The chemical content of raindrops as a function of drop radius .3. A new method to measure the mean aerosol particle size of different inorganic species in the atmosphere

A new method is presented to measure the mean aerosol particle size of different inorganic species in atmosphere by collecting raindrops as a Function of drop radius. In previous measurements of inorganic species (e.g. Na+, Mg2+, Cl- or SO42-) of size classified raindrops, an interdependence was obtained between concentration of these compounds in raindrops and drop size (''c/r-dependence''). A decrease in concentration with increasing drop radius was found at the beginning of precipitation. Further measurements of size classified raindrops at later precipitation times (t > 10 min) showed a concentration maximum at a specific drop radius. However, the same c/r-dependence was always measured for elements located on aerosol particles near ground (e.g. Al, Fe, Mn or Pb). These species show the same curve shape with no dependence on sampling time. New results concerning these elements show that this is not true. Their c/r-dependencies also changed during long precipitation times (t > 3 h). This is caused by the changing medium particle size during long precipitation times, whereas other below-cloud processes have a strong influence on the c/r-dependence of the elements first mentioned (e.g. Na+, Mg2+, Cl- or SO42-). The rapidly changing c/r-dependence of these species at the beginning of precipitation is caused by evaporation of the smallest raindrops. Results of a tracer experiment clearly indicated that the c/r-dependence of elements located on aerosol particles near ground is mainly influenced by the size of scavenged aerosol particles on which these elements are located. This experiment establishes a connection between raindrop radius showing concentration maximum and scavenged aerosol particle size. Therefore, it is also possible to measure size classified raindrops at real rain events in order to get the mean scavenged particle size of inorganic species in atmosphere. Copyright (C) 1996