Hygroscopicity of nitrogen-containing organic carbon compounds: o-aminophenol and p-aminophenol

Nitrogen-containing Organic Carbon (NOC) is a major constituent of atmospheric aerosols and they have received significant attention in the atmospheric science community. While extensive research and advancements have been made regarding their emission sources, concentrations, and their secondary formation in the atmosphere, little is known about their water uptake efficiencies and their subsequent role in climate, air quality, and visibility. In this study, we investigated the water uptake of two sparingly soluble aromatic NOCs: o-aminophenol (oAP) and p-aminophenol (pAP) under subsaturated and supersaturated conditions using a Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) and a Cloud Condensation Nuclei Counter (CCNC), respectively. Our results show that oAP and pAP are slightly hygroscopic with comparable hygroscopicities to various studied organic aerosols. The supersaturated single hygroscopicity parameter (κ_CCN) was measured and reported to be 0.18 ± 0.05 for oAP and 0.04 ± 0.02 for pAP, indicating that oAP is more hygroscopic than pAP despite them having the same molecular formulae. The observed disparity in hygroscopicity is attributed to the difference in functional group locations, interactions with gas phase water molecules, and the reported bulk water solubilities of the NOC. Under subsaturated conditions, both oAP and pAP aerosols showed size dependent water uptake. Both species demonstrated growth at smaller dry particle sizes, and shrinkage at larger dry particle sizes. The measured growth factor (G_f) range, at RH = 85%, for oAP was 1.60-0.74 and for pAP was 1.53-0.74 with increasing particle size. The growth and shrinkage dichotomy is attributed to morphological particle differences verified by TEM images of small and large particles. Subsequently, aerosol physicochemical properties must be considered to properly predict the droplet growth of NOC aerosols in the atmosphere.