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Chemical characterization of water-soluble components of PM10 and PM2.5 atmospheric aerosols in five locations of Nanjing, China
AE International – Asia Atmospheric Environment 37 (2003) 2893–2902
Chemical characterization of water-soluble components of PM10 and PM2.5 atmospheric aerosols in five locations of Nanjing, China Gehui Wang*, Hui Wang, Yajuan Yu, Shixiang Gao, Jianfang Feng, Songting Gao, Liansheng Wang School of the Environment, State Key Laboratory of Pollution Control and Resources Reuse, Nanjing University, Nanjing 210093, People’s Republic of China Received 10 October 2002; accepted 31 March 2003
Abstract A comprehensive survey for atmospheric pollution was carried out from February to December 2001 in Nanjing city in order to better control the problem. As part of the work, the current study mainly aims at the chemical characterization of water-soluble species of PM10 and PM2.5 atmospheric aerosols, together with their spatial variations. Much heavier particle loadings is observed in Nanjing city, which is more than 4–6 times of NAAQS of the USA, especially fine particles. During the sampling time, 63–77% of PM10 mass is in the PM2.5 fraction. Water-soluble fractions of PM10 and PM2.5 are acidic, and the acidity of PM2.5 is stronger than that of PM10. Water-soluble organic carbon, accounting for about 10% of particle mass, is the most significant component of the water-soluble fraction and 2 2+ shows no clear spatial variations. NO , K+ and Na+ are also abundant 3 , SO4 , NHx (ammonia and ammonium), Ca in PM10 and PM2.5 aerosols. Among the detected water-soluble inorganic chemicals SO2 4 presented the first highest level of concentration in PM10 (16.70–23.51 mg/m3, 3.3–10.9% of the PM10 mass) and in PM2.5 (13.19–20.24 mg/m3, 3.8– 2+ 11.2% of the PM2.5 mass), NO , K+ and Na+. 3 and NHx are the second highest level of chemicals followed by Ca Nearly all those water-soluble inorganic components displayed the higher level of concentrations in the traffic center (SY) and the downtown area (FZ), and presented the lower level of concentrations in the scenery district and the residential area. The total mass of water-soluble fraction, including inorganic species and organic chemicals, also presented the higher concentrations (78.97–108.68 mg/m3, 15.7–42.9% of particle mass) at both sites of SY and FZ. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: PM10 and PM2.5 aerosols; Water-soluble organic carbon; Inorganic ions; Site difference
1. Introduction Atmospheric aerosol is termed as tiny liquid or solid particulate matter suspended in the air. In the last decades atmospheric aerosols have received much attention due to their potential influence on the global *Corresponding author. Chemistry Department, University of California-Davis, Davis, CA 95616, USA. Tel.: +1-530-7523397; fax: +1-530-752-8995. E-mail address: [email protected] (G. Wang).
radiation budget by acting as negative forcings and adverse impact on human health by inducing chronic respiratory illness, cancer and premature death. Watersoluble fraction of atmospheric aerosol contains many important compounds, which can change the size, composition, number and lifetime of aerosol owing to their hygroscopic nature (Jacobson et al., 2000; Intergovenmental Panel on Climate Change (IPCC), 1995; Environmental Protection Agency (USEPA), 1996; Novakov and Penner, 1993). On the other hand, water-soluble components increase the solubility of
1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00271-1
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toxic organic compounds, such as n-alkanes and polycyclic aromatic hydrocarbons (PAHs), by acting as surface active reagents and therefore increase their toxicity to human health. Many epidemic studies have linked airborne concentrations of PM10 (particulate matter with aerodynamic diameter less than 10 mm) and PM2.5 (particulate matter with aerodynamic diameter less than 2.5 mm) with a variety of health outcomes, including both morbidity and mortality. On average about 185–483 106 ton global aerosol per year is caused by anthropogenic sources including transportation, stationary combustion, industrial process, solid waste disposal and other miscellaneous sources (Bridgman, 1990). Most of which is concentrated in the Northern Hemisphere, especially downwind of industrial sites, slash-and-burn agriculture regions, and overgrazed grasslands. Chinese economic, industrial and demographic characteristics are different from those of North America and Europe (Zhang and Friedlander, 2000), which make characteristics of its atmospheric pollution differ from those in western countries. Higher level of ambient particulate loadings has been a persistent and the most serious air pollution problem in Nanjing city, a conurbation located in the eastern China with over 3 million of population. A comprehensive survey of PM10 and PM2.5 atmospheric aerosols was performed in Nanjing city from February to December 2001 in order to better control the problem. Many studies have concerned about the organic fraction (Kadowaki, 1990; Zhang et al., 2000) and heavy metals (Testa and Mosello, 1996; Smirnov et al., 1998; Pina et al., 2000) of atmospheric aerosols. However, they have mainly focused on the water-insoluble fractions of the particulate matter. The current study is devoted to the chemical characterization of water-soluble fractions of PM10 and PM2.5 in five locations of Nanjing city, involving pH, conductivity (K), water-soluble organic carbon (WSOC), water-soluble inorganic carbon (WSIC), total water-soluble carbon (TWSC) and ions, as well as to their spatial variations in the urban area.
2. Experimental part 2.1. Sampling locations Five sites were chosen for this study: (1) Suyuan Hotel (SY, 32 030 18.200 N, 118 460 44.300 E), (2) Fuzi Temple (FZ, 32 010 20.000 N, 118 470 04.700 E), (3) Ruijin Village (RJ, 32 010 58.200 N, 118 480 21.500 E), (4) Zhongshan Mausoleum (ZS, 32 030 42.900 N, 118 500 47.900 E), (5) Yangzi School (YZ, 32 030 43.100 N, 118 500 48.000 E). SY is located nearby Zhujiang highway, a heavy traffic center of Nanjing. RJ is a big residential area and located with FZ, a downtown, in the southern areas of
the city. ZS is a scenery district and located in the east of Nanjing. YZ is a small residential area and lies in a large-scale petrochemical district, which is located in the northwestern Nanjing city. 2.2. Collection materials and sampling protocols Two kinds of samplers have been used during the campaign. KC-2000 for PM10 (made in China) and Anderson GT22001 for PM2.5 (made in USA) are high volume air samplers, of which flow rates are 1.05 and 1.12 m3/min, respectively. The glass fiber filters (200 250 mm2) was baked at 500 C for 2 h in order to eliminate organic species, conditioned in a desiccator for 24 h, and then weighted with Mettler AE240 electronic balance (limit of detection is 10 mg). After sampling, the filters were also equilibrated in the desiccator for 24 h prior to weight, and stored at 18 C before analysis (Wang et al., 2002). PM10 and PM2.5 were simultaneously collected for 8 h from 8:30 a.m. to 4:30 p.m. at each site, which was used to reflect characteristics of the air pollution in the daytime. The heights of the two air inlets are 1.5 m above the ground. For shedding light on the characteristics of mass distribution of atmospheric aerosols in Nanjing city, the Flow Sensor sampler (made in USA), a multi-orifice, multi-stage inertial impactor, was used in campus of Nanjing University to collect ambient particles according to 9 kinds of different sizes. The aerodynamic cutoff points at the flow rate 1.70 m3/h are: X9.0, 9.0–6.0, 6.0– 4.6, 4.6–3.3, 3.3–2.2, 2.2–1.1, 1.1–0.7, 0.7–0.41 and 0.41– 0 mm. The aerosol collection had been conducted in the four seasons. The sampling date which we chosen is based on following meteorological aspects: (1) there was no rain over three-consecutive days prior to sampling, (2) the atmospheric temperature was of seasonal characteristic. 2.3. Determination for conductivity, pH, TWSC, WSIC, WSOC and ions of water-soluble species of the aerosols One-fourth filter was cut into pieces, ultrasonically extracted with 25 ml deionized water for 30 min, and then filtered through 0.45 mm millipore filters into two parts. One part was determined at 25 C for pH, conductivity, TWSC and WSIC of water-soluble species with Rize pH meter, DDSI-308A Mode Conductivitimeter (made in China) and Shimadzu TOC-5000 total organic analyzer (made in Japan), respectively. WSOC was obtained by subtracting WSIC from TWSC. Another part was determined for cations by inductively coupled plasma spectroscopy—AES, and then determined for anions with ion chromatography (Dionex-300) and NHx (ammonia and ammonium) with spectrophotometer (UV-2200)). The reported data of conductivity, TWSC,
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exceeding 65 mg/m3 (NAAQS) at the same five sampling sites were 100%, 100%, 37.5%, 100% and 100%, respectively. Compared with other metropolitan areas located in East Asia, the atmospheric particles concentrations observed in Nanjing city were much higher. For example, PM10 concentrations in Seoul, Korea was about 75 mg/m3 between 1987 and 1991 (Hashimoto et al., 1994) and in Hongkong was average of 55.3 mg/m3 between 1990 and 1994 (Qin et al., 1997). The mass size distribution of atmospheric aerosols in campus of Nanjing University was shown in Fig. 1, along with the geometric standard deviation (GSD) and mass median diameter (MMD) relative to each mode. Atmospheric aerosols in the 4 months has all displayed the bimodal distribution: accumulation (Dp o1 mm) and coarse (Dp >1 mm) modes. Although the distribution of atmospheric aerosols can be multimodal, only an accumulation mode with equivalent aerodynamic diameter below 1 mm and a coarse mode with equivalent aerodynamic diameter above 1 mm can be clearly distinguished for most ambient aerosols. Similar values of GSD and MMD demonstrated the same characteristics of aerosol particles distribution in the 3 months (i.e. March, May and December 2001) other than in the September. Accumulation mode of aerosol in the September presented much larger value of GSD and less value of MMD, which was mainly attributed to much lower mass concentration of fine particles. Most of fine particles are secondary aerosols largely formed through the reactions of gas in the atmosphere and in the submicrometer size range. While most of coarse particles are primary aerosols directly emitted into the atmosphere and generally in the submicrometer size range.
WSOC, WSIC, and ion concentrations of samples were all corrected by field blank sample.
3. Results and discussion 3.1. PM10 and PM2.5 mass Table 1 shows the PM10 and PM2.5 mass concentrations during the survey. Of the five sites the first highest level of particle concentrations was observed at site SY with averages of 423 mg/m3 for PM2.5, 209 mg/m3 for PM2.5–10 and 644 mg/m3 for PM10. The second highest level of particle mass were observed at site FZ with mean values of 215 mg/m3 for PM2.5, 89 mg/m3 for PM2.5–10 and 304 mg/m3 for PM10. The other three sites had the similar level of particle loadings. Sampling site SY lies in Zhujiang motorway, the traffic center of Nanjing city, in which numerous motor vehicles emitted a great deal of exhaust. In Nanjing city a kind of vehicle named doublestroke autobike is widely being used for personal traffic, of which engine burns a mixture of gasoline and diesel oil. The way to make power releases a great deal of pollutants due to incomplete combustion. Enormous volume of traffic and low efficient engine combustion are attribute to the high level of particle concentrations at site SY. FZ sampling site is located in a downtown area of Nanjing, where there is a heavy traffic highway about 500 m away. The other three sampling sites are all covered with good vegetation and far away from traffic motorways, which resulted in the lower particle loadings in the air. From Table 1 it could be found that PM2.5 accounted for 63–77% of PM10, which suggests that fine particles with aerodynamical diameter less than 2.5 mm is the major component of atmospheric particulate matter in Nanjing. Table 1 also indicates that coarse particles with aerodynamical diameter from 2.5 to 10 mm accounted for 24–37% of PM10. Frequencies of PM10 concentrations exceeding 150 mg/ m3 (NAAQS) at SY, RJ, ZS, FZ and YZ were 100%, 75%, 62.5%, 100% and 50%, respectively, during the survey. While frequencies of PM2.5 concentrations
3.2. Conductivity (K), pH, TWSC, WSIC and WSOC of PM10 and PM2.5 Table 2 shows the average, minimum, maximum and standard deviation of pH, K; TWSC, WSIC and WSOC of the water-soluble fractions. Conductivity (K) of
Table 1 PM10 and PM2.5 loadings at each site during the sampling period Site
SY RJ ZS FZ YZ
N
8 8 8 8 8
PM2.5 (mg/m3)
PM2.5–10 (mg/m3)
PM10 (mg/m3)
PM2.5/PM10
PM2.5–10/PM10
Min
Max
Mean
Min
Max
Mean
Min
Max
Mean
Min
Max
Mean
Min
Max
Mean
203 85 44 110 70
586 236 299 359 334
423 149 144 215 180
58 31 24 69 20
141 66 57 88 112
209 45 73 89 54
344 116 68 198 90
644 302 356 428 446
632 194 217 304 234
0.54 0.70 0.26 0.43 0.67
0.79 0.86 0.88 0.84 0.86
0.66 0.76 0.63 0.70 0.77
0.21 0.14 0.12 0.16 0.14
0.46 0.30 0.74 0.57 0.33
0.34 0.24 0.37 0.30 0.33
N: number of samples.
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
2896 350
May 23−27, 2001 March 14−18, 2001
dC/dlog(Dp)(µg.m-3/µm)
300
GSD: 1.53 µm MMD: 5.5 µm
GSD: 1.54 µm MMD: 0.61 µm
250
GSD: 1.56 µm MMD: 5.5 µm
200 150 GSD: 1.56 µm MMD: 0.59 µm
100 50 0 0.1
1 Particle size (Dp,µm)
(A) 350
September 19−23, 2001 December 23−27, 2001
300 dC/dlogDp (µg.m-3/µm)
10
250
GSD: 1.6 µm MMD: 0.57 µm
200
GSD: 1.6 µm MMD: 4.9 µm
150 100 GSD: 6.6 µm MMD: 0.12 µm
50
GSD: 1.4 µm MMD: 6.5 µm
0 0.1
1
(B)
10
Particle size (Dp,µm)
Fig. 1. Mass distribution in different size.
Table 2 Summary of pH, K; TWSC, WSIC and WSOC of PM10 and PM2.5 (N=40) PM10
K (ms/cm) DpH TWSC (mg/m3) WSIC (mg/m3) WSOC (mg/m3)
PM2.5
PM2.5/PM10
Min
Max
Mean
SD
Min
Max
Mean
SD
PM2.5=mPM10+c
54 3.25 17.14 0.07 16.71
1223 0.07 50.93 2.02 49.50
470 1.10 28.61 0.80 27.79
378 0.65 8.42 0.53 8.54
12 3.43 14.73 0.03 14.36
1211 0.25 40.13 1.21 40.11
433 1.35 23.62 0.45 23.15
370 0.66 7.61 0.35 7.65
Y Y Y Y Y
¼ 0:98X ¼ 0:92X ¼ 0:78X ¼ 0:54X ¼ 0:78X
þ 31 þ 0:16 þ 0:88 þ 0:34 þ 1:13
r2 0.97 0.93 0.75 0.76 0.75
N: number of samples; conductivity K ¼ K of sample—K of blank; DpH=pHs DpHb, pHs=pH of sample, pHb=pH of blank; in this study: K of blank=329 ms/cm, pHb=7.96; SD: standard deviation.
water-soluble matters of PM10 and PM2.5 aerosols varied over a wide range. The maximum of K was 1223 ms/cm for PM10 and 1211 ms/cm for PM2.5, and the minimum of K was 54 ms/cm for PM10 and 12 ms/cm for PM2.5.While mean value of K was 470 ms/cm for PM10 and 433 ms/cm for PM2.5, respectively. Conductivity of water-soluble matters is dependent upon the ions content. From the following text (see Section 3.3) it could be found that the total ions contents of all samples
are widely covered in a range from 2.9 to 73.3 mg/m3. From Table 2 it was found that conductivities of watersoluble matters between PM10 and PM2.5 showed a strongly linear relationship by the equation Y ¼ 0:89X þ 31; r2 ¼ 0:97 (Y : conductivity of PM2.5, X : conductivity of PM10), which is in agreement with our previous study (Wang et al., 2002). As shown in Table 2, DpH are all negative and ranged from 3.43 to 0.07 with a average of 1.10 for PM10
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
700 600 500 400 300 200 100 0
PM10
3.3. Ions of PM10 and PM2.5 Table 3 shows the statistical summary of ions of PM10 and PM2.5. Of all anions sulfate is the highest component followed by nitrate, while concentrations of fluorate and chloride are much lower. The concentra3 tion of SO2 4 varied from 7.13 to 37.52 mg/m for PM10 3 and from 5.66 to 32.84 mg/m for PM2.5, along with the 2
PM2.5
PM10
PM2.5
1.5 1 0.5 0 SY
FZ
2
ZS PM10
RJ
YZ
SY
PM2.5 WSOC (µg/m3)
WSIC (µg/m3)
lowest conductivity. Similar values of conductivity were observed at SY and YZ. Generally, conductivity is dependent upon the number of total ions of sample. In other words, the conductivity of atmospheric aerosols has the positive and linear correlation with the concentration of its total ions. As shown in Fig. 2 DpH at all sites presented negative values, indicating the acidic nature of water-soluble fraction of PM10 and PM2.5 aerosols in Nanjing city. DpH displayed larger value at SY and less value at YZ. Heaviest loadings of PM10 and PM2.5 were observed at SY, where the aerosols is more basic due to its higher level of street dust and resuspended soil. It is in agreement with the concentration of WSIC observed at the same site, which was much larger than those at other sites. In contrast to SY, DpH values of the samples collected at YZ were least and the WSIC concentrations were lowest, which suggested that PM10 and PM2.5 aerosols at YZ had the most acidic water-soluble fractions and the least components of soil dust. YZ site is surrounded by a large-scale of petrochemical plant, where heavy volume of organic compounds are emitted into atmosphere and subsequently take part in the photochemical reactions. Organic gases in the atmosphere are generally degraded into water-soluble organic compounds, most of which are organic acid such as monocarboxylic acids (e.g. formic acid and acetic acid) and dicarboxylic acids (e.g. oxalic acid, malonic acid and succinic acid), as well as polyacidic compounds (Decesari et al. 2001; Wang et al., 2002).
-∆pH
Conductivity (µs/cm)
and 1.35 for PM2.5, respectively. It means that pH values of all samples are less than the blanks and watersoluble matters of PM10 and PM2.5 are acidic. It could be found in Table 2 that the mean value of DpH of PM2.5 is less than that of PM10, suggesting that pH of PM2.5, compared with that of PM10, is further lower. It indicates that the acidity of water-soluble fraction of PM2.5 is stronger than PM10. It has long been known that acidic matters such as sulfate, nitrate and carboxylic anions are mainly enriched in fine particulates (Andreae et al., 1987, 1988; Talbot et al., 1988; Li and Winchester, 1989). Whereas, basic water-soluble matters such as Ca2+, Mg2+, K+, Na+ and so on, caused by airslake of the earth’s surface and dust storm, are mainly gathered in coarse particulates (Jacobson et al., 2000). This phenomenon results in the difference pH values between PM10 and PM2.5 (Galloway et al., 1976; Chameides and Davis, 1983). WSOC is the major component of TWSC and much more than WSIC (see Table 2). Concentrations of WSOC varied from 16.71 to 49.50 mg/m3 for PM10 and from 14.36 to 40.11 mg/m3 for PM2.5, along with average of 27.79 mg/m3 for PM10 and 23.15 mg/m3 for PM2.5, respectively. WSOC is increasingly concerned with implication on nucleation during rain and cloud growth, although water-soluble inorganic salts such as sulfate has been taken by classical theory as major factor of cloud nucleation. Due to their high water-solubility, water-soluble organic compounds have the potential to modify the hygroscopic properties of atmospheric particles including particle matter size and cloud condensation nuclei activity, as well as pH of rain. Fig. 2 shows the spatial variations of conductivity, DpH, WSIC and WSOC of PM10 and PM2.5 at the five locations. On average, the highest value of conductivity was observed at site FZ, a downtown area of Nanjing city where lots of restaurants and hotels emit high volume of inorganic ions and organic species leading to the highest level of conductivity. ZS and RJ sites had the
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1.5 1 0.5 0 SY
FZ
ZS
RJ
YZ
FZ
35 30 25 20 15 10 5 0
ZS PM10
SY
FZ
ZS
RJ
YZ
PM2.5
RJ
Fig. 2. Spatial variations of conductivity, DpH, WSIC and WSOC.
YZ
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Table 3 Summary of ions of PM10 and PM2.5 (mg/m3, N=40) PM2.5
PM10
F Cl SO2 4 NO 3 NHx Ca2+ K+ Mg2+ Na+ Total
PM2.5/PM10
Min
Max
Mean
SD
Min
Max
Mean
SD
PM25=mPM10+c
r2
Nd 0.18 7.13 2.66 0.80 0.72 0.26 Nd 1.62 —
1.82 4.80 37.52 29.38 36.05 15.07 14.51 1.10 5.59 —
0.37 1.51 18.13 9.19 10.81 4.58 3.41 0.38 3.77 52.03
0.48 1.04 7.84 6.30 8.76 4.14 3.42 0.26 1.02 —
Nd Nd 5.66 0.44 0.72 0.67 0.18 Nd 0.95 —
1.28 4.73 32.84 26.96 26.24 9.39 12.41 0.55 4.02 —
0.18 1.08 16.34 7.46 9.49 1.72 3.29 0.14 2.36 42.18
0.36 1.03 7.09 5.80 6.67 2.18 3.74 0.16 0.94 —
Y ¼ 0:70X Y ¼ 0:77X Y ¼ 0:81X Y ¼ 0:91X Y ¼ 0:60X Y ¼ 0:41X Y ¼ 0:86X Y ¼ 0:33X Y ¼ 0:47X —
0.88 0.57 0.67 0.89 0.74 0.60 0.88 0.30 0.26 —
þ 0:08 0:09 þ 0:91 1:39 þ 0:98 0:19 0:19 0:01 þ 0:50
N: number of samples; Nd: not detected.
average of 18.13 mg/m3 for PM10 and 16.34 mg/m3 for PM2.5, respectively. The concentration of NO 3 varied from 2.66 to 29.38 mg/m3 for PM10 and from 0.44 to 26.96 mg/m3 for PM2.5, together with the average of 9.19 mg/m3 for PM10 and 7.46 mg/m3 for PM2.5, respectively. Averages of concentrations of F and Cl were about 0.5 and 1.0 mg/m3, respectively. The ratios of anion concentrations of PM2.5 to those of PM10 were all greater than 0.7, especially for SO2 (0.91) and NO3 4 (0.81). It indicates that sulfate and nitrate largely distribute in fine particles. With respect to cations it could be found that NHx (ammonia and ammonium) was the most component, of which mean values were 10.81 mg/m3 for PM10 and 9.49 mg/m3 for PM2.5. The NHx ratio of PM2.5/PM10 is 0.60, suggesting that particulate NHx largely concentrates in fine particles. Ammonia (NH3) and ammonium (NH+ 4 ) are important components. The former is the most abundant gaseous alkaline component in the atmosphere. Particulate NH+ 4 found in the atmosphere originates from NH3 by the neutralization between ammonia and acidic species. The major sources for ammonia gas include animal farming, fertilizers and organic decomposition. Ammonia is not transported very long distances as it is rapidly converted into NH+ 4 aerosols at a rate 30% h1 (Asman and Van Jaarsveld, 1991). Many studies performed at different places in the world have pointed out that this cation accumulates in the fine particle ranges and correlates well with sulfate, suggesting that ammonium sulfate must be the major ammonium salt (Khemani et al., 1982; Lewin et al., 1986). From Table 3 it could be found that other cations (Ca2+, K+ and Na+) of PM10 and PM2.5 also had the higher level of concentrations, which is about 2–5 mg/m3 on average. Concentration of Mg2+ is much lower and below 0.4 mg/m3. Generally, the ratios of concentration of Ca2+, Mg2+ and Na+ between PM10 and PM2.5 were less than 0.47. That is to say that most of these crustal
ions are enriched in coarse particulate matters. Concentration of K+ of PM2.5 correlates well with that of PM10, and the equation is Y ¼ 0:86 0:19; r2 ¼ 0:88 (Y : ion concentration of PM2.5, X : ion concentration of PM10 ), indicating that K+ is mostly concentrated in fine particles ranges. SO2 4 , NO3 and NHx of PM10 and PM2.5 in Nanjing city showed a strong spatial variation with the highest concentrations at site FZ and the lowest concentrations at site RJ (seen in Fig. 3). Additionally, Ca2+ and K+ of PM10 and PM2.5 also display a similar spatial distribution. Due to much lower concentrations no obviously spatial variations is observed for F, Cl and Mg2+. With respect to SO24, the highest concentrations were obtained at FZ, which were 37.52 and 32.84 mg/m3 for PM10 and PM2.5, respectively. Conversely, the lowest concentrations of SO24 were presented at ZS, which were 8.15 mg/m3 for PM10 and 5.73 mg/m3 for PM2.5, respectively. Of the five sites all ions have the first highest concentrations at FZ and the second highest concentrations at SY, except for Na+. In spite of lower particle loadings YZ has the similar levels of ions, which are attribute to the emissions coming from the surrounding factories such as a petroleum refinery plant, a company of chemical industry and a metallurgical factory. In addition to the sea salt contribution, other sources + that contribute to the concentrations of SO2 and 4 , K 2+ Ca can be estimated using the concentrations of nonsea-salt sulphate (nss-SO2 4 ), non-sea-salt calcium ion (nss-Ca2+) and non-sea-salt potassium ion (nss-K+), which are calculated by the following equations and by assuming that the chemical compositions of sea-salt particles are the same as those of sea water, and that the soluble Na+ in particulate samples comes only from seasalts (Morales et al., 1998). 2 þ nss-SO2 4 ¼ ½SO4 ½Na 0:2455;
ð1Þ
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
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Fig. 3. Spatial variations of ions of PM10 and PM2.5 at five sampling sites.
nss-Ca2þ ¼ ½Ca2þ ½Naþ 0:0:0373;
ð2Þ
nss-Kþ ¼ ½Kþ ½Naþ 0:0355:
ð3Þ
+ The nss-SO2 and nss-Ca2+ concentrations at 4 , nss-K different sites are shown in Table 4. As for PM10, the three kind of nss-ions all presented the higher level of concentrations at sites SY and FZ and the lower level of concentrations at other sites. As for PM2.5, the three kinds of nss-ions displayed the different spatial distribution. The highest concentration of nss-SO2 was 4 observed at YZ and the lowest concentration of nssSO2 4 was observed at RJ. The highest concentration of nss-Ca2+ was observed at SY and the lowest concentration of nss-Ca2+ was observed at ZS. The highest concentration of nss-K+ was observed at FZ and the lowest concentration of nss-K+ was observed at ZS.
Sea-salt particles are coarse and have a short atmospheric life-time, which results in the rapid deposition. Nanjing is an inland city and is about 350 km far from the western Pacific Ocean. Therefore, low level of seasalt particles was found. As shown in Table 4, nss-ions account for above 80% of the soluble particulate ions. Nss-SO2 4 is the most abundant nss-ions and much more than nss-K+ and nss-Ca2+, both of which had the similar level of concentrations. 3.4. Relations between water-soluble components and particle mass A mass balance on the major water-soluble species of PM10 and PM2.5 was conducted. WSOC concentration is multiplied by 1.3 to account for hydrogen and oxygen present in the water-soluble HCs and arrive at the
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
nss-SO2 4 SO2 4 (%)
86 82 90 94 92
nss-SO2 4 (mg/m3)
14.95 16.54 13.19 12.54 17.78
3.76 1.64 0.79 1.57 0.98
97 95 98 93 92
3.26 6.08 1.77 1.82 2.66
97 98 94 93 97
concentrations of organic carbonaceous materials. The value of 1.3 is taken here because the value ranges from 1.2 to 1.4 (Countess et al., 1980; Kim et al., 2000; White and Robberts, 1977). WSIC is taken here as CO2 3 and/ or HCO 3 , and multiplied by 5, the ratio of the molecular weight of CO2 3 and/ or HCO3 to the atom weight of C. Major categories of the annual average PM10 and PM2.5 at five sites during the campaign are summarized in Table 5. In terms of the total mass concentration, the high levels of water-soluble components were observed at sites SY and FZ. Conversely, in terms of ratio of mass of water-soluble fraction to particle mass, the highest level of water-soluble components was observed at YZ, where nearly moiety of atmospheric particles is the water-soluble. Whilst the site SY had the lowest percentage due to the highest particle loadings, at which about sixth mass of PM10 and/or PM2.5 is the watersoluble. ZS and RJ had the same levels of mass concentrations and percentages, at both of which about third particle mass is water soluble, indicating the similar characteristics of atmospheric aerosols at the two sampling locations.
4. Summary and conclusions
94 99 97 95 96 97 98 95 95 96 N: number of samples.
87 81 98 96 92 19.58 19.05 16.33 16.26 17.88 8 8 8 8 8
(%)
SY FZ ZS RJ YZ
(mg/m )
6.82 6.12 2.81 2.32 1.35
nss-Ca (mg/m3)
nss-SO2 4 SO2 4 nss-SO2 4 3
PM10 N Site
Table 4 The concentrations of nss-ions in the five locations
2+
nss-Ca Ca2þ (%)
2þ
3.26 7.20 1.87 1.96 2.77
+
nss-K nss-K (mg/m3) Kþ (%)
þ
PM2.5
nss-Ca2+ (mg/m3)
nss-Ca2þ Ca2þ (%)
nss-K+ nss-Kþ (mg/m3) Kþ (%)
2900
On average, 63–77% of PM10 is in the PM2.5 fraction, which indicates that fine particles is the major component of atmospheric aerosols in Nanjing city. PM10 and PM2.5 showed strong spatial variations, which presented higher concentration at both the downtown area (FZ) and the traffic center (SY), whereas presented lower concentration at the residential area (RJ), the scenery district (ZS) and the industrial area (YZ). The highest level of PM10, 644 mg/m3, was observed at SY, and the lowest concentration of PM10 68 mg/m3, was observed at ZS. The heaviest PM2.5 loading varied from 144 mg/m3 at ZS to 423 mg/m3 at SY. Particulate matters displayed bimodal mass distribution, accumulation and coarse modes, with similar values of MMAD and GSD in the three seasons other than in the summer (19–23 September 2001). Compared with NAAQS of the USA, the PM10 and PM2.5 concentrations during the survey were more than 4–6 times of the standard, which suggests that particles pollution in Nanjing is rather severe, especially fine particles pollution. Water-soluble fraction of atmospheric aerosols in Nanjing is acidic, and the acidity of water-soluble matters of PM2.5 is stronger than that of PM10 owing to the acidic substances concentrated in the fine particles. Conductivity of the samples is relative to total ions concentration in the atmospheric aerosols, which ranged from 54 to 1223 ms/cm. WSOC, accounting for about 10% of particle mass, is the most significant component of the water-soluble fraction and showed no clear spatial variations.
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
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Table 5 Relationship between water-soluble species concentrations and particle mass at each site (N ¼ 8) SY
FZ
ZS
RJ
YZ
Mass (mg/ Percent m3) (%)
Mass (mg/ Percent m3) (%)
Mass (mg/ Percent m3) (%)
Mass (mg/ Percent m3) (%)
Mass (mg/ Percent m3) (%)
PM10 mass
682
300
220
213
178
NO 3 SO2 4 NHx Ca2+ K+ Na+ WSOC WSIC Total
11.25 22.62 11.36 7.00 3.46 5.02 37.26 8.98 106.95
PM2.5 mass
451
NO 3 SO2 4 NHx Ca2+ K+ Na+ WSOC WSIC Total
7.97 17.33 8.94 3.87 3.36 2.90 31.57 5.29 81.23
1.7 3.3 1.7 1.0 0.5 0.7 5.5 1.3 15.7
12.58 23.51 14.57 6.27 7.28 3.90 38.55 3.09 108.68
4.2 7.8 4.9 2.1 2.4 1.3 12.9 1.0 36.2
210 1.8 3.8 2.0 0.9 0.7 0.6 7.0 1.2 18.0
10.30 20.24 13.59 1.73 6.22 2.22 32.90 1.88 90.12
9.25 16.70 10.11 2.96 1.92 3.16 30.50 3.38 77.95
4.2 7.6 4.6 1.3 0.9 1.4 13.94 1.5 35.4
133 4.9 9.6 6.5 0.8 3.0 1.1 15.7 0.9 42.9
5.29 14.72 10.05 0.81 1.88 1.35 25.97 1.39 59.97
7.60 16.92 8.48 2.45 2.07 3.52 34.85 4.48 80.23
3.6 7.9 4.0 1.2 1.0 1.7 16.4 2.1 37.7
161 4.0 11.1 7.6 0.6 1.4 1.0 19.5 1.0 45.1
5.66 13.35 6.85 1.68 1.95 2.84 27.71 2.46 62.62
9.35 19.41 8.91 1.40 2.88 3.09 37.17 1.70 83.55
5.3 10.9 5.0 0.8 1.6 1.7 20.9 1.0 46.9
168 3.5 8.3 4.3 1.0 1.2 1.8 17.2 1.5 38.9
8.16 19.23 6.60 1.06 2.74 2.24 31.09 1.60 73.06
4.9 11.2 3.9 0.6 1.6 1.3 18.5 1.0 43.5
N: number of samples.
2+ SO2 , K+ and Na+ are also 4 , NO3 , NHx, Ca abundant in PM10 and PM2.5 and showed a clear spatial variation in Nanjing city. Nearly all of them presented the higher level of concentrations at SY and FZ and the lower level of concentrations at ZS and RJ. The total mass of water-soluble fraction, including inorganic ions and organic chemicals, also presented the highest level in SY and FZ (78.97–108.68 mg/m3, 15.7–42.9% of particle mass) and the lowest concentration in ZS (55.97– 77.95 mg/m3, 35.4–45.1% of particle mass), respectively. Because Nanjing city is far from the Pacific Ocean, nonsea-salt ions are the dominating components, which account for over 80% of total water-soluble ions. Among the detected ions SO2 is the largest chemical 4 component of both PM10 (16.70–23.51 mg/m3, 3.3– 10.9% of the PM10 mass) and PM2.5 (13.19–20.24 mg/ m3, 3.8–11.2% of the PM2.5 mass). NO 3 and NHx are the second highest water-soluble components followed by Ca2+, K+ and Na+.
Acknowledgements We are indebted to the Commission of Science and Technology and the Department of Environment
Protection of Jiangsu Province, People’s Republic of China, who funded a great deal of the work in this paper.
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Chemical characterization of water-soluble components of PM10 and PM2.5 atmospheric aerosols in five locations of Nanjing, China Gehui Wang*, Hui Wang, Yajuan Yu, Shixiang Gao, Jianfang Feng, Songting Gao, Liansheng Wang School of the Environment, State Key Laboratory of Pollution Control and Resources Reuse, Nanjing University, Nanjing 210093, People’s Republic of China Received 10 October 2002; accepted 31 March 2003
Abstract A comprehensive survey for atmospheric pollution was carried out from February to December 2001 in Nanjing city in order to better control the problem. As part of the work, the current study mainly aims at the chemical characterization of water-soluble species of PM10 and PM2.5 atmospheric aerosols, together with their spatial variations. Much heavier particle loadings is observed in Nanjing city, which is more than 4–6 times of NAAQS of the USA, especially fine particles. During the sampling time, 63–77% of PM10 mass is in the PM2.5 fraction. Water-soluble fractions of PM10 and PM2.5 are acidic, and the acidity of PM2.5 is stronger than that of PM10. Water-soluble organic carbon, accounting for about 10% of particle mass, is the most significant component of the water-soluble fraction and 2 2+ shows no clear spatial variations. NO , K+ and Na+ are also abundant 3 , SO4 , NHx (ammonia and ammonium), Ca in PM10 and PM2.5 aerosols. Among the detected water-soluble inorganic chemicals SO2 4 presented the first highest level of concentration in PM10 (16.70–23.51 mg/m3, 3.3–10.9% of the PM10 mass) and in PM2.5 (13.19–20.24 mg/m3, 3.8– 2+ 11.2% of the PM2.5 mass), NO , K+ and Na+. 3 and NHx are the second highest level of chemicals followed by Ca Nearly all those water-soluble inorganic components displayed the higher level of concentrations in the traffic center (SY) and the downtown area (FZ), and presented the lower level of concentrations in the scenery district and the residential area. The total mass of water-soluble fraction, including inorganic species and organic chemicals, also presented the higher concentrations (78.97–108.68 mg/m3, 15.7–42.9% of particle mass) at both sites of SY and FZ. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: PM10 and PM2.5 aerosols; Water-soluble organic carbon; Inorganic ions; Site difference
1. Introduction Atmospheric aerosol is termed as tiny liquid or solid particulate matter suspended in the air. In the last decades atmospheric aerosols have received much attention due to their potential influence on the global *Corresponding author. Chemistry Department, University of California-Davis, Davis, CA 95616, USA. Tel.: +1-530-7523397; fax: +1-530-752-8995. E-mail address: [email protected] (G. Wang).
radiation budget by acting as negative forcings and adverse impact on human health by inducing chronic respiratory illness, cancer and premature death. Watersoluble fraction of atmospheric aerosol contains many important compounds, which can change the size, composition, number and lifetime of aerosol owing to their hygroscopic nature (Jacobson et al., 2000; Intergovenmental Panel on Climate Change (IPCC), 1995; Environmental Protection Agency (USEPA), 1996; Novakov and Penner, 1993). On the other hand, water-soluble components increase the solubility of
1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00271-1
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toxic organic compounds, such as n-alkanes and polycyclic aromatic hydrocarbons (PAHs), by acting as surface active reagents and therefore increase their toxicity to human health. Many epidemic studies have linked airborne concentrations of PM10 (particulate matter with aerodynamic diameter less than 10 mm) and PM2.5 (particulate matter with aerodynamic diameter less than 2.5 mm) with a variety of health outcomes, including both morbidity and mortality. On average about 185–483 106 ton global aerosol per year is caused by anthropogenic sources including transportation, stationary combustion, industrial process, solid waste disposal and other miscellaneous sources (Bridgman, 1990). Most of which is concentrated in the Northern Hemisphere, especially downwind of industrial sites, slash-and-burn agriculture regions, and overgrazed grasslands. Chinese economic, industrial and demographic characteristics are different from those of North America and Europe (Zhang and Friedlander, 2000), which make characteristics of its atmospheric pollution differ from those in western countries. Higher level of ambient particulate loadings has been a persistent and the most serious air pollution problem in Nanjing city, a conurbation located in the eastern China with over 3 million of population. A comprehensive survey of PM10 and PM2.5 atmospheric aerosols was performed in Nanjing city from February to December 2001 in order to better control the problem. Many studies have concerned about the organic fraction (Kadowaki, 1990; Zhang et al., 2000) and heavy metals (Testa and Mosello, 1996; Smirnov et al., 1998; Pina et al., 2000) of atmospheric aerosols. However, they have mainly focused on the water-insoluble fractions of the particulate matter. The current study is devoted to the chemical characterization of water-soluble fractions of PM10 and PM2.5 in five locations of Nanjing city, involving pH, conductivity (K), water-soluble organic carbon (WSOC), water-soluble inorganic carbon (WSIC), total water-soluble carbon (TWSC) and ions, as well as to their spatial variations in the urban area.
2. Experimental part 2.1. Sampling locations Five sites were chosen for this study: (1) Suyuan Hotel (SY, 32 030 18.200 N, 118 460 44.300 E), (2) Fuzi Temple (FZ, 32 010 20.000 N, 118 470 04.700 E), (3) Ruijin Village (RJ, 32 010 58.200 N, 118 480 21.500 E), (4) Zhongshan Mausoleum (ZS, 32 030 42.900 N, 118 500 47.900 E), (5) Yangzi School (YZ, 32 030 43.100 N, 118 500 48.000 E). SY is located nearby Zhujiang highway, a heavy traffic center of Nanjing. RJ is a big residential area and located with FZ, a downtown, in the southern areas of
the city. ZS is a scenery district and located in the east of Nanjing. YZ is a small residential area and lies in a large-scale petrochemical district, which is located in the northwestern Nanjing city. 2.2. Collection materials and sampling protocols Two kinds of samplers have been used during the campaign. KC-2000 for PM10 (made in China) and Anderson GT22001 for PM2.5 (made in USA) are high volume air samplers, of which flow rates are 1.05 and 1.12 m3/min, respectively. The glass fiber filters (200 250 mm2) was baked at 500 C for 2 h in order to eliminate organic species, conditioned in a desiccator for 24 h, and then weighted with Mettler AE240 electronic balance (limit of detection is 10 mg). After sampling, the filters were also equilibrated in the desiccator for 24 h prior to weight, and stored at 18 C before analysis (Wang et al., 2002). PM10 and PM2.5 were simultaneously collected for 8 h from 8:30 a.m. to 4:30 p.m. at each site, which was used to reflect characteristics of the air pollution in the daytime. The heights of the two air inlets are 1.5 m above the ground. For shedding light on the characteristics of mass distribution of atmospheric aerosols in Nanjing city, the Flow Sensor sampler (made in USA), a multi-orifice, multi-stage inertial impactor, was used in campus of Nanjing University to collect ambient particles according to 9 kinds of different sizes. The aerodynamic cutoff points at the flow rate 1.70 m3/h are: X9.0, 9.0–6.0, 6.0– 4.6, 4.6–3.3, 3.3–2.2, 2.2–1.1, 1.1–0.7, 0.7–0.41 and 0.41– 0 mm. The aerosol collection had been conducted in the four seasons. The sampling date which we chosen is based on following meteorological aspects: (1) there was no rain over three-consecutive days prior to sampling, (2) the atmospheric temperature was of seasonal characteristic. 2.3. Determination for conductivity, pH, TWSC, WSIC, WSOC and ions of water-soluble species of the aerosols One-fourth filter was cut into pieces, ultrasonically extracted with 25 ml deionized water for 30 min, and then filtered through 0.45 mm millipore filters into two parts. One part was determined at 25 C for pH, conductivity, TWSC and WSIC of water-soluble species with Rize pH meter, DDSI-308A Mode Conductivitimeter (made in China) and Shimadzu TOC-5000 total organic analyzer (made in Japan), respectively. WSOC was obtained by subtracting WSIC from TWSC. Another part was determined for cations by inductively coupled plasma spectroscopy—AES, and then determined for anions with ion chromatography (Dionex-300) and NHx (ammonia and ammonium) with spectrophotometer (UV-2200)). The reported data of conductivity, TWSC,
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
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exceeding 65 mg/m3 (NAAQS) at the same five sampling sites were 100%, 100%, 37.5%, 100% and 100%, respectively. Compared with other metropolitan areas located in East Asia, the atmospheric particles concentrations observed in Nanjing city were much higher. For example, PM10 concentrations in Seoul, Korea was about 75 mg/m3 between 1987 and 1991 (Hashimoto et al., 1994) and in Hongkong was average of 55.3 mg/m3 between 1990 and 1994 (Qin et al., 1997). The mass size distribution of atmospheric aerosols in campus of Nanjing University was shown in Fig. 1, along with the geometric standard deviation (GSD) and mass median diameter (MMD) relative to each mode. Atmospheric aerosols in the 4 months has all displayed the bimodal distribution: accumulation (Dp o1 mm) and coarse (Dp >1 mm) modes. Although the distribution of atmospheric aerosols can be multimodal, only an accumulation mode with equivalent aerodynamic diameter below 1 mm and a coarse mode with equivalent aerodynamic diameter above 1 mm can be clearly distinguished for most ambient aerosols. Similar values of GSD and MMD demonstrated the same characteristics of aerosol particles distribution in the 3 months (i.e. March, May and December 2001) other than in the September. Accumulation mode of aerosol in the September presented much larger value of GSD and less value of MMD, which was mainly attributed to much lower mass concentration of fine particles. Most of fine particles are secondary aerosols largely formed through the reactions of gas in the atmosphere and in the submicrometer size range. While most of coarse particles are primary aerosols directly emitted into the atmosphere and generally in the submicrometer size range.
WSOC, WSIC, and ion concentrations of samples were all corrected by field blank sample.
3. Results and discussion 3.1. PM10 and PM2.5 mass Table 1 shows the PM10 and PM2.5 mass concentrations during the survey. Of the five sites the first highest level of particle concentrations was observed at site SY with averages of 423 mg/m3 for PM2.5, 209 mg/m3 for PM2.5–10 and 644 mg/m3 for PM10. The second highest level of particle mass were observed at site FZ with mean values of 215 mg/m3 for PM2.5, 89 mg/m3 for PM2.5–10 and 304 mg/m3 for PM10. The other three sites had the similar level of particle loadings. Sampling site SY lies in Zhujiang motorway, the traffic center of Nanjing city, in which numerous motor vehicles emitted a great deal of exhaust. In Nanjing city a kind of vehicle named doublestroke autobike is widely being used for personal traffic, of which engine burns a mixture of gasoline and diesel oil. The way to make power releases a great deal of pollutants due to incomplete combustion. Enormous volume of traffic and low efficient engine combustion are attribute to the high level of particle concentrations at site SY. FZ sampling site is located in a downtown area of Nanjing, where there is a heavy traffic highway about 500 m away. The other three sampling sites are all covered with good vegetation and far away from traffic motorways, which resulted in the lower particle loadings in the air. From Table 1 it could be found that PM2.5 accounted for 63–77% of PM10, which suggests that fine particles with aerodynamical diameter less than 2.5 mm is the major component of atmospheric particulate matter in Nanjing. Table 1 also indicates that coarse particles with aerodynamical diameter from 2.5 to 10 mm accounted for 24–37% of PM10. Frequencies of PM10 concentrations exceeding 150 mg/ m3 (NAAQS) at SY, RJ, ZS, FZ and YZ were 100%, 75%, 62.5%, 100% and 50%, respectively, during the survey. While frequencies of PM2.5 concentrations
3.2. Conductivity (K), pH, TWSC, WSIC and WSOC of PM10 and PM2.5 Table 2 shows the average, minimum, maximum and standard deviation of pH, K; TWSC, WSIC and WSOC of the water-soluble fractions. Conductivity (K) of
Table 1 PM10 and PM2.5 loadings at each site during the sampling period Site
SY RJ ZS FZ YZ
N
8 8 8 8 8
PM2.5 (mg/m3)
PM2.5–10 (mg/m3)
PM10 (mg/m3)
PM2.5/PM10
PM2.5–10/PM10
Min
Max
Mean
Min
Max
Mean
Min
Max
Mean
Min
Max
Mean
Min
Max
Mean
203 85 44 110 70
586 236 299 359 334
423 149 144 215 180
58 31 24 69 20
141 66 57 88 112
209 45 73 89 54
344 116 68 198 90
644 302 356 428 446
632 194 217 304 234
0.54 0.70 0.26 0.43 0.67
0.79 0.86 0.88 0.84 0.86
0.66 0.76 0.63 0.70 0.77
0.21 0.14 0.12 0.16 0.14
0.46 0.30 0.74 0.57 0.33
0.34 0.24 0.37 0.30 0.33
N: number of samples.
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
2896 350
May 23−27, 2001 March 14−18, 2001
dC/dlog(Dp)(µg.m-3/µm)
300
GSD: 1.53 µm MMD: 5.5 µm
GSD: 1.54 µm MMD: 0.61 µm
250
GSD: 1.56 µm MMD: 5.5 µm
200 150 GSD: 1.56 µm MMD: 0.59 µm
100 50 0 0.1
1 Particle size (Dp,µm)
(A) 350
September 19−23, 2001 December 23−27, 2001
300 dC/dlogDp (µg.m-3/µm)
10
250
GSD: 1.6 µm MMD: 0.57 µm
200
GSD: 1.6 µm MMD: 4.9 µm
150 100 GSD: 6.6 µm MMD: 0.12 µm
50
GSD: 1.4 µm MMD: 6.5 µm
0 0.1
1
(B)
10
Particle size (Dp,µm)
Fig. 1. Mass distribution in different size.
Table 2 Summary of pH, K; TWSC, WSIC and WSOC of PM10 and PM2.5 (N=40) PM10
K (ms/cm) DpH TWSC (mg/m3) WSIC (mg/m3) WSOC (mg/m3)
PM2.5
PM2.5/PM10
Min
Max
Mean
SD
Min
Max
Mean
SD
PM2.5=mPM10+c
54 3.25 17.14 0.07 16.71
1223 0.07 50.93 2.02 49.50
470 1.10 28.61 0.80 27.79
378 0.65 8.42 0.53 8.54
12 3.43 14.73 0.03 14.36
1211 0.25 40.13 1.21 40.11
433 1.35 23.62 0.45 23.15
370 0.66 7.61 0.35 7.65
Y Y Y Y Y
¼ 0:98X ¼ 0:92X ¼ 0:78X ¼ 0:54X ¼ 0:78X
þ 31 þ 0:16 þ 0:88 þ 0:34 þ 1:13
r2 0.97 0.93 0.75 0.76 0.75
N: number of samples; conductivity K ¼ K of sample—K of blank; DpH=pHs DpHb, pHs=pH of sample, pHb=pH of blank; in this study: K of blank=329 ms/cm, pHb=7.96; SD: standard deviation.
water-soluble matters of PM10 and PM2.5 aerosols varied over a wide range. The maximum of K was 1223 ms/cm for PM10 and 1211 ms/cm for PM2.5, and the minimum of K was 54 ms/cm for PM10 and 12 ms/cm for PM2.5.While mean value of K was 470 ms/cm for PM10 and 433 ms/cm for PM2.5, respectively. Conductivity of water-soluble matters is dependent upon the ions content. From the following text (see Section 3.3) it could be found that the total ions contents of all samples
are widely covered in a range from 2.9 to 73.3 mg/m3. From Table 2 it was found that conductivities of watersoluble matters between PM10 and PM2.5 showed a strongly linear relationship by the equation Y ¼ 0:89X þ 31; r2 ¼ 0:97 (Y : conductivity of PM2.5, X : conductivity of PM10), which is in agreement with our previous study (Wang et al., 2002). As shown in Table 2, DpH are all negative and ranged from 3.43 to 0.07 with a average of 1.10 for PM10
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
700 600 500 400 300 200 100 0
PM10
3.3. Ions of PM10 and PM2.5 Table 3 shows the statistical summary of ions of PM10 and PM2.5. Of all anions sulfate is the highest component followed by nitrate, while concentrations of fluorate and chloride are much lower. The concentra3 tion of SO2 4 varied from 7.13 to 37.52 mg/m for PM10 3 and from 5.66 to 32.84 mg/m for PM2.5, along with the 2
PM2.5
PM10
PM2.5
1.5 1 0.5 0 SY
FZ
2
ZS PM10
RJ
YZ
SY
PM2.5 WSOC (µg/m3)
WSIC (µg/m3)
lowest conductivity. Similar values of conductivity were observed at SY and YZ. Generally, conductivity is dependent upon the number of total ions of sample. In other words, the conductivity of atmospheric aerosols has the positive and linear correlation with the concentration of its total ions. As shown in Fig. 2 DpH at all sites presented negative values, indicating the acidic nature of water-soluble fraction of PM10 and PM2.5 aerosols in Nanjing city. DpH displayed larger value at SY and less value at YZ. Heaviest loadings of PM10 and PM2.5 were observed at SY, where the aerosols is more basic due to its higher level of street dust and resuspended soil. It is in agreement with the concentration of WSIC observed at the same site, which was much larger than those at other sites. In contrast to SY, DpH values of the samples collected at YZ were least and the WSIC concentrations were lowest, which suggested that PM10 and PM2.5 aerosols at YZ had the most acidic water-soluble fractions and the least components of soil dust. YZ site is surrounded by a large-scale of petrochemical plant, where heavy volume of organic compounds are emitted into atmosphere and subsequently take part in the photochemical reactions. Organic gases in the atmosphere are generally degraded into water-soluble organic compounds, most of which are organic acid such as monocarboxylic acids (e.g. formic acid and acetic acid) and dicarboxylic acids (e.g. oxalic acid, malonic acid and succinic acid), as well as polyacidic compounds (Decesari et al. 2001; Wang et al., 2002).
-∆pH
Conductivity (µs/cm)
and 1.35 for PM2.5, respectively. It means that pH values of all samples are less than the blanks and watersoluble matters of PM10 and PM2.5 are acidic. It could be found in Table 2 that the mean value of DpH of PM2.5 is less than that of PM10, suggesting that pH of PM2.5, compared with that of PM10, is further lower. It indicates that the acidity of water-soluble fraction of PM2.5 is stronger than PM10. It has long been known that acidic matters such as sulfate, nitrate and carboxylic anions are mainly enriched in fine particulates (Andreae et al., 1987, 1988; Talbot et al., 1988; Li and Winchester, 1989). Whereas, basic water-soluble matters such as Ca2+, Mg2+, K+, Na+ and so on, caused by airslake of the earth’s surface and dust storm, are mainly gathered in coarse particulates (Jacobson et al., 2000). This phenomenon results in the difference pH values between PM10 and PM2.5 (Galloway et al., 1976; Chameides and Davis, 1983). WSOC is the major component of TWSC and much more than WSIC (see Table 2). Concentrations of WSOC varied from 16.71 to 49.50 mg/m3 for PM10 and from 14.36 to 40.11 mg/m3 for PM2.5, along with average of 27.79 mg/m3 for PM10 and 23.15 mg/m3 for PM2.5, respectively. WSOC is increasingly concerned with implication on nucleation during rain and cloud growth, although water-soluble inorganic salts such as sulfate has been taken by classical theory as major factor of cloud nucleation. Due to their high water-solubility, water-soluble organic compounds have the potential to modify the hygroscopic properties of atmospheric particles including particle matter size and cloud condensation nuclei activity, as well as pH of rain. Fig. 2 shows the spatial variations of conductivity, DpH, WSIC and WSOC of PM10 and PM2.5 at the five locations. On average, the highest value of conductivity was observed at site FZ, a downtown area of Nanjing city where lots of restaurants and hotels emit high volume of inorganic ions and organic species leading to the highest level of conductivity. ZS and RJ sites had the
2897
1.5 1 0.5 0 SY
FZ
ZS
RJ
YZ
FZ
35 30 25 20 15 10 5 0
ZS PM10
SY
FZ
ZS
RJ
YZ
PM2.5
RJ
Fig. 2. Spatial variations of conductivity, DpH, WSIC and WSOC.
YZ
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
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Table 3 Summary of ions of PM10 and PM2.5 (mg/m3, N=40) PM2.5
PM10
F Cl SO2 4 NO 3 NHx Ca2+ K+ Mg2+ Na+ Total
PM2.5/PM10
Min
Max
Mean
SD
Min
Max
Mean
SD
PM25=mPM10+c
r2
Nd 0.18 7.13 2.66 0.80 0.72 0.26 Nd 1.62 —
1.82 4.80 37.52 29.38 36.05 15.07 14.51 1.10 5.59 —
0.37 1.51 18.13 9.19 10.81 4.58 3.41 0.38 3.77 52.03
0.48 1.04 7.84 6.30 8.76 4.14 3.42 0.26 1.02 —
Nd Nd 5.66 0.44 0.72 0.67 0.18 Nd 0.95 —
1.28 4.73 32.84 26.96 26.24 9.39 12.41 0.55 4.02 —
0.18 1.08 16.34 7.46 9.49 1.72 3.29 0.14 2.36 42.18
0.36 1.03 7.09 5.80 6.67 2.18 3.74 0.16 0.94 —
Y ¼ 0:70X Y ¼ 0:77X Y ¼ 0:81X Y ¼ 0:91X Y ¼ 0:60X Y ¼ 0:41X Y ¼ 0:86X Y ¼ 0:33X Y ¼ 0:47X —
0.88 0.57 0.67 0.89 0.74 0.60 0.88 0.30 0.26 —
þ 0:08 0:09 þ 0:91 1:39 þ 0:98 0:19 0:19 0:01 þ 0:50
N: number of samples; Nd: not detected.
average of 18.13 mg/m3 for PM10 and 16.34 mg/m3 for PM2.5, respectively. The concentration of NO 3 varied from 2.66 to 29.38 mg/m3 for PM10 and from 0.44 to 26.96 mg/m3 for PM2.5, together with the average of 9.19 mg/m3 for PM10 and 7.46 mg/m3 for PM2.5, respectively. Averages of concentrations of F and Cl were about 0.5 and 1.0 mg/m3, respectively. The ratios of anion concentrations of PM2.5 to those of PM10 were all greater than 0.7, especially for SO2 (0.91) and NO3 4 (0.81). It indicates that sulfate and nitrate largely distribute in fine particles. With respect to cations it could be found that NHx (ammonia and ammonium) was the most component, of which mean values were 10.81 mg/m3 for PM10 and 9.49 mg/m3 for PM2.5. The NHx ratio of PM2.5/PM10 is 0.60, suggesting that particulate NHx largely concentrates in fine particles. Ammonia (NH3) and ammonium (NH+ 4 ) are important components. The former is the most abundant gaseous alkaline component in the atmosphere. Particulate NH+ 4 found in the atmosphere originates from NH3 by the neutralization between ammonia and acidic species. The major sources for ammonia gas include animal farming, fertilizers and organic decomposition. Ammonia is not transported very long distances as it is rapidly converted into NH+ 4 aerosols at a rate 30% h1 (Asman and Van Jaarsveld, 1991). Many studies performed at different places in the world have pointed out that this cation accumulates in the fine particle ranges and correlates well with sulfate, suggesting that ammonium sulfate must be the major ammonium salt (Khemani et al., 1982; Lewin et al., 1986). From Table 3 it could be found that other cations (Ca2+, K+ and Na+) of PM10 and PM2.5 also had the higher level of concentrations, which is about 2–5 mg/m3 on average. Concentration of Mg2+ is much lower and below 0.4 mg/m3. Generally, the ratios of concentration of Ca2+, Mg2+ and Na+ between PM10 and PM2.5 were less than 0.47. That is to say that most of these crustal
ions are enriched in coarse particulate matters. Concentration of K+ of PM2.5 correlates well with that of PM10, and the equation is Y ¼ 0:86 0:19; r2 ¼ 0:88 (Y : ion concentration of PM2.5, X : ion concentration of PM10 ), indicating that K+ is mostly concentrated in fine particles ranges. SO2 4 , NO3 and NHx of PM10 and PM2.5 in Nanjing city showed a strong spatial variation with the highest concentrations at site FZ and the lowest concentrations at site RJ (seen in Fig. 3). Additionally, Ca2+ and K+ of PM10 and PM2.5 also display a similar spatial distribution. Due to much lower concentrations no obviously spatial variations is observed for F, Cl and Mg2+. With respect to SO24, the highest concentrations were obtained at FZ, which were 37.52 and 32.84 mg/m3 for PM10 and PM2.5, respectively. Conversely, the lowest concentrations of SO24 were presented at ZS, which were 8.15 mg/m3 for PM10 and 5.73 mg/m3 for PM2.5, respectively. Of the five sites all ions have the first highest concentrations at FZ and the second highest concentrations at SY, except for Na+. In spite of lower particle loadings YZ has the similar levels of ions, which are attribute to the emissions coming from the surrounding factories such as a petroleum refinery plant, a company of chemical industry and a metallurgical factory. In addition to the sea salt contribution, other sources + that contribute to the concentrations of SO2 and 4 , K 2+ Ca can be estimated using the concentrations of nonsea-salt sulphate (nss-SO2 4 ), non-sea-salt calcium ion (nss-Ca2+) and non-sea-salt potassium ion (nss-K+), which are calculated by the following equations and by assuming that the chemical compositions of sea-salt particles are the same as those of sea water, and that the soluble Na+ in particulate samples comes only from seasalts (Morales et al., 1998). 2 þ nss-SO2 4 ¼ ½SO4 ½Na 0:2455;
ð1Þ
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
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Fig. 3. Spatial variations of ions of PM10 and PM2.5 at five sampling sites.
nss-Ca2þ ¼ ½Ca2þ ½Naþ 0:0:0373;
ð2Þ
nss-Kþ ¼ ½Kþ ½Naþ 0:0355:
ð3Þ
+ The nss-SO2 and nss-Ca2+ concentrations at 4 , nss-K different sites are shown in Table 4. As for PM10, the three kind of nss-ions all presented the higher level of concentrations at sites SY and FZ and the lower level of concentrations at other sites. As for PM2.5, the three kinds of nss-ions displayed the different spatial distribution. The highest concentration of nss-SO2 was 4 observed at YZ and the lowest concentration of nssSO2 4 was observed at RJ. The highest concentration of nss-Ca2+ was observed at SY and the lowest concentration of nss-Ca2+ was observed at ZS. The highest concentration of nss-K+ was observed at FZ and the lowest concentration of nss-K+ was observed at ZS.
Sea-salt particles are coarse and have a short atmospheric life-time, which results in the rapid deposition. Nanjing is an inland city and is about 350 km far from the western Pacific Ocean. Therefore, low level of seasalt particles was found. As shown in Table 4, nss-ions account for above 80% of the soluble particulate ions. Nss-SO2 4 is the most abundant nss-ions and much more than nss-K+ and nss-Ca2+, both of which had the similar level of concentrations. 3.4. Relations between water-soluble components and particle mass A mass balance on the major water-soluble species of PM10 and PM2.5 was conducted. WSOC concentration is multiplied by 1.3 to account for hydrogen and oxygen present in the water-soluble HCs and arrive at the
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
nss-SO2 4 SO2 4 (%)
86 82 90 94 92
nss-SO2 4 (mg/m3)
14.95 16.54 13.19 12.54 17.78
3.76 1.64 0.79 1.57 0.98
97 95 98 93 92
3.26 6.08 1.77 1.82 2.66
97 98 94 93 97
concentrations of organic carbonaceous materials. The value of 1.3 is taken here because the value ranges from 1.2 to 1.4 (Countess et al., 1980; Kim et al., 2000; White and Robberts, 1977). WSIC is taken here as CO2 3 and/ or HCO 3 , and multiplied by 5, the ratio of the molecular weight of CO2 3 and/ or HCO3 to the atom weight of C. Major categories of the annual average PM10 and PM2.5 at five sites during the campaign are summarized in Table 5. In terms of the total mass concentration, the high levels of water-soluble components were observed at sites SY and FZ. Conversely, in terms of ratio of mass of water-soluble fraction to particle mass, the highest level of water-soluble components was observed at YZ, where nearly moiety of atmospheric particles is the water-soluble. Whilst the site SY had the lowest percentage due to the highest particle loadings, at which about sixth mass of PM10 and/or PM2.5 is the watersoluble. ZS and RJ had the same levels of mass concentrations and percentages, at both of which about third particle mass is water soluble, indicating the similar characteristics of atmospheric aerosols at the two sampling locations.
4. Summary and conclusions
94 99 97 95 96 97 98 95 95 96 N: number of samples.
87 81 98 96 92 19.58 19.05 16.33 16.26 17.88 8 8 8 8 8
(%)
SY FZ ZS RJ YZ
(mg/m )
6.82 6.12 2.81 2.32 1.35
nss-Ca (mg/m3)
nss-SO2 4 SO2 4 nss-SO2 4 3
PM10 N Site
Table 4 The concentrations of nss-ions in the five locations
2+
nss-Ca Ca2þ (%)
2þ
3.26 7.20 1.87 1.96 2.77
+
nss-K nss-K (mg/m3) Kþ (%)
þ
PM2.5
nss-Ca2+ (mg/m3)
nss-Ca2þ Ca2þ (%)
nss-K+ nss-Kþ (mg/m3) Kþ (%)
2900
On average, 63–77% of PM10 is in the PM2.5 fraction, which indicates that fine particles is the major component of atmospheric aerosols in Nanjing city. PM10 and PM2.5 showed strong spatial variations, which presented higher concentration at both the downtown area (FZ) and the traffic center (SY), whereas presented lower concentration at the residential area (RJ), the scenery district (ZS) and the industrial area (YZ). The highest level of PM10, 644 mg/m3, was observed at SY, and the lowest concentration of PM10 68 mg/m3, was observed at ZS. The heaviest PM2.5 loading varied from 144 mg/m3 at ZS to 423 mg/m3 at SY. Particulate matters displayed bimodal mass distribution, accumulation and coarse modes, with similar values of MMAD and GSD in the three seasons other than in the summer (19–23 September 2001). Compared with NAAQS of the USA, the PM10 and PM2.5 concentrations during the survey were more than 4–6 times of the standard, which suggests that particles pollution in Nanjing is rather severe, especially fine particles pollution. Water-soluble fraction of atmospheric aerosols in Nanjing is acidic, and the acidity of water-soluble matters of PM2.5 is stronger than that of PM10 owing to the acidic substances concentrated in the fine particles. Conductivity of the samples is relative to total ions concentration in the atmospheric aerosols, which ranged from 54 to 1223 ms/cm. WSOC, accounting for about 10% of particle mass, is the most significant component of the water-soluble fraction and showed no clear spatial variations.
G. Wang et al. / Atmospheric Environment 37 (2003) 2893–2902
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Table 5 Relationship between water-soluble species concentrations and particle mass at each site (N ¼ 8) SY
FZ
ZS
RJ
YZ
Mass (mg/ Percent m3) (%)
Mass (mg/ Percent m3) (%)
Mass (mg/ Percent m3) (%)
Mass (mg/ Percent m3) (%)
Mass (mg/ Percent m3) (%)
PM10 mass
682
300
220
213
178
NO 3 SO2 4 NHx Ca2+ K+ Na+ WSOC WSIC Total
11.25 22.62 11.36 7.00 3.46 5.02 37.26 8.98 106.95
PM2.5 mass
451
NO 3 SO2 4 NHx Ca2+ K+ Na+ WSOC WSIC Total
7.97 17.33 8.94 3.87 3.36 2.90 31.57 5.29 81.23
1.7 3.3 1.7 1.0 0.5 0.7 5.5 1.3 15.7
12.58 23.51 14.57 6.27 7.28 3.90 38.55 3.09 108.68
4.2 7.8 4.9 2.1 2.4 1.3 12.9 1.0 36.2
210 1.8 3.8 2.0 0.9 0.7 0.6 7.0 1.2 18.0
10.30 20.24 13.59 1.73 6.22 2.22 32.90 1.88 90.12
9.25 16.70 10.11 2.96 1.92 3.16 30.50 3.38 77.95
4.2 7.6 4.6 1.3 0.9 1.4 13.94 1.5 35.4
133 4.9 9.6 6.5 0.8 3.0 1.1 15.7 0.9 42.9
5.29 14.72 10.05 0.81 1.88 1.35 25.97 1.39 59.97
7.60 16.92 8.48 2.45 2.07 3.52 34.85 4.48 80.23
3.6 7.9 4.0 1.2 1.0 1.7 16.4 2.1 37.7
161 4.0 11.1 7.6 0.6 1.4 1.0 19.5 1.0 45.1
5.66 13.35 6.85 1.68 1.95 2.84 27.71 2.46 62.62
9.35 19.41 8.91 1.40 2.88 3.09 37.17 1.70 83.55
5.3 10.9 5.0 0.8 1.6 1.7 20.9 1.0 46.9
168 3.5 8.3 4.3 1.0 1.2 1.8 17.2 1.5 38.9
8.16 19.23 6.60 1.06 2.74 2.24 31.09 1.60 73.06
4.9 11.2 3.9 0.6 1.6 1.3 18.5 1.0 43.5
N: number of samples.
2+ SO2 , K+ and Na+ are also 4 , NO3 , NHx, Ca abundant in PM10 and PM2.5 and showed a clear spatial variation in Nanjing city. Nearly all of them presented the higher level of concentrations at SY and FZ and the lower level of concentrations at ZS and RJ. The total mass of water-soluble fraction, including inorganic ions and organic chemicals, also presented the highest level in SY and FZ (78.97–108.68 mg/m3, 15.7–42.9% of particle mass) and the lowest concentration in ZS (55.97– 77.95 mg/m3, 35.4–45.1% of particle mass), respectively. Because Nanjing city is far from the Pacific Ocean, nonsea-salt ions are the dominating components, which account for over 80% of total water-soluble ions. Among the detected ions SO2 is the largest chemical 4 component of both PM10 (16.70–23.51 mg/m3, 3.3– 10.9% of the PM10 mass) and PM2.5 (13.19–20.24 mg/ m3, 3.8–11.2% of the PM2.5 mass). NO 3 and NHx are the second highest water-soluble components followed by Ca2+, K+ and Na+.
Acknowledgements We are indebted to the Commission of Science and Technology and the Department of Environment
Protection of Jiangsu Province, People’s Republic of China, who funded a great deal of the work in this paper.
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