Hygroscopic Characteristics of Alkylaminium Carboxylate Aerosols

The hygroscopic growth factor (HGF) and cloud condensation nuclei (CCN) activity for a series of alkylaminium carboxylate aerosols have been measured using a hygroscopicity tandem differential mobility analyzer coupled to a condensation particle counter and a CCN counter. The particles, consisting of the mixtures of mono- (acetic, propanoic, p-toluic, and cis-pinonic acid) and di-carboxylic (oxalic, succinic, malic, adipic, and azelaic acid) acid with alkylamine (mono-, di-, and tri-methylamines), represent those commonly found under diverse environmental conditions. The hygroscopicity parameter (k) of the alkylaminium carboxylate aerosols was derived from the HGF and CCN results and theoretically calculated. The HGF at 90% RH is in the range of 1.3 to 1.8 for aminium monocarboxylates and 1.1 to 2.2 for aminium dicarboxylates, dependent on the molecular weight and functionality (i.e., the carboxylic or OH functional group in organic acids and methyl substitution in alkylamines). The k value for all aminium carboxylates is in the range of 0.06-1.37 derived from the HGF measurements at 90% RH, 0.05 – 0.49 derived from the CCN measurements, and 0.22-0.66 theoretically calculated. The measured hygroscopicity for the alkylaminium carboxylates increases with decreasing acidity. A deliquescence point is apparent for several of the aminium di-carboxylates, but not for the aminium mono-carboxylates. Our results reveal that alkylaminium carboxylate aerosols exhibit distinct hygroscopic and deliquescent characteristics that are dependent on their molecular functionality, hence regulating their impacts on human health, air quality, and direct and indirect radiative forcing on climate.