Chemistry of carbonyl compounds in Po Valley fog water

The Science of the Total Environment, 91 (1990) 79-86 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

79

CHEMISTRY OF CARBONYL COMPOUNDS IN PO VALLEY FOG WATER

MARIA CRISTINA FACCHINI, JOHN LIND, GIORDANO ORSI and SANDRO FUZZI Istituto F I S B A T - - C.N.R., Via de' Castagnoli 1, 40126 Bologna (Italy)

(Received May 17th, 1989; accepted June llth, 1989)

ABSTRACT The importance of carbonyl compounds in Po Valley fog water chemistry is discussed. High concentrations of formaldehyde (HCHO) were detected in fog water (from 16 to 567 ~M; average, 130 #M). The oxidant-limiting conditions during the fog season (fall-winter months) favour the presence of a large fraction of HCHO in the form of adducts with hydroxymethansulfonate: 85% of the total HCHO on average. Other carbonyl compounds were detected in the fog water: acetaldehyde, acrolein and acetone, but typically in much lower concentrations than formaldehyde. These other carbonyl compounds do not appear to be present in a bound form. INTRODUCTION H i g h c o n c e n t r a t i o n s of aldehydes and k e t o n e s h a v e been observed in fog (Grosjean and Wright, 1983; M u n g e r et al., 1984, 1986; S t e i n b e r g and Kaplan, 1984; F a c c h i n i et al., 1986; J a c o b et al., 1986). F o r m a l d e h y d e is usually the most a b u n d a n t , b u t o t h e r species, such as a c e t a l d e h y d e , acrolein, benzaldehyde, glyoxal and p y r u v a l d e h y d e , h a v e also been detected (Grosjean and Wright, 1983; S t e i n b e r g and Kaplan, 1984). T h e i m p o r t a n c e of aldehyde-bisulfite adducts, in p a r t i c u l a r the role of hyd r o x y m e t h a n s u l f o n a t e (HMSA) in S(IV) a q u e o u s chemistry, is well known: a d d u c t f o r m a t i o n effectively e n h a n c e s the solubility of SO2 in the liquid phase with respect to H e n r y ' s law predictions, and also c o n t r i b u t e s to the overall acidity of fog droplets ( M u n g e r et al., 1984). T h e s e a d d u c t s can t h e n serve as a r e s e r v o i r for b o t h a l d e h y d e s and S(IV) in fog droplets, since b o u n d S(IV) is impervious to H202 o x i d a t i o n (Richards et al., 1983; K o k et al., 1986). High c o n c e n t r a t i o n s of H M S A h a v e been detected in fog samples ( M u n g e r et al., 1984, 1986; J a e s c h k e et al., 1988), and a c o m p a r i s o n of the t o t a l formaldehyde, t o t a l S(IV) and H M S A c o n c e n t r a t i o n s supports the i m p o r t a n c e of the formaldehyde-bisulfite adduct. M u n g e r et al. (1986) r e a s o n e d t h a t the " i d e a l " e n v i r o n m e n t for H M S A f o r m a t i o n is c h a r a c t e r i z e d by (a) high p r e c u r s o r c o n c e n t r a t i o n s , (b) n e a r n e u t r a l pH, and (c) low o x i d a n t c o n c e n t r a t i o n s . Po V a l l e y fogs can be c o n s i d e r e d as an " i d e a l " e n v i r o n m e n t for S(IV) a d d u c t f o r m a t i o n since all t h r e e c o n d i t i o n s are satisfied: high c o n c e n t r a t i o n s of p o l l u t a n t s are p r e s e n t

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80 and, in particular, S(IV) and formaldehyde concentrations are comparable to those measured in urban areas (Fuzzi et al., 1988; Jaeschke et al., 1988); the pH range is very wide (from 2.5 up to 7); and we observe a general lack of oxidants (Fuzzi et al., 1988). In previous experiments carried out in November 1985, 1986 and 1987 (Fuzzi, 1986, 1987) we measured formaldehyde by a modification of the Nash technique (Smith and Erhardt, 1975) and formaldehyde plus other carbonyl compounds as 2,4-dinitrophenylhydrazine derivatives. From a comparison of the analyses performed by these two different techniques we obtained strong evidence that HMSA, present in the fog droplets, negatively interfered with total formaldehyde determinations in both methods, especially when HMSA concentrations were high. Therefore, the total carbonyl concentrations were underestimated in the real samples. Similar observations were recently reported by Munger et al. (1986) for the Nash derivatization procedure, and by Ang et al. (1987) for the DNPH derivatization reaction. In the present paper we describe measurements of free and bound carbonyl compounds in Po Valley fog water samples. Following a procedure suggested by Ang et al. (1987) we determined free, and then total carbonyls after cleaving the possible adducts by NaOH addition. The hydroxymethansulfonic acid concentration is inferred as the difference between the two measurements, making the assumption that hydroxyalkanesulfonic acids are by far the most abundant adducts of carbonyl compounds in the atmospheric liquid samples. EXPERIMENTAL The procedure developed by Ang et al. (1987) is based on HPLC separation of the original aqueous-phase reaction mixture containing DNPH derivatized formaldehyde. They found that HMSA does not react with the derivatizing reagent at pH < 2, so that only free HCHO is determined. The total formaldehyde concentration was then determined after raising the pH to 13 by the addition of NaOH, conditions under which the HMSA adduct is no longer stable. We improved upon this technique by carrying out the analysis in a twophase system (water/iso-octane). This procedure improves the derivatization yield, especially for the higher molecular weight carbonyls (Selim, 1977), and eliminates the large peaks due to DNPH reagent, which strongly increases the time of analysis (Facchini et al., 1986).

Fog sample collection Fog samples were collected at the field station of S. Pietro Capofiume in the eastern part of the Po Valley during the period J a n u a r y - M a r c h 1989. This period was characterized by particularly high fog occurrence: 31 fog episodes were observed and 95 fog samples collected. The fog string collector used has been described elsewhere (Fuzzi et al., 1988). The fog samples were usually analyzed within 24 h after collection.

81

Analytical procedure A V a r i a n 2010 H P L C i s o c r a t i c p u m p e q u i p p e d w i t h a V a r i a n 2550 UV-VIS d e t e c t o r , at a w a v e l e n g t h of 360 nm, was used for the a n a l y s e s . T h e s a m p l e s w e r e a u t o m a t i c a l l y i n j e c t e d by a Gilson 231-401 A u t o s a m p l i n g injector, equipped w i t h a R h e o d y n e 7010 i n j e c t i o n v a l v e (10#1 loop), a n d w i t h a r a c k for 0.8ml vials. A c o m m e r c i a l E r b a s i l C18 c o l u m n (10ttm, 250 × 4 . 6 m m i.d.) was used for all m e a s u r e m e n t s . T h e m o b i l e p h a s e was 72:28 m e t h a n o l ( H P L C g r a d e ) / w a t e r (Milli-Q, 18 M ~ ) a t a flow r a t e of 1.2 ml min-1. T w o h u n d r e d m i c r o l i t e r s of a q u e o u s acidic D N P H s o l u t i o n (5 × 10 -3 M in 2 N HC1) was added to 200 pl of sample; a 400 #l a l i q u o t of i s o - o c t a n e was t h e n added. T h e r e a c t i o n vials w e r e t h e n c a p p e d a n d s h a k e n s e v e r a l times; a f t e r 45 m i n the o r g a n i c l a y e r (in the u p p e r p a r t of t h e solution) w a s d i r e c t l y injected by the a u t o s a m p l e r . C a l i b r a t i o n s t a n d a r d s a n d b l a n k s w e r e o b t a i n e d w i t h the s a m e m i c r o s c a l e d e r i v a t i z a t i o n t e c h n i q u e , as p r e v i o u s l y d e s c r i b e d ( F a c c h i n i et al., 1986). T h e s a m e p r o c e d u r e was used for t o t a l c a r b o n y l d e t e r m i n a t i o n s , b u t a 50ttl a l i q u o t of N a O H ( 1 . 5 N ) w a s added to t h e s a m p l e s before r e a g e n t addition, i n c r e a s i n g t h e pH to 13, t h u s d e c o m p o s i n g c a r b o n y l - S ( I V ) adducts. In o r d e r to n e u t r a l i z e t h e added b a s e a n d allow the d e r i v a t i z a t i o n r e a c t i o n to occur, t h e H ÷ c o n t e n t of the D N P H s o l u t i o n was modified (5 × 10 3 M D N P H in 2 . 7 N HCI). I t s h o u l d be n o t e d t h a t by u s i n g the a u t o - i n j e c t o r system, we g r e a t l y r e d u c e d the a m o u n t of fog w a t e r n e c e s s a r y for the analysis. RESULTS AND DISCUSSION F o u r c a r b o n y l c o m p o u n d s w e r e identified as m a j o r c o m p o n e n t s of the fog samples, a l t h o u g h s e v e r a l m i n o r p e a k s w e r e o f t e n p r e s e n t in the c h r o m a t o grams. C o n c e n t r a t i o n r a n g e s and q u a r t i l e s of free a n d t o t a l f o r m a l d e h y d e , a c e t a l d e h y d e a n d acrolein, a r e r e p o r t e d in T a b l e 1: f o r m a l d e h y d e is t h e m o s t a b u n d a n t c a r b o n y l c o m p o u n d w i t h a c o n c e n t r a t i o n t y p i c a l l y 10-20 t i m e s h i g h e r t h a n a c e t a l d e h y d e a n d acrolein. T o t a l a n d free a c e t o n e c o n c e n t r a t i o n s w e r e below the d e t e c t i o n limit ( < 4 # M ) in all cases: this a n a l y t i c a l l i m i t a t i o n TABLE 1 Free (f) and total (t) carbonyl compound concentration (ttM) range and quartiles

Min. 25th percentile 50th percentile 75th percentile Max.

HCHOf

HCHOt

C H 3C H O f

C H 3C H O t

Acrolf

Acrolt

< DL" < DL 17 42 80

16 99 130 171 567

< DL < DL 6 11 15

< DL < DL 6 12 24

< DL < DL < DL < DL 4

< DL < DL < DL 3 8

"DL = detection limit: HCHO, 3ttM; CH3CHO, 5ttM; acrolein, 2ttM. Analytical error: HCHO, 1 #M; CH3CHO, 2ttM; acrolein, 1 #M.

82 TABLE 2 Free (f) a n d t o t a l (t) f o r m a l d e h y d e c o n c e n t r a t i o n ( # M ) of i n d i v i d u a l fog s a m p l e s collected d u r i n g t h e period F e b r u a r y - M a r c h 1989. B o u n d (b) f o r m a l d e h y d e is c a l c u l a t e d by difference, pH a n d bound-to-total f o r m a l d e h y d e r a t i o s are also r e p o r t e d Date

4 Feb.

5 Feb.

6 Feb.

7 Feb.

8 Feb.

19 Feb.

20 Feb.

22 Feb.

23 Feb.

Sampling interval

pH

HCHOf

HCHO t

HCHO b

HCHOb/HCHO t

2116-2138 2200-2248 2250-2338 2340-0028 0121-0209 0212-0300 2226-2238 2249-2327 2339-0027 0029-0037 0048-0111 0122-0149 0224-0255 0306~318 0343~353 1909-1957 2000-2038 1835-1923 1926-2014 2016-2104 2106-2154 2157-2245 2247-2335 2338--0026 0028~116 0118-0206 0209-0257 2006-2054 2057-2145 2147-2218 2318-0006 0025~052 0037-0216 0259-0347 0532-0620 0646-0733 0736-0824 082~0849 2053-2157 0308-0412 0437-0541 0610-0704 0732-0804 0426-0530 0532-0636

5.3 6.1 5.0 4.9 4.6 4.7 4.7 4.4 3.7 3.9 4.2 4.3 4.4 4.4 4.5 4.6 3.6 5.0 4.9 4.8 4.8 4.8 4.8 4.7 4.6 4.4 5.1 6.3 5.4 5.6 5.8 5.9 6.1 6.3 6.4 6.6 6.5 6.5 6.8 3.4 3.7 4.1 3.9 5.0 5.0

11 4 3 3 4 5 5 3 46 33 3 4 3 3 3 13 52 3 3 3 3 3 3 3 3 3 3 33 42 52 47 60 63 62 44 42 53 53 3 80 62 43 56 3 4

567 353 181 140 172 232 184 140 160 148 143 129 137 132 143 203 176 200 199 229 214 168 163 109 103 106 117 171 104 114 93 95 97 96 79 99 98 96 233 202 148 121 117 129 115

556 348 179 137 168 227 179 137 114 115 140 125 135 129 140 190 124 197 196 226 211 165 160 106 100 103 114 138 62 63 46 35 34 34 35 57 45 43 230 122 86 78 61 126 111

0.98 0.99 0.98 0.98 0.98 0.98 0.97 0.98 0.71 0.78 0.98 0.97 0.99 0.98 0.98 0.94 0.70 0.99 0.98 0.99 0.99 0.98 0.98 0.97 0.97 0.97 0.97 0.81 0.60 0.55 0.49 0.37 0.35 0.35 0.44 0.58 0.46 0.45 0.99 0.60 0.58 0.64 0.52 0.98 0.97

83 TABLE 2

Date

6 Mar.

13 Mar. 16 Mar. 17 Mar.

18 Mar.

(continued) Sampling interval

pH

HCHO~

HCHOt

HCHOb

HCHOb/HCHOt

0638-0742 0744~848 0850-0854 0904-0906 2233-2337 2339~036 0304-0330 0425-0529 0531-0600 0457-0601 0603-0707 225{~2354 2356~100 0102-0206 0208-0312 0332-0436 0439~543 0545-0642 0532-0636

5.6 5.7

24 35 22 33 17 20 12 19 17 17 23 6 7 3 9 6 7 7 10

130 127 122 149 118 84 46 39 33 77 69 121 108 101 83 30 16 20 26

106 92 100 116 101 64 34 20 16 60 46 115 101 98 74 24 9 13 16

0.82 0.72 0.82 0.78 0.86 0.76 0.74 0.51 0.48 0.78 0.67 0.95 0.94 0.97 0.89 0.80 0.56 0.65 0.62

6.9 7.0 7.1 7.0 6.7 6.5 6.8 5.9 6.2 6.1 6.1 6.6 6.6 6.6 7.1

was principally due to the blank values, which were always about 2-3#M. Errors and detection limits associated with all analyses are also reported in Table 1 and were calculated as previously described (Facchini et al,, 1986). In Table 2 we report free and total HCHO concentration in individual fog samples. Bound HCHO concentration is given by the difference between the free and total concentrations, pH values and the ratio HCHOb/HCHOt (b, bound; t, total) are also reported. As previously stated we assume that HCHOb corresponds to HMSA. It can clearly be seen from Table 2, that free formaldehyde represents only a minor fraction of the total HCHO concentration: 15% on average. The pH of the fog samples varies over a wide range: from 3.4 to 7.0. The ratio HCHOb/HCHOt is shown in Fig. 1 as a function of pH. Experimental kinetic studies and theoretical discussions relevant to fog and cloud water samples postulate HMSA stability to be maximized between pH 4.0 and 5.0, as suggested by the data presented in Fig. 1 (Dasgupta et al., 1980; Dong and Dasgupta, 1986; Kok et al., 1986). This pH dependence is due to several factors which have been extensively discussed in the literature. The formation rate of HMSA increases roughly by a factor of 10 with each pH unit under acidic to neutral conditions (Boyce and Hoffman, 1984; Munger et al., 1984, 1986; Olson and Hoffman, 1986). Since the solubility of SO2 also decreases with increasing acidity (Schwartz and Freiberg, 1981), conversion of gas-phase precursors to HMSA is strongly inhibited at lower pH. At fog water pH of 4.0-5.0, and under polluted conditions, instantaneous conversion rates of several hundred percent per hour are possible, so

84

1.0

]

OoaO

o0J~o~2

o

,

]o

,

o

I

0

0

0.8

~

0

0

0

o

I

DO

D

0 0

o 0

•-r- 0.6

o

o

0

0 0

0

0 nO

0

~g 0

0

O.4 0

0

o

"10.2

0

, 3

I 4

,

1 5 pH

,

] 6

,

] 7

Fig. 1. Trend of the ratio, bound formaldehyde/total formaldehyde, as a function of pH.

that chemical equilibrium can be established in the course of a fog event. At ' lower pH the low solubility of SO2, the competition for aqueous-phase S(IV) by H202, and unfavourable kinetics make establishment of HMSA equilibrium from gas-phase precursors unlikely. The decomposition rate of HMSA also increases with increasing pH (Deister et al., 1986; Kok et al., 1986). Because free, aqueous-phase S(IV) species are rather labile, losses of free S(IV) due to oxidation in fog droplets, or in the sample storage containers, will continually deplete the reservoir of HMSA. This depletion will be accelerated at high pH since chemical equilibrium is more rapidly established. As shown in Fig. 1 the H C H O J H C H O t ratio is close to 1.0 for fog samples having pH between 4.0 and 5.0, indicating that HCHO is the limiting reagent, and also that, at the time of sample collection, aqueous equilibrium was probably established. At 4.0 < pH < 5.0, this ratio was typically < 1.0, though variable. This could indicate that S(IV) was the limiting reagent, but from the previous discussion it seems more likely that at low pH the formation of HMSA was kinetically unfavourable, and that at high pH losses of HMSA occurred either during the fog event or in the storage bottles before analysis, or both. In either case, the data illustrate that, once formed, HMSA remains stable at pH < 5.0. Without other ancillary measurements (HSO~, SO2(g), HCHO(g), etc.) further interpretation of the data is difficult. In the case of acetaldehyde and acrolein, total and free concentrations are comparable, an indication that acetaldehyde and acrolein do not form stable adducts. These observations are supported by the thermodynamic predictions of Betterton et al. (1988). They observed that many aldehydes are unable to form adducts with S(IV) in solution, both because of the thermodynamic and kinetic characteristics of the reaction with S(IV) and also since they are not usually present in sufficiently high concentration in the aqueous phase.

85 CONCLUSIONS

F o g w a t e r s a m p l e s c o l l e c t e d a t o u r e x p e r i m e n t a l s t a t i o n in the Po V a l l e y d u r i n g t h e m o n t h s J a n u a r y - M a r c h w e r e a n a l y z e d for t o t a l a n d free c a r b o n y l compounds using the two-phase DNPH derivatization and HPLC separation. We c o n c l u d e t h a t : (i) M e a s u r e d c o n c e n t r a t i o n s of c a r b o n y l c o m p o u n d s a r e c o m p a r a b l e to o t h e r studies of u r b a n fog a n d c l o u d w a t e r . F o r m a l d e h y d e is by far t h e m o s t a b u n d a n t w i t h a n a v e r a g e c o n c e n t r a t i o n of 130 pM. A c r o l e i n a n d a c e t a l d e h y d e o c c u r at c o n c e n t r a t i o n s less t h a n o n e - t e n t h t h a t of H C H O . (ii) T h e g e n e r a l l a c k of o x i d a n t s in t h e Po V a l l e y d u r i n g t h e fog s e a s o n f a v o u r s the f o r m a t i o n of S ( I V ) - H C H O adducts. H i g h c o n c e n t r a t i o n s of b o u n d H C H O w e r e m e a s u r e d , a c c o u n t i n g for 85% of t o t a l H C H O , on a v e r a g e . No a d d u c t s w i t h o t h e r c a r b o n y l s w e r e observed. (iii) T h e p H d e p e n d e n c e of the H C H O b / H C H O t r a t i o s u g g e s t s t h a t H M S A f o r m a t i o n , a n d s u b s e q u e n t stability, is m a x i m i z e d b e t w e e n p H 4.0 a n d 5.0. I n t e r p r e t a t i o n of d a t a at l o w e r a n d h i g h e r p H is difficult w i t h o u t m o r e c o m p l e t e measurements. ACKNOWLEDGEMENTS A n n a m a r i a C o r r e g g i a r i a n d S t e f a n o M i s e r o c c h i a r e t h a n k e d for t h e i r a s s i s t a n c e d u r i n g t h e field o p e r a t i o n s . T h e s t u d y w a s s p o n s o r e d j o i n t l y by the C o m m i s s i o n of the E u r o p e a n C o m m u n i t i e s ( c o n t r a c t EV4V-0084-C) a n d t h e E n t e N a z i o n a l e E n e r g i a E l e t t r i c a - - C e n t r o di R i c e r c a T e r m i c a e N u c l e a r e ( c o n t r a c t 2RTII0326).

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