the relative amounts of scattering and absorption. The high OC abundance implied that light scattering of carbonaceous aerosol should be one of the major factors causing visibility impairment in Tianjin. Total carbonaceous aerosol (TCA) was calculated by the sum of organic matter (OM = 1.6 × OC) and EC according to Turpin and Lim (2001). As Table 2 shows, TCA contributed an
Table 3 Comparison of OC and EC concentrations (g/m3 ), OC/EC ratio at Tianjin with results from other cities. Location
Sampling period
OC
EC
OC/EC
References
Tianjina , China Beijing, China Shanghai, China Guangzhou, China Taiyuanb , China Hong Kongc , China Chongju, Korea Seoul, Korea Los Angeles, USA Helsinki, Finlandb Milan, Italyb
January, April, July 2007 July 1999–June 2000 March 1999–March 2000 January–February 2002 December 2005–February 2006 1998–2001 October 1995–August 1996 March 2003–February 2005 January 1995–February 1996 July 2000–July 2001 August 2002–December 2003
22.7 23.9 14.6 22.6 28.9 8.9 5.0 10.2 7.7 3.0 9.2
5.1 8.8 6.1 8.3 4.8 4.7 4.4 4.1 3.8 1.2 1.4
4.4 2.7 2.4 2.7 7.0 1.9 1.1 2.5 2.0 2.5 6.5
This study He et al. (2001) Ye et al. (2003) Cao et al. (2003) Meng et al. (2007) Yu et al. (2004) Lee and Kang (2001) Kim, Huh, Hopke, Holsen, and Yi (2007) Kim, Teffera, and Zeldin (2000) Viidanoja et al. (2002) Lonati, Ozgen, and Giugliano (2007)
a b c
Measurement at the urban site. TOT method was used in these three studies, and TOR was used in others. Carbon content in PM10 .
W. Li, Z. Bai / Particuology 7 (2009) 432–437
average 40.0% in winter, 33.8% in spring and 31.4% in summer to the total fine particle mass, indicating that carbonaceous aerosol is a significant contributor to PM2.5 pollution in Tianjin. The fine carbonaceous fractions in this study were similar to those (41.6% in winter, 30.6% in summer) measured at Tianjin by Cao et al. (2007), and also in agreement with what was reported (20–50%) for urban aerosols in the world (Duan et al., 2005). 3.3. Relationship between OC and EC The origin of OC and EC can be evaluated by the relationship between OC and EC (Chow et al., 1996; Turpin & Huntzicker, 1995). As shown in Fig. 2, good OC–EC correlations, differentiated according to seasons, were obtained with correlation coefficient (R) of 0.89, 0.93 and 0.84 for winter, spring and summer, respectively. These correlations indicated the presence of common dominant sources for OC and EC (e.g. coal combustion, motor vehicle exhaust)
435
because the relative rates of EC and OC emission would be proportional to each other. OC and EC had their strongest correlation in April 2007, implying that the majority of OC was primary and SOC formation might be minor. The different linear regression equations in various seasons could be related to the seasonal fluctuation of source emissions and SOC formation. The mass ratio of OC to EC (OC/EC) can be used to interpret the emission and transformation characteristics of carbonaceous aerosol. The daily OC/EC ratios of this study were in the range of 2.1–9.1, 2.3–5.3 and 2.3–6.0, with the averages of 5.1, 3.6 and 3.9 during winter, spring and summer, respectively. The elevated OC/EC ratios in winter could be attributed to several reasons. First, coal consumption for winter heating contributes more to OC than EC, and also increases the emission of volatile organic precursors. Second, low temperature leads to the adsorption and condensation of semi-volatile organic compounds onto existing solid particles. Third, the low mixing layer height in winter would enhance the SOC formation (Strader, Lurman, & Pandis, 1999). This seasonal pattern of higher wintertime OC/EC ratio was also observed in Beijing (Dan et al., 2004), Guangzhou and Hong Kong (Duan et al., 2007). The OC/EC ratios also displayed some spatial differences as a result of the influence of distinct local emissions. For example, the wintertime OC/EC ratio at the industrial site was higher than those at other two sites, which might be related to the emission pattern of higher OC and lower EC emission rates of local sources. The summertime OC/EC ratios at the coastal site were much lower compared with those at other sites because of higher EC concentrations, which could be attributed to the port emissions. Both marine container vessels and related land-based transportation activities at the port are mainly diesel-powered, and EC is the major exhaust of diesel-fueled vehicles (Watson et al., 1994). It is also found that the impact of the port emissions on local carbonaceous aerosol was stronger in the summer than other seasons. Many studies have related the OC/EC ratio to secondary organic particle formation. A primary OC/EC ratio of 2.2 or 2.0 was usually regarded as an indication of the presence of secondary organic particles. In other words, the additional OC that causes the OC/EC ratio exceeding 2.2 or 2.0 can be considered to be secondary in origin. According to these hypotheses, secondary organic carbon (SOC) might play an important role in carbonaceous pollution in Tianjin. However, it is difficult to conclude the presence of SOC from the absolute values of OC/EC alone because the relative abundance of OC and EC varies among sources and is also location- and meteorology-influenced (Cao et al., 2005; Chow et al., 2004). 3.4. Estimation of secondary organic carbon OC consists of a complicated mixture of species from both primary and secondary sources. The separation and quantification of primary and secondary OC is of great importance in understanding secondary aerosol formation as well as in controlling particulate carbon pollution. Since EC is predominantly emitted from primary combustion sources, it has often been used as a tracer of primary OC in evaluation of the SOC concentrations. The ratio of OC/EC in source emissions when compared to the same ratio in atmospheric samples will be indicative of the presence of secondary organic aerosol (SOA) formation. In the EC tracer method (Turpin & Huntzicker, 1995), SOC is estimated by means of the following equation:
OC
OCsec = OCtot − EC
Fig. 2. Correlation between OC and EC concentrations in PM2.5 in Tianjin during winter (a), spring (b) and summer (c).
EC
pri
,
(1)
where OCsec is the secondary OC, and OCtot the measured total OC. The primary organic carbon (POC) could be calculated from the formula EC(OC/EC)pri . However, it is challenging to know the exact contribution of different primary sources without performing
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a comprehensive source analysis. Castro, Pio, Harrison, and Smith (1999) suggested that the primary OC/EC ratio could be replaced by the minimum OC/EC ratio observed in atmospheric samples which contain exclusively primary carbonaceous compounds. Thus, SOC can be calculated as
OC
OCsec = OCtot − EC
EC
min
.
(2)
An estimation of SOC in Tianjin is made according to Eq. (2). It must be noted that this method can give only semi-quantitative information because of the high uncertainties associated with them. The observed values of (OC/EC) in this study (as Fig. 2 shows) were 2.1 in winter, 2.3 in spring and summer. These values were in the range of 1.1–2.4 reported for primary aerosols but larger than the minimum OC/EC ratios of 1.68 in Beijing (Yang et al., 2005), 1.3 in the PRDR (Cao et al., 2003), 1.5 at Kaohsiung City (Lin & Tai, 2001) and 1.1 in Birmingham (Castro et al., 1999). Given that small proportions of SOC may still exist in the samples with minimum OC/EC ratios, this OC/EC method provides a lower limit for SOC content. As listed in Table 2, the estimated SOC concentrations were 29.9, 6.6 and 5.1 g/m3 , accounting for 46.9%, 35.3% and 40.2% of the total OC in the winter, spring and summer, respectively. These results suggest that secondary organic aerosol may be a significant contributor to fine organic particles throughout the year in Tianjin. The seasonality of SOC was similar to that of the total OC with elevated concentrations during heating season, which could be attributed to the enhanced emission of organic precursors, as well as the low mixing layer height that results in the SOC precursors’ stagnation and SOC formation (Dan et al., 2004; Duan et al., 2005; Strader et al., 1999). 3.5. Distribution of eight carbon fractions According to the IMPROVE TOR protocol, the carbon content in aerosol samples can be separated into seven thermally defined carbon fractions (OC1, OC2, OC3, OC4, EC1, EC2, EC3), corresponding to the temperature plateaus through an analysis process, plus one fraction of optically detected pyrolyzed carbon (OP). The IMPROVE protocol defines OC as OC1 + OC2 + OC3 + OC4 + OPC and EC as EC1 + EC2 + EC3 − OPC. The distribution of eight carbon fractions provides some information on the sources of carbonaceous aerosol. For instance, in the particulate source profiles of Texas, USA (Chow et al., 2004), OC1 is enriched in the vegetative burning profile, OC3 and OC4 are enriched in the road dust profile. In the source samples collected in Xi’an, China (Cao et al., 2005), OC2 was the most abundant carbon fraction in coal combustion samples, and EC1 constituted a major fraction of TC in motor vehicle exhaust. Yu, Xu, and Yang (2002) observed a larger OP fraction for polar organic compounds extracted in water. The percentages of the eight carbon fractions averaged for each sampling period are shown in Fig. 3. The greatest variability among the three seasons is found for OC1 that represents semi-volatile
organic compounds. The percentage of OC1 reached 17% in winter and only about 7% and 3% in spring and summer, respectively. This result suggested that, apart from the enhanced emissions of biofuel burning for heating supply, more semi-volatile OCs tend to condense on existing aerosols under low temperature thus contributing to the high wintertime OC concentrations. The abundance of EC1, mainly from vehicle exhaust (Cao et al., 2005), was relatively stable in all the seasons. OC2 in summer is more than 5% higher than in other seasons, possibly due to photochemical SOC formation. The high OP fraction, ranging from 20.8% to 29.6%, implied that substantial water-soluble polar compounds may be present in Tianjin atmosphere. Increases of OC3 and OC4 in spring may be indicative of the impacts of road dust (Chow et al., 2004). Development of the source profiles of 8 carbon fractions in Tianjin is necessary for accurate source apportionment of carbonaceous aerosol. 4. Conclusions PM2.5 aerosol samples were simultaneously collected at an urban, an industrial and a coastal site of Tianjin in January, April and July of 2007 to investigate the characteristics of fine carbonaceous aerosols. Major findings are as follows: (1) The average OC concentrations in PM2.5 were 45.0 ± 32.8, 18.6 ± 7.6 and 13.0 ± 6.2 g/m3 , and EC were 7.9 ± 2.5, 6.1 ± 4.7 and 3.8 ± 3.1 g/m3 during winter, spring and summer, respectively. The seasonality of OC and EC is similar to other studies conducted in China. Elevated particulate carbon concentrations in the winter resulted from enhanced emissions from coal combustion heating coupled with poor atmospheric dispersion. Abundant rainfall in the summer can lead to efficient removal of carbonaceous aerosol by wet scavenging. (2) OC and EC concentrations for the three sites ranked in the order of industrial > urban > coastal during winter and spring. In summer, OC and EC at the coastal site were about 28% and 68%, respectively, higher than the 3-site average values due to influences of the port emissions. EC concentrations at the coastal site showed little seasonal variation indicating its relatively constant emission rate. (3) Carbonaceous aerosol was a significant contributor to fine particle pollution in Tianjin, accounting for 40.0% in winter, 33.8% in spring and 31.4% in summer of PM2.5 mass. About 80% carbonaceous species existed in the form of OC. (4) The average OC/EC ratios were 5.1, 3.6 and 3.9 during winter, spring and summer, respectively. SOC may be an important contributor to fine organic aerosol in Tianjin, whose estimated concentrations accounted for about 35–47% of the total OC with elevated contributions observed in the winter. (5) The distribution of eight carbon fractions suggested that semivolatile organic compounds tend to condense on existing aerosols under low temperature, contributing to the high wintertime OC concentrations. Acknowledgements This project is supported by The National Natural Science Foundation of China (Grant no. 20677030) and The Commonweal Project of National Environment Protection (Grant no. 200709013). References
Fig. 3. Average percentages of total carbon contributed by eight carbon fractions in different seasons.
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