Temperature and Pressure Studies of the Reactions of CH₃O₂, HO₂, and 1,2‑C₄H₉O₂ with NO₂

A novel technique has been developed for the detection of peroxy radicals in order to study their kinetics with NO₂. Peroxy radicals (RO₂, where R = H, CH₃, and 1,2-C₄H₉) were produced by laser flash photolysis and were probed by photodissociation of the RO₂ and the subsequent detection of either OH or CH₃O photofragments by laser-induced fluorescence. Reaction , CH₃O₂ + NO₂ + M ⇌ CH₃O₂NO₂ + M (M = N₂), was studied between 25 and 400 Torr at 295 K, giving results in excellent agreement with the literature. At temperatures between 333 and 363 K, equilibration was observed and yielded Δ_r<i>H</i>^⊖₂₉₈(1) = −93.5 ± 0.3 kJ mol^–1. Reaction , HO₂ + NO₂ + M ⇌ HO₂NO₂ + M (M = N₂), was studied at 295 K and showed kinetics in fair agreement with the literature. Equilibration at higher temperatures was obscured by an additional loss of HO₂NO₂ from the system. In addition, the OH quantum yield from photolysis of HO₂NO₂ at 248 nm was determined to be 0.15 ± 0.03. Reaction , 1,2-C₄H₉O₂ + NO₂ + M ⇌ 1,2-C₄H₉O₂NO₂ + M (M = He), was studied between 241 and 341 K, and at the higher temperatures equilibration was observed, which yielded Δ_r<i>H</i>^⊖₂₉₈(3) = −93.5 ± 0.6 kJ mol^–1. The low uncertainties in the enthalpies of formation for both CH₃O₂ and 1,2-C₄H₉O₂ are a result of using a master equation method that allows global analysis of all the available rate data (present measurements and literature values) for forward and reverse reactions under all conditions of temperature and pressure

Comment on “When Rate Constants Are Not Enough”

Comment on “When Rate Constants Are Not Enough

OH and HO2 chemistry during NAMBLEX: roles of oxygenates, halogen oxides and heterogenous uptake

Several zero-dimensional box-models with different levels of chemical complexity, based on the Master Chemical Mechanism (MCM), have been used to study the chemistry of OH and HO2 in a coastal environment in the Northern Hemisphere. The models were constrained to and compared with measurements made during the NAMBLEX campaign (Mace Head, Ireland) in summer 2002. The base models, which were constrained to measured CO, CH4 and NMHCs, were able to reproduce [OH] within 25%, but overestimated [HO2] by about a factor of 2. Agreement was improved when the models were constrained to oxygenated compounds (acetaldehyde, methanol and acetone), highlighting their importance for the radical budget. When the models were constrained to measured halogen monoxides (IO, BrO) and used a more detailed, measurements-based, treatment to describe the heterogeneous uptake, modelled [OH] increased by up to 15% and [HO2] decreased by up to 30%. The actual impact of halogen monoxides on the modelled concentrations of HOx was dependant on the uptake coefficients used for HOI, HOBr and HO2. Better agreement, within the combined uncertainties of the measurements and of the model, was achieved when using high uptake coefficients for HO2 and HOI (γHO2=1, γHOI=0.6). A rate of production and destruction analysis of the models allowed a detailed study of OH and HO2 chemistry under the conditions encountered during NAMBLEX, showing the importance of oxygenates and of XO (where X=I, Br) as coreactants for OH and HO2 and of HOX photolysis as a source for OH

Ozone photochemistry during the UK heat wave of August 2003

A wide range of chemical and physical parameters have been observed over the course of a severe Europe-wide air pollution episode in August 2003. Detailed surface observations made at the rural perimeter edge of London, U.K. indicated significantly elevated levels of primary VOCs, ozone (>110ppb), other photochemical by-products such as PAN, HCHO, and higher oxygenates but not NOx. Reactive organic tracers in combination with surface Doppler wind radar and back trajectory analysis have been used to establish that initial rapid morning rises in O3 during the episode were caused by entrainment of air from aloft, polluted on regional scales from mainland Europe. Total VOC reactivity to OH approximately doubled during this episode, with similar distribution between functional groups, but showing a temperature dependant exponentially increasing contribution from biogenic isoprene (max 1.2 ppbV). In addition to entrainment of regional air pollution, ozone formation rates within the U.K. boundary layer on the day of observation have been determined using measured peroxy radicals in combination with other chemical data. Under episodic conditions total peroxy radicals in excess 120pptV were observed in late afternoon with strong correlation to a later and higher peak in ozone when compared to non-episodic conditions. During the daytime under episodic conditions alkyl peroxy radical formation was dominated by PAN thermolysis, whose afternoon lifetime averaged only 18.3 min, but which was sustained at a concentration greater than 750 pptV. Low episodic wind speeds resulted in a relatively small possible spatial footprint for emissions of reactive precursors to PAN, and a strong correlation between isoprene and PAN production rate suggest this species may have contributed to the later afternoon increases seen in surface O3