Droplet activation of wet particles: development of the Wet CCN approach

S Nakao,S M Kreidenweis,M D Petters,S R Suda,M Camp

doi:10.5194/amt-7-2227-2014

Abstract

Abstract. Relationships between critical supersaturation required for activation and particle dry diameter have been the primary means for experimentally characterizing cloud condensation nuclei (CCN) activity; however, use of the dry diameter inherently limits the application to cases where the dry diameter can be used to accurately estimate solute volume. This study challenges the requirement and proposes a new experimental approach, Wet CCN, for studying CCN activity without the need for a drying step. The new approach directly measures the subsaturated portion of the Köhler curves. The experimental setup consists of a humidity-controlled differential mobility analyzer and a CCN counter; wet diameter equilibrated at known relative humidity is used to characterize CCN activity instead of the dry diameter. The experimental approach was validated against ammonium sulfate, glucose, and nonspherical ammonium oxalate monohydrate. Further, the approach was applied to a mixture of nonspherical iodine oxide particles. The Wet CCN approach successfully determined the hygroscopicity of nonspherical particles by collapsing them into spherical, deliquesced droplets. We further show that the Wet CCN approach offers unique insights into the physical and chemical impacts of the aqueous phase on CCN activity; a potential application is to investigate the impact of evaporation/co-condensation of water-soluble semivolatile species on CCN activity.

Highlights

In addition to their key roles in the prediction of climate changes (IPCC, 2007), clouds are important drivers of atmospheric chemistry, impacting aerosol formation, modification, and removal (Graedel and Weschler, 1981; Blando and Turpin, 2000; Ervens et al, 2011)
The Wet cloud condensation nuclei (CCN) approach was applied to ammonium oxalate monohydrate particles as an example system of nonspherical crystals (Hori et al, 2003), for which the Dry CCN method will overestimate solute volume and underestimate κ
This study developed the conceptual basis of the Wet CCN technique, based on simple modifications of existing conventional approaches, which we term Dry CCN methods

Summary

Introduction

In addition to their key roles in the prediction of climate changes (IPCC, 2007), clouds are important drivers of atmospheric chemistry, impacting aerosol formation, modification, and removal (Graedel and Weschler, 1981; Blando and Turpin, 2000; Ervens et al, 2011). The number concentration of available CCN influences cloud droplet size, and thereby reflectivity (Twomey, 1974), as well as cloud lifetime (Albrecht, 1989). The equilibrium between water vapor and a watercontaining atmospheric aerosol particle depends on the particle (or droplet) size and its physicochemical parameters, i.e., moles of dissolved molecules (or dissociated ions), solution non-ideality, and surface tension (Pruppacher and Klett, 1997; Seinfeld and Pandis, 2006); for solid particles that take up water via an adsorption process, surface properties are important (Sorjamaa and Laaksonen, 2007; Kumar et al, 2009, 2011). Organic compounds are recognized as important constituents of CCN due to their abundance and water solubility (Novakov and Penner, 1993). The chemical composition of organic-containing CCN remains largely unknown since aerosol-phase organics comprise numerous compounds and only around 10–30 % of these can be identified by current analytical techniques (Hallquist et al, 2009)

Objectives

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