Abstract. HONO formation has been proposed as an important OH radical source in simulation chambers for more than two decades. Besides the heterogeneous HONO formation by the dark reaction of NO 2 and adsorbed water, a photolytic source has been proposed to explain the elevated reactivity in simulation chamber experiments. However, the mechanism of the photolytic process is not well understood so far. As expected, production of HONO and NO x was also observed inside the new atmospheric simulation chamber SAPHIR under solar irradiation. This photolytic HONO and NO x formation was studied with a sensitive HONO instrument under reproducible controlled conditions at atmospheric concentrations of other trace gases. It is shown that the photolytic HONO source in the SAPHIR chamber is not caused by NO 2 reactions and that it is the only direct NO y source under illuminated conditions. In addition, the photolysis of nitrate which was recently postulated for the observed photolytic HONO formation on snow, ground, and glass surfaces, can be excluded in the chamber. A photolytic HONO source at the surface of the chamber is proposed which is strongly dependent on humidity, on light intensity, and on temperature. An empirical function describes these dependencies and reproduces the observed HONO formation rates to within 10 %. It is shown that the photolysis of HONO represents the dominant radical source in the SAPHIR chamber for typical tropospheric O 3 /H 2 O concentrations. For these conditions, the HONO concentrations inside SAPHIR are similar to recent observations in ambient air.

More than 200 in situ measurements of several selected nonmethane hydrocarbons (NMHCs) were made in the remote marine atmosphere over the Atlantic between 40øS and 50øN. Ethane, the by far longest lived of the NMHCs, showed southern hemispheric mixing ratios around 280 ppt, comparable in magnitude to most other previous measurements. The mixing ratios of ethene, propene, propane, and i-and n-butane in the southern hemisphere were in the range of 10-30 ppt. For i-pentane and n-pentane the atmospheric mixing ratios in the southern hemisphere were below the detection limit of 15-20 ppt. These values are rather low compared to other published measurements in the marine atmosphere. This is due to the remoteness of the measuring locations and the very low biological activity in the surrounding ocean areas. For all alkanes the latitudinal profiles exhibit a considerable decrease from north to south. The relative hydrocarbon patterns show that the alkanes in the northern hemisphere are primarily due to long-range transport from continental or coastal areas. In general the transport times exceed several days. Consequently, the observed northern hemispheric ethene and propene mixing ratios must be, with few exceptions, primarily the result of oceanic emissions. The average difference of these compounds by a factor of 2 between the southern and the northern hemisphere can be explained by change of the phytoplankton concentration in ocean water. There is little or no indication for the existence of significant diurnal cycles for these two alkenes.

During the cruise ANT VIII/1 of the German R/V Polarstern in August/September 1989 the latitudinal distributions of the atmospheric concentrations of methylchloride, dichloromethane, trichloroethene, and tetrachloroethene were measured over the Atlantic between 45øN and 30øS by in situ gas chromatography. With the exception of trichloroethene they showed mixing ratios well above the lower limit of detection. The methylchloride distribution was uniform with average mixing ratios of 532 _+ 8 and 550 _+ 12 ppt in the northern and southern Hemispheres, respectively. Dichloromethane increased linearly between the Intertropical Convergence Zone and 45øN with average mixing ratios of 36 -+ 6 ppt and was almost constant in the southern hemisphere with an average of 18 _+ 1 ppt. Tetrachloroethene mixing ratios were between less than 1 and 10 ppt in the northern hemisphere and always below 3 ppt in the southern hemisphere. Similar to dichloromethane, tetrachloroethene was nearly constant in the southern hemisphere and increased linearly toward northern latitudes. This is compatible with the predominantly industrial origin of these compounds. Trichloroethene varied between 0.3 ppt and about 15 ppt in the northern hemisphere with an average of 3 _ 1 ppt and was generally lower than i ppt in the southern hemisphere with mixing ratios often near or below the detection limit of 0.1 ppt. For CH3C1 we estimate a global turnover of 3.5 x 1012 g/yr which is compatible with previous results Using a simple model calculation our measurements imply a global 12 12 turnover for CH2C12 and C2C14 •)f 0.9 x 10 g/yr and 0.6 x 10 g/yr, respectively. global budget of CH3C1. The anthropogenic source of methylchloride seems to be negligible, since methylchloride is almost entirely used as a chemical intermediate in the production of other compounds and does not reach the atmosphere by direct emission. In contrast to methylchloride dichloromethane, trichloroethene and tetrachloroethene are predominantly man-made. They are primarily used as solvents and degreasing agents [Pearson, 1982]. Thus they can be used as tracers for the source distribution and transport mechanisms of anthropogenic trace gases and supply qualitative and quantitative information about the impact of anthropogenic emissions on the remote atmosphere. Furthermore, chlorinated hydrocarbons such as methylchloroform, CH3CC13, have been used to study the concentration of atmospheric OH [Fraser and Hyson, 1986; Prinn et al., 1987, 1992]. An investigation of the compounds discussed here can contribute to the knowledge of the variations in the OH distribution. Yet, data sets of the global distribution of halocarbons in the atmosphere are very limited. In this paper we provide measurements of the halocarbons over the Atlantic between 45øN and 30øS. EXPERIMENT The halocarbons were measured by gas chromatography aboard the German R/V Polarstern. The cruise started on August 5, 1989, at Bremerhaven, Germany, and ended on September 5, 1989, in Puerto Madryn, Argentina. From 30øN to 10øN the...

Abstract. During the cruise ANT VII/I (September/October 1988) of the German research vessel Po/arstern the latitudinal distributions of several nonmethane hydrocarbons were measured over the Atlantic between 45" N and 30' S by in-situ gas chromatography.On the average, the highest mixing ratios of ethane, propane, i-and n-butane, ethene and acetylene were observed in the Northern Hemisphere around 40" N and just north of the intertropical convergence zone, respectively. South of the equator, a bulge in the mixing ratios of ethane and acetylene was observed indicating aged biomass burning emissions. This observation coincided with enhanced tropospheric ozone found in this region at this season. On the average ethane and acetylene mixing ratios were around 500 and 100 ppt, respectively, whereas the levels of the other NMHC were in the range of some ppt up to 100 ppt.Compared with the results of the cruise ANT V/5 (March/April, 1987), the ethane mixing ratios in September/October proved to be a factor of 3 lower in the Northern Hemisphere and a factor of 2 higher in the Southern Hemisphere, probably due to seasonal effects. Possible causes are the higher OH radical concentrations in summer, which result in a faster removal of ethane or stronger emission from biomass burning which also peaks in the dry season.The relative pattern of the hydrocarbons just north of the ITCZ was very similar for both measurement series. In this region, the NMHC were advected by long-range transport from the continent, whereas generally the ocean itself acts as a major NMHC source. This is supported by the results of a balance calculation between oceanic emissions and atmospheric removal rates.

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