Time Resolution In Precipitation and Cloud Sampling

Abstract

Measurements of the composition of precipitation and clouds provide information which can be used to characterise deposition and aid the understanding of the wet chemical processes.

This paper reviews some of the measurements made by CERL in an attempt to characterise the processes determining the ionic composition of cloud and precipitation. The measurements described include the continuous chemical monitoring of rain composition, aircraft sampling of rain and precipitation and ground based cloud studies involving deliberate attempts to alter cloud chemistry.

A common theme throughout these experiments is the emphasis on temporal resolution of information. It is the correlation in time of ionic behaviour of precipitation and cloud water which is most useful both for interpreting the changes in chemical composition and for comparison with computer model predictions. The best opportunity at present for elucidating chemical processes in clouds appears to be that provided by those experiments in which cloud composition is altered directly by controlled emissions. These experiments are most easily conducted at well characterised hill top sites. However, they necessitate good temporal resolution of the ionic composition of clouds if useful information is to be obtained.

These studies demonstrate the importance of making detailed temporal measurements in the gas, aqueous and aerosol phases as a means of elucidating chemical transformation processes.