Abstract
Abstract. Water activity is a key factor in aerosol thermodynamics and hygroscopic growth. We introduce a new representation of water activity (aw), which is empirically related to the solute molality (μs) through a single solute specific constant, νi. Our approach is widely applicable, considers the Kelvin effect and covers ideal solutions at high relative humidity (RH), including cloud condensation nuclei (CCN) activation. It also encompasses concentrated solutions with high ionic strength at low RH such as the relative humidity of deliquescence (RHD). The constant νi can thus be used to parameterize the aerosol hygroscopic growth over a wide range of particle sizes, from nanometer nucleation mode to micrometer coarse mode particles. In contrast to other aw-representations, our νi factor corrects the solute molality both linearly and in exponent form x · ax. We present four representations of our basic aw-parameterization at different levels of complexity for different aw-ranges, e.g. up to 0.95, 0.98 or 1. νi is constant over the selected aw-range, and in its most comprehensive form, the parameterization describes the entire aw range (0–1). In this work we focus on single solute solutions. νi can be pre-determined with a root-finding method from our water activity representation using an aw−μs data pair, e.g. at solute saturation using RHD and solubility measurements. Our aw and supersaturation (Köhler-theory) results compare well with the thermodynamic reference model E-AIM for the key compounds NaCl and (NH4)2SO4 relevant for CCN modeling and calibration studies. Envisaged applications include regional and global atmospheric chemistry and climate modeling.
Highlights
The gas-liquid-solid partitioning of atmospheric particles and precursor gases determines to a large degree the composition and water uptake of atmospheric aerosol particles, which affect human and ecosystem health, clouds and climate (e.g. Kunzli et al, 2000; IPCC, 2007)
We present four different representations of our basic aw-parameterization to accommodate different aw-ranges relevant for general circulation models (GCMs) applications. νi is constant for a given temperature over the aw-range, and in its most comprehensive form, the parameterizations describes the entire aw range (0–1). νi is predetermined with a root-finding method using RH of deliquescence (RHD) and solubility measurements
A closer inspection of the numerics used by these methods shows that they have in common the use of one class of fitting function type that is combined with a parameter to correct the solute molality μs for non-ideality, i.e. the Van’t Hoff factor model (VH) and activity coefficient model (AC) models use a rational function approach, whereas the osmotic coefficient model (OS) model uses an exponential fit
Summary
The gas-liquid-solid partitioning of atmospheric particles and precursor gases determines to a large degree the composition and water uptake of atmospheric aerosol particles, which affect human and ecosystem health, clouds and climate (e.g. Kunzli et al, 2000; IPCC, 2007). Three types of methods have been used to account for hygroscopic growth of atmospheric aerosols in general circulation models (GCMs): (i) the f (RH) method, (ii) Kohlertheory and (iii) thermodynamic equilibrium models. This method accounts for the hygroscopic nature of water-soluble aerosol particles and, has been used for first-order estimates of aerosol HG and the corresponding radiative forcing of climate. The second method explicitly accounts for the hygroscopic nature, since the Kohler equation is based on the Raoult-term Both methods do not explicitly account for gas-liquid-solid partitioning and deliquescence that accompanies aerosol hygroscopic growth. Models that account for the gas-liquid-solid partitioning of single and mixed solute solutions can calculate the RHD based HG factor (HGF) of single and mixed solutions, which usually includes various inorganic, organic and non-soluble compounds. A comprehensive box model inter-comparison of major inorganic aerosol thermodynamic properties of mixed solutions predicted by EQSAM4, which applies the parameterizations presented here, and EQUISOLV II (Jacobson et al, 1996, 1999) is the subject of a separate publication
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