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Chemical composition of fogwater in an urban area: Strasbourg (France)
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Environmental Pollution, Vol. 94, No. 3, pp. 345-354, 1996 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0269-7491/96 $15.00+0.00
ELSEVIER
CHEMICAL COMPOSITION OF FOGWATER IN A N U R B A N AREA: STRASBOURG (FRANCE) Maurice Millet, Astrid Sanusi & Henri Wortham* Centre de Gdochimie de la surface, CNRS UPR 6251, Universitd Louis Pasteur, 28, rue Goethe, 67083, Strasbourg Cedex, France
(Received 22 May 1995; accepted 10 May 1996)
Abstract
fogwater, such as PAHs (Capel et al., 1991), phenols and nitrophenols (Richartz et al., 1990), aldehydes and organic acids (Igawa et al., 1989) and pesticides (Glotfelty et al., 1987). In the majority of cases, the fogwater articles that study the inorganic fraction, focus on some fog events (2~) and try to determine the mechanisms of concentration and acidification of the whole droplet range (Jacob et al., 1985, 1987; Johnson et al., 1987; Post et al., 1991; Brook et al., 1992; Fuzziet al., 1992). Moreover, these works generally study coastal fogs (Jacob et al., 1985, 1987; Post et al., 1991). Conversely, the purpose of the present paper is to measure the ionic composition of two inhalable size fractions (2-6/zm and 5-8/zm diameter) of a continental fog and to discuss what processes could control the composition and its variability. This study concerns the 18 fog events that occurred during 1991, with a view to identifying potential health effects for future investigations.
To investigate the acidity and to identify the predominant compounds, this work presents the chemical analysis of 18 fogwater samples collected during the year 1991 in Strasbourg, in the east of France. For each fog event, two droplet size categories (2-6 lzm and 5~8 Izm) have been separately collected and 16 ionic components have been analysed. These two fraction sizes were chosen because they correspond approximately to the size range that can penetrate the human lung and they may have possible health effects. The dominant species were NH4 +, NO3-, S042- and CI-, with a maximum level of 12640, 17270, 21 620 and 13540/zeq litre -1, respectively. For most of the fog events the highest concentrations of all analysed species were observed in the 2-6/zm droplets, pH values ranged between 2.79 and 5.70 and the fogwater acidity was governed by three strong acids, H2S04, HN03 and HCI and was partially neutralised by NH3 and probably by the presence of CaC03 in the "loess', which is the major constituent of soils in the upper Rhine valley. In other respects the acetate/formate ratio (methanoate/ ethanoate), generally lower than 1, indicates an important pollution due to automobile exhaust, although the Pb concentrations are moderate due to the general use of unleaded gasoline in France since 1989. © 1997 Elsevier Science Ltd. All rights reserved
MATERIAL AND METHODS Fogwater sampling Samples were collected from the roof of the Botanical Institute of Strasbourg University (height 30 m). It is situated nearly in the centre of the town and 3000 m away from industrial sites and highways. In this region, which is densely populated and industrialised, radiation fogs at ground level are frequently observed during autumn and winter months. Commonly, they are formed during the night, but they can occasionally remain the whole day. Fogwater was collected by using two heads, first described by Berner in 1988, connected to pumps. Air containing fog droplets was drawn up through the sampling heads and droplets with sufficient movement quantity were captured by impaction on cones." Fog droplets are generally between 1 and 100/zm in diameter, with the majority (in volume) in the range 1050/zm (Fuzzi et al., 1984, 1992; Johnson et al., 1987); but in this work, due to the sampling method, only small droplets ranging between 2 and 6/zm and 5 and
Keywords: Fogwater, ionic composition, organic acids, metals, atmospheric acidity.
INTRODUCTION In recent studies, the scientific community has been interested in fog as a potential mechanism for scavenging atmospheric contaminants. In 1980 Hoffmann and co-workers demonstrated the high acidity of fogwater in Southern California (Munger et al., 1983; Jacob et al., 1985). It was found that the southern Californian fogs were frequently 100 times more acidic than rain in the same area (Hileman, 1983). This observation has led to many studies in various countries to determine the concentration of inorganic and organic compounds in *To whom correspondence should be addressed. 345
346
M . Millet et al.
8/zm diameter were sampled. The operating details of these collectors have been described elsewhere (Berner, 1988; Hoffmann & Metzig, 1991; Lammel & Metzig, 1991; Gieray et al., 1993; Millet, 1994; Millet et al., 1995). The collectors were started manually when fog was sufficiently dense to obscure the cathedral belfry of Strasbourg from the laboratory (about 500 m) and were also stopped manually when the belfry reappeared. With this empirical method, all the weak fog events were missed but for these, the experiment shows that it was impossible to collect a sufficient volume of liquid to be analysed. This method makes it possible to always start and stop the sampling for the same fog density. Nevertheless, depending on the rapidity of the appearance of the fog events, the delay between the beginning of the fog and the start of the sampling could vary between 0.5 and 2 h. The periods of collection varied between 3 and 11 h. The liquid water contents (LWCs) (see Table 1) were estimated from the sampling period, the pump flow and the volume of the sample. Thus the LWC represents solely the liquid water contained in the small fog droplets (2-6/zm and 5-8 #m) and not the quantity of water per volume of air as commonly defined. Before sample collection, the collectors were washed thoroughly with deionised water. Analytical procedure At the end of each sampling session, the fogwater volumes of each head were measured and the pH was immediately determined by using a pH-meter Consort P407 equipped with a combination micro electrode (6 mm diameter) Ingold. In order to prevent chemical reactions and the microbial decomposition of the organic acids (Keene et al., 1983), the samples were stored in a freezer at -18°C until they were analysed as recommended by Colin et al. (1990). Before each analysis, the samples were filtered with Millex SLGS025NB (0.22/zm) Waters (Millipore) to extract insoluble matter (Czuczwa et al., 1988; Lim et al., 1991; Millet et al., 1995; Sanusi et al., 1996). Major inorganic anions (CI-, NO3- and SO42-) were determined by ion chromatography with a Waters Powerline 600E pump, a Wescan 213A conductivity detector and an IC-Pak A/HR Waters column. The eluant was Borate-Gluconate/CH3CN/n-Butanol. Major monovalent cations (Na ÷, NH4 ÷ and K ÷) were analysed by ion chromatography using an IC-Pak CM/D Waters column. The eluant was 0.1 mM HNO3/ 3 mM EDTA-acid. Polyvalent cations (Mg 2÷ , Ca 2÷ , Fe, Mn, Zn and Pb) were determined by atomic absorption using a Varian AA-875 spectrophotometer. An air/acetylene flame was used for Mg 2÷, Fe, Mn, Zn and Pb, while an N20/ acetylene was used for Ca 2÷ in order to avoid interference (Munger et al., 1990). Organic acids, formate (methanoate), acetate (ethanoate) and butyrate (butanoate), were measured by UV absorption at 214 nm with a Waters IC-Pak Ion exclusion column and a Waters UV486 detector. The eluant was H3PO4 (0.1%) (Millet, 1994).
For the chromatographic technique, an internal standard was used: PO43- for the anions, Li + for the cations and succinate for the organic acids. The detection limits were in the order of 0.5/xg ml -~ for ion chromatography with conductivity detection, 0.01 /zg m1-1 for atomic absorption and 0.4/zg m1-1 for ion chromatography with UV detection, with a coefficient of variation less than 9%, 8% and 10°/0, respectively. Evaluation of seasalt concentrations In order to evaluate the marine contribution, for compounds such as sulphate, chloride and calcium, the standard method was used according to previous works (Riley & Chester, 1971; Jacob et al., 1985; Colin et al., 1989). With this method, Na is considered as a tracer for the marine source. However, for this compound, a small contribution of terrigenic origin arises due to the collection of sodium crustal aerosols. To evaluate this contribution, one can look at a relationship between the sodium concentration and a tracer element of purely terrigenic origin, such as A1. Two studies have been conducted by Colin et al. (1989) and by Sanusi et al. (1996), at a site (Le Donon, France) situated approximately 50 km from the present sampling area. Analysing the rain and snow collected made it possible to determine enrichment factors relative to seawater. Using their data for A1 concentration, it can be concluded that Na of crustal origin corresponds roughly to a maximum contribution of a few/zeq litre-1 and these low values were not taken into account, although it may have a certain importance for air masses originating from the north east (Colin et al., 1989). Even though these results have been obtained in rainwater and snow, one can consider that the crustal contribution of Na in fogwater samples is also low. With this assumption, one can evaluate the marine contribution of an element X by calculating the [X]/[Na] ratio. Statistical analysis and validation of results In order to determine the possible links between components and to identify their possible sources, statistical analysis (correlation matrix) was applied to the data from the composite 2-6/zm and 5-8 tzm fractions. The ion balance was calculated by the following formula: (~anions- Ecations)/[Z(anions) + E(cations)] (Fuzzi et aL, 1992) and only samples with a sufficient volume for complete chemical analysis were included. The average of the ion balance ratio was -7.7 with a standard deviation 10.9. The anion deficit was probably due to the non-analysis of some compounds, such as carbonate, nitrite, fluoride etc. None individually was present in great quantity, but the sum total could explain this deficit. The coefficients of variation of the ion balance ratio, calculated for each sample from the analytical errors, were between 17.5 and 18.1%. If one considers simultaneously the anion deficit and the analytic errors, the good ion balances would be included between -25.8 and + 10.4 with regard to the perfect ion balance. For the present samples, one fog event had a
5-8 2-6 5-8 2-6 5-8 2-6 5-8 2-6 5 8 5-8 2-6 5-8 2-6 5-8 5-8 2-6 5-8 2-6 5-8 2-6 5-8 5-8 2-6 5-8 2-6 5-8 2-6 5-8 2-6
5-8 2-6 5-8 2-6
n.d. n.d. 0.04 0.02 n.d. n.d. 0.03 0.01 n.d. 0.017 n.d. 0.05 0.02 0.002 0.002 n.d. 0.02 0.006 0.04 0.02 0.003 0.03 0.02 0.003 n.d. 0.004 n.d. 0.004 n.d.
0.05 0.02 n.d. n.d.
5.50 5.50 9.40 9.40 3.30 3.30 6.15 6.15 2.30 10.15 10.15 11 11 9.25 10.30 10.30 14 14 14 14 7 13.30 13.30 8.30 8.30 15 15 11 11
4 4 3 3
3795 (3676) 2270 (2137) 1400 (1367) 1895 n.d. 1680(1669) 1460(1436) 860(831) 3480(3458) 935(865) 1360 n.d. 1560(1521) 3150(3121) 1340 n.d. 1310(1256) 4035(4021) 980(854) 1750(1686) 3460(3079) 7450(7088) 21620 n.d. 2665n.d. 10770(10660) 1980(1941) 4275(4233) 1810(1763) 1485(1434) 4410 (n.d.). 2050(1940) 4400(4266) 1780(1699) 5445 n.d. 2150(2068) 4990(4946)
3780 1570 1250 2340 620 1240 110 5020 640 1140 400 1550 1780 1110 3460 590 980 2290 2640 17270 800 4220 830 1650 580 660 1750 1060 2060 1245 3500 980 3410
150 60 105 610 10 n.d. 75 320 180 15 220 0 10 20 40 130 60 250 60 340 75 50 480 110 570 60 490 190 n.d.
130 70 100 1130 470 30 290 670 15 30 120 160 350 10 910 25 40 70 55 230 90 300 125 710 65 100 325 80 140 110 790 70 n.d.
350 40 35 100 550 40 190 230 10 n.d. 70 170 70 10 120 10 10 30 30 50 0 90 n.d. 60 60 n.d. 290 50 140 0 100 40 n.d.
70 10 140 290 90 200 240 180 580 n.d. 320 240 n.d. 450 120 1040 530 3150 2990 n.d. 0 910 320 350 390 420 0 910 1105 670 n.d. 680 365
980 1100 280 n.d. 2725(2721) 1302(1293) 120(109) 559(551) 178(153) n.d.n.d, 987(977) 2455(2444) n.d.n.d, 414(394) 2996(2990) 209(163) 108(85) 3648(3509) 7086(6954) n.dn.d, 2013(n.d.) 5619 n.d. 555 (541) 614(599) 1227(1210) 1422(1403) 2739(n.d.) 1504(1464) 2685(2690) 1318(1289) 3373 n.d. 955 (925) 200(2184)
530 (508) 658 (433) 3119 (3076) 160(136) 420(395) 1218(1170) 45(39) 62(0) 1491(1479) n.d.n.d, n.d.n.d, n.d.n.d,
990 60(58) 107(86) 1280 30(26) 58(12) 2560 50(45) 41(0) 5330 250(246) 8(0) 1130 130(117) 222(89) n.d. n.d.n.d, n.d.n.d, 1530 160(153) 230(156) 4090 435(430) 362(307) n.d. n.d.n.d, n.d.n.d, 2310 230(220) 181(77) 4160 410(407) 329(301) 1530 600(577) 222(0) 2870 170(128) 263(141) 2850 750(681) 798(73) 4670 1010(944) 1457(769) n.d. n.d.n.d, n.d.n.d, 2690 260 n.d. 321 n.d. 12640 1260(1240) 864(655) 2040 170(163) 255(181) 5520 420(412) 864(783) 2495 420(111) 189 (99) 1670 90 (81) 214(117) 4910 440 (n.d.) 469(n.d) 3410 250(230) 337(128) 4310 670(646) 576(322) 3210 380(366) 305(151) 5430 570 n.d. 535 n.d. 3720 200(185) 305(151) 5450 430(422) 337(233)
3140 1835 630 n.d. 4.1 n.d. 0.2 0.1 0.2 n.d. 0.2 2.3 n.d. 0.6 1.8 0.2 1.5 0~5 0.5 n.d. 0.1 1.8 0.7 0.5 0.3 0.1 0.8 n.d. n.d. 0.8 n.d. 2.7 n.d.
n.d. 3.6 0.2 n.d. 3 n.d. 3 13.1 3 n.d. 11.3 11.9 n.d. 0.6 3 0.6 0.6 6.0 5.8 n.d. 31.6 304.4 9.6 17.3 0.6 5.4 22.1 n.d. n.d. 1.8 n.d. 3 n.d.
n.d. 3 0.6 n.d.
4.8 n.d. 0.7 1.9 0.2 n.d. 0.6 1.2 n.d. 0.2 1.2 0.2 0.2 1.2 1.2 n.d. 1.5 1.2 0.2 0.2 0.2 0.4 1.2 n.d. n.d. 0.2 n.d. 24.1 n.d.
n.d. 6 0.2 n.d.
22.9 n.d. 7 13.5 5.2 n.d. 16.7 25.2 n.d. 9.2 174.3 9.9 17.0 16.4 117.4 n.d. 47.9 52.2 18.4 43.9 3.3 44.4 96.4 n.d. n.d. 17.4 n.d 19.1 n.d
n.d. 11.7 2.2 n.d.
C1 NO3SO42Formate Acetate Butyrate Na + NH4 ÷ K-Mg 2+ Ca 2~ Mn Fe Pb Zn t~eq litre i /zeq litre ~ /zeq litre ~ #eq litre i #eq litre i/~eq litre ~/zeq litre ~#eq litre 1 #eqlitre ~ tteqlitre- ~ #eq litre 1 #mol litre ~/zmol litre ~/zmol litre I#mol litre 1
3.20 2200(1053) 2.90 1230 (887) 4.40 745 (417) 3.30 12502840 n.d. 4.35 820(715) 3.6 850(616) 3.70 630(349) 3.70 1060(849) 4.70 540(0) 3.70 500 n.d. 3.50 1660(1287) 3.50 3620(3339) 3.80 10950 n.d. 4.85 1020(494) 4.50 1200(1060) 5.80 360(0) 5.80 1495(875) 4.35 2160(0) 3.20 3640(142) n.d. 13540n.d. 3.25 2400 n.d. 2.80 6940(5905) 3.50 1540(1166) 3.00 3650(3240) 4.10 740(284) 3.90 855(364) 3.30 2330 (n.d.). 3.90 1080 (15) n.d. 2850(1557) 3.90 860 (76) 3.40 2700 n.d. 3.90 1695(899) 3.30 2290(1863)
Diameter LWC Sampling pH /zm gm 3 time h
LWC: Liquid Water Content for fog droplets ranging between 2 and 6/zm and 5 and 8 ~tm. n.d.: not determined.
29/30 .11.91 30.11/ 01.12.91
08.10.91 08/09 .10.91 09/10 .10.91 28.10.91 25/26 .11.91 26/27 .11.91 27/28 .11.91 28.11.91 28/ 29.11.91 29.11.91
12.03.91
27/28 .02.91 04.03.91
27.02.91
26.02.91
20.02.91
Date
Table 1. Concentrations of anions and cations and pH values during fog events in Strasbourg in 1991 for the (2-6 pm) and (5-8 ttm) fractions. Values in parentheses correspond to non-seasalt components
4~
348
M . M i l l e t et al.
higher value (-30.8 for the sample 9/10.10.91 5-8/zm), so, only this sample was discarded and not considered in the further data analyses.
RESULTS
Gas concentrations and meteorological conditions The concentrations of major air pollutants (SO2, NO, NO2 and O3), temperature and wind velocity were measured by the Association pour la Surveillance de la Pollution Atmosph6rique (ASPA) at a location situated 400 m from the fogwater sampling site. The most important compounds were N O and NO2 (concentrations of 38-286 # g m -3 and 49-145 ttg m -3, respectively), while SO2 had a concentration o f 11-129 /zg m -3. The low concentrations of 03 (1-3.8/zg m -3) indicate a very small photochemical activity. During fog events there was a temperature ranging between - 1 and 13°C and minimal air movements, with a wind speed of 0.1-1.2 m s -1. It is noticeable that these wind velocities are insufficient to determine a wind direction, which can probably change several times during the same fog event. Fogwater composition Table 1 gives, for each event, the LWC, the sampling time, the pH values and the ionic concentrations. The present data can be compared with those recorded in previous studies (Table 2). Firstly, the present concentrations are generally higher but it is useful to recall that, unlike the others, only a very limited fraction of the fog droplet spectrum was collected. Consequently, it
is better to compare the relative concentrations rather than the absolute values. As in previous data from the US and Italy (Jacob et al., 1986; Fuzzi, 1988; Munger et al., 1983, 1990), Strasbourg fogwater was dominated by NH4 +, CI-, NO3- and SO42- (Table 2). On the other hand, in the present study, C1- and NO3- were still in the same order of magnitude, whereas C1- was generally at a much lower concentration than NO3- (Table 2). The major cation present in fogwater was NH4 ÷ and this represented 41 and 52% of the total cation loading for the 2 ~ / z m and 5-8 ttm fractions, respectively. A similar result has been observed by Munger et al. (1990) in Pasadena aerosols. In order to dispose of pollution tracer as proposed by Talbot et al. (1988), three weak organic acids (formate, acetate and butyrate) were analysed. The total concentration of (dissociated and undissociated) organic acids is reported in Table 1. As found by Winiwarter et al. (1988), in the present fog samples the acetate concentration was often similar or higher than that of formate, except for a few cases.
DISCUSSION
Droplet-size dependence of fogwater composition For a given LWC, the sample concentrations are influenced by the variability in the concentrations of dissolved pollutant per unit volume of air (obtained by multiplying the total concentration by the sample volume and dividing by the volume of air pumped). Moreover, because of the partial LWC due to the fixed droplet size range and on the assumption that the
Table 2. Averageconcentrationsfor the fogwatersamplescollectedin Strasbourg in 1991 (pet] litre-t) and comparisonwithother studies Compounds
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(A)
(B)
CINO3SO42H÷ Na ÷ NH4 ÷ K+ Mg 2+ Ca 2+ Fe Mn Pb Zn Forma~ Acetate Butyrate pH
87 6 19
200 555 910 320 40 1580 50 20 60 8 2
240 11820 2740
130 1780 780 1180 80 1120 17 40 280 60 3 8
120 820 2070 2 42 3270
430 1020 890 140 60 2110 40 30 280 21 1 1 20
2590 440 610
4.80
4.02
1880 1240 2150 198 840 2390 310 350 1700 6 1 2 25 84 135 90 4.02
3110 3410 5020 570 510 4820 440 420 2120 47 2 2 50 360 350 120 3.50
60 19 6 14 14
2 8 5.60
10
70 14540 40 170
320 130 4.30
(1) Post et al. (1991): Barrington, Australia. (2) Fuzzi (1988): Po Valley, Italy. (3) Munger et al. (1990): Riverside, California, US. (4) Jacob et al. (1985): Lennox, US. (5) Jacob et al. (1986): Bakersfield, California, US. (6) Joos & Baltensperger (1991): Switzerland. (7) Johnson et al. (1987): Diibendorf, Switzerland. (A) 5-8/zm fractions: Strasbourg, France. (B) 2-6/zm fractions: Strasbourg, France.
33 170 20 1 1
30 1160 15 22 220 4 10
40 150 3.08
Chemical composition of fogwater volume mean diameter within that size range remains constant, it can be concluded that, for a given concentration of pollutant per unit volume of air, if the LWC decreases (which means a decrease of the number of droplets per unit volume of air) the concentrations in the fogwater samples increase. These two observations indicate that, for a given droplet size, the concentrations of the liquid phase are governed by the ratio between the concentrations of dissolved pollutant per unit volume of air and the number of droplets per unit volume of air. Furthermore, we observe, in general, higher concentrations in the 2-6/~m fraction than in the 5-8/zm fraction (Table 1). The increased concentration in the small droplets, previously observed in cloudwater and fogwater (Ogren et al., 1992; Collett et al., 1993), results from the size-dependent chemical composition of the aerosol particles that act as condensation nuclei (Ogren & Charlson, 1992), from the dissolution of the aerosol particles in smaller amounts of water (Johnson et al., 1987) and from the higher surface/volume ratio of the small droplets, which promote greater gas/liquid exchange and, consequently, chemical reactions (Schwartz, 1988). Nevertheless, even if the absolute concentrations varied according to the size of the droplets, it is found that no systematic droplet size dependence of fogwater composition was observed. Thus, although the droplet sizes are different, the chemistry of the droplets are equivalent.
Fog droplet-size distribution Assuming that the droplets are spherical, the density of fogwater droplets (D) can be calculated for each LWC and for each droplet-size category as follows: D = LWC f~2 n(r)dr 4 yr f~2 n(r)r 3dr
(1)
349
noticed that the calculated LWC is near the lower limit of the measured LWC interval, probably because only the thick fog events have been collected. Using the droplet size distribution presented by Bott (1991), the density of droplets for the two size categories studied can be evaluated. Thus, for an equivalent LWC, the density of droplets is about 400 times higher for the 2-6 #m fraction than for the 5-8/zm fraction. This conclusion is in agreement with experimental results observed previously by Fuzzi et al. (1992) in the Po Valley: the density decreases when the droplet size increases, even though the volume of fogwater increases with the droplet size.
Automobile exhaust Like Winiwarter et al. (1988) in the Po Valley, our acetate concentration is often similar to, or higher than, that of formate, and this phenomenon is attributed to automobile traffic. Indeed, there are four possible sources of organic acids: terrestrial vegetation emissions (Keene & Galloway, 1986), liquid phase reaction in the atmosphere between formaldehyde and hydroxyl radical (Jacob, 1986), emission from motor vehicle exhaust (Kawamura et al., 1985; Talbot et al., 1988) and biomass burning (Talbot et al., 1988). But only in the two latter cases, is acetate concentration typically higher than that of formate (Talbot et al., 1988). If one considers that biomass burning is negligible in urban areas, organic acids would come essentially from the automobile exhaust. This conclusion is corroborated by the high levels of gaseous NOx and by the significant correlation between NO3- and formic and acetic acids (Table 3), but it seems to be in contradiction with Pb concentrations, which are lower than those observed by others (Jacob et al., 1985; Johnson et al., 1987). A possible explanation is the general use of unleaded gasoline in France since 1989 (Nriagu, 1990). Consequently, Pb 10 s
where rl and r2 are the lower and upper limits of the droplet size categories and where r and n are, respectively, the radius and the number density of the droplet (which is a function of r). To determine the droplet-size distribution, the results of a simulation model of a typical droplet spectrum of fog formed over an urban area (Bott, 1991) can be used. With this model, in which the size of the droplets varied between 0.5 and 30 tzm radius (Fig. 1), the theoretical LWC and number of droplets obtained for the two droplet fractions studied in the present work can be computed. By these calculations, the following was obtained for the 2-6/zm and 5-8/zm (or 1-3 #m and 2 . 5 4 / z m in radius) fractions, respectively: 2.3x10 -3 and 3.5× 10-3 g of water per cubic metre of air for the LWC and 9× 107 and 3.5× 105 droplets per cubic metre of air. These results are compatible with the present experimental data, in which the LWC ranged between 6x 10-3 and 2x10 -3 and between 2x10 -3 and 5×10 -2, respectively, for the 2-6 and 5-8 ttm fractions (Table 1). It is
10 4
10 3
lo2 ~
10 I
'~
100
10-~
10-2
10-3 1
10 Radius r Oam)
Fig. 1. Fog droplet-size distribution in the range 0.5-30 ~m (Bott, 1991).
CI-
NO3-
SO4 2-
H+
Na +
NH4 +
1.000 0.826 0.839 0.142 0.076 -0.140 0.669 -0.020 0.433 -0.119 0.339 0.370 -0.201 0.356
K+
Ca 2÷
Form.
Acet.
Buty.
Fe
Mn
Zn
Pb
1.000 0.774 1.000 -0.022 0.050 1.000 0.001 0.062 0.672 1.000 -0.246 0.125 0.502 0.479 1.000 0.338 0.482 0.192 0.099 0.002 1.000 -0.004 0.158 0.066 0.127 0.415 0.107 1.000 0.477 0.595 0.238 0.522 0.081 0.115 0.054 1.000 -0.044 -0.054 0.087 -0.084 0.038 -0.059 0.515 -0.079 1.000 0.303 0.375 0.032 -0.032 -0.017 0.511 0.134 0.108 -0.107 0.378 0.372 -0.054 -0.189 -0.175 0.426 0.102 0.092 -0.132 -0.195 -0.068 0.113 0.028 0.280 0.251 0.364 -0.254 -0.049 0.515 0.389 -0.370 -0.308 -0.381 -0.021 -0.094 0.256 -0.346
Mg 2+
LWC: Liquid Water Content for droplet ranging between 2 and 6/zm and 5 and 8/zm.
1.000 -0.413 1.000 -0.412 0.466 1.000 SO4 2- -0.496 0.894 0.705 1.000 H+ -0.174 0.769 0.449 0.705 1.000 Na + -0.352 0.275 0.228 0.384 0.104 1.000 NH4 + -0.433 0.859 0.722 0.899 0.696 0.123 1.000 K+ -0.469 0.827 0.594 0.917 0.538 0.601 0.797 Mg 2+ -0.475 0.714 0.337 0.762 0.515 0.737 0.521 0.533 Ca 2÷ -0.592 0.661 0.469 0.815 0.391 0.601 Form. -0.164 0.281 0.615 0.348 0.360 -0.334 0.502 Acet. -0.114 0.121 0.567 0.274 0.163 -0.316 0.360 Buty. -0.030 0.066 0.133 0.016 -0.062 -0.316 0.009 Fe -0.202 0.796 0.484 0.779 0.665 0.045 0.850 Mn -0.368 0.112 0.007 0.087 0.233 -0.045 0.067 Zn -0.337 0.305 0.426 0.528 0.139 0.116 0.351 Pb -0.269 -0.031 -0.073 -0.057 -0.015 0.032 0.012 SO2 -0.141 0.510 0.069 0.467 0.705 -0.056 0.358 NO -0.152 0.542 0.504 0.431 0.682 0.012 0.299 NO2 0.293 0.102 0.510 0.037 0.544 -0.307 0.013 03 -0.129 0.262 -0.073 0.240 0.164 0.412 -0.040
LWC C1NO3-
LWC
1.000 0.923 0.651 0.179
SO2
NO2
1.000 0.530 1.000 0.437 -0.064
NO
Table 3. Correlations between compounds analysed during fog events in Strnsbourg in 1991 (n = 22). Correlations in bold type are significant (p < 0.05)
1.000
03
Chemical composition o f f ogwater is no longer a reliable tracer of automobile pollution. This result is corroborated by the absence of significative correlation between organic acids and Pb (Table 3).
NO3-, 5042- and
NH4 + origins The presence of nitrate and ammonium ions can be attributed to a direct input of gaseous nitric acid and ammonia, as well as to an input due to the collection of nitrate and ammonia contained in aerosols. However, the problem is more complicated for sulphate, since it also has a marine source. For seawater, the ratio of SO42-/Na ÷ is 0.12, with 5042- and Na + expressed in /zeq litre -1 (Riley & Chester, 1971; Jacob et al., 1985). Whatever the fogwater sample, the SO42-/Na + ratio is always larger than 0.12 (Fig. 2), so the marine contribution is either weak or negligible. This result is corroborated by the absence of a significant correlation between SO42- and Na + (Table 3). Then, the presence of the most of the sulphate can be attributed to an input of SO2 followed by its oxidation to 5042-, as well as to an input due to the collection of sulphate contained in aerosols. In order to determine whether the gas to particle conversion occurred before or after the formation of the fog, a second statistical analysis has been made with the loading of ions per unit volume of air. These loads were calculated by multiplying the aqueous concentration by the sample volume and dividing by the volume of air pumped. If the first statistical analysis gives the greater correlations, the SO42- and NO3- concentrations in liquid phase are controlled by gaseous SO2 and NO× concentrations which means that scavenging is controlled by phase equilibrium. On the other hand, if the second statistical analysis gives the greater correlation, liquid concentrations are water-volume-dependent and the scavenging is controlled by mass transfer. In the present study, both results are almost equivalent (0.467 against
351
0.415 for SO2-SO42-; 0.504 and 0.510 against 0.476 and 0.5 for NO-NO3- and NO2-NO3-, respectively), which may indicate a combined action of the two phenomena. The same results are obtained with the study of the correlations with LWC since negative correlations were obtained between LWC and compounds such as Cl-, NO 3-, SO42-, NH4 +, K +, Mg 2+ and Ca 2+ (Table 3 and Fig. 3), which indicates that an important part of the dissolved material enters solution from the aerosol particles. Nevertheless, all these negative correlations are low (in absolute values), so the direct input from the gas phase after the fog has formed is probably not negligible. Indeed, with the present method of collection, a variation of the LWC induces a variation of the number of droplets and, consequently, a variation of air-liquid interface, which promotes the mass transfer phenomenon. Simultaneously, the high fogwater concentrations probably induce a control of the scavenging by the phenomenon of phase equilibrium. CI- origin
For fog of marine origin, the CI-/Na ÷ ratio should reflect the composition of seawater, i.e. Cl-/Na÷ ~ 1.17 (C1- and Na + expressed in/~eq litre -1) (Riley & Chester, 1971; Jacob et al., 1985). As can be seen in Fig. 4, the CI-/Na + ratio often exceeds 1.17, but in four cases the ratio is smaller than this value. The ratio smaller than 1.17 can partially be explained by the presence of Na of terrigenic origin. However, using the rough estimate for Na + of crustal origin found in the previous sections, one finds that the C1/Na ratios indeed increase but remain below 1.17. Another possibility would mean the replacement of C1- by NO3- or SO42- (Robbins et al., 1959; Eriksson, 1960; Clegg & Brimblecombe, 1985, 1986; Keene et al., 1990) but we have no way to compute its importance. The ratios larger than 1.17 mean that C1- has not only a marine origin, but also originates from human activities. It may result from the automobile exhaust, since gasoline contains lead bromo chloride as an additive
34
m
30 20
I
/
1.0-
I 5-8
2-6~./
0.8 0.6
16
0.4-
_~
!
0.2-
0.0
8
-0.4 -0.6 0.12
i LWC LWC
D~es of ~ g ~ r
J
CI" ~
Cl" ~
i
NO3" NO~" !
i
i
SO42" SO~2" ~
i
NH4* ~4"
i
H* ~
Ca 2+
H÷ ~
Ca ~
~pl~
Fig. 3. Correlations between some compounds analysed
Fig. 2. SO42-/Na + ratio in fogwater samples.
during fog events in Strasbourg in 1991 (n = 22).
352
M . Millet et al.
of HC1 per cubic meter. The total liquid content is typically 0.06 ml m -3 (Johnson et al., 1987); then the C1- concentration in solution is about 2300/zeq litre -], which is comparable to the non-seasalt chloride presently observed (Table 1). Consequently, the non-seasalt chloride seems to come mainly from the refuse incinerator.
16
5-Spin 2-6pro
14 12 10 +al 8
Ca 2+ origin
rO 6 4
2 0
,01J , N0
~1.17
o Datesof fogwatcrsampling
Fig. 4. C1-/Na + ratio in fogwater samples. (Friedlander, 1973; Fuzzi et al., 1984). Nevertheless, since 1989 the use of unleaded gasoline seems to be general in France and, as in the case of Pb, chloride produced in this manner becomes less and less important. It may also be produced, in the form of HC1 in the gas phase, by the paper industry, which uses CI- for whitening paper, but this type of industry does not exist in the Strasbourg region. Lastly, CI- may result from the stack gases of refuse incinerators. Indeed, the combustion of organochlorine compounds, such as polyvinylchloride, produces HCI in the gas phase (Johnson et al., 1987; Sigg et al., 1987). A refuse incinerator exists in Strasbourg, situated about 4 km away to the south of the sampling site. CI- could come from this plant. This hypothesis is supported by the results obtained by DRIRE-Alsace (1992), which estimated that this incinerator emitted 1980 tonnes of HC1 per year. On the assumption that all the HC1 emitted is dissolved in fogwater, the observed chloride loading and the known HC1 emission (1980 tonnes per year or 63 gs -]) from the incinerator can be compared as follows: On account of the variation of the wind direction during individual fog events, the emissions are spread in all directions around the incinerator. Since in fog conditions the height of the mixing layer is about 500 m and the plume moves at a maximum wind speed of 1 m s -], the HC1 concentration at 4000 m away from the incinerator is: [63/(2 x zr× 4000 x 500)] x 1 = 5 x 10 -6 g
Ca may have a marine origin, but, in the Strasbourg region, this contribution is low (Table 1). A second possible origin for Ca 2+ is the long-range transport of Saharan dust. However, Colin et al. (1989) have shown that, at a site situated at about 50 km to the west of Strasbourg, this input of Ca 2 + is very weak, even for air masses that originate in North Africa. A third explanation is that Ca 2 + could be emitted by human activities, such as traffic, cement factory, etc. A fourth possibility is revealed by the study of the soil composition in the Strasbourg region. This study indicates that the soil is composed of 'loess', which is very rich in Ca 2 +. Because of its formation by the freezing-defreezing process, the 'loess' consists of many very fine dust particles easily spread by the wind. Fogwater acidity
Several compounds, such as H2SO4, HNO3, HC1, NH3 and CaCO3, contribute to the acid-base balance of fogwater. In order to compare the acidifying contributions, a series of linear correlations has been calculated between the major cations H +, NH4 + plus non-seasalt Ca 2+ (nss-Ca2+), and the anions non-seasalt C1- (nssC1-), non-seasalt SO42- (nss-SO42-) plus NO3(Table 4). For the correlation of H + + NH4 + versus nss-SO42-+ NO3-, a correlation coefficient of 0.892 was obtained. Then, free acidity in fogwater samples was controlled essentially by nss-sulphate and nitrate acidity, and neutralised by ammonium. Yet, the addition of nss-Ca 2+ and nss-C1- increases the correlation coefficients (0.941), and these two compounds contribute to a smaller extent to the acid-base balance. According to the literature, HNO3 and H2SO4 are strongly involved in the acidity of urban fogwater (Johnson et al., 1987; Sigg et al., 1987; Joos & Baltensperger, 1991; Post et al., 1991). In contrast, HC1 is generally little involved in the acidity of precipitation of fog (Colin et al., 1989), even if it can be an important acidifying agent in certain exceptional situations (Martin & Barber, 1978; Jaffrezo & Colin, 1987; Sigg et al., 1987). Thus, in Switzerland, Johnson et al. (1987)
Table 4. Acid-base balances and correlation coefficients for the fogwater samples collected in Strasbourg in 1991
Ions H + + NH4 + H + + NH4 + H + + NH4 + H + + NH4 + H + + NH4 + H + + NH4 +
vs nss-SO42-+ NO3+ nss-Ca2+ vs nss-SO42- + NO3vs nss-SO42-+ nss-C1- + NO3+ nss-Ca 2+ vs nss-SO42-+ nss-C1- + N O 3 vs nss-SO42-+ nss-C1+ nss-Ca2+ vs nss-SO42-+ nss-C1-
Correlation coef.
Significance
0.892 0.909 0.891 0.941 0.939 0.885
0.05 0.05 0.05 0,05 0.05 0.05
Chemical composition o f fogwater
and Sigg et al. (1987) attribute the high acidity to an input of gaseous HC1 emitted by a refuse incinerator. In Strasbourg fogwater samples, whatever the samples, the NO3- and nss-SO42- are always a very major component (Table 1) and the nss-Cl- is also, except in a few cases, a relatively important component compared to the two previous species (NO3- and nss-SO42-) (Table 1). Moreover, as presented previously, the higher coefficient of correlation is obtained when nss-C1- is included in the acid-base balance. Consequently, it seems that the Strasbourg fogwater acidity is increased by HC1, which could be emitted by a refuse incinerator, as in Switzerland.
CONCLUSION Results presented concern the chemical analysis of fog events collected during a 1-year sampling campaign in Strasbourg, a town in the east of France. A sampling procedure was adopted that made it possible to collect two size fractions (2-6 # m and 5-8/zm). A total of 18 fog events were collected during 1991 in Strasbourg; this represents 33 samples, because sometimes the 2 - 6 / z m fractions were not available. It was observed that the smaller fraction is the most concentrated and has the most acidic pH. This work has allowed the identification of two probable important sources of acidity in Strasbourg. The first is automobile traffic, characterised by the ratio of formate and acetate, which in this case indicates that the second compound is typically higher than or similar to the formate concentrations. It was observed also that the general use of unleaded gasoline reduces the atmospheric Pb concentration, because, in spite of heavy traffic and a study of small droplet size, Pb concentrations are moderate. The second source of acidity is probably a refuse incinerator near the town, which produces HCI in the gas phase. In association with H2SO4 and HNO3, HC1 is an important factor in the acidity of the fogwater. So, fitting a filter in the chimney stack of this refuse incinerator could improve the air quality in Strasbourg. These acids are partially neutralised by some compounds, such as NH4 and Ca, which may originate from the rural area near the town and human activities, such as cement production. Since November 1995, a washing system of the refuse incinerator exhaust has been installed. A new study of the fogwater composition would permit an evaluation of the influence of this installation on the air quality of Strasbourg.
ACKNOWLEDGEMENTS This work has been supported by I F A R E (Institut Franco-Allemand pour la Recherche en Environement). The gas concentrations were measured and supplied by ASPA (Association pour la Surveillance de la Pollution
353
Atmosph6rique). The fogwater collectors were supplied by Dr G. Metzig from the Kernforschungzentrum (Karlsruhe, Germany). All are gratefully acknowledged.
REFERENCES Berner, A. (1988). The collection of fog droplets by a jet impaction stage. Sci. Total Environ., 73, 217-228. Bott, A. (1991). On the influence of the physico-chemical properties of aerosols on the life cycle of radiation fogs. Boundary-Layer Met., 56, 1-31. Brook, J. R., Wiebe, A. H., Summers, P. W. & Li, S. M. (1992). Aerosol acidity in Canada and the link with respiratory health. J. Aerosol Sci., 23(1), $977-$981. Capel, P. D., Leuenberger, C. & Giger, W. (1991). Hydrophobic organic chemicals in urban fog. Atmos. Environ., 25A, 1335-1346. Clegg, S. L. & Brimblecombe, P. (1985). Potential degassing of hydrogen chloride from acidified sodium chloride droplets. Atmos. Environ., 19, 465-470. Clegg, S. L. & Brimblecombe, P. (1986). The dissociation constant and Henry's law constant of HC1 in aqueous solution. Atmos. Environ., 20, 2483-2485. Colin, J. L., Renard, D., Lescoat, V., Jaffrezo, J. L., Gros, J. M. & Srauss, B. (1989). Relationship between rain and snow acidity and air mass trajectory in eastern France. Atmos. Environ., 23, 1487-1498. Colin, J. L., Jaffrezo, J. L. & Gros, J. M. (1990). Solubility of major species in precipitation: factors of variation. Atmos. Environ., 24A, 1939-1944. Collett, J., Jr, Oberholzer, B. & Staehelin, J. (1993). Cloud chemistry at Mt Rigi, Switzerland: dependence on drop size and relationship to precipitation chemistry. Atmos. Environ., 27A, 33-42. Czuczwa, J., Leuenberger, Ch. & Giger, W. (1988). Seasonal and temporal changes of organic compounds in rain and snow. Atmos. Environ., 22, 907-916. DRIRE-Alsace (1992). Les Rejets Industriels en Alsace. Eriksson, E. (1960). The yearly circulation of chloride and sulfur in nature, meteorological, geochemical and pedological implications. Tellus, 12, 63-109. Friedlander, S. K. (1973). Chemical element balances and identification of air pollution sources. Environ. Sci. Technol., 7, 235-240. Fuzzi, S. (1988). Fog chemistry and deposition in the Po valley. In Acid Deposition at High Elevation Sites, ed. M. H. Unsworth & D. Fowler, Kluwer, Dordrecht, The Netherlands, pp. 443-452. Fuzzi, S., Castillo, R. A., Jiusto, J. E. & Lala, G. C. (1984). Chemical composition of radiation fog at Albany, New York and its relationship to fog microphysics. J. Geophys. Res., 85, 7159-7164. Fuzzi, S., Facchini, M. C., Orsi, G., Lind, J. A., Wobrock, W., Kessel, M., Maser, R., Jeaschke, W., Enderle, K. H., Arends, B. G., Berner, A., Solly, I., Kruisz, C., Reischl, G., Pahl, S., Kaminski, U., Winckler, P., Ogren, J. A., Noone, K. J., Hallberg, A., Fierlinger-Oberlinninger, H., Puxbaum, H., Marzorati, A., Hansson, H.-C., Wiednsohler, A., Svenningsson, I. B., Martisson, B. G., Schell, D. & Georgii, H. W. (1992). The Po valley fog experiment 1989, An overview. Tellus, 44B, 448-469. Gieray, R., Lammel, G., Metzig, G. & Wieser, P. (1993). Size dependent single particle and chemical bulk analysis of droplets and interstitial particles in an orographic cloud. Atmos. Res., 30, 263-293. Glotfelty, D. E., Seiber, J. N. & Liljedahl, L. A. (1987). Pesticides in fog. Nature, 325, 602-605. Hileman, B. (1983). Acid fog. Environ. Sci. Technol., 17, 117A-120A.
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Hoffmann, J. & Metzig, G. (1991). Calibration of fog water " collectors, or. Aerosol Sci., 22(Suppl. 1), 5291-5294. Igawa, M., Munger, J. W. & Hoffmann, M. R. (1989). Analysis of aldehydes in cloud and fogwater samples by HPLC with a poscolumn reaction detector. Environ. Sci. Technol., 23, 556--561. Jacob, D. J. (1986). Chemistry of OH remote clouds and its role in the production of formic acid and peroxy monosulfate. J. Geophys. Res., 91, 9807-9866. Jacob, D. J., Waldman, J. M., Munger, J. W. & Hoffmann, M. R. (1985). Chemical composition of fogwater collected along the California coast. Environ. Sci. Technol., 19, 730736. Jacob, D. J., Munger, J. W., Waldman, J.-D. & Hoffmann, M. (1986). The H2SO4-HNO3-NH3 system at high humidities and in fogs: 1. Spatial and temporal patterns in the San Joaquin Valley of California. J. Geophys. Res., 91, 10731088. Jacob, D. J., Shair, F. H., Waldman, J. M., Munger, J. M. & Hoffman, M. R. (1987). Transport and oxidation of SO2 in a stagnant foggy valley. Atmos. Environ., 21, 1305-1314. Jaffrezo, J. L. & Colin, J. L. (1987). Construction and exploitation of an automatic sequential wet-only collector. Environ. Technol. Lett., 8, 467--474. Johnson, C. A., Sigg, L. & Zobrist, J. (1987). Case studies on the chemical composition of fogwater: the influence of local gaseous emissions. Atmos. Environ., 21, 2365-2374. Joos, F. & Baltensperger, U. (1991). A field study on chemistry S(IV) oxidation rates and vertical transport during fog conditions. Atmos. Environ., 25A, 217-230. Kawamura, K., Ng, L.-L. & Kaplan, I. R. (1985). Determination of organic acids (C1-C10) in the atmosphere, motor exhausts, and engine oils. Environ. Sci. Technol., 19, 10821086. Keene, W. C. & Galloway, J. N. (1986). Considerations regarding sources for formic and acetic acids in the troposphere, at. Geophys. Res., 91, 14466-14474. Keene, W. C., Galloway, J. N. & Holden, J. D. (1983). Measurements of weak organic acidity in precipitation from remote areas of the world. J. Geophys. Res., 88, 5122-5130. Keene, W. C., Pszenny, A. A. P., Jacob, D. J., Duce, R. A., Galloway, J. N., Schultz-Tokos, J. J., Sievering, H. & Boatman, J. F. (1990). The geochemical cycling of reactive chlorine through the marine troposphere. Global Biogeochem. Cycles, 4, 407-430. Lammel, G. & Metzig, G. (1991). Multiphase chemistry of orographic clouds: observations at subalpine mountain sites. Fresenius J. Anal. Chem., 3411, 564-574. Lim, B., Jickells, T. D. & Davis, T. D. (1991). Sequential sampling of particles, major ions and total trace metal in wet deposition. Atmos. Environ., 25A, 745-762.
Martin, A. & Barber, R. (1978). Some observations of acidity and sulfur in rainwater from rural sites in central England and Wales. Atmos. Environ., 12, 1481-1487. Millet, M. (1994). PhD Thesis, Universit6 Louis Pasteur, Strasbourg, France. Millet, M., Wortham, H. & Mirabel, Ph. (1995). Solubility of polyvalent cations in fogwater at an urban site in Strasbourg (France). Atmos. Environ., 29, 2625-2633. Munger, J. W., Jacob, D. J., Waldman, J. M. & Hoffmann, M. R. (1983). Fogwater chemistry in an urban atmosphere. J. Geophys. Res., 88, 5109-5121. Munger, J. W., Collett, J., Daube, B. & Hoffmann, M. R. (1990). Fogwater chemistry at Riverside, California. Atmos. Environ., 34B, 185-205. Nriagu, G. O. (1990). The rise and fall of leaded gasoline. Sci. Total Environ., 92, 13-28. Ogren, J. A. & Charlson, R. J. (1992). Implications for models and measurements of chemical in homogeneities among cloud droplets. Tellus, 44B, 208-226. Ogren, J. A., Noone, K. J., Hallberg, A., Schell, D., Berner, A., Solly, I., Kruisz, C., Arends, B. G. & Wobrock, W. (1992). Measurements of the size dependence of the concentration of non-volatile in fog droplets. Tellus, 44B, 570-580. Post, D., Bridgman, H. A. & Ayers, G. P. (1991). Fog and rainwater composition in rural SE Australia. J. Atmos. Chem., 13, 83-95. Richartz, H., Reischl, A., Trautner, F. & Hutzinger, O. (1990). Nitrated phenols in fog. Atmos. Environ., 24A, 3067-3071. Riley, J. P. & Chester, R. (1971). Introduction to Marine Chemistry, Academic Press, London, UK, 81 pp. Robbins, R. C., Cadle, R. D. & Eckhardt, D. L. (1959). The conversion of sodium chloride to hydrogen chloride in the atmosphere. J. Met., 16, 53-56. Sanusi, A., Wortham, H., Millet, M. & Mirabel, Ph. (1996). Chemical composition of rainwater in eastern France. Atmos. Environ., 30, 59-71. Schwartz, S. E. (1988). Mass-transport limitation to the rate of in-cloud oxidation of SO2: re-examination in the light of new data. Atmos. Environ., 22, 2491-2499. Sigg, L., Stumm, W., Zobrist, J. & Ziircher, F. (1987). The chemistry of fog: factors regulating its composition. Chimia, 41, 159-165. Talbot, R. W., Beecher, K. M., Charris, R. & Cofer, W. R. (1988). Atmospheric geochemistry of formic and acetic acids at a mid-latitude temperature site. J. Geophys. Res., 93, 1638-1652. Winiwarter, H., Puxbaum, H., Facchini, M. C., Orsi, G., Beltz, N., Enderle, K. & Jaeschke, W. (1988). Organic acids gas and liquid phase measurements in the Po Valley fallwinter conditions in the presence of fog. Tellus, 40B, 348357.
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Environmental Pollution, Vol. 94, No. 3, pp. 345-354, 1996 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0269-7491/96 $15.00+0.00
ELSEVIER
CHEMICAL COMPOSITION OF FOGWATER IN A N U R B A N AREA: STRASBOURG (FRANCE) Maurice Millet, Astrid Sanusi & Henri Wortham* Centre de Gdochimie de la surface, CNRS UPR 6251, Universitd Louis Pasteur, 28, rue Goethe, 67083, Strasbourg Cedex, France
(Received 22 May 1995; accepted 10 May 1996)
Abstract
fogwater, such as PAHs (Capel et al., 1991), phenols and nitrophenols (Richartz et al., 1990), aldehydes and organic acids (Igawa et al., 1989) and pesticides (Glotfelty et al., 1987). In the majority of cases, the fogwater articles that study the inorganic fraction, focus on some fog events (2~) and try to determine the mechanisms of concentration and acidification of the whole droplet range (Jacob et al., 1985, 1987; Johnson et al., 1987; Post et al., 1991; Brook et al., 1992; Fuzziet al., 1992). Moreover, these works generally study coastal fogs (Jacob et al., 1985, 1987; Post et al., 1991). Conversely, the purpose of the present paper is to measure the ionic composition of two inhalable size fractions (2-6/zm and 5-8/zm diameter) of a continental fog and to discuss what processes could control the composition and its variability. This study concerns the 18 fog events that occurred during 1991, with a view to identifying potential health effects for future investigations.
To investigate the acidity and to identify the predominant compounds, this work presents the chemical analysis of 18 fogwater samples collected during the year 1991 in Strasbourg, in the east of France. For each fog event, two droplet size categories (2-6 lzm and 5~8 Izm) have been separately collected and 16 ionic components have been analysed. These two fraction sizes were chosen because they correspond approximately to the size range that can penetrate the human lung and they may have possible health effects. The dominant species were NH4 +, NO3-, S042- and CI-, with a maximum level of 12640, 17270, 21 620 and 13540/zeq litre -1, respectively. For most of the fog events the highest concentrations of all analysed species were observed in the 2-6/zm droplets, pH values ranged between 2.79 and 5.70 and the fogwater acidity was governed by three strong acids, H2S04, HN03 and HCI and was partially neutralised by NH3 and probably by the presence of CaC03 in the "loess', which is the major constituent of soils in the upper Rhine valley. In other respects the acetate/formate ratio (methanoate/ ethanoate), generally lower than 1, indicates an important pollution due to automobile exhaust, although the Pb concentrations are moderate due to the general use of unleaded gasoline in France since 1989. © 1997 Elsevier Science Ltd. All rights reserved
MATERIAL AND METHODS Fogwater sampling Samples were collected from the roof of the Botanical Institute of Strasbourg University (height 30 m). It is situated nearly in the centre of the town and 3000 m away from industrial sites and highways. In this region, which is densely populated and industrialised, radiation fogs at ground level are frequently observed during autumn and winter months. Commonly, they are formed during the night, but they can occasionally remain the whole day. Fogwater was collected by using two heads, first described by Berner in 1988, connected to pumps. Air containing fog droplets was drawn up through the sampling heads and droplets with sufficient movement quantity were captured by impaction on cones." Fog droplets are generally between 1 and 100/zm in diameter, with the majority (in volume) in the range 1050/zm (Fuzzi et al., 1984, 1992; Johnson et al., 1987); but in this work, due to the sampling method, only small droplets ranging between 2 and 6/zm and 5 and
Keywords: Fogwater, ionic composition, organic acids, metals, atmospheric acidity.
INTRODUCTION In recent studies, the scientific community has been interested in fog as a potential mechanism for scavenging atmospheric contaminants. In 1980 Hoffmann and co-workers demonstrated the high acidity of fogwater in Southern California (Munger et al., 1983; Jacob et al., 1985). It was found that the southern Californian fogs were frequently 100 times more acidic than rain in the same area (Hileman, 1983). This observation has led to many studies in various countries to determine the concentration of inorganic and organic compounds in *To whom correspondence should be addressed. 345
346
M . Millet et al.
8/zm diameter were sampled. The operating details of these collectors have been described elsewhere (Berner, 1988; Hoffmann & Metzig, 1991; Lammel & Metzig, 1991; Gieray et al., 1993; Millet, 1994; Millet et al., 1995). The collectors were started manually when fog was sufficiently dense to obscure the cathedral belfry of Strasbourg from the laboratory (about 500 m) and were also stopped manually when the belfry reappeared. With this empirical method, all the weak fog events were missed but for these, the experiment shows that it was impossible to collect a sufficient volume of liquid to be analysed. This method makes it possible to always start and stop the sampling for the same fog density. Nevertheless, depending on the rapidity of the appearance of the fog events, the delay between the beginning of the fog and the start of the sampling could vary between 0.5 and 2 h. The periods of collection varied between 3 and 11 h. The liquid water contents (LWCs) (see Table 1) were estimated from the sampling period, the pump flow and the volume of the sample. Thus the LWC represents solely the liquid water contained in the small fog droplets (2-6/zm and 5-8 #m) and not the quantity of water per volume of air as commonly defined. Before sample collection, the collectors were washed thoroughly with deionised water. Analytical procedure At the end of each sampling session, the fogwater volumes of each head were measured and the pH was immediately determined by using a pH-meter Consort P407 equipped with a combination micro electrode (6 mm diameter) Ingold. In order to prevent chemical reactions and the microbial decomposition of the organic acids (Keene et al., 1983), the samples were stored in a freezer at -18°C until they were analysed as recommended by Colin et al. (1990). Before each analysis, the samples were filtered with Millex SLGS025NB (0.22/zm) Waters (Millipore) to extract insoluble matter (Czuczwa et al., 1988; Lim et al., 1991; Millet et al., 1995; Sanusi et al., 1996). Major inorganic anions (CI-, NO3- and SO42-) were determined by ion chromatography with a Waters Powerline 600E pump, a Wescan 213A conductivity detector and an IC-Pak A/HR Waters column. The eluant was Borate-Gluconate/CH3CN/n-Butanol. Major monovalent cations (Na ÷, NH4 ÷ and K ÷) were analysed by ion chromatography using an IC-Pak CM/D Waters column. The eluant was 0.1 mM HNO3/ 3 mM EDTA-acid. Polyvalent cations (Mg 2÷ , Ca 2÷ , Fe, Mn, Zn and Pb) were determined by atomic absorption using a Varian AA-875 spectrophotometer. An air/acetylene flame was used for Mg 2÷, Fe, Mn, Zn and Pb, while an N20/ acetylene was used for Ca 2÷ in order to avoid interference (Munger et al., 1990). Organic acids, formate (methanoate), acetate (ethanoate) and butyrate (butanoate), were measured by UV absorption at 214 nm with a Waters IC-Pak Ion exclusion column and a Waters UV486 detector. The eluant was H3PO4 (0.1%) (Millet, 1994).
For the chromatographic technique, an internal standard was used: PO43- for the anions, Li + for the cations and succinate for the organic acids. The detection limits were in the order of 0.5/xg ml -~ for ion chromatography with conductivity detection, 0.01 /zg m1-1 for atomic absorption and 0.4/zg m1-1 for ion chromatography with UV detection, with a coefficient of variation less than 9%, 8% and 10°/0, respectively. Evaluation of seasalt concentrations In order to evaluate the marine contribution, for compounds such as sulphate, chloride and calcium, the standard method was used according to previous works (Riley & Chester, 1971; Jacob et al., 1985; Colin et al., 1989). With this method, Na is considered as a tracer for the marine source. However, for this compound, a small contribution of terrigenic origin arises due to the collection of sodium crustal aerosols. To evaluate this contribution, one can look at a relationship between the sodium concentration and a tracer element of purely terrigenic origin, such as A1. Two studies have been conducted by Colin et al. (1989) and by Sanusi et al. (1996), at a site (Le Donon, France) situated approximately 50 km from the present sampling area. Analysing the rain and snow collected made it possible to determine enrichment factors relative to seawater. Using their data for A1 concentration, it can be concluded that Na of crustal origin corresponds roughly to a maximum contribution of a few/zeq litre-1 and these low values were not taken into account, although it may have a certain importance for air masses originating from the north east (Colin et al., 1989). Even though these results have been obtained in rainwater and snow, one can consider that the crustal contribution of Na in fogwater samples is also low. With this assumption, one can evaluate the marine contribution of an element X by calculating the [X]/[Na] ratio. Statistical analysis and validation of results In order to determine the possible links between components and to identify their possible sources, statistical analysis (correlation matrix) was applied to the data from the composite 2-6/zm and 5-8 tzm fractions. The ion balance was calculated by the following formula: (~anions- Ecations)/[Z(anions) + E(cations)] (Fuzzi et aL, 1992) and only samples with a sufficient volume for complete chemical analysis were included. The average of the ion balance ratio was -7.7 with a standard deviation 10.9. The anion deficit was probably due to the non-analysis of some compounds, such as carbonate, nitrite, fluoride etc. None individually was present in great quantity, but the sum total could explain this deficit. The coefficients of variation of the ion balance ratio, calculated for each sample from the analytical errors, were between 17.5 and 18.1%. If one considers simultaneously the anion deficit and the analytic errors, the good ion balances would be included between -25.8 and + 10.4 with regard to the perfect ion balance. For the present samples, one fog event had a
5-8 2-6 5-8 2-6 5-8 2-6 5-8 2-6 5 8 5-8 2-6 5-8 2-6 5-8 5-8 2-6 5-8 2-6 5-8 2-6 5-8 5-8 2-6 5-8 2-6 5-8 2-6 5-8 2-6
5-8 2-6 5-8 2-6
n.d. n.d. 0.04 0.02 n.d. n.d. 0.03 0.01 n.d. 0.017 n.d. 0.05 0.02 0.002 0.002 n.d. 0.02 0.006 0.04 0.02 0.003 0.03 0.02 0.003 n.d. 0.004 n.d. 0.004 n.d.
0.05 0.02 n.d. n.d.
5.50 5.50 9.40 9.40 3.30 3.30 6.15 6.15 2.30 10.15 10.15 11 11 9.25 10.30 10.30 14 14 14 14 7 13.30 13.30 8.30 8.30 15 15 11 11
4 4 3 3
3795 (3676) 2270 (2137) 1400 (1367) 1895 n.d. 1680(1669) 1460(1436) 860(831) 3480(3458) 935(865) 1360 n.d. 1560(1521) 3150(3121) 1340 n.d. 1310(1256) 4035(4021) 980(854) 1750(1686) 3460(3079) 7450(7088) 21620 n.d. 2665n.d. 10770(10660) 1980(1941) 4275(4233) 1810(1763) 1485(1434) 4410 (n.d.). 2050(1940) 4400(4266) 1780(1699) 5445 n.d. 2150(2068) 4990(4946)
3780 1570 1250 2340 620 1240 110 5020 640 1140 400 1550 1780 1110 3460 590 980 2290 2640 17270 800 4220 830 1650 580 660 1750 1060 2060 1245 3500 980 3410
150 60 105 610 10 n.d. 75 320 180 15 220 0 10 20 40 130 60 250 60 340 75 50 480 110 570 60 490 190 n.d.
130 70 100 1130 470 30 290 670 15 30 120 160 350 10 910 25 40 70 55 230 90 300 125 710 65 100 325 80 140 110 790 70 n.d.
350 40 35 100 550 40 190 230 10 n.d. 70 170 70 10 120 10 10 30 30 50 0 90 n.d. 60 60 n.d. 290 50 140 0 100 40 n.d.
70 10 140 290 90 200 240 180 580 n.d. 320 240 n.d. 450 120 1040 530 3150 2990 n.d. 0 910 320 350 390 420 0 910 1105 670 n.d. 680 365
980 1100 280 n.d. 2725(2721) 1302(1293) 120(109) 559(551) 178(153) n.d.n.d, 987(977) 2455(2444) n.d.n.d, 414(394) 2996(2990) 209(163) 108(85) 3648(3509) 7086(6954) n.dn.d, 2013(n.d.) 5619 n.d. 555 (541) 614(599) 1227(1210) 1422(1403) 2739(n.d.) 1504(1464) 2685(2690) 1318(1289) 3373 n.d. 955 (925) 200(2184)
530 (508) 658 (433) 3119 (3076) 160(136) 420(395) 1218(1170) 45(39) 62(0) 1491(1479) n.d.n.d, n.d.n.d, n.d.n.d,
990 60(58) 107(86) 1280 30(26) 58(12) 2560 50(45) 41(0) 5330 250(246) 8(0) 1130 130(117) 222(89) n.d. n.d.n.d, n.d.n.d, 1530 160(153) 230(156) 4090 435(430) 362(307) n.d. n.d.n.d, n.d.n.d, 2310 230(220) 181(77) 4160 410(407) 329(301) 1530 600(577) 222(0) 2870 170(128) 263(141) 2850 750(681) 798(73) 4670 1010(944) 1457(769) n.d. n.d.n.d, n.d.n.d, 2690 260 n.d. 321 n.d. 12640 1260(1240) 864(655) 2040 170(163) 255(181) 5520 420(412) 864(783) 2495 420(111) 189 (99) 1670 90 (81) 214(117) 4910 440 (n.d.) 469(n.d) 3410 250(230) 337(128) 4310 670(646) 576(322) 3210 380(366) 305(151) 5430 570 n.d. 535 n.d. 3720 200(185) 305(151) 5450 430(422) 337(233)
3140 1835 630 n.d. 4.1 n.d. 0.2 0.1 0.2 n.d. 0.2 2.3 n.d. 0.6 1.8 0.2 1.5 0~5 0.5 n.d. 0.1 1.8 0.7 0.5 0.3 0.1 0.8 n.d. n.d. 0.8 n.d. 2.7 n.d.
n.d. 3.6 0.2 n.d. 3 n.d. 3 13.1 3 n.d. 11.3 11.9 n.d. 0.6 3 0.6 0.6 6.0 5.8 n.d. 31.6 304.4 9.6 17.3 0.6 5.4 22.1 n.d. n.d. 1.8 n.d. 3 n.d.
n.d. 3 0.6 n.d.
4.8 n.d. 0.7 1.9 0.2 n.d. 0.6 1.2 n.d. 0.2 1.2 0.2 0.2 1.2 1.2 n.d. 1.5 1.2 0.2 0.2 0.2 0.4 1.2 n.d. n.d. 0.2 n.d. 24.1 n.d.
n.d. 6 0.2 n.d.
22.9 n.d. 7 13.5 5.2 n.d. 16.7 25.2 n.d. 9.2 174.3 9.9 17.0 16.4 117.4 n.d. 47.9 52.2 18.4 43.9 3.3 44.4 96.4 n.d. n.d. 17.4 n.d 19.1 n.d
n.d. 11.7 2.2 n.d.
C1 NO3SO42Formate Acetate Butyrate Na + NH4 ÷ K-Mg 2+ Ca 2~ Mn Fe Pb Zn t~eq litre i /zeq litre ~ /zeq litre ~ #eq litre i #eq litre i/~eq litre ~/zeq litre ~#eq litre 1 #eqlitre ~ tteqlitre- ~ #eq litre 1 #mol litre ~/zmol litre ~/zmol litre I#mol litre 1
3.20 2200(1053) 2.90 1230 (887) 4.40 745 (417) 3.30 12502840 n.d. 4.35 820(715) 3.6 850(616) 3.70 630(349) 3.70 1060(849) 4.70 540(0) 3.70 500 n.d. 3.50 1660(1287) 3.50 3620(3339) 3.80 10950 n.d. 4.85 1020(494) 4.50 1200(1060) 5.80 360(0) 5.80 1495(875) 4.35 2160(0) 3.20 3640(142) n.d. 13540n.d. 3.25 2400 n.d. 2.80 6940(5905) 3.50 1540(1166) 3.00 3650(3240) 4.10 740(284) 3.90 855(364) 3.30 2330 (n.d.). 3.90 1080 (15) n.d. 2850(1557) 3.90 860 (76) 3.40 2700 n.d. 3.90 1695(899) 3.30 2290(1863)
Diameter LWC Sampling pH /zm gm 3 time h
LWC: Liquid Water Content for fog droplets ranging between 2 and 6/zm and 5 and 8 ~tm. n.d.: not determined.
29/30 .11.91 30.11/ 01.12.91
08.10.91 08/09 .10.91 09/10 .10.91 28.10.91 25/26 .11.91 26/27 .11.91 27/28 .11.91 28.11.91 28/ 29.11.91 29.11.91
12.03.91
27/28 .02.91 04.03.91
27.02.91
26.02.91
20.02.91
Date
Table 1. Concentrations of anions and cations and pH values during fog events in Strasbourg in 1991 for the (2-6 pm) and (5-8 ttm) fractions. Values in parentheses correspond to non-seasalt components
4~
348
M . M i l l e t et al.
higher value (-30.8 for the sample 9/10.10.91 5-8/zm), so, only this sample was discarded and not considered in the further data analyses.
RESULTS
Gas concentrations and meteorological conditions The concentrations of major air pollutants (SO2, NO, NO2 and O3), temperature and wind velocity were measured by the Association pour la Surveillance de la Pollution Atmosph6rique (ASPA) at a location situated 400 m from the fogwater sampling site. The most important compounds were N O and NO2 (concentrations of 38-286 # g m -3 and 49-145 ttg m -3, respectively), while SO2 had a concentration o f 11-129 /zg m -3. The low concentrations of 03 (1-3.8/zg m -3) indicate a very small photochemical activity. During fog events there was a temperature ranging between - 1 and 13°C and minimal air movements, with a wind speed of 0.1-1.2 m s -1. It is noticeable that these wind velocities are insufficient to determine a wind direction, which can probably change several times during the same fog event. Fogwater composition Table 1 gives, for each event, the LWC, the sampling time, the pH values and the ionic concentrations. The present data can be compared with those recorded in previous studies (Table 2). Firstly, the present concentrations are generally higher but it is useful to recall that, unlike the others, only a very limited fraction of the fog droplet spectrum was collected. Consequently, it
is better to compare the relative concentrations rather than the absolute values. As in previous data from the US and Italy (Jacob et al., 1986; Fuzzi, 1988; Munger et al., 1983, 1990), Strasbourg fogwater was dominated by NH4 +, CI-, NO3- and SO42- (Table 2). On the other hand, in the present study, C1- and NO3- were still in the same order of magnitude, whereas C1- was generally at a much lower concentration than NO3- (Table 2). The major cation present in fogwater was NH4 ÷ and this represented 41 and 52% of the total cation loading for the 2 ~ / z m and 5-8 ttm fractions, respectively. A similar result has been observed by Munger et al. (1990) in Pasadena aerosols. In order to dispose of pollution tracer as proposed by Talbot et al. (1988), three weak organic acids (formate, acetate and butyrate) were analysed. The total concentration of (dissociated and undissociated) organic acids is reported in Table 1. As found by Winiwarter et al. (1988), in the present fog samples the acetate concentration was often similar or higher than that of formate, except for a few cases.
DISCUSSION
Droplet-size dependence of fogwater composition For a given LWC, the sample concentrations are influenced by the variability in the concentrations of dissolved pollutant per unit volume of air (obtained by multiplying the total concentration by the sample volume and dividing by the volume of air pumped). Moreover, because of the partial LWC due to the fixed droplet size range and on the assumption that the
Table 2. Averageconcentrationsfor the fogwatersamplescollectedin Strasbourg in 1991 (pet] litre-t) and comparisonwithother studies Compounds
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(A)
(B)
CINO3SO42H÷ Na ÷ NH4 ÷ K+ Mg 2+ Ca 2+ Fe Mn Pb Zn Forma~ Acetate Butyrate pH
87 6 19
200 555 910 320 40 1580 50 20 60 8 2
240 11820 2740
130 1780 780 1180 80 1120 17 40 280 60 3 8
120 820 2070 2 42 3270
430 1020 890 140 60 2110 40 30 280 21 1 1 20
2590 440 610
4.80
4.02
1880 1240 2150 198 840 2390 310 350 1700 6 1 2 25 84 135 90 4.02
3110 3410 5020 570 510 4820 440 420 2120 47 2 2 50 360 350 120 3.50
60 19 6 14 14
2 8 5.60
10
70 14540 40 170
320 130 4.30
(1) Post et al. (1991): Barrington, Australia. (2) Fuzzi (1988): Po Valley, Italy. (3) Munger et al. (1990): Riverside, California, US. (4) Jacob et al. (1985): Lennox, US. (5) Jacob et al. (1986): Bakersfield, California, US. (6) Joos & Baltensperger (1991): Switzerland. (7) Johnson et al. (1987): Diibendorf, Switzerland. (A) 5-8/zm fractions: Strasbourg, France. (B) 2-6/zm fractions: Strasbourg, France.
33 170 20 1 1
30 1160 15 22 220 4 10
40 150 3.08
Chemical composition of fogwater volume mean diameter within that size range remains constant, it can be concluded that, for a given concentration of pollutant per unit volume of air, if the LWC decreases (which means a decrease of the number of droplets per unit volume of air) the concentrations in the fogwater samples increase. These two observations indicate that, for a given droplet size, the concentrations of the liquid phase are governed by the ratio between the concentrations of dissolved pollutant per unit volume of air and the number of droplets per unit volume of air. Furthermore, we observe, in general, higher concentrations in the 2-6/~m fraction than in the 5-8/zm fraction (Table 1). The increased concentration in the small droplets, previously observed in cloudwater and fogwater (Ogren et al., 1992; Collett et al., 1993), results from the size-dependent chemical composition of the aerosol particles that act as condensation nuclei (Ogren & Charlson, 1992), from the dissolution of the aerosol particles in smaller amounts of water (Johnson et al., 1987) and from the higher surface/volume ratio of the small droplets, which promote greater gas/liquid exchange and, consequently, chemical reactions (Schwartz, 1988). Nevertheless, even if the absolute concentrations varied according to the size of the droplets, it is found that no systematic droplet size dependence of fogwater composition was observed. Thus, although the droplet sizes are different, the chemistry of the droplets are equivalent.
Fog droplet-size distribution Assuming that the droplets are spherical, the density of fogwater droplets (D) can be calculated for each LWC and for each droplet-size category as follows: D = LWC f~2 n(r)dr 4 yr f~2 n(r)r 3dr
(1)
349
noticed that the calculated LWC is near the lower limit of the measured LWC interval, probably because only the thick fog events have been collected. Using the droplet size distribution presented by Bott (1991), the density of droplets for the two size categories studied can be evaluated. Thus, for an equivalent LWC, the density of droplets is about 400 times higher for the 2-6 #m fraction than for the 5-8/zm fraction. This conclusion is in agreement with experimental results observed previously by Fuzzi et al. (1992) in the Po Valley: the density decreases when the droplet size increases, even though the volume of fogwater increases with the droplet size.
Automobile exhaust Like Winiwarter et al. (1988) in the Po Valley, our acetate concentration is often similar to, or higher than, that of formate, and this phenomenon is attributed to automobile traffic. Indeed, there are four possible sources of organic acids: terrestrial vegetation emissions (Keene & Galloway, 1986), liquid phase reaction in the atmosphere between formaldehyde and hydroxyl radical (Jacob, 1986), emission from motor vehicle exhaust (Kawamura et al., 1985; Talbot et al., 1988) and biomass burning (Talbot et al., 1988). But only in the two latter cases, is acetate concentration typically higher than that of formate (Talbot et al., 1988). If one considers that biomass burning is negligible in urban areas, organic acids would come essentially from the automobile exhaust. This conclusion is corroborated by the high levels of gaseous NOx and by the significant correlation between NO3- and formic and acetic acids (Table 3), but it seems to be in contradiction with Pb concentrations, which are lower than those observed by others (Jacob et al., 1985; Johnson et al., 1987). A possible explanation is the general use of unleaded gasoline in France since 1989 (Nriagu, 1990). Consequently, Pb 10 s
where rl and r2 are the lower and upper limits of the droplet size categories and where r and n are, respectively, the radius and the number density of the droplet (which is a function of r). To determine the droplet-size distribution, the results of a simulation model of a typical droplet spectrum of fog formed over an urban area (Bott, 1991) can be used. With this model, in which the size of the droplets varied between 0.5 and 30 tzm radius (Fig. 1), the theoretical LWC and number of droplets obtained for the two droplet fractions studied in the present work can be computed. By these calculations, the following was obtained for the 2-6/zm and 5-8/zm (or 1-3 #m and 2 . 5 4 / z m in radius) fractions, respectively: 2.3x10 -3 and 3.5× 10-3 g of water per cubic metre of air for the LWC and 9× 107 and 3.5× 105 droplets per cubic metre of air. These results are compatible with the present experimental data, in which the LWC ranged between 6x 10-3 and 2x10 -3 and between 2x10 -3 and 5×10 -2, respectively, for the 2-6 and 5-8 ttm fractions (Table 1). It is
10 4
10 3
lo2 ~
10 I
'~
100
10-~
10-2
10-3 1
10 Radius r Oam)
Fig. 1. Fog droplet-size distribution in the range 0.5-30 ~m (Bott, 1991).
CI-
NO3-
SO4 2-
H+
Na +
NH4 +
1.000 0.826 0.839 0.142 0.076 -0.140 0.669 -0.020 0.433 -0.119 0.339 0.370 -0.201 0.356
K+
Ca 2÷
Form.
Acet.
Buty.
Fe
Mn
Zn
Pb
1.000 0.774 1.000 -0.022 0.050 1.000 0.001 0.062 0.672 1.000 -0.246 0.125 0.502 0.479 1.000 0.338 0.482 0.192 0.099 0.002 1.000 -0.004 0.158 0.066 0.127 0.415 0.107 1.000 0.477 0.595 0.238 0.522 0.081 0.115 0.054 1.000 -0.044 -0.054 0.087 -0.084 0.038 -0.059 0.515 -0.079 1.000 0.303 0.375 0.032 -0.032 -0.017 0.511 0.134 0.108 -0.107 0.378 0.372 -0.054 -0.189 -0.175 0.426 0.102 0.092 -0.132 -0.195 -0.068 0.113 0.028 0.280 0.251 0.364 -0.254 -0.049 0.515 0.389 -0.370 -0.308 -0.381 -0.021 -0.094 0.256 -0.346
Mg 2+
LWC: Liquid Water Content for droplet ranging between 2 and 6/zm and 5 and 8/zm.
1.000 -0.413 1.000 -0.412 0.466 1.000 SO4 2- -0.496 0.894 0.705 1.000 H+ -0.174 0.769 0.449 0.705 1.000 Na + -0.352 0.275 0.228 0.384 0.104 1.000 NH4 + -0.433 0.859 0.722 0.899 0.696 0.123 1.000 K+ -0.469 0.827 0.594 0.917 0.538 0.601 0.797 Mg 2+ -0.475 0.714 0.337 0.762 0.515 0.737 0.521 0.533 Ca 2÷ -0.592 0.661 0.469 0.815 0.391 0.601 Form. -0.164 0.281 0.615 0.348 0.360 -0.334 0.502 Acet. -0.114 0.121 0.567 0.274 0.163 -0.316 0.360 Buty. -0.030 0.066 0.133 0.016 -0.062 -0.316 0.009 Fe -0.202 0.796 0.484 0.779 0.665 0.045 0.850 Mn -0.368 0.112 0.007 0.087 0.233 -0.045 0.067 Zn -0.337 0.305 0.426 0.528 0.139 0.116 0.351 Pb -0.269 -0.031 -0.073 -0.057 -0.015 0.032 0.012 SO2 -0.141 0.510 0.069 0.467 0.705 -0.056 0.358 NO -0.152 0.542 0.504 0.431 0.682 0.012 0.299 NO2 0.293 0.102 0.510 0.037 0.544 -0.307 0.013 03 -0.129 0.262 -0.073 0.240 0.164 0.412 -0.040
LWC C1NO3-
LWC
1.000 0.923 0.651 0.179
SO2
NO2
1.000 0.530 1.000 0.437 -0.064
NO
Table 3. Correlations between compounds analysed during fog events in Strnsbourg in 1991 (n = 22). Correlations in bold type are significant (p < 0.05)
1.000
03
Chemical composition o f f ogwater is no longer a reliable tracer of automobile pollution. This result is corroborated by the absence of significative correlation between organic acids and Pb (Table 3).
NO3-, 5042- and
NH4 + origins The presence of nitrate and ammonium ions can be attributed to a direct input of gaseous nitric acid and ammonia, as well as to an input due to the collection of nitrate and ammonia contained in aerosols. However, the problem is more complicated for sulphate, since it also has a marine source. For seawater, the ratio of SO42-/Na ÷ is 0.12, with 5042- and Na + expressed in /zeq litre -1 (Riley & Chester, 1971; Jacob et al., 1985). Whatever the fogwater sample, the SO42-/Na + ratio is always larger than 0.12 (Fig. 2), so the marine contribution is either weak or negligible. This result is corroborated by the absence of a significant correlation between SO42- and Na + (Table 3). Then, the presence of the most of the sulphate can be attributed to an input of SO2 followed by its oxidation to 5042-, as well as to an input due to the collection of sulphate contained in aerosols. In order to determine whether the gas to particle conversion occurred before or after the formation of the fog, a second statistical analysis has been made with the loading of ions per unit volume of air. These loads were calculated by multiplying the aqueous concentration by the sample volume and dividing by the volume of air pumped. If the first statistical analysis gives the greater correlations, the SO42- and NO3- concentrations in liquid phase are controlled by gaseous SO2 and NO× concentrations which means that scavenging is controlled by phase equilibrium. On the other hand, if the second statistical analysis gives the greater correlation, liquid concentrations are water-volume-dependent and the scavenging is controlled by mass transfer. In the present study, both results are almost equivalent (0.467 against
351
0.415 for SO2-SO42-; 0.504 and 0.510 against 0.476 and 0.5 for NO-NO3- and NO2-NO3-, respectively), which may indicate a combined action of the two phenomena. The same results are obtained with the study of the correlations with LWC since negative correlations were obtained between LWC and compounds such as Cl-, NO 3-, SO42-, NH4 +, K +, Mg 2+ and Ca 2+ (Table 3 and Fig. 3), which indicates that an important part of the dissolved material enters solution from the aerosol particles. Nevertheless, all these negative correlations are low (in absolute values), so the direct input from the gas phase after the fog has formed is probably not negligible. Indeed, with the present method of collection, a variation of the LWC induces a variation of the number of droplets and, consequently, a variation of air-liquid interface, which promotes the mass transfer phenomenon. Simultaneously, the high fogwater concentrations probably induce a control of the scavenging by the phenomenon of phase equilibrium. CI- origin
For fog of marine origin, the CI-/Na ÷ ratio should reflect the composition of seawater, i.e. Cl-/Na÷ ~ 1.17 (C1- and Na + expressed in/~eq litre -1) (Riley & Chester, 1971; Jacob et al., 1985). As can be seen in Fig. 4, the CI-/Na + ratio often exceeds 1.17, but in four cases the ratio is smaller than this value. The ratio smaller than 1.17 can partially be explained by the presence of Na of terrigenic origin. However, using the rough estimate for Na + of crustal origin found in the previous sections, one finds that the C1/Na ratios indeed increase but remain below 1.17. Another possibility would mean the replacement of C1- by NO3- or SO42- (Robbins et al., 1959; Eriksson, 1960; Clegg & Brimblecombe, 1985, 1986; Keene et al., 1990) but we have no way to compute its importance. The ratios larger than 1.17 mean that C1- has not only a marine origin, but also originates from human activities. It may result from the automobile exhaust, since gasoline contains lead bromo chloride as an additive
34
m
30 20
I
/
1.0-
I 5-8
2-6~./
0.8 0.6
16
0.4-
_~
!
0.2-
0.0
8
-0.4 -0.6 0.12
i LWC LWC
D~es of ~ g ~ r
J
CI" ~
Cl" ~
i
NO3" NO~" !
i
i
SO42" SO~2" ~
i
NH4* ~4"
i
H* ~
Ca 2+
H÷ ~
Ca ~
~pl~
Fig. 3. Correlations between some compounds analysed
Fig. 2. SO42-/Na + ratio in fogwater samples.
during fog events in Strasbourg in 1991 (n = 22).
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M . Millet et al.
of HC1 per cubic meter. The total liquid content is typically 0.06 ml m -3 (Johnson et al., 1987); then the C1- concentration in solution is about 2300/zeq litre -], which is comparable to the non-seasalt chloride presently observed (Table 1). Consequently, the non-seasalt chloride seems to come mainly from the refuse incinerator.
16
5-Spin 2-6pro
14 12 10 +al 8
Ca 2+ origin
rO 6 4
2 0
,01J , N0
~1.17
o Datesof fogwatcrsampling
Fig. 4. C1-/Na + ratio in fogwater samples. (Friedlander, 1973; Fuzzi et al., 1984). Nevertheless, since 1989 the use of unleaded gasoline seems to be general in France and, as in the case of Pb, chloride produced in this manner becomes less and less important. It may also be produced, in the form of HC1 in the gas phase, by the paper industry, which uses CI- for whitening paper, but this type of industry does not exist in the Strasbourg region. Lastly, CI- may result from the stack gases of refuse incinerators. Indeed, the combustion of organochlorine compounds, such as polyvinylchloride, produces HCI in the gas phase (Johnson et al., 1987; Sigg et al., 1987). A refuse incinerator exists in Strasbourg, situated about 4 km away to the south of the sampling site. CI- could come from this plant. This hypothesis is supported by the results obtained by DRIRE-Alsace (1992), which estimated that this incinerator emitted 1980 tonnes of HC1 per year. On the assumption that all the HC1 emitted is dissolved in fogwater, the observed chloride loading and the known HC1 emission (1980 tonnes per year or 63 gs -]) from the incinerator can be compared as follows: On account of the variation of the wind direction during individual fog events, the emissions are spread in all directions around the incinerator. Since in fog conditions the height of the mixing layer is about 500 m and the plume moves at a maximum wind speed of 1 m s -], the HC1 concentration at 4000 m away from the incinerator is: [63/(2 x zr× 4000 x 500)] x 1 = 5 x 10 -6 g
Ca may have a marine origin, but, in the Strasbourg region, this contribution is low (Table 1). A second possible origin for Ca 2+ is the long-range transport of Saharan dust. However, Colin et al. (1989) have shown that, at a site situated at about 50 km to the west of Strasbourg, this input of Ca 2 + is very weak, even for air masses that originate in North Africa. A third explanation is that Ca 2 + could be emitted by human activities, such as traffic, cement factory, etc. A fourth possibility is revealed by the study of the soil composition in the Strasbourg region. This study indicates that the soil is composed of 'loess', which is very rich in Ca 2 +. Because of its formation by the freezing-defreezing process, the 'loess' consists of many very fine dust particles easily spread by the wind. Fogwater acidity
Several compounds, such as H2SO4, HNO3, HC1, NH3 and CaCO3, contribute to the acid-base balance of fogwater. In order to compare the acidifying contributions, a series of linear correlations has been calculated between the major cations H +, NH4 + plus non-seasalt Ca 2+ (nss-Ca2+), and the anions non-seasalt C1- (nssC1-), non-seasalt SO42- (nss-SO42-) plus NO3(Table 4). For the correlation of H + + NH4 + versus nss-SO42-+ NO3-, a correlation coefficient of 0.892 was obtained. Then, free acidity in fogwater samples was controlled essentially by nss-sulphate and nitrate acidity, and neutralised by ammonium. Yet, the addition of nss-Ca 2+ and nss-C1- increases the correlation coefficients (0.941), and these two compounds contribute to a smaller extent to the acid-base balance. According to the literature, HNO3 and H2SO4 are strongly involved in the acidity of urban fogwater (Johnson et al., 1987; Sigg et al., 1987; Joos & Baltensperger, 1991; Post et al., 1991). In contrast, HC1 is generally little involved in the acidity of precipitation of fog (Colin et al., 1989), even if it can be an important acidifying agent in certain exceptional situations (Martin & Barber, 1978; Jaffrezo & Colin, 1987; Sigg et al., 1987). Thus, in Switzerland, Johnson et al. (1987)
Table 4. Acid-base balances and correlation coefficients for the fogwater samples collected in Strasbourg in 1991
Ions H + + NH4 + H + + NH4 + H + + NH4 + H + + NH4 + H + + NH4 + H + + NH4 +
vs nss-SO42-+ NO3+ nss-Ca2+ vs nss-SO42- + NO3vs nss-SO42-+ nss-C1- + NO3+ nss-Ca 2+ vs nss-SO42-+ nss-C1- + N O 3 vs nss-SO42-+ nss-C1+ nss-Ca2+ vs nss-SO42-+ nss-C1-
Correlation coef.
Significance
0.892 0.909 0.891 0.941 0.939 0.885
0.05 0.05 0.05 0,05 0.05 0.05
Chemical composition o f fogwater
and Sigg et al. (1987) attribute the high acidity to an input of gaseous HC1 emitted by a refuse incinerator. In Strasbourg fogwater samples, whatever the samples, the NO3- and nss-SO42- are always a very major component (Table 1) and the nss-Cl- is also, except in a few cases, a relatively important component compared to the two previous species (NO3- and nss-SO42-) (Table 1). Moreover, as presented previously, the higher coefficient of correlation is obtained when nss-C1- is included in the acid-base balance. Consequently, it seems that the Strasbourg fogwater acidity is increased by HC1, which could be emitted by a refuse incinerator, as in Switzerland.
CONCLUSION Results presented concern the chemical analysis of fog events collected during a 1-year sampling campaign in Strasbourg, a town in the east of France. A sampling procedure was adopted that made it possible to collect two size fractions (2-6 # m and 5-8/zm). A total of 18 fog events were collected during 1991 in Strasbourg; this represents 33 samples, because sometimes the 2 - 6 / z m fractions were not available. It was observed that the smaller fraction is the most concentrated and has the most acidic pH. This work has allowed the identification of two probable important sources of acidity in Strasbourg. The first is automobile traffic, characterised by the ratio of formate and acetate, which in this case indicates that the second compound is typically higher than or similar to the formate concentrations. It was observed also that the general use of unleaded gasoline reduces the atmospheric Pb concentration, because, in spite of heavy traffic and a study of small droplet size, Pb concentrations are moderate. The second source of acidity is probably a refuse incinerator near the town, which produces HCI in the gas phase. In association with H2SO4 and HNO3, HC1 is an important factor in the acidity of the fogwater. So, fitting a filter in the chimney stack of this refuse incinerator could improve the air quality in Strasbourg. These acids are partially neutralised by some compounds, such as NH4 and Ca, which may originate from the rural area near the town and human activities, such as cement production. Since November 1995, a washing system of the refuse incinerator exhaust has been installed. A new study of the fogwater composition would permit an evaluation of the influence of this installation on the air quality of Strasbourg.
ACKNOWLEDGEMENTS This work has been supported by I F A R E (Institut Franco-Allemand pour la Recherche en Environement). The gas concentrations were measured and supplied by ASPA (Association pour la Surveillance de la Pollution
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Atmosph6rique). The fogwater collectors were supplied by Dr G. Metzig from the Kernforschungzentrum (Karlsruhe, Germany). All are gratefully acknowledged.
REFERENCES Berner, A. (1988). The collection of fog droplets by a jet impaction stage. Sci. Total Environ., 73, 217-228. Bott, A. (1991). On the influence of the physico-chemical properties of aerosols on the life cycle of radiation fogs. Boundary-Layer Met., 56, 1-31. Brook, J. R., Wiebe, A. H., Summers, P. W. & Li, S. M. (1992). Aerosol acidity in Canada and the link with respiratory health. J. Aerosol Sci., 23(1), $977-$981. Capel, P. D., Leuenberger, C. & Giger, W. (1991). Hydrophobic organic chemicals in urban fog. Atmos. Environ., 25A, 1335-1346. Clegg, S. L. & Brimblecombe, P. (1985). Potential degassing of hydrogen chloride from acidified sodium chloride droplets. Atmos. Environ., 19, 465-470. Clegg, S. L. & Brimblecombe, P. (1986). The dissociation constant and Henry's law constant of HC1 in aqueous solution. Atmos. Environ., 20, 2483-2485. Colin, J. L., Renard, D., Lescoat, V., Jaffrezo, J. L., Gros, J. M. & Srauss, B. (1989). Relationship between rain and snow acidity and air mass trajectory in eastern France. Atmos. Environ., 23, 1487-1498. Colin, J. L., Jaffrezo, J. L. & Gros, J. M. (1990). Solubility of major species in precipitation: factors of variation. Atmos. Environ., 24A, 1939-1944. Collett, J., Jr, Oberholzer, B. & Staehelin, J. (1993). Cloud chemistry at Mt Rigi, Switzerland: dependence on drop size and relationship to precipitation chemistry. Atmos. Environ., 27A, 33-42. Czuczwa, J., Leuenberger, Ch. & Giger, W. (1988). Seasonal and temporal changes of organic compounds in rain and snow. Atmos. Environ., 22, 907-916. DRIRE-Alsace (1992). Les Rejets Industriels en Alsace. Eriksson, E. (1960). The yearly circulation of chloride and sulfur in nature, meteorological, geochemical and pedological implications. Tellus, 12, 63-109. Friedlander, S. K. (1973). Chemical element balances and identification of air pollution sources. Environ. Sci. Technol., 7, 235-240. Fuzzi, S. (1988). Fog chemistry and deposition in the Po valley. In Acid Deposition at High Elevation Sites, ed. M. H. Unsworth & D. Fowler, Kluwer, Dordrecht, The Netherlands, pp. 443-452. Fuzzi, S., Castillo, R. A., Jiusto, J. E. & Lala, G. C. (1984). Chemical composition of radiation fog at Albany, New York and its relationship to fog microphysics. J. Geophys. Res., 85, 7159-7164. Fuzzi, S., Facchini, M. C., Orsi, G., Lind, J. A., Wobrock, W., Kessel, M., Maser, R., Jeaschke, W., Enderle, K. H., Arends, B. G., Berner, A., Solly, I., Kruisz, C., Reischl, G., Pahl, S., Kaminski, U., Winckler, P., Ogren, J. A., Noone, K. J., Hallberg, A., Fierlinger-Oberlinninger, H., Puxbaum, H., Marzorati, A., Hansson, H.-C., Wiednsohler, A., Svenningsson, I. B., Martisson, B. G., Schell, D. & Georgii, H. W. (1992). The Po valley fog experiment 1989, An overview. Tellus, 44B, 448-469. Gieray, R., Lammel, G., Metzig, G. & Wieser, P. (1993). Size dependent single particle and chemical bulk analysis of droplets and interstitial particles in an orographic cloud. Atmos. Res., 30, 263-293. Glotfelty, D. E., Seiber, J. N. & Liljedahl, L. A. (1987). Pesticides in fog. Nature, 325, 602-605. Hileman, B. (1983). Acid fog. Environ. Sci. Technol., 17, 117A-120A.
354
M. Millet et al.
Hoffmann, J. & Metzig, G. (1991). Calibration of fog water " collectors, or. Aerosol Sci., 22(Suppl. 1), 5291-5294. Igawa, M., Munger, J. W. & Hoffmann, M. R. (1989). Analysis of aldehydes in cloud and fogwater samples by HPLC with a poscolumn reaction detector. Environ. Sci. Technol., 23, 556--561. Jacob, D. J. (1986). Chemistry of OH remote clouds and its role in the production of formic acid and peroxy monosulfate. J. Geophys. Res., 91, 9807-9866. Jacob, D. J., Waldman, J. M., Munger, J. W. & Hoffmann, M. R. (1985). Chemical composition of fogwater collected along the California coast. Environ. Sci. Technol., 19, 730736. Jacob, D. J., Munger, J. W., Waldman, J.-D. & Hoffmann, M. (1986). The H2SO4-HNO3-NH3 system at high humidities and in fogs: 1. Spatial and temporal patterns in the San Joaquin Valley of California. J. Geophys. Res., 91, 10731088. Jacob, D. J., Shair, F. H., Waldman, J. M., Munger, J. M. & Hoffman, M. R. (1987). Transport and oxidation of SO2 in a stagnant foggy valley. Atmos. Environ., 21, 1305-1314. Jaffrezo, J. L. & Colin, J. L. (1987). Construction and exploitation of an automatic sequential wet-only collector. Environ. Technol. Lett., 8, 467--474. Johnson, C. A., Sigg, L. & Zobrist, J. (1987). Case studies on the chemical composition of fogwater: the influence of local gaseous emissions. Atmos. Environ., 21, 2365-2374. Joos, F. & Baltensperger, U. (1991). A field study on chemistry S(IV) oxidation rates and vertical transport during fog conditions. Atmos. Environ., 25A, 217-230. Kawamura, K., Ng, L.-L. & Kaplan, I. R. (1985). Determination of organic acids (C1-C10) in the atmosphere, motor exhausts, and engine oils. Environ. Sci. Technol., 19, 10821086. Keene, W. C. & Galloway, J. N. (1986). Considerations regarding sources for formic and acetic acids in the troposphere, at. Geophys. Res., 91, 14466-14474. Keene, W. C., Galloway, J. N. & Holden, J. D. (1983). Measurements of weak organic acidity in precipitation from remote areas of the world. J. Geophys. Res., 88, 5122-5130. Keene, W. C., Pszenny, A. A. P., Jacob, D. J., Duce, R. A., Galloway, J. N., Schultz-Tokos, J. J., Sievering, H. & Boatman, J. F. (1990). The geochemical cycling of reactive chlorine through the marine troposphere. Global Biogeochem. Cycles, 4, 407-430. Lammel, G. & Metzig, G. (1991). Multiphase chemistry of orographic clouds: observations at subalpine mountain sites. Fresenius J. Anal. Chem., 3411, 564-574. Lim, B., Jickells, T. D. & Davis, T. D. (1991). Sequential sampling of particles, major ions and total trace metal in wet deposition. Atmos. Environ., 25A, 745-762.
Martin, A. & Barber, R. (1978). Some observations of acidity and sulfur in rainwater from rural sites in central England and Wales. Atmos. Environ., 12, 1481-1487. Millet, M. (1994). PhD Thesis, Universit6 Louis Pasteur, Strasbourg, France. Millet, M., Wortham, H. & Mirabel, Ph. (1995). Solubility of polyvalent cations in fogwater at an urban site in Strasbourg (France). Atmos. Environ., 29, 2625-2633. Munger, J. W., Jacob, D. J., Waldman, J. M. & Hoffmann, M. R. (1983). Fogwater chemistry in an urban atmosphere. J. Geophys. Res., 88, 5109-5121. Munger, J. W., Collett, J., Daube, B. & Hoffmann, M. R. (1990). Fogwater chemistry at Riverside, California. Atmos. Environ., 34B, 185-205. Nriagu, G. O. (1990). The rise and fall of leaded gasoline. Sci. Total Environ., 92, 13-28. Ogren, J. A. & Charlson, R. J. (1992). Implications for models and measurements of chemical in homogeneities among cloud droplets. Tellus, 44B, 208-226. Ogren, J. A., Noone, K. J., Hallberg, A., Schell, D., Berner, A., Solly, I., Kruisz, C., Arends, B. G. & Wobrock, W. (1992). Measurements of the size dependence of the concentration of non-volatile in fog droplets. Tellus, 44B, 570-580. Post, D., Bridgman, H. A. & Ayers, G. P. (1991). Fog and rainwater composition in rural SE Australia. J. Atmos. Chem., 13, 83-95. Richartz, H., Reischl, A., Trautner, F. & Hutzinger, O. (1990). Nitrated phenols in fog. Atmos. Environ., 24A, 3067-3071. Riley, J. P. & Chester, R. (1971). Introduction to Marine Chemistry, Academic Press, London, UK, 81 pp. Robbins, R. C., Cadle, R. D. & Eckhardt, D. L. (1959). The conversion of sodium chloride to hydrogen chloride in the atmosphere. J. Met., 16, 53-56. Sanusi, A., Wortham, H., Millet, M. & Mirabel, Ph. (1996). Chemical composition of rainwater in eastern France. Atmos. Environ., 30, 59-71. Schwartz, S. E. (1988). Mass-transport limitation to the rate of in-cloud oxidation of SO2: re-examination in the light of new data. Atmos. Environ., 22, 2491-2499. Sigg, L., Stumm, W., Zobrist, J. & Ziircher, F. (1987). The chemistry of fog: factors regulating its composition. Chimia, 41, 159-165. Talbot, R. W., Beecher, K. M., Charris, R. & Cofer, W. R. (1988). Atmospheric geochemistry of formic and acetic acids at a mid-latitude temperature site. J. Geophys. Res., 93, 1638-1652. Winiwarter, H., Puxbaum, H., Facchini, M. C., Orsi, G., Beltz, N., Enderle, K. & Jaeschke, W. (1988). Organic acids gas and liquid phase measurements in the Po Valley fallwinter conditions in the presence of fog. Tellus, 40B, 348357.