A method for studies of the interaction between atmospheric aerosol particles and cloud and fog droplets

J. Aerosol Sci. Vol, 30, Suppl. 1, pp. $581-S582, 1999 © 1999 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/99/$ * see front matter

Pergamon

A Method for Studies of the Interaction between Atmospheric Aerosol Particles and Cloud and Fog Droplets G6ran Frank, Sven-Inge Cederfelt and Bengt G. Martinsson Div. of Nuclear Physics, Lund Institute of Technology, Lund University, Sweden

Keywords Droplet Aerosol Analyser, DAA, unipolar charging, cloud droplet, fog droplet

Introduction Studies of clouds and fogs, how they form, develop, persist, evaporate, precipitate and on which parameters this is depending, are important in order to understand the role of clouds and fogs in the atmosphere and especially for understanding climate forcing by aerosol particles. The Droplet Aerosol Analyser (DAA) (Martinsson, 1996) is an instrument specially developed for studies of cloud and fog droplet formation and growth and their interaction with atmospheric aerosol particles. The instrument together with recent calibration results will be presented at the conference. The DAA measures the ambient size of individual droplets and interstitial particles (particles that have not grown to droplets), the size of the dry residual particles after evaporation of the droplets (or interstitial particle) and the number of dry residual particles. This gives a unique three-parameter data set (ambient diameter, dry residual particle diameter and number concentration). Having access to these parameters, a number of related aerosol/cloud parameters, can be determined: • characterisation of the droplet activation as defined by the K6hler equation • the size dependent scavenging of particles due to activation • concentration of soluble matter in the individual droplets (solute concentration) • size and number distribution of ambient diameters for droplets and interstitial particles • size and number distribution of dry residual particles

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Abstracts of the 1999 EuropeanAerosol Conference

The results from the DAA are well suited for comparisons with modelling of cloud and fog droplet formation. The instrument has until now been used in six field experiments at five different locations, both in clouds and fogs (Cederfelt et ah, 1997, Martinsson et ah, 1997, Frank et ah, 1998, Martinsson et al., 1999).

Short description of the DAA The DAA is based on a concept where the aerosol is processed in several steps by aerosol charging mechanisms, diffusion drying of droplets and electrostatic aerosol spectrometry, in order to obtain the desired ability to relate the ambient size to the dry residue size, see figure 1. The aerosol spectrometry is based on the use of DMA's, Differential Mobility Analysers. The DAA mode o f operation is such that the ambient (droplet) diameters connected to twelve different dry residue diameters (six at a time) are determined. The residue diameters are set fixed (DMA 2a-f are fixed) and the DAA (DMA 1) scans over the droplet diameter. After a complete scan over the droplet sizes, the six residue diameters are changed to six new residue sizes according to a computer-controlled, repetitive schedule comprising two different settings and thus twelve dry residue sizes. The number o f particles o f each dry size is counted during the scan and the result obtained is a droplet size distribution for each measured dry particle size.

Calibration The unipolar charging unit has been calibrated with respect to the number of charges and the charge distribution obtained as a function of droplet diameter. Monodisperse droplets were generated with a TSI Vibrating Orifice Aerosol Generator. The charge distribution was measured with a Differential Mobility Analyser (DMA) designed for large droplets. The droplet size and concentration were measured with a TSI Aerodynamic Particle Sizer (APS).

References: Cederfelt S-l, B. G. Martinsson, B. Svermingsson, G. Frank, H-C Hansson, E. Swietlicki, A. Wiedensohler, M. Wendisch, K. M. Beswick, K. N. Bower, M. W. Gallagher, S. Pahl, R. Maser and D. Schell, 1997: Field validation of the droplet aerosol analyser: Atmospheric Environment, 31, no 16, 2657-2670 Frank G., B. G. Martinsson, S-I Cederfelt, O. H. Berg, E. Swietlicki, M. Wendish, B. Yuskiewicsz, J. Heintzenberg, D. Orsini, F. Stratmann, P. Laj P. and L. Ricci, 1998: Droplet Formation and Growth in Polluted Fogs: Contr. Atmos. Phys., 71, 65-85. Martinsson B.G., 1996: Physical basis for a droplet aerosol analysing method: J. Aerosol Sci. 27, 9971013. Martinsson B. G., S-I Cederfelt, B. Svenningsson, G. Frank, H-C Hansson, E. Swietlicki, A. Wiedensohler, M. Wendisch, M. W. Gallagher, R. N. Colvile, K. M. Beswick, T. W. Choularton and K. N. Bower, 1997: Experimental determination of the connection between cloud droplet size and its dry residue size: Atmospheric Environment, 31, no 16, 2477-2490. Martinsson B. G., G. Frank, S-I Cederfelt, E. Swietlicki, O. H. Berg, J. Zhou, K. N. Bower, C. Bradbury, W. Birmili, A. Wiedensohler, B. Yuskiewicsz, 1999: Droplet Nucleation and Growth in Orographic Clouds in relation to the Aerosol Population: Atmos. Res., 50, 289-315