Peter Wiesen

Heterogeneous conversion of NO2 on carbonaceous surfaces

In the present study the heterogeneous conversion of NO2 on commercial soot, freshly prepared flame soot and commercial soot treated with sulfuric acid has been investigated. The experimental results indicate a complex reaction mechanism in which the soot has to be treated as a reactant leading to deactivation of active sites on the surface with increasing NO2 consumption. Mean uptake coefficients γ for the first few minutes of the reaction of ∽10-6 have been determined for a consumption of ∽1013 NO2 cm-2. The γ-values decrease to <10-8 for a consumption of ∽1015 NO2 cm-2, indicating that the influence of soot on the partitioning of NOx in the atmosphere is negligible. Nitrous acid (HONO) and nitric oxide (NO) were found to be the major gas phase products. HONO yields up to 80% are observed, clearly showing that the nature of the reaction is non-catalytic. The humidity dependence of the reaction system was studied and indicates that water is necessary for HONO formation. In the reaction of pure HONO on soot surfaces, high yields of NO were observed, indicating a consecutive process: NO2→HONO→NO. When soot was coated with sulfuric acid the yield of HONO decreased while that of NO increased. This behaviour is caused by an increasing reactivity of the intermediate HONO on the modified surface. The experimental results indicate that without any recycling mechanism HONO formation on soot surfaces is not of major importance in the atmosphere.

Nonintrusive optical measurements of aircraft engine exhaust emissions and comparison with standard intrusive techniques

Nonintrusive systems for the measurement on test rigs of aeroengine exhaust emissions required for engine certification (CO, NO(x), total unburned hydrocarbon, and smoke), together with CO(2) and temperature have been developed. These results have been compared with current certified intrusive measurements on an engine test. A spectroscopic database and data-analysis software has been developed to enable Fourier-transform Infrared measurement of concentrations of molecular species. CO(2), CO, and NO data showed agreement with intrusive techniques of approximately ?30%. A narrow-band spectroscopic device was used to measure CO(2) (with deviations of less than ?10% from the intrusive measurement), whereas laser-induced incandescence was used to measure particles. Future improvements to allow for the commercial use of the nonintrusive systems have been identified and the methods are applicable to any measurement of combustion emissions.

Nitrous Oxide (N2O) Emissions from Vehicles

N2O is an important greenhouse gas and accurate emission data are required to assess its impact on global climate. It is well established that automobiles, particularly those equipped with 3-way catalysts, emit N2O. However, the vehicle contribution to the global N2O budget is uncertain. We report results of N2O emission measurements performed in a road tunnel in Germany and using a chassis dynamometer system in the USA. We estimate that the global vehicle fleet emits (0.12±0.06) Tg yr−1 of N2O. From the emission factor (g N2O/g CO2) determined an annual N2O emission of (0.12±0.06) Tg yr−1 of N2O (0.08±0.04 Tg N yr−1) for the global vehicle fleet has been estimated which represents 1–4% of the atmospheric growth rate of this species.

Verification of the Contribution of Vehicular Traffic to the Total NMVOC Emissions in Germany and the Importance of the NO3 Chemistry in the City Air

In 1997 and 1998 several field campaigns for monitoring non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NO x ) were carried out in a road traffic tunnel and in the city center of Wuppertal, Germany. C2–C10 aliphatic and aromatic hydrocarbons were monitored using a compact GC instrument. DOAS White and long path systems were used to measure aromatic hydrocarbons and oxygenated aromatic compounds. A formaldehyde monitor was used to measure formaldehyde. Chemiluminescence NO analysers with NO2 converter were used for measuring NO and NO2. The high mixing ratios of the NMVOCs observed in the road traffic tunnel, especially 2.9 ppbv phenol, 1.5 ppbv para-cresol and 4.4 ppbv benzaldehyde, in comparison with the measured background concentration clearly indicate that these compounds were directly emitted from road traffic. Para-Cresol was for the first time selectively detected as primary pollutant from traffic. From the measured data a NMVOC profile of the tunnel air and the city air, normalised to benzene (ppbC/ppbC), was derived. For most compounds the observed city air NMVOC profile is almost identical with that obtained in the traffic tunnel. Since benzene originates mainly from road traffic emission, the comparison of the normalised emission ratios indicate that the road traffic emissions in Wuppertal have still the largest impact on the city air composition, which is in contrast to the German emission inventory. In both NMVOC profiles, aromatic compounds have remarkably large contributions of more than 40 ppbC%. In addtion, total NMVOC/NO x ratios from 0.6 up to 3.0 ppbC/ppb in the traffic tunnel air and 3.4 ± 0.5 in the city air of Wuppertal were obtained. From the observed para-cresol/toluene and ortho-cresol/toluene ratios in the city air, evidence was found that also during daytime NO3 radical reactions play an important role in urban air.

Atmospheric Photooxidation of Fluoroacetates as a Source of Fluorocarboxylic Acids

A 1080 L environmental chamber with in situ FTIR spectroscopy detection was used to study the product distribution and the mechanism of the Cl-initiated photooxidation of a series of fluoroacetates. The gas-phase reactions of Cl atoms with ethyl trifluoroacetate (CF(3)C(O)OCH(2)CH(3)), methyl trifluoroacetate (CF(3)C(O)OCH(3)), and methyl difluoroacetate (CF(2)HC(O)OCH(3)) were investigated at 296 +/- 2 K and atmospheric pressure (approximately 760 Torr) of synthetic air. The fate of the fluoroalkoxy radicals formed in the reaction with Cl atoms mainly occurs through (i) an H-atom abstraction by reaction with O(2,) to produce the corresponding fluoroanhydride and (ii) an alpha-ester rearrangement via a five-membered ring intermediate to give the corresponding fluoroacetic acid. The yields of fluoroacids (CF(2)XC(O)OH, with X = H, F) obtained were as follows: 78 +/- 5, 23 +/- 2, and 30 +/- 5% for CF(3)C(O)OCH(2)CH(3), CF(3)C(O)OCH(3), and CF(2)HC(O)OCH(3,), respectively. Yields of <or=20, <or= 80, and <or=55% have been estimated for the anhydride formation from CF(3)C(O)OCH(2)CH(3), CF(3)C(O)OCH(3), and CF(2)HC(O)OCH(3), respectively. Formation of CF(2)O, with yield of 13 +/- 2% has been observed for the reaction of Cl with CF(2)HC(O)OCH(3). The measured yields are rationalized in terms of mechanisms consisting of competitive reaction channels for the radicals formed in the oxidation, that is, reaction with O(2), alpha-ester rearrangement and a decomposition pathway. The stability of the five-membered transition state of the alpha-ester rearrangement is correlated with the acid yields observed for the different fluoroacetates. Atmospheric implications, especially with regard to the fluorocarboxylic acid formation, are discussed.

Mechanistic study of the heterogeneous conversion of NO2 into HONO and N2O on acid surfaces

The heterogeneous conversion of NO2 into HONO and N2O on acid surfaces has been investigated in an 11 l Pyrex glass reactor under simulated atmospheric conditions and in a 64 l quartz glass reactor at reduced pressure. NO2, N2O and HONO were monitored with time by long-path tunable IR diode laser absorption spectroscopy using single vibrational rotational lines at 1641.0, 1254.47 and 1254.46 cm–1, respectively.The experimental results show that NO2 is heterogeneously converted on acid surfaces into HONO and then into N2O. HONO formation in the reaction system has been found to occur by the heterogeneous reaction of NO2 with adsorbed water on the surface and does not require NO. The experimental findings indicate that the nitrosyl ion probably plays an important role in converting HONO into N2O and HNO3. A mechanism is proposed which can explain the experimental findings.

Primary NO2 emissions and their impact on air quality in traffic environments in Germany

BackgroundThe decreasing NOX concentrations at urban measurement stations in Germany are in agreement with the reduction of NOX emissions from vehicular traffic. However, the measured NO2 concentrations are stagnating nationwide. In 2010, at more than the half of the urban measurement stations in Germany, annual mean values for NO2 exceeded the new Europe-wide limit value of 40 μg/m3 (20 ppbv) NO2. Similar findings are reported from many other member states of the European Union.ResultsThe observed trend of the airborne NO2 concentrations has different reasons. Firstly, the NO2/NOx emission ratio has increased significantly during the last two decades. Furthermore, secondary NO2, caused by the titration reactions of NO with ozone (O3) and peroxy radicals (RO2), is responsible for the major fraction (approximately 70%) of the measured NO2. However, secondary NO2 shows a highly nonlinear dependency on NOx and thus, is decreasing much more slowly than expected from the decreasing NOx levels. Based on the results from the present study, the increased NO2/NOX emission ratio can only explain a minor fraction of the observed high airborne NO2 concentration in the city center.ConclusionsA further reduction of primary NO2 emissions, due to improved exhaust gas treatment, will not have a strong influence on urban NO2 levels, and a further significant reduction of the NOX emissions, in particular from vehicular traffic, is necessary in order to meet the annual mean limit value for NO2 of about 20 ppb in the future.

OH-Initiated Degradation of Unsaturated Esters in the Atmosphere: Kinetics in the Temperature Range of 287−313 K

The kinetics of the gas-phase reactions of hydroxyl radicals (OH) with methyl methacrylate (k(1)), butyl methacrylate (k(2)), butyl acrylate (k(3)), and vinyl acetate (k(4)) have been investigated for the first time as a function of temperature using the relative technique. The experiments were performed in a 1080 L quartz glass photoreactor over the temperature range (T = 287-313 K) at a total pressure of 760 +/- 10 Torr synthetic air using in situ FTIR absorption spectroscopy to monitor the concentration-time behaviors of reactants. OH radicals were produced by the 254 nm photolysis of hydrogen peroxide (H(2)O(2)). The following Arrhenius expressions (in units of cm(3) molecule(-1) s(-1)) adequately describe the measured rate coefficients as a function of temperature: k(1) = (1.97 +/- 0.95) x 10(-12) exp[(921 +/- 52)/T], k(2) = (1.65 +/- 1.05) x 10(-11) exp[(413 +/- 34)/T], k(3) = (4.4 +/- 2.5) x 10(-13) exp[(1117 +/- 105)/T], and k(4) = (4.06 +/- 2.02) x 10(-12) exp[(540 +/- 49)/T]. All of the rate coefficients display a negative temperature dependence and low pre-exponential factor, which supports an addition mechanism for the reactions involving reversible OH-adduct formation. The rate coefficients (in units of cm(3) molecule(-1) s(-1)) determined at room temperature (298 K) were as follows: k(1) = (4.30 +/- 0.98) x 10(-11), k(2) = (6.63 +/- 1.42) x 10(-11), k(3) = (2.17 +/- 0.48) x 10(-11), and k(4) = (2.48 +/- 0.61) x 10(-11). The results are compared with previous values of the rate coefficients reported in the literature, which were mainly measured at room temperature. The reactivity of the various unsaturated esters toward the OH radical is discussed in terms of structure activity relationships and parallels are drawn with the OH-radical activities of structurally similar compounds. Using the kinetic parameters determined in this work, residence times of the esters in the atmosphere with respect to their reaction with OH have been determined and are compared with other possible degradation pathways. Possible atmospheric implications of the various degradation pathways studied are discussed.

Investigations on the gas-phase photolysis and OH radical kinetics of methyl-2-nitrophenols

Methyl-2-nitrophenols can be emitted directly to the atmosphere or can be formed in situ via the oxidation of aromatic hydrocarbons. Nitrophenols possess phytotoxic properties and recent studies indicate their photooxidation is effective in producing secondary organic aerosols. Therefore, investigations on the major photooxidation pathways of these compounds with respect to assessing their environmental impacts and effects on human health are highly relevant. Presented here are determinations of the rate coefficients for the reactions of OH radicals with four methyl-2-nitrophenol isomers using a relative kinetic technique. The experiments were performed in a 1080 l photoreactor at (760 +/- 10) Torr total pressure of synthetic air at (296 +/- 3) K. The following rate coefficients (in units of cm(3) molecule(-1) s(-1)) have been obtained: 3-methyl-2-nitrophenol, (3.69 +/- 0.70) x 10(-12); 4-methyl-2-nitrophenol, (3.59 +/- 1.17) x 10(-12); 5-methyl-2-nitrophenol, (6.72 +/- 2.14) x 10(-12); 6-methyl-2-nitrophenol, (2.70 +/- 0.57) x 10(-12). Photolysis of the methyl-2-nitrophenols with the superactinic fluorescent lamps (320 < lambda < 480 nm, lambda(max) = 360 nm) used in the experiments was observed. Photolysis frequencies measured for the methyl-2-nitrophenols in the photoreactor have been determined and scaled to atmospheric conditions. The results suggest that photolysis rather than the reaction with OH radicals will be the dominant gas phase atmospheric loss process for methyl-2-nitrophenols.

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.