Influence of organic compound functionality on aerosol hygroscopicity: dicarboxylic acids, alkyl-substituents, sugars and amino acids

Aleksandra Marsh,Jonathan P Reid,Rachael E H Miles,Lucy Nandy,Cari S Dutcher,Grazia Rovelli,Alexander G Cowling

doi:10.5194/acp-17-5583-2017

Abstract

Abstract. Hygroscopicity data for 36 organic compounds, including amino acids, organic acids, alcohols and sugars, are determined using a comparative kinetics electrodynamic balance (CK-EDB). The CK-EDB applies an electric field to trap-charged aqueous droplets in a chamber with controlled temperature and relative humidity (RH). The dual micro dispenser set-up allows for sequential trapping of probe and sample droplets for accurate determination of droplet water activities from 0.45 to > 0.99. Here, we validate and benchmark the CK-EDB for the homologous series of straight-chain dicarboxylic acids (oxalic–pimelic) with measurements in better agreement with Universal Quasichemical Functional Group Activity Coefficients (UNIFAC) predictions than the original data used to parametrise UNIFAC. Furthermore, a series of increasingly complex organic compounds, with subtle changes to molecular structure and branching, are used to rigorously assess the accuracy of predictions by UNIFAC, which does not explicitly account for molecular structure. We show that the changes in hygroscopicity that result from increased branching and chain length are poorly represented by UNIFAC, with UNIFAC under-predicting hygroscopicity. Similarly, amino acid hygroscopicity is under-predicted by UNIFAC predictions, a consequence of the original data used in the parametrisation of the molecular subgroups. New hygroscopicity data are also reported for a selection of alcohols and sugars and they show variable levels of agreement with predictions.

Highlights

The hygroscopicity of an aerosol can be defined as the capacity of an aerosol particle to absorb water, and it quantifies the equilibrium partitioning of water between the gas and condensed phases (Krieger et al, 2012)
The activation of cloud condensation nuclei (CCN) to form cloud droplets is governed by hygroscopic response as well as aerosol size distribution, leading to the indirect effect of aerosols on climate (Farmer et al, 2015; Lohmann and Feichter, 2005)
The comparative kinetics electrodynamic balance (CK-electrodynamic balance (EDB)) can be used to probe the hygroscopic growth of aerosol particles from low to high water activities (< 0.45 to > 0.99) with a greater accuracy (< ±0.2 % error in water activity at water activities > 0.8 and ±1 % error in water activity at water activities < 0.8) than can be achieved in conventional approaches (Rovelli et al, 2016)

Summary

Introduction

The hygroscopicity of an aerosol can be defined as the capacity of an aerosol particle to absorb water, and it quantifies the equilibrium partitioning of water between the gas and condensed phases (Krieger et al, 2012). Aerosol hygroscopic growth directly impacts the radiative balance of the atmosphere, with the size and refractive index of aerosol particles influencing their light scattering and absorption cross sections (Ravishankara et al, 2015; Moise et al, 2015). Hygroscopic growth on inhalation can influence the depth of penetration of aerosol into the respiratory system, with consequences for the impact of ambient aerosol and particulate matter on rates of morbidity and mortality (Haddrell et al, 2015; Pöschl and Shiraiwa, 2015). An improved characterisation and quantification of the hygroscopic response of ambient aerosol is important for more accurate predictions of the radiative forcing of aerosol, their impact on air quality and their consequences for human health

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Conclusion