Envlron. Sci. Technol. 1993, 27, 1403-1407
Oxocarboxylic and a,w-Dicarboxylic Acids: Photooxidation Products of Biogenic Unsaturated Fatty Acids Present in Urban Aerosols Eurlpldes G. Stephanou' and Nlkolaos Stratlgakls
Department of Chernlstry, Division of Environmental Chemlstry, University of Crete, 71409 Heraclion, Qreece, and Institut of Marine Biology of Crete, 71003 Heraclion, Greece ~~~~~~~~
w-Oxocarboxylicand a,w-dicarboxylicacids in the free and salt form have been determined in airborne and nearground particles in an urban coastal area in the eastern Mediterranean Sea. They range from c6 to c26, with a maximum concentration at Cg and Cg. The sum of the concentrations of Cg and Cs, w-oxo- and a,w-dicarboxylic acids, represents an important proportion (10-36%) of the acidic fraction of aeolian particulates extracts. The compounds C6 and Cy are believed to be photooxidation products of anthropogenic cyclic olefins, while those ranging from C12 to c26 are considered likely to be of biogenic origin. The l,&octanedioic and 1,9-nonanedioic acids are proposed to be oxidation products unsaturated fatty acids. The simultaneous presence of the corresponding 8-oxooctanoic and 9-oxononanoic acids as well as of the 9,lO-epoxyoctadecanoicacid of l-nonanal and of nonanoic acid, when correlated to the very low concentration (or even absence) of unsaturated fatty acids, represents additional evidence for the above hypothesis. A mechanism of this atmosphericphotooxidation reaction is proposed. Introduction Aliphatic and aromatic hydrocarbons, fatty alcohols, and carboxylic acids have been identified in aeolian particles in urban, rural, and marine areas (1-6). Adsorbed organic compounds on the surface of sea-salt particles, such as fatty acids, were reported as major componentsof marine aerosols in environments like the tropical North Pacific Ocean (4),the eastern Mediterranean Sea (61,and the western Mediterranean Sea (7). Unsaturated fatty acids, which are important components of marine and terrestrial plants, have been detected in some marine aerosols in very low concentrations, with the concomitant presence of a,w-dicarboxylic acids (8,9) in the same samples. The distribution pattern of the CgCl2 dicarboxylic acids, observed in the solvent-extractable fraction of Los Angeles rainwater, WBB considered to arise by oxidation of unsaturated fatty acids emitted from biogenic sources to the atmosphere (IO). In contrast, the cSc7 o-oxoacids (maximizing at c6) (111,observed in aerosol samples collected in Pasadena, CA (121, are considered to be products of anthropogenic cyclic olefins. In order to study the atmosphericchemistry of long-chain alkenes, a series of smog chamber experiments were used to examine the photooxidation reactions of l-octene with O(3P), *OH,and 0 s (13). During these experiments the following results were obtained (a) the O(sP) reaction gave mainly hexyl oxirane and heptanal after -75% of the l-octene had reacted; (b) the 'OHinitiated reaction was found to produce only -20%
* Author to whom correspondance should be addreeaed at the University of Creta. 0013-936X/93/0927-1403$04.00/0
@ 1993 American Chemical Society
Table I. Some Atmospheric Conditions Prevailing during the Sampling Period in the City of Heracliona 4/24/1991
5/411991
8/28/1991
wind (m/s) N (0.9) SW, SE (1.6) W,NW (2.4) 2.8 6.0 os [ P P h ] 3.5 2.6 0.9 NOz[ppbl 1.8 17.0-29.0 8.0-21.5 tamp [ O C I 7.619.5 57-67 57-75 57-67 humidity [ % ] 9 sunlight [h] 9 12 a N, W, and NW are winds from the sea, while SW and SE are from the land.
heptanal; and (c) the Oa/l-octene reaction was found to produce -80% heptanal and -10% heptanoic acid. The latter compound was postulated to arise from the isomerization of a thermally stabilized Criege biradical. The high levels of ozone in the lower atmosphere are an issue of concern, because of their effects on plant productivity and human health. The reactions of ozone and other oxidants with biogenic or anthropogeniccornpounds in the troposphere are an area of research still to be explored. We have performed analyses of a series of aerosol and ground particulates that were collected from a coastal urban area. The acidic fractions of these samples were examined in order to determine the photooxidation products of unsaturated fatty acids with various atmospheric oxidants and especially ozone, predicted by the above experiments. We present our findings in this paper. Experimental Section
Sampling. Aeolian particulates were collected on a sampling station 15 m high, located in the center of the city of Heraclion (north coast of the Island of Crete). The samplingperiod started April 1991and ended August 1991. The winds in this area are variable, with north, northwest, and south winds prevailing during the year. Table I contains the atmosphericconditionsprevailing during the sampling period of this study. Air particulates were sampled on a preextracted glass fiber filter (20 X 25 cm, having a collection efficiencyhigher than 99 % for particles kith radius higher than 0.3 Mm, at the flow rate 90m3h-l) mounted in a high-volume air sampling device (Model 2000, General Metals Works, Cleves, OH, 2000-3000 m3 air sampled). Particles were also collected, 30 cm from the ground, on a preextracted AP4003705 Millipore filter, using a Millipore pumping device (10-14 L/min pumping capacity, 14 m3 of air sampled). Isolation and Analysis. Sample storage, materials preparation, control of procedural blanks, extraction, and carboxylic acids (as free acids or as salts) isolation have been presented in detail elsewhere (6). Neutral lipids, among them n-aldehydes,were separated into different compound classes by using column flash chromatography: 1.5 g of silica gel (230-400 mesh,backed Envlron. Sci. Technol.,Voi. 27, No. 7,
1993
1403
e
c9
C18
A
x
E 2 !
a,w-
4.l
0
10.0
E
f
20.0
c24
30.0
rmin 1
c6
40.0
0
c11
10.0
12.0
14.0
16.0
18.0
[min]
Flgure 1. (A) mlz 74 ion chromatogram of the methylatedacidic extract of an aeolian particle sample (C, = carbon numbers of fatty acids methyl esters, C,, = carbon numbers of unsaturatedacid methyl esters, and a,w-Cg = 1,Q-nonanedioicdimethyl ester); (B) combined Ion chromatograms for m/z 97, 111, and 112 (speclfic ions for w-oxo- and a,wdicarboxylicfatty aclds) of the same methylatedacidic fractlon ('C, = carbon numbers of w-oxo- and C, = carbon numbers of a,odicarboxyilc acids).
for 3 h at 150 "C), in 15 mL of n-hexane in a column 30 X 0.7 cm. Nitrogen pressure was 1.4 mL/min. The solvent gradient and the compound class obtained in each fraction were (i) 15 mL of n-hexane (n-alkanes), (ii) 15 mL of n-hexane (63%)/toluene (37% ) (polycyclic aromatic hydrocarbons),and (iii)15mL of n-hexane (50%)/methylene chloride (50% (n-aldehydes) (14). The GUMS analyses (for identification and quantification) and the GC/FID analyses (for quantification) were performed as described in ref 6. All quantities given here have been corrected by taking into account the results of the application of the analytical methodology (column chromatographyperformance and relativeresponse factors in GC/FID and GC/MS) that included standard compounds and subtraction of the quantities of blanks. Results and Discussion The extractable acidic fraction identified in this study consists of fatty acids, a,w-dicarboxylic acids, and w-oxocarboxylic acids in both the free and the salt forms. In Figure 1 are presented the mlz 74 ion chromatogram (Figure 1A) and the m/z 97,98,111, and 112 (specificions for w-oxocarboxylic acids and a,w-dicarboxylic acids) combined ion chromatograms (Figure 1B) of the methylated acidicfraction of an aeolianparticle sample. Figure 2 shows the electron-impact induced mass spectra of the methyl-9,lO-epoxyoctadecanoate(Figure 2A) and of the methyl-9-oxononanoate (Figure2B). The mass spectrum of methyl-9,lO-epoxyoctadecanoateand the mass spectra of the dimethyl esters of 1,9-nonanedioic and 1,8-octanedioic acids,which were determined as salts in amarine 1404
Envlron. Sci. Technol., Vol. 27, No. 7, 1993
aerosol of the same coastal area, have been thoroughly presented (9). Table I1containsthe concentrationsfor different ranges, ,,C and carbon preference index (CPI), of the totalfatty acids in the free (FFA) and salt (FAS) form and the individual concentrations of unsaturated fatty acids of a,w-dicarboxylic acids and of w-oxocarboxylicacids, all in free and salt form, determined in aeolian (EP) and nearground particles (GP) extracts. In Table I1 are also given the concentrations of l-nonanal and nonanoic acid that were determined in the same samples, and the ratios of the s u m of the concentrationsof C g and Cg a,w-dicarboxylic and o-oxocarboxylicacids to sum of those of CUM,Cle:~, and ClS2 unsaturated fatty acids, [C[cu,w + ol/C[unsl) are given. The ratio of the concentration of octadecanoic acid to that of 9-octadecenoicacid ([CI~]/[CI~:II) is also given. Finally, Table I1 includes the sum of the concentration of Cg and CSa,@-dicarboxylicand w-oxocarboxylic acidsas percentagesof the total acid fractionconcentration (% [a,w
+ 03).
In the present study, two forms of fatty acids were detected and determined, namely, alkanoic acids in the free form and alkanoic acids in the salt form. This fact indicates an aerosol of both marine (containing acids in the salt form) and land origin (containing acids in free form). In both forms of alkanoic acids found here, one can distinguish two main categories: (a) intermediate molecular weight compounds, ranging from Cg to Cig and maximizing at c16 and c14 and (b) high molecular weight alkanoic acids, ranging from CZO to C32 and maximizing at C22, c24, (228, and C ~ OBoth . categories have CPI values
A /O\
-
CH CH-(CH:,)~-CO:,GH,
CH,-(CH,),-
100.0 j5 100.0 156
87
50.0
50.0
0
0 0
I50
200
250
300
mlz
h 60
111
155 158 11111
100
I
IIII
ll1IIlI
140
IIIIi
M+ 186
180
mlz
Flgure 2. (A) Mass spectrum of methyC9,l O-epoxyoctadecanoate;(6) mass spectrum of methyCQoxononanoate.
greater than 2. The n-alkanoic acids, such as nonanionic acid, with carbon atom numbers less than 12 were determined in concentrations almost equal to or lower than those determined in the blanks. Alkanoic acids ranging from C g to C1g are believed to arise from recent biogenic sources (microorganisms,plant waxes, marine organisms) and vehicular emissions (15, 16);those of higher molecular weight (>Cm) usually derive from epicuticular waxes of vascular plant folliage. Alkanoic acids of higher range (c20-c32) are found in larger concentrations and with higher ,C during the summer months (6). We found the same in this study, especially regarding the alkanoic acids in the free form (Table 11). In some of the samples, we detected (Table 11)unsaturated fatty acids, such as C16:1(palmitoleic), CIS:^ (oleic), and C18:2 (linoleic). These compounds are indicators of recent biogenesis (3). Their absence or low concentrations result from their instability under the oxidative environmental conditions that prevail in this area (Table I). The ratio of the concentrations of saturated vs unsaturated fatty acids ([Cl8]/ [CU:~])cogently illustrates the more rapid decomposition for the unsaturated acids in the salt form, as well as their faster degradation during summer, at which time ozone concentration is very high (Tables I and 11). Samples collected during a cruise in the western Mediterranean (7)show lower ([c181/[c18:ll) ratios (in the range 0.6-2.4 for fatty acid salts and 0.4-2.6 for free fatty acids), indicating less degraded unsaturated acids than ours (Table 11). a,@-Dicarboxylicacids, ranging from c6 to C27 and with C g being in all cases the most abundant homologue, were determined in all air particle samples that we studied (Table 11). Our detection of the compounds of this class with higher carbon numbers in the salt form points to their marine origin. Indeed, a,w-dicarboxylicacids ranging from C20to c 2 8 have been detected in the sea grass Zosteru marina L. ( 17),which is very abundant in the area. It should be noted that the presence of higher homologues
(>C20) is related more often with winds coming from the sea (Tables I and 11). Long-chain dicarboxylic acids ranging from CIOto c24, with a strong even-carbon number predominance and maxima at C12, CIS, and C20, have been reported in air samples (3). Their presence was attributed to a biogenic source or to the degradation of w-hydroxyfatty acids found in vascular plants. The presence of shorter dicarboxylic acids, ranging from C5 to C7, has been related to the photooxidation of anthropogenic cyclic olefins, whose reactions with ozone have been studied in special smog chambers ( 1 1 ) . a,w-Dicarboxylic and w-oxocarboxylicacids, with 8 and 9 carbon atoms, are considered to be photooxidation products of unsaturated fatty acids. These compounds have been already detected in marine aerosolsin the South Pacific (8) and the eastern Mediterranean Sea (9). The compound identified as the 9,lO-epoxyodadecanoic acid has been detected only in one sample in the acid salt form (EP4/24/91). This compound had been determined previously only in a marine aerosol sample in a nonurban area at the same coast (9). It was found to be very labile (9). The presence of l-nonanal and nonanoic acid in all the samples in which the above mentioned c8 and CS a,wdicarboxylic and o-oxocarbocylic acids are present and the presence in these samples of 9,lO-epoxyoctadecanoic acid, when correlated with the absenceor low concentration of palmitoleic, oleic, and linoleic acids, are consonantwith the mechanism proposed below (Schemes 1-111) for the oxidation of these unsaturated acids with 0 3 , O(3P),and
*OH. From the known reactions of l-octene and cyclohexene with atmospheric oxidants such as O(3P) and 'OH (11, 131, one would expect oleic acid, for example, to give 9,10-epoxyoctadecanoicacid (A), l-nonanal (B), nonanoic acid (J),and 9-oxononanoic acid (C)(Scheme I). Envlron. Scl. Technoi., Vol. 27, No. 7, l S S 3
1405
Table 11. Concentrations (ng/m*) of Monocarboxylic Fatty Acids (with CPI and L), Unsaturated Fatty Acids (una), a,o-Dicarboxylic (a,w-dicarb),w-Oxocarboxylic (w-oxocarb),and I-Nonanal in Aeolian Particulate (EP) and Ground Particulate (GP) Samples. EP 4/24/91 FFA FAS rC9-Clel
c-
CPI rc20-Cs21
~,,
17.8
96.7
31.7
135.6
31.1
4746.0
994.0
cl6
13.1 18.5
cl6
cl4
6.7 24.9
12.6 43.2 cz2 1.3
6.0 17.2
Cl6
c16
23.1 69.6
>30 5.1
Cl6
ClS
7.9 3110.0
c2s
C28
21.0 135.0 CZO >21.0
c 2 2
CU
2.9
C1&1 C1&2 Cl61
4.7 6.6 1.8
0.3
a,w-dicarb
*
0.27 0.28 2.27 12.85 1.12 1.41 0.49 2.20 0.38 0.93 1.07
ClO c11 Cl2 c13 Cl4 C16 c 1 6 Cll
1.02 13.67 1.48 1.59 0.61 1.52 0.53 1.02 1.46 1.02 0.83
*
ClS . .
c19
*
Cn, C2l c22 c2S
GP 8/28/91 FFA FAS
112.1
2.1
C8 c9
EP 8/28/91 FFA FAS
cl6
CPI uns
C6 CI
EP 5/4/91 FFA FAS
1.91
CU C2S C26 c21
c22
3.6
2.5
3.0 5.9 1.3
c30
9.4
0.006
16.9 70
* 0.02 0.89 7.97 0.57 0.73 0.55 2.64 0.90 1.97 2.78
2.4
1.14 9.86 0.97 1.08
*
0.94 0.52
*
1.07 0.92 3.72 25.88 1.49 2.62 1.01 0.82 2.36
1.61 0.80 2.88 18.92 1.13 1.34 0.61
68 65
0.3 1.4
66.0
81
0.87
*
*
0.60
0.86 1.29 0.75 1.47 1.61 1.67 0.99 1.60
0.80
*
* 2.30 2.11 1.47
w-oxocarb C8
1.3 0.1 0.08 1.2 4.4 10.5
Ce 1-nonanal nonanoic acid C[a,o+ wl/Z[unsl [cldcls:lI
% ra,w
+ wl
0.5
* 50.7 14.4 19.6
0.35 0.85 0.06 0.09 1.0 8.0 10.1
1.0
* ** ** 18.0
0.30 0.30 0.02
** **
14.0
8900 763.3 36.1
* ** ** 2.5
0.9 6.7 6.3
a CPI, carbon preference index, summed from C12 to CISand from C20 to Cas; FFA, free fatty acids; FAS, fatty acid salta; Z[a,o + wl/Z[unsl, ratio of the sum of the concentrations of C8 and Cg a,@-dicarboxylicand w-oxocarboxylicacids to sum of those of unsaturated reported here; [C1$Cls:1], ratio of the concentration of octadecanoic acid to this of 9-octadecenoic acid; % [a,o+ w ] , sum of concentrations of Ca and Cg a,o-dicarboxylic and 0-oxocarboxylicacids a~ percentage of the total acid fraction concentration; (*), compounds identified but not quantified, or compounds found in concentrations below those of blanks; (**), ratios approaching an infinite value.
Scheme I (AQ)
R-CH =CH -(CH2)7-COOH
(R=CH3(CH2)7:01ei~acid)
importancein the atmosphere. CompoundsB and C under these severe oxidation conditions (Table I) should further produce the 1,9-nonanedioicacid (I) and nonanoic acid (J).
+ R-CHO
+ (B)
OHC-(CH2),-COOH
(C)
+ OHC-(CH2)7-COOH
(C)
A - B + C B, C HOOC-(CH2)7-COOH
(I) + RCOOH (J)
We characterizedthe compoundsB and C in all samples. Compound A,which is very unstable, is expected to be further oxidized to compounds B and C. The reaction between O(3P)and unsaturated acids should be of minor 1406 Envlron. Sci. Technol., Vol. 27, No. 7, 1993
The reaction of oleic acid with ozone (Scheme 11) in addition to compounds B and C is expected to give the stable Criegee biradicals E and F (13)(Scheme 11). These biradicals should give in the presence of water (13)both 1,9-nonanedioic(I) and nonanoic (J)acids and lead to compoundsB and C (131,which are further oxidized to I and J. The above results also agreewith those obtained in the reaction of cyclohexene with ozone (13). The formation of 1,8-oxooctanoic (H)and 1,8-octanedioic acids (K) may be rationalizedas shown in Scheme 111. As 1-nonanaland nonanoic acid may have other sources (vegetation and biomass combustion (18-20)) than the photooxidation reactions of A9 unsaturated alkanoic acids,
ic, and 1-oxononanoic acids in the free form and 1,9nonanedioic acid in its salt in near-groundparticles. Again the compounds considered as photooxidation products of the A9 unsaturated fatty acids are in larger proportion in the acidic fraction obtained from in the salt form than in the free form (Table 11). On the other hand, the a,wdicarboxylic and w-oxocarboxylic acids are in lower proportion in near-ground particlesthan in air particles (Table 11). In near-ground aerosols, where larger particles dominate, we found less oxidized material. The longer residence time of small particles, in the air, is therefore connected to more oxidized material.
Scheme I1 R-CH =CH--(CH&-COOH
(AQ)
t
I
ACHO
I 7-0'
'0-2
Acknowledgments
(b) E, F
Scheme I11 OHC-(CH&-C02H
-k
(C)
-
*COCH2--(CH,)6--COOH G
- E, C
*OH
OHC-(CH2),-COOH
This research is supported by the European Community CT92-0824SCIENCE programme. We thank NATO for supporting with a Collaborative Travel Grant.
1,J
Literature Cited
*CO-(CH2),-COOH
*CH2-(CH2),-COOH
+ HzO
+ CO
HOOC-(CH2),-COOH
02
(HI
(K)
we focus further the discussion of results on the w-oxoand a,@-dicarboxylicacids. If we consider the ratio of the sum of a,w-dicarboxylic acids and w-oxocarboxylic acids (CSand Cg homologues, arising from the photooxidationof unsaturated fatty acids) to the sum of palmitoleic, oleic, and linoleic acids (C[a,o wl/~[unsl)(Table 11),we concludethat in air particulate samplesunsaturated fatty acids undergo faster degradation in the salt than in the free form. The same is true taking into consideration the ozone concentration: this ratio is higher when the ozone in the air is higher. We reach the same conclusion, if we consider the percentages of the above compounds in the acidic fractions (% [ a , w + wl), where the values are generally higher for higher 03 concentrations and for compounds in the salt form than for compounds in the free form (Table 11). Long-chain carboxylic acids in their salt form may be considerd as surfactants in aqueous atmospheric aerosols that form an organic coating over the aerosol surface (21) and thus may increase the lifetime of such particles in the atmosphere. Therefore, in this mixed (marine and land derived) urban aerosol, unsaturated acid salts possess long enough time to be oxidized by various atmospheric oxidants to form the detected oxo- and dicarboxylic acids in concentrations higher than those found in the free form. The detection of a,o-dicarboxylic acids (CS and CS) in marine sediments (9, 22) and sediment trap (22) was interpreted as indicative of atmospherictransport of these photooxidation products rather than of an autochthonous formation. We determined 1,8-octanedioic,1,g-nonanedio-
+
Simoneit, B. R. T.; Chester, R.; Eglinton, G. Nature 1977, 267,682. Simoneit, B. R. T. Mar. Chem. 1977,5,443. Simoneit, B. R. T.; Mazurek, M. A. Atmos. Environ. 1982, 16, 2139. Gagosian, R.B.; Peltzer, E.; Zafiriou, 0.Nature 1981,291, 312. Sicre, M. A.; Marty, J. C.; Saliot, A.; Aparicio, X.; Grimalt, J.; Albaiges, J. Atmos. Environ. 1987,21,2247. Stephanou, E. Atmos. Enuiron. 1992,26A,2821. Sicre, M. A.; Marty, J. C.; Saliot, A. J. Geophys. Res. 1990, 95,3649. Kawamura, K.; Gagosian, R. B. Nature 1987,325,330. Stephanou, E. Naturwissenschaften 1992,79, 128. Kawamura, K.; Kaplan, I. Environ. Sci. Technol. 1983,17, 497. Hatakeyama, S.;Tanokaka, T.; Weng, J. H.; Bandow, H.; Takagi, H.; Akimoto, H. Environ. Sci. Technol. 1985,19, 935. Schuetzle, D.;Cronn, D.; Crittenden, A. L. Environ. Sci. Technol. 1975,9,838. Paulson, S.E.; Seinfeld,J. H. Environ. Sci. Technol. 1992, 26, 1165. Stephanou, E.; Stratigakis, N. University of Crete, 1993 (in preparation). Simoneit, B. R. T. Atmos. Environ. 1984,18, 51. Mazurek, M. A.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci. Technol. 1991,25,684. Hollerbach, A. Grundlagen der Organischen Geochemie; Springer-Verlag: Berlin, 1985;p 46. Graedel, T. E.; Hawkins, D. T.; Claxton, L. D. In Atmofpheric Chemical Compounds; Academic Press: Orlando, PL, 1986;pp 304 and 351. (19) Pereira, W. E.; Rostod, C. E.; Taylor, H. E.; Klein, J. M. Environ. Sci. Technol. 1982, 16, 387. (20) Greenberg, J. P.; Zimmerman, R. R.; Heidt, L.; Pollok, W. J. Geophys. Res. 1984,89,1350. (21) Toossi, R.;Novakov, T. Atmos. Environ. 1985,19, 127. (22) Kawamura,K.;Hand, N.; Nozaki, Y. Geochem. J. 1990,24, 217.
Received for review November 9, 1992.Revised manuscript received February 23,1993.Accepted March 8,1993.
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