Characterization of Organic Constituents in Aerosols in Relation to Their rigin and Transport: A Review

Abstract

Organic matter in aerosols is derived from two major sources and is admixed depending on the geographic area. These sources are biogenic detritus (e.g. plant wax, microbes, etc.) and anthropogenic emissions (e.g. oils, soot, etc.). Both biogenic detritus and some of the anthropogenic emissions contain organic materials (C14- C40 +) extractable by solvent, which have unique and distinguishable compound distribution patterns. Molecular composition analysis has been applied to such extracts after suitable chemical separation into subfractions (i.e. hydrocarbons, ketones, fatty acids, fatty alcohols, and wax esters). Microbial and vascular plant lipids are the dominant biogenic residues and petroleum hydrocarbons, with lesser amounts of the pyrogenic polynuclear aromatic hydrocarbons (PAH), are the major anthropogenic residues. These lipid fractions are quantified and characterized in terms of their contents of both homologous compound series and specific molecular markers. Samples from rural sites contain predominantly vascular plant wax (as hydrocarbons, fatty acids and fatty alcohols) and lesser amounts of higher plant resin residues. Urban samples and, to varying degrees, some rural samples contain primarily higher weight residues of petroleum products. Molecular marker compounds from flora (e.g. phytosterols, terpenes, wax esters) are also present. Petroleum residues are comprised of hydrocar- bons and are found mainly in urbanized areas, with PAH from combustion at varying concentrations. Oceanic aerosols contain traces of lipids derived primarily from terrestrial plant waxes and fatty acids (C13-C18) from mainly marine sources. The sources of the lipid compound classes in aerosols have been reconciled by the application of the same analytical techniques to composited vegetation samples and emissions from vehicles (both gasoline and diesel) and from fires. This detailed approach to the molecular composition of organic aerosols indicates that primary biogenic residues are significant components of the troposphere over continental locations in the Northern Hemisphere and over oceanic areas. These organic materials are important constituents in the global cycling and flux of organic carbon. The comprehensive molecular analysis of all aerosol lipid fractions permits the definition of specific geographic source areas of air parcels and the interaction and admixing of anthropogenic versus natural components.