Atmospheric Trends and Lifetime of CH3CCI3 and Global OH Concentrations

Mechanistic and kinetic studies on the OH-initiated atmospheric oxidation of fluoranthene

The atmospheric oxidation of polycyclic aromatic hydrocarbons (PAHs) can generate toxic derivatives which contribute to the carcinogenic potential of particulate organic matter. In this work, the mechanism of the OH-initiated atmospheric oxidation of fluoranthene (Flu) was investigated by using high-accuracy molecular orbital calculations. All of the possible oxidation pathways were discussed, and the theoretical results were compared with the available experimental observation. The rate constants of the crucial elementary reactions were evaluated by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The main oxidation products are a range of ring-retaining and ring-opening chemicals containing fluoranthols, fluoranthones, fluoranthenequinones, nitro-fluoranthenes, dialdehydes and epoxides. The overall rate constant of the OH addition reaction is 1.72×10(-11) cm(3) molecule(-1) s(-1) at 298 K and 1 atm. The atmospheric lifetime of Flu determined by OH radicals is about 0.69 days. This work provides a comprehensive investigation of the OH-initiated oxidation of Flu and should help to clarify its atmospheric conversion.

Rainwater trifluoroacetic acid (TFA) in Guangzhou, South China: levels, wet deposition fluxes and source implication

The origin of trifluoroacetic acid (TFA) occurring in hydrosphere has long been a controversial issue. Hydrochlorofluorocarbons and hydrofluorocarbons (HCFCs/HFCs) as replacements of chlorofluorocarbons (CFCs) are precursors of TFA in the atmosphere, their contribution to rainwater TFA is a concern as their ambient mixing ratios are continually growing. Here we present rainwater TFA monitored from April 2007 to March 2008 in urban Guangzhou, a central city in south China's highly industrialized and densely populated Pearl River Delta region. Rainwater TFA levels ranged 45.8-974 ng L(-1) with a median of 166 ng L(-1). TFA levels negatively correlated with rainfall amount, the yearly rainfall-weighted average for TFA was 152 ng L(-1). The annual TFA wet deposition flux was estimated to be 229 g km(-2) yr(-1), and the total wet deposition of TFA reached ~1.7 tyr(-1) in Guangzhou. The Two-Box model was applied to estimate attributions of HCFCs/HFCs and fluoropolymers to rainwater TFA assuming TFA generated was proportional to gross domestic product (GDP), gross industrial product (GIP) or number of private cars. The results revealed that the degradation of HCFCs/HFCs and fluoropolymers could explain 131.5-152.4 ng L(-1) rainwater TFA, quite near the observed rainfall-weighted annual mean of 152 ng L(-1), suggesting rainwater TFA in Guangzhou was predominantly originated from these anthropogenic precursors. HCFCs/HFCs accounted for 83.3-96.5% of rainwater TFA observed, while fluoropolymers' contributions were minor (~5%). HFC-134a alone could explain 55.9-90.0% of rainwater TFA, and its contribution would be greatly enhanced with its wide use in mobile air conditioning systems and rapid increase in ambient mixing ratios.

OH-initiated oxidation mechanisms and kinetics of 2,4,4'-Tribrominated diphenyl ether

2,4,4'-Tribromodiphenyl ether (BDE-28) was selected as a typical congener of polybrominated diphenyl ethers (PBDEs) to examine its fate both in the atmosphere and in water solution. All the calculations were obtained at the ground state. The mechanism result shows that the oxidations between BDE-28 and OH radicals are highly feasible especially at the less-brominated phenyl ring. Hydroxylated dibrominated diphenyl ethers (OH-PBDEs) are formed through direct bromine-substitution reactions (P1∼P3) or secondary reactions of OH-adducts (P4∼P8). Polybrominated dibenzo-p-dioxins (PBDDs) resulting from o-OH-PBDEs are favored products compared with polybrominated dibenzofurans (PBDFs) generated by bromophenols and their radicals. The complete degradation of OH adducts in the presence of O2/NO, which generates unsaturated ketones and aldehydes, is less feasible compared with the H-abstraction pathways by O2. Aqueous solution reduces the feasibility between BDE-28 and the OH radical. The rate constant of BDE-28 and the OH radical is determined to be 1.79 × 10(-12) cm(3) molecule(-1) s(-1) with an atmospheric lifetime of 6.7 days.

Atmospheric chemical reactions of 2,3,7,8-tetrachlorinated dibenzofuran initiated by an OH radical: mechanism and kinetics study

Reactions with the OH radical are expected to be the dominant removal processes for gas-phase polychlorinated dibenzo-p-dioxins and dibenzofuran (PCDD/Fs). The OH-initiated atmospheric chemical reaction mechanism and kinetics of 2,3,7,8-tetrachlorinated dibenzofuran (TCDF) are researched using the density functional theory and canonical variational transition state theory. The reaction mechanism of TCDF with the OH radical and ensuing reactions including bond cleavage of furan ring, O(2) addition or abstraction, dechlorination process, bimolecular reaction of TCDF-OH-O(2) peroxy radical with NO, and reaction of carbonyl free radicals TCDF-OH-O with H(2)O are investigated. In the subsequent reactions of TCDF-OH, O(2) abstraction and dechlorination are most likely to predominate the process. As the main products, the HO(2) radical and the Cl atom are active and may play important roles in the atmospheric oxidation processes. The rate constants of TCDF with the OH radical are calculated, which are consistent with the reported data.

Time trend analysis of atmospheric POPs concentrations in the Great Lakes region since 1990

Using a multiple linear regression model of the concentrations of several persistent organic pollutants in the atmospheric vapor and particle phases and in precipitation, we have analyzed a data set of about 700,000 values to determine the rate at which these concentrations are decreasing. These concentrations were measured as part of the Integrated Atmospheric Deposition Network (IADN), which has operated several sites near the North American Great Lakes since 1991. The pollutants measured include 83 polychlorinated biphenyl congeners, 17 polycyclic aromatic hydrocarbons, and 24 organochlorine pesticides. In the approach used here, for each of the three phases, the concentrations of a specific chemical at all the sites were combined and fitted with a regression incorporating the sine and cosine of the Julian Day (relative to 1 January 1990 and with a periodicity of one year) and the population living and working within a 25-km radius of the sampling site. Partial residuals were then calculated for each datum, all of the residuals for the three phases were combined, and an overall halving time was calculated from them. This relatively simple approach indicated that the concentrations of PCBs in air around the Great Lakes are decreasing with an overall halving time of 17 ± 2 years, which is slow for a substance that was banned about 35 years ago. Phenanthrene, chrysene, and endosulfan showed halving times on the order of 10 years. The concentrations of several organochlorine pesticides were decreasing more rapidly; for example α- and γ-HCH (lindane) have halving times of about 3.5 years.

Theoretical Study of the HOCH2OO• + HO2 • Reaction: Detailed Molecular Mechanisms of the Three Reaction Channels

The HO2 • + HOCH2OO• reaction was theoretically investigated, using various high-level, single-reference Complete Basis Set methods including CBS-QB3, CBS-QCI/APNO and CBS-Q(MPW1B95) and a new multi-reference CI-PT2 approach. Three major product channels under atmospheric conditions were identified and their molecular mechanisms elucidated in great detail by Intrinsic Reaction Coordinate Analyses (IRC) at the B3LYP/6–311G(d,p) level: (i) Direct head-to-tail H-atom abstraction from the hydroperoxy radical by the alkylperoxy, occurring on the triplet Potential Energy Surface (PES) leading to HOCH2OOH + O2; (ii) A two-step rearrangement of the initial singlet HOCH2OOOOH tetroxide complex to form HC(O)OH + •OH + HO2 •; (iii) A multi-step rearrangement of the initial HOCH2OOOOH tetroxide to yield HC(O)OH + O2(1Δ) + H2O, about twice as fast as the former channel on the singlet-surface. The findings provide an explanation for the observed hydroxyl radical formation in this reaction (Jenkin et al., Phys. Chem. Chem. Phys. 9 (2007) 3149) and rationalize the high overall rate and its pronounced negative temperature dependence (Veyret et al., J. Phys. Chem. 93 (1989) 2368).

Rate coefficients for the reaction of OH with CF3CH2CH3 (HFC-263fb) between 200 and 400 K: ab initio, DFT, and transition state theory calculations

Rate coefficients for the reaction of the hydroxyl radical with CF(3)CH(2)CH(3) (HFC-263fb) were computed using ab initio methods, viz. MP2, G3MP2, and G3B3 theories between 200 and 400 K. Structures of the reactants in the ground state (GS) and transition state (TS) were optimized at MP2(FULL)/6-31G*, MP2(FULL)/6-311+ +G**, and B3LYP/6-31G* level of theories. Seven TSs were identified for the title reaction in the above theories. However, five out of seven TSs were found to be symmetrically distinct. The kinetic parameters due to these five different TSs are presented in this manuscript. Intrinsic reaction coordinate (IRC) calculations were performed to confirm the existence of transition states. The contributions of all the individual hydrogens in the substrate for the reaction are estimated and compared with the results obtained using Structure Additivity Relationships. The rate coefficients for the title reaction were computed to be k = (7.96 +/- 0.93) x 10(-13) exp [-(2245 +/- 30)/T] cm(3) molecule(-1) s(-1) at MP2, k = (9.50 +/- 0.93) x 10(-13) exp [-(1162 +/- 30)/T] cm(3) molecule(-1) s(-1) at G3MP2, and k = (7.01 +/- 0.88) x 10(-13) exp [-(753 +/- 35)/T] cm(3) molecule(-1) s(-1)at G3B3 theories. The theoretically computed rate coefficients are found to be in excellent agreement with the experimentally determined ones. The OH-driven atmospheric lifetimes of this compound are computed to be 132, 2.2, and 0.7 years at, MP2, G3MP2, and G3B3 level of theories, respectively.

Atmospheric chemistry of acetylacetone

A kinetic study on the reactions of the OH radical and ozone with acetylacetone (AcAc) has been performed in a 1080 L quartz glass reaction chamber using in situ FTIR spectroscopy analysis. Temperature dependent rate coefficients for the reaction of AcAc with the OH radical were determined over the temperature range 285-310 K using the relative kinetic method. The following Arrhenius expression was derived: k = 3.35 x 10(-12) exp((983 +/- 130)/T) cm3 molecule(-1) s(-1), where the indicated error is the two least-squares deviation. A rate coefficient (in units of cm3 molecule(-1) s(-1)) of (1.03 +/- 0.31) x 10(-18) has been obtained at (298 +/- 3) K for the reaction of ozone with AcAc. A product investigation on the gas-phase reaction of OH radical with AcAc was conducted in a 405 L borosilicate glass chamber using in situ FTIR spectroscopy to monitor reactants and products. Methylglyoxal, acetic acid, peroxy acetic nitrate (PAN) were positively identified as products with molar yields of (20.8 +/- 4.5)%, (16.9 +/- 3.4)%, and (2.0 +/- 0.5)%, respectively. From the residual infrared spectrum the main products are attributed to 2,3,4-pentantrione (CH3-CO-CO-CO-CH3) and its hydrated analogue pentan-2,3-dione-4-diol (CH3-CO-CO-C(OH)2-CH3). Based on the observed products, a simplified mechanism for the reaction of the OH radical with AcAc is proposed.

Observations of long-lived anthropogenic halocarbons at the high-Alpine site of Jungfraujoch (Switzerland) for assessment of trends and European sources

Anthropogenic halocarbons, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), bromocarbons (halons) and long-lived chlorinated solvents have been measured continuously at the high-Alpine site of Jungfraujoch (Switzerland) since January 2000. Chloro- and bromo-containing halocarbons are responsible for the stratospheric ozone depletion and will be globally banned from usage within the next years. With the exception of the stable CFC-12 (CF2 Cl2), all major CFCs and chlorinated solvents show a negative trend in recent years in their background concentrations at Jungfraujoch. HCFCs, as their first-generation substitute, are still increasing with a few percent per year. However, the frequency and the strength of HCFCs pollution events, which are caused by regional European emissions, are already declining. This can be seen as a sign of the impending ban of these gases within the next years in Europe. On the other hand, HFCs as the second-generation substitutes, are increasing with relative rates of at least 10% per year (e.g. almost 5 ppt per year for HFC-134a). An allocation of European sources was attempted by combining measured concentrations with trajectories of air masses reaching the Jungfraujoch during pollution events. Potential source regions could be detected in Italy, France, Spain and Germany.

An evaluation of the current radiative forcing benefit of the Montreal Protocol at the high-Alpine site Jungfraujoch

A combination of reconstructed histories, long-term time series and recent quasi-continuous observations of non-CO2 greenhouse gases at the high-Alpine site Jungfraujoch is used to assess their current global radiative forcing budget and the influence of regulations due to the Montreal Protocol on Substances that Deplete the Ozone Layer in terms of climate change. Extrapolated atmospheric greenhouse gases trends from 1989 assuming a business-as-usual scenario, i.e. no Montreal Protocol restriction, are presented and compared to the observations. The largest differences between hypothetical business-as-usual mixing ratios and current atmospheric observations over the last 16 years were found for chlorinated species, in particular methyl chloroform (CH3CCl3) at 167 to 203 ppt and chlorofluorocarbon-12 (CFC-12) at 121 to 254 ppt. These prevented increases were used to estimate the effects of their restrictions on the radiative forcing budget. The net direct effect due to the Montreal Protocol regulations reduces global warming and offsets about 14 to 30% of the positive greenhouse effect related to the major greenhouse gases CO2, CH4, N2O and also SF6, and about 12 to 22% of the hypothetical current radiative forcing increase without Montreal Protocol restrictions. Thus, the Montreal Protocol succeeded not only in reducing the atmospheric chlorine content in the atmosphere but also dampened global warming. Nevertheless, the Montreal Protocol controlled species still add to global warming.

Deposition versus photochemical removal of PBDEs from Lake Superior air

Analysis of a sediment core collected from Siskiwit Lake, located on a remote island in Lake Superior, provides evidence that polybrominated diphenyl ethers (PBDEs) are removed effectively from the atmosphere via deposition processes during long-range transport. A mass balance model based on photochemical rate constants and data from atmospheric samples was created to understand the relative importance of various photochemical and deposition processes in removing PBDEs from the atmosphere. Photolysis rate constants were derived from UV absorption spectra of 25 PBDEs recorded in isooctane over the range of 280-350 nm at 298 K. Photolysis decays measured for BDE-3 and -7 in the gas phase were substantial compared to a well-defined chemical actinometer, indicating that their photolysis quantum yields are significant. Dibenzofuran production was observed when PBDE congeners containing ortho-bromines were photolyzed in helium. From estimates of removal rates of PBDEs from the lower troposphere, we find that wet and dry deposition accountfor >95% of the removal of BDE-209, while photolysis accounts for -90% of the removal of gas-phase congeners such as BDE-47. These results help explain the deposition patterns of PBDEs found in lake and river sediments and have important implications concerning the inclusion of photolysis as a fate process in multimedia models.

Atmospheric Hexachlorocyclohexanes in the North Pacific Ocean and the adjacent Arctic region: spatial patterns, chiral signatures, and sea-air exchanges

During the 2003 Chinese Arctic Research Expedition (CHINARE2003) from the Bohai Sea to the high Arctic (37 degrees N to 80 degrees N) aboard the icebreaker Xuelong (Snow Dragon), air samples were collected for the analysis of hexachlorocyclohexanes (HCHs) in the North Pacific Ocean and adjacent Arctic region. The sigma HCHs (alpha-HCH + gamma-HCH) ranged from 2.3 to 95.1 pg/m3 with the highest levels observed in Far East Asia (32.5 pg/m3), followed by the North Pacific Ocean (17.0 pg/m3) and the Arctic (7.3 pg/ m3). Compared to previous studies in the same areas in 1990s, our measurements were approximately 1 order of magnitude lower. Because of disproportionate chemical reduction and physical fractioning during long-range transport, the ratios of alpha-HCH to gamma-HCH (alpha/gamma-HCH) showed a significant increasing trend from low to high latitudes, suggesting thatthe alpha/gamma-HCH range of 4-7 could not be used to identify sources of technical HCHs especially in remote areas. The ratios of (+)-alpha-HCH to the sum of (+)-alpha-HCH and (-)-alpha-HCH were on average much more biased from 0.5 compared to previous observations in mid-1990s, indicating the exchange of atmospheric alpha-HCH with those in the oceans, where (+)-alpha-HCH was selectively depleted in biological degradation processes. Estimated fugacity ratios based on available data for both alpha-HCH and gamma-HCH further implied their net volatilization from seawater to air in the Arctic Ocean.

Changes in tropospheric composition and air quality due to stratospheric ozone depletion and climate change

It is well-understood that reductions in air quality play a significant role in both environmental and human health. Interactions between ozone depletion and global climate change will significantly alter atmospheric chemistry which, in turn, will cause changes in concentrations of natural and human-made gases and aerosols. Models predict that tropospheric ozone near the surface will increase globally by up to 10 to 30 ppbv (33 to 100% increase) during the period 2000 to 2100. With the increase in the amount of the stratospheric ozone, increased transport from the stratosphere to the troposphere will result in different responses in polluted and unpolluted areas. In contrast, global changes in tropospheric hydroxyl radical (OH) are not predicted to be large, except where influenced by the presence of oxidizable organic matter, such as from large-scale forest fires. Recent measurements in a relatively clean location over 5 years showed that OH concentrations can be predicted by the intensity of solar ultraviolet radiation. If this relationship is confirmed by further observations, this approach could be used to simplify assessments of air quality. Analysis of surface-level ozone observations in Antarctica suggests that there has been a significant change in the chemistry of the boundary layer of the atmosphere in this region as a result of stratospheric ozone depletion. The oxidation potential of the Antarctic boundary layer is estimated to be greater now than before the development of the ozone hole. Recent modeling studies have suggested that iodine and iodine-containing substances from natural sources, such as the ocean, may increase stratospheric ozone depletion significantly in polar regions during spring. Given the uncertainty of the fate of iodine in the stratosphere, the results may also be relevant for stratospheric ozone depletion and measurements of the influence of these substances on ozone depletion should be considered in the future. In agreement with known usage and atmospheric loss processes, tropospheric concentrations of HFC-134a, the main human-made source of trifluoroacetic acid (TFA), is increasing rapidly. As HFC-134a is a potent greenhouse gas, this increasing concentration has implications for climate change. However, the risks to humans and the environment from substances, such as TFA, produced by atmospheric degradation of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are considered minimal. Perfluoropolyethers, commonly used as industrial heat transfer fluids and proposed as chlorohydrofluorocarbon (CHFC) substitutes, show great stability to chemical degradation in the atmosphere. These substances have been suggested as substitutes for CHFCs but, as they are very persistent in the atmosphere, they may be important contributors to global warming. It is not known whether these substances will contribute significantly to global warming and its interaction with ozone depletion but they should be considered for further evaluation.