ARTICLE pubs.acs.org/est
Lignin-Derived Phenols in Houston Aerosols: Implications for Natural Background Sources Kabindra M. Shakya,†,* Patrick Louchouarn,‡ and Robert J. Griffin† † ‡
Department of Civil and Environmental Engineering, Rice University, 6100 Main St., Houston, Texas 77005, United States Department of Marine Sciences and Department of Oceanography, Texas A&M University at Galveston, 1001 Texas Clipper Rd, Galveston, Texas 77554, United States
bS Supporting Information ABSTRACT:
Solvent-extractable monomeric methoxyphenols in aerosol samples conventionally have been used to indicate the influence of biomass combustion. In addition, the presence of lignin oxidation products (LOP), derived from the CuO oxidation of vascular plant organic matter, can help trace the source and inputs of primary biological particles in aerosols. Ambient aerosols (coarse and fine) collected in Houston during summer 2010 were analyzed by gas chromatographymass spectrometry to characterize monomeric and polymeric sources of LOPs. This is the first time polymeric forms of the LOPs have been characterized in ambient aerosols. The absence or small concentrations of solvent-extractable monomeric LOPs and levoglucosan isomers point to the limited influence of biomass burning during the sampling period. The trace levels of anhydrosugar concentrations most likely result from long-range transport. This observation is supported by the absence of co-occurring lignin monomers that undergo photochemical degradation during transport. The larger concentration (142 ng m3) of lignin polymers in coarse aerosols shows the relative importance of primary biological aerosol particles, even in the urban atmosphere. The LOP parameters suggest a predominant influence from woody tissue of angiosperms, with minor influence from soft tissues, gymnosperms, and soil organic matter.
’ INTRODUCTION Organic aerosols play an important role in atmospheric chemistry (affecting volatile organic compounds and ozone; heterogeneous reactions), climate (direct and indirect effects), visibility, and human health. Important primary organic aerosol sources include plant materials (fragments; pollens), biomass burning, road, soil and mineral dust, and sea salt.1 The natural background contribution to urban organic aerosol levels is not well studied because its determination is hindered by analytical challenges.1 Natural biological debris is estimated to be emitted to the atmosphere at a rate of 50 Tg yr1 globally.2 However, the global flux might be significantly larger when accounting for primary biological aerosol particles (PBAP) that are compositionally not well-defined.3 Lignin, the second-most abundant naturally occurring polymer (after cellulose), is composed of collective macromolecules that form the major structure of vascular plants, contributing about 28% and 20% of softwood and hardwood biomass, respectively.4,5 Because vascular plants are exclusively terrestrial and lignin possesses greater resistance to biodegradation than hemicellulose and cellulose,610 it serves as an ideal unambiguous tracer of terrigenous organic matter (OM).1121 Lignin is composed of three aromatic alcohols (p-coumaryl, coniferyl, and sinapyl alcohols) that give rise to phenolic aldehydes, ketones, and acids upon oxidation or pyrolysis.5,11,22 These r 2011 American Chemical Society
lignin-derived phenols include three vanillyls (V), three syringyls (S), and two cinnamyls (C). Three p-hydroxyl phenols (P) (acid, aldehyde, and ketone) and 3,5-dihydroxybenzoic acid (3,5 Bd) also are formed upon oxidation, although these last four species have additional sources (bacteria, phytoplankton, and macroalgae).13,23 These eight methoxyphenols used in this study are a subset of the organic molecules produced from the oxidation of ligno-cellulose and were chosen because of their application in geochemistry studies for more than three decades in multiple environmental systems.20 To our knowledge, this is the first time this method has been applied to study aerosols. Thus, the use of the standardized CuO method permits a comparison between the values for the selected methoxyphenols and their ratios obtained from this study and those that already exist in a large body of literature. Anhydrosugars such as levoglucosan and its isomers are known markers of biomass burning.2430 Levoglucosan occurs in aerosol particles from combustion of cellulose-containing materials, and the emission of levoglucosan and its isomers varies widely depending on types of biomass being combusted.2628 Received: May 16, 2011 Accepted: August 30, 2011 Revised: July 29, 2011 Published: August 30, 2011 8268
dx.doi.org/10.1021/es201668y | Environ. Sci. Technol. 2011, 45, 8268–8275
Environmental Science & Technology Methoxyphenols are relatively unstable compared to levoglucosan due to their higher sensitivity to photodegradation during long-distance transport.14,24,29,31 Previous studies have used solvent-extractable monomeric lignin derived methoxyphenols in aerosols (without postsampling oxidation) to characterize the contribution of biomass burning.14,3235 However, if postcollection oxidation (e.g., with CuO) is performed, the presence of lignin oxidation products (LOPs) can be used to estimate the primary biological contribution to organic aerosols and to identify the taxonomic class and tissue type of source vascular plant material because of the difference in distribution of LOP species in major plant classes and tissues.11,17,22 The LOPs also provide insight into various characteristics such as fresh vs altered (based on acid to aldehyde ratio), gymnosperms vs angiosperms (based on the presence or absence of S), and herbaceous vs woody species (based on the presence or absence of C).11,17,22,36 Additionally, p-hydroxyl phenols and 3,5 Bd provide information on grass sources and humified soil organic matter, respectively.1418 Recently, Louchouarn et al.20 identified these species in polymeric form in aerosol reference materials. The current work thus applied the CuO oxidation method cited in Louchouarn et al.20 for the first time to characterize lignin material in ambient urban organic aerosols. The main objective of this paper is to determine the natural sources that contribute to organic aerosol loadings in coarse and fine fractions in a major urban area using LOPs. Temporal variation, composition, and relative ratios of the LOPs in Houston as well as the application of LOPs as tracers for identification of natural plant materials or plant combustion sources in aerosols are investigated.
’ MATERIALS AND METHODS Sampling. Aerosols were collected for every 24 h on prebaked (550 °C for 24 h), 47 mm quartz fiber filters (Pall Life Sciences, NY) from 07/20/2010 to 08/09/2010. A Versatile Air Pollutant Sampler (VAPS, URG, NC) with channels drawing 2 L per minute and 15 L per minute was used for collecting coarse (diameter range of 2.510 μm) and fine (diameter
