Compound-Specific Carbon Isotopic Composition of Ethanol in Brazil and US Vehicle Emissions and Wet Deposition.

Global atmospheric ethanol budget models include large uncertainties in the magnitude of ethanol emission sources and sinks. To apply stable isotope techniques to constrain ethanol emission sources, a headspace solid phase microextraction gas chromatograph-combustion-isotope ratio mass spectrometry method (HS-SPME-GC-C-IRMS) was developed to measure the carbon isotopic composition of aqueous phase ethanol at natural abundance levels (1-30 µM) with a precision of 0.4‰. The method was applied to determine the carbon isotope signatures (δ13C) of vehicle ethanol emission sources in Brazil (-12.8 ± 2.4‰) and the US (-9.8 ± 2.5‰), and to measure the carbon isotope composition of ethanol in wet deposition (-22.6 to -12.7‰). A two end-member isotope mixing model was developed using anthropogenic and biogenic end members and fractionation scenarios to estimate ethanol source contributions to wet deposition collected in Brazil and US. Mixing model results indicate anthropogenic sources contribute two and a half to four times more ethanol to the atmosphere than previously predicted in modeled global ethanol inventories. As established and developing countries continue to rapidly increase ethanol fuel consumption and subsequent emissions, understanding the magnitude of all ethanol sources and sinks will be essential for modeling future atmospheric chemistry and air quality impacts.

Variability of ethanol concentration in rainwater driven by origin versus season in coastal and inland North Carolina, USA.

Rainwater ethanol concentrations were measured for one year (June 2013-May 2014) in central (Elon, NC) and coastal (Wilmington, NC) North Carolina, allowing for a comparison of the effects of coastal and marine rain on ethanol concentration and deposition both at the coast and 250 km inland. Rain samples were collected on an event basis and analyzed using enzyme oxidation and headspace solid-phase microextraction (HS-SPME). The volume-weighted average ethanol concentration at Elon (609 ±â€¯116 nM) was higher than at Wilmington (208 ±â€¯21 nM). Rainfall influenced by air masses originating over the Atlantic Ocean has previously been observed to be lower in ethanol concentration than terrestrial rain at the Wilmington location, and this was true during this study as well. Lower-ethanol marine and coastal air masses did not affect the concentration of ethanol in Elon rain, 250 km from the coast. This is likely due to the rapid supply of locally emitted ethanol to air masses moving over the land. No difference in rainwater ethanol concentrations was observed for Elon rain based on air mass back trajectories, most likely because all the rain was impacted by both anthropogenic and biogenic terrestrial sources typical of most inland areas. Seasonal variation in ethanol concentrations was significant in the inland location with elevated ethanol concentrations observed in fall; no seasonal variation was observed in coastal location rain. This study presents for the first time the different drivers for ethanol concentrations in rainwater from a coastal and a proximal inland location.

Use of experimentally determined Henry's Law and salting-out constants for ethanol in seawater for determination of the saturation state of ethanol in coastal waters.

The Henry's law constant for ethanol in seawater was experimentally determined to be 221 ± 4 M/atm at 22 °C compared with 247 ± 6 M/atm in pure water. The salting out coefficient for ethanol was 0.13 M-1. In seawater ln(KH) = -(12.8 ± 0.7) + (5310 ± 197)/T where KH is in M atm-1 and temperature is in K. This plus the salting out coefficient allow calculation of KH for any estuarine or sea water between 1 and 35 °C. High concentrations of dissolved organic carbon do not affect KH values in fresh or seawater. Nearshore surface waters were usually undersaturated with respect to gas phase ethanol except when air concentrations decreased, whereas surface seawater 40 km from shore was supersaturated. The percent saturation in surface waters is driven primarily by changes in air concentrations because these change quickly (hours) and more extensively than surface water. This study allows calculation of ethanol saturation states from air and surface water concentrations which is a necessary step to define the role of surface oceans in the global biogeochemical cycling of ethanol both now and in the future as use of ethanol biofuel continues to grow.

Surface waters as a sink and source of atmospheric gas phase ethanol.

This study reports the first ethanol concentrations in fresh and estuarine waters and greatly expands the current data set for coastal ocean waters. Concentrations for 153 individual measurements of 11 freshwater sites ranged from 5 to 598 nM. Concentrations obtained for one estuarine transect ranged from 56 to 77 nM and levels in five coastal ocean depth profiles ranged from 81 to 334 nM. Variability in ethanol concentrations was high and appears to be driven primarily by photochemical and biological processes. 47 gas phase concentrations of ethanol were also obtained during this study to determine the surface water degree of saturation with respect to the atmosphere. Generally fresh and estuarine waters were undersaturated indicating they are not a source and may be a net sink for atmospheric ethanol in this region. Aqueous phase ethanol is likely converted rapidly to acetaldehyde in these aquatic ecosystems creating the undersaturated conditions resulting in this previously unrecognized sink for atmospheric ethanol. Coastal ocean waters may act as either a sink or source of atmospheric ethanol depending on the partial pressure of ethanol in the overlying air mass. Results from this study are significant because they suggest that surface waters may act as an important vector for the uptake of ethanol emitted into the atmosphere including ethanol from biofuel production and usage.

Temporal and spatial variability of trace volatile organic compounds in rainwater.

This study presents the first detailed concentration profile of trace VOCs in atmospheric waters. Analytes were detected and quantified in 111 unique rain events in Wilmington, NC, USA over a one-year period. Headspace solid phase microextraction was optimized for detection of these compounds at sub-nanomolar levels. Distinct seasonality in both the occurrence and concentration of compounds was observed with the lowest abundance occurring during low irradiance winter months. In contrast to other rainwater components studied at this location, VOCs did not show any correlation between rainfall amount and concentrations. There was significant spatial variation with regards to air-mass back-trajectory for methyfuran with higher concentrations observed in terrestrial events during the growing season. Air mass back trajectory also impacted CCl4 concentrations in rainwater with evidence of a possible oceanic input. However there was no significant impact of air-mass back-trajectory on the concentration of BTEX observed in rain indicating that storm origin is not the controlling factor driving concentrations of these analytes in precipitation. Members of the BTEX family did, however, have significant correlations with each other occurring in ratios aligned closely with ratios reported in the literature for gas-phase BTEX. Using available gas-phase data from locations with similar anthropogenic sources and Henry's Law constants, calculated concentrations agreed with VOC levels found in Wilmington rain. Results of this study indicate local gas-phase scavenging is the major source of VOCs in rain and wet deposition is not an efficient removal mechanism (<0.1%) of VOCs from the atmosphere.

Characterization of carbohydrates in rainwater from the southeastern North Carolina.

Carbohydrates have been widely reported in atmospheric aerosols, but have not previously been quantified in rainwater. We have identified and quantified a series of 11 specific compounds including monosaccharides (glucose, fructose, arabinose, galactose and pinitol), disaccharides (sucrose and trehalose), sugar alcohols (arabitol, dulcitol and mannitol) and the anhydrosaccharide levoglucosan. Rainwater analyzed in this study includes 52 distinct precipitation events in Wilmington, NC between June 2011 and October 2012. Our analysis indicates carbohydrates typically contribute <1% of total dissolved organic carbon in rain, but can account for as much as 10-35% during periods of high pollen or local fires. Concentrations of individual carbohydrates reached as high as 5.8 µM, with glucose and sucrose typically being the predominant species. The distribution of carbohydrates exhibited a distinct seasonal pattern, with higher concentrations of most carbohydrates, especially sucrose, in spring and summer, driven primarily by increased biogenic inputs during the growing season. Concentrations of carbohydrates were an order of magnitude higher in storms of terrestrial origin compared to marine events, further supporting a terrestrial biogenic origin of most species. Sequential sampling of Hurricane Irene showed significant quantities of carbohydrates present at the end of the storm when air mass back trajectories traversed over land. The highest level of levoglucosan, a compound associated with biomass burning, was detected in rain with an air mass back trajectory that traveled over a region affected by wildfires. When compared to aerosol concentrations reported by others, the sugar concentrations in rain demonstrate wet deposition is an important removal mechanism of this water-soluble and bioavailable fraction of atmospheric particulate organic matter.

Controls on the redox potential of rainwater.

Hydrogen peroxide acting as a reductant affects the redox potential of rainwater collected at the Bermuda Atlantic Time Series Station, the South Island of New Zealand, the contiguous USA, and the primary study site in Wilmington, NC. Analytical measurements of both halves of redox couples for dissolved iron, mercury, and the nitrate-nitrite-ammonium system can predict the rainwater redox potential measured directly by a platinum electrode. Measurements of these redox couples along with the pH in rain yields pe⁻ between 8 and 11; the half reaction for hydrogen peroxide acting as a reductant using typical rainwater conditions of 15 µM H2O2 at pH 4.7 gives pe⁻ = 9.12, where pe⁻ = negative log of the activity of hydrated electrons. Of the six rainwater redox systems investigated, only manganese speciation appeared to be controlled by molecular oxygen (pe⁻ = 15.90). Copper redox speciation was consistent with superoxide acting as a reductant (pe⁻ = 2.7). The concentration of H2O2 in precipitation has more than doubled over the preceding decade due to a decrease in SO2 emissions, which suggests the redox chemistry of rainwater is dynamic and changing, potentially altering the speciation of many organic compounds and trace metals in atmospheric waters.

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.

Changing chemical composition of precipitation in Wilmington, North Carolina, U.S.A.: implications for the Continental U.S.A.

The H+(aq) concentration in Wilmington, NC, precipitation has decreased by approximately 50% during the preceding two decades, similar to trends seen nationwide. The decrease in acidity is important because solution pH plays a key role in atmospheric reactions, and because the change is so large. This study presents the first long-range study of dissolved organic carbon (DOC) levels in precipitation which demonstrates that DOC concentrations have decreased by approximately half in Wilmington, NC, precipitation. The concentrations of H+(aq) and DOC are highly correlated primarily because small organic acids contribute to both DOC and H+(aq) in precipitation. Ammonium ion concentrations in precipitation have increased due to increased agricultural activities, and this also affects precipitation pH. The reduction of SO2 emissions in 1995 imposed by the Clean Air Act Amendment, better control of emissions of volatile organic compounds, and the increase in ammonia emissions all contribute to the decreasing H+(aq) in precipitation nationwide. These compositional changes in precipitation have many environmental implications, such as decreased acid deposition to lakes, changing speciation for trace metals in precipitation, increased ammonium deposition to coastal waters, and decreased DOC transport to the open ocean.

Redox speciation of copper in rainwater: temporal variability and atmospheric deposition.

Copper speciation was determined in 68 rainwater samples collected in Wilmington, NC, from August 25, 2000, to September 24, 2002. Volume-weighted average concentrations of Cu(total), dissolved Cu(II), and dissolved Cu(I) were 5.3, 3.2, and 1.4 nM, respectively, with a significantly higher ratio of Cu(II)/Cu(I) in summer relative to winter events. The concentrations of all Cu species were higher in storms of continental origin relative to marine-dominated events, suggesting anthropogenic and/or terrestrial sources are important contributors of Cu in precipitation. Concentrations of strong Cu-complexing ligands were consistently lower than dissolved Cu concentrations, indicating a significant portion, but not all, of the dissolved Cu in rainwater is strongly complexed. A portion of these ligands, in addition to the sulfite and chloride in precipitation, may be Cu(I)-complexing ligands, which may explain the resistance of Cu(I) against oxidation in rainwater. Using our rainwater concentration data along with other published rainwater Cu concentrations and an estimate for total global annual rain, the total global flux of Cu removed from the atmosphere via wet deposition is 150 x 106 kg yr(-1). This represents complete removal of the estimated Cu input into the troposphere and indicates essentially all Cu released into the global atmosphere is removed by rain.

Chromium speciation in rainwater: temporal variability and atmospheric deposition.

The concentrations of the various chromium species were determined in 89 rainwater samples collected in Wilmington, NC, from October 1, 1999, to December 31, 2001. Volume-weighted annual average concentrations of Cr(total), particulate Cr, Cr(III)(aq), and Cr(VI)(aq) were 4.6, 2.2, 0.8, and 1.2 nM, respectively. There was distinct seasonal and diurnal variability in the concentrations of the various chromium species. Chromium emissions to the global atmosphere by both natural and anthropogenic sources are estimated to be 2.2 x 10(9) mol/yr. Using our rainwater concentration data along with other published rainwater Cr concentrations and an estimate for total global annual rain, the total global flux of chromium removed from the atmosphere via wet deposition is 2.1 x 10(9) mol/yr. This represents complete removal of Cr and indicates that essentially all chromium released into the global atmosphere is removed via rain. About half this chromium is dissolved with roughly equal concentrations of toxic Cr(VI) and relatively harmless Cr(III) species.