Cloud processing of continental aerosol particles:: Experimental investigations for different drop sizes

Atmospheric aerosol particles are activated and grow into drops during the formation of a cloud. Subsequently, they are delivered from the dissipating cloud drops back to the atmosphere. During the cloud lifetime, the drops scavenge water-soluble trace gases, leading to an increase in size and solubility of the particles emerging from the evaporating cloud drops. This processing of aerosol particles by clouds has an influence on the microphysics of the following cloud and its probability to rain as well as on the cooling effect of the direct and indirect aerosol forcing of climate. To measure the cycling history (particle activation and gas scavenging in drops followed by processing of the activated particles followed by emerging processed particles) of continental aerosol particles passing through cloud drops of different sizes, a new method is developed and applied during three field experiments carried out on the Mount Kleiner Feldberg/Ts., Germany, in 1990, 1993, and 1995. The typical droplet spectra of most of the observed stratus clouds is weakly bimodal, with mode 1 at drop sizes between 3 and 5 mu m and mode 2 between 5 and 10 mu m radius. Cloud drops in this overall size range are subject to growth only by condensation, while coalescence can be neglected. Therefore the observed processing is related solely to gas scavenging and in-cloud chemical reactions. We found that the processing of particles is different for the two modes of the cloud drop size spectrum: Small activated particles mostly grow to the small drops of mode 1, while larger particles can grow further to the larger drops of mode 2. Likewise, the mass of scavenged gas is, on average, lower for the small than for the larger drops. Vice versa, the ratio of scavenged gas to particle mass, the parameter quantifying the particle processing, is, on average, found to be higher in the small drop mode containing the smaller particles. The reason is that the degree of processing is mainly inversely linked to the mass of the activated particles. Therefore the strongest modification of particles takes place in smaller drops and affects mainly the smaller activated particles (r(ap) less than or equal to 0.1 mu m). Their radii can increase by up to a factor of 3 and, consequently, their nucleation as well as radiative properties change significantly. The consequence for the aerosol climate forcing is that the cooling can be, to an unknown extent, intensified with increasing atmospheric amount of water-soluble trace gases such as HNO3, NH3, and SO2, counteracting the warming effect of the greenhouse gases.