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Diurnal variations in major rainwater components at a coastal site in North Carolina
Atmospheric Environment 35 (2001) 3927–3933
Diurnal variations in major rainwater components at a coastal site in North Carolina G. Brooks Avery Jr.*, Joan D. Willey, Robert J. Kieber Department of Chemistry and Marine Science Program, University of North Carolina at Wilmington, Wilmington, NC 28403-3297, USA Received 8 November 2000; accepted 17 March 2001
Abstract Concentrations of several major rainwater components were determined in rain events occurring during the early morning hours (12:00 midnight to 6:00 a.m.) and during the afternoon (12:00 noon to 6:00 p.m.) to examine possible diurnal variations. Generally, rainwater components with gas phase origins (H+, NO@ 3 , formaldehyde, H2O2, formic acid, acetic acid, pyruvic acid, oxalic acid, and lactic acid) had higher concentrations during p.m. rain events compared to a.m. events. Although source strengths of both biogenic and anthropogenic rainwater components are generally higher during the daytime, nocturnal removal of a wide variety of components in similar proportions (approximately 2–3 less at night) indicates a physical rather than a chemical process affecting diurnal variations. Rainwater components with aerosol origins (Cl@, and SO2@ ) displayed the opposite diurnal pattern or showed no diurnal 4 variation. Possible reasons for these variations include one or both of the following scenarios: (1) the formation of dew at night removes gas phase atmospheric gasses but not aerosols or (2) during the night, a marine air mass containing lower concentrations of all analytes and higher concentrations of Cl@ is advected into the area. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Dew; Rainwater; Diurnal variations
1. Introduction Major components of rainwater in our region include: 2@ H+, NO@ , NH+ 3 , SO4 4 (Willey and Kiefer, 1993), organic acids (Avery, 1989; Avery et al., 1991; Tang, 1998), hydrogen peroxide (Willey et al., 1996; Kieber et al., 2001), formaldehyde (Kieber et al., 1999a, b), and sea salts composed mainly of NaCl and MgSO4 (Willey and Kiefer, 1993). With the exception of the sea salts and precursors to H2SO4, which exist in the atmosphere as aerosols, the majority of these components or their precursors are found in the gas phase (Seinfeld and Pandis, 1998). When rain events occur, a fraction of both gas phase and particulate phase components are
*Corresponding author. Tel.: +1-910-962-7388; fax: +1910-962-3013. E-mail addresses: [email protected] (G. B. Avery Jr.).
incorporated into raindrops and are deposited as wet deposition (Seinfeld and Pandis, 1998). Atmospheric concentrations of rainwater components vary both spatially and temporally. For example, concentrations of terrestrial derived rainwater components associated with biogenic and anthropogenic emissions vary with proximity to landmass (e.g., Arlander et al., 1990) and population centers (e.g., Dawson et al., 1980; Likens et al., 1987; Nolte et al., 1999), respectively. Many biogenic components vary with season and are found in higher concentrations during the growing season (e.g., Keene and Galloway, 1984; Tanner and Meng, 1984; Avery et al., 1991). Some of these components undergo short-term temporal variations. For example, the concentrations of formic and acetic acids in the gas phase undergo diurnal variations characterized by elevated afternoon concentrations and lowered concentrations at night (Puxbaum et al., 1988; Talbot et al., 1988; Sakugawa et al., 1993).
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Dicarboxylic acid gas phase and particulate concentrations also display diurnal variations (Kawamura and Kaplan, 1987). Pollution derived components, such as nitric acid, also undergo diurnal variations linked to traffic volume and solar intensity and duration. At night, dry deposition of these components, combined with decreases in the production processes mentioned above, are thought to lower concentrations of the acids in the gas phase. Although diurnal variations in gas phase concentrations of several of these rainwater components have been well documented, no link has been established between these variations and resulting rainwater concentrations. The purpose of this study was to investigate diurnal variations in concentrations of rainwater components having primarily aerosol or gas phase sources. In order to accomplish this objective, rainwater concentrations of components originating as gas phase and aerosols were compared in storms occurring during early morning hours (a.m.) and during the afternoon (p.m.). Rainwater concentrations that were determined for components which originate primarily in the gas phase, or with precursors that originate in the gas phase included: hydrogen ion, NO@ 3 , H2O2, formaldehyde and the anions of organic acids, formate, acetate, lactate, pyruvate, and oxalate. Rainwater components scavenged primarily as aerosols included Cl@ and SO2@ ). 4
2.2. Season, a.m. p.m. definitions, and data sets
2. Methods
Aerosol components were defined as rainwater constituents that exist in the atmosphere primarily as aerosols. Gas phase components were defined as rainwater components with the majority of their atmospheric concentrations in the gas phase. The only exception would be our classification of the dicarboxylic acids as gas phase components. Kawamura and Kaplan (1987) reported an average of 62% of dicarboxylic acids as aerosols in a highly urbanized location. At our study site, the dicarboxylic acids behaved more like gas phase components than particulates. Seasons were defined as non-growing (1 October–31 March) and growing (1 April–30 September) (Avery, 1989). a.m. storms were defined as starting after 12:00 midnight local time and ending before 6:00 a.m. p.m. storms were defined as starting after 12:00 noon and ending before 6:00 p.m.. A total of 33 storms fit these criteria. Non-growing a.m. and p.m. events had 10 storms each, growing season p.m. and a.m. had 8 and 5 storms, respectively. Rainwater component concentration averages and standard deviations were volume weighted which minimizes effects of small rain events on averages and is the mathematical equivalent to combining all rain samples into one container prior to analysis (Topol et al., 1985). Four large anomalous events were eliminated from the data sets to prevent them from dominating volumeweighted averages. After these large events were removed, average depths of rain events for the four categories were very similar (range 13–20 mm).
2.1. Rain sampling
2.3. Formaldehyde
The primary rain sampling site used in this study was an open area of longleaf pine, wire grass, and turkey oak on the campus of the University of North Carolina at Wilmington (34113.90 N, 77152.70 W, 8.5 km from the Atlantic Ocean). This site complies with US EPA specifications for rain gauge and sampler placement (Topol et al., 1985). All rainwater event samples were collected using an Aerochem Metrics (ACM) Model 301 Automatic Sensing Wet/Dry Precipitation Collector which housed a 4 l muffled glass beaker placed within a HDPE plastic bucket. For formaldehyde analysis, 5 ml rainwater samples were withdrawn, placed in separate 7 ml Teflon vials, derivatized with 100 ml DNPH and refrigerated at 41C. Samples for organic acid analysis were placed in 30 ml Teflon vials, preserved with chloroform, and stored in a refrigerator (41C) immediately after collection to stop biological activity (Keene and Galloway, 1984). Chloroform blanks showed no organic acids contamination. (Tang, 1998). Hydrogen peroxide and pH measurements were conducted immediately upon collection.
Formaldehyde concentrations in rainwater samples were determined by the method of Kieber and Mopper (1990). In this rapid and extremely sensitive analysis, samples and standards are reacted with 2,4-dinitrophenylhydrazine for one hour in the dark forming a hydrazone which is separated from interfering substances by HPLC and quantified by UV detection at 308 nm. The detection limit of the method is 10 nm with a precision of 5% at typical rainwater formaldehyde concentrations. 2.4. Hydrogen peroxide Hydrogen peroxide was analyzed by a fluorescence decay technique involving the peroxidase mediated oxidation of the fluorophore scopoletin by H2O2 in a phosphate buffered (0.1 M) sample at pH 7 (Kieber and Helz, 1986, 1995). Each sample was analyzed at least three times. Calibration curves were obtained by recording the decrease in fluorescence upon addition of dilutions of hydrogen peroxide stock solution to the sample. The method has an analytical precision of 2%
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RSD with a detection limit of 2 nm which was more than sufficient for rainwater samples. @
2.5. H+, Cl@, NO3 , SO4
2@
Anions were determined using suppressed ion chromatography and quantified against synthetic rain standards with a precision of 3% RSD (EPA, 1981; Fitchett, 1983). Rainwater pH was determined using a Ross Model 81-02 electrode calibrated with low ionic strength 4.10/6.97 buffers (Orion Research Incorporated, Boston, MA). pHix ionic strength adjuster (Orion) was added to each sample to match ionic strength of samples to buffers. Duplicate pH analyses agreed within 0.02 pH units. 2.6. Organic acids Formate and acetate standards were prepared from sodium salts. Standards were prepared daily from a concentrated stock prepared every other month. Organic acid concentrations were measured with a Dionex 4000i/ SP ion chromatograph with a SP4290 integrator, Dionex IonPacR AS11 4 mm analytical column, AG11 4 mm Guard column and anion micromembrance Suppressor Model AMMS-11 (Avery, 2001). Under the conditions used, this column is capable of resolving 34 different anions of organic acids.
3. Results and discussion 3.1. Diurnal variations With the exception of the aerosols Cl@ and SO2@ 4 (Fig. 1), concentrations of all analytes were higher in growing season p.m. rain compared to a.m. rain (Figs. 2 and 3). Cl@ concentrations displayed the opposite trend while SO2@ a.m. and p.m. concentrations were very 4 similar (Fig. 1). Although not as pronounced as during the growing season, the concentrations of all analytes during the non-growing season were higher in p.m. rain compared to the a.m. rain with the exceptions of formaldehyde, oxalic acid and pyruvic acid (Fig. 2). Diurnal variations in concentrations of rainwater constituents are driven by diurnal variations in their gas phase and/or aerosol concentrations. Changes in gas phase and aerosol concentrations, in turn, reflect variations in strengths of sources and/or sinks of individual components. In order for the diurnal variation observed in this study to occur, sources of rainwater components must be greater than sinks during the daytime and sinks must be greater than sources at night. Processes capable of causing these relatively short-term variations must be rapid enough to affect concentrations on a daily basis and be linked to differences associated with day- and nighttime conditions.
Fig. 1. Volume-weighted concentrations of rainwater components associated with particles during a.m. and p.m. growing and non-growing season events.
3.2. Sources Sources of rainwater components to the atmosphere generally include direct biogenic and anthropogenic emissions, as well as production by chemical and/or photochemical reactions from precursors resulting from these emissions. All analytes presented in this study result from these sources, with the exception of the sea salt component Cl@, which is emitted by physical processes at our coastal location and sulfate, which is affected by aerosol formation and scavenging. Maximum emissions or production via these sources (excluding seasalt components) are favored during growing season conditions which is evident from previous seasonal studies of rainwater components discussed later. Growing season conditions are characterized by high temperatures and intense long-duration solar radiation. On a daily basis, these conditions are optimized during the daytime. Therefore, the occurrence of higher concentrations of rainwater components in PM storms versus AM storms is consistent with an increase in source strengths during the daytime. For example, biogenic emissions from plants, which are known to emit both precursors of organic acids and the organic acids directly occur to a greater extent during the growing season when solar radiation and temperature is maximum (Keene and Galloway, 1986, 1988; Andreae et al., 1988; Talbot et al., 1988, 1990; Avery et al., 1991; Servant et al., 1991). This results in higher concentrations of organic acids in growing versus
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Fig. 2. Volume-weighted concentrations of water-soluble low molecular weight rainwater components with gas phase origins during a.m. and p.m. growing and non-growing season events.
non-growing season rain. The same patterns likely exist for short-term temporal variations in biogenic emission. Biogenic source strengths likely peak during the daytime when temperature and solar radiation is maximum. Anthropogenic emissions at our sampling location display a similar pattern. Rainwater components associated with pollution (i.e., NO@ 3 , non-sea salt sulfate, and acetic acid from automobile exhausts) occur to a greater extent during the growing season (Willey and Kiefer, 1993; Kieber et al., 1999a, b; Avery et al., 2001) at our coastal location when population is the greatest. Automobile exhausts, an important anthropogenic source, directly impact both gas phase and rainwater concentrations of acetic acid at our sampling location (Willey and Wilson, 1993; Avery et al., 2001), and likely affect other rainwater components associated with combustion. Automobile emissions, like biogenic emissions, are also maximum during the daytime hours and
likely impact gas phase concentrations of compounds emitted from them (e.g., Kawamura and Kaplan, 1987). Similarly, both chemical and photochemical reactions should be higher during the daytime when solar radiation and temperatures are the greatest. Sakugawa et al. (1993) reported that rainwater concentrations of H2O2, NO@ 3 , SO2@ 4 , aldehydes, and organic acids peaked in the afternoon hours during three long-duration rain events in Los Angeles displaying the importance of source strength variations on rainwater concentrations. Similarly, source strength variations at our location would support the increases in rainwater concentrations observed during the PM events in this study. 3.3. Sinks Variations in source strengths alone cannot explain diurnal variations of rainwater components observed in
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Fig. 3. Volume-weighted concentrations of inorganic rainwater components with gas phase origins during a.m. and p.m. growing and non-growing season events.
this study. There must also be a rapid removal mechanism capable of drawing down gas phase concentrations for a wide variety of rainwater components on a nightly basis. The consistent diurnal pattern observed for all constituents in this study (except Cl@ and SO2@ ), suggests a physical rather than chemical 4 removal process as the nighttime sink for these rainwater components. The similar impact this removal mechanism has on the various components is further displayed by the consistent amount of nighttime decrease. With the exception of pyruvic acid (Fig. 2), all components displayed a 2–3 decrease in concentration at night during the growing season (Figs. 2 and 3). Two such mechanisms supported by the data in the current study are removal of water-soluble gas phase components via dew formation and/or advection of low concentration air into the area at night.
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3.3.1. Dew With the exception of lactic and pyruvic acids, all components discussed in the current study have been previously identified in dew (Pierson and Brachaczek, 1990 and references therein, lactic and pyruvic acid concentrations may not have been sought in any of the studies referenced). The volume-weighted concentrations of many of the components identified in dew were similar to or greater than concentrations for rainwater reported in the current study. Dry deposition, which includes dew, has been reported to account for greater than 90% of the deposition of organic acids on an annual basis in southern California (Grosjean, 1989; Sakugawa et al., 1993). This indicates that dew may act as an important sink for water-soluble atmospheric gases removing them before they can be incorporated into rain. This would explain lower concentrations of constituents in a.m. rain since dew formation is typically a nighttime phenomenon. It would also explain the lack of a diurnal pattern in Cl@ and SO2@ concentrations 4 (Fig. 1) since these components are associated with particles and would not be removed from the atmosphere by dissolving in dew like gas phase components. Studies reporting large decreases in gas phase concentrations of formic and acetic acids at night illustrate the potential impact that dew formation may have on diurnal variations of gas phase concentrations of watersoluble species. For example, Hartman et al. (1991) found concentrations of organic acids in the gas phase over a Venezuelan scrub-grass and semi-deciduous forest below their detection limit at night indicating efficient removal of acids present during the daytime. This is consistent with the findings of Helas et al. (1992) who predicted (based on calculations) that 97% of gas phase formic acid and 81% of gas phase acetic acids would be removed at night via dew under realistic summertime atmospheric conditions. Talbot et al. (1988) reported up to a tenfold decrease in gaseous formic acid concentrations at night possibly due in part to removal via dew. There is also evidence in the literature suggesting that diurnal variations in gas phase concentrations of formic and acetic acids are not observed when dew formation does not occur. Servant et al. (1991) observed no diurnal variations in the gas phase formic and acetic acid concentrations in the Mayombe forest, Congo. Their mountainous study site was described as having ‘‘almost 100% cloud cover and weak sunlight limited to only about an hour per day’’. It is possible that the concentrations of gas phase formic and acetic acids did not display diurnal variations because the air was always moist and essentially dew was always present. Similarly, Puxbaum et al. (1988) observed summertime diurnal variations in gaseous concentrations of formic and acetic acids in Austria, but observed no such variations during winter conditions with snow cover. The less pronounced diurnal variations in concentra-
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tions of rainwater constituents observed during the nongrowing season rain in the current study may reflect decreased formation of dew during the winter when relative humidity is much lower. 3.3.2. Advection of marine air The other possible explanation for diurnal variations in concentrations of rainwater constituents supported by the data in the current study is nighttime advection of air that has not been greatly impacted by biogenic and anthropogenic sources. At our coastal location, marine air masses to the east contain lower concentrations of all rainwater constituents with the exception of Cl@, which is found in higher concentrations (Willey and Kiefer, 1993). Also, diurnal on- and off-shore wind variations are common during the growing season, meeting the criteria for the cause of the diurnal variations in rainwater constituent concentrations to be linked to day/night cycles. The advection of ‘‘clean’’ marine air at night would explain both the lower AM concentrations of most of the rainwater constituents and the higher Cl@ concentrations associated with marine aerosols. Advection of clean marine air is most likely not the only sink because diurnal variations in gas phase concentrations of components such as acetic and formic acid have been identified in areas well removed from the coast (Puxbaum et al., 1988; Talbot et al., 1988; Hartman et al., 1991). At sites like these, lower concentration air from aloft may advect into the lower atmosphere causing the observed diurnal variations (Helas et al., 1992; Talbot et al., 1988). Two other possible removal mechanisms that could lower nighttime gas phase concentrations are the predominance of marine storms at night and chemical reactions. Both these scenarios however, are not supported by data from the current study. Volumeweighted averages for the four categories of rain events presented in the current study were recalculated after removing marine storms from the data sets. Using only storms with terrestrial influence, the patterns of diurnal variations were identical. The second possible scenario, nightly removal via chemical reactions, are not rapid enough to explain the decrease in atmospheric concentrations observed for some of the rainwater constituents displaying diurnal variations in this study. For example, the removal of formic and acetic acids by reaction with hydroxyl radical is very slow resulting in calculated residence times of several weeks (Grosjean, 1989 and references therein). It is also highly unlikely that all these analytes would undergo chemical reactions leading to the same 2–3 loss at night. If chemical reactions were occurring, there would be much more variability in the loss at night. The minor role chemical reactions play as a removal mechanism can be best shown by the persistence of a diurnal cycle of pyruvic and oxalic acid concentrations during the growing season. These acids are thought to be rapidly consumed by photolysis and
should have residence times of only a few hours (Grosjean, 1989), yet they still display higher daytime concentrations in rainwater collected during the growing season. This further illustrates the strength of the previously described sinks and their impact on gas phase concentrations of atmospheric components. 4. Conclusions Diurnal variations in rainwater concentrations of several water-soluble atmospheric components have been observed in this study. Both anthropogenic and biogenic sources are highest during the day, but a rapid removal mechanism at night must also be invoked to explain the observed diurnal variations. Nocturnal removal of a wide variety of components in similar proportions (approximately 2–3 less at night) indicates a physical rather than a chemical process affecting diurnal variations. The opposite diurnal pattern for Cl@ concentration and the lack of a growing season diurnal pattern for SO2@ supports two different scenarios: (1) 4 the formation of dew at night removes the gas phase atmospheric gasses but not aerosols (Cl@ and SO2@ ) or 4 (2) during the night, a marine air mass containing lower concentrations of all the analytes and higher concentrations of Cl@ is advected into the area. This study demonstrates the importance of accounting for variations in sinks of analytes as well sources when considering causes of diurnal variations in concentrations of atmospheric constituents of rainwater.
Acknowledgements The Atmospheric Chemistry Division of the National Science Foundation supported this work through Grants ATM-9530069 and NSF ATM9729425. G. Brooks Avery Jr. was supported through the Camille and Henry Dreyfus Scholar/Fellow program for undergraduate institutions. The MACRL group at UNCW assisted with collection and analysis.
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G. B. Avery Jr. et al. / Atmospheric Environment 35 (2001) 3927–3933 Avery Jr., G.B., Tang, Y., Kieber, R.J., Willey, J., 2001. Impact of recent urbanization on formic and acetic acid concentrations in coastal North Carolina rainwater. Atmospheric Environment, in press. Avery Jr., G.B., Willey, J.D., Wilson, C.A., 1991. Formic and acetic acids in coastal North Carolina rainwater. Environmental Science and Technology 25, 1875–1879. Dawson, G.A., Farmer, J.C., Moyers, J.L., 1980. Formic and acetic acids in the atmosphere of the southwest U.S.A. Geophysical Research Letters 7, 725–728. EPA, 1981. Operations and maintenance manual for precipitation measurement systems. EPA-600/4-82-042b, Research Triangle Park, North Carolina. Fitchett, A.W., 1983. Analysis of Rain by Ion Chromatography, Vol. 823. ASTM Special Technique Publication, Philadelphia, PA. Grosjean, D., 1989. Organic acids in southern California air: ambient concentrations, mobile source emissions, in situ formation and removal processes. Environmental Science and Technology 23, 1506–1514. Hartman, W.R., Santana, M., Hermoso, M., Andreae, M.O., Sanhueza, E., 1991. Diurnal cycles of formic and acetic acids in the northern part of the Guayana Shield, Venezuela. Journal of Atmospheric Chemistry 13, 63–72. Helas, G., Andreae, M.O., Hartmann, W.R., 1992. Behavior of atmospheric formic and acetic acid in the presence of hydrometeors. Journal of Atmospheric Chemistry 15, 101–115. Kawamura, K., Kaplan, I.R., 1987. Motor exhaust emissions as a primary source for dicarboxylic acids in Los Angeles ambient air. Environmental Science and Technology 21, 105–110. Keene, W.C., Galloway, J.N., 1984. Organic acidity in precipitation of North America. Atmospheric Environment 18, 2491–2497. Keene, W.C., Galloway, J.N., 1986. Considerations regarding the sources for formic and acetic acids in the troposphere. Journal of Geophysical Research 91, 14466–14474. Keene, W.C., Galloway, J.N., 1988. The biogeochemical cycling of formic and acetic acids through the troposphere: an overview of our current understanding. Tellus 40B, 322–334. Kieber, R.J., Cooper, W.J., Willey, J.D., Avery Jr., G.B., 2001. Hydrogen peroxide at the Bermuda Atlantic time series station. Part 1: temporal variability of atmospheric hydrogen peroxide and its influence on seawater concentrations. Journal of Atmospheric Chemistry, in press. 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., Helz, R.G., 1995. Temporal and seasonal variations of hydrogen peroxide in estuarine waters. Estuarine and Coastal Shelf Science 40, 495–503. Kieber, R.J., Mopper, K., 1990. Determination of picomolar concentrations of carbonyl compounds in natural waters, including seawater, by liquid chromatography. Environmental Science and Technology 24, 1477–1481. Kieber, R.J., Rhines, M.F., Willey, J.D., Avery Jr., G.B., 1999a. Nitrite variability in coastal North Carolina rainwater and its impact on the nitrogen cycle in rain. Environmental Science and Technology 33 (3), 373–377.
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Kieber, R.J., Rhines, M.F., Willey, J.D., Avery Jr., G.B., 1999b. Rainwater formaldehyde: concentration, deposition and photochemical formation. Atmospheric Environment 33, 3659–3667. Likens, G.E., Keene, W.C., Miller, J.M., Galloway, J.N., 1987. Chemistry of precipitation from a remote terrestrial site in Australia. Journal of Geophysical Research 92, 13299–13314. Nolte, C.G., Matthew, M.P., Cass, G.R., 1999. Gas phase C2–C10 organic acid concentrations in the Los Angeles atmosphere. Environmental Science and Technology 33, 540–545. Pierson, W.R., Brachaczek, 1990. Dew chemistry and acid deposition in Glendora, California, during the 1986 carbonaceous species method comparison study. Aerosol Science and Technology 12, 8–27. Puxbaum, H., Rosenburg, C., Gregori, M., Lanzerstorfer, C., Ober, E., Winiwarter, W., 1988. Atmospheric concentrations of formic acid and acetic acid and related compounds in eastern and northern Austria. Atmospheric Environment 22, 2841–2850. Sakugawa, H., Kaplan, I.R., Shepard, L.S., 1993. Measurements of H2O2, aldehydes and organic acids in Los Angeles rainwater: their sources and deposition rates. Atmospheric Environment 27B, 203–219. Seinfeld, J.H., Pandis, S.N., 1998. Acid Deposition, Atmospheric Chemistry and Physics. Wiley, New York. Servant, J., Kouadio, G., Cros, B., Delmas, R., 1991. Carboxylic monacids in the air of Mayombe forest (Congo): role of the forest as a source or sink. Journal of Atmospheric Chemistry 12, 367–380. Talbot, R.W., Andreae, M.O., Barresheim, H., Jacob, D.J., Beecher, K.M., 1990. Sources and sinks of formic, acetic, and pyruvic acids over central Amazonia 2, wet season. Journal of Geophysical Research 95, 16799–16811. Talbot, R.W., Beecher, K.M., Harris, R.C., Cofer, W.R. III., 1988. Atmospheric geochemistry of formic and acetic acids at a mid-latitude temperate site. Journal of Geophysical Research 93, 1638–1652. Tang, Y., 1998. Organic acids in North Carolina rainwater. Masters Thesis, University of North Carolina at Wilmington. Tanner, R.L., Meng, Z., 1984. Seasonal variations in ambient atmospheric levels of formaldehyde and acetaldehyde. Environmental Science and Technology 18, 723–726. Topol, L.E., Levon, M., Flanagan, J., Schwall, R.J., Jackson, A.E., 1985. Quality assurance management for precipitation systems. EPA/600/4-82-042a, Environmental Protection Agency, Research Triangle Park, North Carolina. Willey, J.D., Kieber, R.J., Lancaster, R., 1996. Coastal rainwater hydrogen peroxide: concentration and deposition. Journal of Atmospheric Chemistry 25, 149–165. Willey, J.D., Kiefer, R.H., 1993. Atmospheric deposition in southeastern North Carolina: composition and quantity. Journal of Elisha Mitchell Scientific Society 109, 1–19. Willey, J.D., Wilson, C., 1993. Formic and acetic acids in atmospheric condensate in Wilmington, North Carolina. Journal of Atmospheric Chemistry 16, 123–133.
Diurnal variations in major rainwater components at a coastal site in North Carolina G. Brooks Avery Jr.*, Joan D. Willey, Robert J. Kieber Department of Chemistry and Marine Science Program, University of North Carolina at Wilmington, Wilmington, NC 28403-3297, USA Received 8 November 2000; accepted 17 March 2001
Abstract Concentrations of several major rainwater components were determined in rain events occurring during the early morning hours (12:00 midnight to 6:00 a.m.) and during the afternoon (12:00 noon to 6:00 p.m.) to examine possible diurnal variations. Generally, rainwater components with gas phase origins (H+, NO@ 3 , formaldehyde, H2O2, formic acid, acetic acid, pyruvic acid, oxalic acid, and lactic acid) had higher concentrations during p.m. rain events compared to a.m. events. Although source strengths of both biogenic and anthropogenic rainwater components are generally higher during the daytime, nocturnal removal of a wide variety of components in similar proportions (approximately 2–3 less at night) indicates a physical rather than a chemical process affecting diurnal variations. Rainwater components with aerosol origins (Cl@, and SO2@ ) displayed the opposite diurnal pattern or showed no diurnal 4 variation. Possible reasons for these variations include one or both of the following scenarios: (1) the formation of dew at night removes gas phase atmospheric gasses but not aerosols or (2) during the night, a marine air mass containing lower concentrations of all analytes and higher concentrations of Cl@ is advected into the area. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Dew; Rainwater; Diurnal variations
1. Introduction Major components of rainwater in our region include: 2@ H+, NO@ , NH+ 3 , SO4 4 (Willey and Kiefer, 1993), organic acids (Avery, 1989; Avery et al., 1991; Tang, 1998), hydrogen peroxide (Willey et al., 1996; Kieber et al., 2001), formaldehyde (Kieber et al., 1999a, b), and sea salts composed mainly of NaCl and MgSO4 (Willey and Kiefer, 1993). With the exception of the sea salts and precursors to H2SO4, which exist in the atmosphere as aerosols, the majority of these components or their precursors are found in the gas phase (Seinfeld and Pandis, 1998). When rain events occur, a fraction of both gas phase and particulate phase components are
*Corresponding author. Tel.: +1-910-962-7388; fax: +1910-962-3013. E-mail addresses: [email protected] (G. B. Avery Jr.).
incorporated into raindrops and are deposited as wet deposition (Seinfeld and Pandis, 1998). Atmospheric concentrations of rainwater components vary both spatially and temporally. For example, concentrations of terrestrial derived rainwater components associated with biogenic and anthropogenic emissions vary with proximity to landmass (e.g., Arlander et al., 1990) and population centers (e.g., Dawson et al., 1980; Likens et al., 1987; Nolte et al., 1999), respectively. Many biogenic components vary with season and are found in higher concentrations during the growing season (e.g., Keene and Galloway, 1984; Tanner and Meng, 1984; Avery et al., 1991). Some of these components undergo short-term temporal variations. For example, the concentrations of formic and acetic acids in the gas phase undergo diurnal variations characterized by elevated afternoon concentrations and lowered concentrations at night (Puxbaum et al., 1988; Talbot et al., 1988; Sakugawa et al., 1993).
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Dicarboxylic acid gas phase and particulate concentrations also display diurnal variations (Kawamura and Kaplan, 1987). Pollution derived components, such as nitric acid, also undergo diurnal variations linked to traffic volume and solar intensity and duration. At night, dry deposition of these components, combined with decreases in the production processes mentioned above, are thought to lower concentrations of the acids in the gas phase. Although diurnal variations in gas phase concentrations of several of these rainwater components have been well documented, no link has been established between these variations and resulting rainwater concentrations. The purpose of this study was to investigate diurnal variations in concentrations of rainwater components having primarily aerosol or gas phase sources. In order to accomplish this objective, rainwater concentrations of components originating as gas phase and aerosols were compared in storms occurring during early morning hours (a.m.) and during the afternoon (p.m.). Rainwater concentrations that were determined for components which originate primarily in the gas phase, or with precursors that originate in the gas phase included: hydrogen ion, NO@ 3 , H2O2, formaldehyde and the anions of organic acids, formate, acetate, lactate, pyruvate, and oxalate. Rainwater components scavenged primarily as aerosols included Cl@ and SO2@ ). 4
2.2. Season, a.m. p.m. definitions, and data sets
2. Methods
Aerosol components were defined as rainwater constituents that exist in the atmosphere primarily as aerosols. Gas phase components were defined as rainwater components with the majority of their atmospheric concentrations in the gas phase. The only exception would be our classification of the dicarboxylic acids as gas phase components. Kawamura and Kaplan (1987) reported an average of 62% of dicarboxylic acids as aerosols in a highly urbanized location. At our study site, the dicarboxylic acids behaved more like gas phase components than particulates. Seasons were defined as non-growing (1 October–31 March) and growing (1 April–30 September) (Avery, 1989). a.m. storms were defined as starting after 12:00 midnight local time and ending before 6:00 a.m. p.m. storms were defined as starting after 12:00 noon and ending before 6:00 p.m.. A total of 33 storms fit these criteria. Non-growing a.m. and p.m. events had 10 storms each, growing season p.m. and a.m. had 8 and 5 storms, respectively. Rainwater component concentration averages and standard deviations were volume weighted which minimizes effects of small rain events on averages and is the mathematical equivalent to combining all rain samples into one container prior to analysis (Topol et al., 1985). Four large anomalous events were eliminated from the data sets to prevent them from dominating volumeweighted averages. After these large events were removed, average depths of rain events for the four categories were very similar (range 13–20 mm).
2.1. Rain sampling
2.3. Formaldehyde
The primary rain sampling site used in this study was an open area of longleaf pine, wire grass, and turkey oak on the campus of the University of North Carolina at Wilmington (34113.90 N, 77152.70 W, 8.5 km from the Atlantic Ocean). This site complies with US EPA specifications for rain gauge and sampler placement (Topol et al., 1985). All rainwater event samples were collected using an Aerochem Metrics (ACM) Model 301 Automatic Sensing Wet/Dry Precipitation Collector which housed a 4 l muffled glass beaker placed within a HDPE plastic bucket. For formaldehyde analysis, 5 ml rainwater samples were withdrawn, placed in separate 7 ml Teflon vials, derivatized with 100 ml DNPH and refrigerated at 41C. Samples for organic acid analysis were placed in 30 ml Teflon vials, preserved with chloroform, and stored in a refrigerator (41C) immediately after collection to stop biological activity (Keene and Galloway, 1984). Chloroform blanks showed no organic acids contamination. (Tang, 1998). Hydrogen peroxide and pH measurements were conducted immediately upon collection.
Formaldehyde concentrations in rainwater samples were determined by the method of Kieber and Mopper (1990). In this rapid and extremely sensitive analysis, samples and standards are reacted with 2,4-dinitrophenylhydrazine for one hour in the dark forming a hydrazone which is separated from interfering substances by HPLC and quantified by UV detection at 308 nm. The detection limit of the method is 10 nm with a precision of 5% at typical rainwater formaldehyde concentrations. 2.4. Hydrogen peroxide Hydrogen peroxide was analyzed by a fluorescence decay technique involving the peroxidase mediated oxidation of the fluorophore scopoletin by H2O2 in a phosphate buffered (0.1 M) sample at pH 7 (Kieber and Helz, 1986, 1995). Each sample was analyzed at least three times. Calibration curves were obtained by recording the decrease in fluorescence upon addition of dilutions of hydrogen peroxide stock solution to the sample. The method has an analytical precision of 2%
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RSD with a detection limit of 2 nm which was more than sufficient for rainwater samples. @
2.5. H+, Cl@, NO3 , SO4
2@
Anions were determined using suppressed ion chromatography and quantified against synthetic rain standards with a precision of 3% RSD (EPA, 1981; Fitchett, 1983). Rainwater pH was determined using a Ross Model 81-02 electrode calibrated with low ionic strength 4.10/6.97 buffers (Orion Research Incorporated, Boston, MA). pHix ionic strength adjuster (Orion) was added to each sample to match ionic strength of samples to buffers. Duplicate pH analyses agreed within 0.02 pH units. 2.6. Organic acids Formate and acetate standards were prepared from sodium salts. Standards were prepared daily from a concentrated stock prepared every other month. Organic acid concentrations were measured with a Dionex 4000i/ SP ion chromatograph with a SP4290 integrator, Dionex IonPacR AS11 4 mm analytical column, AG11 4 mm Guard column and anion micromembrance Suppressor Model AMMS-11 (Avery, 2001). Under the conditions used, this column is capable of resolving 34 different anions of organic acids.
3. Results and discussion 3.1. Diurnal variations With the exception of the aerosols Cl@ and SO2@ 4 (Fig. 1), concentrations of all analytes were higher in growing season p.m. rain compared to a.m. rain (Figs. 2 and 3). Cl@ concentrations displayed the opposite trend while SO2@ a.m. and p.m. concentrations were very 4 similar (Fig. 1). Although not as pronounced as during the growing season, the concentrations of all analytes during the non-growing season were higher in p.m. rain compared to the a.m. rain with the exceptions of formaldehyde, oxalic acid and pyruvic acid (Fig. 2). Diurnal variations in concentrations of rainwater constituents are driven by diurnal variations in their gas phase and/or aerosol concentrations. Changes in gas phase and aerosol concentrations, in turn, reflect variations in strengths of sources and/or sinks of individual components. In order for the diurnal variation observed in this study to occur, sources of rainwater components must be greater than sinks during the daytime and sinks must be greater than sources at night. Processes capable of causing these relatively short-term variations must be rapid enough to affect concentrations on a daily basis and be linked to differences associated with day- and nighttime conditions.
Fig. 1. Volume-weighted concentrations of rainwater components associated with particles during a.m. and p.m. growing and non-growing season events.
3.2. Sources Sources of rainwater components to the atmosphere generally include direct biogenic and anthropogenic emissions, as well as production by chemical and/or photochemical reactions from precursors resulting from these emissions. All analytes presented in this study result from these sources, with the exception of the sea salt component Cl@, which is emitted by physical processes at our coastal location and sulfate, which is affected by aerosol formation and scavenging. Maximum emissions or production via these sources (excluding seasalt components) are favored during growing season conditions which is evident from previous seasonal studies of rainwater components discussed later. Growing season conditions are characterized by high temperatures and intense long-duration solar radiation. On a daily basis, these conditions are optimized during the daytime. Therefore, the occurrence of higher concentrations of rainwater components in PM storms versus AM storms is consistent with an increase in source strengths during the daytime. For example, biogenic emissions from plants, which are known to emit both precursors of organic acids and the organic acids directly occur to a greater extent during the growing season when solar radiation and temperature is maximum (Keene and Galloway, 1986, 1988; Andreae et al., 1988; Talbot et al., 1988, 1990; Avery et al., 1991; Servant et al., 1991). This results in higher concentrations of organic acids in growing versus
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Fig. 2. Volume-weighted concentrations of water-soluble low molecular weight rainwater components with gas phase origins during a.m. and p.m. growing and non-growing season events.
non-growing season rain. The same patterns likely exist for short-term temporal variations in biogenic emission. Biogenic source strengths likely peak during the daytime when temperature and solar radiation is maximum. Anthropogenic emissions at our sampling location display a similar pattern. Rainwater components associated with pollution (i.e., NO@ 3 , non-sea salt sulfate, and acetic acid from automobile exhausts) occur to a greater extent during the growing season (Willey and Kiefer, 1993; Kieber et al., 1999a, b; Avery et al., 2001) at our coastal location when population is the greatest. Automobile exhausts, an important anthropogenic source, directly impact both gas phase and rainwater concentrations of acetic acid at our sampling location (Willey and Wilson, 1993; Avery et al., 2001), and likely affect other rainwater components associated with combustion. Automobile emissions, like biogenic emissions, are also maximum during the daytime hours and
likely impact gas phase concentrations of compounds emitted from them (e.g., Kawamura and Kaplan, 1987). Similarly, both chemical and photochemical reactions should be higher during the daytime when solar radiation and temperatures are the greatest. Sakugawa et al. (1993) reported that rainwater concentrations of H2O2, NO@ 3 , SO2@ 4 , aldehydes, and organic acids peaked in the afternoon hours during three long-duration rain events in Los Angeles displaying the importance of source strength variations on rainwater concentrations. Similarly, source strength variations at our location would support the increases in rainwater concentrations observed during the PM events in this study. 3.3. Sinks Variations in source strengths alone cannot explain diurnal variations of rainwater components observed in
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Fig. 3. Volume-weighted concentrations of inorganic rainwater components with gas phase origins during a.m. and p.m. growing and non-growing season events.
this study. There must also be a rapid removal mechanism capable of drawing down gas phase concentrations for a wide variety of rainwater components on a nightly basis. The consistent diurnal pattern observed for all constituents in this study (except Cl@ and SO2@ ), suggests a physical rather than chemical 4 removal process as the nighttime sink for these rainwater components. The similar impact this removal mechanism has on the various components is further displayed by the consistent amount of nighttime decrease. With the exception of pyruvic acid (Fig. 2), all components displayed a 2–3 decrease in concentration at night during the growing season (Figs. 2 and 3). Two such mechanisms supported by the data in the current study are removal of water-soluble gas phase components via dew formation and/or advection of low concentration air into the area at night.
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3.3.1. Dew With the exception of lactic and pyruvic acids, all components discussed in the current study have been previously identified in dew (Pierson and Brachaczek, 1990 and references therein, lactic and pyruvic acid concentrations may not have been sought in any of the studies referenced). The volume-weighted concentrations of many of the components identified in dew were similar to or greater than concentrations for rainwater reported in the current study. Dry deposition, which includes dew, has been reported to account for greater than 90% of the deposition of organic acids on an annual basis in southern California (Grosjean, 1989; Sakugawa et al., 1993). This indicates that dew may act as an important sink for water-soluble atmospheric gases removing them before they can be incorporated into rain. This would explain lower concentrations of constituents in a.m. rain since dew formation is typically a nighttime phenomenon. It would also explain the lack of a diurnal pattern in Cl@ and SO2@ concentrations 4 (Fig. 1) since these components are associated with particles and would not be removed from the atmosphere by dissolving in dew like gas phase components. Studies reporting large decreases in gas phase concentrations of formic and acetic acids at night illustrate the potential impact that dew formation may have on diurnal variations of gas phase concentrations of watersoluble species. For example, Hartman et al. (1991) found concentrations of organic acids in the gas phase over a Venezuelan scrub-grass and semi-deciduous forest below their detection limit at night indicating efficient removal of acids present during the daytime. This is consistent with the findings of Helas et al. (1992) who predicted (based on calculations) that 97% of gas phase formic acid and 81% of gas phase acetic acids would be removed at night via dew under realistic summertime atmospheric conditions. Talbot et al. (1988) reported up to a tenfold decrease in gaseous formic acid concentrations at night possibly due in part to removal via dew. There is also evidence in the literature suggesting that diurnal variations in gas phase concentrations of formic and acetic acids are not observed when dew formation does not occur. Servant et al. (1991) observed no diurnal variations in the gas phase formic and acetic acid concentrations in the Mayombe forest, Congo. Their mountainous study site was described as having ‘‘almost 100% cloud cover and weak sunlight limited to only about an hour per day’’. It is possible that the concentrations of gas phase formic and acetic acids did not display diurnal variations because the air was always moist and essentially dew was always present. Similarly, Puxbaum et al. (1988) observed summertime diurnal variations in gaseous concentrations of formic and acetic acids in Austria, but observed no such variations during winter conditions with snow cover. The less pronounced diurnal variations in concentra-
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tions of rainwater constituents observed during the nongrowing season rain in the current study may reflect decreased formation of dew during the winter when relative humidity is much lower. 3.3.2. Advection of marine air The other possible explanation for diurnal variations in concentrations of rainwater constituents supported by the data in the current study is nighttime advection of air that has not been greatly impacted by biogenic and anthropogenic sources. At our coastal location, marine air masses to the east contain lower concentrations of all rainwater constituents with the exception of Cl@, which is found in higher concentrations (Willey and Kiefer, 1993). Also, diurnal on- and off-shore wind variations are common during the growing season, meeting the criteria for the cause of the diurnal variations in rainwater constituent concentrations to be linked to day/night cycles. The advection of ‘‘clean’’ marine air at night would explain both the lower AM concentrations of most of the rainwater constituents and the higher Cl@ concentrations associated with marine aerosols. Advection of clean marine air is most likely not the only sink because diurnal variations in gas phase concentrations of components such as acetic and formic acid have been identified in areas well removed from the coast (Puxbaum et al., 1988; Talbot et al., 1988; Hartman et al., 1991). At sites like these, lower concentration air from aloft may advect into the lower atmosphere causing the observed diurnal variations (Helas et al., 1992; Talbot et al., 1988). Two other possible removal mechanisms that could lower nighttime gas phase concentrations are the predominance of marine storms at night and chemical reactions. Both these scenarios however, are not supported by data from the current study. Volumeweighted averages for the four categories of rain events presented in the current study were recalculated after removing marine storms from the data sets. Using only storms with terrestrial influence, the patterns of diurnal variations were identical. The second possible scenario, nightly removal via chemical reactions, are not rapid enough to explain the decrease in atmospheric concentrations observed for some of the rainwater constituents displaying diurnal variations in this study. For example, the removal of formic and acetic acids by reaction with hydroxyl radical is very slow resulting in calculated residence times of several weeks (Grosjean, 1989 and references therein). It is also highly unlikely that all these analytes would undergo chemical reactions leading to the same 2–3 loss at night. If chemical reactions were occurring, there would be much more variability in the loss at night. The minor role chemical reactions play as a removal mechanism can be best shown by the persistence of a diurnal cycle of pyruvic and oxalic acid concentrations during the growing season. These acids are thought to be rapidly consumed by photolysis and
should have residence times of only a few hours (Grosjean, 1989), yet they still display higher daytime concentrations in rainwater collected during the growing season. This further illustrates the strength of the previously described sinks and their impact on gas phase concentrations of atmospheric components. 4. Conclusions Diurnal variations in rainwater concentrations of several water-soluble atmospheric components have been observed in this study. Both anthropogenic and biogenic sources are highest during the day, but a rapid removal mechanism at night must also be invoked to explain the observed diurnal variations. Nocturnal removal of a wide variety of components in similar proportions (approximately 2–3 less at night) indicates a physical rather than a chemical process affecting diurnal variations. The opposite diurnal pattern for Cl@ concentration and the lack of a growing season diurnal pattern for SO2@ supports two different scenarios: (1) 4 the formation of dew at night removes the gas phase atmospheric gasses but not aerosols (Cl@ and SO2@ ) or 4 (2) during the night, a marine air mass containing lower concentrations of all the analytes and higher concentrations of Cl@ is advected into the area. This study demonstrates the importance of accounting for variations in sinks of analytes as well sources when considering causes of diurnal variations in concentrations of atmospheric constituents of rainwater.
Acknowledgements The Atmospheric Chemistry Division of the National Science Foundation supported this work through Grants ATM-9530069 and NSF ATM9729425. G. Brooks Avery Jr. was supported through the Camille and Henry Dreyfus Scholar/Fellow program for undergraduate institutions. The MACRL group at UNCW assisted with collection and analysis.
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