Identification and emission factors of molecular tracers in organic aerosols from biomass burning Part 2. Deciduous trees

Smoke particulate matter from deciduous trees (angiosperms) subjected to controlled burning, both under smoldering and flaming conditions, was sampled by high volume air filtration on precleaned quartz fiber filters. The filtered particles were extracted with dichloromethane and the crude extracts were methylated for separation by thin layer chromatography into hydrocarbon, carbonyl, carboxylic acid ester and polar fractions. Then, the total extract and individual fractions were analyzed by gas chromatography and gas chromatography–mass spectrometry. The major organic components directly emitted in smoke particles were straight chain aliphatic compounds from vegetation wax and triterpenoid acids (biomarkers) from gums and mucilages. The major natural products altered by combustion included derivatives from phenolic (lignin) and monosaccharide (cellulose) biopolymers and oxygenated and aromatic products from triterpenoids. Steroid biomarkers and polycyclic aromatic hydrocarbons (PAH) were also present, however, as minor constituents. Although the concentrations of organic compounds in smoke aerosols are highly variable and dependent on combustion temperature, the biomarkers and their combustion alteration products are in these cases source specific. The major components are adsorbed or trapped on particulate matter and thus may be utilized as molecular tracers in the atmosphere for determining fuel type and source contributions from biomass burning.     

Identification and emission factors of molecular tracers in organic aerosols from biomass burning Part 1. Temperate climate conifers

Smoke particulate matter from conifers subjected to controlled burning, both under smoldering and flaming conditions, was sampled by high volume air filtration on precleaned quartz fiber filters. The filtered particles were extracted with dichloromethane and the crude extracts were methylated for separation by thin layer chromatography into hydrocarbon, carbonyl, carboxylic acid ester and polar fractions. Then, the total extract and individual fractions were analyzed by gas chromatography and gas chromatography–mass spectrometry. The major organic components directly emitted in smoke particles were straight chain aliphatic compounds from vegetation wax and diterpenoid acids (biomarkers) from resin. The major natural products altered by combustion included derivatives from phenolic (lignin) and monosaccharide (cellulose) biopolymers and oxygenated and aromatic products from diterpenoids. Other biomarkers present as minor components included phytosterols, both the natural and altered products, and unaltered high molecular weight wax esters. Polycyclic aromatic hydrocarbons (PAH) were also present, however, only as minor constituents. Although the concentrations of organic compounds in smoke aerosols are highly variable and dependent on combustion temperature, the biomarkers and their combustion alteration products are source specific. The major components are adsorbed or trapped on particulate matter and thus may be utilized as molecular tracers in the atmosphere for determining fuel type and source contributions from biomass burning.     

Biomass burning — a review of organic tracers for smoke from incomplete combustion

Biomass combustion is an important primary source of particles with adsorbed biomarker compounds in the global atmosphere. The introduction of natural product organic compounds into smoke occurs primarily by direct volatilization/steam stripping and by thermal alteration based on combustion temperature. Although the molecular compositions of organic matter in smoke particles are highly variable, the molecular tracers are generally still source specific. Dehydroabietic acid is typically the major tracer for conifer smoke in the atmosphere. Degradation products from biopolymers (e.g. levoglucosan from cellulose, methoxyphenols from lignin) are also excellent tracers. Additional markers of thermally-altered and directly-emitted natural products in smoke have been defined which aids the assessment of the organic matter types and input from biomass combustion to aerosols. The precursor to product approach of compound characterization by organic geochemistry has also been applied successfully to provide source specific tracers for studying the chemistry and dispersion of ambient aerosols and the intermingling of natural with anthropogenic emissions and with smoke plumes. A brief review of the organic matter composition in aerosols derived from the major sources is also given, with emphasis on the detection of biomass burning components. These major sources are the natural background from biogenic detritus (e.g. plant wax, microbes, etc.) and anthropogenic particle emissions (e.g. oils, soot, synthetics, compounds, etc.). The emissions of organic constituents in coal smoke particulate matter are also reviewed and depend on combustion temperature, ventilation, burn time, and coal rank (geologic maturity). The components of peat and brown coal and to a lesser degree semi-bituminous coal consist mainly of hydrocarbons, biomarkers, and aromatic components, quite similar to burning of contemporary biomass. Dispersion from the source and long range transport of smoke particulate matter with the associated organic compounds is also discussed.     

Size distributions of hydrocarbons in suspended particles from the Yellow River

Suspended particle samples from the Yellow River estuary were sorted into five grain size fractions to explore the effect of grain size distribution on organic matter content and composition. The n-alkanes and PAHs were determined for each size fraction. PAHs and n-alkanes were more abundant in the finer fractions and the loading decreases steadily with increasing of grain size. However, the total n-alkanes or PAHs normalized to organic C were lower in the smaller size fractions than those in the larger size fractions, suggesting n-alkanes or PAHs may be diluted by the addition of organic matter or gradually decreased by degradation in the smaller size fractions. The particulate n-alkanes in the Yellow River estuary consist of a mixture of compounds from terrigenous and riverine biogenic n-alkanes and more biogenic n-alkanes accumulate in finer particles. Particulate PAHs are related to combustion/pyrolysis processes of coal/wood, and the relative contribution of petrogenic PAHs increase with increasing grain size. The total particulate n-alkane and PAH discharges passing the Lijin Station are about 3.94 t d⁻¹ and 0.52 t d⁻¹, respectively. Fine particles (<32 μm) play a significant role in organic matter transfer.     

Organic geochemistry of suspended and settling particulate matter in Lake Michigan

Organic matter contained in particulate matter in Lake Michigan waters and sediments has been characterized by $\text{C}\text{N}$ ratios and by distributions of biomarker fatty acids, alkanols, sterols, and aliphatic hydrocarbons. Differences in organic constituents of particulate matter from various depths and distances from shore indicate a complex interaction of production, transformation, and destruction of the organic matter contained in sinking particles. Near-surface material contains important contributions of landderived organic matter, presumably of eolian input. Midwater particles have predominantly aquatic organic material of algal origin. At the sediment-water interface, selective suspension of the finer fractions of surficial sediments enriches bottom nepheloid layers with these sediment size classes. As a result, near-bottom particulate matter has an aquatic biomarker character. Organic matter associated with sinking particles undergoes substantial degradation during passage to the bottom of Lake Michigan, and aquatic components are selectively destroyed relative to terrigenous components.     

Characteristics of lipid tracer compounds transported to the Arabian Gulf by runoff from rivers and atmospheric dust transport

River runoff and atmospheric fallout (dust and air particulate matter) are major input sources of natural and anthropogenic terrestrial organic and inorganic components to the Arabian seas. In this study, we report on the various lipid tracer compounds that might be transported to the Arabian Gulf by rivers, dust, and air particulate matter. These are based on geochemical analysis of sediment, dust, and particulate samples collected from Iraq, Kuwait, and Saudi Arabia. The samples were extracted with a dichloromethane/methanol mixture and analyzed by gas chromatography-mass spectrometry. The extractable organic compounds (lipids) in the samples include n-alkanes, n-alkanoic acids, n-alkanols, methyl n-alkanoates, steroids, triterpenoids, carbohydrates, and petroleum hydrocarbons. The steroids and triterpenoids were major components in river and wetland samples. The major sources of these lipids were from natural vegetation, microbial (plankton and bacteria) residues in the sediments, sand, and soils, with some contribution from anthropogenic sources. Accordingly, these sources could be major inputs to the Arabian seas besides the autochthonous marine products. Future studies of the organic and inorganic biogeochemistry on river, dust, and coastal areas are needed to characterize the various regional sources, transformation, and diagenetic processes of the organic matter en route to the marine environment.     

Organic compound tracers of fine soil and sand particles during summer in the metropolitan area of Riyadh,Saudi Arabia

Soil and sand fine particles, which may be resuspended as fine dust in the atmosphere, contain a variety of anthropogenic and natural organic components. Samples of fine soil and sand particles (sieved to <125 μM) were collected from the Riyadh area in the summer of 2003 and extracted with a mixture of dichloromethane and methanol (3:1, v:v). The derivatized total extracts were analyzed by gas chromatography–mass spectrometry in order to characterize the composition and sources of the organic components. Both anthropogenic and natural biogenic inputs were the major sources of the organic compounds in these extracts. Discarded plastics and vehicular emission products were the major anthropogenic sources in the fine particles from populated areas of the city. Their tracers were plasticizers, UCM, n-alkanes, hopanes and traces of steranes. Vegetation was the major natural source of organic compounds in samples from outside Riyadh and included n-alkanols, n-alkanoic acids, n-alkanes, methyl alkanoates, sterols and triterpenoids. Carbohydrates had high concentrations (42–54%) in all samples and indicate sources from decomposition of cellulose and/or the presence of viable microbiota such as bacteria and fungi. The results were also compared with the data obtained in winter 2002 and showed that anthropogenic inputs were higher in summer than in winter, whereas the opposite trend was observed for natural inputs.     

Matrix protected organic matter in a river dominated margin: A possible mechanism to sequester terrestrial organic matter?

The provenance of organic matter in surface sediments from the northern Gulf of Mexico was investigated by analyzing the compositions of lipid biomarkers (n-alkanes, fatty acids, sterols) liberated after a series of chemical treatments designed to remove different organo-mineral matrix associations (i.e. freely extractable, base-hydrolyzable, unhydrolyzable). Bulk analyses of the organic matter (carbon content, carbon:nitrogen ratios, stable and radiocarbon isotopic analyses) were also performed on the intact sediments and their non-hydrolyzable, demineralized residue. We found recognizable lipids from distinct sources, including terrestrial vascular plants, bacteria and marine algae and zooplankton, within each of the isolated fractions. Based on the lipid signatures and bulk compositions, the organic matter within the unhydrolyzable fractions appeared to be the most diagenetically altered, was the oldest in age, and had the highest abundance of terrigenous lipids. In contrast, the base-hydrolyzable fraction was the most diagentically unaltered, had the youngest ages and was most enriched in N and marine lipids. Our results indicate that fresh, autochthonous organic matter is the most important contributor to base-hydrolyzable lipids, whereas highly altered allochthonous sources appear to be predominant source of unhydrolyzable lipids in the surface sediments from the Atchafalaya River shelf. Overall, the lipid biomarker signatures of intact sediments were biased towards the autochthonous source because many of the organic compounds indicative of degraded, terrigenous sources were protected from extraction and saponification by organo-mineral matrices. It is only after these protective matrices were removed by treatment with HCl and HF that these compounds became evident.     

Hydrocarbons generated by hydropyrolysis of suspended marine particulate organic matter: Relationship to the oceanography of the Black Sea and the Rhodes Gyre

The suspended particulate organic matter, SPOM, in the autumnal Black Sea has been characterised using catalytic hydropyrolysis (HyPy) of the total (bound plus free) lipid material. The technique, which generates maximum yields of volatile products from sediments, kerogens and phytoplankton, was followed using gas chromatography and gas chromatography/mass spectrometry. The generated alkanes were dominated by n-C₁₈, hypothesised to arise predominantly from unsaturated C₁₈ fatty acids. Steranes were generated from reductive conversion of free and bound sterols. The generation of branched alkanes and especially of hopanes provided formal evidence for the participation of bacteria in the mineralisation of the SPOM. Whereas similar distributions of n-alkanes were generated from SPOM sampled from different depths of the comparatively well-stirred Rhodes Gyre (eastern Mediterranean), mineralisation of the SPOM at each depth of the central Black Sea produced characteristic changes in the composition and concentration of the HyPy products. Depth profiles of the n-alkanes generated from SPOM in the region of the Rim Current were affected by the local hydrography. Polynuclear aromatic hydrocarbons (PAHs) of anthropogenic origin were present in the surface waters of the central Black Sea. Some methyl benzenes, thiophenes and pyrroles were also generated.     

Influence of car body repair activities on particulate matter pollution in a suburban area (Bouzaréah City,Algiers)

This work was carried out in order to study the impact of car body repair activities in Bouzaréah City (suburban area located at the plateau of Algiers) on the air quality. Atmospheric particulate matter was targeted using high-volume sampler. Precisely, the particulate organic matter such as organophosphate esters, especially triphenyl phosphate and tributyl phosphate, was evaluated by the implementation of accelerated solvent extraction. The results of the major extractable organic components revealed that tributyl phosphate and triphenyl phosphate were found at 5.46 and 1.78 ng m^?3, respectively. On the other hand, the classical pollutants such as n-alkanes, n-alkanoic monocarboxylic acids and polycyclic aromatic hydrocarbons were also measured, respectively, at average total concentrations of 35.8, 107.4 and 9.8 ng m^?3. This work confirms that sanding and painting car bodies may contribute to air pollution, especially after detection of diethylnitrosamine which remains the most surprising compound in the particulate matter due to its volatility and can be considered as emerging contaminant.     

Some chemical structural features of coals of differing maceral distributions and stratigraphy

Six North American bituminous coals (four Carboniferous and two Cretaceous) and eight maceral concentrates from Carboniferous British seams have been heated with tetralin and hydrogen to 400°C. The hexane-soluble parts of the products were separated by column chromatography and the fractions were then analyzed by gas chromatography/mass spectrometry. In all cases the aromatic hydrocarbon fraction was a very complex mixture containing a number of alkylated biphenyls, diphenylmethanes and -ethanes, and polycyclic compounds. However single ioncchromatography (m/z = 142) showed that, in addition to these, the products from five of the six American coals contained homologous series of long chain n-alkylnapththalenes with alkyl groups extending to C₁₆. In other fractions from the American coals, homologous series of n-alkylfurans and alkylphenols were found to be major components, as also were many alkylcyclohexenones related to isophorone. Extensive homologous series of n-alkyl aromatics were not detected in products from any of six vitrinites from British seams, though series of n- and iso-alkylphenols were found in the products from two sporinite concentrates. A number of polycyclic ketones apparently related to sterols were found in the products from the British vitrinites but not from any American coals. Reasons for what appear to be systematic structural differences between the American and British coals are discussed. Differences in stratigraphy are noted and it seems not impossible that differences in source vegetation are significant.     

Hydrothermal petroleum of Lake Chapala,Citala Rift,western Mexico: Bitumen compositions from source sediments and application of hydrous pyrolysis

Hydrothermal simulation experiments were performed with contemporary sediments from Lake Chapala to assess the source of the lake tars. The precursor-product relationships of the organic compounds were determined for the source sediments and their hydrous pyrolysis products. The pyrolysis products contained major unresolved complex mixtures of branched and cyclic hydrocarbons, low amounts of n-alkanes, dinosterane, gammacerane, and immature and mature hopane biomarkers derived from lacustrine biomass sources. The results support the proposal that the tar manifestations in the lake are not biodegraded petroleum, but were hydrothermally generated from lacustrine organic matter at temperatures not exceeding about 250 °C over brief geological times.     

Lipid,sterol and saccharide sources and dynamics in surface soils during an annual cycle in a temperate climate region

Multi-biomarkers were characterized in surface soils with different vegetation during an annual cycle in Oregon, U.S.A., to study the composition and dynamics of soil organic matter (SOM). The major compound classes identified include saccharides, steroids, terpenoids, and homologous series of aliphatic lipids (n-alkanoic acids, n-alkanols, and n-alkanes). Saccharides, n-alkanoic acids, and sterols were the most dominant compound groups identified in Ryegrass field soils, whereas n-alkanoic acids, n-alkanols, and sterols were dominant in soils under conifer and deciduous vegetation. Plant species, instead of microbial organisms, was found to be the primary source influencing the concentration and distribution of the major biomarker tracers in the studied surface soils. Over an annual cycle, concentrations of higher plant lipids such as monoacyl glycerides, sterols, n-alkanoic acids and total wax were higher during summer (especially June–August). During fall into winter, the concentrations of all compounds decreased to steady state levels due to cessation of de novo synthesis and concomitant biodegradation and remineralization of detritus. Sucrose and glucose reached maximum concentrations during spring (especially March–May), which could be related with plant growth, especially rootlets in the soils. Mycose, the microbial/fungal metabolite, maximized during late summer, suggesting the concomitant increase of microbial/fungal activity with the increasing primary production. The composition and variation of biomarkers observed over an annual cycle improved our understanding of SOM dynamics in temperate soils, which could also be linked to regional and global carbon cycles.     

Finely dispersed brown carbon in a smoggy atmosphere

It is shown that the absorption capacity of smoke aerosol during mass forest and forest–peat fires is determined to a considerable degree by light absorbing organic compounds or brown carbon. According to the data from the AERONET global network of stations [1], the absorption spectra of smoke aerosol vary significantly if airborne particulate matter is contained in brown carbon. It is established that in several cases, the absorption spectra of smoke aerosol are approximated with satisfactory accuracy by exponents. It is shown that the finely dispersed (submicron) fraction of the smoke aerosol makes a major contribution to its optical characteristics in the 0.44–1.02 μm spectral region. Strong variation in the single scattering albedo is discovered in the presence of brown carbon in the smoke aerosol. It is shown that the optical characteristics of coarsely dispersed and finely dispersed fractions of smoke aerosol differ considerably.     

Lipid biomarker analysis of cyanobacteria-dominated microbial mats in meltwater ponds on the McMurdo Ice Shelf,Antarctica

The lipid biomarker composition of microbial mat communities from three meltwater ponds from the McMurdo Ice Shelf, Antarctica, was investigated for the first time. Hydrocarbons, ether-linked components, fatty acids (FAs), wax esters, hopanols and sterols from Fresh, Orange and Salt Ponds were analysed. The dominance of cyanobacteria and the presence of bacterial sulfate reducers were confirmed using signature FAs in all three mats. Wax ester analysis suggested the presence of Chloroflexus spp. The dominance of short chain hydrocarbons, wax esters and FAs indicated that microorganisms are the major source of organic matter in these meltwater ponds. A variety of sterols were present in different relative abundances. The greatest diversity of sterols was in Salt Pond, followed by Fresh Pond, which was attributed to differences in the present eukaryotic diversity. Lipid profiles of the three communities were similar despite the presence of a salinity gradient. Analysis of lipid biomarkers allowed the creation of profiles for these unique Antarctic cryo-ecosystems. This will assist in the comparison of present and past microbial communities and in the monitoring of Antarctic biodiversity in response to global climate change and other environmental perturbations.     

Extractable and bound lipid components in a freshwater sediment

Lipids directly extracted by organic solvents from sediment of a small eutrophic lake (Crose Mere) comprise free hydrocarbons, alcohols, sterols and monocarboxylic acids together with an esterified component yielding additional amounts of alcohols, sterols and monocarboxylic acids on saponification. The carbon-number distributions of the free lipid compound classes indicate a major contribution from allochthonous sources whereas the distributions within compound classes released on saponification are attributable to autochthonous sources. The more advanced state of decomposition of terrestrial biota relative to aquatic biota and the composition of the source organisms are postulated to be factors leading to the differences in distribution of corresponding free and esterified lipid classes. Acid hydrolysis of solvent-extracted sediment liberates additional lipid constituents belonging to the above compound classes. Of these bound lipids, the n-alkanes and branched/cyclic alkanoic acids show a more significant bacterial contribution than do the corresponding components obtained by solvent extraction alone.     

Organic geochemical studies of a recent Inner Great Barrier Reef sediment—I. Assessment of input sources

The concentration of solvent-extractable (SE) and bound hydrocarbons, ketones, alcohols, sterols, monocarboxylic acids, hydroxyacids and α,ω-dicarboxylic acids of a surface sample (0–2.5 cm) from a recent carbonate tropical sediment taken at 60ft (18m) depth north-west of the Low Isles, North Queensland, are reported in detail. n-Alkanes are a minor constituent in the SE and bound hydrocarbon fractions with the major component being unresolved complex hydrocarbon material which is not an anthropogenic input. 6,10,14-Trimethylpentadecan-2-one derived from chlorophyll, as well as phytol, were identified as major components of the ketone and alcohol constituents. No chlorophyll pigments or pigmented degradation products were present, implying degradation before incorporation into the sediment. This conclusion is consistent with evidence for higher-plant input in which all sensitive lipids have been degraded before incorporation. SE and bound fractions have been studied in detail and show considerable differences probably arising from the importance of viable biomass in the SE fraction. Inputs to the sediment are determined as higher plants, algae, bacteria, fungi and meiofauna in descending order of importance.     

Biogeochemistry of particulate organic matter from lakes of different trophic levels in Switzerland

Biomarker compositions of particulate organic matter (POM) from the oligotrophic Lake Brienz and the eutrophic Lake Lugano (both Switzerland) are compared, in order to obtain information about organic matter (OM) production and transformation processes in relation to water column stratification. Eutrophic conditions in Lake Lugano are reflected by enhanced alkalinity, elevated total organic carbon (TOC) and chlorin contents compared with Lake Brienz. Lower δ¹³C values of dissolved inorganic carbon (DIC) in Lake Lugano reflect enhanced OM respiration in the water column.Differences in OM dynamics between both lakes, as well as seasonal variations, are evidenced by TOC-normalised concentration profiles of total fatty acids (FAs) and total neutrals. In Lake Brienz, the results reflect the relative contributions of primary productivity and refractory, allochthonous OM to POM, governed by particle load and interflows due to density stratification. The depth trends at Lake Lugano are a result of high primary productivity, water column stratification and associated particle load in the upper layers, as well as microbially induced degradation close to the chemocline and greater preservation under anoxic conditions. Minor differences exist with regard to the OM composition. In both lakes, FA distributions and the composition of n-alkanols indicate a predominant autochthonous OM source (algae, zooplankton, bacteria). Inputs of OM from diatoms are reflected in highly-branched isoprenoid (HBI) alkenes, 16:1 n-FAs and 24-methylcholesta-5,22-dien-3β-ol (either epibrassicasterol or brassicasterol). Differences in relative proportions of n-C₁₆ vs. n-C₁₈ FAs and alkanols, respectively, as well as in the percentages of C₂₇, C₂₈ and C₂₉ sterols relative to the sum of sterols are related to differences in the abundances of chrysophytes, diatoms and green algae within the euphotic zone of both lakes as well as in bacterial activity and soil in-wash. High relative proportions of cholesterol in the autumn samples, most pronounced at Lake Lugano, were attributed to an increased input from zooplankton grazing in the water column.Differences in OM degradation processes are reflected in slightly higher chlorin index values and higher relative proportions of saturated vs. unsaturated n-FAs in Lake Lugano. Higher contents of branched chain FAs, 16:1ω7 n-FA, and enhanced 18:1ω7/18:1ω9 n-FA ratios suggest enhanced bacterial biomass in the water column of Lake Lugano close to the chemocline. Increasing proportions of saturated n-FAs and n-alkanols with increasing water depth, most distinct in the autumn for both lakes, argue for intensified bacterial activity and degradation of OM during autumn. High relative contents of sterols and low n-alkanol concentrations in POM close to the chemocline at Lake Lugano during spring are interpreted to reflect higher primary productivity in the photic zone, OM export to the deeper parts and enhanced degradation rates of more labile constituents (i.e. C₁₃–C₂₀ n-alkanols), as compared to Lake Brienz.     

Composition and sources of extractable organic matter from a sediment core in Lake Kivu,East African rift valley

Lake Kivu is a gas-charged East African rift lake with currently anoxic bottom water. The extractable compounds and residual organic matter of a short sediment core have δ¹³C values typical of lacustrine microbial detritus. The total extracts consist primarily of polar compounds such as n-alkanoic acids, hydroxyalkanoic acids, triterpenoids, steroids and monosaccharides, with minor amounts of n-alkanes and n-alkanols. These tracer compounds and δ¹³C values indicate that the organic matter in the surficial and deeper sedimentary record was dominated by bacterial sources. The sapropelic sediment between these horizons contains organic matter from primarily algal with lesser bacterial input. Terrestrial organic markers are minor in all samples. The major fractions of the compounds in the total extracts were oxidized in the upper water column prior to transit through the anoxic bottom water to sedimentary deposition. The sapropelic horizon may reflect lake water turnover with ventilation or hydrothermal activity and consequently increased algal blooms.     

Composition of particulate organic matter in the southern Chesapeake Bay: Sources and Reactivity

The distribution of two classes of lipid biomarker compounds (fatty acids and sterols) was used in conjunction with several bulk parameters (total suspended solids, chlorophyll a, and particulate carbon and nitrogen concentrations) to examine spatial and temporal variability in the sources of particulate organic matter (POM) important to southern Chesapeake Bay. Based on these geochemical parameters, we found that suspended and sedimentary organic matter in the southern Chesapeake Bay is derived from autochthonous sources including a mixture of fresh and detrital phytoplankton, zooplankton, and bacteria. The dominant factor contributing to temporal variability during our study was phytoplankton productivity. Enrichments in particulate organic carbon, chlorophyll a, total fatty acids, total sterols, and a number of biomarkers specific to phytoplankton sources were found in particles collected from surface (1 m) and deep (1 m above the bottom) portions of the water column at several sites during the spring bloom in March 1996 and during a localized bloom in July 1995. Comparison of sites at the mouths of two tributaries (York and Rappahannock rivers) to southern Chesapeake Bay with two sites located in the bay mainsterm indicates spatial variation in the composition of POM was not significant in this region of the bay. The energetic nature of this region of the Chesapeake Bay most likely contributes to the observed homogeneity. Comparison with biomarker studies conducted in other estuaries suggests the high levels of productivity characteristic of the Chesapeake Bay contribute to high background levels of POM.