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Size distributions of hydrophilic and hydrophobic fractions of water-soluble organic carbon in an urban atmosphere in Hong Kong
Atmospheric Environment 166 (2017) 110e119
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Size distributions of hydrophilic and hydrophobic fractions of water-soluble organic carbon in an urban atmosphere in Hong Kong Nijing Wang a, Jian Zhen Yu a, b, * a b
Fok Ying Tung Graduate School, Hong Kong University of Science & Technology, Hong Kong, China Department of Chemistry and Division of Environment, Hong Kong University of Science & Technology, Hong Kong, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Both hydrophilic and hydrophobic fractions of water-soluble organic carbon (WSOC) had a dominant droplet-mode. Organic materials contributed more than the inorganic ions to the watersoluble mass on particles smaller than 0.32 mm. The hydrophobic WSOC component on the coarse mode were found to come from condensation mode particles through coagulation. The hydrophobic WSOC component was well-correlated with sulfate in all size modes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2017 Received in revised form 28 June 2017 Accepted 4 July 2017 Available online 6 July 2017
Water-soluble organic carbon (WSOC) is a significant part of ambient aerosol and plays an active role in contributing to aerosol's effect on visibility degradation and radiation budget through its interactions with atmospheric water. Size-segregated aerosol samples in the range of 0.056e18 mm were collected using a ten-stage impactor sampler at an urban site in Hong Kong over one-year period. The WSOC samples were separated into hydrophilic (termed WSOC_h) and hydrophobic fractions (i.e., the humiclike substances (HULIS) fraction) through solid-phase extraction procedure. Carbon in HULIS accounted for 40 ± 14% of WSOC. The size distribution of HULIS was consistently characterized in all seasons with a dominant droplet mode (46e71%) and minor condensation (9.0e18%) and coarse modes (20 e35%). The droplet mode had a mass median aerodynamic diameter in the range of 0.7e0.8 mm. This size mode showed the largest seasonal variation in abundance, lowest in the summer (0.41 mg/m3) and highest in the winter (3.3 mg/m3). WSOC_h also had a dominant droplet mode, but was more evenly distributed among different size modes. Inter-species correlations within the same size mode suggest that the condensation-mode HULIS was partly associated with combustion sources and the dropletmode was strongly associated with secondary sulfate formation and biomass burning particle aging processes. There is evidence to suggest that the coarse-mode HULIS largely originated from coagulation of condensation-mode HULIS with coarse soil/sea salt particles. The formation process and possible sources of WSOC_h was more complicated and multiple than HULIS and need further investigation. Our
Keywords: Size distribution HULIS Hydrophilic WSOC Urban Hong Kong
* Corresponding author. Department of Chemistry, Hong Kong University of Science & Technology, Hong Kong, China. E-mail address: [email protected] (J.Z. Yu). http://dx.doi.org/10.1016/j.atmosenv.2017.07.009 1352-2310/© 2017 Elsevier Ltd. All rights reserved.
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measurements indicate that WSOC components contributed a dominant fraction of water-soluble aerosol mass in particles smaller than 0.32 mm while roughly 20e30% in the larger particles. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Water soluble organic carbon (WSOC) is a major fraction of organic carbon in ambient aerosol formation. Analytically it is feasible to further divide WSOC into two fractions, hydrophilic part and hydrophobic part using solid-phase extraction (SPE). Such a demarcation is useful in disentangling the role of organics in interacting with atmospheric water. The approach using an SPE step was initially developed to remove the inorganic salts, which are major aerosol constituents and co-exist with WSOC, as the inorganic salts would interfere chemical characterization of the aerosol organics in many advanced analytical instruments (e.g., LC/ MS). The commonly employed SPE stationary phases include silica with carbon chain (alkyl- or aryl-bonded silicas), function-bonded silica, alumina etc. (Hennion, 1999). In practice, this SPE separation step also removes the more hydrophilic fraction WSOC. The fraction that is retained on the SPE and later eluted with an organic solvent (e.g., methanol) is the hydrophobic part of WSOC, also called humic-like substances (HULIS) as they share similar spectroscopic properties to macromolecular humic substances in terrestrial and aquatic environments (Graber and Rudich, 2006; Kiss et al., 2002; Sannigrahi et al., 2006; Zappoli et al., 1999). Due to their surface active property and light absorbing ability (Dinar et al., 2006; Hoffer et al., 2006; Kiss et al., 2005), HULIS is expected to be an active component in cloud condensation nuclei (CCN) process (Gysel et al., 2004) and contribute to atmospheric light absorption (Lukacs et al., 2007). Recent studies also showed that HULIS contains redox active components contributing to health effects by ambient PM through catalyzing the generation of reactive oxygen species (Dou et al., 2015; Lin and Yu, 2011; Verma et al., 2012). The ambient measurements of HULIS in PM10 and PM2.5 for different locations have suggested that biomass burning and secondary formation through in-cloud processing as major sources for HULIS (Havers et al., 1998; Decesari et al., 2001; Gelencser et al., csy et al., 2008; 2002; Kiss et al., 2002; Cavalli et al., 2004; Kriva Salma et al., 2008; Lin et al., 2010a; Kuang et al., 2015; Park and Son, 2016). Most of the previous studies focused on the hydrophobic part of WSOC (i.e., HULIS) (Lin et al., 2010b; Kuang et al., 2015), the hydrophilic part in WSOC (here named as WSOC_h) was rarely commented on. Size distribution of ambient aerosol is important in understanding aerosol properties, possible sources and formation pathways. Up to now, size distributions of organic and elemental carbon (OCEC), WSOC and ion species have been extensively studied (e.g., McMurry and Zhang, 1989; Yu et al., 2004a,b, 2010; Huang and Yu, 2008; Park et al., 2013; Wang et al., 2013; Bian et al., 2014). In comparison, only three studies reported the size distribution of HULIS (Lin et al., 2010b; Salma et al., 2013; Park and Son, 2016) and the measurement period was limited to only 1e2 weeks to one month. There is no research reporting the ambient size distribution of hydrophilic part of WSOC. In this study, one-year size-resolved aerosol samples were collected at an urban site in Hong Kong. Size distribution information of both hydrophobic and hydrophilic WSOC, together with size-segregated data of OCEC and major ion species, were obtained. The main objective of this work is to identify the size distribution characteristics of HULIS and WSOC_h
in an urban environment and to determine the size-specific relative importance of organic and inorganic water-soluble mass, providing data for further understanding the role of water-soluble organics in visibility impairment and CCN formation. An additional objective is to gain knowledge of the sources and possible formation pathways of the two WSOC fractions in different size range through comparison with the size distribution data for other major species (WSOC, OC, EC and ion species). 2. Method 2.1. Sample collection The ambient size-resolved aerosol were collected in Tsuen Wan (22 2201800 N, 114 060 5200 E), a general air quality monitoring station of Hong Kong Environmental Protection Department (HKEPD). The sampler was located on the roof top of a 17 m building. Sizesegregated samples were collected onto pre-baked 47 mm quartz fiber filters using a micro-orifice uniform deposit impactor (MOUDI, MSP Corp., Shoreview, MN, USA), operating in a nonrotating mode and at a flow rate of 30 L min1. The sampler has 10 size stages in an aerodynamic diameter range of 0.056e18 mm. In order to compensate the reduced space between each adjacent impactor plate caused by quartz filters, special spacers of 0.127 cm (MSP Corp., Shoreview, MN, USA) were used in the sampler. The MOUDI sampler was returned to the laboratory for cleaning by alcohol after each sampling. The duration of each sample was 48 h (0:00 a.m. to 0:00 a.m.) and one sample was collected every 12 days from September 2013 to August 2014. Due to operational mistakes, samples collected in April 2014 were not valid. In late August and early September 2013, additional four sets of samples were collected. The sampling program yielded a total of 32 valid sets of samples. Two field blank samples were collected in each sampling trip. The collected filters were stored in polycarbonate petri-slide dishes lined with prebaked aluminum foil and placed in a refrigerator at 18 C before analysis. 2.2. Chemical analysis Half of each 47-mm substrate filter was used for WSOC and HULIS analysis, while the other half was used for analysis of ions and OCEC. The filter portion was extracted with 15 mL ultrapure water in an ultrasonic bath for 45 min. The extracts were filtered through a 0.45 mm PTFE (polytetrafluorethylene) syringe filter (Millipore, Billerica, MA, USA) and analyzed for WSOC using a TOC analyzer (Shimadzu TOC-VCPH, Japan). The instrument response was calibrated using sucrose. Due to relatively lower WSOC concentration for size-segregated samples, large volume of injection (200 mL) was applied. The method for HULIS isolation and qualification was adopted from our previous work (Lin et al., 2010b), which was modified from the one developed by Varga et al. (2001). In brief, 4.0 mL of the water extract was acidified to pH ¼ 2 using HCl and loaded on a SPE cartridge (Oasis HLB, 30 mm, 60 mg, Waters, USA). Before use, the cartridge was activated, cleaned and equilibrated by methanol, pure water and 0.01 M HCl in succession. The sample-loaded
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cartridge was rinsed with 2 1.0 mL pure water and this effluent solution contained the hydrophilic substances (inorganic ions, low molecular weight organic acids, sugar compounds etc.). The cartridge was then eluted with 3 0.5 mL methanol containing 2% ammonia (w/w) to obtain the HULIS eluate. Subsequently, the eluate was evaporated under a gentle N2 stream to dryness, redissolved in 0.5 mL water and quantified for HULIS mass concentration using a HPLC system equipped with an evaporative light scattering detector (ELSD) without any analytical column (ELSD3300, Alltech, USA). SRFA (Suwannee River Fulvic Acid, International Humic Substances Society) and NAFA (Nordic Aqueous Fulvic Acid) were chosen as calibration standards for ELSD. The two fulvic acid mixtures showed similar detection response on the ELSD, demonstrating that ELSD as a mass detector is insensitive to sample chemical composition (Lin et al., 2010b). For OC and EC determination, a quarter of each sample filter was loaded into an aerosol carbon analyzer (Sunset Laboratory, Tigard, OR, USA) and analyzed using a thermal/optical transmittance (TOT) method. The non-uniform particle deposition of the MOUDI samples prevents the use of laser correction to determine the split point between OC and EC (Huang and Yu, 2008). Hence, we manually set the split point at the point when the carrier gas was switched to 2% oxygen in helium. Ions were quantified using ion chromatography (IC; Dionex DX500, Thermos Fisher Scientific, MA, USA). A CS-12A column and methanesulfonic acid (MSA) as elution solution were used for 2þ separation in the analysis of the cations (i.e., Naþ, Kþ, NHþ 4 , Mg and Ca2þ). An AS-11 column and a gradient elution solution of 2 NaOH were used in the analysis of the anions (i.e., Cl, NO 3 , SO4 and Oxalate) (Yang et al., 2005). The concentrations of all chemical species (WSOC, HULIS, OC, EC, ion species) were corrected for blank levels determined using the field blanks. 3. Results and discussion 3.1. Size distribution characterization Size distribution of ambient aerosol is typically characterized by several partially superimposed size modes, each mode associated with different sources and formation pathways. Following the same approach used in previous studies (Dong et al., 2004; Dzubay and Hasan, 1990), we fit the measurement data for HULIS, WSOC and ion species assuming three log-normally distributed modes, that is, a condensation mode, a droplet mode and a coarse mode. Seasonal variation of size distribution was examined, with 6 sets of samples falling in spring (March to May), 10 sets in summer (June to August), 9 sets in fall (September to November), and 7 sets in winter (December to February). The individual sampling dates are listed in Table S1 in the supporting information (SI). Table 1 summarizes the seasonal average modal concentrations and mass median aerodynamic diameter (MMAD) for EC, OC, WSOC, HULIS, WSOC_h, and sulfate. Data for the other ion species are given in Table S3. 3.1.1. Size distribution of ion species Sources and size distribution characteristics of ions species are better known and can serve as valuable references for the less 2 þ known aerosol constituents. NHþ 4 , K , SO4 and oxalate shared a similar modal pattern characterized by a dominant droplet mode with MMAD ranging from 0.72 to 0.96 mm and minor condensation þ 2 and coarse modes (Fig. S1a in SI). For NHþ 4 , K and SO4 , the droplet mode concentrations were most prominent in winter (3.9, 0.39, and 12 mg m3, respectively) and oxalate in the droplet mode had similar abundance in fall and winter (0.26 mg m3). The lowest droplet mode concentration appeared in summer for all the four ion species (0.83, 0.03, 2.8, and 0.09 mg m3, respectively).
A second group of species, including Naþ, Mg2þ, and Ca2þ, had a distinctly different size distribution from the above-discussed group. They had a dominant coarse mode and a minor droplet mode (Table S3 and Fig. S1b), reflecting sea salt and crustal material as their dominant source origins. The MMAD of coarse-mode Ca2þ was 5.2e5.9 mm, slightly larger than that for Naþ and Mg2þ (4.6e5.1 mm). These ion species were more abundant in spring or fall with Naþ at 1.2 mg m3 in spring, Mg2þ at 0.16 mg m3 in spring and fall, and Ca2þ at 0.73 mg m3 in fall. Cl and NO 3 were in the third group, with their size distributions in summer and fall in common with those of Naþ, Mg2þ, and Ca2þ, but in spring and winter the fine mode became more significant (Table S3 and Fig. S1c). Fine-mode nitrate in winter time contributed 45% (2.1 mg m3) of the total nitrate. The characteristic of variable size distribution among different samples for nitrate is known to be a combined result of the semi-volatile nature of nitrate and its affinity for alkali sea salt and dust particles (Xue et al., 2014; Griffith et al., 2015). The same chemical properties underlying the variable size distributions of nitrate are also shared by chloride, and therefore they fall in the same group in their size distribution pattern. 3.1.2. Size distribution of EC, OC and WSOC The size distributions of OC and EC were dominated by the droplet mode (Fig. 1a and b). EC had a more significant droplet mode, accounting for 70% of total EC, compared with 50% for OC. While more of OC was present in condensation mode and coarse mode (16% and 34%, respectively) than EC (11% and 19%, respectively). Seasonally, EC in all the three modes and OC in droplet mode had much higher concentrations in winter compared with other seasons (Table 1). In urban areas, EC in condensation mode is most likely related to freshly emitted particles from vehicle emissions and previous studies reported the MMAD varied from 0.1 mm to 0.4 mm depending on vehicles types and operating conditions (Huang et al., 2006a; Yu and Yu, 2009). We observed comparable MMAD values (0.13e0.18 mm) and modal concentrations for the condensation-mode EC from spring to fall. But in winter, the condensation mode MMAD of EC shifted to 0.27 mm and the concentration was more than two times as much as the concentration in other seasons, suggesting additional EC sources in this season. WSOC was characterized with a dominant droplet mode (Fig. 1c), with 65% in this size mode. Significantly less WSOC mass was associated with the condensation mode (13%) and coarse mode (22%). Seasonally, WSOC concentration in summer (1.8 mg m3) was the lowest and its size distribution was more evenly distributed among the different size modes, with a comparable coarse mode (40%) and droplet mode fraction (36%); the highest WSOC concentration (5.2 mg m3) occurred in the winter and the droplet mode was the most prominent, accounting for 70% (Table 1). This seasonality is consistent with a previous study conducted in Shenzhen, an urban coastal city near Hong Kong, which also recorded higher fraction in winter (52%) than in summer (43%) (Huang et al., 2006b). 3.1.3. Size distribution of HULIS HULIS and WSOC_h are two comparable sub-fractions of WSOC in our samples. The HULIS fraction was quantified as its mass concentration, accounting for carbon and other non-carbon mass (Lin et al., 2010b). To facilitate the abundance comparison with WSOC_h, we applied a multiplier factor of 1.9 to calculate the concentration of carbon in HULIS (i.e., HULIS-C). This mass-tocarbon multiplier was derived from our previous research (Lin et al., 2010b) and the same value was reported by Kiss et al. (2002) for HULIS samples from a rural site in Hungary. On annual average, 40 ± 14% of WSOC was HULIS-C, summing over the entire
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Table 1 Modal concentrations (mg m3), mass median aerodynamic diameters (MMAD, mm) and geometric standard deviation (GSD) (s) of elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC), humic-like substances (HULIS), hydrophilic water-soluble organic carbon (WSOC_h) and SO2 4 in four seasons (spring, N ¼ 6 for HULIS and N ¼ 4 for WSOC_h; summer, N ¼ 10; fall, N ¼ 9; winter, N ¼ 7). Condensation
EC Spring Summer Fall Winter OC Spring Summer Fall Winter WSOC Spring Summer Fall Winter HULIS Spring Summer Fall Winter WSOC_h Spring Summer Fall Winter SO2 4 Spring Summer Fall Winter
Droplet
Coarse
C (mg/m )
MMAD (mm)
GSD (s)
C (mg/m )
MMAD (mm)
GSD (s)
C (mg/m3)
MMAD (mm)
GSD (s)
0.30 0.42 0.45 1.07
0.13 0.18 0.18 0.27
1.41 1.61 1.58 1.85
3.69 1.95 3.36 5.17
0.64 0.60 0.72 0.74
1.82 1.70 1.66 1.61
0.95 0.73 0.61 1.56
4.84 4.27 3.81 3.19
1.73 1.74 2.06 1.93
0.69 0.63 1.14 1.15
0.11 0.15 0.18 0.18
1.59 1.60 1.91 1.87
2.47 1.76 3.14 3.76
0.64 0.67 0.76 0.77
2.07 2.25 1.70 1.75
1.47 1.30 2.54 2.06
5.05 5.06 4.58 4.09
1.76 1.62 2.09 1.89
0.49 0.43 0.40 0.54
0.22 0.16 0.15 0.27
1.80 1.59 1.72 1.89
1.86 0.64 3.12 3.60
0.80 0.62 0.74 0.80
1.75 1.50 1.73 1.62
0.70 0.71 0.73 1.03
4.00 3.63 4.60 4.02
1.95 1.80 1.65 1.98
0.35 0.16 0.69 0.43
0.21 0.17 0.23 0.21
1.55 1.98 1.83 1.62
2.08 0.41 2.28 3.26
0.77 0.74 0.79 0.79
1.70 1.54 1.65 1.50
0.73 0.31 0.95 0.93
5.07 5.26 4.75 4.01
1.89 1.68 1.79 1.93
0.46 0.32 0.28 0.11
0.20 0.13 0.18 0.10
1.97 1.69 1.79 1.46
1.29 0.59 1.58 2.30
0.78 0.64 0.78 0.85
1.70 1.66 1.55 2.03
0.48 0.55 0.51 0.60
3.10 3.85 3.35 5.58
1.91 1.85 2.11 2.09
0.51 0.18 1.09 0.69
0.34 0.20 0.31 0.29
1.63 1.61 1.70 1.66
8.04 2.76 8.25 11.99
0.87 0.72 0.89 0.89
1.50 1.44 1.57 1.60
1.72 0.75 1.56 1.98
3.26 4.10 3.06 3.33
1.81 1.71 1.91 1.60
3
3
size range. For fine particles (smaller than 3.2 mm), 42 ± 13% of WSOC were HULIS-C. This percent value was similar to measurements made for PM2.5 at different locations in previous studies csy et al., 2008; Lin et al., 2010b; Kuang (Salma et al., 2008; Kriva et al., 2015). Fig. 1d shows the average size distributions of HULIS in different seasons. The sample-by-sample distribution of HULIS in three size modes are shown in Fig. S2. HULIS had a similar seasonal variation in its size distribution to those of EC, WSOC and ion species of NHþ 4, Kþ, SO2 4 and oxalate. On the annual average, the droplet mode was most prominent, accounting for 64% (2.0 mg m3), with the condensation mode making up 13% (0.41 mg m3) and the coarse mode 23% (0.73 mg m3). In comparison with three previous studies reporting HULIS size distribution, our result had more in common with those recorded in Lin et al. (2010b) and Park and Son (2016). Lin et al. (2010b) reported 81% of HULIS associated with the droplet mode in aerosol collected in a rural location in the Pearl River Delta (PRD) region in November. Park and Son (2016) measured 58% of HULIS in the size range of 0.32e1.0 mm in aerosol collected at an urban site in Gwangju, Korea in February 2015. In the study by Salma et al. (2013), MOUDI samples were collected during April to May at a curbside site in central Budapest. They observed that the condensation-mode HULIS-C was more abundant (51%) than the droplet mode (31%), possibly due to the elevated influence from vehicle-related sources. Seasonally, the highest droplet-mode concentration (3.3 mg m3) appeared in winter accounting for 71% of total HULIS mass; the lowest (0.41 mg m3) was observed in summer; similar concentrations were recorded in spring (2.1 mg m3) and fall (2.3 mg m3). The seasonality of the HULIS droplet mode abundance 2 was similar to EC, Kþ, NHþ 4 and SO4 , suggesting possible source
commonality among those species. This result is consistent with many previous studies reporting evidence for secondary formation and biomass burning as two major HULIS formation sources (Altieri et al., 2008; Gelencser et al., 2003; Hoffer et al., 2004; Holmes and Petrucci, 2006; Lin et al., 2010b). 3.1.4. Size distribution of hydrophilic organic carbon of WSOC (WSOC_h) WSOC_h was obtained by subtracting the hydrophobic organic carbon from WSOC (WSOC_h ¼ WSOC-HULIS-C). Fig. 1e shows the seasonal average size distributions and Table 1 lists the modal concentrations and MMAD values. Among the three size modes of WSOC_h, the coarse mode had a relatively higher fraction in summer while the droplet mode dominated in winter when the total WSOC_h was most abundant (3.0 mg m3). The MMAD of the three modes were in the similar range to those of HULIS and WSOC. WSOC_h also shared the same seasonality in its droplet-mode concentration as HULIS (winter > fall > spring > summer), which indicated droplet-mode HULIS and WSOC_h might have similar sources. A few dissimilarities were found between HULIS and WSOC_h in their size distribution characteristics. First, WSOC_h in condensation mode showed a seasonality different from that of HULIS. For HULIS, its condensation mode was at a higher concentration in fall and at a lower level in summer. For WSOC_h, the lowest concentration level was in winter (0.11 mg m3) and comparable concentrations (0.28e0.46 mg m3) were in other seasons. Second, WSOC_h in summer was roughly evenly distributed among the three modes while HULIS in summer had its mass mainly distributed in the coarse mode and droplet modes (37% and 40%, respectively) and little in the condensation mode. Third, in
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Fig. 1. Lognormal Size distribution of MOUDI samples in Tsuen Wan, Hong Kong in four seasons (2013e2014): (a) elemental carbon (EC); (b) organic carbon (OC); (c) water-soluble organic carbon (WSOC); (d) humic-like substances (HULIS); (e) hydrophilic water-soluble organic carbon (WSOC_h).
comparison with HULIS and WSOC, the droplet mode of WSOC_h was broader in winter with a standard deviation (s) of 2.0.
size-mode based on 32 sets MOUDI one-year data are shown in Fig. 2a and 2b and tabulated in Tables S6a-S6c.
3.2. Correlations of HULIS and WSOC_h with other species in three modes
3.2.1. Condensation mode Particles in condensation mode was associated with substances that are freshly emitted plus those condensates added soon after emissions. HULIS had a moderate correlation with EC (R2 ¼ 0.40, Fig. 3a) suggesting EC and HULIS residing in this size mode may partly share emission sources. Kuang et al. (2015) carried out source apportionment of PM2.5 HULIS-C at one urban site and one suburban site near a port in the PRD region and found vehicle emissions
The larger data set obtained in this study allows us to probe the source origin of HULIS and WSOC_h through examining interspecies correlations in the same size mode and inter-size mode correlations for the same species. The coefficient of determination (R2) among species in each
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sharing common sources between the two. Bian et al. (2014) illustrated the possible source for condensation-mode sulfate was freshly formed from oxidation of SO2. It is plausible that the condensation mode sulfate particles could be effective in contributing to secondary HULIS formation through promoting heterogeneous reactions with oxygenated atmospheric oxidation products of volatile organic compounds (e.g., Surratt et al., 2008). The moderate correlations of HULIS with sulfate and EC suggested the condensation mode HULIS may come from combined primary EC emission sources and secondary formation involving freshly formed sulfate particles. WSOC_h had little or weak correlations with other measured chemical species (Fig. 2b). Consequently, chemical data obtained in this work was not informative in suggesting possible sources or insights into primary vs secondary origin for the condensationmode WSOC_h. We note that lack of correlation is not a definite indication of different source origins. For example, two secondary species formed from two primary pollutants of a common emission source may be poorly correlated as a result of multiple transformation in the atmosphere.
Fig. 2. Coefficient of determination (R2) of (a) HULIS and (b) WSOC_h with other species.
contributed little to HULIS-C but a part of HULIS-C was associated with ship emissions. The sampling site in this work is located near Hong Kong's container terminal, which is among the busiest shipping ports in the world (Yu et al., 2004a). Contribution of ship emissions as a common source was likely the underlying cause for the correlation between EC and HULIS. The concentration of Kþ in condensation mode was generally low (<0.1 mg m3) and for several samples no Kþ was detected on stages of smaller particle size bins. Kþ in the condensation mode had weak correlation with HULIS (R2 ¼ 0.22). Therefore, biomass burning was unlikely a significant source for HULIS in condensation mode. EC and Kþ were poorly correlated with each other, indicating biomass burning was not a major source for EC in the condensation mode either. HULIS was moderately correlated with sulfate in the condensation mode (R2 ¼ 0.49, Fig. 3b). Sulfate had no correlation with EC (R2 ¼ 0.05) in the condensation mode, negating the possibility of
3.2.2. Droplet mode In the droplet mode, HULIS and WSOC_h had good correlations 2 with the same set of species (e.g., EC, Kþ, NHþ 4 , SO4 and oxalate) (Fig. 2) and were moderately correlated with each other (R2 ¼ 0.45). 2 HULIS was strongly correlated with SO2 4 (R ¼ 0.83, Fig. 4a) and 2 þ NH4 (R ¼ 0.71). The results support the suggestion that secondary formation involving in-cloud processing might be a major source for HULIS in this region. In winter, secondary sulfate was much more abundant than summer and the relative humidity in winter (63e84%) was slightly lower than summer (75e84%) in Tsuen Wan. This is likely a result of more abundant precursors driving enhanced secondary formation in winter time, despite an anticipated lower level of photochemical oxidants in this season. Deng et al. (2013) also found that secondary aerosol formation was more significant on polluted days with PM2.5 exceeding 65 mg m3 in the PRD region. Unlike the condensation mode, HULIS and Kþ in the droplet mode were well-correlated (R2 ¼ 0.78, Fig. 4c). It was possibly because biomass burning particles were easy to be cloud activated and subsequently became larger after addition of sulfate (Huang et al., 2006b). EC in the droplet mode also had a better correlation with HULIS (R2 ¼ 0.67, Fig. 4b) and Kþ (R2 ¼ 0.77). The strong correlations suggest that Kþ, EC, and HULIS in the droplet size were likely associated with the same atmospheric aging processes, such as growth from condensation mode to droplet mode through cloud processing and subsequent cloud evaporation (Decesari et al., 2002; Huang and Yu, 2008; Lin et al., 2010b; Reid et al., 2005). Compared with HULIS, WSOC_h had weaker but still good
Fig. 3. Correlation between (a) HULIS and EC; (b) HULIS and SO2 4 in condensation mode.
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þ Fig. 4. Correlation between (a) HULIS and SO2 4 ; (b) HULIS and EC; (c) HULIS and K in droplet mode.
correlation with the same suite of species mentioned above (Fig. 2b). It is worth noting that the correlations of WSOC_h with the secondary species and Kþ were not as strong as the correlations for HULIS. In contrast, WSOC_h and EC had a slightly better correlation (R2 ¼ 0.69) than HULIS and EC (R2 ¼ 0.67). This appears to suggest that there may be EC related sources contributing to droplet-mode WSOC_h but not to HULIS. Recent work from our group on source apportioning of PM2.5 in Tsuen Wan indicated that a significant part of WSOC_h was associated with vehicle emissions (Kuang, 2017). Previous research also identified vehicular exhaust as a source for WSOC_h in PM2.5 in an urban site in Guangzhou and it was suggested that high fuel combustion efficiency in vehicles resulted in emitting low molecular weight WSOC, i.e. hydrophilic WSOC (Kuang et al., 2015). Two studies reporting size distribution characteristics of EC in our study region found two EC condensation modes, peaked at both 0.1 mm and 0.4 mm. It was suggested that both sizes of EC originated from primary vehicle emissions but from different vehicle fleet and/or vehicles operating at different road
loadings (Huang et al., 2006a; Yu and Yu, 2009). In this study, EC and WSOC_h similarly peaked at around 0.2 mm and 0.7 mm in condensation mode and droplet mode, respectively, with an overlap ranging from 0.2 to 0.6 mm (Fig. 1a and 1e). The 0.4 mm EC size mode was not resolved in our work due to limited number of size bins. It would have been lumped into the droplet mode in our three-mode size distribution description, explaining that the good correlation of WSOC_h and EC was a result of common vehicular emission source. 3.2.3. Coarse mode A significant fraction (23 ± 7%) of HULIS was in the coarse mode. No previous studies have explained the origin of HULIS on the coarse particles due to limited data. The correlation analysis revealed a moderate correlation of HULIS in the coarse mode with non-sea salt sulfate (nss-sulfate) (R2: 0.54) and ammonia (R2: 0.43) (Fig. 5). Nss-sulfate was the dominant source of sulfate in the coarse mode in our samples
þ Fig. 5. Correlations between (a) HULIS and non-sea salt SO2 4 ; (b) HULIS and NH4 in coarse mode.
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(74 ± 22% on an annual average). A possible formation pathway for coarse-mode nss-sulfate was coagulation of ammonium sulfate in the fine mode with sea salt/dust particles in the coarse mode (Bian et al., 2014). Small particles are more active in Brownian motion and thereby have a higher tendency for coagulation (Hinds, 1999;
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Seinfeld, 2006). Our calculation indicates that the coagulation coefficient between particles of 0.1 mm and 10 mm was approximately 20 times higher than the coefficient between particles of 1 mm and 10 mm (Kuang et al., 2016). We further examined the correlations between HULIS in coarse mode and other two modes (Fig. 6), which
Fig. 6. HULIS correlations between (a) condensation mode and coarse mode; (b) droplet mode and coarse mode.
Fig. 7. Size-specific relative abundance of water-soluble inorganic and organic fractions. (a) mass concentrations in individual size bins, (b) percent distributions of the watersoluble fractions in individual size bins, (c) mass concentrations in three size modes, and (d) percent distributions of the water-soluble fractions in three size modes.
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revealed a much better correlation of the coarse mode with the condensation mode (R2 ¼ 0.5) than with the droplet mode (R2 ¼ 0.2). This result of inter-size mode correlations supports the coagulation origin of the coarse mode HULIS. Kuang et al. (2016) reported similar positive inter-size mode correlations for individual organosulfate compounds, which are known HULIS constituents. WSOC_h in the coarse mode had poor correlations with other species in this size range (Fig. 2b and Table S6c). The lack of correlations may indicate multiple sources of comparable importance, however, the measured species in this work was not useful in indicating sources for WSOC_h. 3.3. Size-specific relative abundance of water-soluble inorganic and organic fractions WSOC co-exist with water-soluble inorganic species (WS_Inorg) (i.e. the inorganic ions) in ambient aerosols. It is necessary to know the size-specific relative abundance of the different water-soluble fractions for modeling and evaluating environmental impacts of aerosols involving interaction with water, as the water affinity property varies among them (Li et al., 2017). Fig. 7 presents the distribution of the three water-soluble fractions (i.e., WS_inorg, HULIS and WSOM_h) in individual size bins of the MOUDI sampler and in the three individual size modes, averaged over the whole data set. WS_Inorg sums up all the measured inorganic species, including sulfate, nitrate, chloride, ammonium, Kþ, Naþ, Ca2þ, and Mg2þ. WSOM_h is calculated to be the difference between WSOM and HULIS, with WSOM estimated from WSOC by multiplying a factor of 2.1 to account for the non-carbon atom mass (Lin et al., 2010b). The relative distribution of the three water-soluble fractions were varied among different size bins. The mass percentage of WS_Inorg had a significant increase in each successively larger size bin, from 19% in the smallest size bin to 74% in the 0.56e1.0 mm size bin, then remained relatively constant (74e82%) in the larger size bins (1.0e18 mm). The two WSOC components dominated the water-soluble aerosol mass for particles smaller than 0.32 mm. Liu et al. (2014) also observed substantial contributions of WSOC to water-soluble mass on ultrafine particles (<0.1 mm) in sizesegregated samples collected in a suburban location between Beijing and Tianjin. They further found significant correlations between the hygroscopicity parameter k and the mass fractions of WSOC. These size-resolved data of WSOC highlight the importance of differentiating hydrophilic and hydrophobic fractions when linking visibility degradation to chemical components. As for the distribution of the water-soluble fractions among different size modes, the water-soluble aerosol mass was dominated by the two WSOC components in the condensation mode while by WS_Inorg in the droplet and coarse modes. This observation is similar to that reported by Lin et al. (2010b) for five sets of size distribution measurements in a rural location of the PRD. 4. Summary This study reported a one-year variation of HULIS and WSOC_h size distribution in the urban environment of Hong Kong. Similar to WSOC, OC, and EC, their size distributions could be described with a superimposition of three size-modes, i.e., condensation, droplet, and coarse modes. The size distributions were characterized by a dominant droplet mode and less significant coarse mode and condensation mode. The seasonal contrast was most significant with the droplet mode, with noticeably higher abundance in winter than in summer, in line with the seasonality of PM pollution in our region.
The large data set obtained in this work allows meaningful exploration of interspecies and inter-size modes correlations to identify associated sources and formation processes. HULIS in all size modes were found to be well correlated with sulfate, suggesting a strong association of HULIS with sulfate formation processes. HULIS in the condensation mode was also moderately correlated with EC, in line with the knowledge that certain combustion processes of less combustion efficiency could emit pollutants falling in the HULIS fraction. HULIS in the coarse mode correlated with HULIS in the condensation mode, supporting the hypothesis of coagulation of HULIS on small particles with sea salt/ dust particles as the origin of the coarse mode HULIS. The sources and formation mechanism of WSOC_h were more multiple and complicated than HULIS, which requires more source-specific information than those measured in this work. The two water-soluble components (HULIS and WSOC_h) consisted of an important part of the water-soluble aerosol mass, especially in particles smaller than 0.32 mm. Their contribution was ~50% in the condensation mode, ~30% in the droplet mode, and ~20% in the coarse mode. Their inclusion is necessary for modeling aerosol properties involving interactions with water. Acknowledgments This study was supported by the Research Grants Council of Hong Kong (M-HKUST609/12 and 621312). We thank Drs. Siyuan Wang and Cheng Wu for making available their custom-made data treatment programs. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2017.07.009 References Altieri, K.E., Seitzinger, S.P., Carlton, A.G., Turpin, B.J., Klein, G.C., Marshall, A.G., 2008. Oligomers formed through in-cloud methylglyoxal reactions: chemical composition, properties, and mechanisms investigated by ultra-high resolution FT-ICR mass spectrometry. Atmos. Environ. 42, 1476e1490. Bian, Q., Huang, X.H.H., Yu, J.Z., 2014. One-year observations of size distribution characteristics of major aerosol constituents at a coastal receptor site in Hong Kong e Part 1: inorganic ions and oxalate. Atmos. Chem. Phys. 14, 9013e9027. Cavalli, F., Facchini, M.C., Decesari, S., Mircea, M., Emblico, L., Fuzzi, S., Ceburnis, D., Yoon, Y.J., O'Dowd, C.D., Putaud, J.P., Dell’Acqua, A., 2004. Advances in characterization of size-resolved organic matter in marine aerosol over the North Atlantic. J. Geophys. Res. Atmos. 109, D24. http://dx.doi.org/10.1029/ 2004jd005137. Decesari, S., Facchini, M.C., Matta, E., Lettini, F., Mircea, M., Fuzzi, S., Tagliavini, E., Putaud, J.P., 2001. Chemical features and seasonal variation of fine aerosol water-soluble organic compounds in the Po Valley, Italy. Atmos. Environ. 35, 3691e3699. Decesari, S., Facchini, M.C., Matta, E., Mircea, M., Fuzzi, S., Chughtai, A.R., Smith, D.M., 2002. Water soluble organic compounds formed by oxidation of soot. Atmos. Environ. 36, 1827e1832. Deng, X., Wu, D., Yu, J., Lau, A.K.H., Li, F., Tan, H., Yuan, Z., Ng, W.M., Deng, T., Wu, C., Zhou, X., 2013. Characterization of secondary aerosol and its extinction effects on visibility over the Pearl River Delta Region, China. J. Air Waste Manage. 63, 1012e1021. Dinar, E., Taraniuk, I., Graber, E.R., Katsman, S., Moise, T., Anttila, T., Mentel, T.F., Rudich, Y., 2006. Cloud Condensation Nuclei properties of model and atmospheric HULIS. Atmos. Chem. Phys. 6, 2465e2481. Dong, Y.J., Hays, M.D., Smith, N.D., Kinsey, J.S., 2004. Inverting cascade impactor data for size-resolved characterization of fine particulate source emissions. J. Aerosol Sci. 35, 1497e1512. Dou, J., Lin, P., Kuang, B.Y., Yu, J.Z., 2015. Reactive oxygen species production mediated by humic-like substances in atmospheric aerosols: enhancement effects by pyridine, imidazole, and their derivatives. Environ. Sci. Technol. 49, 6457e6465. Dzubay, T.G., Hasan, H., 1990. Fitting multimodal lognormal size distributions to cascade impactor data. Aerosol Sci. Tech. 13, 144e150. Gelencser, A., Hoffer, A., Kiss, G., Tombacz, E., Kurdi, R., Bencze, L., 2003. In-situ formation of light-absorbing organic matter in cloud water. J. Atmos. Chem. 45, 25e33.
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Size distributions of hydrophilic and hydrophobic fractions of water-soluble organic carbon in an urban atmosphere in Hong Kong Nijing Wang a, Jian Zhen Yu a, b, * a b
Fok Ying Tung Graduate School, Hong Kong University of Science & Technology, Hong Kong, China Department of Chemistry and Division of Environment, Hong Kong University of Science & Technology, Hong Kong, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Both hydrophilic and hydrophobic fractions of water-soluble organic carbon (WSOC) had a dominant droplet-mode. Organic materials contributed more than the inorganic ions to the watersoluble mass on particles smaller than 0.32 mm. The hydrophobic WSOC component on the coarse mode were found to come from condensation mode particles through coagulation. The hydrophobic WSOC component was well-correlated with sulfate in all size modes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2017 Received in revised form 28 June 2017 Accepted 4 July 2017 Available online 6 July 2017
Water-soluble organic carbon (WSOC) is a significant part of ambient aerosol and plays an active role in contributing to aerosol's effect on visibility degradation and radiation budget through its interactions with atmospheric water. Size-segregated aerosol samples in the range of 0.056e18 mm were collected using a ten-stage impactor sampler at an urban site in Hong Kong over one-year period. The WSOC samples were separated into hydrophilic (termed WSOC_h) and hydrophobic fractions (i.e., the humiclike substances (HULIS) fraction) through solid-phase extraction procedure. Carbon in HULIS accounted for 40 ± 14% of WSOC. The size distribution of HULIS was consistently characterized in all seasons with a dominant droplet mode (46e71%) and minor condensation (9.0e18%) and coarse modes (20 e35%). The droplet mode had a mass median aerodynamic diameter in the range of 0.7e0.8 mm. This size mode showed the largest seasonal variation in abundance, lowest in the summer (0.41 mg/m3) and highest in the winter (3.3 mg/m3). WSOC_h also had a dominant droplet mode, but was more evenly distributed among different size modes. Inter-species correlations within the same size mode suggest that the condensation-mode HULIS was partly associated with combustion sources and the dropletmode was strongly associated with secondary sulfate formation and biomass burning particle aging processes. There is evidence to suggest that the coarse-mode HULIS largely originated from coagulation of condensation-mode HULIS with coarse soil/sea salt particles. The formation process and possible sources of WSOC_h was more complicated and multiple than HULIS and need further investigation. Our
Keywords: Size distribution HULIS Hydrophilic WSOC Urban Hong Kong
* Corresponding author. Department of Chemistry, Hong Kong University of Science & Technology, Hong Kong, China. E-mail address: [email protected] (J.Z. Yu). http://dx.doi.org/10.1016/j.atmosenv.2017.07.009 1352-2310/© 2017 Elsevier Ltd. All rights reserved.
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measurements indicate that WSOC components contributed a dominant fraction of water-soluble aerosol mass in particles smaller than 0.32 mm while roughly 20e30% in the larger particles. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Water soluble organic carbon (WSOC) is a major fraction of organic carbon in ambient aerosol formation. Analytically it is feasible to further divide WSOC into two fractions, hydrophilic part and hydrophobic part using solid-phase extraction (SPE). Such a demarcation is useful in disentangling the role of organics in interacting with atmospheric water. The approach using an SPE step was initially developed to remove the inorganic salts, which are major aerosol constituents and co-exist with WSOC, as the inorganic salts would interfere chemical characterization of the aerosol organics in many advanced analytical instruments (e.g., LC/ MS). The commonly employed SPE stationary phases include silica with carbon chain (alkyl- or aryl-bonded silicas), function-bonded silica, alumina etc. (Hennion, 1999). In practice, this SPE separation step also removes the more hydrophilic fraction WSOC. The fraction that is retained on the SPE and later eluted with an organic solvent (e.g., methanol) is the hydrophobic part of WSOC, also called humic-like substances (HULIS) as they share similar spectroscopic properties to macromolecular humic substances in terrestrial and aquatic environments (Graber and Rudich, 2006; Kiss et al., 2002; Sannigrahi et al., 2006; Zappoli et al., 1999). Due to their surface active property and light absorbing ability (Dinar et al., 2006; Hoffer et al., 2006; Kiss et al., 2005), HULIS is expected to be an active component in cloud condensation nuclei (CCN) process (Gysel et al., 2004) and contribute to atmospheric light absorption (Lukacs et al., 2007). Recent studies also showed that HULIS contains redox active components contributing to health effects by ambient PM through catalyzing the generation of reactive oxygen species (Dou et al., 2015; Lin and Yu, 2011; Verma et al., 2012). The ambient measurements of HULIS in PM10 and PM2.5 for different locations have suggested that biomass burning and secondary formation through in-cloud processing as major sources for HULIS (Havers et al., 1998; Decesari et al., 2001; Gelencser et al., csy et al., 2008; 2002; Kiss et al., 2002; Cavalli et al., 2004; Kriva Salma et al., 2008; Lin et al., 2010a; Kuang et al., 2015; Park and Son, 2016). Most of the previous studies focused on the hydrophobic part of WSOC (i.e., HULIS) (Lin et al., 2010b; Kuang et al., 2015), the hydrophilic part in WSOC (here named as WSOC_h) was rarely commented on. Size distribution of ambient aerosol is important in understanding aerosol properties, possible sources and formation pathways. Up to now, size distributions of organic and elemental carbon (OCEC), WSOC and ion species have been extensively studied (e.g., McMurry and Zhang, 1989; Yu et al., 2004a,b, 2010; Huang and Yu, 2008; Park et al., 2013; Wang et al., 2013; Bian et al., 2014). In comparison, only three studies reported the size distribution of HULIS (Lin et al., 2010b; Salma et al., 2013; Park and Son, 2016) and the measurement period was limited to only 1e2 weeks to one month. There is no research reporting the ambient size distribution of hydrophilic part of WSOC. In this study, one-year size-resolved aerosol samples were collected at an urban site in Hong Kong. Size distribution information of both hydrophobic and hydrophilic WSOC, together with size-segregated data of OCEC and major ion species, were obtained. The main objective of this work is to identify the size distribution characteristics of HULIS and WSOC_h
in an urban environment and to determine the size-specific relative importance of organic and inorganic water-soluble mass, providing data for further understanding the role of water-soluble organics in visibility impairment and CCN formation. An additional objective is to gain knowledge of the sources and possible formation pathways of the two WSOC fractions in different size range through comparison with the size distribution data for other major species (WSOC, OC, EC and ion species). 2. Method 2.1. Sample collection The ambient size-resolved aerosol were collected in Tsuen Wan (22 2201800 N, 114 060 5200 E), a general air quality monitoring station of Hong Kong Environmental Protection Department (HKEPD). The sampler was located on the roof top of a 17 m building. Sizesegregated samples were collected onto pre-baked 47 mm quartz fiber filters using a micro-orifice uniform deposit impactor (MOUDI, MSP Corp., Shoreview, MN, USA), operating in a nonrotating mode and at a flow rate of 30 L min1. The sampler has 10 size stages in an aerodynamic diameter range of 0.056e18 mm. In order to compensate the reduced space between each adjacent impactor plate caused by quartz filters, special spacers of 0.127 cm (MSP Corp., Shoreview, MN, USA) were used in the sampler. The MOUDI sampler was returned to the laboratory for cleaning by alcohol after each sampling. The duration of each sample was 48 h (0:00 a.m. to 0:00 a.m.) and one sample was collected every 12 days from September 2013 to August 2014. Due to operational mistakes, samples collected in April 2014 were not valid. In late August and early September 2013, additional four sets of samples were collected. The sampling program yielded a total of 32 valid sets of samples. Two field blank samples were collected in each sampling trip. The collected filters were stored in polycarbonate petri-slide dishes lined with prebaked aluminum foil and placed in a refrigerator at 18 C before analysis. 2.2. Chemical analysis Half of each 47-mm substrate filter was used for WSOC and HULIS analysis, while the other half was used for analysis of ions and OCEC. The filter portion was extracted with 15 mL ultrapure water in an ultrasonic bath for 45 min. The extracts were filtered through a 0.45 mm PTFE (polytetrafluorethylene) syringe filter (Millipore, Billerica, MA, USA) and analyzed for WSOC using a TOC analyzer (Shimadzu TOC-VCPH, Japan). The instrument response was calibrated using sucrose. Due to relatively lower WSOC concentration for size-segregated samples, large volume of injection (200 mL) was applied. The method for HULIS isolation and qualification was adopted from our previous work (Lin et al., 2010b), which was modified from the one developed by Varga et al. (2001). In brief, 4.0 mL of the water extract was acidified to pH ¼ 2 using HCl and loaded on a SPE cartridge (Oasis HLB, 30 mm, 60 mg, Waters, USA). Before use, the cartridge was activated, cleaned and equilibrated by methanol, pure water and 0.01 M HCl in succession. The sample-loaded
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cartridge was rinsed with 2 1.0 mL pure water and this effluent solution contained the hydrophilic substances (inorganic ions, low molecular weight organic acids, sugar compounds etc.). The cartridge was then eluted with 3 0.5 mL methanol containing 2% ammonia (w/w) to obtain the HULIS eluate. Subsequently, the eluate was evaporated under a gentle N2 stream to dryness, redissolved in 0.5 mL water and quantified for HULIS mass concentration using a HPLC system equipped with an evaporative light scattering detector (ELSD) without any analytical column (ELSD3300, Alltech, USA). SRFA (Suwannee River Fulvic Acid, International Humic Substances Society) and NAFA (Nordic Aqueous Fulvic Acid) were chosen as calibration standards for ELSD. The two fulvic acid mixtures showed similar detection response on the ELSD, demonstrating that ELSD as a mass detector is insensitive to sample chemical composition (Lin et al., 2010b). For OC and EC determination, a quarter of each sample filter was loaded into an aerosol carbon analyzer (Sunset Laboratory, Tigard, OR, USA) and analyzed using a thermal/optical transmittance (TOT) method. The non-uniform particle deposition of the MOUDI samples prevents the use of laser correction to determine the split point between OC and EC (Huang and Yu, 2008). Hence, we manually set the split point at the point when the carrier gas was switched to 2% oxygen in helium. Ions were quantified using ion chromatography (IC; Dionex DX500, Thermos Fisher Scientific, MA, USA). A CS-12A column and methanesulfonic acid (MSA) as elution solution were used for 2þ separation in the analysis of the cations (i.e., Naþ, Kþ, NHþ 4 , Mg and Ca2þ). An AS-11 column and a gradient elution solution of 2 NaOH were used in the analysis of the anions (i.e., Cl, NO 3 , SO4 and Oxalate) (Yang et al., 2005). The concentrations of all chemical species (WSOC, HULIS, OC, EC, ion species) were corrected for blank levels determined using the field blanks. 3. Results and discussion 3.1. Size distribution characterization Size distribution of ambient aerosol is typically characterized by several partially superimposed size modes, each mode associated with different sources and formation pathways. Following the same approach used in previous studies (Dong et al., 2004; Dzubay and Hasan, 1990), we fit the measurement data for HULIS, WSOC and ion species assuming three log-normally distributed modes, that is, a condensation mode, a droplet mode and a coarse mode. Seasonal variation of size distribution was examined, with 6 sets of samples falling in spring (March to May), 10 sets in summer (June to August), 9 sets in fall (September to November), and 7 sets in winter (December to February). The individual sampling dates are listed in Table S1 in the supporting information (SI). Table 1 summarizes the seasonal average modal concentrations and mass median aerodynamic diameter (MMAD) for EC, OC, WSOC, HULIS, WSOC_h, and sulfate. Data for the other ion species are given in Table S3. 3.1.1. Size distribution of ion species Sources and size distribution characteristics of ions species are better known and can serve as valuable references for the less 2 þ known aerosol constituents. NHþ 4 , K , SO4 and oxalate shared a similar modal pattern characterized by a dominant droplet mode with MMAD ranging from 0.72 to 0.96 mm and minor condensation þ 2 and coarse modes (Fig. S1a in SI). For NHþ 4 , K and SO4 , the droplet mode concentrations were most prominent in winter (3.9, 0.39, and 12 mg m3, respectively) and oxalate in the droplet mode had similar abundance in fall and winter (0.26 mg m3). The lowest droplet mode concentration appeared in summer for all the four ion species (0.83, 0.03, 2.8, and 0.09 mg m3, respectively).
A second group of species, including Naþ, Mg2þ, and Ca2þ, had a distinctly different size distribution from the above-discussed group. They had a dominant coarse mode and a minor droplet mode (Table S3 and Fig. S1b), reflecting sea salt and crustal material as their dominant source origins. The MMAD of coarse-mode Ca2þ was 5.2e5.9 mm, slightly larger than that for Naþ and Mg2þ (4.6e5.1 mm). These ion species were more abundant in spring or fall with Naþ at 1.2 mg m3 in spring, Mg2þ at 0.16 mg m3 in spring and fall, and Ca2þ at 0.73 mg m3 in fall. Cl and NO 3 were in the third group, with their size distributions in summer and fall in common with those of Naþ, Mg2þ, and Ca2þ, but in spring and winter the fine mode became more significant (Table S3 and Fig. S1c). Fine-mode nitrate in winter time contributed 45% (2.1 mg m3) of the total nitrate. The characteristic of variable size distribution among different samples for nitrate is known to be a combined result of the semi-volatile nature of nitrate and its affinity for alkali sea salt and dust particles (Xue et al., 2014; Griffith et al., 2015). The same chemical properties underlying the variable size distributions of nitrate are also shared by chloride, and therefore they fall in the same group in their size distribution pattern. 3.1.2. Size distribution of EC, OC and WSOC The size distributions of OC and EC were dominated by the droplet mode (Fig. 1a and b). EC had a more significant droplet mode, accounting for 70% of total EC, compared with 50% for OC. While more of OC was present in condensation mode and coarse mode (16% and 34%, respectively) than EC (11% and 19%, respectively). Seasonally, EC in all the three modes and OC in droplet mode had much higher concentrations in winter compared with other seasons (Table 1). In urban areas, EC in condensation mode is most likely related to freshly emitted particles from vehicle emissions and previous studies reported the MMAD varied from 0.1 mm to 0.4 mm depending on vehicles types and operating conditions (Huang et al., 2006a; Yu and Yu, 2009). We observed comparable MMAD values (0.13e0.18 mm) and modal concentrations for the condensation-mode EC from spring to fall. But in winter, the condensation mode MMAD of EC shifted to 0.27 mm and the concentration was more than two times as much as the concentration in other seasons, suggesting additional EC sources in this season. WSOC was characterized with a dominant droplet mode (Fig. 1c), with 65% in this size mode. Significantly less WSOC mass was associated with the condensation mode (13%) and coarse mode (22%). Seasonally, WSOC concentration in summer (1.8 mg m3) was the lowest and its size distribution was more evenly distributed among the different size modes, with a comparable coarse mode (40%) and droplet mode fraction (36%); the highest WSOC concentration (5.2 mg m3) occurred in the winter and the droplet mode was the most prominent, accounting for 70% (Table 1). This seasonality is consistent with a previous study conducted in Shenzhen, an urban coastal city near Hong Kong, which also recorded higher fraction in winter (52%) than in summer (43%) (Huang et al., 2006b). 3.1.3. Size distribution of HULIS HULIS and WSOC_h are two comparable sub-fractions of WSOC in our samples. The HULIS fraction was quantified as its mass concentration, accounting for carbon and other non-carbon mass (Lin et al., 2010b). To facilitate the abundance comparison with WSOC_h, we applied a multiplier factor of 1.9 to calculate the concentration of carbon in HULIS (i.e., HULIS-C). This mass-tocarbon multiplier was derived from our previous research (Lin et al., 2010b) and the same value was reported by Kiss et al. (2002) for HULIS samples from a rural site in Hungary. On annual average, 40 ± 14% of WSOC was HULIS-C, summing over the entire
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Table 1 Modal concentrations (mg m3), mass median aerodynamic diameters (MMAD, mm) and geometric standard deviation (GSD) (s) of elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC), humic-like substances (HULIS), hydrophilic water-soluble organic carbon (WSOC_h) and SO2 4 in four seasons (spring, N ¼ 6 for HULIS and N ¼ 4 for WSOC_h; summer, N ¼ 10; fall, N ¼ 9; winter, N ¼ 7). Condensation
EC Spring Summer Fall Winter OC Spring Summer Fall Winter WSOC Spring Summer Fall Winter HULIS Spring Summer Fall Winter WSOC_h Spring Summer Fall Winter SO2 4 Spring Summer Fall Winter
Droplet
Coarse
C (mg/m )
MMAD (mm)
GSD (s)
C (mg/m )
MMAD (mm)
GSD (s)
C (mg/m3)
MMAD (mm)
GSD (s)
0.30 0.42 0.45 1.07
0.13 0.18 0.18 0.27
1.41 1.61 1.58 1.85
3.69 1.95 3.36 5.17
0.64 0.60 0.72 0.74
1.82 1.70 1.66 1.61
0.95 0.73 0.61 1.56
4.84 4.27 3.81 3.19
1.73 1.74 2.06 1.93
0.69 0.63 1.14 1.15
0.11 0.15 0.18 0.18
1.59 1.60 1.91 1.87
2.47 1.76 3.14 3.76
0.64 0.67 0.76 0.77
2.07 2.25 1.70 1.75
1.47 1.30 2.54 2.06
5.05 5.06 4.58 4.09
1.76 1.62 2.09 1.89
0.49 0.43 0.40 0.54
0.22 0.16 0.15 0.27
1.80 1.59 1.72 1.89
1.86 0.64 3.12 3.60
0.80 0.62 0.74 0.80
1.75 1.50 1.73 1.62
0.70 0.71 0.73 1.03
4.00 3.63 4.60 4.02
1.95 1.80 1.65 1.98
0.35 0.16 0.69 0.43
0.21 0.17 0.23 0.21
1.55 1.98 1.83 1.62
2.08 0.41 2.28 3.26
0.77 0.74 0.79 0.79
1.70 1.54 1.65 1.50
0.73 0.31 0.95 0.93
5.07 5.26 4.75 4.01
1.89 1.68 1.79 1.93
0.46 0.32 0.28 0.11
0.20 0.13 0.18 0.10
1.97 1.69 1.79 1.46
1.29 0.59 1.58 2.30
0.78 0.64 0.78 0.85
1.70 1.66 1.55 2.03
0.48 0.55 0.51 0.60
3.10 3.85 3.35 5.58
1.91 1.85 2.11 2.09
0.51 0.18 1.09 0.69
0.34 0.20 0.31 0.29
1.63 1.61 1.70 1.66
8.04 2.76 8.25 11.99
0.87 0.72 0.89 0.89
1.50 1.44 1.57 1.60
1.72 0.75 1.56 1.98
3.26 4.10 3.06 3.33
1.81 1.71 1.91 1.60
3
3
size range. For fine particles (smaller than 3.2 mm), 42 ± 13% of WSOC were HULIS-C. This percent value was similar to measurements made for PM2.5 at different locations in previous studies csy et al., 2008; Lin et al., 2010b; Kuang (Salma et al., 2008; Kriva et al., 2015). Fig. 1d shows the average size distributions of HULIS in different seasons. The sample-by-sample distribution of HULIS in three size modes are shown in Fig. S2. HULIS had a similar seasonal variation in its size distribution to those of EC, WSOC and ion species of NHþ 4, Kþ, SO2 4 and oxalate. On the annual average, the droplet mode was most prominent, accounting for 64% (2.0 mg m3), with the condensation mode making up 13% (0.41 mg m3) and the coarse mode 23% (0.73 mg m3). In comparison with three previous studies reporting HULIS size distribution, our result had more in common with those recorded in Lin et al. (2010b) and Park and Son (2016). Lin et al. (2010b) reported 81% of HULIS associated with the droplet mode in aerosol collected in a rural location in the Pearl River Delta (PRD) region in November. Park and Son (2016) measured 58% of HULIS in the size range of 0.32e1.0 mm in aerosol collected at an urban site in Gwangju, Korea in February 2015. In the study by Salma et al. (2013), MOUDI samples were collected during April to May at a curbside site in central Budapest. They observed that the condensation-mode HULIS-C was more abundant (51%) than the droplet mode (31%), possibly due to the elevated influence from vehicle-related sources. Seasonally, the highest droplet-mode concentration (3.3 mg m3) appeared in winter accounting for 71% of total HULIS mass; the lowest (0.41 mg m3) was observed in summer; similar concentrations were recorded in spring (2.1 mg m3) and fall (2.3 mg m3). The seasonality of the HULIS droplet mode abundance 2 was similar to EC, Kþ, NHþ 4 and SO4 , suggesting possible source
commonality among those species. This result is consistent with many previous studies reporting evidence for secondary formation and biomass burning as two major HULIS formation sources (Altieri et al., 2008; Gelencser et al., 2003; Hoffer et al., 2004; Holmes and Petrucci, 2006; Lin et al., 2010b). 3.1.4. Size distribution of hydrophilic organic carbon of WSOC (WSOC_h) WSOC_h was obtained by subtracting the hydrophobic organic carbon from WSOC (WSOC_h ¼ WSOC-HULIS-C). Fig. 1e shows the seasonal average size distributions and Table 1 lists the modal concentrations and MMAD values. Among the three size modes of WSOC_h, the coarse mode had a relatively higher fraction in summer while the droplet mode dominated in winter when the total WSOC_h was most abundant (3.0 mg m3). The MMAD of the three modes were in the similar range to those of HULIS and WSOC. WSOC_h also shared the same seasonality in its droplet-mode concentration as HULIS (winter > fall > spring > summer), which indicated droplet-mode HULIS and WSOC_h might have similar sources. A few dissimilarities were found between HULIS and WSOC_h in their size distribution characteristics. First, WSOC_h in condensation mode showed a seasonality different from that of HULIS. For HULIS, its condensation mode was at a higher concentration in fall and at a lower level in summer. For WSOC_h, the lowest concentration level was in winter (0.11 mg m3) and comparable concentrations (0.28e0.46 mg m3) were in other seasons. Second, WSOC_h in summer was roughly evenly distributed among the three modes while HULIS in summer had its mass mainly distributed in the coarse mode and droplet modes (37% and 40%, respectively) and little in the condensation mode. Third, in
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Fig. 1. Lognormal Size distribution of MOUDI samples in Tsuen Wan, Hong Kong in four seasons (2013e2014): (a) elemental carbon (EC); (b) organic carbon (OC); (c) water-soluble organic carbon (WSOC); (d) humic-like substances (HULIS); (e) hydrophilic water-soluble organic carbon (WSOC_h).
comparison with HULIS and WSOC, the droplet mode of WSOC_h was broader in winter with a standard deviation (s) of 2.0.
size-mode based on 32 sets MOUDI one-year data are shown in Fig. 2a and 2b and tabulated in Tables S6a-S6c.
3.2. Correlations of HULIS and WSOC_h with other species in three modes
3.2.1. Condensation mode Particles in condensation mode was associated with substances that are freshly emitted plus those condensates added soon after emissions. HULIS had a moderate correlation with EC (R2 ¼ 0.40, Fig. 3a) suggesting EC and HULIS residing in this size mode may partly share emission sources. Kuang et al. (2015) carried out source apportionment of PM2.5 HULIS-C at one urban site and one suburban site near a port in the PRD region and found vehicle emissions
The larger data set obtained in this study allows us to probe the source origin of HULIS and WSOC_h through examining interspecies correlations in the same size mode and inter-size mode correlations for the same species. The coefficient of determination (R2) among species in each
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sharing common sources between the two. Bian et al. (2014) illustrated the possible source for condensation-mode sulfate was freshly formed from oxidation of SO2. It is plausible that the condensation mode sulfate particles could be effective in contributing to secondary HULIS formation through promoting heterogeneous reactions with oxygenated atmospheric oxidation products of volatile organic compounds (e.g., Surratt et al., 2008). The moderate correlations of HULIS with sulfate and EC suggested the condensation mode HULIS may come from combined primary EC emission sources and secondary formation involving freshly formed sulfate particles. WSOC_h had little or weak correlations with other measured chemical species (Fig. 2b). Consequently, chemical data obtained in this work was not informative in suggesting possible sources or insights into primary vs secondary origin for the condensationmode WSOC_h. We note that lack of correlation is not a definite indication of different source origins. For example, two secondary species formed from two primary pollutants of a common emission source may be poorly correlated as a result of multiple transformation in the atmosphere.
Fig. 2. Coefficient of determination (R2) of (a) HULIS and (b) WSOC_h with other species.
contributed little to HULIS-C but a part of HULIS-C was associated with ship emissions. The sampling site in this work is located near Hong Kong's container terminal, which is among the busiest shipping ports in the world (Yu et al., 2004a). Contribution of ship emissions as a common source was likely the underlying cause for the correlation between EC and HULIS. The concentration of Kþ in condensation mode was generally low (<0.1 mg m3) and for several samples no Kþ was detected on stages of smaller particle size bins. Kþ in the condensation mode had weak correlation with HULIS (R2 ¼ 0.22). Therefore, biomass burning was unlikely a significant source for HULIS in condensation mode. EC and Kþ were poorly correlated with each other, indicating biomass burning was not a major source for EC in the condensation mode either. HULIS was moderately correlated with sulfate in the condensation mode (R2 ¼ 0.49, Fig. 3b). Sulfate had no correlation with EC (R2 ¼ 0.05) in the condensation mode, negating the possibility of
3.2.2. Droplet mode In the droplet mode, HULIS and WSOC_h had good correlations 2 with the same set of species (e.g., EC, Kþ, NHþ 4 , SO4 and oxalate) (Fig. 2) and were moderately correlated with each other (R2 ¼ 0.45). 2 HULIS was strongly correlated with SO2 4 (R ¼ 0.83, Fig. 4a) and 2 þ NH4 (R ¼ 0.71). The results support the suggestion that secondary formation involving in-cloud processing might be a major source for HULIS in this region. In winter, secondary sulfate was much more abundant than summer and the relative humidity in winter (63e84%) was slightly lower than summer (75e84%) in Tsuen Wan. This is likely a result of more abundant precursors driving enhanced secondary formation in winter time, despite an anticipated lower level of photochemical oxidants in this season. Deng et al. (2013) also found that secondary aerosol formation was more significant on polluted days with PM2.5 exceeding 65 mg m3 in the PRD region. Unlike the condensation mode, HULIS and Kþ in the droplet mode were well-correlated (R2 ¼ 0.78, Fig. 4c). It was possibly because biomass burning particles were easy to be cloud activated and subsequently became larger after addition of sulfate (Huang et al., 2006b). EC in the droplet mode also had a better correlation with HULIS (R2 ¼ 0.67, Fig. 4b) and Kþ (R2 ¼ 0.77). The strong correlations suggest that Kþ, EC, and HULIS in the droplet size were likely associated with the same atmospheric aging processes, such as growth from condensation mode to droplet mode through cloud processing and subsequent cloud evaporation (Decesari et al., 2002; Huang and Yu, 2008; Lin et al., 2010b; Reid et al., 2005). Compared with HULIS, WSOC_h had weaker but still good
Fig. 3. Correlation between (a) HULIS and EC; (b) HULIS and SO2 4 in condensation mode.
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þ Fig. 4. Correlation between (a) HULIS and SO2 4 ; (b) HULIS and EC; (c) HULIS and K in droplet mode.
correlation with the same suite of species mentioned above (Fig. 2b). It is worth noting that the correlations of WSOC_h with the secondary species and Kþ were not as strong as the correlations for HULIS. In contrast, WSOC_h and EC had a slightly better correlation (R2 ¼ 0.69) than HULIS and EC (R2 ¼ 0.67). This appears to suggest that there may be EC related sources contributing to droplet-mode WSOC_h but not to HULIS. Recent work from our group on source apportioning of PM2.5 in Tsuen Wan indicated that a significant part of WSOC_h was associated with vehicle emissions (Kuang, 2017). Previous research also identified vehicular exhaust as a source for WSOC_h in PM2.5 in an urban site in Guangzhou and it was suggested that high fuel combustion efficiency in vehicles resulted in emitting low molecular weight WSOC, i.e. hydrophilic WSOC (Kuang et al., 2015). Two studies reporting size distribution characteristics of EC in our study region found two EC condensation modes, peaked at both 0.1 mm and 0.4 mm. It was suggested that both sizes of EC originated from primary vehicle emissions but from different vehicle fleet and/or vehicles operating at different road
loadings (Huang et al., 2006a; Yu and Yu, 2009). In this study, EC and WSOC_h similarly peaked at around 0.2 mm and 0.7 mm in condensation mode and droplet mode, respectively, with an overlap ranging from 0.2 to 0.6 mm (Fig. 1a and 1e). The 0.4 mm EC size mode was not resolved in our work due to limited number of size bins. It would have been lumped into the droplet mode in our three-mode size distribution description, explaining that the good correlation of WSOC_h and EC was a result of common vehicular emission source. 3.2.3. Coarse mode A significant fraction (23 ± 7%) of HULIS was in the coarse mode. No previous studies have explained the origin of HULIS on the coarse particles due to limited data. The correlation analysis revealed a moderate correlation of HULIS in the coarse mode with non-sea salt sulfate (nss-sulfate) (R2: 0.54) and ammonia (R2: 0.43) (Fig. 5). Nss-sulfate was the dominant source of sulfate in the coarse mode in our samples
þ Fig. 5. Correlations between (a) HULIS and non-sea salt SO2 4 ; (b) HULIS and NH4 in coarse mode.
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(74 ± 22% on an annual average). A possible formation pathway for coarse-mode nss-sulfate was coagulation of ammonium sulfate in the fine mode with sea salt/dust particles in the coarse mode (Bian et al., 2014). Small particles are more active in Brownian motion and thereby have a higher tendency for coagulation (Hinds, 1999;
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Seinfeld, 2006). Our calculation indicates that the coagulation coefficient between particles of 0.1 mm and 10 mm was approximately 20 times higher than the coefficient between particles of 1 mm and 10 mm (Kuang et al., 2016). We further examined the correlations between HULIS in coarse mode and other two modes (Fig. 6), which
Fig. 6. HULIS correlations between (a) condensation mode and coarse mode; (b) droplet mode and coarse mode.
Fig. 7. Size-specific relative abundance of water-soluble inorganic and organic fractions. (a) mass concentrations in individual size bins, (b) percent distributions of the watersoluble fractions in individual size bins, (c) mass concentrations in three size modes, and (d) percent distributions of the water-soluble fractions in three size modes.
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revealed a much better correlation of the coarse mode with the condensation mode (R2 ¼ 0.5) than with the droplet mode (R2 ¼ 0.2). This result of inter-size mode correlations supports the coagulation origin of the coarse mode HULIS. Kuang et al. (2016) reported similar positive inter-size mode correlations for individual organosulfate compounds, which are known HULIS constituents. WSOC_h in the coarse mode had poor correlations with other species in this size range (Fig. 2b and Table S6c). The lack of correlations may indicate multiple sources of comparable importance, however, the measured species in this work was not useful in indicating sources for WSOC_h. 3.3. Size-specific relative abundance of water-soluble inorganic and organic fractions WSOC co-exist with water-soluble inorganic species (WS_Inorg) (i.e. the inorganic ions) in ambient aerosols. It is necessary to know the size-specific relative abundance of the different water-soluble fractions for modeling and evaluating environmental impacts of aerosols involving interaction with water, as the water affinity property varies among them (Li et al., 2017). Fig. 7 presents the distribution of the three water-soluble fractions (i.e., WS_inorg, HULIS and WSOM_h) in individual size bins of the MOUDI sampler and in the three individual size modes, averaged over the whole data set. WS_Inorg sums up all the measured inorganic species, including sulfate, nitrate, chloride, ammonium, Kþ, Naþ, Ca2þ, and Mg2þ. WSOM_h is calculated to be the difference between WSOM and HULIS, with WSOM estimated from WSOC by multiplying a factor of 2.1 to account for the non-carbon atom mass (Lin et al., 2010b). The relative distribution of the three water-soluble fractions were varied among different size bins. The mass percentage of WS_Inorg had a significant increase in each successively larger size bin, from 19% in the smallest size bin to 74% in the 0.56e1.0 mm size bin, then remained relatively constant (74e82%) in the larger size bins (1.0e18 mm). The two WSOC components dominated the water-soluble aerosol mass for particles smaller than 0.32 mm. Liu et al. (2014) also observed substantial contributions of WSOC to water-soluble mass on ultrafine particles (<0.1 mm) in sizesegregated samples collected in a suburban location between Beijing and Tianjin. They further found significant correlations between the hygroscopicity parameter k and the mass fractions of WSOC. These size-resolved data of WSOC highlight the importance of differentiating hydrophilic and hydrophobic fractions when linking visibility degradation to chemical components. As for the distribution of the water-soluble fractions among different size modes, the water-soluble aerosol mass was dominated by the two WSOC components in the condensation mode while by WS_Inorg in the droplet and coarse modes. This observation is similar to that reported by Lin et al. (2010b) for five sets of size distribution measurements in a rural location of the PRD. 4. Summary This study reported a one-year variation of HULIS and WSOC_h size distribution in the urban environment of Hong Kong. Similar to WSOC, OC, and EC, their size distributions could be described with a superimposition of three size-modes, i.e., condensation, droplet, and coarse modes. The size distributions were characterized by a dominant droplet mode and less significant coarse mode and condensation mode. The seasonal contrast was most significant with the droplet mode, with noticeably higher abundance in winter than in summer, in line with the seasonality of PM pollution in our region.
The large data set obtained in this work allows meaningful exploration of interspecies and inter-size modes correlations to identify associated sources and formation processes. HULIS in all size modes were found to be well correlated with sulfate, suggesting a strong association of HULIS with sulfate formation processes. HULIS in the condensation mode was also moderately correlated with EC, in line with the knowledge that certain combustion processes of less combustion efficiency could emit pollutants falling in the HULIS fraction. HULIS in the coarse mode correlated with HULIS in the condensation mode, supporting the hypothesis of coagulation of HULIS on small particles with sea salt/ dust particles as the origin of the coarse mode HULIS. The sources and formation mechanism of WSOC_h were more multiple and complicated than HULIS, which requires more source-specific information than those measured in this work. The two water-soluble components (HULIS and WSOC_h) consisted of an important part of the water-soluble aerosol mass, especially in particles smaller than 0.32 mm. Their contribution was ~50% in the condensation mode, ~30% in the droplet mode, and ~20% in the coarse mode. Their inclusion is necessary for modeling aerosol properties involving interactions with water. Acknowledgments This study was supported by the Research Grants Council of Hong Kong (M-HKUST609/12 and 621312). We thank Drs. Siyuan Wang and Cheng Wu for making available their custom-made data treatment programs. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2017.07.009 References Altieri, K.E., Seitzinger, S.P., Carlton, A.G., Turpin, B.J., Klein, G.C., Marshall, A.G., 2008. Oligomers formed through in-cloud methylglyoxal reactions: chemical composition, properties, and mechanisms investigated by ultra-high resolution FT-ICR mass spectrometry. Atmos. Environ. 42, 1476e1490. Bian, Q., Huang, X.H.H., Yu, J.Z., 2014. One-year observations of size distribution characteristics of major aerosol constituents at a coastal receptor site in Hong Kong e Part 1: inorganic ions and oxalate. Atmos. Chem. Phys. 14, 9013e9027. Cavalli, F., Facchini, M.C., Decesari, S., Mircea, M., Emblico, L., Fuzzi, S., Ceburnis, D., Yoon, Y.J., O'Dowd, C.D., Putaud, J.P., Dell’Acqua, A., 2004. Advances in characterization of size-resolved organic matter in marine aerosol over the North Atlantic. J. Geophys. Res. Atmos. 109, D24. http://dx.doi.org/10.1029/ 2004jd005137. Decesari, S., Facchini, M.C., Matta, E., Lettini, F., Mircea, M., Fuzzi, S., Tagliavini, E., Putaud, J.P., 2001. Chemical features and seasonal variation of fine aerosol water-soluble organic compounds in the Po Valley, Italy. Atmos. Environ. 35, 3691e3699. Decesari, S., Facchini, M.C., Matta, E., Mircea, M., Fuzzi, S., Chughtai, A.R., Smith, D.M., 2002. Water soluble organic compounds formed by oxidation of soot. Atmos. Environ. 36, 1827e1832. Deng, X., Wu, D., Yu, J., Lau, A.K.H., Li, F., Tan, H., Yuan, Z., Ng, W.M., Deng, T., Wu, C., Zhou, X., 2013. Characterization of secondary aerosol and its extinction effects on visibility over the Pearl River Delta Region, China. J. Air Waste Manage. 63, 1012e1021. Dinar, E., Taraniuk, I., Graber, E.R., Katsman, S., Moise, T., Anttila, T., Mentel, T.F., Rudich, Y., 2006. Cloud Condensation Nuclei properties of model and atmospheric HULIS. Atmos. Chem. Phys. 6, 2465e2481. Dong, Y.J., Hays, M.D., Smith, N.D., Kinsey, J.S., 2004. Inverting cascade impactor data for size-resolved characterization of fine particulate source emissions. J. Aerosol Sci. 35, 1497e1512. Dou, J., Lin, P., Kuang, B.Y., Yu, J.Z., 2015. Reactive oxygen species production mediated by humic-like substances in atmospheric aerosols: enhancement effects by pyridine, imidazole, and their derivatives. Environ. Sci. Technol. 49, 6457e6465. Dzubay, T.G., Hasan, H., 1990. Fitting multimodal lognormal size distributions to cascade impactor data. Aerosol Sci. Tech. 13, 144e150. Gelencser, A., Hoffer, A., Kiss, G., Tombacz, E., Kurdi, R., Bencze, L., 2003. In-situ formation of light-absorbing organic matter in cloud water. J. Atmos. Chem. 45, 25e33.
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