Intercepted rainfall may be evaporated during or after the rain event. Intercepted rain is generally determined as the difference between rainfall measurements outside and inside the forest. Such measurements are often used to discriminate between water storage and evaporation during rain as well. Two well-accepted methods underestimate water storage by a factor two as compared to direct observations. The underestimation of storage is compensated by an overestimation of evaporation during rain by a factor of three. The direct observations of water storage and evaporation appear to agree with previous direct observations. Thus, it is concluded that these observations are representative. Also, our results based on methods using only rainfall measurements inside and outside the forest appear to agree with previous results. This would result in the conclusion that the common methods systematically underestimate water storage and overestimate evaporation during rain. Indeed, the systematic errors can be explained by the neglect of drainage before saturation. Water storage is better simulated assuming an exponential saturation of a larger storage capacity. A smaller evaporation can be simulated using an appropriate resistance to vapour transport. The observations in dense coniferous forest showed water storage to be the dominant process in rainfall interception, but this conclusion should not be generalized to other forests and climates. Direct observations of water storage and evaporation are recommended to build a realistic set of parameters for rainfall interception studies of the main vegetation types. ᭧

[1] The contribution of ocean-derived DMS to the atmospheric burdens of a variety of sulphur compounds (DMS, MSA, SO 2 , and nss SO 4 = ) is quantified from season to season. Such quantification, especially for nss SO 4 = (the climate-relevant product of DMS oxidation), is essential for the quantification of the radiative forcing of climate that may be attributable to marine phytoplankton under possible future climate conditions. Threedimensional chemical transport modeling up to the stratosphere is used as a tool in realizing this aim. Global data sets on oceanic and terrestrial sulphur sources are used as input. We find that the contribution of ocean-leaving DMS to the global annually averaged column burdens of the modeled compounds is considerable: 11.9 mmol m À2 (98% of total global burden) for DMS; 0.95 mmol m À2 (94% of total global burden) for MSA; 2.8 mmol m À2 (32% of total global burden) for SO 2 and 2.5 mmol m À2 (18% of total global burden) for nss SO 4 = . The mean annual contribution of DMS to the climaterelevant nss SO 4 = column burden is greatest in the relatively pristine Southern Hemisphere, where it is estimated at 43%. This contribution is only 9% in the Northern Hemisphere, where anthropogenic sulphur sources are overwhelming. The marine algal-derived input of the other modeled sulphur compounds (DMS, MSA, and SO 2 ) is also greatest in the Southern Hemisphere where a lower oxidative capacity of the atmosphere, a larger sea-toair transfer of DMS and a larger emission surface area lead to an elevation of the atmospheric DMS burden.

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[1] The MSA:nss SO 4 = ratio, which is a measure of the relative marine biogenic contribution to the total atmospheric sulphur burden, has long been measured in various parts of the globe. Transect studies and observations from a network of stations have provided some idea of the spatial and temporal behavior of the ratio in various regions, but gaps in knowledge still exist in other parts of the globe. Here we present results of a global 3-D chemical transport modeling study which complement these measurements and provide a globe-wide picture of the spatial variation and distribution of this ratio.Comparison of modeled versus measured data on the MSA:nss SO 4 = ratio resulting from all sulphur sources considered shows fair model performance (i.e., a general overestimation of 23%; degrees of freedom = 90) in all areas of the globe where actual measurements of the ratio have been made. On the other hand, the model-observation comparisons for the MSA:nss SO 4 = ratio derived solely from the oceanic DMS source are not as satisfactory (an overall overestimation of a factor of 3; degrees of freedom = 50). The MSA:nss SO 4 = ratio that is derived from the oceanic DMS source alone provides information on the relative yields of MSA and SO 4 = from atmospheric DMS oxidation. Our model results are consistent with measurements, showing that the ratio is highest around the polar regions and lowest within the tropics. This spatial trend is attributed to the fact that MSA production occurs best under low temperatures (maximum ambient temperature of 27°C). Despite MSA being preferably produced under low temperatures, observations at high latitudes have consistently shown summer maxima and winter minima in the MSA:nss SO 4 = ratio. This has raised many questions on the robustness of the theory of the MSA production mechanism. Diminished marine biological activity and low seawater DMS conditions in winter have widely been cited as the cause of this observed trend. In this study, we further propose that since photochemical hydroxyl radical (OH) production during the dark winter months at polar latitudes is non-existent, reduced wintertime oxidation of DMS by OH to form MSA results in summer maxima and winter minima in MSA concentrations at these latitudes. Temperature and marine biological activity are, therefore, not the only major determining factors for MSA production at high latitudes on a seasonal scale. Light conditions are also important. Throughout the year, the highest ratios occur in the Southern Hemisphere, where the atmospheric DMS burden is highest. This is in agreement with both short-and long-term measurements in literature.

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