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Atmospheric Environment 40 (2006) 7539–7545 www.elsevier.com/locate/atmosenv

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Carbon isotopic characterization of dissolved organic carbon in rainwater: Terrestrial and marine influences G.B. Avery Jr., J.D. Willey, R.J. Kieber Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, NC 28403-5932, USA Received 10 January 2006; received in revised form 29 June 2006; accepted 4 July 2006

Abstract This study presents the first published 14C and 13C isotopic data for dissolved organic carbon (DOC) in rainwater. The C fraction modern (Fm) values for rain DOC ranged from 0.9051 to 1.0569. Between 4% and 24% of the rainwater DOC was of fossil fuel origin based on a two end member calculation assuming biogenic carbon has the same 14C content as the contemporary atmosphere (Fm ¼ 1.11–1.19) and fossil fuel carbon is devoid of 14C. The d13C content of rain DOC ranged from
21.8% to
28.2% spanning the terrestrial and fossil fuels average (
26%) to marine (
18% to
20%) values. Rain events with more continental influences had d13C values closer to the terrestrial and fossil fuels signal and higher concentrations of fossil fuel derived DOC. Rain events with stronger oceanic influences had d13C values shifting towards those typical of marine systems and had lower concentrations of fossil fuel derived DOC. The influence air mass back trajectory has on the terrestrial and marine characteristics of rainwater DOC identifies air mass recent history as an important factor controlling the sources of DOC in precipitation. It also indicates water soluble volatile organic carbon (VOC’s) have relatively short residence times on the order of days with minimal global transport. However, residence times of days, and the presence of both marine and terrestrial DOC in rainwater, suggests that precipitation represents an important exchange of carbon between land and the oceans especially in coastal regions. r 2006 Elsevier Ltd. All rights reserved. 14

Keywords: Rain precipitation; Dissolved organic carbon; Isotopes;

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1. Introduction Rainwater is a globally important removal mechanism for atmospheric dissolved organic carbon (DOC) with a carbon flux (0.3 Gt yr
1) equal to approximately 6% of the fossil fuel flux influx (5.5 Gt yr
1) to the atmosphere (Willey et al., 2000). Previous work has suggested that sources of rain DOC include anthropogenic emissions (Kawamura Corresponding author. Tel.: +1 910 962 7388; fax: +1 910 962 3013. E-mail address: [email protected] (G.B. Avery Jr.).

1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.07.014

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et al., 1985; Kawamura and Kaplan, 1986, 1987; Talbot et al., 1988; Fraser et al., 1998; Avery et al., 2001) as well as marine and terrestrial biogenic emissions (Kawamura and Kaplan, 1986; Keene and Galloway, 1988; Talbot et al., 1988; Andreae, 1988; Avery et al., 2006). However, it is difficult to constrain and confirm these sources by simply measuring concentrations of DOC and its components. Isotopic data provides important information relating to sources of compounds in the environment. 14C measurements of organic carbon can be used to distinguish between biogenic carbon using a

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simple two end-member calculation since fossil fuel carbon is devoid of 14C, and biogenic carbon has a 14 C–12C ratio similar to that of atmospheric CO2. This approach has been used in earlier studies to identify the anthropogenic and biogenic fraction of carbon in atmospheric methane (Quay et al., 1991), volatile organic carbon (VOC) (Klouda et al., 1996), carbonyl compounds (Larsen et al., 1998), and carbon monoxide (Klouda and Connolly, 1995). The 13C content of organic carbon also provides important source information. Biogenic organic carbon d13C values range from approximately
21% for marine carbon to
26% for terrestrial carbon (Williams and Gordon, 1970). Fossil fuel carbon d13C values range from approximately
26% to
30% with an average value of
28% for the United States (Andres et al., 1996). Therefore, more negative d13C values indicate fossil fuel and terrestrial components whereas more positive d13C values suggest a stronger marine source. The purpose of this study was to apply both stable (13C) and radioactive (14C) carbon isotopic analysis to elucidate sources and examine transport of rainwater DOC. The data represent the first published 13 C and 14C values for rainwater DOC and provide new evidence for the exchange of organic carbon between marine and terrestrial environments via precipitation. 2. Methods 2.1. Study site The UNCW rainwater collection site (34113.20 N, 77151.80 W) is located in a large open area (approximately 1 ha) within a turkey oak, long leaf pine, and wire grass community on the UNCW campus. It is approximately 8.5 km from the Atlantic Ocean. Samples were collected using two Aerochem Metrics (ACM) Model 301 automatic sensing wet/dry precipitation collectors in muffled (550 1C, 5 h) 4 L glass beakers placed inside each wet deposition sampling bucket. Samples were collected and prepared for isotopic analysis within 12 h following cessation of the event. 2.2. Analysis of DOC All water used in preparation of standards and samples was obtained from a Milli-Q Plus Ultra Pure Water System, which produces deionized water with a resistivity of 18 megaohm cm
1. Water

obtained from this purification system has extremely low carbon blanks (2.2 mM), which is required for the DOC analysis. DOC was determined on rainwater samples with a Shimadzu TOC 5000 carbon analyzer equipped with an ASI 5000 autosampler. 2.3. Preparation of rainwater DOC for analysis

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The method for converting DOC in rainwater to CO2 for mass spectrometer analysis is a UV oxidation method similar to those employed for preparation of seawater DOC samples for 14C analysis (Williams and Druffel, 1987; Druffel et al., 1992; Bauer et al., 1998). The rainwater (approximately 500–800 mL depending on DOC concentration) was placed in a 1 L quartz flask containing a CO2-free N2 headspace which minimizes exposure of the samples to air. Both DOC and DIC were monitored before and after each step of the preparation procedure to check for contamination and for balance of total carbon (DOC and DIC). The samples presented in this study had no measurable contamination during processing. The rainwater was acidified with 2 M HCl to a pHo2.00 and purged with CO2-free N2 for 10 min which proved adequate to remove inorganic carbon but did not alter DOC concentration. The DOC content was determined on an aliquot of the inorganic carbon-free sample for comparison to post-UV oxidation. Two sodium hydroxide pellets were added to the flasks creating a basic solution (pH410) allowing the inorganic carbon formed during the breakdown of the DOC to exist as the carbonate ion and remain in solution. The DOC content of the basic rainwater was reanalyzed to ensure no inorganic carbon was added during the addition of sodium hydroxide. The rainwater sample was then UV oxidized (1.2 kW, Hg arc UV lamp) for 12 h converting the DOC to CO2 which is kept in solution as carbonate under the basic conditions of the experiment Completeness of DOC oxidation to CO2 was confirmed by acidification of a sample aliquot, purging to remove CO2, and measuring the final DOC concentration. Samples were then shipped to the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility for analysis where the CO2 was trapped on a vacuum line and converted to a graphite target for AMS analysis (see NOSAMS web page http://www.nosams.whoi.edu for details on the 14C analysis).

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2.4. Reporting of

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C data

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C data are reported as fraction modern (Fm). NOSAMS describes Fm on their website (http:// www.nosams.whoi.edu/clients/data.html) as follows ‘‘Fraction Modern is a measurement of the deviation of the 14C/12C ratio of a sample from ‘‘Modern’’. Modern is defined as 95% of the radiocarbon concentration (in AD 1950) of NBS oxalic acid I normalized to d13CVPDB ¼
19 per mil (Olsson, 1970).’’ The 13C content of the DOC are reported using the standard notation where d13C ¼ [(13C/12Csample/13C/12Cstandard)
1]  1000, the standard being PDB (Pee Dee Bellemnite). 3. Results and discussion 3.1. Rain events The 14C and 13C content of rainwater DOC was determined for five Wilmington, North Carolina rain events. The five events were selected to

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incorporate a range of marine and terrestrial influences. Events were characterized by their relative exposure to land during the 48 h preceding the rain event based on previously published air mass back-trajectory classifications (Avery et al., 2006). The events included, in order of increasing terrestrial influence, a strictly marine event (Emar), three mixed marine and terrestrial events (Emix1, Emix2, and Emix3) and one strictly terrestrial event (Eter) (Fig. 1). 3.2.

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C content of rainwater DOC

3.2.1. Fossil fuel rainwater DOC The fraction modern (Fm) values for the rain DOC ranged from 0.9051 to 1.0569 (Table 1). These values were lower than previously published Fm’s for contemporary living material which range from 1.11 to 1.19 (Klouda et al., 1996; Klouda and Connolly, 1995; Larsen et al., 1998), and for atmospheric CO2 determined at the rain collection site during the current study (1.0994). These lower

Fig. 1. Map of Eastern United States showing air mass back trajectories of rain events used in this study. Map shows air mass path at 500 m altitude 48 h prior to rain event.

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Table 1 Fraction modern and d13C values for five rain samples and atmospheric CO2

a

Rain sample

Depth of rain event (cm)

pH

DOC (mM)

Fraction modern

d13C (%)

Emar Emix1 Emix2 Emix3 Eter Atmospheric CO2

0.88 3.45 1.32 2.9 1.35

4.74 4.58 4.52 4.43 4.59

132 309 540 302 351

0.9481 1.0569 1.0501 0.9405 0.9051 1.0994


20.8
23.8
24.8
25.4
28.2
17.7

a

Represents dissolved inorganic carbon.

25

Fossil Fuel DOC (%)

20

15

10

5

0 Emar

Emix1

Emix2 Rain Event

Emix3

Eter

Fig. 2. Percentage of rain fossil fuel derived DOC based on a two end member calculation assuming biogenic carbon has the same content as the contemporary atmosphere (average Fm ¼ 1.15) and fossil fuel carbon is devoid of 14C.

values for rain DOC indicate a dilution of contemporary biogenic DOC by 14C depleted fossil fuels. The fraction of fossil fuel rain DOC (f) can be calculated using a two-component mass balance calculation below, Fmrain ¼ f ðFmff Þ þ ð1
f ÞðFmatm Þ, where Fmrain is the measured Fm of the rainwater DOC, Fmff is the Fm of fossil fuel carbon (0.0), and Fmatm is the average value (1.15) from previously reported values of contemporary living material (Klouda et al., 1996; Klouda and Connolly, 1995; Larsen et al., 1998). On the basis of these calculations, 4–24% of the total rainwater DOC was of fossil fuel origin (Fig. 2). The percentage of rain fossil fuel DOC did not vary systematically with terrestrial influence. However, the concentration of rain fossil fuel DOC, calculated by using the percentage of fossil fuel calculated above and the

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total rain DOC for each event, was significantly higher in storms with air mass back-trajectories influenced by land (Fig. 3). These results suggest that the abundance of fossil fuel DOC in rain is directly influenced by proximity to terrestrial anthropogenic combustion sources but the percentage of fossil fuel DOC (Fig. 2) is more complex because of variations in biogenic sources of DOC. The importance of biogenic sources in influencing the fraction fossil fuel DOC is not surprising considering that on the basis of the current study, the majority of DOC in rain (76–96%) is biogenic. 3.2.2. Fossil fuel DOC fluxes Deposition of fossil fuel DOC for the five rain events ranged from 2  10
4 to 18  10
4 mol C m
2. Deposition of fossil fuel DOC did not display a distinct pattern with variations in terrestrial influence

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70

Fossil Fuel DOC (uM)

60 50 40 30 20 10 0 Emar

Emix1

Emix2 Rain Event

Emix3

Eter

Fig. 3. Concentration of fossil fuel derived DOC in rain events calculated based on the percentage fossil fuel DOC from 14C measurements and total DOC concentrations for each rain event.

although the lowest deposition did occur in the strictly marine event (Emar). One of the most important aspects of the data presented here is that it can be used to provide an estimate of the global flux of fossil fuel derived DOC removed via rainwater each year. Taking the estimated 0.33 Gt C removed from the atmosphere by rainwater DOC each year (Willey et al., 2000 and references therein) and multiplying it by the average percent fossil fuel derived DOC from the current study (14%) yields a net removal of 0.046 Gt C per year. This fossil fuel derived DOC flux represents removal of incompletely combusted fossil fuels as rainwater DOC. The amount of carbon emitted to the atmosphere each year as the result of incomplete combustion processes can be approximated using current fossil fuel flux values and fossil fuel combustion efficiency estimates. On the basis of energy production data, 5.5 Gt C yr
1 is released to the atmosphere via fossil fuel combustion the majority of which is CO2. Approximately 1% of gasoline is not completely combusted to CO2 and enters the atmosphere as VOC (Fraser et al., 1998). Assuming this efficiency of combustion for fossil fuels in general, approximately 0.055 Gt C yr
1 of incompletely combusted fossil fuels (1% of total fossil fuel flux) is emitted to the atmosphere each year. This is remarkably similar to our estimate of 0.046 Gt C yr
1 for removal of fossil fuel DOC in rain and suggests that the majority of incompletely combusted fossil fuels emitted to the atmosphere each year are removed via rainwater.

3.3. 13C content of rainwater DOC: terrestrial vs. marine influences Rain DOC d13C values ranged from
21.8% to
28.2% spanning the range from terrestrial and fossil fuels (average 
26%) to marine (
18% to
20%) values (Table 1). Rain events with more continental influences, as indicated by air mass back trajectories, had d13C values closer to terrestrial and fossil fuels signals, while increasing oceanic influences resulted in d13C values shifting towards those typical of marine systems (Fig. 4). Eter had a d13C value of
28.2% indicating terrestrial and/or fossil fuel sources while Emar had a d13C of
21.8% characteristic of marine organic matter. Emix1, Emix2, and Emix3 had intermediate d13C values ranging from
23.8% to
25.4%. The d13C values of these mixed storms also showed systematic variations towards 13C depleted values with increasing terrestrial and marine influence. The influence of air mass back trajectory on the terrestrial and marine characteristics of rainwater DOC identifies short-term history of the air mass as an important factor controlling the sources of DOC in precipitation. It also indicates relatively short residence times on the order of days for water soluble VOC’s and minimal global transport. However, residence times of days, and the presence of both marine and terrestrial DOC in rainwater, suggests that precipitation represents an important exchange of carbon between land and the oceans, especially in coastal regions.

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Fig. 4. Delta

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C values for rain events.

Acknowledgements The Atmospheric Chemistry Division of the National Science Foundation supported this work through Grants, ATM-9729425, ATM-0096878 and ATM-0342420. The UNCW MACRL group assisted with rain collection and analyses. We would like to offer a special thanks to the National Ocean Sciences Accelerator Mass Spectrometry facility for analyzing our samples as part of their Research Initiatives program.

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