Kinetics of the Reactions of Chlorinated Very Short-Lived Substances (VSLSs) with Chlorine Atoms

Chlorinated very short-lived substances (VSLSs), which are not controlled by the Montreal Protocol, are of current concern with regard to recovery of stratospheric ozone. Further study is needed on the temperature dependences of chlorinated VSLSs relevant to atmospheric conditions. Here, the kinetics of chlorinated VSLSs, such as chloroform (CHCl3), dichloromethane (CH2Cl2), dichloroethane (CH2ClCH2Cl), and trichloroethene (C2HCl3) reacting with chlorine atoms, were investigated between 180 and 400 K, expanding the range of temperatures relative to previous studies. RRKM/Master Equation and Canonical Variational Transition State Theory were utilized to calculate the rate coefficients using the MultiWell suite of programs. CCSD(T), QCISD(T), and M062X with aug-cc-pV(T+d)Z levels of theory were used to calculate the kinetic parameters. Arrhenius equations obtained from fits to the calculated rate coefficients are k1 = (2.66 ± 0.7) × 10-12 exp [(-927 ± 131)/T] cm3 molecule-1 s-1, k2 = (8.99 ± 0.3) × 10-12 exp [(-957 ± 19)/T] cm3 molecule-1 s-1, k3 = (1.51 ± 0.16) × 10-11 exp [(-714 ± 54)/T] cm3 molecule-1 s-1, and k4 = (9.17 ± 1.8) × 10-12 exp [(612 ± 101)/T] cm3 molecule-1 s-1 for the reactions of CHCl3, CH2Cl2, CH2ClCH2Cl, and C2HCl3 with Cl atoms, respectively. The rate coefficients for the reactions of chlorinated VSLSs with Cl atoms from this study are compared with the most recent recommended values from the NASA/JPL and IUPAC evaluations and with literature values. The reactivity trends of the reactions are discussed.

Recent Trends in Stratospheric Chlorine From Very Short-Lived Substances

Very short-lived substances (VSLS), including dichloromethane (CH2Cl2), chloroform (CHCl3), perchloroethylene (C2Cl4), and 1,2-dichloroethane (C2H4Cl2), are a stratospheric chlorine source and therefore contribute to ozone depletion. We quantify stratospheric chlorine trends from these VSLS (VSLCltot) using a chemical transport model and atmospheric measurements, including novel high-altitude aircraft data from the NASA VIRGAS (2015) and POSIDON (2016) missions. We estimate VSLCltot increased from 69 (±14) parts per trillion (ppt) Cl in 2000 to 111 (±22) ppt Cl in 2017, with >80% delivered to the stratosphere through source gas injection, and the remainder from product gases. The modeled evolution of chlorine source gas injection agrees well with historical aircraft data, which corroborate reported surface CH2Cl2 increases since the mid-2000s. The relative contribution of VSLS to total stratospheric chlorine increased from ~2% in 2000 to ~3.4% in 2017, reflecting both VSLS growth and decreases in long-lived halocarbons. We derive a mean VSLCltot growth rate of 3.8 (±0.3) ppt Cl/year between 2004 and 2017, though year-to-year growth rates are variable and were small or negative in the period 2015-2017. Whether this is a transient effect, or longer-term stabilization, requires monitoring. In the upper stratosphere, the modeled rate of HCl decline (2004-2017) is -5.2% per decade with VSLS included, in good agreement to ACE satellite data (-4.8% per decade), and 15% slower than a model simulation without VSLS. Thus, VSLS have offset a portion of stratospheric chlorine reductions since the mid-2000s.

Fate of Chloromethanes in the Atmospheric Environment: Implications for Human Health, Ozone Formation and Depletion, and Global Warming Impacts

Among the halogenated hydrocarbons, chloromethanes (i.e., methyl chloride, CH₃Cl; methylene chloride, CH₂Cl₂; chloroform, CHCl₃; and carbon tetrachloride, CCl₄) play a vital role due to their extensive uses as solvents and chemical intermediates. This article aims to review their main chemical/physical properties and commercial/industrial uses, as well as the environment and health hazards posed by them and their toxic decomposition products. The environmental properties (including atmospheric lifetime, radiative efficiency, ozone depletion potential, global warming potential, photochemical ozone creation potential, and surface mixing ratio) of these chlorinated methanes are also reviewed. In addition, this paper further discusses their atmospheric fates and human health implications because they are apt to reside in the lower atmosphere when released into the environment. According to the atmospheric degradation mechanism, their toxic degradation products in the troposphere include hydrogen chloride (HCl), carbon monoxide (CO), chlorine (Cl₂), formyl chloride (HCOCl), carbonyl chloride (COCl₂), and hydrogen peroxide (H₂O₂). Among them, COCl₂ (also called phosgene) is a powerful irritating gas, which is easily hydrolyzed or thermally decomposed to form hydrogen chloride.

Microbial removal of atmospheric carbon tetrachloride in bulk aerobic soils

Atmospheric concentrations of carbon tetrachloride (CCl(4)) were removed by bulk aerobic soils from tropical, subtropical, and boreal environments. Removal was observed in all tested soil types, indicating that the process was widespread. The flux measured in field chamber experiments was 0.24 ± 0.10 nmol CCl(4) (m(2) day)(-1) (average ± standard deviation [SD]; n = 282). Removal of CCl(4) and removal of methane (CH(4)) were compared to explore whether the two processes were linked. Removal of both gases was halted in laboratory samples that were autoclaved, dry heated, or incubated in the presence of mercuric chloride (HgCl(2)). In marl soils, treatment with antibiotics such as tetracycline and streptomycin caused partial inhibition of CCl(4) (50%) and CH(4) (76%) removal, but removal was not affected in soils treated with nystatin or myxothiazol. These data indicated that bacteria contributed to the soil removal of CCl(4) and that microeukaryotes may not have played a significant role. Amendments of methanol, acetate, and succinate to soil samples enhanced CCl(4) removal by 59%, 293%, and 72%, respectively. Additions of a variety of inhibitors and substrates indicated that nitrification, methanogenesis, or biological reduction of nitrate, nitrous oxide, or sulfate (e.g., occurring in possible anoxic microzones) did not play a significant role in the removal of CCl(4). Methyl fluoride inhibited removal of CH(4) but not CCl(4), indicating that CH(4) and CCl(4) removals were not directly linked. Furthermore, CCl(4) removal was not affected in soils amended with copper sulfate or methane, supporting the results with MeF and suggesting that the observed CCl(4) removal was not significantly mediated by methanotrophs.

Photolysis of Trichloronitromethane (Chloropicrin) under Atmospheric Conditions

An experimental investigation on the photolysis of the pesticide chloropicrin, (trichloronitromethane, CCl3NO2), under atmospheric conditions was carried out at the outdoor European Photoreactor, (EUPHORE), in Valencia, Spain. The photodissociation rate coefficient, J obs(CCl3NO2), was determined directly under sunlight conditions during spring and summer months. Values in the range J obs(CCl3NO2) = (3.9–5.1)×10−5 s−1 were obtained, corresponding to photolysis lifetimes of 7.1–5.4 hours. Absorption cross-sections for chloropicrin were determined over the wavelength range 260–370 nm, and together with the measured solar flux intensity, were used to calculate the maximum photolysis rate for chloropicrin, J max. Comparison of the observed photolysis rate with the calculated maximum photolysis rate showed that the effective quantum yield of photodissociation, Φ = J obs(CCl3NO2)/J max, was 0.94±0.08. Photolysis of chloropicrin in air or nitrogen gave phosgene as the major carbon-containing product with a yield close to unity based on the loss of chloropicrin. The product yield data were shown to be consistent with a mechanism in which the photolysis channel produces a CCl3 radical and NO2. Kinetic studies on the reactions of hydroxyl radicals and ozone with chloropicrin suggest that, as expected, loss of CCl3NO2 by reaction with these species will be negligible under atmospheric conditions compared to photolysis. Photolysis of chloropicrin in air in the presence of isopropanol gave significant yields of ozone and is consistent with the generation of Cl atoms and NOx following the photodissociation of CCl3NO2. The atmospheric implications of the use of chloropicrin as a pesticide are discussed.

Investigating ozone-induced decomposition of surface-bound permethrin for conditions in aircraft cabins

The reaction of ozone with permethrin can potentially form phosgene. Published evidence on ozone levels and permethrin surface concentrations in aircraft cabins indicated that significant phosgene formation might occur in this setting. A derivatization technique was developed to detect phosgene with a lower limit of detection of 2 ppb. Chamber experiments were conducted with permethrin-coated materials (glass, carpet, seat fabric, and plastic) exposed to ozone under cabin-relevant conditions (150 ppb O(3), 4.5/h air exchange rate, <1% relative humidity, 1700 ng/cm(2) of permethrin). Phosgene was not detected in these experiments. Reaction of ozone with permethrin appears to be hindered by the electron-withdrawing chlorine atoms adjacent to the double bond in permethrin. Experimental results indicate that the upper limit on the reaction probability of ozone with surface-bound permethrin is approximately 10(-7). Extrapolation by means of material-balance modeling indicates that the upper limit on the phosgene level in aircraft cabins resulting from this chemistry is approximately 1 microg/m(3) or approximately 0.3 ppb. It was thus determined that phosgene formation, if it occurs in aircraft cabins, is not likely to exceed relevant, health-based phosgene exposure guidelines.

PRACTICAL IMPLICATIONS

Phosgene formation from ozone-initiated oxidation of permethrin in the aircraft cabin environment, if it occurs, is estimated to generate levels below the California Office of Environmental Health Hazard Assessment acute reference exposure level of 4 microg/m(3) or approximately 1 ppb.

The flux of chloroform and tetrachloromethane along an elevational gradient of a coastal salt marsh, East China

The fluxes of trichloromethane (CHCl(3), CM) and tetrachloromethane (CCl(4), TCM) were seasonally measured using static flux chambers over an annual cycle in a coastal salt marsh, East China. The salt marsh presented as a large sink for both the compounds in the growing season (from April to October), but it was a minor source for the gas species in the non-growing season. Generally, the cordgrass marsh acted as a sink of CM and TCM. The net consumption of CM and TCM observed in the study marsh may result from the high ambient atmospheric concentrations and enriched soil organic matter that result in anoxic sediments. Higher plants were suggested to be an important sink for CM and TCM in the growing season, but a net source in the non-growing season. However, the mechanism responsible for the plant removal process is not clear.

Atmospheric fate of dichlorvos: photolysis and OH-initiated oxidation studies

The OH-initiated oxidation of dichlorvos (a widely used insecticide) has been investigated under atmospheric conditions at the large outdoor European photoreactor (EUPHORE) in Valencia, Spain. The rate constant of OH reaction with dichlorvos, k, was measured by using a conventional relative rate technique where 1,3,5-trimethylbenzene (TMB) and cyclohexane were taken as references. With the use of the rate constants of 5.67 x 10(-11) and of 6.97 x 10(-12) cm3 molecule(-1) s(-1) for the reactions OH + TMB and OH + cyclohexane, respectively, the resulting value of the OH reaction rate constant with dichlorvos was derived to be k = (2.6 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1). The tropospheric lifetime of dichlorvos with respect to reaction with OH radical has been estimated to be around 11 h. The major carbon-containing products observed for the OH reaction with dichlorvos in air under sunlight condition were phosgene and carbon monoxide. The formation of a very stable toxic primary product such as phosgene associated with the relatively short lifetime of dichlorvos may make the use of this pesticide even more toxic for humans when released into the atmosphere.

A Kinetic and Mechanistic Study of the Reactions of OH Radicals and Cl Atoms with 3,3,3-Trifluoropropanol under Atmospheric Conditions

Product distribution studies of the OH radical and Cl atom initiated oxidation of CF3CH2CH2OH in air at 1 atm and 298 +/- 5 K have been carried out in laboratory and outdoor atmospheric simulation chambers in the presence and absence of NOx. The results show that CF3CH2CHO is the only primary product and that the aldehyde is fairly rapidly removed from the system. In the absence of NOx the major degradation product of CF3CH2CHO is CF3CHO, and the combined yields of the two aldehydes formed from CF3CH2CH2OH are close to unity (0.95 +/- 0.05). In the presence of NOx small amounts of CF3CH2C(O)O2NO2 were also observed (<15%). At longer reaction times CF3CHO is removed from the system to give mainly CF2O. The laser photolysis-laser induced fluorescence technique was used to determine values of k(OH + CF3CH2CH2OH) = (0.89 +/- 0.03) x 10(-12) and k(OH + CF3CH2CHO) = (2.96 +/- 0.04) x 10(-12) cm3 molecule(-1) s(-1). A relative rate method has been employed to measure the rate coefficients k(OH + CF3CH2CH2OH) = (1.08 +/- 0.05) x 10(-12), k(OH + C6F13CH2CH2OH) = (0.79 +/- 0.08) x 10(-12), k(Cl + CF3CH2CH2OH) = (22.4 +/- 0.4) x 10(-12), and k(Cl + CF3CH2CHO) = (25.7 +/- 0.4) x 10(-12) cm3 molecule(-1) s(-1). The results from this investigation are discussed in terms of the possible importance of emissions of fluorinated alcohols as a source of fluorinated carboxylic acids in the environment.

Photolysis of chloral under atmospheric conditions

The photolysis of chloral under atmospheric conditions was studied at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. The photodissociation rate coefficient, J(chloral), was measured directly under different sunlight conditions during April 1999. Values in the range of J(chloral) = (4.61-6.11) x 10(-5) s(-1) were obtained, yielding an average value of J(chloral)/J(NO2) = (6.15 +/- 0.62) x 10(-3). This corresponds to a photolysis lifetime of 4.5-6 h under conditions appropriate to the solar flux during summer months and confirms that atmospheric photolysis is the major degradation pathway for chloral. The overall quantum efficiency of photolysis under atmospheric conditions was determined to be 1.00 +/- 0.05. The atmospheric photolysis of chloral produced phosgene, CO, and Cl atoms with molar yields of 0.83 +/- 0.04, 1.01 +/- 0.05, and 1.18 +/- 0.06, respectively. The product yield data are consistent with a mechanism in which the primary photolysis channel produces a Cl atom and a CCl2CHO radical. The latter species is converted to the oxy radical OCCl2CHO,which decomposes by both C-C and C-Cl bond fission. A chemical mechanism for the photolysis of chloral by sunlight is proposed, and the atmospheric implications are discussed.

Review of concentrations and chemistry of trichloroacetate in the environment

This paper reviews the concentrations of trichloroacetate (TCA) in the atmosphere-plant-soil system. Data originate mainly from Europe. The median TCA concentration in rainwater and canopy drip decreased until 1995. From then the median TCA concentration in rainwater remains rather constant while for canopy drip later data are not available. The same seems to hold for concentrations in air although a very limited data set is available. The median concentrations in coniferous needles and groundwater are constant for the period observed. The median TCA concentrations in soil decreased until 1992 and then remained constant.The TCA formation from chlorinated solvents in the atmosphere may explain a substantial percentage of the TCA amount in the atmosphere. The TCA concentrations in rainwater and canopy drip indicate that there will be other sources contributing to 10-50%. Waste incineration, biomass burning and natural formation in the marine boundary layer are potential candidate sources of TCA, but nothing can be said as yet on their TCA emission rates. Anthropogenic emissions of chlorine could also be a source.TCA can be formed from chlorinated solvents by biota. However, for coniferous trees the uptake of TCA from soil may be the predominant route. Biotic and abiotic reactions can cause to formation of TCA in soil, but also formation of TCA from chlorinated solvents by biota that excrete TCA, may contribute. Mass balance calculations of the bioactive soil top layer show that the production rate of TCA in certain soil types could be substantial. The mass balance calculations could not distinguish between natural and anthropogenic sources in soil.

Joint persistence of transformation products in chemicals assessment: case studies and uncertainty analysis

The joint persistence (JP) quantifies the environmental persistence of a parent compound and a selection of relevant transformation products. Here, the importance as well as the uncertainty of the JP in comparison to the persistence of the parent compound alone (primary persistence, PP) are investigated. To demonstrate the effect of transformation products on the environmental persistence of organic chemicals, three case studies of parent compounds (nonylphenol ethoxylates, perchloroethylene, atrazine) and transformation products are investigated in detail with a multimedia fate model. Comparison of the PP and JP values shows that transformation products can significantly increase the persistence. In addition to the point estimates of PP and JP, the associated uncertainties are investigated. For each of the case studies, the chemical-specific input parameters of all compounds are varied and the corresponding variance of the PP and JP is determined by Monte Carlo simulations. Interestingly, the higher number of input parameters required for the JP does not necessarily increase the uncertainty of the JP as compared to that of the PP alone. An exact mathematical expression specifying the contribution of each transformation product to the JP is given. When transformation products are grouped in different generations, it becomes discernible that the first generation increases the JP most; the later generations are of decreasing importance. Finally, the effect of incomplete knowledge of the transformation products and their properties on the JP results is discussed. For reliable JP estimates, knowledge of the first generation transformation products and their degradation rate constants is required.

Trichloroacetic acid in the environment

Suppositions that the trichloroacetic acid (TCA, CCl3C(O)OH) found in nature was a consequence solely of the use of chlorinated hydrocarbon solvents prompted this critical review of the literature on its environmental fluxes and occurrences. TCA is widely distributed in forest soils (where it was rarely used as an herbicide) and measurements suggest a soil flux of 160 000 tonnes yr(-1) in European forests alone. TCA is also produced during oxidative water treatment and the global flux could amount to 55 000 tonnes yr(-1) (from pulp and paper manufacture, potable water and cooling water treatments). By contrast, the yields of TCA from chlorinated hydrocarbon solvents are small: from tetrachloroethene 13 600 tonnes yr(-1) and from 1,1,1-trichloroethane 4300 tonnes yr(-1) on a global basis, at the atmospheric burdens and removal rates typical of the late 1990s. TCA is ubiquitous in rainwater and snow. Its concentrations are highly variable and the variations cannot be connected with location or date. However, there is no significant difference between the concentrations found in Chile and in eastern Canada (by the same analysts), or between Malawi and western Canada, or between Antarctica and Switzerland, nor any significant difference globally between the concentrations in cloud, rain and snow (although local enhancement in fog water has been shown). TCA is present in old ice and firn. At the deepest levels, the firn was deposited early in the 19th century, well before the possibility of contamination by industrial production of reactive chlorine, implying a non-industrial background. This proposition is supported by plume measurements from pulp mills in Finland. TCA is ubiquitous in soils; concentrations are very variable but there are some indications that soils under coniferous trees contain higher amounts. The concentrations of TCA found in plant tissue are region-specific and may also be plant-specific, to the extent that conifers seem to contain more than other species. TCA is removed from the environment naturally. There is abundant evidence that soil microorganisms dehalogenate TCA and it is lost from within spruce needles with a half-life of 10 days. There is also recent evidence of an abiotic aqueous decarboxylation mechanism with a half-life of 22 days. The supposedly widespread effects of TCA in conifer needles are not shown in controlled experiments. At concentrations in the needles of Scots pine similar to those observed in needles in forest trees, changes consequent on TCA treatment of field laboratory specimens were almost all insignificant.

Fractionation of halogenated organic matter present in rain and snow

Organic matter in samples of rain and snow from Sweden, Poland, Germany and the Republic of Ireland was fractionated by employing a series of filtration, purging, evaporation and extraction steps. Determinations of the group parameter AOX (adsorbable organic halogens) in aqueous phases and EOX (extractable organic halogens) in organic phases showed that halogenated organic matter present in bulk precipitation is composed of several different groups of compounds. The largest amounts of organically bound halogens were found in fractions of relatively polar and non-volatile to semivolatile compounds. In particular, a significant part of the AOX could be attributed to alkaline-labile organic bases. Gas chromatographic analysis of different organic extracts in the chlorine channel of an atomic emission detector (AED) resulted in chromatograms with few distinct peaks, and analysis in the bromine channel did not produce any distinct peaks. Chlorinated acetic acids were the most abundant halogenated organic acids, and chlorinated alkyl phosphates were normally responsible for the largest peaks in the chlorine chromatogram of neutral, hexane-extractable compounds. When analysing volatiles, 1,4-dichlorobenzene and a thus far unidentified chloroorganic compound often caused the largest response in the chlorine channel of the AED system.

Balloon observations of organic and inorganic chlorine in the stratosphere: the role of HClO4 production on sulfate aerosols

Simultaneous observations of stratospheric organic and inorganic chlorine were made in September 1993 out of Fort Sumner, New Mexico, using JPL balloon-borne MkIV interferometer. Between 15 and 20 km, a significant fraction (20-60%) of the inorganic chlorine could not be accounted for by the sum of measured HCl, ClONO2, and HOCl. Laboratory measurements of the reaction of ClO radicals on sulfuric acid solutions have indicated that, along with HCl, small amounts of perchloric acid, HClO4, were formed. Very little is known about the fate of HClO4 in the stratosphere and we use a photochemical box model to determine the impact of this new species on the partitioning of inorganic chlorine in the stratosphere. Assuming that HClO4 is photochemically stable, it is shown that in the enhanced aerosol loading conditions resulting from Mt. Pinatubo's eruption, HClO4 could represent a significant reservoir of chlorine in the lower stratosphere, sequestering up to 0.2 ppbv (or 50%) of the total inorganic chlorine at 16 km. The occurrence of this new species could bring to closure the inorganic chlorine budget deficiency made apparent by recent ER-2 aircraft in situ measurements of HCl.