Pergamon

J. Aemol Sci. Vol. 30. Suppl. I, pp. S837--S838. 1999 0 1999 Published by Elwier Science Ltd. All rights reserved

Printed in Great Britain OOZI-8502/99/S . see front matter

HYGROSCOPIC GROWTH OF AEROSOL PARTICLES IN THE SOUTHERN ATLANTIC OCEAN AND INDIAN OCEAN

Andreas Massling, Alfred Wiedensohler, Bernhard Busch

Institute for Tropospheric Research, Permoserstr. 15, D-04303 Leipzig, Germany

KEYWORDS aerosols, hygroscopic growth, INDOEX, atlantic ocean

INTRODUCTION Aerosol particles influence the radiative budget of the earth directly by scattering and absorbing the incoming light and indirectly by serving as cloud nucleation nuclei. Particles have a relatively short lifetime in contrast to greenhouse gases and they only occur on a regional scale. It is supposed that the influence of aerosol particles on the radiative budget of the earth is as large as the radiative effect of greenhouse gases. Up to now there is only little knowledge about the specific properties of aerosol particles.

Cruise Track

Therefore a determination of the aerosol particles properties is necessary in order to reduce the uncertainties that still exist in computer models that are developed to make a forecast of the earths radiative balance. One of the important properties of aerosol particles is the hygroscopic growth.

. . The hygroscopic growth measurements R resented here were performed during the AEROSOLS campaign (14* January till 20’ February 1999 from Norfolk, USA, via Cape Town to Mauritius) and during INDOEX (22”d February till 1” of April in the region between Mauritius and the Maledives )

HYGROSCOPIC GROWTH Many measurements of the chemical and physical properties of the aerosol have been performed with particles in the dry state. In the atmosphere particles are in equilibrium with the ambient relative humidity. The hygroscopic properties of the particles are required to correct these measurements for ambient conditions. Apart from that hygroscopic data can give information about the mixing state (internal/external) of the particles.

METHODS A Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) have been used to determine the hygroscopic properties for particles within a diameter size range of 50 - 250nm, which covers the Aitken and accumulation mode. The HTDMA consists of two Differential Mobility Analyzers (DMA) in series and an aerosol humidifier in between. The

S837

S838 Abstracts of the 1999 European Aerosol Conference

first DMA selects a narrow particle size fraction from the aerosol. This fraction of the aerosol is humidified in the aerosol conditioner to a well defined humidity and the altered size distribution due to a possible water vapor uptake is then determined with a combination of the second DMA and a Condensation Particle Counter (CPC). The size spectra have been measured at a relative humidity of 30, 55,75 and 90%, respectively.

3,O I I

5 hygroscopic growth of aerosol particles in the Atlantic Ocean

p 23 (Dp=l50 nm, RH=90 %, marine aerosol)

14 16 16 20 22 24 26 26 30 32 34 36

day of the year

Fig. 1: hygroscopic growth factors during days with a more and a less hygroscopic component

3.0 .

: hygroscopic growth of aerosol particles in the Atlantic Ocean

E 215: (Dp=150 nm, RH=90 %, marine aerosol)

.F 2,0-

8 : A

AA A Ah a b 1,5- P - l l @

l . 0

l,O ..(..,..,.,,..I..,..,..,.., 26,0 26,4 26.6 27.2 27,6 2&O 2634 26,6 29,2 29,6

day of the year

Fig. 2: hygroscopic growth factors during days with a more and a less hygroscopic component

RESULTS Fig. 1 shows the temporal behavior of the growth factor (particle diameter at 90 5% rh divided by the initial dry particle diameter) for marine particles with an initial diameter of 150 nm. In this case only particles with the same hygroscopic behavior (growth factor 1.9) could be found, whereas for continental ah-masses (Fig. 2) two fractions with different hygroscopic behavior (1.8 and 1.4) were present.