Reducing Soil-Emitted Nitrous Acid as a Feasible Strategy for Tackling Ozone Pollution.

Severe ozone (O3) pollution has been a major air quality issue and affects environmental sustainability in China. Conventional mitigation strategies focusing on reducing volatile organic compounds and nitrogen oxides (NOx) remain complex and challenging. Here, through field flux measurements and laboratory simulations, we observe substantial nitrous acid (HONO) emissions (FHONO) enhanced by nitrogen fertilizer application at an agricultural site. The observed FHONO significantly improves model performance in predicting atmospheric HONO and leads to regional O3 increases by 37%. We also demonstrate the significant potential of nitrification inhibitors in reducing emissions of reactive nitrogen, including HONO and NOx, by as much as 90%, as well as greenhouse gases like nitrous oxide by up to 60%. Our findings introduce a feasible concept for mitigating O3 pollution: reducing soil HONO emissions. Hence, this study has important implications for policy decisions related to the control of O3 pollution and climate change.

Ozonolysis can produce long-lived greenhouse gases from commercial refrigerants.

Hydrofluoroolefins are being adopted as sustainable alternatives to long-lived fluorine- and chlorine-containing gases and are finding current or potential mass-market applications as refrigerants, among a myriad of other uses. Their olefinic bond affords relatively rapid reaction with hydroxyl radicals present in the atmosphere, leading to short lifetimes and proportionally small global warming potentials. However, this type of functionality also allows reaction with ozone, and whilst these reactions are slow, we show that the products of these reactions can be extremely long-lived. Our chamber measurements show that several industrially important hydrofluoroolefins produce CHF3 (fluoroform, HFC-23), a potent, long-lived greenhouse gas. When this process is accounted for in atmospheric chemical and transport modeling simulations, we find that the total radiative effect of certain compounds can be several times that of the direct radiative effect currently recommended by the World Meteorological Organization. Our supporting quantum chemical calculations indicate that a large range of exothermicity is exhibited in the initial stages of ozonolysis, which has a powerful influence on the CHF3 yield. Furthermore, we identify certain molecular configurations that preclude the formation of long-lived greenhouse gases. This demonstrates the importance of product quantification and ozonolysis kinetics in determining the overall environmental impact of hydrofluoroolefin emissions.

The fate of methyl salicylate in the environment and its role as signal in multitrophic interactions.

Phytohormones emitted into the atmosphere perform many functions relating to the defence, pollination and competitiveness of plants. To be effective, their atmospheric lifetimes must be sufficient that these signals can be delivered to their numerous recipients. We investigate the atmospheric loss processes for methyl salicylate (MeSA), a widely emitted plant volatile. Simulation chambers were used to determine gas-phase reaction rates with OH, NO3, Cl and O3; photolysis rates; and deposition rates of gas-phase MeSA onto organic aerosols. Room temperature rate coefficients are determined (in units of cm3 molecule-1 s-1) to be (3.20 ± 0.46) × 10-12, (4.19 ± 0.92) × 10-15, (1.65 ± 0.44) × 10-12 and (3.33 ± 2.01) × 10-19 for the reactions with OH, NO3, Cl and O3 respectively. Photolysis is negligible in the actinic range, despite having a large reported near-UV chromophore. Conversely, aerosol uptake can be competitive with oxidation under humid conditions, suggesting that this compound has a high affinity for hydrated surfaces. A total lifetime of gas-phase MeSA of 1-4 days was estimated based on all these loss processes. The competing sinks of MeSA demonstrate the need to assess lifetimes of semiochemicals holistically, and we gain understanding of how atmospheric sinks influence natural communication channels within complex multitrophic interactions. This approach can be extended to other compounds that play vital roles in ecosystems, such as insect pheromones, which may be similarly affected during atmospheric transport.

FC(O)C(O)F, FC(O)CF2C(O)F, and FC(O)CF2CF2C(O)F: Ultraviolet and Infrared Absorption Spectra and 248 nm Photolysis Products.

Perfluorodicarbonyl (PFDC) compounds may be emitted directly into the atmosphere or formed in the atmospheric degradation of trace fluorinated gases, such as unsaturated perfluoro cyclic compounds. A potential atmospheric removal process for PFDCs is UV photolysis, which is presently not well-characterized. In this work, UV and infrared absorption spectra of FC(O)C(O)F, FC(O)CF2C(O)F, and FC(O)CF2CF2C(O)F (three of the simplest PFDCs) and their 248 nm photolysis products are reported. UV spectra were measured at 296 K between 190 and 320 nm using single wavelength and broadband diode array spectroscopic measurement techniques. Infrared absorption spectra were measured at 296 K using Fourier transform infrared spectroscopy between 500 and 4000 cm-1. The PFDCs are shown to be potent greenhouse gases with radiative efficiencies (well-mixed) of 0.142, 0.218, and 0.293 W m-2 ppb-1 for FC(O)C(O)F, FC(O)CF2C(O)F, and FC(O)CF2CF2C(O)F, respectively. Photolysis product yields (248 nm) were measured using pulsed laser photolysis combined with infrared absorption detection of radical products scavenged to stable bromides by reaction with Br2. BrC(O)F was identified as a major stable end product in all systems with a yield greater than ∼90%. The infrared spectrum of BrC(O)F is reported as part of this study. FC(O)CBrF2 and FC(O)CF2CBrF2 were also identified as products in the photolysis of FC(O)CF2C(O)F and FC(O)CF2CF2C(O)F, respectively, by comparison with theoretically calculated infrared absorption spectra. A carbonyl difluoride (CF2O) primary photolysis yield of ∼10% was measured in the photolysis of FC(O)C(O)F.

Experimental and computational studies of Criegee intermediate reactions with NH3 and CH3NH2.

Ammonia and amines are emitted into the troposphere by various natural and anthropogenic sources, where they have a significant role in aerosol formation. Here, we explore the significance of their removal by reaction with Criegee intermediates, which are produced in the troposphere by ozonolysis of alkenes. Rate coefficients for the reactions of two representative Criegee intermediates, formaldehyde oxide (CH2OO) and acetone oxide ((CH3)2COO) with NH3 and CH3NH2 were measured using cavity ring-down spectroscopy. Temperature-dependent rate coefficients, k(CH2OO + NH3) = (3.1 ± 0.5) × 10-20T2 exp(1011 ± 48/T) cm3 s-1 and k(CH2OO + CH3NH2) = (5 ± 2) × 10-19T2 exp(1384 ± 96/T) cm3 s-1 were obtained in the 240 to 320 K range. Both the reactions of CH2OO were found to be independent of pressure in the 10 to 100 Torr (N2) range, and average rate coefficients k(CH2OO + NH3) = (8.4 ± 1.2) × 10-14 cm3 s-1 and k(CH2OO + CH3NH2) = (5.6 ± 0.4) × 10-12 cm3 s-1 were deduced at 293 K. An upper limit of ≤2.7 × 10-15 cm3 s-1 was estimated for the rate coefficient of the (CH3)2COO + NH3 reaction. Complementary measurements were performed with mass spectrometry using synchrotron radiation photoionization giving k(CH2OO + CH3NH2) = (4.3 ± 0.5) × 10-12 cm3 s-1 at 298 K and 4 Torr (He). Photoionization mass spectra indicated production of NH2CH2OOH and CH3N(H)CH2OOH functionalized organic hydroperoxide adducts from CH2OO + NH3 and CH2OO + CH3NH2 reactions, respectively. Ab initio calculations performed at the CCSD(T)(F12*)/cc-pVQZ-F12//CCSD(T)(F12*)/cc-pVDZ-F12 level of theory predicted pre-reactive complex formation, consistent with previous studies. Master equation simulations of the experimental data using the ab initio computed structures identified submerged barrier heights of -2.1 ± 0.1 kJ mol-1 and -22.4 ± 0.2 kJ mol-1 for the CH2OO + NH3 and CH2OO + CH3NH2 reactions, respectively. The reactions of NH3 and CH3NH2 with CH2OO are not expected to compete with its removal by reaction with (H2O)2 in the troposphere. Similarly, losses of NH3 and CH3NH2 by reaction with Criegee intermediates will be insignificant compared with reactions with OH radicals.

Ethylenediurea (EDU) mitigates the negative effects of ozone in rice: Insights into its mode of action.

Monitoring of ozone damage to crops plays an increasingly important role for the food security of many developing countries. Ethylenediurea (EDU) could be a tool to assess ozone damage to vegetation on field scale, but its physiological mode of action remains unclear. This study investigated mechanisms underlying the ozone-protection effect of EDU in controlled chamber experiments. Ozone sensitive and tolerant rice genotypes were exposed to ozone (108 ppb, 7 hr day-1 ) and control conditions. EDU alleviated ozone effects on plant morphology, foliar symptoms, lipid peroxidation, and photosynthetic parameters in sensitive genotypes. Transcriptome profiling by RNA sequencing revealed that thousands of genes responded to ozone in a sensitive variety, but almost none responded to EDU. Significant interactions between ozone and EDU application occurred mostly in ozone responsive genes, in which up-regulation was mitigated by EDU application. Further experiments documented ozone degrading properties of EDU, as well as EDU deposits on leaf surfaces possibly related to surface protection. EDU application did not mitigate the reaction of plants to other abiotic stresses, including iron toxicity, zinc deficiency, and salinity. This study provided evidence that EDU is a surface protectant that specifically mitigates ozone stress without interfering directly with the plants' stress response systems.

Temperature-Dependence of the Rates of Reaction of Trifluoroacetic Acid with Criegee Intermediates.

The rate coefficients for gas-phase reaction of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide, decrease with increasing temperature in the range 240-340 K. The rate coefficients k(CH2 OO + CF3 COOH)=(3.4±0.3)×10-10  cm3 s-1 and k((CH3 )2 COO + CF3 COOH)=(6.1±0.2)×10-10  cm3 s-1 at 294 K exceed estimates for collision-limited values, suggesting rate enhancement by capture mechanisms because of the large permanent dipole moments of the two reactants. The observed temperature dependence is attributed to competitive stabilization of a pre-reactive complex. Fits to a model incorporating this complex formation give k [cm3 s-1 ]=(3.8±2.6)×10-18  T2 exp((1620±180)/T) + 2.5×10-10 and k [cm3 s-1 ]=(4.9±4.1)×10-18  T2 exp((1620±230)/T) + 5.2×10-10 for the CH2 OO + CF3 COOH and (CH3 )2 COO + CF3 COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates.

Experimentally Determined Site-Specific Reactivity of the Gas-Phase OH and Cl + i-Butanol Reactions Between 251 and 340 K.

Product branching ratios for the gas-phase reactions of i-butanol, (CH3)2CHCH2OH, with OH radicals (251, 294, and 340 K) and Cl atoms (294 K) were quantified in an environmental chamber study and used to interpret i-butanol site-specific reactivity. i-Butyraldehyde, acetone, acetaldehyde, and formaldehyde were observed as major stable end products in both reaction systems with carbon mass balance indistinguishable from unity. Product branching ratios for OH oxidation were found to be temperature-dependent with the α, ß, and γ channels changing from 34 ± 6 to 47 ± 1%, from 58 ± 6 to 37 ± 9%, and from 8 ± 1 to 16 ± 4%, respectively, between 251 and 340 K. Recommended temperature-dependent site-specific modified Arrhenius expressions for the OH reaction rate coefficient are (cm3 molecule-1 s-1): kα(T) = 8.64 × 10-18 × T1.91exp(666/T); kß(T) = 5.15 × 10-19 × T2.04exp(1304/T); kγ(T) = 3.20 × 10-17 × T1.78exp(107/T); kOH(T) = 2.10 × 10-18 × T2exp(-23/T), where kTotal(T) = kα(T) + kß(T) + kγ(T) + kOH(T). The expressions were constrained using the product branching ratios measured in this study and previous total phenomenological rate coefficient measurements. The site-specific expressions compare reasonably well with recent theoretical work. It is shown that use of i-butanol would result in acetone as the dominant degradation product under most atmospheric conditions.

1,2-Dichlorohexafluoro-cyclobutane (1,2-c-C4F6Cl2, R-316c) a potent ozone depleting substance and greenhouse gas: atmospheric loss processes, lifetimes, and ozone depletion and global warming potentials for the (E) and (Z) stereoisomers.

The atmospheric processing of (E)- and (Z)-1,2-dichlorohexafluoro-cyclobutane (1,2-c-C4F6Cl2, R-316c) was examined in this work as the ozone depleting (ODP) and global warming (GWP) potentials of this proposed replacement compound are presently unknown. The predominant atmospheric loss processes and infrared absorption spectra of the R-316c isomers were measured to provide a basis to evaluate their atmospheric lifetimes and, thus, ODPs and GWPs. UV absorption spectra were measured between 184.95 to 230 nm at temperatures between 214 and 296 K and a parametrization for use in atmospheric modeling is presented. The Cl atom quantum yield in the 193 nm photolysis of R-316c was measured to be 1.90 ± 0.27. Hexafluorocyclobutene (c-C4F6) was determined to be a photolysis co-product with molar yields of 0.7 and 1.0 (±10%) for (E)- and (Z)-R-316c, respectively. The 296 K total rate coefficient for the O((1)D) + R-316c reaction, i.e., O((1)D) loss, was measured to be (1.56 ± 0.11) × 10(-10) cm(3) molecule(-1) s(-1) and the reactive rate coefficient, i.e., R-316c loss, was measured to be (1.36 ± 0.20) × 10(-10) cm(3) molecule(-1) s(-1) corresponding to a ~88% reactive yield. Rate coefficient upper-limits for the OH and O3 reaction with R-316c were determined to be <2.3 × 10(-17) and <2.0 × 10(-22) cm(3) molecule(-1) s(-1), respectively, at 296 K. The quoted uncertainty limits are 2σ and include estimated systematic errors. Local and global annually averaged lifetimes for the (E)- and (Z)-R-316c isomers were calculated using a 2-D atmospheric model to be 74.6 ± 3 and 114.1 ± 10 years, respectively, where the estimated uncertainties are due solely to the uncertainty in the UV absorption spectra. Stratospheric photolysis is the predominant atmospheric loss process for both isomers with the O((1)D) reaction making a minor, ~2% for the (E) isomer and 7% for the (Z) isomer, contribution to the total atmospheric loss. Ozone depletion potentials for (E)- and (Z)-R-316c were calculated using the 2-D model to be 0.46 and 0.54, respectively. Infrared absorption spectra for (E)- and (Z)-R-316c were measured at 296 K and used to estimate their radiative efficiencies (REs) and GWPs; 100-year time-horizon GWPs of 4160 and 5400 were obtained for (E)- and (Z)-R-316c, respectively. Both isomers of R-316c are shown in this work to be long-lived ozone depleting substances and potent greenhouse gases.

Gas-phase rate coefficients for the OH + n-, i-, s-, and t-butanol reactions measured between 220 and 380 K: non-Arrhenius behavior and site-specific reactivity.

Butanol (C4H9OH) is a potential biofuel alternative in fossil fuel gasoline and diesel formulations. The usage of butanol would necessarily lead to direct emissions into the atmosphere; thus, an understanding of its atmospheric processing and environmental impact is desired. Reaction with the OH radical is expected to be the predominant atmospheric removal process for the four aliphatic isomers of butanol. In this work, rate coefficients, k, for the gas-phase reaction of the n-, i-, s-, and t-butanol isomers with the OH radical were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH radicals and laser induced fluorescence to monitor its temporal profile. Rate coefficients were measured over the temperature range 221-381 K at total pressures between 50 and 200 Torr (He). The reactions exhibited non-Arrhenius behavior over this temperature range and no dependence on total pressure with k(296 K) values of (9.68 ± 0.75), (9.72 ± 0.72), (8.88 ± 0.69), and (1.04 ± 0.08) (in units of 10(-12) cm(3) molecule(-1) s(-1)) for n-, i-, s-, and t-butanol, respectively. The quoted uncertainties are at the 2σ level and include estimated systematic errors. The observed non-Arrhenius behavior is interpreted here to result from a competition between the available H-atom abstraction reactive sites, which have different activation energies and pre-exponential factors. The present results are compared with results from previous kinetic studies, structure-activity relationships (SARs), and theoretical calculations and the discrepancies are discussed. Results from this work were combined with available high temperature (1200-1800 K) rate coefficient data and room temperature reaction end-product yields, where available, to derive a self-consistent site-specific set of reaction rate coefficients of the form AT(n) exp(-E/RT) for use in atmospheric and combustion chemistry modeling.

Determination of gas-phase ozonolysis rate coefficients of C(8-14) terminal alkenes at elevated temperatures using the relative rate method.

The rates of ozonolysis of a suite of terminal alkenes ranging from C(8-14) are determined in the gas phase at an elevated temperature of 395.9 ± 1.2 K and a pressure of ∼650 Torr using the EXTreme RAnge chamber (EXTRA). Rates are found to be invariant with carbon number, whilst literature measurements conducted under ambient conditions exhibited an increase in rate coefficient after 10 carbon atoms. These earlier findings appear to contradict the intuitive notion that the inductive effect is a short-range process operating over a maximum distance of a few carbon atoms. These new measurements support the hypothesis that operating under ambient conditions, kinetic measurements of condensable species can be influenced adversely by heterogeneous processes and should therefore be treated with caution.

Structure-activity relationship (SAR) for the prediction of gas-phase ozonolysis rate coefficients: an extension towards heteroatomic unsaturated species.

Heteroatomic unsaturated volatile organic compounds (HUVOCs) are common trace components of the atmosphere, yet their diverse chemical behaviour presents difficulties for predicting their oxidation kinetics using structure-activity relationships (SARs). An existing SAR is adapted to help meet this challenge, enabling the prediction of ozonolysis rates with unprecedented accuracy. The new SAR index, x(H), correlates strongly with available literature measurements of ozonolysis rate coefficients (R(2) = 0.87), a database representing 110 species. It was found that capturing the inductive effect rather than the steric effect is of primary importance in predicting the reactivity of these species, which is to be anticipated since HUVOCs can possess a variety of functional groups with a range of electron-withdrawing and donating tendencies. New experimental measurements of ozonolysis rate coefficients were conducted for 1-penten-3-ol, 3-methyl; ethene, 1,1-dimethoxy; E-2-pentenoic acid; E-1,2-dichloroethene; Z-1,2-dichloroethene; trichloroethene; tetrachloroethene; 1-butene, 3-chloro and 2-chloropropene, and were determined to be 5.15 × 10(-18), 4.82 × 10(-16), 3.07 × 10(-18), 8.05 × 10(-20), 4.88 × 10(-21), 6.04 × 10(-22), 1.56 × 10(-24), 2.26 × 10(-18) and 1.13 × 10(-19) cm(3) molecule(-1) s(-1), respectively. The index of the inductive effect, i(H), is compared with other indices of the electron-withdrawing capacity of a substitution, notably the Taft σ* constants and the rate of reaction of a given species with the hydroxyl radical, both of which are expected to be unaffected by steric factors. i(H) correlates strongly in both cases and suggests a universal response by olefinic species towards electrophilic addition.

Temperature-dependent ozonolysis kinetics of selected alkenes in the gas phase: an experimental and structure-activity relationship (SAR) study.

The kinetics of the reactions of ozone with several alkenes have been measured at atmospheric pressure between 217 and 301 K using EXTRA (EXTreme RAnge chamber). This work represents the first kinetic determinations of the system and focuses on the temperature-dependence of alkene ozonolysis, which is an important tropospheric process impacting upon climate and human health, yet few studies have investigated these reactions as a function of temperature. Temperature-dependent rate coefficients have been established for 3,3-dimethyl-1-butene, 2,4,4-trimethyl-1-pentene and 4-methyl-1-pentene at 217-301 K and atmospheric pressure. The derived Arrhenius expressions are as follows: k = (2.68+2.23-1.23) x 10-15 exp[-(16.29 +/- 1.20/RT)], k = (7.31+9.39-4.05) x 10-15 exp[-(15.33 +/- 1.84/RT)] and k = (5.21+2.85-1.85) x 10-15 exp[-(15.66 +/- 0.87/RT)] cm3 molecule-1 s-1 for 3,3-dimethyl-1-butene, 2,4,4-trimethyl-1-pentene and 4-methyl-1-pentene, respectively.A strong linear correlation has been observed between a simple structure-activity relationship (SAR) and the activation energy, Ea, possessing an R2 value of 0.90. However, no significant correlation was observed for the A-factor. Notwithstanding, with accurate predictions of the SAR for Ea and log k298, values for the A-factor can be retrieved, and hence the prediction of k at any temperature. The newly acquired data agree well with the original SAR and suggest that the factors controlling the rate of ozonolysis reaction are captured accurately by the SAR index.

Structure-activity relationship (SAR) for the gas-phase ozonolysis of aliphatic alkenes and dialkenes.

The configuration of alkyl substituents about carbon-carbon unsaturated bonds exerts a controlling influence on the rate of the ozonolysis reaction. Alkyl substituents can increase (via the inductive effect) and decrease (via the steric effect) the activity of unsaturated bonds, and an accurate description of this information ought to correlate with the ozonolysis rate coefficient. A strong linear relationship is observed (R2 = 0.94), providing the basis of our SAR method. SAR estimates were tested against literature measurements of ozonolysis rate coefficients for 48 aliphatic alkenes and dialkenes, and were found to be accurate to within a factor of 2.3 of the measured value for the entire dataset. This represents a significant improvement over methods reported in the literature, where quoted predictions are at best accurate to within a factor of 6.5. Rates of gas-phase ozonolysis of alkenes and dialkenes can now be predicted with unprecedented accuracy using a simple SAR. The SAR was then validated against new experimental data. Absolute rate coefficients for the gas-phase reaction of ozone with a series of alkenes were determined in a simulation chamber at 295 +/- 2 K and atmospheric pressure by monitoring the loss of ozone in the presence of excess alkene. The rate coefficients (in units of 1 x 10(-18) cm3 molecule(-1) s(-1)) are: 5.12 +/- 0.93 for 1-pentene, 2,3-dimethyl; 406 +/- 49 for 2-pentene, 2-methyl; 151 +/- 5 for (E)-2-hexene, 14.5 +/- 1.0 for 1,5-hexadiene and 20.7 +/- 3.1 for 1,5-hexadiene, 2-methyl. There is good agreement between the experimental and predicted values and the adjustable parameters of the SAR are shown to be insensitive to the inclusion of the new data. The use of the SAR in atmospheric chemical modelling is investigated. Ozonolysis and OH radical rate coefficients are estimated for each alkene and dialkene present in the MCM v3.1. The effects of error within predicted rate coefficients upon modelled concentrations of a number of key species, including O3, OH, HO2, NO and NO2 were rather small and is not in itself a major cause of uncertainty in modelled concentrations.

Is hydrogen abstraction an important pathway in the reaction of alkenes with the OH radical?

In this paper, we report the first temperature dependent kinetic measurements of the reaction of C(> or =5) 1-alkenes with OH radicals and also report the first room temperature measurements for 1-alkenes between C(8-11). The rate coefficients at room temperature and between 75 and 100 Torr for the reaction of OH with 1-pentene, 1-octene, 1-nonene, 1-decene and 1-undecene are (2.74 +/- 0.38) x 10(-11), (3.62 +/- 0.68) x 10(-11), (4.20 +/- 0.41) x 10(-11), (7.00 +/- 0.96) x 10(-11) and (12.44 +/- 1.62) x 10(-11) cm(3) molecule(-1) s(-1), respectively, at 298 K. No pressure dependence is observed, suggesting that these reactions are at their high pressure limit. Inspection of the variation of rate coefficient with temperature over the temperature range 262-300 K shows a weak negative temperature dependence, suggesting that adduct formation is taking place, however, a curved Arrhenius plot is clearly seen for 1-decene and theoretical analysis suggests that abstraction by OH is a non-negligible channel. An estimation of products formed from OH oxidation suggests that abstraction will lead to products that have a lower photochemical ozone creation potential than those formed via addition.

Kinetics and branching ratio studies of the reaction of C2H5O2 + HO2 using chemical ionisation mass spectrometry.

The overall rate coefficient for the reaction of C(2)H(5)O(2) with HO(2) was determined using a turbulent flow chemical ionization mass spectrometer (TF-CIMS) system over the pressure range of 75 to 200 Torr and temperatures between 195 and 298 K. The temperature dependence of the overall rate coefficient for the reaction between C(2)H(5)O(2) and HO(2) was fitted using the following Arrhenius expression: k(T) = (2.08) x 10(-13) exp [(864 +/- 79)/T] cm(-3) molecule(-1) s(-1). The upper limits for the branching ratios for reactive channels leading to O(3) and OH production were quantified for the first time. A tropospheric model has been used to assess the impact of the experimental error of the rate coefficients determined in this study on predicted concentrations of a number of key species, including O(3), OH, HO(2), NO and NO(2). In all cases it is found that the propagated error is very small and will not in itself be a major cause of uncertainty in modelled concentrations. However, at low temperatures, where there is a wide discrepancy between existing kinetic studies, modelling using the range of kinetic data in the literature shows a small but significant variation for [C(2)H(5)O(2)], [C(2)H(5)OOH], [NO(x)] and the HO(2) : OH ratio. Furthermore, a structure-activity relationship (SAR) was developed to rationalise the reactivity of the reaction between RO(2) and HO(2).

Can topological indices be used to predict gas-phase rate coefficients of importance to tropospheric chemistry? Reactions of alkenes with OH, NO3 and O3.

Rate coefficients for the gas-phase reactions of unsaturated hydrocarbons with OH, NO(3) and O(3) are of vital importance to atmospheric modelling. Many of these rate coefficients are unknown--possibly resulting from the variety of these compounds and the inherent expense of deriving these data experimentally--and a reliable method for their prediction would therefore be of value to such models. This study presents a method for estimating rate coefficients for C(2)-C(10) olefins. Measured rate coefficients for the reaction of unsaturated hydrocarbons with OH and NO(3) radicals and O(3) are correlated with the Randic topological descriptor and an established correlation parameter, i.e. ionization potentials calculated using the frontier molecular orbital (FMO) approach. Although the ionization potential method produces better correlations in general, OH correlations of aliphatic species, subdivided into groups of varying total number of primary (1 degree), secondary (2 degrees) and tertiary (3 degrees) carbon atoms (Sigma x degrees (x)) produced several new trends with the Randic index that were not present in the ionization potential correlations. Strong correlation was observed with a Randic-type index optimized to include a term for side chain length (Sigma x degrees (x)). The presence of strong relationships, made predictable by molecular constitution, makes topological descriptors a useful and accessible tool for estimating rate coefficients for the reactions of OH with aliphatic species. This study indicates that alkene reactivity is affected strongly by constitution and that abstraction as well as addition may be important for some classes of compound.