Atmospheric Hydroxyl Radical Reaction Rate Coefficient and Total Environmental Lifetime of α-Endosulfan.

Endosulfan is a persistent organochlorine pesticide that was globally distributed before it was banned and continues to cycle in the Earth system. The chemical kinetics of the gas-phase reaction of α-endosulfan with the hydroxyl radical (OH) was studied by means of pulsed vacuum UV flash photolysis and time-resolved resonance fluorescence (FP-RF) as a function of temperature in the range of 348-395 K and led to a second-order rate coefficient kOH = 5.8 × 10-11 exp(-1960K/T) cm3 s-1 with an uncertainty range of 7 × 10-12 exp(-1210K/T) to 4 × 10-10 exp(-2710K/T) cm3 s-1. This corresponds to an estimated photochemical atmospheric half-life in the range of 3-12 months, which is much longer than previously assumed (days to weeks). Comparing the atmospheric concentrations observed after the global ban of endosulfan with environmental multimedia model predictions, we find that photochemical degradation in the atmosphere is slower than the model-estimated biodegradation in soil or water and that the latter limits the total environmental lifetime of endosulfan. We conclude that the lifetimes typically assumed for soil and aquatic systems are likely underestimated and should be revisited, in particular, for temperate and warm climates.

Modeling the Formation, Degradation, and Spatiotemporal Distribution of 2-Nitrofluoranthene and 2-Nitropyrene in the Global Atmosphere.

Polycyclic aromatic hydrocarbons (PAHs) are common atmospheric pollutants and known to cause adverse health effects. Nitrated PAHs (NPAHs) are formed in combustion activities and by nitration of PAHs in the atmosphere and may be equally or more toxic, but their spatial and temporal distribution in the atmosphere is not well characterized. Using the global EMAC model with atmospheric chemistry and surface compartments coupled, we investigate the formation, abundance, and fate of two secondarily formed NPAHs, 2-nitrofluoranthene (2-NFLT) and 2-nitropyrene (2-NPYR). The default reactivity scenario, the model with the simplest interpretation of parameters from the literature, tends to overestimate both absolute concentrations and NPAH/PAH ratios at observational sites. Sensitivity scenarios indicate that NO2-dependent NPAH formation leads to better agreement between measured and predicted NPAH concentrations and that photodegradation is the most important loss process of 2-NFLT and 2-NPYR. The highest concentrations of 2-NFLT and 2-NPYR are found in regions with strong PAH emissions, but because of continued secondary formation from the PAH precursors, these two NPAHs are predicted to be spread across the globe.

Fast Formation of Nitro-PAHs in the Marine Atmosphere Constrained in a Regional-Scale Lagrangian Field Experiment.

Polycyclic aromatic hydrocarbons (PAHs) and some of their nitrated derivatives, NPAHs, are seemingly ubiquitous in the atmospheric environment. Atmospheric lifetimes may nevertheless vary within a wide range, and be as short as a few hours. The sources and sinks of NPAH in the atmosphere are not well understood. With a Lagrangian field experiment and modeling, we studied the conversion of the semivolatile PAHs fluoranthene and pyrene into the 2-nitro derivatives 2-nitrofluoranthene and 2-nitropyrene in a cloud-free marine atmosphere on the time scale of hours to 1 day between a coastal and an island site. Chemistry and transport during several episodes was simulated by a Lagrangian box model i.e., a box model coupled to a Lagrangian particle dispersion model, FLEXPART-WRF. It is found that the chemical kinetic data do capture photochemical degradation of the 4-ring PAHs under ambient conditions on the time scale of hours to 1 day, while the production of the corresponding NPAH, which sustained 2-nitrofluoranthene/fluoranthene and 2-nitropyrene/pyrene yields of (3.7 ± 0.2) and (1.5 ± 0.1)%, respectively, is by far underestimated. Predicted levels of NPAH come close to observed ones, when kinetic data describing the reactivity of the OH-adduct were explored by means of theoretically based estimates. Predictions are also underestimated by 1-2 orders of magnitude, when NPAH/PAH yields reported from laboratory experiments conducted under high NOx conditions are adopted for the simulations. It is concluded that NPAH sources effective under low NOx conditions, are largely underestimated.

Heterogeneous OH Oxidation, Shielding Effects, and Implications for the Atmospheric Fate of Terbuthylazine and Other Pesticides.

Terbuthylazine (TBA) is a widely used herbicide, and its heterogeneous reaction with OH radicals is important for assessing its potential to undergo atmospheric long-range transport and to affect the environment and public health. The apparent reaction rate coefficients obtained in different experimental investigations, however, vary by orders of magnitude depending on the applied experimental techniques and conditions. In this study, we used a kinetic multilayer model of aerosol chemistry with reversible surface adsorption and bulk diffusion (KM-SUB) in combination with a Monte Carlo genetic algorithm to simulate the measured decay rates of TBA. Two experimental data sets available from different studies can be described with a consistent set of kinetic parameters resolving the interplay of chemical reaction, mass transport, and shielding effects. Our study suggests that mass transport and shielding effects can substantially extend the atmospheric lifetime of reactive pesticides from a few days to weeks, with strong implications for long-range transport and potential health effects of these substances.

Exploring the planetary boundary for chemical pollution.

Rockström et al. (2009a, 2009b) have warned that humanity must reduce anthropogenic impacts defined by nine planetary boundaries if "unacceptable global change" is to be avoided. Chemical pollution was identified as one of those boundaries for which continued impacts could erode the resilience of ecosystems and humanity. The central concept of the planetary boundary (or boundaries) for chemical pollution (PBCP or PBCPs) is that the Earth has a finite assimilative capacity for chemical pollution, which includes persistent, as well as readily degradable chemicals released at local to regional scales, which in aggregate threaten ecosystem and human viability. The PBCP allows humanity to explicitly address the increasingly global aspects of chemical pollution throughout a chemical's life cycle and the need for a global response of internationally coordinated control measures. We submit that sufficient evidence shows stresses on ecosystem and human health at local to global scales, suggesting that conditions are transgressing the safe operating space delimited by a PBCP. As such, current local to global pollution control measures are insufficient. However, while the PBCP is an important conceptual step forward, at this point single or multiple PBCPs are challenging to operationalize due to the extremely large number of commercial chemicals or mixtures of chemicals that cause myriad adverse effects to innumerable species and ecosystems, and the complex linkages between emissions, environmental concentrations, exposures and adverse effects. As well, the normative nature of a PBCP presents challenges of negotiating pollution limits amongst societal groups with differing viewpoints. Thus, a combination of approaches is recommended as follows: develop indicators of chemical pollution, for both control and response variables, that will aid in quantifying a PBCP(s) and gauging progress towards reducing chemical pollution; develop new technologies and technical and social approaches to mitigate global chemical pollution that emphasize a preventative approach; coordinate pollution control and sustainability efforts; and facilitate implementation of multiple (and potentially decentralized) control efforts involving scientists, civil society, government, non-governmental organizations and international bodies.

Iron(III)-induced activation of chloride and bromide from modeled salt pans.

The photochemistry of halides in sea spray aerosol, on salt pans, and on other salty surfaces leads to a formation of reactive halogen species. We investigated the photochemical formation of atomic chlorine (Cl) and bromine (Br) in the gas phase in the presence of laboratory-modeled salt pans consisting of sodium chloride doped with iron(III) chloride hexahydrate (0.5 and 2 wt %). The samples were spread on a Teflon sheet and exposed to simulated sunlight in a Teflon smog chamber in purified, humidified air in the presence of a test mixture of hydrocarbons at the ppb level to determine Cl, Br, and OH formation by the radical clock method. Driven by the photolytic reduction of Fe(III) to Fe(II), the production rates of the Fe(III)-doped NaCl salt samples (up to10(7) atoms cm(-3) s(-1)) exceeded the release of Cl above a pure NaCl sample by more than an order of magnitude in an initially O3-free environment at low NOX. In bromide-doped samples (0.5 wt % NaBr), a part of the Cl release was replaced by Br when Fe(III) was present. Additions of sodium sulfate, sodium oxalate, oxalic acid, and catechol to NaCl/FeCl3 samples were found to restrain the activation of chloride.

Reversible addition of the OH radical to p-cymene in the gas phase: multiple adduct formation. Part 2.

A flash photolysis-resonance fluorescence (FP-RF) system was used to study the p-cymene (PC) + OH reaction at temperatures between 299 and 349 K in helium. Triexponential functions were fitted to groups of observed OH decay curves according to a model considering a reversible addition to form two adducts as thermolabile reservoirs of OH. Compared to Part 1 of this paper, consideration of a second adduct strongly improved the fits to our measurements, and the rate constants for the major pathways were optimized between 299 and 349 K. The Arrhenius expression for the rate constant of the sum of OH addition and H-atom abstraction pathways was found to be kOH = 1.9 × 10(-12) exp[(610 ± 210) K/T] cm(3) s(-1). Rate constants of unimolecular decomposition reactions of the adducts were similar to other aromatic compounds with the following Arrhenius expressions: 1 × 10(12) exp[(-7600 ± 800) K/T] s(-1) for adduct 1 and 4 × 10(11) exp[(-8000 ± 300) K/T] s(-1) for adduct 2. Adduct yields increased and decreased with temperature for adduct 1 and 2, respectively, but were similar (∼0.4) around room temperature. Equilibrium constants yielded values for reaction enthalpies and entropies of adduct formations. While for one adduct reasonable agreement was obtained with theoretical predictions, there were significant deviations for the other adduct. This indicates the presence of more than two adduct isomers that were not accounted for in the reaction model. Quantum chemical calculations (DFT M06-2X/6-31G(d,p)) and RRKM kinetics were employed with the aim of clarifying the mechanism of the OH addition to PC. These calculations show that formation of adducts with OH in ortho positions to the isopropyl and methyl substituents is predominant (55% and 24%) to those with OH in ipso positions (21% and 3%). A large fraction (>90%) of the ipso-C3H7 adduct is predicted to react by dealkylation forming p-cresol (in the absence of oxygen) and isopropyl radicals. These theoretical results agree well with the interpretation of the experimental results showing that the two ortho adducts (which appeared as OH reservoirs in the experiment) have been observed.

Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint.

Gaseous nitrogen dioxide (NO2) represents an oxidant that is present in relatively high concentrations in various indoor settings. Remarkably increased NO2 levels up to 1.5 ppm are associated with homes using gas stoves. The heterogeneous reactions of NO2 with adsorbed water on surfaces lead to the generation of nitrous acid (HONO). Here, we present a HONO source induced by heterogeneous reactions of NO2 with selected indoor paint surfaces in the presence of light (300 nm<λ<400 nm). We demonstrate that the formation of HONO is much more pronounced at elevated relative humidity. In the presence of light (5.5 W m(-2)), an increase of HONO production rate of up to 8.6·10(9) molecules cm(-2) s(-1) was observed at [NO2]=60 ppb and 50% relative humidity (RH). At higher light intensity of 10.6 (W m(-2)), the HONO production rate increased to 2.1·10(10) molecules cm(-2) s(-1). A high NO2 to HONO conversion yield of up to 84% was observed. This result strongly suggests that a light-driven process of indoor HONO production is operational. This work highlights the potential of paint surfaces to generate HONO within indoor environments by light-induced NO2 heterogeneous reactions.

Reversible addition of the OH radical to p-cymene in the gas phase: kinetic analysis assuming formation of a single adduct. Part 1.

A flash photolysis-resonance fluorescence (FP-RF) technique was employed to study the kinetics and mechanism of the reaction of OH radicals with p-cymene at temperatures between 297 and 413 K in helium buffer gas. FP-RF experiments involved time-resolved detection of OH radicals by RF following vacuum-UV flash photolysis of H2O-p-cymene-He and H2O-He mixtures. Biexponential functions were fitted to decays of OH radicals according to reversible addition of OH radicals to p-cymene to form a single adduct. A rate constant of (15.7 ± 1.1) × 10(-12) is obtained (in units of cm(3) s(-1)) at room temperature (298 K) for the sum of the addition and abstraction channels (k1a + k1b) according to this simplified model. The Arrhenius plot reveals the step function typical of other aromatics and can be described using the expressions: 2 × 10(-13) exp(+1300 K/T) at temperatures between 297 K and 324 K and 10(-11) exp(-250 K/T) at temperatures between 345 K and 413 K. After consideration of the abstraction channel an equilibrium constant of k1a/k-1a = 6 × 10(-26) exp(+9700 K/T) cm(3) is obtained at temperatures between 297 and 325 K and 2 × 10(-36) exp(+17,000 K/T) cm(3) at temperatures between 325 and 380 K.

Trends of PCDD/F and PCB concentrations and depositions in ambient air in Northwestern Germany.

Time series of polychlorinated dioxins and furans (PCDD/F) and polychlorinated biphenyls (PCB) in ambient air of a large conurbation in North-Western Germany are presented and analyzed. The trend of PCDD/F concentrations, starting from as early as 1988, shows a pronounced decrease by at least one order of magnitude, demonstrating that the emission reductions were effective. The PCDD/F depositions also have decreased by a factor of 5 since 1992. However, both trends have leveled out since 2005. Time series of PCB concentrations and depositions starting in 1994 show only slight decreases for the concentrations and almost no decrease for the depositions. From the decay rates following first order kinetics, half-lives in the order of 5-15 years for the PCDD/F and 15-31 years for the sum of the six indicator PCB could be calculated, which are much longer than the half-lives estimated from their reactivity towards the OH radical. Apparently, small fresh emissions (PCDD/F), considerable secondary emissions and evaporation from contaminated soils slow down their decay in the atmosphere of big conurbations. Analyzing the decay rates of individual PCB congeners shows that the lower chlorinated and more volatile ones are removed faster than the higher chlorinated congeners, probably via gas phase reactions with the OH radical. It can be concluded from the present study that the input of PCDD/F and PCB into the food chain via the air path will continue for another one or two decades in big conurbations.

Halogen-induced organic aerosol (XOA): a study on ultra-fine particle formation and time-resolved chemical characterization.

The concurrent presence of high values of organic SOA precursors and reactive halogen species (RHS) at very low ozone concentrations allows the formation of halogen-induced organic aerosol, so-called XOA, in maritime areas where high concentrations of RHS are present, especially at sunrise. The present study combines aerosol smog-chamber and aerosol flow-reactor experiments for the characterization of XOA. XOA formation yields from alpha-pinene at low and high concentrations of chlorine as reactive halogen species (RHS) were determined using a 700 L aerosol smog-chamber with a solar simulator. The chemical transformation of the organic precursor during the aerosol formation process and chemical aging was studied using an aerosol flow-reactor coupled to an FTIR spectrometer. The FTIR dataset was analysed using 2D correlation spectroscopy. Chlorine induced homogeneous XOA formation takes place at even 2.5 ppb of molecular chlorine, which was photolysed by the solar simulator. The chemical pathway of XOA formation is characterized by the addition of chlorine and abstraction of hydrogen atoms, causing simultaneous carbon-chlorine bond formation. During further steps of the formation process, carboxylic acids are formed, which cause a SOA-like appearance of XOA. During the ozone-free formation of secondary organic aerosol with RHS a special kind of particulate matter (XOA) is formed, which is afterwards transformed to SOA by atmospheric aging or degradation pathways.

Kinetics and mechanism of the reaction of OH with the trimethylbenzenes--experimental evidence for the formation of adduct isomers.

The reversible gas-phase addition of OH radicals to the trimethylbenzenes was investigated in pulsed experiments utilizing VUV flash-photolysis resonance-fluorescence of H(2)O in the temperature range of 275-340 K. Triexponential OH decays were observed in the presence of the trimethylbenzenes, indicating the participation of more than one adduct species. Analytical solutions for the system of differential equations with two adduct isomers were derived, and the OH decay curves were evaluated based on this reaction model. This led to significant improvements of fit qualities and notable changes in OH rate constants compared to a previous model with a single adduct species. The detailed analysis was confined to 1,3,5-trimethylbenzene where reversible formation of two OH-aromatic ortho- and ipso-adduct isomers is feasible in accordance with the extended reaction model. Only after inclusion of additional isomerization reactions, consistent thermochemical data were obtained from the fitted rate constants. Reaction enthalpies of -83 ± 7 kJ mol(-1) and -35 ± 22 kJ mol(-1) were derived for the formation of one adduct isomer and the isomerization into the other, respectively. Based on literature data, the more and less stable adducts were assigned to ipso- and ortho-adduct isomers, respectively. The potential isomerization precluded the determination of primary yields of adduct isomers but formation of the ipso-adduct in any case is a minor process. For the rate constants of the OH + 1,3,5-trimethylbenzene reaction an Arrhenius expression k(OH) = 1.32 × 10(-11) cm(3) s(-1) exp(450 ± 50 K/T) was obtained. Based on the same approach, the rate constants of the OH reactions with 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene were derived as k(OH) = 3.61 × 10(-12) cm(3) s(-1) exp(620 ± 80 K/T) and k(OH) = 2.73 × 10(-12) cm(3) s(-1) exp(730 ± 70 K/T), respectively.

Temperature-dependent kinetics study of the reactions of OH with C2H5I, n-C3H7I, and iso-C3H7I.

Flash photolysis (FP) coupled with resonance fluorescence (RF) was used to measure the absolute rate coefficients for the reactions of OH(X(2)Π) radicals with C(2)H(5)I (k(1)), n-C(3)H(7)I (k(2)), and iso-C(3)H(7)I (k(3)) at temperatures between 297 and 372 K in 188 Torr of He; this represents the first temperature-dependent kinetics studies for the title reactions. The experiments involved time-resolved RF detection of the OH (A(2)Σ(+) → X(2)Π transition at λ = 308 nm) radicals following FP of H(2)O/C(2)H(5)I/He, H(2)O/n-C(3)H(7)I/He, and H(2)O/iso-C(3)H(7)I/He mixtures. The OH(X(2)Π) radicals were produced by FP of H(2)O in vacuum-UV at wavelengths λ > 120 nm. Decays of OH radicals in the presence of C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I were observed to be exponential, and the decay rates were found to be linearly dependent on the C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I concentrations. The results are described by the following Arrhenius expressions (units of cm(3) molecule(-1) s(-1)): k(1)(297-372 K) = (5.55 ± 3.20) × 10(-12) exp[-(830 ± 90) K/T], k(2)(300-370 K) = (1.65 ± 0.90) × 10(-11) exp[-(780 ± 90) K/T] and k(3)(299-369 K) = (7.58 ± 3.70) × 10(-12) exp[-(530 ± 80) K/T]. Reported errors in E/R and in the pre-exponential factors are 2σ random errors, returned by the weighted (by 1/σ(2)) least-squares fits to the kinetic data. The implications of the reported kinetic results for understanding both atmospheric and nuclear safety interests of C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I are discussed.

Atmospheric photosensitized heterogeneous and multiphase reactions: from outdoors to indoors.

This proposal involves direct photolysis processes occurring in the troposphere incorporating photochemical excitation and intermolecular energy transfer. The study of such processes could provide a better understanding of ·OH radical formation pathways in the atmosphere and in consequence, of a more accurate prediction of the oxidative capacity of the atmosphere. Compounds that readily absorb in the tropospheric actinic window (ionic organic complexes, PAHs, aromatic carbonyl compounds) acting as potential photosensitizers of atmospheric relevant processes are explored. The impact of hotosensitation on relevant systems which could act as powerful atmospheric reactors,that is, interface ocean-atmosphere, urban and forest surfaces and indoor air environments is also discussed.

Kinetic study of the reaction of OH with CH2I2.

Flash photolysis (FP) coupled to resonance fluorescence (RF) was used to measure the absolute rate coefficients (k(1)) for the reaction of OH(X(2)Π) radicals with diiodomethane (CH(2)I(2)) over the temperature range 295-374 K. The experiments involved time-resolved RF detection of the OH (A(2)Σ(+)→X(2)Π transition at λ = 308 nm) following FP of the H(2)O/CH(2)I(2)/He mixtures. The OH(X(2)Π) radicals were produced by FP of H(2)O in the vacuum-UV at wavelengths λ > 120 nm. Decays of OH radicals in the presence of CH(2)I(2) are observed to be exponential, and the decay rates are found to be linearly dependent on the CH(2)I(2) concentration. The results are described by the Arrhenius expression k(1)(T) = (4.2 ± 0.5) × 10(-11) exp[-(670 ± 20)K/T] cm(3) molecule(-1) s(-1). The implications of the reported kinetic results for understanding the atmospheric chemistry of CH(2)I(2) are discussed.

Photolysis and heterogeneous reaction of coniferyl aldehyde adsorbed on silica particles with ozone.

The pseudo-first-order loss of coniferyl aldehyde, adsorbed on silicon dioxide particles, upon heterogeneous ozonolysis was monitored at various ozone mixing ratios in the absence and presence of simulated sunlight. For the first time we investigated the effect of light on the heterogeneous ozonolysis of coniferyl aldehyde adsorbed on silica particles. We found that UV-VIS light (λ>300 nm) does not impact the degradation of coniferyl aldehyde by ozone but induces an additional, slow photolysis of the aldehyde with a photolytic rate constant of ~10(-5) s(-1). In both cases, that is, in presence and/or absence of light, the heterogeneous ozonation kinetics are well described by an immediate gas-surface reaction formalism with light-independent rate constants of k(2nd)=(7.2±0.9)×10(-19) cm(3) molec(-1) s(-1) and (7.6±1.7)×10(-19) cm(3) molec(-1) s(-1) in the absence and presence of light, respectively. Five oxidation products: glycolic acid, oxalic acid, vanillin, vanillic acid and 3,4-dihydroxybenzoic acid were identified and confirmed by their corresponding standards. Vanillin and vanillic acid absorb light in the region λ>300 nm and thus can further participate in the direct and indirect photolysis processes of atmospheric relevance. A reaction mechanism is proposed in order to elucidate the ozonolysis reaction and to explain the reaction products.

Circular multireflection cell for optical spectroscopy.

We constructed a circular multireflection (CMR) cell, allowing multireflection around the center of the cell. This is caused by a skewed adjustment of the entering beam (equivalent to a simple parallel shift/offset), avoiding the center of the cell, thus leading to multiple reflections. The experimental setup with a cell with an inner diameter of 6 cm showed up to 17.5 beam passes on polished aluminum and attained path lengths up to 105 cm, demonstrated by Fourier transform infrared measurements of CO(2) gas between 2283 and 2400 cm(-1). The circular concept, i.e., the centering of the reflections, is useful for absorption spectroscopy on trace gases and aerosols. The optical alignment of the cell can completely be performed from outside the experimental setup, e.g., an aerosol flow reactor or a vacuum system. The variation of the path length is easily possible by adjusting the position of the cell with respect to the entering light beam.

Quantitative 1H NMR-analysis of technical octabrominated diphenylether DE-79 and UV spectra of its components and photolytic transformation products.

A quantitative analysis of the technical octabromo diphenylether mixture DE-79 is performed by (1)H NMR, avoiding any separation technique. The mass fractions are 36% BDE183, 19% BDE197, 13.1% BDE207, 9.1% BDE196, 7.3% BDE153, 6.2% BDE203, 2.2% BDE180, 1.6% BDE171, 1.2% BDE154 and 0.7% BDE206, considering an additional mass fraction of 1.3% BDE209 determined by HPLC analysis with a diode array detector (DAD). HPLC chromatograms of the BDEs in commercial decaBDE, octaBDE DE-79 and pentaBDE DE-71 and UV spectra of the components of DE-79 are presented. The photolysis of the octaBDE mixture DE-79, dissolved in tetrahydrofuran, by simulated sunlight is monitored by HPLC-DAD and observed to proceed mainly via debromination. Polybrominated dibenzofurans are identified from their UV spectra as significant intermediates.