The effects of clouds on particle size distributions at the high-alpine site Jungfraujoch

Pergamon

J. Aerosol Sci. Vol. 28, Suppl. I, pp. $573-$574, 1997 ©1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

PII: S0021-8502 (97) 00335 -2

0021-85o2/97$17.00+0.00

T H E EFFECTS OF CLOUDS ON PARTICLE SIZE DISTRIBUTIONS AT THE HIGH-ALPINE SITE JUNGFRAUJOCH M. Schwikowski, E. Weingartner, U. Baltensperger and H.W. G~iggeler Paul Scherrer Institut, CH-5232 Villigen-PSI, Switzerland

KEYWORDS Activation, aerosol, CCN, Jungfraujoch, high-alpine, modal diameter

INTRODUCTION High-elevation mountain sites can be used as platforms to monitor various parameters of the continental aerosol. In addition, they allow to study the aerosol-cloud relationship in precipitating and non-precipitating clouds, since they are located at cloud level. The main effect of cloud processing on the aerosol size distribution discussed so far, is the increase of particle volume due to in-cloud oxidation of SO2 (see e.g. Raes et al., 1995). However, our experiments indicate that the effect of precipitating clouds could also be a reduction of the modal diameter.

EXPERIMENTS AND DATA ANALYSIS Various aerosol and cloud parameters were measured under clear-sky and cloudy conditions at a high-alpine site (Jungfraujoch, Switzerland, 3450 m asl) during field experiments in April and May 1992 as well as October and November 1993. Number size distributions of aerosol particles with optical diameters d from 0.1 to 7.5 I.tm were recorded by an optical particle counter (LAS-X, PMS), total number concentrations by a condensation particle counter (CPC, 3022, TSI), cloud liquid water contents (LWC) by a particulate volume monitor (PVM-100, Gerber Scientific), and cloud droplet number concentrations and size distributions by a laser spectrometer (FSSP-100, PMS). The inlet system of the particle counters was designed to prevent the sampling of cloud droplets. From the number size distributions, surface and volume distributions were calculated assuming spherical shapes of the particles. The aerosol number size distributions could be fitted well with lognormal distributions if only data with d < 0.75 ~tm were considered, since the volume distribution exhibited a bimodal shape with relatively high coarse particle concentrations at d > I ~tm. The integrals of the fitted lognormal distributions usually agreed well with the total number concentration obtained by the CPC.

RESULTS AND DISCUSSION Cloud events were often characterised by a depletion of the aerosol concentration in the size range 0.1 < d < 1 ~tm, indicating an efficient activation of the particles to cloud droplets (Baltensperger et al., 1997). In order to quantify the activation, the number concentrations of cloud droplets (Ned) measured by the FSSP were compared with the total number concentrations including interstitial aerosol as well as cloud droplets (Ntot) for four precipitation events in May 1992 (Fig. 1). A good correlation was found between Ned and Ntot when only particles with d > 0.2 Ixm were considered, with a mean activated fraction of 83% (r=0.96, circles in Fig. 1). When particles with d > 0.1 ~tm were included in the analysis, the correlation was less pronounced and

$573

$574

Abstracts of the 1997 EuropeanAerosolConference

a mean activated fraction of only 48% resulted (r=0.82, squares in Fig. 1). This indicates that under the prevailing supersaturations mainly particles with d > 0.2 I.tm were activated. The mean activated fraction was independent of the total number concentration, which was also found by Gillani et al. (1995) for a comparable range of Ntot- The activated fraction increased with increasing LWC.

400 /

/

300v

200-

il

"0 u Z 100-

W 0

'--I

' 100

I 2O0

'

I 300

'

I 400

'

I 500

'

I~ 0

60O

Ul~<20n~l"8

(1(]2 (1

L~C>3~On'g~'~ 0.1

0.16 (118 02

Ntot (cm ~)

Fig. 1: Correlation between cloud droplet number concentration (Ncd) and total particle concentration (Ntot) for particles with d > 0.2 ~tm (circles) and with d > 0.1 lain (squares). Data consist of 15 min averages from four snowfall events.

Fig. 2: Frequency distribution of the fitted modal diameters of the number distributions for clear-sky and cloudy conditions characterised by different LWC's. Data are 1 hour averages from May 1992.

In order to look at the effects of the clouds on the particle size distribution, the fitted modal diameters of the number distributions were classified according to the LWC to discriminate between clear-sky and cloudy conditions. The frequency distributions of the fitted modal diameters for the different classes LWC < 20 mg/m3 (clear-sky), and LWC > 20, > 90, and > 200 mg/m 3 (cloudy conditions) are shown in Fig. 2. Clear-sky conditions were characterised by modal diameters between 90 and 160 nm, whereas with increasing LWC modal diameters of about 50 nm became most frequent. Thus, the interstitial aerosol is characterised by smaller modal diameters corroborating the findings that mainly particles with d > 0.2 ~m were activated during cloud formation. The results suggest that clouds may alter the aerosol size distribution towards smaller modal diameters if they remove activated aerosol particles by precipitation.

REFERENCES Baltensperger U., Schwikowski M., Jost D.T., G~iggeler H.W. and Poulida O. (1997) Scavenging of atmospheric constituents in mixed phase clouds at the high-alpine site Jungfraujoch; Part I: Basic concept and cloud scavenging. Submitted to Atmospheric Environment. Gillani N.V., Schwartz S.E., Leaitch W.R., Strapp J.W. and Isaac G.A. (1995) Field observations in continental stratiform clouds: partitioning of cloud particles between droplets and unactivated interstitial aerosols. J. Geophys. Res. 100, 18687-18706. Raes F., Wilson J. and van Dingenen R. (1995) Aerosol dynamics and its implication for the global aerosol climatology. In: R.J. Charlson & J. Heintzenberg (eds.) Aerosol Forcing of Climate, John Wiley, Chichester, pp. 153-169.