Abstract
Abstract. Climatic effects of newly-formed atmospheric secondary aerosol particles are to a large extent determined by their condensational growth rates. However, all the vapours condensing on atmospheric nanoparticles and growing them to climatically relevant sizes are not identified yet and the effects of particle phase processes on particle growth rates are poorly known. Besides sulfuric acid, organic compounds are known to contribute significantly to atmospheric nanoparticle growth. In this study a particle growth model MABNAG (Model for Acid-Base chemistry in NAnoparticle Growth) was developed to study the effect of salt formation on nanoparticle growth, which has been proposed as a potential mechanism lowering the equilibrium vapour pressures of organic compounds through dissociation in the particle phase and thus preventing their evaporation. MABNAG is a model for monodisperse aqueous particles and it couples dynamics of condensation to particle phase chemistry. Non-zero equilibrium vapour pressures, with both size and composition dependence, are considered for condensation. The model was applied for atmospherically relevant systems with sulfuric acid, one organic acid, ammonia, one amine and water in the gas phase allowed to condense on 3–20 nm particles. The effect of dissociation of the organic acid was found to be small under ambient conditions typical for a boreal forest site, but considerable for base-rich environments (gas phase concentrations of about 1010 cm−3 for the sum of the bases). The contribution of the bases to particle mass decreased as particle size increased, except at very high gas phase concentrations of the bases. The relative importance of amine versus ammonia did not change significantly as a function of particle size. While our results give a reasonable first estimate on the maximum contribution of salt formation to nanoparticle growth, further studies on, e.g. the thermodynamic properties of the atmospheric organics, concentrations of low-volatility organics and amines, along with studies investigating the applicability of thermodynamics for the smallest nanoparticles are needed to truly understand the acid-base chemistry of atmospheric nanoparticles.
Highlights
Atmospheric aerosol particles affect the climate by scattering solar radiation and by acting as cloud condensation nuclei (CCN)
We focus on four research questions: (1) what concentrations of organic acid and amine are needed to explain the atmospheric nanoparticle growth rates when acidbase chemistry is taken into account and what should the saturation vapour pressure of the organic acid be; (2) what are the relative roles of ammonia and amine in the salt formation and particle growth; (3) how does the relative humidity affect the salt formation and particle growth; and (4) how do the properties of the organic acid affect the salt formation and particle growth
In simulation set 1, concentrations of the organic acid and amine and saturation vapour pressure of the organic acid were varied while concentrations of sulfuric acid and ammonia and relative humidity (RH) were kept constant in order to study the concentrations of organic acid and amine needed for atmospheric nanoparticle growth
Summary
Atmospheric aerosol particles affect the climate by scattering solar radiation and by acting as cloud condensation nuclei (CCN). Many of the oxidation products of organic vapours identified in the atmosphere have higher saturation vapour pressures than required for condensation on nanoparticles (Goldstein and Galbally, 2007), and short-chain organic acids as well as aliphatic amines that have higher saturation vapour pressures have been observed in nanoparticles (Smith et al, 2010; Laitinen et al, 2011) This suggests that gas phase oxidation and reversible condensation are not the only processes explaining nanoparticle growth (see Donahue et al, 2011; Pierce et al, 2011) and that particle phase processes, like polymerization (Limbeck et al, 2003) and salt formation (Barsanti et al, 2009), may have an important role in lowering the volatility of condensing organic compounds. We focus on four research questions: (1) what concentrations of organic acid and amine are needed to explain the atmospheric nanoparticle growth rates when acidbase chemistry is taken into account and what should the saturation vapour pressure of the organic acid be; (2) what are the relative roles of ammonia and amine in the salt formation and particle growth; (3) how does the relative humidity affect the salt formation and particle growth; and (4) how do the properties of the organic acid affect the salt formation and particle growth
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