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Water activity is a key factor in aerosol thermodynamics and hygroscopic growth. We introduce a new representation of water activity (αw), 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 solutemolality both linearly and in exponent form x · αx. 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 αw-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 αw 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. [ABSTRACT FROM AUTHOR]

The article discusses a study which evaluates the feasibility and quality of Eddy-Covariance flux measurements from a weight-shift microlight aircraft (WSMA). It examines the precision of the wind measurement, the lynchpin of flux calculations from aircraft. It compares measurements of wind, temperature, humidity and respective fluxes between a tall tower and the WSMA.

The article presents a study on the fourth version of Equilibrium Simplified Aerosol Model (EQSAM4). It mentions that the model is part of an aerosol chemistry-microphysics module (GMXe) and chemistry-climate model (EMAC). It notes the atmospheric aerosols' relative humidity of deliquescence (RHD) based water uptake which is significant for atmospheric chemistry and climate modeling. An overview of its parameterization of the aerosols through gas-liquid-solid partitioning is also presented.

Modeling atmospheric aerosol and cloud microphysics is rather complex, even if chemical and thermodynamical equilibrium is assumed. We show, however, that the thermodynamics can be considerably simplified by reformulating equilibrium to consistently include water, and transform laboratory-based concepts to atmospheric conditions. We generalize the thermodynamic principles that explain hydration and osmosis -- merely based on solute solubilities -- to explicitly account for the water mass consumed by hydration. As a result, in chemical and thermodynamical equilibrium the relative humidity (RH) suffices to determine the saturation molality, including solute and solvent activities (and activity coefficients), since the water content is fixed by RH for a given aerosol concentration and type. As a consequence, gas/liquid/solid aerosol equilibrium partitioning can be solved analytically and non-iteratively. Our new concept enables an efficient and accurate calculation of the aerosol water mass and directly links the aerosol hygroscopic growth to fog, haze and cloud formation. We apply our new concept in the 3rd Equilibrium Simplified Aerosol Model (EQSAM3) for use in regional and global chemistry-transport and climate models. Its input is limited to the species' solubilities from which a newly introduced stoichiometric coefficient for water is derived. Analogously, we introduce effective stoichiometric coefficients for the solutes to account for complete or incomplete dissociation. We show that these coefficients can be assumed constant over the entire activity range and calculated for various inorganic, organic and non-electrolyte compounds, including alcohols, sugars and dissolved gases. EQSAM3 calculates the aerosol composition and gas/liquid/solid partitioning of mixed inorganic/organic multicomponent solutions and the associated water uptake for almost 100 major compounds. It explicitly accounts for particle hygroscopic growth by computing aerosol properties such as single solute molalities, molal based activities, including activity coefficients for volatile compounds, efflorescence and deliquescence relative humidities of single solute and mixed solutions. Various applications and a model inter-comparison indicate that a) the application is not limited to dilute binary solutions, b) sensitive aerosol properties such as hygroscopic growth and the pH of binary and mixed inorganic/organic salt solutions up to saturation can be computed accurately, and c) aerosol water is central in modeling atmospheric chemistry, visibility, weather and climate. [ABSTRACT FROM AUTHOR]