The chemical content of raindrops as a function of drop radius—I. Field measurements at the cloud base and below the cloud

In previous field studies it was found that raindrops of different sizes show a strong dependence between their chemical content and their drop radius. In all these cases drops were sampled at the ground far below the cloud base using the Guttalgor method. Interpretations of these early results were constrained by the difficulty of quantifying the different processes leading to the measured dependencies. The aim of this paper is to present more detailed information about the causes controlling these dependencies. The main point is to distinguish between in-cloud and below-cloud influences. This is possible by taking samples simultaneously from within the clouds and far below the cloud base. Field studies were performed at the top (inside cloud or near cloud base) and at the bottom (far below cloud base) of the mountain “Weisser Stein” (Dossenheim, Germany). By taking samples from inside the clouds we are able to exclude the influence of below-cloud processes. Comparing the c/r-dependencies we measured at both stations, information about the importance of below-cloud phenomena is derived. To show that our results reflect a general correlation we performed five field experiments over a period of two years. Our results show that all raindrops sampled at the base of stratus clouds have approximately the same cation and anion concentration independent of their radius. This is contrary to measurements made far below the cloud during previous field experiments and also to those made during this study performed at the bottom of “Weisser Stein”. Usually, a maximum concentration at a middle drop radius was found for these cases. Based on our results we are able to conclude that the interdependence between the chemical content of raindrops and their radius is mainly due to below-cloud processes.