Joan D. Willey

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.

Natural fluorescence as a tracer for distinguishing between piedmont and coastal plain river water in the nearshore waters of Georgia and North Carolina

Natural fluorescence, dissolved silica and salinity were investigated as possible tracers to distinguish between piedmont river and coastal plain river waters discharged into the ocean off North Carolina and Georgia. In the Georgia study area, dissolved silica was not suitable for use as a tracer because silica concentrations were variable and did not mix conservatively with seawater. In the North Carolina study area, dissolved silica concentrations exhibited too much short-term variability for tracer use. In both areas, natural fluorescence was a suitable tracer. Additional investigations relevant to tracer application were made of the method for determining natural fluorescence; these include dependence on temperature of analysis, pH dependence, sample storage effect, sensitivity, correlation with total organic carbon and possible interferences from chlorophyll a , lignin sulfonates, detergents, petroleum and iron.

The effect of seawater magnesium on natural fluorescence during estuarine mixing, and implications for tracer applications

Abstract Natural fluorescence, which is thought to result from low molecular weight humic and fulvic compounds, can be used as a tracer to distinguish between individual river waters. Natural fluorescence exhibits conservative mixing with seawater, except for a slight fluorescence increase which is sometimes observed in the low salinity range (0–5‰). This increase is not due to the inner filter effect (internal quenching). Laboratory experiments can reproduce this low-salinity natural fluorescence increase. Of the major seawater ions, only magnesium can cause a similar natural fluorescence increase. Variation in sample pH, ionic strength, or particle content cannot explain the natural fluorescence increase, nor does it appear to be related to the estuarine flocculation of humic material. Addition of seawater magnesium to the fluorescent material with subsequent loss of hydrogen ions could enhance fluorescence by adding crosslinking to the structure. Replacement of a fluorescence-depressing metal like copper or iron by magnesium could also enhance fluorescence, essentially by removing the quenching effect of the metal. Experimental data in this study are consistent with both of these possible mechanisms. Calcium also enhances fluorescence, however the effect of seawater calcium during estuarine mixing is not as apparent as the magnesium effect. The implications of this low-salinity natural fluorescence increase with respect to estuarine and coastal tracer applications depend on whether individual rivers mix in the high or low salinity region of an estuary or coastal area.

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.

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.

The effect of pressure on the solubility of amorphous silica in seawater at 0°C

The solubility of amorphous silica in seawater at 0°C and from 1 to 1,220 atm. was found to be a linear function of pressure above 270 atm., but to deviate from linearity below that pressure. Using a quadratic derivation of Plancks equation, ΔV for the dissolution was found to be −16.5 cm3mole−1, and Δk was found to be −4.4 · 10−2 cm3 mole−1 atm−1∂Δk/∂P was found to be 27.2 · 10−5 cm3 mole−1 atm−2 which is too significant a factor to allow the commonly made assumption that ∂Δk/∂P =0. Norths (1973) model of hydration suggests that this non-zero ∂Δk/∂P may indicate that the silicic acid molecule is more extensively hydrated at lower pressures. If the pressure in an experiment is suddenly lowered to atmospheric pressure after equilibrium solubility had been attained at the higher pressure, the precipitation that occurs to reduce the resulting supersaturation is complete within one hour in the experimental system used in this study.

Enhancement of chlorophyll a production in Gulf Stream surface seawater by rainwater nitrate

Abstract The concentration of nitrate commonly found in rain from continental storm systems passing over eastern North Carolina was sufficient to stimulate chlorophyll a production in surface Gulf Stream water collected over the continental slope near Cape Hatteras, NC, in mid-October 1989. Two bioassay experiments were conducted on board ship by mixing Gulf Stream surface seawater with synthetic rainwater (5% rainwater). These bioassays indicated an increased production of chlorophyll a in response to rainwater nitrate but not phosphate. The response was apparent after incubation for 2 days, and the chlorophyll a concentrations increased approximately 2.5 times relative to controls in both experiments.

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.