Robert J. Kieber

Rainwater dissolved organic carbon: concentrations and global flux.

Dissolved organic carbon (DOC) is a major component of both marine (23 μM) and continental (161 μM) rain, present in concentrations greater than nitric and sulfuric acids combined. Rain is a significant source of DOC to surface seawater (90 × 1012 g C yr−1), equivalent to the magnitude of river input of DOC to the open ocean and half the magnitude of carbon buried in marine sediments per year on a global scale. Current models of global carbon cycling focus primarily on inorganic forms of carbon and are unable to account for approximately 20% of the global carbon dioxide, suggesting a significant missing carbon sink. Quantification of the average DOC concentration in marine rain allows calculation of the global rainwater flux of DOC of 430 ± 150 × 1012 g C yr−1. When inorganic carbon is included, this rainwater carbon flux becomes 510 ± 170 × 1012 g C yr−1, which, although not the same carbon, is equivalent in magnitude to over one third of the missing carbon sink.

Iron speciation in coastal rainwater: concentration and deposition to seawater

More than half of the dissolved iron in rain collected in Wilmington, NC, USA, occurred as Fe(II)(aq). More than 80% of the dissolved iron in marine rain from several marine storms in both North Carolina and New Zealand was Fe(II)(aq). In almost all rain events Fe(II)(aq) was in excess of Fe(III)(aq). Rainwater is a significant source of iron to surface seawater and contributes approximately 1010 mol year−1 of dissolved plus particulate iron to surface seawater on a global scale, which is more than 30 times the amount of iron resident in the surface 10 m of seawater. The length of time atmospherically deposited dissolved iron remains in surface seawater is critical to its role as a phytoplankton nutrient because it is predominately the soluble form of Fe that is bioavailable. Earlier studies have demonstrated that Fe(II)(aq) oxidizes rapidly in seawater. Our experiments utilizing authentic rainwater with ambient concentrations and speciation of iron clearly demonstrate, however, that rainwater Fe(II)(aq) is stabilized against oxidation for more than 4 h in seawater and rainwater Fe(III)(aq) is protected against rapid precipitation when added to coastal or oligotrophic seawater. These results are significant because they show rainwater deposited Fe does not behave as previously thought based on earlier kinetic work on non-rainwater Fe(II) oxidation in seawater. Rainwater, therefore, is an important source of soluble, stable Fe(II)(aq) to surface seawater.

Rainwater formaldehyde: concentration, deposition and photochemical formation

Formaldehyde (HCHO) concentrations were measured in 116 rain samples in Wilmington, NC from June 1996 to February 1998. Concentrations ranged from below the detection limit of 10 nM, to 13 μM, in the range of HCHO levels reported at other locations worldwide. The volume-weighted annual average rainwater formaldehyde concentration was 3.3±0.3 μM and comprised approximately 3% of the measured dissolved organic carbon. Using the volume weighted average HCHO concentration and annual precipitation of 1.4 m, an annual formaldehyde deposition of 4.6 mmol m−2 yr−1 was determined. Rainwater is a significant source of formaldehyde to surface waters and may contribute as much as 30 times the resident amount found in natural waters of southeastern North Carolina during the summer. Formaldehyde concentrations did not correlate with precipitation volume suggesting continuous supply during rain events. Evidence is presented which indicates part of this supply may be from direct photochemical production in the aqueous phase. Formaldehyde levels exhibited a distinct seasonal oscillation, with higher concentrations during the summer. This pattern is similar to that observed with other rainwater parameters at this site including pH, nitrate, and ammonium, and is most likely the result of increased photochemical production, as well as biogenic and anthropogenic emissions during summer months. The concentration of formaldehyde in both winter El Nino rains and summer tropical rains was less than half its concentration in non-El Nino or non-tropical events, suggesting significant terrestrial input. Formaldehyde was correlated with hydrogen peroxide and non-sea-salt sulfate deposition suggesting a relationship between HCHO, H2O2, S(VI) within the troposphere.

Coastal rainwater hydrogen peroxide: Concentration and deposition

Correlation analysis between rainwater component concentrations (hydrogen peroxide, hydrogen ion, nitrate, nonseasalt sulfate and chloride ion) was used to investigate patterns of variation in hydrogen peroxide concentrations in rain collected in Wilmington, North Carolina, a coastal southeastern United States location, between October 1992, and October 1994. Rainwater hydrogen peroxide concentrations in general correlated positively with the pollutant components (hydrogen ion, nitrate and non-seasalt sulfate). This pattern suggests that destruction of hydrogen peroxide by sulfur dioxide is not the dominant factor controlling the concentration of hydrogen peroxide in this rainwater, with the possible exception of winter rain from coastal storms where an inverse correlation between hydrogen peroxide and nonseasalt sulfate was observed. Sequential sampling indicates rapid production of hydrogen peroxide and incorporation into rain within time periods of hours during summer daytime rains.Rain is an important transport mechanism for removal of atmospheric hydrogen peroxide, which may affect the oxidizing capacity of surface waters that receive the rain. During this study time, the annual deposition of hydrogen peroxide by rain was 12 mmole m-2 yr-1. An average rain event added approximately half of the resident amount of hydrogen peroxide to the shallow lakes typical of eastern North Carolina; extreme rain events can triple the amount normally present. The episodic nature of rain contributes to the variability in hydrogen peroxide concentration in surface waters. Higher hydrogen peroxide concentrations and greater rainfall amounts cause wet deposition of hydrogen peroxide to be approximately seven times greater during the warm season than the cold season.

A chemiluminescence method for the analysis of H2O2 in natural waters

Abstract Hydrogen peroxide (H 2 O 2 ) was determined at nanomolar levels in natural waters by a chemiluminescent method involving reaction of hydrogen peroxide with an acridinium ester 10-methyl-9-( p -formylphenyl)-acridinium carboxylate trifluoromethanesulfonate. The method is simple, rapid, requires no catalyst or metal ion complexes, and has an analytical precision of 4% RSD at typical natural water concentrations. The analysis also produces a linear response over the concentration range, 5×10 −9 to 60×10 −6 M, simply by changing the pH of the solution prior to addition of the acridinium compound or by varying the concentration of the acridinium ester at constant pH. The detection limit is 5 nM and is limited primarily by the capability to obtain H 2 O 2 -free blank water. Analytical results were verified in distilled and a variety of natural water matrices by intercomparison with a completely independent fluorescence decay technique involving the hydrogen peroxide oxidation of the fluorophore scopoletin. The two methods produced results with no statistical differences in the data at the 99.9% confidence level. In addition, the method does not suffer from background fluorescence matrix effects in organic-rich environments, which hinders the applicability of commonly used natural water hydrogen peroxide analyses. Application of the method to these highly fluorescent waters is also presented.

Temporal Variability of Iron Speciation in Coastal Rainwater

Iron occurs in rain as particulateand dissolved Fe and includes both Fe(II) and Fe(III)species. Model calculations and correlation analysisindicate Fe(II)(aq) occurs almost exclusively as thefree ion whereas Fe(III)(aq) occurs as both ironoxalate and Fe(OH)2+(aq) with largevariations over the pH range from 4.0 to 5.0. Complexation with humic-like compounds may also beimportant for Fe(III)(aq); however, the concentrationand structural characteristics of these compounds haveyet to be determined. 112 rain samples were collectedfor iron analysis in Wilmington, North Carolina,between 1 July 1997, and 30 June 1999. Total iron,particulate iron and Fe(III)(aq) were higher inconcentration in summer and spring rain relative towinter and autumn rain. Fe(II)(aq) concentrations, incontrast, did not vary seasonally. Particulate iron,which was approximately half the total rainwater iron,was highest between noon and 6 p.m. (EST), probably dueto more intense regional convection including land-seabreezes during that time. The ratio ofFe(II)(aq)/Fe(III)(aq) was also highest in rainreceived between noon and 6 p.m., which most likelyreflects photochemical reduction of Fe(III)(aq)complexes to form Fe(II)(aq). A conceptual modeldepicting the interplay between iron species, lightintensity and organic ligands in rainwater ispresented.

Iron speciation and hydrogen peroxide concentrations in New Zealand rainwater

Abstract Rain was collected on the southern portion of the South Island of New Zealand during the summer of 1999 (January–March) during which time significant losses of ozone and increased UV were reported in the stratosphere over New Zealand. Iron and hydrogen peroxide concentrations were measured in rainwater because these analytes are directly influenced by photochemical processes and therefore are particularly susceptible to increasing UV levels. The absolute concentration of dissolved Fe(II) in New Zealand samples was very similar to summertime rain received in Wilmington, NC however the relative contribution of Fe(II) to total Fe was approximately twice as great in New Zealand samples. The larger percentage of reduced iron may reflect higher UV levels in New Zealand since Fe(II) is generated via photochemical reduction of particulate or dissolved Fe(III). No dissolved Fe(III) was detected in New Zealand rainwater, in contrast to the Wilmington site, where summertime Fe(III) concentrations are approximately equal to Fe(II) concentrations. Summertime hydrogen peroxide concentrations and diel variability in New Zealand were similar to other coastal and marine values in both the northern and southern hemispheres suggesting the increasing UV in New Zealand is not significantly increasing hydrogen peroxide concentrations at this location. Any excess photochemically produced hydrogen peroxide in New Zealand may be consumed through oxidation of Fe(II) which is rapidly reformed from photochemical reduction of Fe(III) by the higher UV levels in New Zealand.

HYDROPHOBIC C18 BOUND ORGANIC COMPLEXES OF CHROMIUM AND THEIR POTENTIAL IMPACT ON THE GEOCHEMISTRY OF CHROMIUM IN NATURAL WATERS

The complexation of aqueous, inorganic chromium by naturally occurring, relatively hydrophobic dissolved organic matter was investigated in a wide variety of natural waters. Levels of complexed chromium ranged from a few picomolar in open ocean organic-poor waters to several nanomolar in inland DOM-richwaters. Organic chromium concentrations were positively correlated to both dissolved organic carbon and UV absorbance. Humic substances appear to be important complexing agents in all waters studied. Photolysis experiments indicated that the organochromium species were photodegradable, with significant degradation occurring even after short-term exposure to ambient sunlight

Diurnal variations in major rainwater components at a coastal site in North Carolina

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+, NO3−, 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 SO42−) displayed the opposite diurnal pattern or showed no diurnal 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.

Long-Term Temporal Variability in Hydrogen Peroxide Concentrations in Wilmington, North Carolina USA Rainwater

Measurements of hydrogen peroxide (H(2)O(2)), pH, dissolved organic carbon (DOC), and inorganic anions (chloride, nitrate, and sulfate) in rainwater were conducted on an event basis at a single site in Wilmington, NC for the past decade in a study that included over 600 individual rain events. Annual volume weighted average (VWA) H(2)O(2) concentrations were negatively correlated (p < 0.001) with annual VWA nonseasalt sulfate (NSS) concentrations in low pH (<5) rainwater. Under these conditions H(2)O(2) is the primary aqueous-phase oxidant of SO(2) in the atmosphere. We attribute the increase of H(2)O(2) to decreasing SO(2) emissions which has had the effect of reducing a major tropospheric sink for H(2)O(2). Annual VWA H(2)O(2) concentrations in low pH (<5) rains showed a significant increase over the time scale of this study, which represents the only long-term continuous data set of H(2)O(2) concentrations in wet deposition at a single location. This compositional change has important implications because H(2)O(2) is a source of highly reactive free radicals so its increase reflects a higher overall oxidation capacity of atmospheric waters. Also, because rainwater is an important mechanism by which H(2)O(2) is transported from the atmosphere to surface waters, greater wet deposition of H(2)O(2) could influence the redox chemistry of receiving watersheds which typically have concentrations 2-3 orders of magnitude lower than rainwater.