Abstract. The uptake of SO 2 onto Saharan mineral dust from the Cape Verde Islands was investigated using a coated wall flow tube coupled to a mass spectrometer. The rate of loss of SO 2 to the dust coating was measured and uptake coefficients were determined using the measured BET surface area of the sample. The uptake of SO 2 , with an initial concentration between (2-40)×10 10 molecule cm −3 (0.62-12 µTorr), was found to be strongly time dependent over the first few hundred seconds of an experiment, with an initial uptake γ 0,BET of (6.6±0.8)×10 −5 (298 K), declining at longer times. The amount of SO 2 adsorbed on the dust samples was measured over a range of SO 2 concentrations and mineral dust loadings. The uptake of SO 2 was found to be up to 98% irreversible over the timescale of these investigations. Experiments were also performed at 258 K, at a relative humidity of 27% and at 298 K in the presence of ozone. The initial uptake and the amount of SO 2 taken up per unit area of BET dust surface was the same within error, irrespective of the conditions used; however the presence of ozone reduced the amount of SO 2 released back into the gas-phase per unit area once exposure of the surface ended. Multiple uptakes to the same surface revealed a loss of surface reactivity, which did not return if the samples were exposed to gas-phase water, or left under vacuum overnight. A mechanism which accounts for the observed uptake behaviour is proposed and numerically modelled, allowing quantitative estimates of the rate and amount of SO 2 removal in the atmosphere to be estimated. Removal of SO 2 by mineral dust is predicted to be significant at high dust loadings.

The reactions of Cl atoms and ClO radicals with CH3-SOCH3 (DMSO) have been studied using the discharge flow method with direct detection of DMSO, CO, and products by mass spectrometry. The absolute rate constant at room temperature measured for reaction 1, (CH3)2SO + Cl --> products, was k(1) = (1.7 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1). For reaction 2, (CH3)2SO + ClO --> products, only an upper limit could be established, k(2) < or = 6 x 10(-14) cm3 molecule(-1) s(-1) Reaction 1 has been found to proceed through adduct formation and further decomposition involving the cleavage of the C-S bound. The pressure effect on the Cl-DMSO reaction from 0.5 to 3 Torr was negligible, and the temperature dependence in the range 273-335 K was also very slight. The results obtained are related to previous studies of sulfur compounds, and the atmospheric implications are also discussed in relation to the homogeneous sinks of DMSO. Tropospheric lifetimes of DMSO based on average Cl and ClO concentrations and the measured rate constants have been calculated showing that the contribution of reaction 1 must be of minor relevance in the marine boundary layer. Reaction 2 is so slow that it does not play any role within the atmospheric sulfur chemistry.

Smog chamber/GC techniques were used to investigate the atmospheric degradation of two unsaturated alcohols, 1-penten-3-ol and (Z)-2-penten-1-ol, by oxidation with chlorine atoms at atmospheric pressure of N(2) or air, as a function of temperature. The rate coefficients at 298 K were (units in cm(3) molecule(-1) s(-1)): (2.35 ± 0.31) × 10(-10) and (3.00 ± 0.49) × 10(-10) for 1-penten-3-ol and (Z)-2-penten-1-ol, respectively. The identified and quantified gas-phase products (with molar yields in brackets) were carbonyl compounds such as chloroacetaldehyde (33 ± 1%), propionaldehyde (39 ± 1%), acetaldehyde (8 ± 3%) and 1-penten-3-one (2%) from 1-penten-3-ol; and chlorobutyraldehyde (19 ± 1%), propionaldehyde (27 ± 1%), acetaldehyde (18 ± 2%) and (Z)-2-pentenal (36 ± 1%) from (Z)-2-penten-1-ol. A parallel theoretical study at the QCISD(T)6-311G**//MP2/6-311G** level was carried out to facilitate understanding of the reaction mechanism. Both the theoretical and experimental studies indicated that addition of Cl to the double bond of the unsaturated alcohol is the dominant reaction pathway, although the H-abstraction channel cannot be excluded. The atmospheric lifetimes of those unsaturated alcohols were calculated and the results are discussed.

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The results of a discharge flow-mass spectrometric (DF-MS) kinetic study of the reaction between Cl and dimethylsulfide (DMS) (1) over the temperature range 259−364 K at low total pressure between 0.5 and 1 Torr with helium as carrier gas are reported. At room temperature and 1.0 Torr the main products of reaction 1 correspond to an abstraction channel leading to HCl and CH3SCH2 with k(1) = (6.9 ± 1.3) × 10-11 cm3 molecule-1 s-1. The association channel has also been confirmed by mass spectroscopic detection of the adduct CH3S(Cl)CH3 with a yield <0.05 under the experimental conditions used. It is now shown that the abstraction channel requires a slight activation energy, k(1) = (2.0 ± 1.2) × 10-10 exp[−(332 ± 173)/T] cm3 molecule-1 s-1. The kinetics and mechanism of the reaction ClO + DMS → products (2) over the temperature range 259−335 K at total pressures between 0.5 and 2 Torr have also been studied by DF-MS. By mass spectroscopic calibration of dimethyl sulfoxide, DMSO, the branching ratio of the channel leading to this product has been measured (0.90 ± 0.49). The rate constant of reaction 2 has been measured under pseudo-first-order conditions in excess of DMS over ClO:  k(2) = (1.2 ± 0.7) × 10-15 exp[(354 ± 163)/T] cm3 molecule-1 s-1 with k(2) = (3.9 ± 1.2) × 10-15 cm3 molecule-1 s-1 at 298 K. The reaction is postulated to proceed through a channel involving a long-lived intermediate [CH3S(OCl)CH3]* which may decompose back to reactants or to products. Finally, the atmospheric implications through the DMS chemistry of both reactions are discussed.