Research Comprehensive Characterization of Atmospheric Organic Matter in Fresno, California Fog Water PIERRE HERCKES Arizona State University, Department of Chemistry and Biochemistry, Tempe, Arizona 85287-1604 JERRY A. LEENHEER U.S. Geological Survey, Building 95, MS408, Federal Center, Denver, Colorado 80225 JEFFREY L. COLLETT, JR.* Colorado State University, Department of Atmospheric Science, Fort Collins, Colorado 80523-1371
Fogwater collected during winter in Fresno (CA) was characterized by isolating several distinct fractions and characterizing them by infrared and nuclear magnetic resonance (NMR) spectroscopy. More than 80% of the organic matter in the fogwater was recovered and characterized. The most abundant isolated fractions were those comprised of volatile acids (24% of isolated carbon) and hydrophilic acids plus neutrals (28%). Volatile acids, including formic and acetic acid, have been previously identified as among the most abundant individual species in fogwater. Recovered hydrophobic acids exhibited some properties similar to aquatic fulvic acids. An insoluble particulate organic matter fraction contained a substantial amount of biological material, while hydrophilic and transphilic fractions also contained material suggestive of biotic origin. Together, these fractions illustrate the important contribution biological sources make to organic matter in atmospheric fog droplets. The fogwater also was notable for containing a large amount of organic nitrogen present in a variety of species, including amines, nitrate esters, peptides, and nitroso compounds.
ticulate matter, commonly referred to as water-soluble organic carbon (WSOC). The most popular approach uses anion exchange chromatography followed by nuclear magnetic resonance spectroscopy (NMR) (21, 22). Other approaches include capillary electrophoresis (CE) followed by UV detection (23) or liquid chromatography (LC) followed by mass spectrometry (MS) (24). An early comprehensive approach to water-soluble organic characterization (25, 26) concluded that extracts of particulate matter show humic-like characteristics and therefore used the phrase humic-like substances (HULIS) to identify this material. Later studies confirmed that the watersoluble organic fraction in aerosols, fog, and cloud droplets has properties (absorbance, fluorescence, acidity, behavior in CE separations, and elution on size exclusion column) similar to humic material and is made up of a polyacidic species (23). Kiss and co-workers (24) used LC followed by UV and MS detection to study organic matter in fogs, concluding that the mass spectrum was similar to that of fulvic acid. Recently, chemical fractionation coupled with NMR characterization was used to investigate the watersoluble part of carbonaceous aerosol particles (27, 28). Despite these valuable efforts, our understanding of the composition and structure of organic matter in fogs and clouds remains primitive. To improve understanding of organic matter composition and structure in the atmosphere, and to suggest fruitful directions for future organic speciation efforts, a new approach, originally developed for organic matter characterization in surface waters (29), was applied to fogwater. Fogs were sampled in Fresno, CA, because this city frequently experiences severe winter stagnation episodes accompanied by radiation fog formation, yielding polluted fogwater high in organic content. Collected fogwater was characterized by fractionating and isolating organic matter into several classes based upon molecule size, solubility, polarity, and acid/base/ neutral characteristics. Resulting organic isolates were then characterized by infrared and 13C nuclear magnetic resonance (NMR) spectrometry and elemental analyses. This is the first study to obtain sufficient sample fractions of this type of fogwater to characterize the organic matter by 13C NMR spectrometry; a similar approach has been recently applied to aerosol samples (30).
Experimental Procedures Introduction In recent years, efforts have increased to characterize organic material present in the atmospheric aqueous phase (cloud/ fog and rain droplets). Early studies often included only measurements of total (TOC) (1-3) or dissolved organic carbon (DOC) (4-6). Most organic speciation studies addressed individual organic species or compound classes, including organic acids (7-9), aldehydes (2, 10-13), pesticides (14, 15), phenols (16-18), n-alkanes (19), and other biomarkers (20). Recently, more extensive speciation efforts suggest that these approaches can account only for a minor part of the TOC present in fogs (20). Some investigators have proposed new approaches to characterize, in a more comprehensive way, the organic matter in cloud or fog droplets and in the water-soluble organic fraction of par* Corresponding author phone: (970)491-8697; fax: (970)491-8449; e-mail:
[email protected]. 10.1021/es0607988 CCC: $37.00 Published on Web 12/09/2006
2007 American Chemical Society
Sampling. Fog and aerosol samples were collected in the winter of 2003/2004 in Fresno, CA, at the experimental farm of California State University, Fresno. The sampling site was situated in a large agricultural plot but relatively close (hundreds of meters) to major highways and residential areas. Samples were collected during a 3 week period with an extra large version of the Caltech Active Strand Cloudwater Collector CASCC (31). This stainless steel collector operates at a flow rate of approximately 40 m3/min, allowing for the collection of large fogwater sample volumes, critical for the type of analyses reported here. Sampling intervals ranged from 1 h to a few hours. All samples were combined in a large 20 L polyethylene carboy to which 48 g of sodium hydrogen sulfate was added as a preservative to lower the sample pH to 1.7 and suppress biological activity. For each sample added to the composite, the weight was recorded and an aliquot was taken for TOC determination. The total cumulative sample volume, 4.95 L, was stored refrigerated and in the dark until analysis, which began 2 weeks after the end of VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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bed column, and 0.5 L XAD-4 resin bed column at the flow rate reported previously. The sample was followed by 1.0 L of deionized water rinse, and the MSC-1H sample was disconnected from the other two columns. The MSC-1H column was rinsed with additional deionized water until the conductivity of the effluent was 1000 ˚C) combustion temperature of the elemental analysis as compared to the TOC combustion (680 ˚C) (34). Low particulate elemental carbon (EC) scavenging efficiencies in the San Joaquin Valley fogs (36), however, suggest that the fog EC content and resulting recovery bias are probably small. The largest carbon mass fractions were observed for the hydrophilic acids plus neutrals fraction and the volatile acids fraction, accounting for 28.3 and 24.3% of the recovered OC, respectively. The particulate organic matter, hydrophobic VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Infrared spectra of organic matter fractions from Fresno fogwater.
FIGURE 4. fogwater.
13
C NMR spectra of organic matter fractions from Fresno
acids, and transphilic acids plus neutral fractions each contained between 9 and 14% of the recovered OC, with contributions of 7.6 and 5.3% for the hydrophobic neutrals and bases fractions. Infrared spectra of organic matter fractions from Fresno fogwater are presented in Figure 3, and the 13C NMR spectra are presented in Figure 4. The particulate organic matter fraction has a substantial inorganic component as indicated 396
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by its low organic carbon percentage (27.87%). Inorganic bands from 1200 to 400 cm-1 in the infrared spectrum of this fraction indicate a mixture of sodium hydrogen sulfate and sodium sulfate, suggesting that the dialysis step to remove the sodium hydrogen sulfate preservative was not complete. The broad bands near 1720, 1650, and 1550 cm-1 are indicative of carboxylic acids and amide groups in proteins and N-acetyl amino sugars. The 13C NMR spectrum of the particulate fraction indicates that the organic matter is predominately an aliphatic hydrocarbon (0-60 ppm) with small amounts of C-O linkages, aromatic carbon, and carbonyl groups in acids and amides. The low aromatic carbon content is surprising because this particulate fraction had the appearance of black soot. The volatile acids fraction represents the second most abundant carbon fraction in the fogwater. The 13C NMR spectrum for this fraction (Figure 4) contains two sharp peaks (26 and 182 ppm) indicative of sodium acetate. The peak at 172 ppm is indicative of sodium formate, although the infrared spectrum (Figure 3) shows a very small peak at 880 cm-1 that indicates sodium carbonate, which occurs at 171 ppm in the 13C NMR spectrum, also is present. Small peaks at 12 and 15 ppm represent terminal methyl groups in sodium salts of longer chain volatile acids such as propanoic and butyric acids. Peaks for sodium sulfate, derived from the sodium bisulfate preservative, also are present in this fraction. The hydrophobic neutral fraction gave several peaks in the infrared spectrum (shown in Figure 3) that are not typically found in infrared spectra of natural organic matter studied in the surface media. Peaks at 1630 and 1280 cm-1 are indicative of nitrate esters. Peaks at 1650 and 1380 cm-1 may indicate nitrite esters, but the intensity ratio of the 16501380 cm-1 peaks does not match published spectra for nitrite esters (37). The broad, intense peak at 1380 cm-1 may indicate nitroso compounds (both N-nitroso and C-nitroso) that give peaks in the 1400-1300 cm-1 range (37). Peaks for sodium sulfate also are found in the infrared spectra. The major peaks in the 13C NMR spectrum for this fraction represent aliphatic (0-60 ppm) and aromatic (100-160 ppm) hydrocarbons that give this fraction its hydrophobic properties. Minor amounts of carboxylic acids (175 ppm and at 1710 cm-1 in the infrared spectrum) also are found in this fraction. The hydrophobic acid fraction also yielded peaks in the infrared spectrum at 1630 and 1280 cm-1 that indicate the presence of nitrate esters, but no peaks indicative of other nitrogen functional groups were found. The other peaks in the infrared spectrum are typical for an aquatic fulvic acid (see typical peak assignments in Table 1 in the Supporting Information). The 13C NMR spectrum of this fraction, however, indicates a smaller carboxylic acid content (at 175 ppm), as compared to the aliphatic and aromatic carbon content, than is typical for an aquatic fulvic acid. The highly polar nitrate ester groups found in this fraction may increase the solubility of this fraction so that it fractionates into the hydrophobic acid fraction rather than into the hydrophobic neutral fraction. The transphilic acid plus neutrals fraction appears more polar than the hydrophobic neutral and hydrophobic acid fractions and has no peaks indicative of nitrate or nitrite esters or nitroso compounds. Observed amide I (1650 cm-1) and amide II (1550 cm-1) bands are indicative of proteins and peptides. The 13C NMR spectrum of this fraction indicates a smaller carboxylic acid content (175 ppm) and hydroxyl content (80 ppm), as compared to aliphatic carbon content, than is typical for this fraction in natural surface waters. The hydrophilic acid plus neutrals fraction composition is dominated by hydroxy-carboxylic acids. The infrared spectrum for this fraction (Figure 3) shows bands that fit the published spectra (37) for lactic acid (1128 and 1046 cm-1) and glycolic acid (1090-1). The 13C NMR spectrum of this
fraction (Figure 4) has a small anomeric carbon peak near 100 ppm, which indicates that carbohydrates compose a small portion of this fraction. The base fraction comprises the largest overall mass of all the isolated fogwater fractions (Table 1) but the lowest fraction of fogwater carbon (Figure 2). Consequently, the base fraction infrared spectrum (Figure 3) represents primarily inorganic substances. The ammonium ion is indicated by bands near 3170 and 1400 cm-1. Central California fogs are known to have high ammonium contents. The amount observed here also reflects a combination of ammonium addition and removal through the sample fractionation process. The large bands from 950 to 750 cm-1 fit a mixture of chromate, vanadate, and molybdate oxyanions, likely reflecting contaminants from the stainless steel sampler chosen for this organic characterization effort. The base fraction 13C NMR spectrum (Figure 4) indicates that the organic bases are a mixture of aliphatic amines (spectral intensity at 40-55 ppm of C-N linkage) and aromatic amines (shoulder at 145 ppm). These are tentative assignments that should be confirmed in future studies by independent methods. Because this is the first application of this specific separation and analysis method to atmospheric samples, it is difficult to directly compare our findings with those from other investigators. The approach most similar to ours (27, 28) examined properties of aerosol organic carbon using a single separation on XAD8 and, in some cases, further characterization by size exclusion chromatography. Measurements of St. Louis winter aerosol by these investigators revealed a greater abundance of hydrophobic material than observed in the Fresno fogs. This difference is not surprising given the expected bias toward scavenging of more hydrophilic particles during fog formation, although differences in aerosol characteristics between St. Louis and Fresno could also be a factor.
Discussion Previous efforts to characterize organic matter in fogwater samples have typically focused on identifying individual organic compounds, leaving a large fraction of the organic matter uncharacterized. One exception is the use of an ion exchange separation, followed by TOC and 1H-NMR measurements of three eluted fractions (21), which provided a broader classification of organic matter in fog samples from Italy. The work presented here represents a more comprehensive fractionation and analysis approach, developed initially for application to natural organic matter in surface waters and soils (29, 32), applied for the first time to fogwater organic matter characterization. Recovery of approximately 80% of the carbon in the fractionated fog sample is an excellent result considering that collecting sufficiently high TOC fogwater for this type of work is challenging and that handling and transfer losses tend to increase percentage carbon losses as the sample mass decreases. Further, volatile neutral compounds, such as alcohols, aldehydes, and ketones, are not recovered by the fractionation and isolation procedure used in this study. Formaldehyde and other volatile compounds are known constituents of organic carbon in fogwater samples (e.g., ref 20). Together, formaldehyde, glyoxal, and methylglyoxal constituted approximately 10% of the dissolved organic carbon in the main Fresno fog events (36), with smaller contributions from other carbonyls. The particulate organic matter fraction appears to contain a substantial amount of biological material. The presence of proteins and straight chain lipids is indicative of bacterial cells. Microorganisms, including bacteria, fungi, and algae, have been reported previously in fog and cloud samples (38, 39). Previous studies have also found that proteins are major
organic nitrogen species in fog samples from California’s Central Valley (40). It is unlikely that the bacterial cells grew in the preserved sample during storage at pH 1.7. The black coloring of this fraction is suggestive of elemental carbon; however, the aromatic carbon content is small. An atomic H/C ratio of 1.56 in the particulate fraction confirms that there is a substantial aliphatic hydrocarbon content. The composition and concentration of the volatile acid fraction was dominated by acetic acid, with much smaller amounts of formic acid and longer chain fatty acids. This pattern is consistent with past measurements of San Joaquin Valley fogs (20, 41). The amounts of acetate and formate in the Fresno fog were confirmed by analysis by ion chromatography. The hydrophobic neutral fraction is minor in mass percentage, but its composition is potentially the most important regarding health concerns. Many nitroso compounds, especially N-nitroso compounds, are carcinogens. Dialkyl nitrosamines and nitrosomorpholine have been previously reported in atmospheric gas samples (42). Preliminary testing of individual fog samples for small chain nitrosamines, including N-nitrosodimethylamine, using solidphase microextraction coupled to gas chromatography with nitrogen chemiluminescence detection (43) found these species present at concentrations up to 240 ng/L. Organic nitrate esters, previously observed in aerosols (44), also were detected in this fraction. The hydrophobic acid fraction also has a substantial content of organic nitrate esters. The bulk composition of the hydrophobic acid fraction is similar to an aquatic fulvic acid derived from the oxidation of terpenoid precursors (44), confirming previous observations of the fulvic acid character of organic fog matter (24). Fresno fog results, however, do not support a 40% contribution as previously proposed in a model mixture (45). Peptides found in the transphilic acid plus neutral fraction and hydroxy acids found in the hydrophilic acid plus neutral fraction suggest a biotic origin for much of the organic material in these fractions. Together, these two fractions comprise about 40% of the carbon in the fog. The ratios of organic mass (OM) to organic carbon (OC) in the transphilic acid plus neutral fraction and the hydrophilic acid plus neutral fraction are 2.2 and 3.1, respectively, considerably greater than ratios typically used to convert organic carbon to organic mass in atmospheric aerosols (46). The fog OM/OC ratios could be slightly overestimated due to inorganic contaminants increasing the OM gravimetric measurement; however, the corresponding IR spectra do not reveal any substantial inorganic constituents. Amines found in the base fraction can be precursors for the nitroso compounds found in the hydrophobic neutral fraction, through their reaction with inorganic nitrites found in California fogs (47, 48), including those sampled here (36). The presence of amines in this fraction is not surprising given high organic nitrogen contents observed in these Fresno fogs (36). Other investigators also have reported substantial organic nitrogen in San Joaquin Valley fogs (40), including small chain amines and amino acids.
Acknowledgments We are grateful to S. Youngster, T. Lee, and A. Simpson for field assistance and to C. Krauter (CSU Fresno) for hosting the measurements. We thank M. Suffet and J. Grebel from UCLA for providing speciated nitrosamine measurements. Support for this work was provided by the National Science Foundation (ATM-0222607). Any use of trade, firm, and product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Supporting Information Available Infrared frequency bands for various structures of organic matter isolates as well as infrared spectral peaks of inorganic solutes and structural assignments for 13C NMR spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review April 4, 2006. Revised manuscript received November 2, 2006. Accepted November 3, 2006. ES0607988
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