Abstract
Abstract. Slightly soluble atmospherically relevant organic compounds may influence particle CCN activity and therefore cloud formation. Adipic acid is a frequently employed surrogate for such slightly soluble organic materials. The 11 published experimental studies on the CCN activity of adipic acid particles are not consistent with each other nor do they, in most cases, agree with the Köhler theory. The CCN activity of adipic acid aerosol particles was studied over a significantly wider range of conditions than in any previous single study. The work spans the conditions of the previous studies and also provides alternate methods for producing "wet" (deliquesced solution droplets) and dry adipic acid particles without the need to produce them by atomization of aqueous solutions. The experiments suggest that the scatter in the previously published CCN measurements is most likely due to the difficulty of producing uncontaminated adipic acid particles by atomization of solutions and possibly also due to uncertainties in the calibration of the instruments. The CCN activation of the small (dm<150 nm) initially dry particles is subject to a deliquescence barrier, while for the larger particles the activation follows the Köhler curve. Wet adipic acid particles follow the Köhler curve over the full range of particle diameters studied. In addition, the effect of adipic acid coatings on the CCN activity of both soluble and insoluble particles has also been studied. When a water-soluble core is coated by adipic acid, the CCN-hindering effect of particle phase is eliminated. An adipic acid coating on hydrophobic soot yields a CCN active particle. If the soot particle is relatively small (dcore≤102 nm), the CCN activity of the coated particles approaches the deliquescence line of adipic acid, suggesting that the total size of the particle determines CCN activation and the soot core acts as a scaffold.
Highlights
As has been discussed in several publications, the effect on climate of aerosol radiative forcing may be as large as that of the greenhouse gases, but opposite in sign and much more uncertain (See for example, Kaufman and Koren, 2006; Schwartz et al, 2007; IPCC, 2007)
cloud condensation nuclei (CCN) activity depends on the shape, size, structure, and composition of the aerosol particles (McFiggans et al, 2005; Dusek et al, 2006)
The results of the experiments lead to the following conclusions: 1. In accord with earlier suggestions (Hori et al, 2003, and Kreidenweis et al, 2006), the CCN activation of small, dry slightly soluble organic particles is governed by the deliquescence of the particles, and CCN activation follows the deliquescence curve for the particles
Summary
As has been discussed in several publications, the effect on climate of aerosol radiative forcing may be as large as that of the greenhouse gases, but opposite in sign and much more uncertain (See for example, Kaufman and Koren, 2006; Schwartz et al, 2007; IPCC, 2007). The high uncertainties are due to the currently inadequate representation of the complex interactions of aerosols with climate due to their intrinsically complex composition, morphology, and size distributions. Atmospheric aerosol particles influence global climate by direct and indirect processes (Lohmann and Feichter, 2005). The direct process of aerosol climate-forcing involves the absorption, reflection, and scattering of incoming solar radiation by atmospheric aerosols. The indirect process involves atmospheric particles serving as cloud condensation nuclei (CCN) on which water vapor condenses to form cloud droplets. CCN activity depends on the shape, size, structure, and composition of the aerosol particles (McFiggans et al, 2005; Dusek et al, 2006). An aspect of CCN activity is the subject of this work
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