Journal of Atmospheric Chemistry 37: 299–329, 2000. 299 © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Single Particle Analysis of Aerosols, Observed in the Marine Boundary Layer during the Monterey Area Ship Tracks Experiment (MAST), with Respect to Cloud Droplet Formation L. A. DE BOCK 1,P.E.JOOS1, K. J. NOONE 2, R. A. POCKALNY 3 and R. E. VAN GRIEKEN 1 1Department of Chemistry, University of Antwerp (UIA), B-2610 Antwerp, Belgium 2Department of Meteorology, Stockholm University, S-106 91 Stockholm, Sweden 3Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, U.S.A. (Received: 10 September 1998; accepted: 12 April 2000) Abstract. The chemical composition of individual particles >0.2 µm sampled during the MAST- experiment were analysed by SEM-EDX, in combination with multivariate techniques. The objective of this experiment was to identify the mechanisms responsible for the modification of marine stra- tocumulus clouds by emissions from ships and in a wider sense to provide information on the global processes involved in atmospheric modification of cloud albedo. Aerosols were examined under different MBL pollution levels (clean, intermediately polluted and moderately polluted) in five different reservoirs: background below-cloud and above-cloud aerosol; background cloud droplet residual particles; below-cloud ship plume aerosol and ship track cloud droplet residual particles. In this study a relation was provided between the aerosol emitted from the ship’s stack to an effect in cloud. Additionally, a large fraction of the ambient aerosol was found to be composed of organic material or other compounds, consisting of low Z-elements, associated with chlorine. Their number fraction was largest in clean marine boundary layers, and decreased with increasing pollution levels. The fraction of ‘transformed sea salt’ (Na, Cl, S), on the other hand, increased with the pollution level in the MBL. Only 20% of the particles fell within the detectable range of the analysis. Key words: aerosols, cloud formation, microanalysis, ship tracks, climate change. 1. Introduction Chemical and physical processes acting in clouds are very complex, since clouds are multiphase systems in which gaseous compounds, interstitial aerosol particles and liquid droplets coexist. All of the processes occurring in clouds are mutually connected and proceed simultaneously. Among the different existing cloud types, marine stratocumulus clouds seem to play a crucial role in the control of the ra- diative budget of the earth’s atmosphere. By reflecting incoming solar radiation they are able to reduce the earth’s surface temperature. Anthropogenic activity may 300 L. A. DE BOCK ET AL. influence cloud reflectivity (albedo), perturbing the radiative balance of the earth (Kim and Cess, 1993). The objective of the Monterey Area Ship Tracks Experiment (MAST), which took place in June 1994 off the coast of central California, U.S.A., was to acquire a better understanding of the mechanisms responsible for the modification of marine stratocumulus clouds by emissions from ships (Durkee et al., 1997). Ever since the launching of the first TIROS satellites, the anomalous cloud lines (or ship tracks) in satellite images have been suspected to be induced by additional aerosol particles produced by ships (Conover, 1966; Twomey, 1968). Moreover, recent evidence for this correlation was provided by King et al. (1993), Radke et al. (1989), Platnick and Twomey (1994), Ferek et al. (1997) and Hindman and Bodowski (1996). The overall objective of the MAST project was to provide information on the processes involved in anthropogenic modification of cloud albedo. For a long time the char- acterisation of aerosols and cloud droplets in the cloud multiphase systems was approached, like several other environmental topics, by bulk analysis techniques. The single particle analysis approach (SPA) is presently recognised as a powerful tool to reveal detailed information, inaccessible by bulk techniques, concerning the particle origin, formation, reactivity, transformation reactions and their environ- mental impact (De Bock et al., 1996). In this study, scanning electron microscopy in combination with energy dispersive X-ray analysis (SEM-EDX, one of the most commonly used micro-analytical techniques in SPA) was applied to determine the composition and size of individual cloud droplet residual particles and ship track and ship plume aerosol particles, with diameters down to 0.2 µm. Droplet residual particles are those particles remaining after cloud droplets have been sampled and evaporated. Compositional differences found between particles collected in these different reservoirs will help to link the aerosol produced by ships to that present in clouds, and to identify the processes by which ship tracks are formed and become recognisable on satellite images. SEM-EDX was considered to be a particularly ap- propriate technique since the instrument is strongly computerised and automated, offering the possibility to analyse several hundreds of particles in a few hours time. In combination with multivariate techniques, the analysed particles can be assigned to specific sources and classified in different types from which the abundances can reveal the source strength. 2. Experimental 2.1. SAMPLING As part of the MAST sampling campaign, 12 research flights with the University of Washington’s C131 aircraft were flown off the coast of central California, U.S.A. In addition to its normal payload, the aircraft was equipped with a number of addi- tional instruments to investigate the nature of the particulate and gaseous emissions from ships and the chemistry and microphysics of clouds. Particle measurements included a continuous-flow cloud condensation nuclei (CCN) spectrometer (Hud- SINGLE PARTICLE ANALYSIS OF AEROSOLS 301 son, 1989), to investigate CCN properties; a radial differential mobility analyser (Zhang et al., 1995) to determine aerosol size distributions, and a counterflow virtual impactor (CVI; Noone et al., 1988; Orgen et al., 1985) to collect residual aerosol particles. This paper will discuss the chemical analysis of particles sampled with the CVI. The chemical composition of droplet residual particles will reflect the com- position of the particles on which the cloud droplets actually formed, plus any non-volatile material produced in the droplets during their lifetime in the cloud. By comparing the composition of cloud droplet residual particles with the composition of particles in (a) the background below-cloud aerosol, (b) the ship plume aerosol, and (c) the aerosol above the subsidence inversion, we can determine the contri- bution of the aerosol in each of these reservoirs to the number of cloud droplets formed in the clouds we sampled. Perturbations in cloud albedo are dependent on the number of additional cloud droplets formed; SPA gives us a particularly powerful tool to determine the source of these additional droplets, since it is a number-based analysis technique. The major disadvantage of the technique in this regard is that the smallest particle which can be analysed is 0.2 µm in size. Öström et al. (1997) stated in a study of residual particle distributions observed in clouds forming in boundary layers of varying degrees of pollution, that typically, only 20% of the number of residual particles found in the cloud droplets are larger than 0.2 µm. While the composition of the particles controlling cloud droplet number can not be definitively determined with SPA in this case, the relative contribution of various different particle classes to the droplet population can nonetheless be determined. It is known that for the small particles, i.e., smaller than 0.2 µm, sulfate is very important. However, these particles are, as said before, not measurable with SPA. Another unavoidable limit- ation in this work is that highly volatile particles might disappear in the vacuum of the SEM-instrument. 2.2. INSTRUMENTATION – ANALYSIS A total of 71 aerosol samples were analysed using a JEOL JSM 6300 SEM (JEOL, Tokyo, Japan). The instrument is equipped with a PGT (Princeton Gamma Tech, Princeton, NJ, U.S.A.) energy dispersive X-ray detector. The PA6300 program was applied for the automated analysis of 500 individual particles per sample at an acceleration voltage of 20 kV and a beam current of 1 nA. This number of particles is statistically (Poisson distribution) sufficient enough to get a reasonable view about the presence of a certain class of compounds. The average total particle number concentration during exactly the time of the sampling for single particle analysis is not known, but the total particle concentration results during the MAST experiment are discussed in Noone et al. (2000). For every sample, 500 particles are analysed; a number concentration of detected particles cannot be given. The X-ray accumulation time was fixed at 20 s. When the PA6300 program is run, 302 L. A. DE BOCK ET AL. the area of the sample to be analysed is subdivided in different fields and the particle analysis starts with the collection of a backscattered electron image of the field. Based on this image, the area, perimeter and diameter of the particles in the field are calculated and compared with a selection criterion. The X-ray spectrum is accumulated by rastering the electron beam on 40 points within the particle’s contour. Spectrum evaluation is performed by a tophat filter and storage on optical disk of images and spectra allows a re-evaluation of the acquired data. Clear drawbacks of SEM-EDX are the poor detection limits (about 0.1%) and the limitation to detect only elements with Z > 10, due to the presence of a Be- window in front of the Si-Li detector in a conventional SEM instrument. The X-rays of elements, like C, O and N are too low in energy to penetrate this window. Therefore, in SEM-EDX a particle group is classified as organic mater- ial, if no characteristic X-rays are collected or, in other words, when the sum of characteristic X-ray peak intensities in this group is very low.