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

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.

Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols

“…Recent measurements by Marsh et al (2017) using the comparative kinetics technique applied in an electrodynamic balance (CK-EDB) also report hygroscopicity data for methylsuccinic acid (see Fig. 1) and the performance of the UNIQUAC Functional-group Activity Coefficients (UNI-FAC) model in comparison to new measurements of pure and substituted dicarboxylic acids, sugars, and amino acids.…”

“…An individual droplet (∼ 10 µm in diameter) was isolated and held in an optical trap, in which it was exposed to a controlled humidity profile ranging from about 25 to 87 % RH at a temperature of 295 K. The droplet size and refractive index, with a measurement precision of ap- proximately 1 nm and 0.05 %, respectively, were monitored using the wavelength position of cavity-enhanced resonances in the Raman spectrum and the sizing algorithms of Preston and Reid (2013). An offline calibration was performed to correlate the mass fraction of solute (mfs) (here the organic component is the solute and water is the solvent) to the refractive index using bulk solutions and a digital refractometer (Atago PAL-RI), along with the refractive index value determined for the liquid-state pure-solute component from the work by Marsh et al (2017). Using the dispersion parameters from the sizing algorithms (Preston and Reid, 2013), the refractive index determined in the AOT was corrected to the wavelength of calibration and the mfs was determined as a function of RH for each measured value of refractive index.…”

“…In these equilibrium measurements, the RH is equivalent to the particle-phase water activity. Methylsuccinic acid is a compound with a deliquescence point at 95.5 % RH upon humidification if the aerosols are initially solid (Marcolli et al, 2004). In the hygroscopicity measurements, methylsuccinic acid droplets absorb or desorb water in a reversible manner in response to the set environmental RH.…”

“…In contrast to the good agreement between experimental data and model predictions in this study, the CK-EDB measurements seem to slightly overpredict the hygroscopicity (i.e., smaller mfs at a given RH) compared to both the AIOM-FAC model and the AOT experimental data, even though the experimental data mostly agree within the stated uncertainties. The CK-EDB method would observe a smaller mfs at a given water activity if subtle particle-phase diffusion limitations slowed the evaporation rate of water from droplets to an extent not considered in the mass flux framework used by Marsh et al (2017), in which homogeneous mixing of organic solute and water is assumed in the droplet bulk during net evaporation conditions. If such effects were to play a role, at a given droplet size and mfs, the evaporation rate would tend to be slower and the data evaluation using the mass flux equation would predict a smaller droplet water activity.…”

“…Depending on the particle composition and environmental conditions, such as RH and temperature, particle-phase water can be an important component of aqueous organic droplets (Peng et al, 2001;Brooks et al, 2002;Braban et al, 2003;Marcolli et al, 2004;Mochida and Kawamura, 2004;Parsons et al, 2004;Chan et al, 2005Chan et al, , 2008Mikhailov et al, 2009;Ma et al, 2013;Ganbavale et al, 2014;Chen et al, 2015;Jing et al, 2016). Upon oxidation, the amount of particlephase water can continuously vary in response to changes in particle composition.…”