Environ. Sci. Technol. 2007, 41, 1628-1634
Hydroxydicarboxylic Acids: Markers for Secondary Organic Aerosol from the Photooxidation of r-Pinene M A G D A C L A E Y S , * ,† R A F A L S Z M I G I E L S K I , † IVAN KOURTCHEV,† PIETER VAN DER VEKEN,† REINHILDE VERMEYLEN,† WILLY MAENHAUT,‡ MOHAMMED JAOUI,§ TADEUSZ E. KLEINDIENST,⊥ MICHAEL LEWANDOWSKI,⊥ JOHN H. OFFENBERG,⊥ AND EDWARD O. EDNEY⊥ Department of Pharmaceutical Sciences, University of Antwerp (Campus Drie Eiken), Universiteitsplein 1, BE-2610 Antwerp, Belgium, Department of Analytical Chemistry, Institute for Nuclear Sciences, Ghent University, Proeftuinstraat 86, BE-9000 Gent, Belgium, Alion Science and Technology, P.O. Box 12313, Research Triangle Park, North Carolina 27709, and National Exposure Research Laboratory, Office of Research and Development, United States Environmental Protection Agency, Research Triangle Park, North Carolina 27711
Detailed organic analysis of fine (PM2.5) rural aerosol collected during summer at K-puszta, Hungary from a mixed deciduous/coniferous forest site shows the presence of polar oxygenated compounds that are also formed in laboratory irradiated R-pinene/NOx/air mixtures. In the present work, two major photooxidation products of R-pinene were characterized as the hydroxydicarboxylic acids, 3-hydroxyglutaric acid, and 2-hydroxy-4-isopropyladipic acid, based on chemical, chromatographic, and mass spectral data. Different types of volatile derivatives, including trimethylsilyl ester/ether, methyl ester trimethylsilyl ether, and ethyl ester trimethylsilyl ether derivatives were measured by gas chromatography/mass spectrometry (GC/MS), and their electron ionization (EI) spectra were interpreted in detail. The proposed structures of the hydroxydicarboxylic acids were confirmed or supported with reference compounds. 2-Hydroxy-4-isopropyladipic acid formally corresponds to a further reaction product of pinic acid involving addition of a molecule of water and opening of the dimethylcyclobutane ring; this proposal is supported by a laboratory irradiation experiment with R-pinene/NOx/ air. In addition, we report the presence of a structurally related minor R-pinene photooxidation product, which was tentatively identified as the C7 homolog of 3-hydroxyglutaric acid, 3-hydroxy-4,4-dimethylglutaric acid. The detection of 2-hydroxy-4-isopropyladipic acid in ambient aerosol provides an explanation for the relatively low atmospheric concentrations of pinic acid found during daytime in forest environments.
* Corresponding author phone: 32 3 820 27 07; fax: 32 3 820 27 34; e-mail:
[email protected]. † University of Antwerp. ‡ Ghent University. § Alion Science and Technology. ⊥ United States Environmental Protection Agency. 1628
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 5, 2007
Introduction It has been firmly established that monoterpenes give rise to secondary organic aerosol (SOA) through reactions of ozone, OH, and NO3 radicals (for a review, see Calogirou et al., ref 1). Monoterpenes are mainly emitted by conifers, having an annual global emission rate estimated at about 127 Tg with R-pinene as the major terpene emitted (2). Most laboratory studies of formation of SOA from monoterpenes have focused on investigations of SOA ozonolysis products of R-pinene including pinic, norpinic, pinonic, and norpinonic acids with pinic acid as a major reaction product (38), and, conversely, field studies mainly concerned the measurement of the latter compounds (9-13). The SOA ozonolysis products of R-pinene were also reported for the OH radical initiated photooxidation of R-pinene (14); however, in recent smog chamber experiments chemical analyses of irradiated R-pinene/NOx mixtures that involve reactions of R-pinene with OH and NO3 as well as with ozone revealed the presence of highly oxidized, acyclic, polar compounds along with the ozonolysis products (15, 16). A number of these highly oxidized compounds have also been observed in the fine fraction of ambient PM samples, although in the absence of standards, these compounds have only been tentatively identified (15-17). The presence of these compounds in apparently high concentrations in ambient PM samples along with the relatively low concentrations of pinic acid during daytime raises the possibility that one or more of the highly oxidized polar compounds could be formed from further reaction of pinic acid. In the present study, we consider the identification of three of these compounds detected in PM2.5 field samples collected in K-puszta, Hungary, a rural site with mixed deciduous, coniferous vegetation. Based on chemical considerations, the synthesis of reference compounds, and the interpretation of mass spectra, the structures of the compounds, all hydroxydicarboxylic acids, are identified with greater confidence than could be achieved in the recent study of Jaoui et al. (16). The compounds, 3-hydroxyglutaric acid, 3-hydroxy-4,4-dimethylglutaric acid, and 2-hydroxy-4-isopropyladipic acid (2-HIPAA), are given in Figure 1. In addition, an R-pinene/NOX smog chamber irradiation was conducted and the concentrations of pinic acid and 2-HIPAA in the aerosol were then compared as a function of time to determine the extent to which pinic acid might be further reacted in the system.
Experimental Section Aerosol Sampling. Field Samples. The ambient aerosol samples were collected at K-puszta, Hungary, during a field campaign between 4 June and 10 July 2003. The sampling station is situated in the clearing of a mixed coniferous/ deciduous forest on the Great Hungarian Plain (46°58′N, 19°33′E, 136 m asl) about 80 km SE of Budapest. The location is believed to be representative for a rural site and to be free from local anthropogenic pollution. The campaign was characterized by stable meteorological conditions and the weather was especially warm and dry. A dichotomous highvolume (Hi-Vol) sampler (placed at about 7 m above ground level) was used to collect samples in two size fractions, a fine (2.5 µm AD) fraction (18). Details about the aerosol sampling and meteorological and ozone data are presented in previous work dealing with the analysis of the same samples but for other aerosol constituents (19). Laboratory Samples. R-Pinene/NOx/air irradiation experiments were performed as described in detail in Jaoui et al. 10.1021/es0620181 CCC: $37.00
2007 American Chemical Society Published on Web 01/23/2007
FIGURE 1. GC/MS TICs obtained for the trimethylsilylated extract of PM2.5 aerosol (a) collected during daytime at K-puszta, Hungary, on 19 June 2003, and (b) an irradiated r-pinene/NOx/air mixture. Inset: mass chromatograms showing the coelution between 2-hydroxyglutaric acid (m/z 321; RT, 37.17 min) and U1 that was identified as 3-hydroxyglutaric acid (m/z 349; RT, 37.23 min). Identifications: (1) succinic acid; (2) 2-methylglyceric acid; (3) glyceric acid; (4) and (6) cis- and trans-2-methyl-1,3,4-trihydroxy1-butene; (5) 3-methyl-2,3,4-trihydroxy-1-butene; (7) malic acid; (8) norpinic acid; (9) 2-methylthreitol; (10) 2-methylerythritol; (11) 2-hydroxyglutaric acid; (12) pinic acid; (13) octanoic acid; (14) levoglucosan; (15) arabitol; (16) tetradecanoic acid; (17) glucose1; (18) mannitol; (19) sorbitol; (20) palmitic acid; (21) glucose2; (22) stearic acid; and (*) unknown. The superscripts 1 and 2 refer to isomeric forms of glucose. Chemical structures of the novel compounds identified in the present study: U1, 3-hydroxyglutaric acid; U2, 3-hydroxy-4,4-dimethylglutaric acid; and U3, 2-hydroxy4-isopropyladipic acid (2-HIPAA).
TABLE 1. Initial Conditions for the r-Pinene Irradiation Experiments Exp ID
chamber operation
NO (ppb)
NOx (ppb)
r-pinene (ppb)
T (C)
initial RH (%)
ER-177 ER-275
dynamica static
317 491
317 501
313 478
27.2 22.8
30.9 30.8
a
Average chamber residence time (τ): 5.9 h.
(16). Briefly, the experiments were carried out in a rectangular 14.5 m3 smog chamber similar to the one described by Kleindienst et al. (20). The chamber is fabricated from stainless steel with interior walls bonded with a 40-µm TFE Teflon coating and equipped with four banks of fluorescent bulbs (40 per bank) that provide irradiation distributed over the UV spectrum from 300 to 400 nm. The smog chamber was operated in the dynamic mode for the collection of the sample used for structural elucidation purposes and in the static mode for the time evolution experiment. The samples were collected by a 47-mm Zefluor filter (Pall Gelman Laboratory, Ann Arbor, MI) and the filters were weighed before and after sampling for gravimetric determination of the SOA mass. After obtaining the SOA mass, the filters were kept in a freezer at -25 °C until GC/MS analysis. The initial conditions for the chamber experiments are given in Table 1 Analyses for Polar Organic Marker Compounds. Field Samples. Only selected fine size fraction (
