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Chemical characteristics of chromophoric dissolved organic matter in rainwater
Atmospheric Environment 43 (2009) 2497–2502
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Chemical characteristics of chromophoric dissolved organic matter in rainwater Carrie Miller, Kelly G. Gordon, Robert J. Kieber*, Joan D. Willey, Pamela J. Seaton Department of Chemistry and Biochemistry, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5932, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 August 2008 Received in revised form 14 January 2009 Accepted 25 January 2009
Proton nuclear magnetic resonance (1H-NMR), UV absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy were used to define the chemical characteristics of chromophoric dissolved organic matter (CDOM) in whole and C18 extracted rainwater. The average total recovery of fluorescence determined from the sum of extract and filtrate fractions relative to the whole was 86% suggesting that 14% of fluorescent CDOM in rainwater is comprised of very hydrophobic material that cannot be eluted from the column. Half the fluorescence of rainwater was recovered in the filtrate fraction which is important because it suggests that 50% of the chromophoric material present in precipitation is relatively hydrophilic. The average spectral slope coefficient was smaller in extracted samples (16.3 9.0 mm1) relative to whole samples (18.9 2.8 mm1) suggesting that the extracted material contains larger molecular weight material. Approximately one-third of the total dissolved organic carbon (DOC) in rainwater exists in the extract fraction suggesting that a large percentage of the uncharacterized DOC in rainwater can be accounted for by these hydrophobic macromolecular species. The fluorescence of extracted samples is strongly correlated with total NMR integration and is most sensitive to aromatic protons suggesting that molecules in this region are the most important in controlling the optical properties of rainwater. The lower removal efficiency of CDOM in rainwater relative to surface waters or the water-soluble fraction of aerosols during solid phase extraction (SPE) suggests that rainwater contains significantly more hydrophilic chromophoric compounds which are compositionally different than found in these other aquatic matrices. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Chromophoric dissolved organic matter CDOM Rain
1. Introduction Chromophoric dissolved organic matter (CDOM) is a ubiquitous component of rainwater which plays a central role in a variety of important processes occurring in atmospheric waters. Previous research has shown that CDOM is the dominant chromophore in rainwater with the absorbance of samples decreasing exponentially between 250 and 550 nm (Kieber et al., 2006) and as such plays an important role in solar radiative transfer in the condensed phase affecting both the attenuation and spectral distribution of sunlight reaching the earth’s surface. Recent studies have also shown that significant photodegradation of CDOM occurs after exposure to simulated sunlight (Kieber et al., 2007) suggesting that these chromophoric compounds could be involved in a variety of photo mediated processes. Additionally, if CDOM constituents are surfaceactive they could have a significant impact on droplet population and consequently cloud albedo, by lowering the surface tension of atmospheric waters (Decesari et al., 2005; Facchini et al., 2000; Kiss et al., 2005). * Corresponding author. Tel.: þ1 910 962 3865; fax: þ1 910 962 3013. E-mail address: [email protected] (R.J. Kieber). 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.01.056
Despite the significance of CDOM to a variety of important processes occurring in atmospheric waters, virtually nothing is known regarding its source and chemical characteristics. In contrast, the composition of CDOM in surface waters and the watersoluble organic carbon (WSOC) fraction of atmospheric aerosols has been extensively studied. The composition of WSOC in atmospheric aerosols in particular has gained a great deal of attention recently because of their influence on cloud condensation (Graber and Rudich, 2006). Comparison of rain CDOM properties to surface waters and WSOC compounds was used to examine their potential importance as sources of CDOM in rainwater. The structural and chromophoric characteristics of CDOM were examined using solid phase extraction (SPE) coupled with spectroscopic analysis including UV absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy. SPE of CDOM from rainwater was accomplished via C18 cartridges employing a previously described extraction technique (Kieber et al., 2006). This method was chosen because earlier studies have demonstrated that C18 extraction is better able to retain the UV–visible and fluorescence characteristics of isolated chromophoric organic material relative to XAD (Amador et al., 1990). SPE utilizing C18 has also been used extensively with surface
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waters and water-soluble organic carbon (WSOC) from aerosol samples to isolate optically active DOM (Amador et al., 1990; Limbeck et al., 2005; Lombardi and Jardim, 1999; Parlanti et al., 2002; Varga et al., 2001). Whole rain and two fractionated samples from the C18 SPE, extract and filtrate were analyzed for each rain event. Comparison of optical properties in these three fractions yields important new information regarding the composition of CDOM because it provides insight into the relative hydrophobicity and molecular weight of material in the various subsets. Proton nuclear magnetic resonance (1H-NMR) spectroscopy was also used to provide structural analysis of the hydrophobic SPE fraction of rainwater CDOM. Integration of regions of the spectra, normalized to an internal standard, provides information regarding hydrogen distributions and major functional groups present in CDOM as well as their relative abundances. This includes the relative aliphatic and aromatic character of CDOM which, in combination with UV–visible and fluorescence analysis, sheds fundamental new insight into the chemical characteristics of CDOM which are most important in controlling its chromophoric properties. Earlier estimates indicate that less than half the dissolved organic carbon (DOC) in rainwater can be accounted for by specific carbon compounds (Likens et al., 1983; Willey et al., 2000). One of the most important uncertainties related to the biogeochemical cycling of DOC in atmospheric waters is the composition of the remaining DOC. Organic acids (primarily acetic and formic) are the largest contributor to rainwater DOC at this location (ca. 40%) while formaldehyde contributes 3%, acetaldehyde contributes 5%, and amino acids contribute 2%. One potentially important class of organic compounds which could contribute to the uncharacterized DOC pool in rainwater is macromolecular substances such as CDOM. A final novel aspect of the present study is that it defines the contribution of these uncharacterized hydrophobic carbon compounds to the DOC pool in rainwater. 2. Method 2.1. Rainwater collection and sample processing Rainwater was collected at the University of North Carolina Wilmington (UNCW) on an event basis between September 15, 2005 and September 6, 2006. The UNCW rainwater site is located approximately 8.5 km from the Atlantic Ocean and receives both terrestrial and marine influenced rainwater. Four AerochemMetrics (SCM) Model 301 Automatic Sensing Wet/Dry Precipitation Collectors, one containing a Pyrex beaker and three containing 2.2 L Teflon bottles, were used to collect rainwater. The Pyrex beaker and the Teflon bottles were replaced after each sampling. The beakers were cleaned by combustion at 450 C for at least 6 h and the Teflon bottles were rinsed with ultra clean water deionized water (MilliQ Plus Ultra-pure water system, 18.2 MU cm1, indicated as DIW) between each sampling. Blanks were periodically collected to ensure that there was no carry over of DOC between sampling events. Samples for DOC, UV absorbance, excitation-emission matrix (EEM) spectroscopy and 1H-NMR analysis were collected from the Teflon bottles and filtered using a 0.2 mm, acid-washed Gelman SuporÒ polysulfonone filter. Filtered samples were stored in the dark at 4 C until analysis. The rainwater samples were fractionated by solid phase extraction (SPE) using C18 Sep-Paks (Waters Chromatography, Milford, MA). The goal of this study was to examine the chemical properties of rain under natural pH conditions (4.92 during the current study) so pH adjustment was not used during the SPE procedure. Extractions on four rainwater samples were performed at natural pH and pH 3 to confirm that the difference in extraction efficiencies in rainwater and surface waters was not the
result of analytical differences in extraction pH (surface waters are typically acidified to approximately pH 3 before extraction). Before extraction, C18 cartridges were conditioned by washing twice with 5 mL of 10% water in methanol solution, followed by two 5 mL water aliquots. Room temperature filtered rainwater was extracted by drawing 50 mL through the C18 cartridge at a flow of 2 mL min1. The portion of the rainwater which was not retained by the C18 cartridge (filtrate) was collected and refrigerated until analysis. Two 5 mL aliquots of DIW were passed through the cartridges after the rainwater to remove residual salts. The samples were then eluted off the C18 cartridges with two 3 mL aliquots of 10% water in methanol. Following elution, the solvent was removed under reduced pressure (Buchi Rotavapor, Model RE 111, Switzerland) for 30 min. The sample was placed under vacuumed conditions for an additional 1 h (Sargent-Welch Model 1400, Skokie, IL) to ensure complete removal of the methanol. DIW was then added to the dried extracts, to bring the samples up to the preextraction volume, and the samples were sonicated for 30 s. This fraction was referred to as the extract fraction of the rainwater. For each rain event, three samples were collected: whole, C18 extract and filtrate. Sample aliquots for DOC analysis were preserved by acidified to pH 2 after the fractionation step (Willey et al., 2000). Samples for optical analysis were not acidified. Both aliquots were stored in the dark at 4 C until analysis. 2.2. Optical analysis The optical activity of the CDOM in rainwater was determined using UV absorbance and EEM fluorescence spectroscopy (Kieber et al., 2006). A Varian Cary IE dual-beam spectrophotometer (2 nm slit width) was used for absorbance measurements between 240 and 800 nm and a 10 cm spectral cell was used for all readings. Since nitrate and hydrogen peroxide absorb UV light, the absorbance of these species was subtracted from the total absorbance so that the reported values reflect the absorbance from the CDOM in the rainwater (Kieber et al., 2006). Absorbance coefficients (a) were determined at each wavelength (a ¼ 2.3 A pathlength1; A ¼ absorbance) and spectral slopes were determined using an exponential fit of the absorbance between 240 and 400 nm (Kieber et al., 2006). Absorbance measurements were only performed on the whole and extract fraction of the rainwater. The filtrate fraction could not be analyzed due to an absorbent contaminant, either from the methanol used to condition the C18 cartridge or from something leaching off of the cartridge, which was detected when DIW was used for extraction blank determination. Excitation-emission matrix (EEM) fluorescence measurements were performed using a Jobin Yvon SPEX Fluoromax-3 scanning fluorometer equipped with a 150 W Xe arc lamp and a R928P detector. EEMs were created using emission scans between 250 and 550 nm and excitation scans between 250 and 500 nm. FLToolbox 1.91 (Wade Sheldon, University of Georgia) developed for MATLABÒ was used for post-processing the fluorescence scans in order to eliminate Rayleigh and Raman scattering peaks (Zepp et al., 2004). The data were normalized to quinine sulfate equivalents (QSE). Fluorescence values presented in the current paper are threedimensional surface integrations of EEM’s of the entire emission over the corrected fluorescence matrix (Moran et al., 2000). 2.3. Dissolved organic carbon and ancillary data Dissolved organic carbon (DOC) was measured by high temperature combustion using a Shimadzu TOC 5000 total organic carbon analyzer equipped with an ASI5000 autosampler. The concentration of DOC was determined on whole and extract rainwater fractions but not on the C18 filtrate since this fraction was
C. Miller et al. / Atmospheric Environment 43 (2009) 2497–2502
contaminated with methanol from conditioning the cartridge. DOC concentrations in extract blanks (pH 4.5 synthetic rainwater treated as a sample in place of rainwater) were very low and near the detection limit indicating that all detectable traces of methanol in the extract as measured by DOC analysis were removed. The detection limit for this method is 5 mM (Willey et al., 2000). Rainwater samples were also analyzed for pH, hydrogen peroxide (Kieber and Helz, 1986) and inorganic anions (chloride, nitrate and sulfate) by suppressed ion chromatography. Volume-weighted averages (VWA) were determined for each analyte using the concentration and volume of rainwater for each rain event. 2.4. Nuclear Magnetic Resonance (NMR) Liquid phase 1H-NMR were recorded on a Bruker Avance 400 MHz NMR spectrometer. The C18 SPE extraction method described above was also used to extract samples for 1H-NMR analysis. The extracts were reconstituted in 0.60 mL of D2O and 20 mL internal standard (187 mM 3-Trimethylsilyl-propionic-2,2,3,3d4 acid, sodium salt). Samples were analyzed using a presaturation experiment (zgpr, 1024 scans) where: pulse angle ¼ 30 (P1 ¼ 9.00 ms), D1 ¼ 3.00 s, and PL ¼ 60 dB using an inverse gradient probe. After the acquisition, the FID was transformed with exponential multiplication (LB ¼ 5). Phasing, calibration, and integration were done identically for all experiments to allow for comparisons of different rain events. After phasing, the trimethylsilyl peak of the standard was set to zero ppm and normalized for each sample. Several regions of the 1H-NMR spectra were integrated including the H–Ar (9.0–6.5 ppm), the H–C–O (4.5–3.4 ppm), the H–C–C¼ (3.3–1.9 ppm), the CH2 (1.9–1.1 ppm), and the CH3 (1.1–0.5 ppm). Samples were all normalized to an internal standard (0.1 to 0.1 ppm) to allow for comparison of different natures of extracted DOM. The 10–9.5 ppm and 6.5–6.0 ppm regions were also integrated to correct for baseline anomalies and drift. The peak area of the internal standard was integrated to one. The two regions that were used to correct for baseline drift were subtracted from the integration of the respected region after being corrected for the range of the region. Total abundances of the respective regions were then calculated as well as a percentage of the entire region.
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CDOM on the C18 SPE cartridges due to the hydrophobic nature of this material. The extract fraction of rainwater contained on average 36% of the total integrated fluorescence in rain samples while 50% of the fluorescence was recovered in the filtrate. This is significant because it suggests that half the chromophoric material present in these rain samples is relatively hydrophilic and is not effectively bound to the nonpolar stationary phase of the C18 cartridge. A significant positive correlation (p < 0.001, r ¼ 0.915) exists between the integrated fluorescence in the whole and extract samples (Fig. 1) suggesting that there is a fixed proportion of fluorescent material being extracted and that the distribution of extractable and nonextractable fluorescence in rainwater co-vary. Comparison of rainwater CDOM optical properties to surface waters and water-soluble atmospheric aerosols (WSOC) can be used to further examine the chemical characteristics of rainwater CDOM. Extractions using C18 cartridges removed 70–90% of the fluorescent material in both fresh and marine surface waters (Amador et al., 1990; Lombardi and Jardim, 1999; Parlanti et al., 2002). The high removal efficiency of CDOM using SPE is one factor which has led to the classification of surface water CDOM as hydrophobic humic-like material. High CDOM removal (75–95%) was also observed with WSOC from atmospheric aerosols (Limbeck et al., 2005). Acidification of rainwater to pH 3, the pH typically used with surface water extractions, increased the extraction efficiency in some rain events but decreased this efficiency in other samples. The difference was always less than 15% in either direction and therefore could not explain the large difference observed in the extraction of CDOM in rainwater relative to other water matrices. Earlier studies have suggested that CDOM in rainwater may be derived from surface waters and atmospheric aerosols (Kieber et al., 2006). The lower removal efficiency of CDOM in rainwater during SPE suggests that rainwater contains relatively more hydrophilic fluorescent compounds compared to surface waters or the watersoluble fraction of aerosols. This indicates that the CDOM compounds in rainwater are compositionally different relative to the chromophoric compounds in surface waters and the WSOC of aerosols. The lower removal efficiency of CDOM in rainwater during SPE compared to surface waters and aerosols also suggests that these are not the sole source of chromophoric material in rainwater.
3. Results and discussion 3.2. Optical activity – spectral slopes 3.1. Optical activity – EEM
Table 1 Volume-weighted average dissolved organic carbon (DOC) concentration, volumeweighted average integrated fluorescence and simple average spectral slopes of the whole, extract and filtrate fractions of rainwater. Integrated fluorescence values are reported as area under the EEM scans. Na: not analyzed. Fraction
DOC (mM)
Integrated Fluorescence (103)
Spectral slope
Whole Extract Filtrate
77 (20) 28 (7) Na
50 (2) 18 (1) 25 (5)
18.9 (2.8) 16.3 (9.0) Na
Spectral characteristics, such as the spectral slope coefficient calculated from non-linear least-square regressions of absorption 80
Extract Fluorescence (x10-3)
The average total recovery of fluorescence determined from the sum of extract and filtrate fractions relative to the whole was 86% (Table 1). This suggests that 14% of fluorescent CDOM in rainwater is comprised of very hydrophobic material that cannot be eluted from the nonpolar C18 column under conditions used in this study. Similar recoveries have been reported for WSOC extracted from atmospheric aerosols in which approximately 25% of light absorbing DOM was not recovered after C18 SPE (Limbeck et al., 2005; Varga et al., 2001). The loss of chromophoric materials in these earlier studies was attributed to the permanent absorption of
60
40
20
0
0
50
100
150
200
250
Whole Fluorescence (x10-3) Fig. 1. Fluorescence in the extract fraction of rainwater as a function of fluorescence in whole rainwater (R ¼ 0.915, slope ¼ 2.81). Fluorescence values are total integrated fluorescence of the EEM scans reported in quinine sulfate equivalents (QSE) in ppb.
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coefficient versus wavelength, can be used to infer compositional differences in whole and extracted rainwater. Previous work on surface waters has suggested an inverse relationship between DOM molecular weight and spectral slope coefficient (Chin et al., 1994; De Haan, 1993). Chin et al. (1994) reported a positive correlation between weight-averaged-molecular weight and specific absorptivity of humic substances at 280 nm. Further investigations have shown the relationship to be fairly robust for whole water DOM and DOM isolated by tangential-flow ultrafiltration and can cautiously be used for ‘‘ballpark estimates’’ of the molecular weight of DOM in the absence of direct molecular weight measurements (Chin et al., 1994; Everett et al., 1999). The average spectral slope coefficient determined for whole rainwater samples in this study (18.9 mm1, Table 1) is very similar to the previously reported spectral slope coefficient (19.3 mm1) for rainwater collected in 2002 and 2003 at this site (Kieber et al., 2006). This suggests that the absorbing compounds in the present study are compositionally similar to rainwater analyzed previously at this location. The average spectral slope coefficient was smaller in extracted samples (16.3 9.0 mm1) relative to the whole (18.9 2.8 mm1) indicating that the extracted material contains larger molecular weight material. This difference in spectral slopes was, however, relatively small suggesting that the hydrophobicity of DOM is not controlled entirely by molecular weight.
DOM may be somewhat greater than the storm-to-storm variability of fluorescent compounds. Organic acids comprise between 14 and 40% of the total DOC in rainwater (Avery et al., 2006; Likens et al., 1983; Williams et al., 1997). Earlier studies at the UNCW rain site demonstrated that the organic acid content was on the high end of the range of observed at other locations (36–40%), however a recent study suggests that small organic acid concentrations may be decreasing in rainwater nationwide including Wilmington, NC (Willey et al., 2006). With a VWA DOC concentration of 77 mM in Wilmington rain, the approximate organic acid concentration would be in the range of tens of mM. Due to the hydrophilic nature of organic acids, these compounds would be present in the filtrate portion of the C18 fractionated rainwater. The DOC concentration in the filtrate was not determined but the concentration can be estimated (approximately 50 mM) from the difference between the DOC concentration in the whole and extracted rainwater fractions. Although organic acids comprise a large fraction of the DOC in the filtrate fraction of rainwater, these compounds cannot explain the optical activity of the organic matter in this fraction since small organic acids do not fluoresce. This estimate of organic acid content in rain suggests the fluorescent activity of DOM in the filtrate portion of rainwater is likely due to compounds that are highly fluorescent but at relatively low concentrations.
3.3. DOC
3.4.
Dissolved organic carbon concentrations were determined for whole and extracted DOM on a subset of samples (n ¼ 18). The VWA DOC concentration measured in this study (77 mM in the subset, 83 mM for rain collected during the whole sampling year) falls within the range of volume-weighted DOC average concentrations for the preceding two years (2003 – 87 mM; 2004 – 56 mM) for Wilmington, NC rainwater (Willey et al., 2006) suggesting this is a normal year for DOC in precipitation at this location. Approximately one-third of the total DOC in rainwater could be accounted for in the extract suggesting a large fraction of DOC in rainwater is comprised of relatively hydrophobic material (Table 1). A significant positive relationship was observed between the whole and extractable DOC concentrations (Fig. 2; p < 0.001, r ¼ 0.711), although this relationship is not as strong as observed with whole and extractable integrated fluorescence (Fig. 1). This suggests that the storm-to-storm variability of the nonfluorescent component of
One important question based on the current understanding of CDOM in rainwater relates to its chemical characteristics. 1H-NMR analysis of the C18 extract fraction in rainwater provides spectra which appear as relatively continuous and broad distribution of signals suggesting the presence of complex mixtures of compounds (Fig. 3) similar to what has been observed in earlier rainwater studies at this location (Kieber et al., 2006). Integration of regions of the spectra, normalized to an internal standard, provides information regarding hydrogen distributions and major functional groups present in CDOM as well as their relative abundances. Integrated regions in the 1H-NMR spectra were of: Ar–H: aromatic protons (9–6.5 ppm), CH–O: protons on carbon atoms singly bound to oxygen or other heteroatoms atoms, such as nitrogen, sulfur or halogens (4.5–3.3 ppm), CH–C]: aliphatic protons on carbon atoms adjacent to carbonyl groups or aromatic rings (3.2–1.9 ppm),
1
H-NMR
Extract DOC (µM)
160
120
80
40
0
0
200
400
600
800
Whole DOC (µM) Fig. 2. Dissolved organic carbon (DOC) in the extract fraction of rainwater as a function of the DOC concentration in the whole rainwater (r ¼ 0.752, slope ¼ 0.23 with the yint forced through zero).
Fig. 3. 1H-NMR spectrum of C18 extracted CDOM.
C. Miller et al. / Atmospheric Environment 43 (2009) 2497–2502
Extracted Fluorescence (x10-3)
60
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observed in surface waters or WSOC from atmospheric aerosols. Organic acids, which are not chromophoric, may account for approximately half of the DOC in the filtrate fraction of the rainwater. This coupled with the large fluorescence measured in the filtrate suggests the presence of a small concentration of highly fluorescent, relatively hydrophilic compounds in rainwater which do not exist in other aquatic matrices. Due to the chromophoric properties of these compounds, they could be extremely important in light attenuation in the atmosphere and in photochemical reactions.
40
20
Acknowledgements 0
0
20
40
60
80
100
120
Total NMR Integration Fig. 4. Total integrated fluorescence of the extract fraction of rainwater as a function of total 1H-NMR integrations (R ¼ 0.739).
CH2 in alkyl chains (1.9–1.1 ppm), and CH3- terminal methyls (1.1–0.5 ppm). Earlier studies which investigated the organic carbon extracted from aerosols by 1H-NMR obtained spectra from the poly acid fraction of their extract which were similar to the rainwater spectra presented here, especially in the aliphatic regions (Decesari et al., 2000, 2005, 2001, 2002). The authors proposed that the extracts consisted of predominantly aliphatic poly-carboxylic acids and also contained a minor component of hydroxyl/ether groups and aromatic protons. The influence of each spectral region on the fluorescence of rainwater was examined in order to evaluate which chemical characteristics are most important in determining the optical properties of CDOM. The total 1H-NMR integration was positively correlated with total integrated fluorescence (r ¼ 0.739, p < 0.001) in the C18 extract of rainwater (Fig. 4). Each integrated region of the spectra was also positively correlated to total integrated fluorescence (Table 2) suggesting that all regions potentially could contribute to CDOM fluorescence by being covalently bound to structural components responsible for fluorescence. There was significant variability in the contribution of each region, as measured by the slope of the correlation line, to CDOM fluorescence with 5–20 fold greater contribution from the aromatic region relative to all other regions. This is not surprising as the most intense fluorescence is found in compounds containing aromatic functional groups with low energy p / p* levels. This suggests that the aromatic region of the 1H-NMR spectra contains the moieties which contribute most strongly to the fluorescence of rainwater, even though this region contributes the least to the overall NMR total integration (Table 2). 3.5. Implications Approximately half the fluorescence in rainwater was detected in the filtrate fraction after C18 extraction, a phenomenon not Table 2 1 H-NMR integration percents (one standard deviation) and slopes of total integrated fluorescence vs. 1H-NMR integration for the various regions of the 1H-NMR spectrum. 1
% Integration
slope
r (n ¼ 17)
p (n ¼ 17)
–CH3 –CH2– –CH–C] –CH–O– Ar–H
11.2 40.2 32.1 15.0 1.4
4.67 1.25 1.70 2.41 25.4
0.875 0.819 0.850 0.865 0.553
<0.001 <0.001 <0.001 <0.001 <0.02
H-NMR Integral Region
(2.2) (2.8) (3.4) (4.1) (1.0)
This work was supported by NSF Grant ATM-0646153. The Marine and Atmospheric Chemistry Research Laboratory at UNC Wilmington assisted with sampling and analyses. References Amador, J.A., Milne, P.J., Moore, C.A., Zika, R.G., 1990. Extraction of chromophoric humic substances from seawater. Marine Chemistry 29, 1–17. Avery, G.B., Kieber, R.J., Witt, M., Willey, J.D., 2006. Rainwater monocarboxylic and dicarboxylic acid concentrations in southeastern North Carolina, USA as a function of air mass back trajectory. Atmospheric Environment 40, 1683–1693. Chin, Y.P., Aiken, G.R., O’loughlin, E., 1994. Molecular weight polydispersity and spectroscopic properties of aquatic humic substances. Environmental Science and Technology 28, 1853–1858. De Haan, H., 1993. Solar UV-light penetration and photodegradation of humic substances in peaty lake water. Limnology and Oceanography 38, 1072–1076. Decesari, S., Facchini, M.C., Fuzzi, S., 2000. Characterization of water soluble organic compounds in atmospheric aerosol: a new approach. Journal of Geophysical Research 105, 1481–1489. Decesari, S., Facchini, M.C., Fuzzi, S., Mcfiggans, G.B., Coe, H., Bower, K.N., 2005. The water soluble organic component of size segregated aerosol, cloud water and wet deposition from Jeju Island during ACE Asia. Atmospheric Environment 39, 211–222. Decesari, S., Facchini, M.C., Matta, E., Lettini, F., Mircea, M., Fuzzi, S., Tagliavini, E., Putaud, J.P., 2001. Chemical features and seasonal variation of fine aerosol water soluble organic compounds in the Po Valley, Italy. Atmospheric Environment 35, 3691–3699. Decesari, S., Facchini, M.C., Matta, E., Mircea, M., Fuzzi, S., Chughtai, A.R., Smith, D.M., 2002. Water soluble organic compounds formed by oxidation of soot. Atmospheric Environment 36, 1827–1832. Everett, C.R., Chin, Y.P., Aiken, G.R., 1999. High pressure size exclusion chromatography analysis of dissolved organic matter isolated by tangential flow ultrafiltration. Limnology and Oceanography 44, 1316–1322. Facchini, M.C., Decesari, S., Mircea, M., Fuzzi, S., Loglio, G., 2000. Surface tension of atmospheric wet aerosol and cloud/fog droplets in relation to their organic carbon content and chemical composition. Atmospheric Environment 34, 4853–4857. Graber, E.R., Rudich, Y., 2006. Atmospheric HULIS: how humic like are they? A comprehensive and critical review. Atmospheric Chemistry and Physics 6, 729–753. Kieber, R.J., Helz, R.G., 1986. Two method verification of hydrogen peroxide determinations in natural waters. Analytical Chemistry 58, 2312–2315. Kieber, R.J., Whitehead, R.F., Willey, J.D., Reid, S., Seaton, P.J., 2006. Chromophoric dissolved organic matter (CDOM) in rainwater, southeastern North Carolina, USA. Journal of Atmospheric Chemistry 54, 21–41. Kieber, R.J., Willey, J.D., Whitehead, R.F., Reid, S., 2007. Photochemistry of chromophoric dissolved organic matter (CDOM) in rainwater. Journal of Atmospheric Chemistry 58, 219–235. Kiss, G., Tombacz, E., Hannsson, H.C., 2005. Surface tension effects of humic like substances in the aqueous extract of tropospheric fine aerosol. Journal of Atmospheric Chemistry 50, 279–294. Likens, G.E., Edgerton, R.S., Galloway, J.N., 1983. The composition and deposition of organic carbon in precipitation. Tellus 35B, 16–24. Limbeck, A., Handler, M., Neuberger, B., Klatzer, B., Puxbaum, H., 2005. Carbonspecific analysis of humic-like substances in atmospheric aerosol and precipitation samples. Analytical Chemistry 77, 7288–7293. Lombardi, A., Jardim, W., 1999. Fluorescence spectroscopy of high performance liquid chromatography fractionated marine and terrestrial organic materials. Water Research 33, 512–520. Moran, M.A., Sheldeon, W.M., Zepp, R.G., 2000. Carbon loss and optical property changes during long term photochemical and biological degradation of estuarine dissolved organic matter. Limnology and Oceanography 45, 1254–1264. Parlanti, E., Morin, B., Vacher, L., 2002. Combined 3D-spectrofluorometry, high performance liquid chromatography and capillary electrophoresis for the characterization of dissolved organic matter in natural waters. Organic Geochemistry 33, 221–236.
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Chemical characteristics of chromophoric dissolved organic matter in rainwater Carrie Miller, Kelly G. Gordon, Robert J. Kieber*, Joan D. Willey, Pamela J. Seaton Department of Chemistry and Biochemistry, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5932, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 August 2008 Received in revised form 14 January 2009 Accepted 25 January 2009
Proton nuclear magnetic resonance (1H-NMR), UV absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy were used to define the chemical characteristics of chromophoric dissolved organic matter (CDOM) in whole and C18 extracted rainwater. The average total recovery of fluorescence determined from the sum of extract and filtrate fractions relative to the whole was 86% suggesting that 14% of fluorescent CDOM in rainwater is comprised of very hydrophobic material that cannot be eluted from the column. Half the fluorescence of rainwater was recovered in the filtrate fraction which is important because it suggests that 50% of the chromophoric material present in precipitation is relatively hydrophilic. The average spectral slope coefficient was smaller in extracted samples (16.3 9.0 mm1) relative to whole samples (18.9 2.8 mm1) suggesting that the extracted material contains larger molecular weight material. Approximately one-third of the total dissolved organic carbon (DOC) in rainwater exists in the extract fraction suggesting that a large percentage of the uncharacterized DOC in rainwater can be accounted for by these hydrophobic macromolecular species. The fluorescence of extracted samples is strongly correlated with total NMR integration and is most sensitive to aromatic protons suggesting that molecules in this region are the most important in controlling the optical properties of rainwater. The lower removal efficiency of CDOM in rainwater relative to surface waters or the water-soluble fraction of aerosols during solid phase extraction (SPE) suggests that rainwater contains significantly more hydrophilic chromophoric compounds which are compositionally different than found in these other aquatic matrices. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Chromophoric dissolved organic matter CDOM Rain
1. Introduction Chromophoric dissolved organic matter (CDOM) is a ubiquitous component of rainwater which plays a central role in a variety of important processes occurring in atmospheric waters. Previous research has shown that CDOM is the dominant chromophore in rainwater with the absorbance of samples decreasing exponentially between 250 and 550 nm (Kieber et al., 2006) and as such plays an important role in solar radiative transfer in the condensed phase affecting both the attenuation and spectral distribution of sunlight reaching the earth’s surface. Recent studies have also shown that significant photodegradation of CDOM occurs after exposure to simulated sunlight (Kieber et al., 2007) suggesting that these chromophoric compounds could be involved in a variety of photo mediated processes. Additionally, if CDOM constituents are surfaceactive they could have a significant impact on droplet population and consequently cloud albedo, by lowering the surface tension of atmospheric waters (Decesari et al., 2005; Facchini et al., 2000; Kiss et al., 2005). * Corresponding author. Tel.: þ1 910 962 3865; fax: þ1 910 962 3013. E-mail address: [email protected] (R.J. Kieber). 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.01.056
Despite the significance of CDOM to a variety of important processes occurring in atmospheric waters, virtually nothing is known regarding its source and chemical characteristics. In contrast, the composition of CDOM in surface waters and the watersoluble organic carbon (WSOC) fraction of atmospheric aerosols has been extensively studied. The composition of WSOC in atmospheric aerosols in particular has gained a great deal of attention recently because of their influence on cloud condensation (Graber and Rudich, 2006). Comparison of rain CDOM properties to surface waters and WSOC compounds was used to examine their potential importance as sources of CDOM in rainwater. The structural and chromophoric characteristics of CDOM were examined using solid phase extraction (SPE) coupled with spectroscopic analysis including UV absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy. SPE of CDOM from rainwater was accomplished via C18 cartridges employing a previously described extraction technique (Kieber et al., 2006). This method was chosen because earlier studies have demonstrated that C18 extraction is better able to retain the UV–visible and fluorescence characteristics of isolated chromophoric organic material relative to XAD (Amador et al., 1990). SPE utilizing C18 has also been used extensively with surface
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waters and water-soluble organic carbon (WSOC) from aerosol samples to isolate optically active DOM (Amador et al., 1990; Limbeck et al., 2005; Lombardi and Jardim, 1999; Parlanti et al., 2002; Varga et al., 2001). Whole rain and two fractionated samples from the C18 SPE, extract and filtrate were analyzed for each rain event. Comparison of optical properties in these three fractions yields important new information regarding the composition of CDOM because it provides insight into the relative hydrophobicity and molecular weight of material in the various subsets. Proton nuclear magnetic resonance (1H-NMR) spectroscopy was also used to provide structural analysis of the hydrophobic SPE fraction of rainwater CDOM. Integration of regions of the spectra, normalized to an internal standard, provides information regarding hydrogen distributions and major functional groups present in CDOM as well as their relative abundances. This includes the relative aliphatic and aromatic character of CDOM which, in combination with UV–visible and fluorescence analysis, sheds fundamental new insight into the chemical characteristics of CDOM which are most important in controlling its chromophoric properties. Earlier estimates indicate that less than half the dissolved organic carbon (DOC) in rainwater can be accounted for by specific carbon compounds (Likens et al., 1983; Willey et al., 2000). One of the most important uncertainties related to the biogeochemical cycling of DOC in atmospheric waters is the composition of the remaining DOC. Organic acids (primarily acetic and formic) are the largest contributor to rainwater DOC at this location (ca. 40%) while formaldehyde contributes 3%, acetaldehyde contributes 5%, and amino acids contribute 2%. One potentially important class of organic compounds which could contribute to the uncharacterized DOC pool in rainwater is macromolecular substances such as CDOM. A final novel aspect of the present study is that it defines the contribution of these uncharacterized hydrophobic carbon compounds to the DOC pool in rainwater. 2. Method 2.1. Rainwater collection and sample processing Rainwater was collected at the University of North Carolina Wilmington (UNCW) on an event basis between September 15, 2005 and September 6, 2006. The UNCW rainwater site is located approximately 8.5 km from the Atlantic Ocean and receives both terrestrial and marine influenced rainwater. Four AerochemMetrics (SCM) Model 301 Automatic Sensing Wet/Dry Precipitation Collectors, one containing a Pyrex beaker and three containing 2.2 L Teflon bottles, were used to collect rainwater. The Pyrex beaker and the Teflon bottles were replaced after each sampling. The beakers were cleaned by combustion at 450 C for at least 6 h and the Teflon bottles were rinsed with ultra clean water deionized water (MilliQ Plus Ultra-pure water system, 18.2 MU cm1, indicated as DIW) between each sampling. Blanks were periodically collected to ensure that there was no carry over of DOC between sampling events. Samples for DOC, UV absorbance, excitation-emission matrix (EEM) spectroscopy and 1H-NMR analysis were collected from the Teflon bottles and filtered using a 0.2 mm, acid-washed Gelman SuporÒ polysulfonone filter. Filtered samples were stored in the dark at 4 C until analysis. The rainwater samples were fractionated by solid phase extraction (SPE) using C18 Sep-Paks (Waters Chromatography, Milford, MA). The goal of this study was to examine the chemical properties of rain under natural pH conditions (4.92 during the current study) so pH adjustment was not used during the SPE procedure. Extractions on four rainwater samples were performed at natural pH and pH 3 to confirm that the difference in extraction efficiencies in rainwater and surface waters was not the
result of analytical differences in extraction pH (surface waters are typically acidified to approximately pH 3 before extraction). Before extraction, C18 cartridges were conditioned by washing twice with 5 mL of 10% water in methanol solution, followed by two 5 mL water aliquots. Room temperature filtered rainwater was extracted by drawing 50 mL through the C18 cartridge at a flow of 2 mL min1. The portion of the rainwater which was not retained by the C18 cartridge (filtrate) was collected and refrigerated until analysis. Two 5 mL aliquots of DIW were passed through the cartridges after the rainwater to remove residual salts. The samples were then eluted off the C18 cartridges with two 3 mL aliquots of 10% water in methanol. Following elution, the solvent was removed under reduced pressure (Buchi Rotavapor, Model RE 111, Switzerland) for 30 min. The sample was placed under vacuumed conditions for an additional 1 h (Sargent-Welch Model 1400, Skokie, IL) to ensure complete removal of the methanol. DIW was then added to the dried extracts, to bring the samples up to the preextraction volume, and the samples were sonicated for 30 s. This fraction was referred to as the extract fraction of the rainwater. For each rain event, three samples were collected: whole, C18 extract and filtrate. Sample aliquots for DOC analysis were preserved by acidified to pH 2 after the fractionation step (Willey et al., 2000). Samples for optical analysis were not acidified. Both aliquots were stored in the dark at 4 C until analysis. 2.2. Optical analysis The optical activity of the CDOM in rainwater was determined using UV absorbance and EEM fluorescence spectroscopy (Kieber et al., 2006). A Varian Cary IE dual-beam spectrophotometer (2 nm slit width) was used for absorbance measurements between 240 and 800 nm and a 10 cm spectral cell was used for all readings. Since nitrate and hydrogen peroxide absorb UV light, the absorbance of these species was subtracted from the total absorbance so that the reported values reflect the absorbance from the CDOM in the rainwater (Kieber et al., 2006). Absorbance coefficients (a) were determined at each wavelength (a ¼ 2.3 A pathlength1; A ¼ absorbance) and spectral slopes were determined using an exponential fit of the absorbance between 240 and 400 nm (Kieber et al., 2006). Absorbance measurements were only performed on the whole and extract fraction of the rainwater. The filtrate fraction could not be analyzed due to an absorbent contaminant, either from the methanol used to condition the C18 cartridge or from something leaching off of the cartridge, which was detected when DIW was used for extraction blank determination. Excitation-emission matrix (EEM) fluorescence measurements were performed using a Jobin Yvon SPEX Fluoromax-3 scanning fluorometer equipped with a 150 W Xe arc lamp and a R928P detector. EEMs were created using emission scans between 250 and 550 nm and excitation scans between 250 and 500 nm. FLToolbox 1.91 (Wade Sheldon, University of Georgia) developed for MATLABÒ was used for post-processing the fluorescence scans in order to eliminate Rayleigh and Raman scattering peaks (Zepp et al., 2004). The data were normalized to quinine sulfate equivalents (QSE). Fluorescence values presented in the current paper are threedimensional surface integrations of EEM’s of the entire emission over the corrected fluorescence matrix (Moran et al., 2000). 2.3. Dissolved organic carbon and ancillary data Dissolved organic carbon (DOC) was measured by high temperature combustion using a Shimadzu TOC 5000 total organic carbon analyzer equipped with an ASI5000 autosampler. The concentration of DOC was determined on whole and extract rainwater fractions but not on the C18 filtrate since this fraction was
C. Miller et al. / Atmospheric Environment 43 (2009) 2497–2502
contaminated with methanol from conditioning the cartridge. DOC concentrations in extract blanks (pH 4.5 synthetic rainwater treated as a sample in place of rainwater) were very low and near the detection limit indicating that all detectable traces of methanol in the extract as measured by DOC analysis were removed. The detection limit for this method is 5 mM (Willey et al., 2000). Rainwater samples were also analyzed for pH, hydrogen peroxide (Kieber and Helz, 1986) and inorganic anions (chloride, nitrate and sulfate) by suppressed ion chromatography. Volume-weighted averages (VWA) were determined for each analyte using the concentration and volume of rainwater for each rain event. 2.4. Nuclear Magnetic Resonance (NMR) Liquid phase 1H-NMR were recorded on a Bruker Avance 400 MHz NMR spectrometer. The C18 SPE extraction method described above was also used to extract samples for 1H-NMR analysis. The extracts were reconstituted in 0.60 mL of D2O and 20 mL internal standard (187 mM 3-Trimethylsilyl-propionic-2,2,3,3d4 acid, sodium salt). Samples were analyzed using a presaturation experiment (zgpr, 1024 scans) where: pulse angle ¼ 30 (P1 ¼ 9.00 ms), D1 ¼ 3.00 s, and PL ¼ 60 dB using an inverse gradient probe. After the acquisition, the FID was transformed with exponential multiplication (LB ¼ 5). Phasing, calibration, and integration were done identically for all experiments to allow for comparisons of different rain events. After phasing, the trimethylsilyl peak of the standard was set to zero ppm and normalized for each sample. Several regions of the 1H-NMR spectra were integrated including the H–Ar (9.0–6.5 ppm), the H–C–O (4.5–3.4 ppm), the H–C–C¼ (3.3–1.9 ppm), the CH2 (1.9–1.1 ppm), and the CH3 (1.1–0.5 ppm). Samples were all normalized to an internal standard (0.1 to 0.1 ppm) to allow for comparison of different natures of extracted DOM. The 10–9.5 ppm and 6.5–6.0 ppm regions were also integrated to correct for baseline anomalies and drift. The peak area of the internal standard was integrated to one. The two regions that were used to correct for baseline drift were subtracted from the integration of the respected region after being corrected for the range of the region. Total abundances of the respective regions were then calculated as well as a percentage of the entire region.
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CDOM on the C18 SPE cartridges due to the hydrophobic nature of this material. The extract fraction of rainwater contained on average 36% of the total integrated fluorescence in rain samples while 50% of the fluorescence was recovered in the filtrate. This is significant because it suggests that half the chromophoric material present in these rain samples is relatively hydrophilic and is not effectively bound to the nonpolar stationary phase of the C18 cartridge. A significant positive correlation (p < 0.001, r ¼ 0.915) exists between the integrated fluorescence in the whole and extract samples (Fig. 1) suggesting that there is a fixed proportion of fluorescent material being extracted and that the distribution of extractable and nonextractable fluorescence in rainwater co-vary. Comparison of rainwater CDOM optical properties to surface waters and water-soluble atmospheric aerosols (WSOC) can be used to further examine the chemical characteristics of rainwater CDOM. Extractions using C18 cartridges removed 70–90% of the fluorescent material in both fresh and marine surface waters (Amador et al., 1990; Lombardi and Jardim, 1999; Parlanti et al., 2002). The high removal efficiency of CDOM using SPE is one factor which has led to the classification of surface water CDOM as hydrophobic humic-like material. High CDOM removal (75–95%) was also observed with WSOC from atmospheric aerosols (Limbeck et al., 2005). Acidification of rainwater to pH 3, the pH typically used with surface water extractions, increased the extraction efficiency in some rain events but decreased this efficiency in other samples. The difference was always less than 15% in either direction and therefore could not explain the large difference observed in the extraction of CDOM in rainwater relative to other water matrices. Earlier studies have suggested that CDOM in rainwater may be derived from surface waters and atmospheric aerosols (Kieber et al., 2006). The lower removal efficiency of CDOM in rainwater during SPE suggests that rainwater contains relatively more hydrophilic fluorescent compounds compared to surface waters or the watersoluble fraction of aerosols. This indicates that the CDOM compounds in rainwater are compositionally different relative to the chromophoric compounds in surface waters and the WSOC of aerosols. The lower removal efficiency of CDOM in rainwater during SPE compared to surface waters and aerosols also suggests that these are not the sole source of chromophoric material in rainwater.
3. Results and discussion 3.2. Optical activity – spectral slopes 3.1. Optical activity – EEM
Table 1 Volume-weighted average dissolved organic carbon (DOC) concentration, volumeweighted average integrated fluorescence and simple average spectral slopes of the whole, extract and filtrate fractions of rainwater. Integrated fluorescence values are reported as area under the EEM scans. Na: not analyzed. Fraction
DOC (mM)
Integrated Fluorescence (103)
Spectral slope
Whole Extract Filtrate
77 (20) 28 (7) Na
50 (2) 18 (1) 25 (5)
18.9 (2.8) 16.3 (9.0) Na
Spectral characteristics, such as the spectral slope coefficient calculated from non-linear least-square regressions of absorption 80
Extract Fluorescence (x10-3)
The average total recovery of fluorescence determined from the sum of extract and filtrate fractions relative to the whole was 86% (Table 1). This suggests that 14% of fluorescent CDOM in rainwater is comprised of very hydrophobic material that cannot be eluted from the nonpolar C18 column under conditions used in this study. Similar recoveries have been reported for WSOC extracted from atmospheric aerosols in which approximately 25% of light absorbing DOM was not recovered after C18 SPE (Limbeck et al., 2005; Varga et al., 2001). The loss of chromophoric materials in these earlier studies was attributed to the permanent absorption of
60
40
20
0
0
50
100
150
200
250
Whole Fluorescence (x10-3) Fig. 1. Fluorescence in the extract fraction of rainwater as a function of fluorescence in whole rainwater (R ¼ 0.915, slope ¼ 2.81). Fluorescence values are total integrated fluorescence of the EEM scans reported in quinine sulfate equivalents (QSE) in ppb.
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coefficient versus wavelength, can be used to infer compositional differences in whole and extracted rainwater. Previous work on surface waters has suggested an inverse relationship between DOM molecular weight and spectral slope coefficient (Chin et al., 1994; De Haan, 1993). Chin et al. (1994) reported a positive correlation between weight-averaged-molecular weight and specific absorptivity of humic substances at 280 nm. Further investigations have shown the relationship to be fairly robust for whole water DOM and DOM isolated by tangential-flow ultrafiltration and can cautiously be used for ‘‘ballpark estimates’’ of the molecular weight of DOM in the absence of direct molecular weight measurements (Chin et al., 1994; Everett et al., 1999). The average spectral slope coefficient determined for whole rainwater samples in this study (18.9 mm1, Table 1) is very similar to the previously reported spectral slope coefficient (19.3 mm1) for rainwater collected in 2002 and 2003 at this site (Kieber et al., 2006). This suggests that the absorbing compounds in the present study are compositionally similar to rainwater analyzed previously at this location. The average spectral slope coefficient was smaller in extracted samples (16.3 9.0 mm1) relative to the whole (18.9 2.8 mm1) indicating that the extracted material contains larger molecular weight material. This difference in spectral slopes was, however, relatively small suggesting that the hydrophobicity of DOM is not controlled entirely by molecular weight.
DOM may be somewhat greater than the storm-to-storm variability of fluorescent compounds. Organic acids comprise between 14 and 40% of the total DOC in rainwater (Avery et al., 2006; Likens et al., 1983; Williams et al., 1997). Earlier studies at the UNCW rain site demonstrated that the organic acid content was on the high end of the range of observed at other locations (36–40%), however a recent study suggests that small organic acid concentrations may be decreasing in rainwater nationwide including Wilmington, NC (Willey et al., 2006). With a VWA DOC concentration of 77 mM in Wilmington rain, the approximate organic acid concentration would be in the range of tens of mM. Due to the hydrophilic nature of organic acids, these compounds would be present in the filtrate portion of the C18 fractionated rainwater. The DOC concentration in the filtrate was not determined but the concentration can be estimated (approximately 50 mM) from the difference between the DOC concentration in the whole and extracted rainwater fractions. Although organic acids comprise a large fraction of the DOC in the filtrate fraction of rainwater, these compounds cannot explain the optical activity of the organic matter in this fraction since small organic acids do not fluoresce. This estimate of organic acid content in rain suggests the fluorescent activity of DOM in the filtrate portion of rainwater is likely due to compounds that are highly fluorescent but at relatively low concentrations.
3.3. DOC
3.4.
Dissolved organic carbon concentrations were determined for whole and extracted DOM on a subset of samples (n ¼ 18). The VWA DOC concentration measured in this study (77 mM in the subset, 83 mM for rain collected during the whole sampling year) falls within the range of volume-weighted DOC average concentrations for the preceding two years (2003 – 87 mM; 2004 – 56 mM) for Wilmington, NC rainwater (Willey et al., 2006) suggesting this is a normal year for DOC in precipitation at this location. Approximately one-third of the total DOC in rainwater could be accounted for in the extract suggesting a large fraction of DOC in rainwater is comprised of relatively hydrophobic material (Table 1). A significant positive relationship was observed between the whole and extractable DOC concentrations (Fig. 2; p < 0.001, r ¼ 0.711), although this relationship is not as strong as observed with whole and extractable integrated fluorescence (Fig. 1). This suggests that the storm-to-storm variability of the nonfluorescent component of
One important question based on the current understanding of CDOM in rainwater relates to its chemical characteristics. 1H-NMR analysis of the C18 extract fraction in rainwater provides spectra which appear as relatively continuous and broad distribution of signals suggesting the presence of complex mixtures of compounds (Fig. 3) similar to what has been observed in earlier rainwater studies at this location (Kieber et al., 2006). Integration of regions of the spectra, normalized to an internal standard, provides information regarding hydrogen distributions and major functional groups present in CDOM as well as their relative abundances. Integrated regions in the 1H-NMR spectra were of: Ar–H: aromatic protons (9–6.5 ppm), CH–O: protons on carbon atoms singly bound to oxygen or other heteroatoms atoms, such as nitrogen, sulfur or halogens (4.5–3.3 ppm), CH–C]: aliphatic protons on carbon atoms adjacent to carbonyl groups or aromatic rings (3.2–1.9 ppm),
1
H-NMR
Extract DOC (µM)
160
120
80
40
0
0
200
400
600
800
Whole DOC (µM) Fig. 2. Dissolved organic carbon (DOC) in the extract fraction of rainwater as a function of the DOC concentration in the whole rainwater (r ¼ 0.752, slope ¼ 0.23 with the yint forced through zero).
Fig. 3. 1H-NMR spectrum of C18 extracted CDOM.
C. Miller et al. / Atmospheric Environment 43 (2009) 2497–2502
Extracted Fluorescence (x10-3)
60
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observed in surface waters or WSOC from atmospheric aerosols. Organic acids, which are not chromophoric, may account for approximately half of the DOC in the filtrate fraction of the rainwater. This coupled with the large fluorescence measured in the filtrate suggests the presence of a small concentration of highly fluorescent, relatively hydrophilic compounds in rainwater which do not exist in other aquatic matrices. Due to the chromophoric properties of these compounds, they could be extremely important in light attenuation in the atmosphere and in photochemical reactions.
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20
Acknowledgements 0
0
20
40
60
80
100
120
Total NMR Integration Fig. 4. Total integrated fluorescence of the extract fraction of rainwater as a function of total 1H-NMR integrations (R ¼ 0.739).
CH2 in alkyl chains (1.9–1.1 ppm), and CH3- terminal methyls (1.1–0.5 ppm). Earlier studies which investigated the organic carbon extracted from aerosols by 1H-NMR obtained spectra from the poly acid fraction of their extract which were similar to the rainwater spectra presented here, especially in the aliphatic regions (Decesari et al., 2000, 2005, 2001, 2002). The authors proposed that the extracts consisted of predominantly aliphatic poly-carboxylic acids and also contained a minor component of hydroxyl/ether groups and aromatic protons. The influence of each spectral region on the fluorescence of rainwater was examined in order to evaluate which chemical characteristics are most important in determining the optical properties of CDOM. The total 1H-NMR integration was positively correlated with total integrated fluorescence (r ¼ 0.739, p < 0.001) in the C18 extract of rainwater (Fig. 4). Each integrated region of the spectra was also positively correlated to total integrated fluorescence (Table 2) suggesting that all regions potentially could contribute to CDOM fluorescence by being covalently bound to structural components responsible for fluorescence. There was significant variability in the contribution of each region, as measured by the slope of the correlation line, to CDOM fluorescence with 5–20 fold greater contribution from the aromatic region relative to all other regions. This is not surprising as the most intense fluorescence is found in compounds containing aromatic functional groups with low energy p / p* levels. This suggests that the aromatic region of the 1H-NMR spectra contains the moieties which contribute most strongly to the fluorescence of rainwater, even though this region contributes the least to the overall NMR total integration (Table 2). 3.5. Implications Approximately half the fluorescence in rainwater was detected in the filtrate fraction after C18 extraction, a phenomenon not Table 2 1 H-NMR integration percents (one standard deviation) and slopes of total integrated fluorescence vs. 1H-NMR integration for the various regions of the 1H-NMR spectrum. 1
% Integration
slope
r (n ¼ 17)
p (n ¼ 17)
–CH3 –CH2– –CH–C] –CH–O– Ar–H
11.2 40.2 32.1 15.0 1.4
4.67 1.25 1.70 2.41 25.4
0.875 0.819 0.850 0.865 0.553
<0.001 <0.001 <0.001 <0.001 <0.02
H-NMR Integral Region
(2.2) (2.8) (3.4) (4.1) (1.0)
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