Abstract. An observationally constrained box model has been constructed in order to investigate the chemistry of the marine boundary layer at Mace Head, a remote location on the west coast of Ireland. The primary aim of the model is to reproduce concentrations of the hydroxyl (OH) and hydroperoxy (HO2) radicals measured by an in situ fluorescence assay by gas expansion (

A simple model, in which deposition is limited by diffusion through a thin laminar film of water, has successfully described the deposition of several gases to the sea. However, the deposition velocity of ozone to seawater is 10–30 times greater than this model predicts. This enhancement is attributed to significant reactions of ozone with halides and other components of seawater within the laminar surface layer, and a modified version of the model for ozone, and possibly other reactive gases, is proposed. To test the model, comparisons were made of predicted and observed deposition velocities for ozone to solutions of sodium sulphite and nitrite. Measurements of the rate constants for the reaction of ozone with these solutions and with some components of seawater were made with a stopped flow apparatus. The reaction with iodide was too rapid for direct observation, but the rate constant was inferred from measurements of the deposition of ozone to iodide solutions. According to the model, iodide makes a substantial contribution to the deposition of ozone to seawater, but an additional, unidentified reaction is necessary to explain fully the deposition rate. A surfactant species may well be involved. The model indicates that the ozone is destroyed and the oxidation products produced in the top few microns of the sea. The production of molecular iodine at the surface may have significant geochemical consequences.

Detailed observations have been made of the seasonal variation in concentration of a wide range of nonmethane hydrocarbons over the North Atlantic Ocean in air with a predominantly polar maritime origin. The results bear out many of the findings of our previous studies [Za 'ghtman et al., 1990] with the observation of a concentration maximum in the winter and a minimum in the summer. The more recent results indicate that the amplitude of the seasonal cycle is remarkably constant. Also the total concentration of reactive carbon in the various forms of nonmethane hydrocarbons probably exceeds 20 ppbC. This constitutes a substantial reservoir of material which could take part in ozone production in the relatively remote troposphere, if sufficient nitrogen oxides are copresent. The hydrocarbon composition of air in winter is strongly dependent on its origin, with tropical air having a composition similar to polar air in the spring months. The relative magnitude of the seasonal cycles for some hydrocarbons is proportional to their rates of reaction with hydroxyl radicals. This is true for straight chain paraffins up to C5, acetylene and benzene, and it suggests that the removal of these molecules from the atmosphere occurs predominantly by reaction with hydroxyl radicals. For other molecules, particularly branched chain paraffins and substituted aromatic molecules, there is evidence that other removal processes are operating in competition with the hydroxyl radical, especially in winter. Arguments are advanced that in the case of the branched chain paraffins this may be caused by reaction with nitrate radicals. 1988; Liu et al., 1987].Nonmethane hydrocarbons are known to have a controlling influence on the production of ozone in the polluted boundary layer of the atmosphere. It is quite possible, however, that their influence is more widespread, particularly because in winter time, photochemical degradation is much less efficient. In winter conditions, many nonmethane hydrocarbons can escape from the boundary layer in source areas and disperse into the free troposphere over large parts of the northern hemisphere.In a recent paper [Lightman et al., 1990] we have drawn attention to the buildup in wintertime of a substantial concentration of hydrocarbons in the free troposphere over the North Atlantic Ocean. Here we wish to extend our observations to the seasonal cycles of a much wider range of hydrocarbons, from (22 to (28, which have been obtained with more sophisticated analytical techniques. The measurement programme which produced the data on the •Now at Department of Chemistry, University of California, Irvine. composition of the free troposphere also examined the detailed enow at Atmospheric Chemistry Research Unit, Imperial College, composition of hydrocarbons in urban plumes, spreading from Silwood Park, Ascot, England. London. Interesting comparisons can be made between the composition of "clean" background air in the northern hemisphere in winter and that recently contaminated with pollutant emissions Copyrig...

Abstract. This paper describes the most extensive set of simultaneous measurements of the concentrations of nitrate (NO3) and peroxy (sum of HO2+RO2, R = alkyl and acyl) radicals to date. The measurements were made in the coastal marine boundary layer over the North Sea, at the Weybourne Atmospheric Observatory on the North Norfolk coast during the spring and autumn of 1994. In spring the average nighttime concentration of NO3 measured by differential optical absorption spectroscopy, was about 10 parts per trillion (ppt) (maximum 25 ppt). The corresponding peroxy radical concentration, measured by the chemical amplifier technique, averaged about 2 ppt (maximum 6 ppt), although this is likely to be an underestimate of the total radical concentration. There is a significant positive correlation between the two sets of radicals, which has not been reported previously. A box model of the marine boundary layer is used to show that this correlation arises from the processing of reactive organic species by NO3. During spring the relatively long lifetime of NO3 (up to 18 min) at night is controlled by reaction with dimethyl sulfide (DMS), and the model predicts significant production of HNO3, methyl tiomethylen (CH3SCH202) and other peroxy radicals, HCHO, and eventually sulfate. A nighttime production rate for the hydroxyl (OH) of about 2 x 104 molecules cm -3 s -• is estimated. During one night in autumn the NO3 lifetime of about 3 min is too short to be explained by reaction with unsaturated hydrocarbons, but is satisfactorily accounted for by the heterogeneous loss ofN20 s on deliquesced aerosols in relatively polluted conditions.

No abstract