Characteristics, sources and formation of aerosol oxalate in an Eastern Asia megacity and its implication to haze pollution

Introduction Conclusions References


Authors Authors
Y Jiang, G Zhuang, Q Wang, T Liu, K Huang, Joshua S. Fu, J Li, Y Lin, R Zhang, and C Deng This article is available at TRACE: Tennessee Research and Creative Exchange: https://trace.tennessee.edu/ utk_civipubs/12

Introduction
Oxalic acid, with oxalate, has attracted a great deal of attention as the most abundant dicarboxylic acid in the tropospheric aerosols (Kawamura and Ikushima, 1993;Facchini et al., 1999a;Mader et al., 2004). As a major identified water-soluble organic compound in aerosol, oxalate could affect hygroscopic properties of aerosol particles 5 (Cruz and Pandis, 1998;Brooks et al., 2002;Kumar et al., 2003). It can act as cloud condensation nuclei (CCN) or reduce the surface tension of particles to form CCN (Facchini et al., 1999b;Kerminen, 2001); the refractive index of aerosol particles can be influenced by the presence of oxalate and further their radiative forcing estimates. Oxalate may also play a role in solubility, photochemistry and bioavailability of transition 10 metals in aerosols (Deguillaume et al., 2005;Jickells et al., 2005). The sources and formation pathways of aerosol oxalate are still under investigation. Oxalate can be emitted directly from fossil fuel combustion, biomass burning, and biogenic activity. However, many studies suggested the existence of a global secondary source in addition to the primary sources of the compound (Kawamura et al., 1995 • 28 E) is located in the western coast of the North Pacific Ocean, east front of the Yangtze River Delta, China. It is one of the largest, urbanized and motorized cities in China with a population of ∼19 000 000. Shanghai has the marine monsoon subtropical climate with an annual average precipitation of 5 ∼1100 mm (http://cdc.cma.gov.cn). Under the influence of the monsoon system, one year in Shanghai is usually divided into the hiemal half-year from November to April with prevailing northwestly continental wind and the estival half-year from May to October with prevailing southeastly wind from the North Pacific Ocean (Yin and Tan, 2003). For the past decade the air quality has been terribly degraded due to the rapid mo- 10 torization, and the heavy haze has been the frequent weather pattern in Shanghai. Organic acids, including oxalic acid, with the organic aerosol have been considered as one of major components of haze. In this paper we report the concentration levels, seasonal variation and relative distribution of aerosol oxalate in PM 2. 5 and TSP, as well as the possible sources and formation pathways of aerosol oxalate. The relation- 15 ships between oxalate, meteorological factors and the regional haze pollution are also discussed. Introduction  -29 November 2007, autumn;(4) 24 December 2007-26 January 2008. Polycarbonate filters were put in the sampler at the sampling site for 24 h (without pumping) and used as the blanks. Four blanks were collected in every season. Samples were placed in polyethylene plastic bags with artificial parchment lining bags right after sampling and reserved in a refrigerator (−18 • C). All of 5 those filters were weighed before and after sampling with an analytical balance (Sartorius 2004 MP, reading precision 10 µg) after stabilizing under constant temperature (20 ± 1 • C) and humidity (40 ± 2 %). A total of 238 aerosol samples were collected and used in this study. All the procedures were strictly quality-controlled to avoid any possible contamination of the samples. 10 Black carbon (BC) measurement was conducted before series of chemical analysis. The concentration of BC in the aerosol samples was measured by a smoke stain reflectometer (model 43D, Diffusion Systems Ltd., London, UK). Five spots on each complete sample filter were selected randomly and evenly as possible for the measurement of BC contents which were calculated from comparing the reflectance of the 15 sample filter with that of a clean filter (Taha et al., 2007).

Ion analysis and pH measurement
One-fourth of each aerosol sample filter and blank filter was extracted ultrasonically by 10 ml water, which was deionized to the resistivity of 18 MΩ cm −1 . After passing 20 through the microporous membranes (mixed cellulose, pore size, 0.45 µm; diameter, 25 mm), each filtrate was stored at 4 • C in a clean tube for analysis. with pH values of 4.00 and 6.86. Detailed procedures were given elsewhere (Yuan et al., 2003).

Element analysis
Half of the sample and blank filter was digested at 170 • C for 4 h in high-pressure gaseous SO 2 and NO 2 in Shanghai were obtained from http://www.semc.gov.cn. The extent of the sampling artifact strongly depended on the volatility of the compounds, the aerosol acidity, the ambient temperature and relative humidity (Clegg et al., 1996;Souza et al., 1999;Limbeck et al., 2001;Pathak and Chan, 2005). The vapor pressure of oxalate is <20 mm Hg at 20 • which reflects a low volatility of this compound. 5 Low ambient temperature, low aerosol acidity and high relative humidity would make oxalate in the atmosphere be more associated with particles. The aerosols in Shanghai are known to be acidic due to the high concentration of SO 2 and NO x (Fu et al., 2008). Results of our measurement also showed that the pH of aerosol filtrates in Shanghai averaged at 3.96-5.41 (Table 1). However, the temperature of the ambient atmosphere in Shanghai was moderate except in summer and the average RH was above ∼65 % throughout the year. Consequently, the oxalate detected in the aerosol samples are expected to be able to represent the characteristics of the majority of the atmospheric oxalate and its measurement in this work is considered not affected by sorption or desorption artifacts under the local condition of Shanghai.   (2002b). Table 2 showed that the concentrations of oxalate in Shanghai were lower than those in Beijing and Hong Kong, but comparable to those in Nanjing, Tokyo, Chiba, Tampa Bay and LA. The higher levels of oxalate in Beijing may be explained by its heavy traffic emissions and winter heating system, while the high relative humidity and thick cloud cover may be responsible for the high values in Hong Kong.

5
The seasonal variation of oxalate concentrations in PM 2.5 and TSP samples collected in 2007 were showed in Table 3. The average concentrations of oxalate indicated a seasonal variation of autumnsummerwinterspring in both PM 2.5 and TSP. Their contributions to the total mass of the aerosols displayed in a slightly different order as summer>autumn>spring>winter. Since the production of aerosol oxalate in the 10 atmosphere is generally a radical (mainly hydroxyl radical) related photo-oxidation or chemical oxidation process (Warneck, 1999;Sorooshian et al., 2006;Martinelango et al., 2007), the high concentrations of oxalate in summer and autumn can be attributed to the generally high ambient temperature and more solar radiation in these two seasons. 15 The relative distribution of oxalate and several selected inorganic ions between PM 2.5 and TSP in each season were analyzed. As shown in Figure 1, oxalate was mainly present in the fine particle mode. The value of PM 2.5 /TSP of oxalate concentration reached its peak in summer, up to 0.88. According to the data from our online monitoring of PM 2.5 and PM 1 in the same sampling site, the ratios of mass concentration 20 of PM 1 to that of PM 2.5 averaged at 80 % in 2009 in Shanghai which suggested that aerosol oxalate probably dominated in finer particles (<1.0 µm). This is consistent with the results of previous studies, in which majority of aerosol oxalate was found appeared in the droplet mode (∼0.7-2.0 µm) with a MMAD around 1.0 µm, while small fraction of it distributed in the coarse mode (>2.0 µm) and condensation mode (∼0.1-0.7 µm) 25 (Yao et al., 2002a;Huang et al., 2006;Martinelango et al., 2007).
It could also be seen clearly in Figure 1 that nss-sulfate (non sea salt sulfate), ammonium and potassium reside dominantly in PM 2.5 rather than in TSP in all four seasons, with an exception of nitrate. The ratios of PM 2. 5  near to 0.5 which may be explained by the high volatility of nitric acid/nitrate and the coastal environment of Shanghai. Gaseous nitric acid (HNO 3 ) and nitrate (NO − 3 ) bound to aerosol particles coexist in the atmosphere. Nitrate in the aerosols can exist in the forms of NH 4 NO 3 , NaNO 3 , Ca(NO 3 ) 2 , etc., depending on the category and concentration of gaseous and aerosol species which nitric acid can obtain to combine with. In 5 the coastal area, the abundant atmospheric sea salt aerosols can partition gas-phase nitric acid to particle-or aqueous-phase NaNO 3 (Dasgupta et al., 2007). Most of the newly formed NaNO 3 still reside in coarse particles of sea salt which consequently increased the PM 2.5 /TSP ratio of nitrate in Shanghai. Martinelango et al. (2007) indicated a high ratio of PM 10 /PM 2.5 of nitrate in a costal environment, while studies carried in 10 Hong Kong (Zhuang et al., 1999;Yao et al., 2002a) also observed a dominant coarse mode of nitrate. More nitric acid would partition to gas phase under hot and acidic ambient environment, which means more coarse mode nitrate could be produced from the gas phase through the sea salt conversion. From our observation, the two lowest PM 2.5 /TSP ratios of nitrate appeared in spring, the most acidic season, and in summer, 15 the season with highest temperature, lend more support to this viewpoint.
The probable dominant droplet mode oxalate and similarity in size distribution between oxalate and the inorganic ions also gave suggestion on the sources and formation pathways of aerosol oxalate which would be discussed further in the following sections.

Case study: aerosol oxalate in a dust event
In an ever-recorded dust day in 2 April 2007, sampling was segmented to five periods (Table 4) to avoid possible overload of the filters and to gain a more lucid profile of the dust event. The most intensive input of the dust occurred in the morning time (09:29-12:10) in which the mass concentration of TSP and oxalate in TSP rushed to 25 1340.4 µg m −3 and 1.14 µg m −3 respectively. At the same time, the PM 2.5 /TSP ratio of mass and oxalate concentration decreased to 0.29 and 0.38, which was nearly half of the values in normal days. The three-day back trajectories exhibited the air mass 22083 Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | movements at the beginning of the dust day ( Fig. 2, left). The dust air mass originated from west/northwest China, where is a major source area of Asian dust, crossed the continent to the coastal regions, over the Bo Sea and Yellow Sea and finally arrived at Shanghai. The transport heights of the air mass were at 2500 m to 3500 m, which were typical of the high altitude transport weather pattern of a dust event. The aerosols This could explain the distinct decrease of PM 2.5 /TSP ratio of oxalate concentration during the dust episode compared to the value of spring average. Intensity of the dust lessened gradually along the time which could be concluded from the aerosol mass concentrations in the five successive sampling periods. The mass concentrations of both PM 2.5 and TSP in the fifth period (22:29, 2 April-09:11, 20 3 April) displayed an obvious decline, suggesting the ending of this dust event. Air masses observed largely came from north China instead and the transport heights were obviously lower than those at the beginning of the dust event (Fig. 2, right). The cloud cover, which was believed to be influenced by the Asian brown cloud (ABC) arisen by the dust air mass to a great extent in this case, also decreased from 104 25 to 66.7 PCT at the ending. However, the PM 2.5 /TSP ratio of aerosol mass and oxalate concentration changed differently to the concentrations themselves. They both reached the minimums at the third period (15:19-19:42) of the sampling, in which the ratios were 0.11 for mass concentration and 0.31 for oxalate concentration respectively. 11,2011 Characteristics of aerosol oxalate in an Eastern Asia megacity Y. Jiang et al. Then in the fourth and fifth periods, the ratios kept rising back to the normal day level. This was in accordance with the fact that dust aerosol was mainly composed of coarse particles and the relative concentration of the coarse particles measured in downwind area would decrease with the weakening of the dust event. Less input of coarse mode particles meant that the aerosol oxalate from local primary/secondary sources, which 5 was mainly resided in fine particle mode in Shanghai, began taking up the predominance.

Source identification by correlation analysis
To explore the possible sources and formation pathways, oxalate together with some species, which could be the indicator or the tracer of the various sources, was subjected to correlation analysis. We selected Al as the indicator for the crustal source, NO − 2 for vehicular emission, nss-SO 2− 4 and NO − 3 for secondary formation through different pathways, and K + for biomass burning. The correlation coefficients between these source-indicating species and oxalate in PM 2.5 were listed in Table 5. 15 Aerosol oxalate exhibited a poor correlation with aluminum with a peak value in spring (r = 0.02-0.53, Table 5). This suggested the contribution from crust or soil to the atmospheric oxalate mass in Shanghai was relatively small. A moderate correlation coefficient of 0.53 between oxalate and Al in spring could be brought by the larger amount of dust/soil aerosol input from north/northwest areas of China to the coastal region in 20 this season. Emissions of oxalate or its precursors from the metabolic processes of various soil fungal species could be another explanation for such a trend of correlation coefficients between oxalate and Al along the seasons (Dutton and Evans, 1999;Gadd, 1999). Obviously higher values were observed in spring (r = 0.53) and summer (r = 0.36) than in autumn (r = 0.02) and winter (r = 0.10), which was the inactive period Introduction Previous researches suggested that combustion processes were important sources of atmospheric HNO 2 and the most effective source of this type was the automobile engine (Kessler and Platt, 1984;Pitts et al., 1984;Wang et al., 2007a). The concentrations of nitrite (NO − 2 ) associated with the atmospheric aerosol are proportional to HNO 2 concentrations in the gas phase according to the Henry's Law. Therefore, here 5 we use aerosol NO − 2 as an indicator for primary vehicular emissions. From the very low correlation coefficients between oxalate and NO − 2 in every season (r = 0.07-0.25), we concluded that the vehicular emission made a very limited contribution to the total amount of aerosol oxalate in Shanghai as a primary source. Hence, the secondary formation in the atmosphere should account for the majority of this compound. 10 A high correlation was found between oxalate and NO − 3 (r = 0.49-0.90) in PM 2.5 , suggesting a similar source or formation pathway between these two species. The atmospheric precursors of NO − 3 and oxalate were NO x and volatile organic matters (e.g. alkenes), respectively, a characteristic source of which in urban city was vehicular exhausts. The good correlation between aerosol NO − 3 and oxalate provided evidence 15 that vehicular emission was a major secondary source for these two compounds in Shanghai. Furthermore, the conversion of NO 2 to NO − 3 , which is activated by ozone or radicals like OH and the formation of oxalate from alkenes which was also supposed to be a radical involved oxidation process, could be parallel and internally linked to each other through gaseous or aqueous radicals. For oxalate resided in particle size 20 range larger than droplet mode (>1.0 µm), it could be produced by surface reactions of gaseous oxalic acid or its precursors with alkaline coarse particles or from the evaporation of oxalate in smaller size range (Kerminen et al., 2000;Yao et al., 2002b). These could be two possible formation pathways for oxalate in higher fine particle mode (1.0-2.5 µm). However, more nitric acid evaporated from fine particles and absorbed by 25 alkaline coarse particles compared to oxalic acid due to their different volatility. This provided a reasonable explanation for the high correlation and distinct size distribution between these two compounds. As a result of a number of the previous studies, the formation of sulfate in the atmosphere through aqueous-phase oxidation is well established. Some researchers concluded from simulations that in-cloud process could convert up to 80 % SO 2 to sulfate in troposphere (McHenry and Dennis, 1994;Warneck, 1999). Evidences were found in recent studies that the secondary formation of oxalate in the troposphere also requires 5 the aqueous medium, with glyoxylate as a key aqueous-phase precursor (Yao et al., 2002a;Sorooshian et al., 2006;Carlton et al., 2007a). In Shanghai, aerosol oxalate showed a strong correlation with nss-sulfate in all four seasons (r = 0.60-0.95), especially in summer (r = 0.95), suggesting a common formation pathway between these two species. The similarity in oxalate and sulfate's distribution between PM 2.5 and TSP 10 also offered support to this hypothesis. As mentioned in the size distribution analysis and a number of studies, aerosol oxalate and sulfate mainly resided in fine particles and both probably represented a dominant droplet particle mode (< 1.0 µm). Based on plenty of previous studies, aqueous phase reactions were necessary to droplet mode sulfate as well as droplet mode secondary organic aerosol productions to occur 15 (Blando and Turpin, 2000). That is to say, attributed to the particle size distribution, in cloud process was suggested to be the most possible common formation pathway of aerosol oxalate and sulfate.

ACPD
Ammonium also showed an obvious positive correlation with oxalate, with the correlation coefficients of 0.52-0.80. One possible explanation of this phenomenon is that 20 ammonium in the atmosphere is mainly produced by the reaction between gaseous NH 3 and acidic sulfate particles (Finlayson- Pitts and Pitts, 2000) and therefore its high correlation with oxalate might be affected (or brought) by sulfate. On the other hand, it could be explained by the presence of ammonium oxalate in aerosols. Lefer and Talbot (2001) suggested that ammonium oxalate aerosol may be directly formed from the 25 gaseous ammonia and oxalic acid. Combined with the result of size distribution analysis, the high correlation between ammonium and oxalate indicated a possible form of existence for aerosol oxalate rather than an evidence for a common source. Since K + is an essential nutrient element for the growth of plants, it has been used as an effective indicator for the source of atmospheric particulates from biomass burning, especially in the fine particle mode (Yamasoe et al., 2000;Yao et al., 2004;Falkovich et al., 2005;Huang et al., 2006). The size distribution characteristics of K + observed in this work also showed that K + dominantly reside in PM 2.5 in every season in urban 5 Shanghai (∼80 %). The results of correlation analysis presented a moderate correlation between oxalate and K + with a peak value in autumn (r = 0.65), being consistent with the fact that the events of biomass burning happened most frequently in this season. If an oxalate/K + correlation suggested the contribution of biomass burning to the aerosol oxalate concentrations, here is a question needed to be addressed: is biomass burning just a primary source or also a secondary source for aerosol oxalate since biogenic VOCs could be released to the atmosphere through the biomass burning activity? The ratio of oxalate to K + (oxalate/K + ) in this work averaged at 0.26 in autumn, which was apparently larger than the reported values for the oxalate directly measured in biomass burning plumes (0.03-0.1) (Yamasoe et al., 2000). On the other hand,, the 15 high content of water soluble compounds (inorganic salts and water-soluble organic matters) in biomass burning aerosols made majority of them could act as CCN (Rogers et al., 1991;Novakov and Corrigan, 1996). Hence, the high oxalate/K + ratio suggested a secondary formation of oxalate from biomass burning emitted VOCs and probably through an in-cloud pathway as well. This is exactly the reason that VOCs precursors of 20 oxalate had a characteristic source from biomass burning in autumn that could partially explain the observation of the highest oxalate concentration and the lowest correlation coefficient between oxalate, nitrate and sulfate in this season.

Linear relationship of oxalate with sulfate/nitrate
The relationships of aerosol oxalate with the two source-indicating species, nss-SO 2− and then oxidized by ozone or hydroperoxides at significant rates. Meanwhile, aqueous phase formation was regarded necessary for droplet mode secondary organic aerosol production to occur (Blando and Turpin, 2000), which was applicable to the majority of aerosol oxalate in Shanghai based on previous discussions. Differently, the oxidation of NO 2 5 to nitric acid in the atmosphere mostly occurs in the gaseous phase, either oxidized by OH radical directly or by ozone through two intermediates, NO 3 radical and nitrogen pentoxide (Warneck, 2000). Previous studies (Warneck, 2003(Warneck, , 2005Carlton et al., 2007a;Martinelango et al., 2007) suggested that atmospheric oxalate is dominantly produced through aqueous oxidation by OH radical with glyoxylic acid, the most immediate precursor, which derived from either gaseous or aqueous oxidation of various primary precursors, ethene, acetylene, isoprene from biogenic source, etc. It was noticed that OH radical was involved in secondary formations of both NO − 3 and oxalate in the atmosphere. That is to say, the productions of NO − 3 and oxalate would be limited by the availability of OH radical. From this point of view, the secondary formations of 15 these two species were internally related.
Summer and winter as representative seasons (Fig. 3), oxalate demonstrated a significantly linear relationship with both sulfate and nitrate, which reflected the good correlations between aerosol oxalate and the two inorganic salts. Further information could be obtained from the slope values of the sulfate-oxalate and nitrate-oxalate lin-20 ear regression curves, which both were higher in winter than in summer. The seasonal variation of the average concentrations of SO 2 and NO 2 may explain the difference. As shown in Table 1, the observed average concentrations of SO 2 and NO 2 in winter were higher than those in summer in Shanghai. However, concentrations of VOCs (alkenes and aldehydes) in Shanghai were much lower (<5 ppbv) than the two inorganic pol- 25 lutant gases and showed no clear seasonal variation (Geng et al., 2008). Therefore, as compounds formed through common or related formation pathways, the increased slope values in winter could be partially attributed to the enhanced difference between the concentrations of SO 2 and NO 2 and those of the VOCs precursors involved in the Introduction

Meteorological evidences for the formation pathway of aerosol oxalate
Meteorological factors were taken into account for the formation of aerosol oxalate in Shanghai. The variation of temperature, relative humidity (RH), cloud cover, wind speed (WS) and wind direction (WD) in the sampling period were compared with that 5 of the oxalate concentrations. Based on the daily average observation values, no significant quantitative correlation was found between oxalate and any of these meteorological factors. However, the oxalate concentration did increase under such weather conditions: (1) clear days with clouds; (2) days with haze, mist or fog and (3) rain or shower or thunderstorm happened in the sampling days. Should the concentration 10 level of aerosol oxalate be simultaneously influenced by multiple meteorological parameters? The time series of relative humidity (RH), cloud cover and oxalate concentration through the sampling period were shown in Fig. 4. It could be concluded from the temporal trends that whenever both the relative humidity and cloud cover behaved declines compared to the values in the preceding day, a decrease of oxalate concentration was 15 observed accordingly and vice versa (see the dash lines in purple in Fig. 4). That is to say, simultaneous increases in ambient RH and cloud cover would indicate raises in oxalate concentration in the same days. The influence brought by the variations of meteorological conditions and the differences between the sources and concentration levels of VOCs precursors of oxalate in 20 such a time scale could be reduced to a large extent by comparing the data of every two neighboring days. These two positive contributing meteorological factors to the ambient concentration of the compound implied that a secondary formation of aerosol oxalate in aqueous medium did exist in Shanghai and probably through an in-cloud pathway. Introduction

Aerosol oxalate and haze pollution
One of the meteorological factors, visibility, was selected for the further study on aerosol oxalate due to its strong relationship with haze pollution. Haze is defined as the weather phenomenon which leads to atmospheric visibility less than 10 km due to the moisture, dust, smoke, and vapor in the atmosphere. Haze pollution has drawn great 5 attention in the past decade for its impact on visibility, public health and even climate change (Okada et al., 2001;Wang and Shooter, 2002;Chen et al., 2003;Yadav et al., 2003). The characteristics and formation mechanism of haze may vary in different regions. However, many studies on haze pollution (Chan et al., 1999;Mysliwiec and Kleeman, 2002;Sun et al., 2006;Fu et al., 2008), including study in Shanghai and its 10 surrounding areas indicated that high mass fractions of water soluble inorganic ions, such as NH + 4 , SO 2− 4 , and NO − 3 in PM 2.5 were observed during the haze episodes. The increases of mass fractions of these hygroscopic species were characteristic during haze episodes, suggesting that contribution of these hygroscopic species to the degradation of visibility were greater than other species in the aerosols. As a water-soluble 15 organic compound (WSOC), the role of oxalate in haze formation was investigated by comparing the variation of oxalate concentration and daily visibility of Shanghai in four seasons. As shown in Fig. 5, visibility of the city showed a strong negative correlation with the oxalate concentration levels in each season. The correlation coefficient r between visibility and oxalate concentration was −0.32 in average and −0.42 at peak 20 value.
In general, aerosol oxalate comprises less than 1 % of the aerosol mass as a water soluble organic compound. The oxalate-WSOC-aerosol OC-aerosol mass relationship was examined to illustrate what the negative correlation between oxalate concentration and atmospheric visibility implied. WSOC usually accounted for 20-70 % of aerosol 25 OC (Decesari et al., 2001(Decesari et al., , 2006. This percentage would be near the high end of the range in large urban center as Shanghai where more water soluble secondary organic aerosol (SOA) comprised a large fraction of ambient aerosol OC, due to the increase in polarity that accompanies the oxidizing formation pathway of SOA (Saxena and Hildemann, 1996;Lim and Turpin, 2002;Hennigan et al., 2008). This also suggested that the secondary formation mechanism of aerosol oxalate was the common and principal formation mechanism of the WSOC in aerosols. Without direct measurement, the OC content of the aerosol was estimated through the concentrations of  (5)); (7) OC * 1.8 = carbonaceous species -EC, here BC was used to represent EC for a rough estimate (Chow et al., 2009). The relative contributions of OC and major inorganic salts in PM 2.5 in Shanghai, 2007 were shown in Fig. 6. Organic carbon was estimated contributing over 30 % of the total PM 2.5 mass concentration in average, which was consistent with results of studies carried in 15 a worldwide range suggesting that ∼20-80 % of fine particle mass was organic (Zhang et al., 2007). Comparably, the sum of the mass concentrations of SO 2− 4 , NO − 3 , NH + 4 and K + took ∼20 % of that of the total PM 2.5 . If the OM (organic matter)/OC ratio of 1.8 and WSOC/OC ratio of 50 % was applied in this study, averagely 27 % of the PM 2.5 mass concentration would be occupied by water soluble organic compounds, which 20 was larger than the portion those major inorganic salts took.
The existence of WSOC in aerosol would make the aerosol be more hygroscopic and thus be more easily to act as condensation nuclei. Being the most abundant identified dicarboxylic acid in the tropospheric aerosol and a water soluble compound, oxalate could be taken as a representative compound of the WSOC in the aerosols. Raise 25 of aerosol oxalate concentration in the atmosphere reflected the raise of WSOC content to a certain extent, which would be more favorable to the formation of haze and, in turn, the degradation of visibility. Furthermore, some of the water-insoluble organics in aerosols can interact with certain water-soluble inorganic compounds, such as , to form an organic-inorganic complex and promote efficient new formation of the both parties in aerosols (Zhang et al., 2004). Therefore, the distinct negative correlation of atmospheric visibility with aerosol oxalate concentration, together with the high percentage of WSOC in the aerosols in Shanghai implied that oxalate being the representative compound in this study, aerosol organic compounds played an important 5 role in the formation of haze as well as in shaping the characteristics of aerosol and air quality in urban cities.

Conclusions
The mass concentrations of oxalate in 2007 sampling year were 0.07-0.41 µg m −3 3 in PM 2.5 and 0.10-0.48 µg m −3 in TSP, respectively. The oxalate concentrations displayed 10 a seasonal variation of autumn>summer>winter>spring in both particle modes. Oxalate was dominantly present in PM 2.5 or even finer particle mode (<1.0 µm) in all samples, and the peak value of PM 2.5 /TSP appeared in summer. Nss-SO 2− 4 , NH + 4 and K + showed a dominant residence in PM 2.5 as oxalate did while NO − 3 had obviously lower values of PM 2.5 /TSP in all four seasons. In a recorded dust day, it was observed that 15 invading dust brought gaseous or particulate oxalate to Shanghai as well as made local oxalate partitioning more to the coarse particle mode. As a result, a sharply increased oxalate level and decreased PM 2.5 /TSP ratio of oxalate concentration was obtained during the dust episode.
Based on the correlation analysis between aerosol oxalate and source-indicating 20 species, crustal source and vehicular emission was found minor contributors to the total amount of oxalate as primary sources. Considering the good correlation between oxalate and K + and high ratio of oxalate/K + , biomass burning was proved to be a secondary source of aerosol oxalate in Shanghai, in addition to urban VOCs sources (vehicular and industrial emissions), especially in autumn. Secondary formation ac- took place through an aqueous-phase oxidation pathway as aerosol SO 2− 4 did. This conclusion was supported by the high correlation of oxalate with nss-SO 2− 4 and K + , dominant residence of oxalate in droplet mode and result of favorable meteorological condition analysis, in which high oxalate concentration was observed when relatively high ambient RH and high cloud cover happened simultaneously. However, since 5 gaseous-phase oxidations happened in the early stages of the production of oxalate from alkenes in the atmosphere, the high correlation of oxalate and NO − 3 suggested that the formation of these two compounds were internally linked through the radicals involved. Due to the large fraction of WSOC in PM 2.5 in Shanghai, oxalate as representative compound of it, its contribution to the haze pollution and visibility degradation 10 of the local environment was comparable to that of the water-soluble inorganic species in the aerosols. The role of oxalate together with other aerosol organic compounds in the haze pollution and urban air quality needs to be further studied.