Incorporation of aerosol particles between 25 and 850 nm into cloud elements: measurements with a new complementary sampling system

The interpretation of the physico-chemical processes in clouds is facilitat ed by segregating in situ cloud elements from their carrier gas and small p articles interstitial aerosol, Thus, the present study focuses on the quant itative phase segregation of interstitial air from cloud phase by two compl ementary samplers with microphysical on-line analysis of the separated phas es. An improved counterflow virtual impactor CVI- was developed for the col lection and subsequent evaporation of the condensed phase, releasing dissol ved gaseous material and residual particles. This sampler operates in the s ize range of few micrometers up to 50 mu m in cloud element diameter and is matched by an interstitial Round Jet Impactor sampling the gas phase with interstitial particles. Calibrations of both samplers verified the calculat ed cut sizes D-50 Of 4, 5, and 6 mu m and quantified the slope of the colle ction efficiency curves. Until this study no direct CVI measurements of the residual particle sizes far below the diameter of 0.1 mu m were available. For the first time a CVI was connected to a Differential Mobility Particle Sizer DMPS scanning between 25 nm and 850 nm, thus, including the entire A itken mode in the residual size, analysis. Cloud studies on the Puy de Dame , France, revealed residual particle sizes including Aitken mode diameter D - 100 nm. and accumulation mode D) 100 nm.. A major feature of the CVI data is expressed by the fact that despite incomplete incorporation of accumula tion mode particles in cloud elements there are contributions of particles with diameters smaller than 0.1 mu m to the number of residual particles. C loud entrainment from height levels above the maximum supersaturation as we lls as the size-dependent chemical composition of the aerosol population mo st likely produced the S-shaped size-dependent partitioning of residual par ticles. Compared to earlier studies the 50% partitioning diameters dropped significantly below 100 nm to roughly 70 nm. (C) 2000 Elsevier Science B.V. All rights reserved.