Publications

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2008
Rhew, RC, Miller BR, Weiss RF.  2008.  Chloroform, carbon tetrachloride and methyl chloroform fluxes in southern California ecosystems. Atmospheric Environment. 42:7135-7140.   10.1016/j.atmosenv.2008.05.038   AbstractWebsite

Chloroform (CHCl3), carbon tetrachloride (CCl4), and methyl chloroform (CH3CCl3) are important carriers of chlorine to the stratosphere and account for an estimated 159 of the total organic chlorine in the troposphere, roughly equivalent to chlorine load due to methyl chloride (CH3Cl). The tropospheric burden of chlorine has declined since 1994, largely due to the restriction of CH3CCl3 and CCl4 use as specified by the Montreal Protocol. However, few field studies have been conducted on the terrestrial-atmosphere exchange of these chlorinated hydrocarbons, leading to uncertainties about the natural cycling of these trace gases. This work shows the results of 75 flux measurements conducted in a variety of southern California ecosystems, including coast sagebrush, chamise chaparral, creosote bush scrub, shoreline, and coastal salt marsh. We find no evidence of a significant soil sink in these ecosystems but rather a small net source of CHCl3 and possibly CCl4. (c) 2008 Elsevier Ltd. All rights reserved.

2004
Simmonds, PG, Derwent RG, Manning AJ, Fraser PJ, Krummel PB, O'Doherty S, Prinn RG, Cunnold DM, Miller BR, Wang HJ, Ryall DB, Porter LW, Weiss RF, Salameh PK.  2004.  AGAGE observations of methyl bromide and methyl chloride at Mace Head, Ireland, and Cape Grim, Tasmania, 1998-2001. Journal of Atmospheric Chemistry. 47:243-269.   10.1023/B:JOCH.0000021136.52340.9c   AbstractWebsite

In situ AGAGE GC-MS measurements of methyl bromide (CH3Br) and methyl chloride (CH3Cl) at Mace Head, Ireland and Cape Grim, Tasmania (1998-2001) reveal a complex pattern of sources. At Mace Head both gases have well-defined seasonal cycles with similar average annual decreases of 3.0% yr(-1) (CH3Br) and 2.6% yr(-1) (CH3Cl), and mean northern hemisphere baseline mole fractions of 10.37 +/- 0.05 ppt and 535.7 +/- 2.2 ppt, respectively. We have used a Lagrangian dispersion model and local meteorological data to segregate the Mace Head observations into different source regions, and interpret the results in terms of the known sources and sinks of these two key halocarbons. At Cape Grim CH3Br and CH3Cl also show annual decreases in their baseline mixing ratios of 2.5% yr(-1) and 1.5% yr(-1), respectively. Mean baseline mole fractions were 7.94 +/- 0.03 ppt (CH3Br) and 541.3 +/- 1.1 ppt (CH3Cl). Although CH3Cl has a strong seasonal cycle there is no well-defined seasonal cycle in the Cape Grim CH3Br record. The fact that both gases are steadily decreasing in the atmosphere at both locations implies that a change has occurred which is affecting a common, major source of both gases (possibly biomass burning) and/or their major sink process (destruction by hydroxyl radical).

2002
Rhew, RC, Miller BR, Bill M, Goldstein AH, Weiss RF.  2002.  Environmental and biological controls on methyl halide emissions from southern California coastal salt marshes. Biogeochemistry. 60:141-161.   10.1023/a:1019812006560   AbstractWebsite

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) emission rates from southern California coastal salt marshes show large spatial and temporal variabilities that are strongly linked to biological and environmental factors. Here we discuss biogeochemical lines of evidence pointing to vegetation as the primary source of CH3Br and CH3Cl emissions from salt marshes. Sediments and macroalgae do not appear to be major producers of these compounds, based on observations that the highest fluxes are not inhibited by soil inundation; their emissions are not correlated with those of certain gases produced in soils; and emissions from mudflat- and macroalgae-dominated sites are relatively small. In contrast, the seasonal and spatial variabilities of methyl halide fluxes in these salt marshes are consistent with the production of these compounds by vascular plants, although the possibility of production by microflora or fungi associated with the salt marsh vegetation is not ruled out. Flux chamber measurements of emission rates are largely correlated to the overall plant biomass enclosed in the chamber, but appear also to be highly dependent on the predominant plant species. Emission rates follow a diurnal trend similar to the trends of ambient air temperature and photosynthetically active radiation, but not surface soil temperature. Diurnal variabilities in the carbon isotope compositions of CH3Cl and CH3Br and their relative ratios of emissions are consistent with simultaneously competing mechanisms of uptake and production.

2001
Rhew, RC, Miller BR, Vollmer MK, Weiss RF.  2001.  Shrubland fluxes of methyl bromide and methyl chloride. Journal of Geophysical Research-Atmospheres. 106:20875-20882.   10.1029/2001jd000413   AbstractWebsite

Flux measurements in coastal sage scrub, chamise chaparral, and creosote bush scrub environments show that methyl bromide (CH(3)Br) and methyl chloride (CH(3)Cl), compounds that are involved in stratospheric ozone depletion, are both produced and consumed by southern California shrubland ecosystems. CH(3)Br and CH(3)Cl 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 CH(3)Cl and CH(3)Br 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 CH(3)Br 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 CH(3)Br and < 20 Gg yr(-1) for CH(3)Cl 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 CH(3)Br budget by adding a new source term and potentially reducing the soil sink term. These results also suggest that while shrubland soil consumption of CH(3)Cl may be small, soils in general may be a globally significant sink for CH(3)Cl.

2000
Rhew, RC, Miller BR, Weiss RF.  2000.  Natural methyl bromide and methyl chloride emissions from coastal salt marshes. Nature. 403:292-295.   10.1038/35002043   AbstractWebsite

Atmospheric methyl bromide (CH3Br) and methyl chloride (CH3Cl), 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(1). Here we show that CH3Br and CH3Cl are released to the atmosphere from all vegetation zones of two coastal salt marshes. We see very large fluxes of CH3Br and CH3Cl per unit area: up to 42 and 570 mu mol m(-2) d(-1), respectively. The fluxes show large diurnal, seasonal and spatial variabilities, but there is a strong correlation between the fluxes of CH3Br 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(2), may produce roughly 10% of the total fluxes of atmospheric CH3Br and CH3Cl.