Harald Geiger

The tropospheric degradation of isoprene : an updated module for the regional atmospheric chemistry mechanism

Abstract A highly condensed reaction scheme for the tropospheric oxidation of isoprene is presented. This mechanism was implemented into the regional atmospheric chemistry mechanism (RACM), which is an established chemical module for regional air quality modelling but contains an isoprene chemistry which is no longer state-of-the-art. The reaction scheme developed here is based on the recently published Mainz isoprene mechanism (MIM) that has been constructed for application in global chemistry transport models. The MIM code was reduced to a size suitable for use in regional atmospheric chemistry models. Redundant reactions were identified and removed from the reaction scheme by means of sensitivity analyses. The revised mechanism was successfully tested against the results of smog chamber experiments carried out in the European photoreactor EUPHORE. A model intercomparison between both the original and the updated RACM mechanism was performed for a number of well-defined scenarios employing conditions ranging from very clean to highly polluted air masses. The calculations revealed large deviations in the concentration–time profiles for key species of the isoprene degradation, particularly under “low-NOx” conditions. The new isoprene chemistry requires only a few additional reactants (7) and chemical reactions (7) and, therefore, offers the possibility for the successful application of the revised reaction scheme in chemistry-transport models (CTM) without an excessive increase in computational efforts.

Smog chamber studies on the influence of diesel exhaust on photosmog formation

Abstract In an outdoor smog chamber, volatile organic compounds (VOC)/NO x /air mixtures were irradiated by natural sunlight in the presence and the absence of diesel exhaust. The VOC mixture contained n -butane, ethene and toluene with a fixed mixing ratio. Diesel exhaust was generated by a diesel engine mounted on a motor test bed directly at the chamber facility. Five different diesel fuel formulations were used. Each experiment was carried out under similar initial conditions for VOC and NO x . In the presence of diesel exhaust, the formation of ozone was significantly increased. Simulation of the experiments performed using a chemical box model yielded good agreement between measured and calculated concentrations for all chamber runs. The increase in ozone formation observed on addition of diesel exhaust was mainly caused by the exhaust concentrations of nitrous acid and formaldehyde, which serve as strong radical sources in the initial phase of each exhaust experiment. A sensitivity analysis showed that the photooxidant formation was not dependent on the formulation of the diesel fuel used. The different ozone formation rates observed for the single exhaust experiments were clearly caused by deviations in initial reactant concentrations as well as photolysis conditions.

Kinetics of the C2(a3Πu) Radical Reacting with Selected Molecules and Atoms

Rate coefficients for reactions of the C2 radical in its a3Πu electronic state with H, N and O atoms and with C3O2, C4F6, H2, NO, N2O, C2H2, C2H4, C3H4 and C6H6 were determined. This work represents the first study of reactions of C2(a3Πu) radicals with the atoms investigated at room temperature and 4 Torr total pressure. The bimolecular rate constants obtained for the atom reactions were kC2+O = (9.8±1.0)× 10-11, kC2+N = (2.8±1.0)× 10-11 and kC2+N<6× 10-14, in units of cm3 s-1. In addition, the reaction C2 + O was found to be independent of total pressure in the range 2-60 Torr. For the reaction C2(a3Πu) + ethene (C2H4) a temperature and pressure independent rate constant of (9.5±1.2)× 10-11 cm3 s-1 was obtained in the temperature range 298-1000 K at 100 Torr total pressure and in the pressure range 5-100 Torr at 298 K. The following rate constants were determined at room temperature and a total pressure of 4 Torr for the reactions of C2(a3Πu) radicals with benzene (C6H6), acetylene (C2H2) and allene (C3H4): kC2+C6H6 = (4.9±0.1)× 10-10, kC2+C2H2 = (1.0±0.1)× 10-10 and kC2+C3H4 = (1.9±0.3)× 10-10, in units of cm3 s-1. The reaction C2 + NO was investigated at room temperature and 100 Torr total pressure, a rate constant kC2+NO = (6.8±0.3)× 10-11 cm3 s-1 was obtained. The reaction C2 + N2O was studied at 4 Torr total pressure in the temperature range 300-700 K for which a temperature independent rate constant kC2+N2O = (3.1±0.4)× 10-14 cm3 s-1 was determined.

Chemical Mechanism Development: Laboratory Studies and Model Applications

Within the German Tropospheric Research Programme (TFS) numerous kinetic and mechanistic studies on the tropospheric reaction/degradation of the following reactants were carried out:• oxygenated VOC, • aromatic VOC, • biogenic VOC, •short-lived intermediates, such as alkoxy and alkylperoxy radicals.At the conception of the projects these selected groups were classes of VOC or intermediates for which the atmospheric oxidation mechanisms were either poorly characterised or totally unknown. The motivation for these studies was the attainment of significant improvements in our understanding of the atmospheric chemical oxidation processes of these compounds, particularly with respect to their involvement in photooxidant formation in the troposphere. In the present paper the types of experimental investigations performed and the results obtained within the various projects are briefly summarised. The major achievements are highlighted and discussed in terms of their contribution to improving our understanding of the chemical processes controlling photosmog formation in the troposphere.

Scenarios for Modeling Multiphase Tropospheric Chemistry

Besides observational data model calculations are a very importanttool for improving our understanding of multiphase chemistryin the troposphere. Before a chemical model can be used for that purposeit is necessary to show that the model does what it is intendedto do. A protocol has been developed thatcan be used as a basis for the verification of the numericsand the correct implementation of thechemical balance equations.The protocol defines meteorological parameters and initial conditionsfor a zerodimensional (box) model. Several scenarios cover the pollutedas well as the remote marine and continental boundary layer and also thefree troposphere. Calculations by different groupswith different modelsand numerical solvers demonstrate that the protocol is clear and complete.The excellent agreement between the results of all groups are a major step of verification of the participating models.The scenarios may also serve as well documented base cases forsensitivity studies.

A product study of the reaction of CH radicals with nitric oxide at 298 K

The product formation of the reaction of CH radicals with NO was studied in the gas phase at 100 Torr total pressure and 298 K. CH radicals were generated by excimer laser photolysis of CHClBr2/Ar mixtures. In the presence of nitric oxide, the formation of NH, CN and NCO radicals in their electronic ground states was monitored by laser-induced fluorescence. From the results of time resolved measurements of the radical concentrations, bimolecular rate coefficients for the reactions CH+X and X+NO [X=NH(X3Σ-), CN(X2Σ+) and NCO(X2Π)] were determined. From their kinetic behaviour, NH and CN radicals were unequivocally identified as direct reaction products of the reaction CH+NO, whereas the formation mechanism of NCO remains unclear.

Estimation of incremental reactivities for multiple day scenarios : an application to ethane and dimethyoxymethane

Abstract Single-day scenarios are used to calculate incremental reactivities by definition (Carter, J. Air Waste Management Assoc. 44 (1994) 881–899.) but even unreactive organic compounds may have a non-negligible effect on ozone concentrations if multiple-day scenarios are considered. The concentration of unreactive compounds and their products may build up over a multiple-day period and the oxidation products may be highly reactive or highly unreactive affecting the overall incremental reactivity of the organic compound. We have developed a method for calculating incremental reactivities for multiple days based on a standard scenario for polluted European conditions. This method was used to estimate maximum incremental reactivities (MIR) and maximum ozone incremental reactivities (MOIR) for ethane and dimethyoxymethane for scenarios ranging from 1 to 6 days. It was found that the incremental reactivities increased as the length of the simulation period increased. The MIR of ethane increased faster than the value for dimethyoxymethane as the scenarios became longer. The MOIRs of ethane and dimethyoxymethane increased but the change was more modest for scenarios longer than 3 days. MOIRs of both volatile organic compounds were equal within the uncertainties of their chemical mechanisms by the 5 day scenario. These results show that dimethyoxymethane has an ozone forming potential on a per mass basis that is only somewhat greater than ethane if multiple-day scenarios are considered.

Kinetic investigation of NCO radicals reacting with selected hydrocarbons

Bimolecular rate coefficients for the reactions of isocyanate radicals (NCO) with ethane (C2H6), allene (C3H4), propene (C3H6), but-1-ene (C4H8) and but-1-yne (C4H6) were measured in argon as carrier gas. NCO radicals have been formed using excimer laser photolysis of chlorine isocyanate (ClNCO) and were detected in their electronic ground state, 2Π, by laser-induced fluorescence. The NCO+C2H6 reaction was investigated at 20 Torr total pressure in the temperature range 297–899 K. The reaction exhibits a positive temperature dependence which is well described by the following modified Arrhenius equation: In addition, measurements carried out at 651 K in the pressure range 5–528 Torr showed no pressure dependence, which supports the view that the NCO+C2H6 reaction proceeds as a simple H atom abstraction. For the reaction of NCO radicals with allene (C3H4), propene (C3H6), but-1-ene (C4H8) and but-1-yne (C4H6) bimolecular rate coefficients were determined at room temperature and at 20 Torr total pressure. Additionally, the reactions of NCO with C3H4 and C4H6 were studied at different total pressures. For both reactions, no pressure dependence was observed. The following rate coefficients were obtained at 298 K (in units of 10-11 cm3 s-1): kNCO+C3H4=(1.39±0.18), kNCO+C3H6=(4.29±0.20), kNCO+C4H8=(6.18±0.46) and kNCO+C4H6=(1.41±0.14). A correlation between the measured bimolecular rate coefficients of the NCO reactions with unsaturated hydrocarbons and their ionisation potentials was established.

The reactions of OH radicals with di-i-propoxymethane and di-sec-butoxymethane: Kinetic measurements and structure activity relationships

The gas-phase reactions of OH(X2Π) radicals with di-i-propoxymethane (DiPM) and di-sec-butoxymethane (DsBM) have been studied in argon in the temperature range 295–700 K at total pressures between 50 and 400 Torr. OH radicals were generated by excimer laser photolysis of H2O2 and were detected by laser-induced fluorescence. Within the investigated ranges, the reactions of OH(X2Π) radicals with DiPM and DsBM were found to be independent of total pressure. Weak dependencies of the rate coefficients on temperature were observed. Bimolecular rate coefficients for the reactions of OH(X2Π) with DiPM and DsBM at 298 K of kOH+DiPM=(3.47±0.20)×10-11 cm3 s-1 and kOH+DsBM=(4.25±0.13)×10-11 cm3 s-1, respectively, have been determined. In order to describe the kinetics of the reactions of OH radicals with DiPM and DsBM as well as analogous acetals, a structure activity relationship (SAR) technique established for other reactant classes has been modified and applied. Compared to the former SAR method, which does not yield satisfying results for oxygenated VOCs (volatile organic compounds), the present calculations lead to much better agreement with the experimental data for dialkylacetals of the type R–O–CH2–O–R.

Effect of Gasoline Formulation on the Formation of Photosmog: A Box Model Study

Abstract Based on exhaust gas analyses from the combustion of five different types of gasoline in a passenger car operated on a chassis dynamometer, box model simulations of the irradiation of exhaust/NOx /air mixtures using an established chemical mechanism for a standardized photo-smog scenario were performed. The fuel matrix used covered wide fractional ranges for paraffinic, olefinic, and aromatic hydrocarbons. Two fuels also contained methyl tertiary butyl ether (MTBE). The different O3 profiles calculated for each run were compared and interpreted. The O3 levels obtained were strongly influenced by the exhaust gas concentrations of aromatic and olefinic hydro-carbons. The higher exhaust content of these compounds caused higher O3 production in the smog system investigated. The conclusion of the present study is that the composition of gasoline cannot be taken directly for the estimation of the emissions’ O3 creation potential from its combustion. Variation of the dilution in the different calculations showed evidence for an additional influence of transport effects. Accordingly, further detailed exhaust gas analyses followed by more complex modeling studies are necessary for a proper characterization of the relationship between fuel blend and gasoline combustion products.