J. N. Crowley

Atmospheric oxidation capacity sustained by a tropical forest

Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere, which are removed by oxidation reactions and deposition of reaction products. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides (NOx = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NOx air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40–80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.

Organic peroxy radicals: Kinetics, spectroscopy and tropospheric chemistry

The present state of knowledge of organic, or carbon-based, peroxy radicals (RO2) is reviewed. Data on the chemical and physical properties of peroxy radicals in the gas-phase is considered, as well as the role of peroxy radicals in tropospheric chemistry and measurements of their concentrations in the atmosphere. Where appropriate, peroxy radicals are grouped together by type (alkyl, acyl, oxygen-substituted, halogen-substituted and aromatic radicals) to facilitate comparison. Data on the hydroperoxy radical (HO2) is included where it is directly relevant to measurements on organic peroxy radicals, eg. absorption cross-sections used in measurements of RO2 + HO2 rate constants. The literature data is critically reviewed and recommendations for absorption cross-sections, rate constants and branching ratios are made where considered appropriate. The laboratory experimental techniques which have been used for the generation and detection of peroxy radicals and the products of their reactions are discussed. The structure, spectroscopy and thermochemistry of the radicals are examined. Although the majority of spectroscopic data concerns the u.v. spectra much used for kinetic studies, near-infrared, infrared and electron spin resonance spectra are also considered. In many cases, peroxy radical u.v. spectra are well-fitted by a Gaussian distribution function, enabling the cross-sections to be easily calculated at any wavelength. For the purpose of this review, the chemical reactions of peroxy radicals are divided into reactions with organic peroxy radicals with HO2, with NO and NO2, and finally with other species. Peroxy radical abstraction and addition reactions with closed-shell species are sufficiently slow to be of negligible importance at temperatures pertinent to the atmosphere and are consequently not covered. Data on both the kinetics and mechanisms of peroxy radical reactions are considered. The role of peroxy radicals as intermediates in the atmospheric degradation of volatile organic compounds and in the production of ozone in the troposphere under both low and high [NOx] conditions is discussed. The involvement of peroxy radicals in night-time oxidation chemistry and the oxidation of halocarbons is also indicated. The techniques used for the difficult measurement of peroxy radical concentrations in the atmosphere are described, together with the results to date. Finally, some tentative suggestions as to further avenues of research are made, based on the data reviewed here and with particular reference to the solution of outstanding problems in atmospheric chemistry. Although a great deal of progress has been made in recent years, it is clear that additional work is needed in most areas covered by this review. New, sensitive and selective laboratory techniques are required for studies of peroxy radical kinetics and high level ab initio calculations would help design laser-based detection techniques. Further product studies of photooxidation systems are needed, particularly as a function of temperature. Recent work has shown that the rate constants for RO2 + HO2 reactions used in modelling studies may be too low; if so, these reactions will be correspondingly more important than previously believed in tropospheric oxidation. Recent kinetic studies of the potentially important reactions of methylperoxy radicals with ClO and NO3 need to be confirmed and mechanistic work is necessary. Although substantial progress has been made towards the monitoring of peroxy radical concentrations in the atmosphere, more work is needed, both on measurements and the development of new techniques.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume VI – heterogeneous reactions with liquid substrates

Atmospheric chemistry is the study of the complex network of thermal and photochemical processes occurring in the gas and condensed (cloud droplets, aerosol particles, ice crystals) phase as well as in multiphase processes. Chemical kinetic modeling is required to interpret observations in the field. This necessitates the availability of a robust, reliable, and regularly updated database of elementary reactions for use in chemical-radiative-transport models. Combustion chemistry and plasma processing for semiconductor applications require the same type of database.

Activation of Br2 and BrCl via uptake of HOBr onto aqueous salt solutions

The reactive uptake of HOBr onto aqueous solutions containing Cl− and Br− has been studied using a wetted-wall flow tube reactor. The uptake was found to be limited by gas phase transport, enabling the measurement of the diffusion coefficient of HOBr in He (319±48 torr cm2 s−1) and N2 (84±7 torr cm2 s−1) at 274 K (errors are 2 σ). The same experiments allowed a lower limit to the accommodation coefficient of α > 1×10−2 to be calculated. Both Br2 and BrCl products were observed in the gas phase, the relative yield of which was dependent on the ratio of Cl− to Br− in the aqueous phase. At [Cl−] = 1M, and [Br−] = 10−3 M, >90% of HOBr taken up by the film was released into the gas phase as Br2. At low Br− concentrations, BrCl was the dominant product. The role of pH in the efficiency of bromine release from the aqueous phase was examined by carrying out HOBr-uptake experiments with 1 M Cl−, 10−3 MBr−, and pH between 4 and 10. At a pH of less than 6.5 at least 90% of the HOBr taken up onto the aqueous Cl− / Br− solution was released into the gas phase as Br2. Bromine was not released following HOBr uptake onto non-acidified solution. These experiments confirm the efficient activation of bromine predicted by recent models of halogen chemistry in the marine boundary layer.

Carbon 13 and D kinetic isotope effects in the reactions of CH4 with O(1 D) and OH: New laboratory measurements and their implications for the isotopic composition of stratospheric methane

Measurements of the 13C and D kinetic isotope effects (KIE) in methane, 13CKIE = k(12CH4)/k(13CH4) and DKIE = k(12CH4)/k(12CH3D), in the reactions of these atmospherically important methane isotopomers with O(1D) and OH have been undertaken using mass spectrometry and tunable diode laser absorption spectroscopy to determine isotopic composition. For the carbon kinetic isotope effect in the reaction with the OH radical, 13CKIEOH = 1.0039 (±0.0004, 2σ) was determined at 296 K, which is significantly smaller than the presently accepted value of 1.0054 (±0.0009, 2 σ). For DKIEOH we found 1.294 (± 0.018, 2σ) at 296 K, consistent with earlier observations. The carbon kinetic isotope effect in the reaction with O(1D) 13CKIEO(1D), was determined to be 1.013, whereas the deuterium kinetic isotope effect is given by DKIEO(1D) = 1.06. Both values are approximately independent of temperature between 223 and 295 K. The room temperature fractionation effect 1000(KIE-1) in the reaction of O(1D) with 12CH4 versus CH4 is thus ≈ 13‰, which is an order of magnitude greater than the previous value of 1‰. In combination with recent results from our laboratory on 13CKIE and DKIE for the reaction of CH4 with Cl, these new measurements were used to simulate the effective kinetic isotope effect for the stratosphere with a two-dimensional, time dependent chemical transport model. The model results show reasonable agreement with field observations of the 13CH4/12CH4 ratio in the lowermost stratosphere, and also reproduce the observed CH3D/CH4 ratio.

Carbon kinetic isotope effect in the reaction of CH4 with Cl atoms

The carbon kinetic isotope effect in the reaction between Cl and CH4 (KIECl) has been measured using tunable diode laser absorption spectroscopy to determine 13CH4/12CH4 ratios. Cl atoms were generated by the irradition of Cl2 in static mixtures of Cl2/CH4/N2 or Cl2/CH4/N2/O2. Both methods resulted in a (KIECl) of 1.066±0.002 at 297 K. The KIECl displayed a slight temperature dependence, increasing to 1.075±0.005 at 223 K. This result suggests a significant influence of the title reaction on the stratospheric CH4 isotopic composition and may help to resolve discrepancies between measurements of stratospheric 13CH4/12CH4 profiles and laboratory measurements of KIEOH.

Isotopic enrichment of nitrous oxide (15N14NO, 14N15NO, 14N14N18O) in the stratosphere and in the laboratory

Nitrous oxide (N2O) extracted from stratospheric whole air samples has been analyzed for its 15N and 18O isotopic composition, and strong enrichments in the heavy isotopes are observed concomitant with decreasing N2O mixing ratio. Notably, the N enrichment is strongly different at the two nonequivalent positions in the molecule. Laboratory broadband photolysis experiments at wavelengths representative for the stratosphere confirm that photolysis is the prime cause for the observed fractionation in the stratosphere. However, the in situ stratospheric fractionation constants are significantly reduced compared to the laboratory data, reflecting the importance of dynamic processes. In addition, small but significant variations in the ratio of the two 15N fractionation constants indicate the influence of additional chemical processes like the oxidation of N2O by O(1 D).

The heterogeneous reactivity of gaseous nitric acid on authentic mineral dust samples, and on individual mineral and clay mineral components

The heterogeneous reaction between HNO3 and various authentic dust, mineral and clay mineral surfaces has been investigated by means of the Knudsen reactor technique at ≈296 K. Authentic dust surfaces from both the Saharan and Chinese dust regions were investigated, as were the minerals and clay minerals: kaolinite, ripidolite, illite, illite/smectite, Ca-montmorillonite, Na-montmorillonite, palygorskite, dolomite and orthoclase. A modified Knudsen reactor design and HNO3 concentrations as low as 109 cm−3 helped eliminate experimental artefacts. In all cases a large and irreversible uptake was observed, with uptake coefficients ranging from 8.1 × 10−2 (orthoclase) to 19.6 × 10−2 (palygorskite). These results strongly suggest that mineral dust can modify tropospheric photochemical cycles involving NOx and NOy.

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

Interaction of methanol, acetone and formaldehyde with ice surfaces between 198 and 223 K

The rate and extent of physical adsorption of methanol, acetone and formaldehyde on ice were measured as a function of concentration and temperature. The gas–ice interaction was analysed by applying adsorption isotherms to determine temperature dependent Langmuir constants, K(T) and saturation surface coverage, Nmax. At low coverage a partitioning constant K#(T) was derived. The dependence of K# on temperature is given by K#(T) = 6.24 × 10−12 exp(6178/T) cm for methanol and K#(T) = 1.25 × 10−10 exp(5575/T) cm for acetone. For formaldehyde a temperature independent expression, K# = 0.7 cm was derived. From these data adsorption enthalpies ΔHads of (−46 ± 7) and (−51 ± 10) kJ mol−1 were obtained for acetone and methanol, respectively. The results were used to calculate the equilibrium partitioning of these trace gases to ice surfaces under conditions relevant to the atmosphere.