Abstract
Abstract. The influence of the properties of fine particles on the formation of clouds and precipitation in the tropical atmosphere is of primary importance to their impacts on radiative forcing and the hydrological cycle. Measurements of aerosol number size distribution, hygroscopicity in both sub- and supersaturated regimes and composition were taken between March and July 2008 in the tropical rainforest in Borneo, Malaysia, marking the first study of this type in an Asian tropical rainforest. Hygroscopic growth factors (GF) at 90 % relative humidity (RH) for the dry diameter range D0 = 32–258 nm, supersaturated water uptake behaviour for the dry diameter range D0 = 45–300 nm and aerosol chemical composition were simultaneously measured using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA), a Droplet Measurement Technologies Cloud Condensation Nuclei counter (CCNc) and an Aerodyne Aerosol Mass Spectrometer (AMS) respectively. The hygroscopicity parameter κ was derived from both CCNc and HTDMA measurements, with the resulting values of κ ranging from 0.05–0.37, and 0.17–0.37, respectively. Although the total range of κ values is in good agreement, there are inconsistencies between CCNc and HTDMA derived κ values at different dry diameters. Results from a study with similar methodology performed in the Amazon rainforest report values for κ within a similar range to those reported in this work, indicating that the aerosol as measured from both sites shows similar hygroscopic properties. However, the derived number of cloud condensation nuclei (NCCN) were much higher in the present experiment than the Amazon, resulting in part from the increased total particle number concentrations observed in the Bornean rainforest. This contrast between the two environments may be of substantial importance in describing the impacts of particles in the tropical atmosphere.
Highlights
Of all the components of anthropogenic forcings, the complex interactions between atmospheric aerosols and cloud formation, properties and lifetime, collectively termed the “aerosol indirect effects” have been identified as having the greatest range of uncertainty (Forster et al, 2007)
From the above canopy measurements it was found that the coarse mode aerosol did not contribute to the number-size distribution, as the number concentrations were extremely low above diameters of 300 nm
Size-resolved hygroscopicity tandem differential mobility analyser (HTDMA) and Cloud Condensation Nuclei counter (CCNc) measurement of atmospheric aerosol in Borneo identified the aerosol to comprise a range of compositions over the measured size range (D0 = 32 − 258 nm), and for the aerosol hygroscopicity to vary with changing air mass back trajectories, largely outside of uncertainty
Summary
Of all the components of anthropogenic forcings, the complex interactions between atmospheric aerosols and cloud formation, properties and lifetime, collectively termed the “aerosol indirect effects” have been identified as having the greatest range of uncertainty (Forster et al, 2007). Cloud condensation nuclei (CCN) are the subset of the atmospheric aerosol which have the ability to nucleate cloud droplets in the presence of a water supersaturated air mass. The number of CCN may indirectly influence the radiative balance of the atmosphere by changing the number of cloud droplets, in turn changing the albedo, longevity and precipitation patterns of clouds (Twomey, 1977; Albrecht, 1989; Lohmann and Feichter, 2005 and Feichter et al, 2004; Andreae and Rosenfeld, 2008). The number of studies conducted in tropics is very low at present, with most limited to Amazonia, where Roberts et al (2001, 2002, 2003); Andreae et al (2004); Rissler et al (2006); Vestin et al (2007); Freud et al (2008); Gunthe et al (2009) classified the Amazonian aerosol as almost “marine”; with CCN
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