Cornelius Zetzsch

Production and decay of ClNO2 from the reaction of gaseous N2O5 with NaCl solution: Bulk and aerosol experiments

The chemistry of N2O5 on liquid NaCl aerosols or bulk NaCl solutions was studied at 291 K by aerosol smog chamber and wetted-wall flow tube experiments. The uptake of N2O5 on deliquescent aerosol was obtained to be (3.2±0.2)×10−2 (1σ error) from the aerosol experiments. In the wetted-wall flow tube we observed that nitryl chloride (ClNO2) is the main product of the reaction at NaCl concentrations larger than approximately 0.5 M and almost the only product at concentrations larger than 1 M. The ClNO2 yield does not depend linearly on the NaCl concentration, especially at small sodium chloride concentrations (i.e., smaller than 1 M). It appeared that a simple mechanism where N2O5 undergoes two reaction channels (hydrolysis and reaction with Cl−) is unable to explain the observed concentration dependence of the product yield. We propose that N2O5 dissociates to NO2+ and NO3− (rate constant kl>104 s−1) mainly. The directly hydrolysis of N2O5 (k3[H2O]) is less than 20% of the total reaction. NO2+ reacts with water to form 2H+ and NO3− (k5) or with Cl− to form ClNO2 (k4). Neglecting the influence of ionic strength we evaluate k4/k5 to be 836±32 (1σ error). Using the wetted-wall flow tube technique, we studied the uptake of nitryl chloride by aqueous solutions containing NaCl. We observed that the uptake coefficient γ decreased from (4.84±0.13)×10−6 on pure water to (0.27±0.02)×10−6 on a 4.6 M NaCl solution. The sharp decrease of γ with increasing salt concentrations is evidence of reversible hydrolysis. ClNO2 dissociates to Cl− + NO2+(k6). In the absence of Cl− we evaluate H ⋅ k61/2 to be 0.44±0.01 mol L−1 atm−1 s−1/2. Finally, we discuss that atomic Cl from photolysis of ClNO2 may play a role in the marine boundary layer at high northern latitudes.

A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH

The design and performance of a smog chamber for the study of photochemical reactions under simulated environmental conditions is described. The chamber is thermostated for aerosol experiments, and it comprises a gas chromatographic sample enrichment system suitable for monitoring hydrocarbons at the ppbv level. By irradiating NOx/alkane-mixtures rate constants for the reaction of OH radicals with n-alkanes are determined from n-pentane to n-hexadecane to be (k±2σ)/10−12 cm3 s−1=4.29±0.16, 6.2±0.6, 7.52 (reference value), 8.8±0.3, 10.2±0.3, 11.7±0.4, 13.7±0.3, 15.1±0.5, 17.5±0.6, 19.3±0.7, 22.3±1.0, and 25.0±1.3, respectively at 312 K. Rate constants, (k±2σ)/10−17 cm3 s−1, for the reaction of ozone with trans-2-butene (21.2±1.0), cis-3-methylpentene-(2) (47.2±1.7), cyclopentene (62.4±3.5), cyclohexene (7.8±0.5), cycloheptene (28.3±1.5), α-pinene (8.6±1.3), and β-pinene (1.4±0.2) are determined in the dark at 297 K using cis-2-butene (13.0) as reference standard.

A smog chamber for studies of the photochemical degradation of chemicals in the presence of aerosols

Abstract A smog chamber, especially designed for the simulation of the tropospheric photochemical degradation of chemicals adsorbed on aerosol surfaces, is described. With optimum aerodynamic diameters of the aerosols (0.5 μm) and by special thermostatization of the chamber residence times of the aerosols up to 2 days in the dark and up to 1 day in the presence of simulated sun irradiation are achieved. Experiments on the degradation of simple alkanes (up to n-nonane) and aromatics in the absence and presence of aerosols (SiO2 and TiO2) are reported. Rate constants for the reactions of OH with n-alkanes from n-butane to n-tridecane are determined to be (k ± 2 σ)/10−12 cm3s−3 = 2.54 ± 0.04, 4.13 ± 0.05, 5.68 ± 0.04, 7.31 ± 0.08, 8.79 (reference value), 10.3 ± 0.2, 12.4 ± 0.2, 13.3 ± 0.2, 13.9 ± 0.2, 15.5 ± 0.2), respectively, at 300 K. These rate constant measurements and the observed concentrations of OH remain unaffected in the presence of SiO2 aerosol at mass concentrations of l mg m−3. In the presence of TiO2 at 2 mg m−3 mass concentration, the steady-state concentration of OH is enhanced by an order of magnitude. In addition, a heterogeneous photodegradation process of the hydrocarbons occurs on the surface of the TiO2 aerosol.

Gas-phase reaction of the OH–benzene adduct with O2: reversibility and secondary formation of HO2

The reaction of OH radicals with benzene and consecutive reactions of benzene–OH adducts with O2 were studied in the gas phase in N2–O2 mixtures at atmospheric pressure and room temperature. OH was produced by pulsed 248 nm photolysis of H2O2. Time-resolved detection of both OH and benzene–OH adducts was performed by continuous-wave (cw) UV-laser long-path absorption at around 308 nm. The reaction: OH+benzene→products [reaction (1)] was not affected by the presence of O2. Rate constants k1=(1.10±0.07)×10-12 cm3 s-1 and (1.06±0.07)×10-12 cm3 s-1 were obtained in N2 and O2, respectively. In N2 addition of NO2 did not change k1, from which an upper limit of 5% is derived for formation of H atoms in reaction (1). An absorption cross-section of σ(308 nm)=(5.8±1.5)×10-18 cm2 and a self-reaction rate constant of (3.4±1.7)×10-11 cm3 s-1 were determined for the benzene–OH adduct. Upper limits of 5×10-15 cm3 s-1, 1×10-14 cm3 s-1 and 5×10-14 cm3 s-1 were obtained for reactions of the adduct with benzene, H2O2 and NO, respectively. The adduct kinetics in the presence of O2 is consistent with the reversible formation of a peroxy radical: adduct+O2↔adduct–O2 [reaction (2a/-2a)]. An equilibrium constant of K2a=(2.7±0.4)×10-19 cm3 was determined and a rate constant of k2a=(2±1)×10-15 cm3 s-1 was roughly estimated. The effective adduct loss from the equilibrium can be explained by (i) an additional irreversible reaction of the adduct with O2 with a rate constant of (2.1±0.2)×10-16 cm3 s-1, or (ii) a unimolecular reaction of the peroxy radical, with a rate constant of (7.6±0.8)×102 s-1. For a reaction of the peroxy radical with O2 an upper limit of 1×10-17 cm3 s-1 is estimated. Addition of NO reveals formation of HO2 in the presence of O2 by recovering OH via HO2+NO. Applying numerical methods, reaction models were tested to describe the observed complex kinetics of OH. The data are consistent with rapid HO2 formation following a peroxy radical+NO reaction with a rate constant of (1.1±0.4)×10-11 cm3 s-1. Extrapolation of HO2 formation rates to [NO]=0 points at a second source of HO2 not preceded by any RO2+NO reaction.

Formation of atomic ci from sea spray via photolysis ofnitryl chloride: determination of the sticking coefficient of N205 on NaCl aerosol

Abstract The sticking coefficient of N205 on NaCl aerosol is determined in a smog chamber (in an inserted teflon bag) at 292 K, and the rate constant for the consecutive chemical reaction leading to formation of ClNO2 is determined at various relative humidities between 71 and 92%. A preliminary evaluation of these first experiments indicates that the sticking coefficient increases with increasing humidity from 0.024 to 0.05 in that range, whereas the portion leading to CINO2 decreases from 0.61 to 0.3.

Interactions of ozone with organic surface films in the presence of simulated sunlight: impact on wettability of aerosols

Heterogeneous reactions between organic films, taken as proxies for atmospheric aerosols, with ozone in presence of simulated sunlight and the photosensitizer 4-carboxybenzophenone (4-CB) were observed to alter surface properties as monitored by contact angle during the reaction. Attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR) was used in addition for product identification. Two types of model surfaces were systematically studied: 4-CB/4-phenoxyphenol and 4-CB/catechol. Solid organic films made of 4-CB/catechol were observed to become hydrophilic by simultaneous exposure to ozone and simulated sunlight, whereas organic films made of 4-CB/4-phenoxyphenol become hydrophobic under the same conditions. These changes in contact angle indicate that photo-induced aging processes involving ozone (such as oligomerisation) not necessarily favour increased hygroscopicity of organic aerosols in the atmosphere. The ratio between hydrophobic and hydrophilic functional groups should reflect the chemical property of organic films with respect to wettability phenomena. Contact angles and surface tensions of the exposed organic film made of 4-CB/4-phenoxyphenol were found to correspond to the hydrophobic/hydrophilic ratios obtained from the FTIR-ATR spectra.

The Influence of NaBr/NaCl Ratio on the Br--Catalysed Production of Halogenated Radicals

The activation of Br- and Cl- to atomic Br and Cl in sea-spray aerosol was investigated in smog-chamber experiments. In the presence of O3, hydrocarbons and NaCl aerosol alone no activation was observed. By adding Br- to the aerosol, the chain reaction: Br + O3 ⇒ BrO, BrO + HO2 ⇒ HOBr, HOBr ⇒ HOBr(aq), HOBr(aq) + H+ + Br- ⇒ Br2 (6), HOBr(aq) + H+ + Cl- ⇒ BrCl (7) was verified. The step from reaction (6) to (7) is accompanied by a decrease of the Br-/Cl- ratio from 1/600 to less than 1/2000. In the absence of sulphate, the chain is initiated by the reaction of OH(aq) with Br-. The pH value decreases to less than 2 during the first minutes of the experiment and later on to almost 1 (in the absence of NOx or SO2). This is caused by the formation of oxalic acid from alkanes and toluene. In stopped flow experiments, the reduction of Br2 by oxalic acid was observed to occur through a two-step mechanism: HC2O4- + Br2 ↔ Br- + BrC2O4H (k22, k-22), BrC2O4H ⇒ Br- + H+ + 2 CO2 (23) with the following rate constants and ratios of rate constants, k ± 2σ: k22k-23 / k-22 = (2.9 ± 0.3) · 10-4 s-1, k-22 / k-23 = 7000 ± 300013000 M-1, k22 = 2 ±-14 M-1 s-1, and k-23 > 0.1 s-1, k-22 > 600 M-1 s-1. Oxalic acid may be responsible for the inhibition of the chain reaction observed at the end of the experiments.

Heterogeneous photochemical formation of Cl atoms from NaCl aerosol, NOx and ozone

The influence of NaCl and simulated sea spray aerosol on photochemical smog is investigated in an aerosol smog chamber experiment in teflon bags in the presence of NOx and ozone. Besides the expected OH, a considerable formation of atomic Cl is observed in the presence of simulated tropospheric sunlight. The photolytic precursor of the atomic Cl is identified to be NO2Cl formed in a dark reaction of N2O5 with NaCl.

A smog chamber study on the impact of aerosols on the photodegradation of chemicals in the troposphere

In the smog chamber the photodegradation of organic compounds (gases and vapours) is observed to be dominated by OH radicals, and rate constants for the reaction of OH with n-alkanes from propane to n-nonane are determined from the simultaneous decay of the hydrocarbons. The presence of SiO2 aerosol at densities up to 4 mg m−3 does not affect the concentration of OH. In the presence of TiO2 aerosol the concentration of OH is increased by one order of magnitude to 108 cm−3. In addition a heterogeneous photodegradation of various alkanes, cycloalkanes, branched-chain alkanes and benzene-aromatics occurs inversely proportional to the vapour pressure. As expected for a heterogeneous process the data can be correlated with gas chromatographic retention times on a column, packed with TiO2. The impact of these processes (which may be due to the formation of OH on the surface of the u.v. absorbing semi-conductor, TiO2) on the chemistry of the troposphere is discussed.

Atmospheric Photosensitized Heterogeneous and Multiphase Reactions: From Outdoors to Indoors

This proposal involves direct photolysis processes occurring in the troposphere incorporating photochemical excitation and intermolecular energy transfer. The study of such processes could provide a better understanding of ·OH radical formation pathways in the atmosphere and in consequence, of a more accurate prediction of the oxidative capacity of the atmosphere. Compounds that readily absorb in the tropospheric actinic window (ionic organic complexes, PAHs, aromatic carbonyl compounds) acting as potential photosensitizers of atmospheric relevant processes are explored. The impact of hotosensitation on relevant systems which could act as powerful atmospheric reactors,that is, interface ocean-atmosphere, urban and forest surfaces and indoor air environments is also discussed.