Methyl Chloroform Research Articles

Steady As She Goes

Most of the chemical oxidation of atmospheric gases and particles is done by the hydroxyl radical (OH). However, the concentration of atmospheric hydroxyl radicals is extremely difficult to measure, so it must be inferred from measurements of the abundance of other species, like methyl chloroform.

Methyl halide and chloroform emissions from a subsiding Sacramento–San Joaquin Delta island converted to rice fields

Net fluxes of methyl halides and chloroform were measured from recently converted rice fields on Twitchell Island, California, using field- and laboratory-based incubations. A stable isotope tracer method was used to demonstrate that the net emissions of CH3Cl and CH3Br during the growing season were predominantly the result of large gross production rates, with gross consumption rates being relatively minor. In agreement with prior studies, the production rates for methyl halides differed significantly at different rice plant growth phases. The Twitchell Island rice field, however, had production rates of CH3Cl during the growth phases that were higher than rates reported at commercial rice fields, presumably because of higher soil chloride concentrations. Laboratory soil incubations showed that the non-flooded rice field soils acted as a small net sink for CH3Cl and as small net sources for CH3Br, CH3I and CHCl3. Despite the higher CH3Cl emissions during the growing season, the overall emissions of halomethanes from the conversion of all potential islands in the San Joaquin Delta to rice paddies is predicted to be an insignificant source of halomethanes.

The global chemistry transport model TM5: description and evaluation of the tropospheric chemistry version 3.0

Abstract. We present a comprehensive description and benchmark evaluation of the tropospheric chemistry version of the global chemistry transport model TM5 (Tracer Model 5, version TM5-chem-v3.0). A full description is given concerning the photochemical mechanism, the interaction with aerosol, the treatment of the stratosphere, the wet and dry deposition parameterizations, and the applied emissions. We evaluate the model against a suite of ground-based, satellite, and aircraft measurements of components critical for understanding global photochemistry for the year 2006. The model exhibits a realistic oxidative capacity at a global scale. The methane lifetime is ~8.9 years with an associated lifetime of methyl chloroform of 5.86 years, which is similar to that derived using an optimized hydroxyl radical field. The seasonal cycle in observed carbon monoxide (CO) is well simulated at different regions across the globe. In the Northern Hemisphere CO concentrations are underestimated by about 20 ppbv in spring and 10 ppbv in summer, which is related to missing chemistry and underestimated emissions from higher hydrocarbons, as well as to uncertainties in the seasonal variation of CO emissions. The model also captures the spatial and seasonal variation in formaldehyde tropospheric columns as observed by SCIAMACHY. Positive model biases over the Amazon and eastern United States point to uncertainties in the isoprene emissions as well as its chemical breakdown. Simulated tropospheric nitrogen dioxide columns correspond well to observations from the Ozone Monitoring Instrument in terms of its seasonal and spatial variability (with a global spatial correlation coefficient of 0.89), but TM5 fields are lower by 25–40%. This is consistent with earlier studies pointing to a high bias of 0–30% in the OMI retrievals, but uncertainties in the emission inventories have probably also contributed to the discrepancy. TM5 tropospheric nitrogen dioxide profiles are in good agreement (within ~0.1 ppbv) with in situ aircraft observations from the INTEX-B campaign over (the Gulf of) Mexico. The model reproduces the spatial and seasonal variation in background surface ozone concentrations and tropospheric ozone profiles from the World Ozone and Ultraviolet Radiation Data Centre to within 10 ppbv, but at several tropical stations the model tends to underestimate ozone in the free troposphere. The presented model results benchmark the TM5 tropospheric chemistry version, which is currently in use in several international cooperation activities, and upon which future model improvements will take place.

Effect of fluorinated substituents on hydrodynamic and conformational properties of hyperbranched polycarbosilane in solutions

Fractions of hyperbranched polycarbosilane modified with fluorine-containing end groups are studied by the methods of molecular hydrodynamics and optics in dilute solutions in hexafluorobenzene, methyl tert-butyl ether, chloroform, tetrahydrofurane, and toluene. The results are compared with the data available for the original polymethylallylcarbosilane. The incorporation of fluorinated groups leads to a change in the hydrodynamic parameters of the hyperbranched polymer. In this case, in thermodynamically better solvents, the patterns of the molecular-mass dependence of intrinsic viscosity for fluorinated and original hyperbranched polycarbosilanes coincide since the macromolecules of the polymers under comparison are characterized by similar shapes and almost the same hydrodynamic sizes. In a thermodynamically worse solvent (toluene), the above-mentioned changes in hydrodynamic parameters and the character of the molecular-mass dependences are related to compaction of macromolecules and a decrease in their shape asymmetry. This tendency is probably associated with the selectivity of this solvent with respect to the fluorine atom.

ChemInform Abstract: Chlorofluorocarbons in the Atmosphere: Trends and Vertical Profiles

Chlorofluorocarbons with extremely long atmospheric lifetimes are responsible for both stratospheric ozone depletion and global warming as they accumulate in the atmosphere. Their average atmospheric concentrations in the northern/southern hemisphere have been monitored since 1979 in order to study their behaviors and the future trends of their concentrations in the atmosphere: 1. An extremely clean analytical system has been developed for accurate analysis of atmospheric chlorofluorocarbons, carbon tetrachloride, methyl chloroform and other halocarbons. Atmospheric samples were collected at the surface level by grab-sampling and analyzed by ECD gas chromatography. 2. The background atmospheric concentrations of CC1,F (CFC-11) and CCl,F, (CFC-12) in the mid-latitude northern hemisphere have been increasing by 4% a year over the past decade. The concentrations of CFC-11 and CFC-12 observed in Antarctica have been 8-10% lower than those observed in the northern hemisphere in accordance with the predominant emission of chlorofluorocarbons in the northern hemisphere and the delay (-2 years) in their diffusion into the southern hemisphere across the Intertropical Convergence Zone. The CC1,FCC1F2 (CFC-113) concentration in the atmosphere tends to increase much faster than those of CFC-11 and CFC-12. 3. The vertical profiles of chlorofluorocarbons in the stratosphere have also been measured by means of balloon grab- sampling and cryogenic sampling. They reflect how efficiently these compounds undergo UV-photolysis to release chlorine atoms and end up with depletion of the ozone layer in the stratosphere.

ChemInform Abstract: Synthesis and Analgesic-Antiinflammatory Activities of Ethyl 2-[3-(1-Phenoxy(methoxy)carbonyl-4-aryl(alkyl)-1,4-dihydropyridyl)] acetates.

Reaction of ethyl 2-(3-pyridyl)acetate 4a or ethyl 2-methyl-2-(3-pyridyl)acetate 4b, with phenyl chloroformate or methyl chloroform ate, afforded the intermediate pyridinium salt 5 which undergoes regioselective nucleophilic attack at C-4 upon reaction with a Grignard reagent in the presence of a cuprous iodide catalyst at −23° to yield the corresponding ethyl 2-[3-(1-phenoxy(methoxy)carbonyl-4-aryl(alkyl)-1,4-dihydropyridyl)]acetates 6a-f in 64–96% chemical yield. No product arising from reaction of the ester substituent of the pyridinium salt 5 with the Grignard reagent was observed. The 1H nmr spectra of 6a-f exhibited dual resonances for the 1,4-dihydropyridyl H-2, H-5 and H-6 protons at 25° in deuteriochloroform. These dual resonaces were attributed to two different rotameric configurations resulting from restricted rotation about the nitrogen-to-carbonyl carbamate bond due to its double bond character. Compound 6 generally exhibited superior analgesic and antiinflammatory activities, compared to the reference drugs aspirin and ibuprofen, respectively. These structure-activity correlations indicate the 1,4-dihydropyridyl ring system present in 6 is a suitable bioisostere for the aryl (heteroaryl) ring present in aryl(heteroaryl)acetic acid non-steroidal antiinflammatory drugs.

Deep air convection in the firn at a zero-accumulation site, central Antarctica

Ice cores provide unique archives of past atmospheres and climate, but interpretation of trapped-gas records and their climatic significance has been hampered by a poor knowledge of the prevalence of air convection in the firn layer on top of polar ice sheets. In particular, the phasing of greenhouse gases and climate from ice cores has been obscured by a discrepancy between empirical and model-based estimates of the age difference between trapped gases and enclosing ice, which may be due to air convection. Here we show that deep air convection (>23m) occurs at a windy, near-zero-accumulation rate site in central Antarctica known informally as the Megadunes site (80.77914°S, 124.48796°E). Deep convection is evident in depth profiles of air withdrawn from the firn layer, in the observed pattern of the nitrogen isotope ratio 15N/14N, the argon isotope ratio 40Ar/36Ar, and in the mixing ratios of the anthropogenic halocarbons methyl chloroform (CH3CCl3) and HFC-134a (CH2FCF3). Transport parameters (diffusivities) were inferred and air was dated using measured carbon dioxide (CO2) and methane (CH4) mixing ratios, by comparing with the Law Dome atmospheric record, which shows that these are the oldest firn air samples ever recovered (CO2 mean age=1863AD). The low accumulation rate and the consequent intense metamorphism of the firn (due to prolonged exposure to seasonal temperature cycling) likely contribute to deep air convection via large grain size and vertical cracks that act as conduits for vigorous air motion. The Megadunes site provides a possible modern analog for the glacial conditions in the Vostok, Dome Fuji, and Dome C ice core records and a possible explanation for lower-than-expected 15N/14N ratios in trapped air bubbles at these times. A general conclusion is that very low accumulation rate causes deep air convection via its effect on firn structural characteristics.

Spatial- and Time-Explicit Human Damage Modeling of Ozone Depleting Substances in Life Cycle Impact Assessment

Depletion of the stratospheric ozone layer is mainly caused by emissions of persistent halocarbons of anthropogenic origin. The resulting increase of solar ultraviolet radiation at the Earth's surface is associated with increased exposure of humans and increased human health damage. Here we assessed the change in human health damage caused by three types of skin cancer and cataract in terms of (healthy) years of life lost per kiloton emission reduction of an ozone-depleting substance (ODS). This so-called characterization factor is used in Life Cycle Assessments (LCAs). Characterization factors are provided for the emissions of five chlorofluorocarbons, three hydrochlorofluorocarbons, three (bromine-containing) halons, carbon tetrachloride, methyl chloroform, and anthropogenic emissions of methyl bromide. We employed dynamic calculations on a global scale for this purpose, taking physical and social geographic data into account such as skin tones, population density, average age, and life expectancy. When emission rates of all ODSs in 2007 are multiplied by our characterization factors, the resulting number of years of life lost may be a factor of 5 higher than reported previously. This increase is merely explained through the global demographic development until 2100 we took into account.

Quantification of the Nanomechanical Stability of Ceramide-Enriched Domains

The quantification of the mechanical stability of lipid bilayers is important in establishing composition-structure-property relations and sheds light on our understanding of the functions of biological membranes. Here, we designed an experiment to directly probe and quantify the nanomechanical stability and rigidity of the ceramide-enriched platforms that play a distinctive role in a variety of cellular processes. Our force mapping results have demonstrated that the ceramide-enriched domains require both methyl beta-cyclodextrin (MbCD) and chloroform treatments to weaken their highly ordered organization, suggesting a lipid packing that is different from that in typical gel states. Our results also show the expulsion of cholesterol from the sphingolipid/cholesterol-enriched domains as a result of ceramide incorporation. This work provides quantitative information on the nanomechanical stability and rigidity of coexisting phase-segregated lipid bilayers with the presence of ceramide-enriched platforms, indicating that that generation of ceramide in cells drastically alters the structural organization and the mechanical property of biological membranes.

Multi-species inversion of CH<sub>4</sub>, CO and H<sub>2</sub> emissions from surface measurements

Abstract. In order to study the spatial and temporal variations of the emissions of greenhouse gases and of their precursors, we developed a data assimilation system and applied it to infer emissions of CH4, CO and H2 for one year. It is based on an atmospheric chemical transport model and on a simplified scheme for the oxidation chain of hydrocarbons, including methane, formaldehyde, carbon monoxide and molecular hydrogen together with methyl chloroform. The methodology is exposed and a first attempt at evaluating the inverted fluxes is made. Inversions of the emission fluxes of CO, CH4 and H2 and concentrations of HCHO and OH were performed for the year 2004, using surface concentration measurements of CO, CH4, H2 and CH3CCl3 as constraints. Independent data from ship and aircraft measurements and satellite retrievals are used to evaluate the results. The total emitted mass of CO is 30% higher after the inversion, due to increased fluxes by up to 35% in the Northern Hemisphere. The spatial distribution of emissions of CH4 is modified by a decrease of fluxes in boreal areas up to 60%. The comparison between mono- and multi-species inversions shows that the results are close at a global scale but may significantly differ at a regional scale because of the interactions between the various tracers during the inversion.

Solubility of Valsartan in Different Organic Solvents and Ethanol + Water Binary Mixtures from (278.15 to 313.15) K

The solubility of valsartan in pure methyl acetate, n-butyl acetate, acetonitrile, N,N-dimethylformamide, dichloromethane, chloroform, and ethanol + water binary mixtures was measured by a synthetic method over the temperature range (278.15 to 313.15) K at atmospheric pressure. The results show that the solubility of valsartan increases with increasing temperature in all six pure solvents and ethanol + water binary mixtures, and it increases with increasing mole fraction of ethanol in the binary mixtures. The experimental data were correlated using the modified Apelblat model, and the agreement with the experimental data was very good.

Preparation of a polymeric membrane with a fine porous structure by dry casting

AbstractWe prepared a polymeric membrane with a fine porous structure from polystyrene (PS), poly(ethylene glycol) (PEG), and solvent solutions by exploiting the phase separation induced in the course of dry casting. To determine the effect of the drying rate and phase separation on the developed porous structure, six different solvents, including toluene, chlorobenzene, tetrahydrofuran, methyl ethyl ketone, 1,4‐dioxane, and chloroform, were used. The pore size and density drastically changed with the different solvents and drying conditions. The polymer concentration at the onset of the phase separation into PEG‐rich and PS‐rich phases also strongly affected the cellular structure. The solubility of PEG into PS and the solvent solutions changed the concentration, which corresponded to the viscosity of the PS‐rich solution at the onset of the phase separation. The higher solubility of PEG in the solutions delayed the onset of phase separation during drying and increased the viscosity. The higher viscosity and the higher drying rate prevented the phase‐separated PEG domains from coalescing and made the resulting pore size smaller and the pore density larger. The finest porous structure, with a pore size of approximately 1 μm and a pore density of 0.08 1/μm2, was prepared from PS/PEG and a 90 wt % chloroform solution. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Tracer test for the measurement of gas diffusion and non-aqueous phase liquid (NAPL) saturation in soil

During soil bioremediation, the diffusion of oxygen into the soil is an important prerequisite for aerobic biodegradation, and the decrease of petroleum products is the ultimate goal. Both processes need to be monitored. The aim of this work was to develop a gas tracer test that yields information on both, gas diffusion and residual saturation with non-aqueous phase liquids (NAPLs) in unsaturated soil heaps. One conservative tracer (methane) and 4 partitioning gas tracers (diethylether, methyl tert-butyl ether, chloroform and n-heptane) were injected as vapors into laboratory columns filled with unsaturated sand with increasing NAPL saturation. Breakthrough curves of gaseous compounds were measured at two points and compared to analytical solutions of an analytical diffusive-reactive transport equation. By fitting of methane data, robust results for effective diffusivity (tortuosity) were obtained. NAPL saturation was most accurately measured by the moderately water soluble tracers (ethers and chloroform). The hydrophobic tracer n-heptane did not partition into water-immersed NAPL. An easy and accurate way to assess air–NAPL partitioning constants from gas chromatography retention times is furthermore reported. It is concluded that gas tracer tests have the potential for measuring two important properties in soil bioremediation systems easily and quickly.

Renewed growth of atmospheric methane

Following almost a decade with little change in global atmospheric methane mole fraction, we present measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) and the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) networks that show renewed growth starting near the beginning of 2007. Remarkably, a similar growth rate is found at all monitoring locations from this time until the latest measurements. We use these data, along with an inverse method applied to a simple model of atmospheric chemistry and transport, to investigate the possible drivers of the rise. Specifically, the relative roles of an increase in emission rate or a decrease in concentration of the hydroxyl radical, the largest methane sink, are examined. We conclude that: 1) if the annual mean hydroxyl radical concentration did not change, a substantial increase in emissions was required simultaneously in both hemispheres between 2006 and 2007; 2) if a small drop in the hydroxyl radical concentration occurred, consistent with AGAGE methyl chloroform measurements, the emission increase is more strongly biased to the Northern Hemisphere.

What can <sup>14</sup>CO measurements tell us about OH?

Abstract. The possible use of 14CO measurements to constrain hydroxyl radical (OH) concentrations in the atmosphere is investigated. 14CO is mainly produced in the upper atmosphere from cosmic radiation. Measurements of 14CO at the surface show lower concentrations compared to the upper atmospheric source region, which is the result of oxidation by OH. In this paper, the sensitivity of 14CO mixing ratio surface measurements to the 3-D OH distribution is assessed with the TM5 model. Simulated 14CO mixing ratios agree within a few molecules 14CO cm−3 (STP) with existing measurements at five locations worldwide. The simulated cosmogenic 14CO distribution appears mainly sensitive to the assumed upper atmospheric 14C source function, and to a lesser extend to model resolution. As a next step, the sensitivity of 14CO measurements to OH is calculated with the adjoint TM5 model. The results indicate that 14CO measurements taken in the tropics are sensitive to OH in a spatially confined region that varies strongly over time due to meteorological variability. Given measurements with an accuracy of 0.5 molecules 14CO cm−3 STP, a good characterization of the cosmogenic 14CO fraction, and assuming perfect transport modeling, a single 14CO measurement may constrain OH to 0.2–0.3×106 molecules OH cm−3 on time scales of 6 months and spatial scales of 70×70 degrees (latitude×longitude) between the surface and 500 hPa. The sensitivity of 14CO measurements to high latitude OH is about a factor of five higher. This is in contrast with methyl chloroform (MCF) measurements, which show the highest sensitivity to tropical OH, mainly due to the temperature dependent rate constant of the MCF–OH reaction. A logical next step will be the analysis of existing 14CO measurements in an inverse modeling framework. This paper presents the required mathematical framework for such an analysis.

Chloroform, carbon tetrachloride and methyl chloroform fluxes in southern California ecosystems

Chloroform (CHCl 3), carbon tetrachloride (CCl 4), and methyl chloroform (CH 3CCl 3) are important carriers of chlorine to the stratosphere and account for an estimated 15% of the total organic chlorine in the troposphere, roughly equivalent to chlorine load due to methyl chloride (CH 3Cl). The tropospheric burden of chlorine has declined since 1994, largely due to the restriction of CH 3CCl 3 and CCl 4 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 CHCl 3 and possibly CCl 4.

Variability of ozone depleting substances as an indication of emissions in the Pearl River Delta, China

The continued production and consumption of five major chlorocarbons, i.e., CFC-11 (CCl 3F), CFC-12 (CCl 2F 2), CFC-113 (CCl 2FCClF 2), CH 3CCl 3, and CCl 4, as allowed by developing nations including China under the Montreal Protocol, were assessed by a method employing concentration variability. Measurements of the five ozone depleting substances (ODS) were measured in downtown Guangzhou and a rural site in the Pearl River Delta (PRD), China by both in situ and flask measurements. In order to post a contrast to PRD with a referencing environment of minimal emissions, in situ measurements were also conducted in Taipei, Taiwan, where a decade long phase-out of CFCs has been implemented. In general, the variability of chlorocarbons in the PRD sites was significantly greater than that of Taipei. While the abundance of the five ODSs in Taipei was relatively uniform with a relative standard deviation (RSD) varying between 3% and 16%, their variability in PRD with the exception of CFC-113 was significantly more pronounced, clearly indicating the significant usage of ODSs. The variability of CFC-113 in both cities, however, was nearly indiscernible from the instrumental precision, suggesting little usage of CFC-113 in China. Methyl chloroform in Guangzhou exhibited a strong link to solvent evaporation as it showed a tight correlation with ambient toluene. Alarmingly, CCl 4 was the most variable of the five major chlorocarbons in Guangzhou, which should arouse a serious concern for public health due to its carcinogenicity.

Halogenated organic species over the tropical South American rainforest

Abstract. Airborne measurements of the halogenated trace gases methyl chloride, methyl bromide and chloroform were conducted over the Atlantic Ocean and about 1000 km of pristine tropical rainforest in Suriname and French Guyana (3–6° N, 51–59° W) in October 2005. In the boundary layer (0–1.4 km), maritime air masses, advected over the forest by southeasterly trade winds, were measured at various distances from the coast. Since the organohalogens presented here have relatively long atmospheric lifetimes (0.4–1.0 years) in comparison to the advection times from the coast (1–2 days), emissions will accumulate in air traversing the rainforest. The distributions of methyl chloride, methyl bromide and chloroform were analyzed as a function of time the air spent over land and the respective relationship used to determine net fluxes from the rainforest for one week within the long dry season. Net fluxes from the rainforest ecosystem have been calculated for methyl chloride and chloroform as 9.5 (±3.8 2σ) and 0.35 (±0.15 2σ)μg m-2 h−1, respectively. No significant flux was observed for methyl bromide within the limits of these measurements. The global budget of methyl chloride contains large uncertainties, in particular with regard to a possible source from tropical vegetation. Our measurements are used in a large-scale approach to determine the net flux from a tropical ecosystem to the planetary boundary layer. The obtained global net flux of 1.5 (±0.6 2σ) Tg yr-1 for methyl chloride is at the lower end of current estimates for tropical vegetation sources, which helps to constrain the range of tropical sources and sinks (0.82 to 8.2 Tg yr-1 from tropical plants, 0.03 to 2.5 Tg yr-1 from senescent/dead leaves and a sink of 0.1 to 1.6 Tg yr-1 by soil uptake). Nevertheless, these results show that the contribution of the rainforest ecosystem is the major source in the global budget of methyl chloride. For chloroform, the extrapolated global net flux from tropical ecosystems is 56 (±23 2σ) Gg yr−1, which is of minor importance compared to the total global sources and might be already contained in the soil emission term.

A quantitative assessment of uncertainties affecting estimates of global mean OH derived from methyl chloroform observations

We estimated the global abundance of OH for the years 1988–1994 by interpreting observations of methyl chloroform (MCF) from two networks using an inverse technique and a 3‐D chemical transport model driven by assimilated meteorology. Our inversion approach optimized both the emissions of MCF and the abundance of OH. Because of an a priori overestimate of the latitudinal gradient of MCF concentration by the model in the standard setup, the inversion lowers global emissions and the global sink due to OH. Optimized emissions are about 10% lower than published inventories on average between 1988 and 1994, and the decrease in the sink suggested by the inversion implies an average lifetime for MCF (with respect to tropospheric OH) of about 6.9 years, 11–21% longer than the 5.7–6.2 years reported in previous optimization studies. Our results are driven by the need to match the observed latitudinal gradient of MCF while balancing the MCF budget. We find that these results depend on the a priori constraint placed on MCF emissions, the rate of interhemispheric mixing in the model, the interhemispheric distribution of OH assumed, and the model simulation of pollution events. Since these factors are highly uncertain, we believe that the level of understanding on global lifetimes of pollutants removed by OH is lower than might be implied by the narrow range of estimates for MCF lifetime in the literature.

Correction to “Trace gas and particulate emissions from the 2003 southern California wildfires”

[1] In the paper ‘‘Trace gas and particulate emissions from the 2003 southern California wildfires’’ by J. Muhle et al. (Journal of Geophysical Research, 112, D03307, doi:10.1029/2006JD007350, 2007), the units for the emission ratios (ER) of methyl chloroform (CH3CCl3) versus carbon dioxide (CO2) in paragraph 27 were mistyped as mmol/mol, but have to be nmol/mol. We thank Paul J. Fraser for pointing this out. None of the conclusions are affected. The corrected sentence is given below: [2] The measuredDCH3CCl3/DCO2 ER of 48 ± 10 nmol/ mol is about 10 times smaller than the ER reported by Rudolph et al. [1995], but within the range of 3–300 nmol/ mol later reported for different fuel types and fire conditions [Rudolph et al., 2000]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D04305, doi:10.1029/2008JD009895, 2008