Robert C. Rhew

Natural methyl bromide and methyl chloride emissions from coastal salt marshes.

Atmospheric methyl bromide (CH3Br) and methyl chloride (CH 3Cl), compounds that are involved in stratospheric ozone depletion, originate from both natural and anthropogenic sources. Current estimates of CH3Br and CH3Cl emissions from oceanic sources, terrestrial plants and fungi, biomass burning and anthropogenic inputs do not balance their losses owing to oxidation by hydroxyl radicals, oceanic degradation, and consumption in soils, suggesting that additional natural terrestrial sources may be important. Here we show that CH3Br and CH 3Cl are released to the atmosphere from all vegetation zones of two coastal salt marshes. We see very large fluxes of CH3Br and CH 3Cl per unit area: up to 42 and 570 µmol m-2 d -1, respectively. The fluxes show large diurnal, seasonal and spatial variabilities, but there is a strong correlation between the fluxes of CH 3Br and those of CH3Cl, with an average molar flux ratio of roughly 1:20. If our measurements are typical of salt marshes globally, they suggest that such ecosystems, even though they constitute less than 0.1% of the global surface area, may produce roughly 10% of the total fluxes of atmospheric CH3Br and CH3Cl.

Shrubland fluxes of methyl bromide and methyl chloride

Flux measurements in coastal sage scrub, chamise chaparral, and creosote bush scrub environments show that methyl bromide (CH3Br) and methyl chloride (CH3Cl), compounds that are involved in stratospheric ozone depletion, are both produced and consumed by southern California shrubland ecosystems. CH3Br and CH3Cl are produced in association with a variety of plants and are consumed by the soils, although there is a large variability in the fluxes, depending on predominant vegetation and environmental conditions. At sites with a net uptake of both compounds the fluxes of CH3Cl and CH3Br show a strong correlation, with a molar ratio of roughly 40:1, pointing to a similar mechanism of consumption. In contrast, the net production rates of these compounds show no apparent correlation with each other. The average observed net CH3Br uptake rates are an order of magnitude smaller than the previously reported average soil consumption rates assigned to shrublands. Extrapolations from our field measurements suggest that shrublands globally have a maximum net consumption of <1 Gg yr−1 for CH3Br and <20 Gg yr−1 for CH3Cl and may, in fact, be net sources for these compounds. Consequently, the measured net fluxes from shrubland ecosystems can account for part of the present imbalance in the CH3Br budget by adding a new source term and potentially reducing the soil sink term. These results also suggest that while shrubland soil consumption of CH3Cl may be small, soils in general may be a globally significant sink for CH3Cl.

Environmental and biological controls on methyl halide emissions from southern California coastal salt marshes

Methyl bromide (CH3Br) and methyl chloride(CH3Cl) emission rates from southernCalifornia coastal salt marshes show largespatial and temporal variabilities that arestrongly linked to biological and environmentalfactors. Here we discuss biogeochemical linesof evidence pointing to vegetation as theprimary source of CH3Br and CH3Clemissions from salt marshes. Sediments andmacroalgae do not appear to be major producersof these compounds, based on observations thatthe highest fluxes are not inhibited by soilinundation; their emissions are not correlatedwith those of certain gases produced in soils;and emissions from mudflat- andmacroalgae-dominated sites are relativelysmall. In contrast, the seasonal and spatialvariabilities of methyl halide fluxes in thesesalt marshes are consistent with the productionof these compounds by vascular plants, althoughthe possibility of production by microflora orfungi associated with the salt marsh vegetationis not ruled out. Flux chamber measurements ofemission rates are largely correlated to theoverall plant biomass enclosed in the chamber,but appear also to be highly dependent on thepredominant plant species. Emission ratesfollow a diurnal trend similar to the trends ofambient air temperature and photosyntheticallyactive radiation, but not surface soiltemperature. Diurnal variabilities in thecarbon isotope compositions of CH3Cl andCH3Br and their relative ratios ofemissions are consistent with simultaneouslycompeting mechanisms of uptake andproduction.

Measuring terrestrial fluxes of methyl chloride and methyl bromide using a stable isotope tracer technique

Author(s): Rhew, Robert C; Aydin, M.; Saltzman, E. S. | Abstract: Atmospheric methyl chloride (CH3Cl) and methyl bromide (CH3Br), compounds involved in the destruction of stratospheric ozone, are simultaneously produced and consumed by the terrestrial biosphere. Here we present a stable isotope incubation technique using13CH3Cl and 13CH3Br as tracers to simultaneously determine production and consumption fluxes in boreal forest soils from Alaska, USA. Measured uptake rates are consistent with previously reported boreal soil results for CH3Br and show a CH3Cl:CH3Br molar consumption ratio of 40:1. Boreal forest soils appear to produce small amounts of these methyl halides as well, but at rates that are negligible in their global budgets. This isotope tracer technique can be applied to laboratory studies of plants and other soils and to field measurements where disturbances to the system can be minimized.

Analysis of low concentration reduced sulfur compounds (RSCs) in air: Storage issues and measurement by gas chromatography with sulfur chemiluminescence detection

Reduced sulfur compounds (RSCs) were measured at low concentrations in small volume air samples using a cryo-trapping inlet system and gas chromatograph outfitted with a sulfur chemiluminescence detector (GC-SCD). The relative sensitivity of the system to the RSCs follows the sequence H(2)S<CH(3)SH<OCS∼DMS<CS(2). The analytical system achieves a detection limit of 120ppt in a 100mL air sample, which is suitable for measuring reactive RSCs (e.g., H(2)S and CH(3)SH) at ambient or near ambient atmospheric concentrations. The inlet system allows for replicate sampling from a stored air sample (sub-sampling), thereby improving estimates of instrumental precision and demonstrating the reproducibility of the analytical method. Although the SCD theoretically provides linear responses equivalent to the sulfur mass injected, we found that the response properties for each RSC differed. At concentrations below 2ppb, the compounds H(2)S and CH(3)SH have diminished responses, leading to larger measurement uncertainties. Two generations of commercially available SilcoCan canisters were tested to evaluate the relative RSC loss due to storage in the canister and loss of inertness because of coating age. The older generation canister (>6 years from initial coating) saw significant loss of H(2)S and CH(3)SH within 2 days, while the more recent generation canister (<1 year from initial coating) yielded percent recoveries of RSCs in the range of 85% (H(2)S and CH(3)SH) to 95% (OCS, DMS and CS(2)) after 7 days of storage, suggesting that these canisters may be suitable for the short-term storage of low level RSCs. The development of this low concentration, low sample volume method is well suited for measuring RSC gas fluxes from natural soils in laboratory incubations and in field flux chamber studies.

Carbonyl sulfide produced by abiotic thermal and photodegradation of soil organic matter from wheat field substrate

Carbonyl sulfide (COS) is a reduced sulfur gas that is taken up irreversibly in plant leaves proportionally with CO2, allowing its potential use as a tracer for gross primary production. Recently, wheat field soil at the Southern Great Plains Atmospheric Radiation Measurement site in Lamont, Oklahoma, was found to be a measureable source of COS to the atmosphere. To understand the mechanism of COS production, soil and root samples were collected from the site and incubated in the laboratory over a range of temperatures (15–34°C) and light conditions (light and dark). Samples exhibited mostly COS net uptake from the atmosphere in dark and cool ( 25°C), consistent with field observations, and at a lower temperature (19°C) when a full spectrum lamp (max wavelength 600 nm) was applied. Sterilized soil and root samples yielded only COS production that increased with temperature, supporting the hypothesis that (a) COS production in these samples is abiotic, (b) production is directly influenced by temperature and light, and (c) some COS consumption in soil and root samples is biotic.

The atmospheric partial lifetime of carbon tetrachloride with respect to the global soil sink

The magnitude of the terrestrial soil sink for atmospheric carbon tetrachloride (CCl4) remains poorly constrained, with the estimated uncertainty range of CCl4 partial lifetimes between ~110 and 910 years. Field observations are sparse, and there are uncertainties in extrapolating these results to the global scale. Here we add to the published CCl4 fluxes with additional field measurements, and we employ a land cover classification scheme based on Advanced Very High Resolution Radiometer measurements that align more closely with the measurement sites to reevaluate the global CCl4 soil sink. We calculate an updated partial lifetime of CCl4 with respect to the soil sink to be 375 (288–536) years, which is 50 to 90% longer than the most recently published best estimates of the soil sink partial lifetime (195 and 245 years). This translates into a longer overall atmospheric lifetime estimate, which is more consistent with the observed atmospheric concentration trend and interhemispheric gradient.

Haloform formation in coastal wetlands along a salinity gradient at South Carolina, United States

Environmental context Natural haloform emissions contribute to stratospheric ozone depletion but there are major unknown or underestimated sources of these gases. This study demonstrates that soil and water at tidal wetlands are important haloform sources, and emissions peak at the forest–marsh transition zone. The low-lying forested wetlands of the south-eastern United States that are facing sea-level rise and seawater intrusion may become hotspots for haloform emission. Abstract Soil haloform emissions are sources of reactive halogens that catalytically deplete ozone in the stratosphere but there are still unknown or underestimated haloform sources. The >200000ha of low-lying tidal freshwater swamps (forests and marshes) in the south-eastern United States could be haloform (CHX3, X=Cl or Br) sources because sea-level rise and saltwater intrusion bring halides inland where they mix with terrestrial humic substances. To evaluate the spatial variation along the common forest–marsh salinity gradient (freshwater wetland, oligohaline wetland and mesohaline saltmarsh), we measured chloroform emissions from in situ chambers and from laboratory incubations of soil and water samples collected from Winyah Bay, South Carolina. The in situ and soil-core haloform emissions were both highest in the oligohaline wetland, whereas the aqueous production was highest in mesohaline saltmarsh. The predominant source shifted from sediment emission to water emission from freshwater wetland to mesohaline saltmarsh. Spreading out soil samples increased soil haloform emission, suggesting that soil pores can trap high amounts of CHCl3. Soil sterilisation did not suppress CHCl3 emission, indicating the important contribution of abiotic soil CHCl3 formation. Surface wetland water samples from eight locations along a salinity gradient with different management practices (natural v. managed) were subjected to radical-based halogenation by Fenton-like reagents. Halide availability, organic matter source, temperature and light irradiation were all found to affect the radical-based abiotic haloform formation from surface water. This study clearly indicates that soil and water from the studied coastal wetlands are both haloform sources, which however appear to have different formation mechanisms.