The Role of Organic Vapor in the Water Uptake of Organic Aerosols

Organic solvents are ubiquitous in the atmosphere and can readily form a coated layer around the particles. While their presence has been widely reported, their effects on cloud droplet formation have been poorly understood. This study elucidated the effect of a prominent organic solvent on the water uptake of organic aerosols under the relevant environmental conditions. Specifically, we investigated the water uptake efficiencies of aerosols obtained from three organic isomers of different solubilities, namely, phthalic acid (PTA; high solubility), isophthalic acid (IPTA; partial solubility), and terephthalic acid (TPTA; low solubility) in the presence of aqueous-phase and gas-phase ethanol (EtOH). Using laboratory-based measurements, the water uptake properties were investigated under supersaturated and subsaturated conditions by using a cloud condensation nuclei counter (CCNC) and a hygroscopicity tandem differential mobility analyzer (H-TDMA), respectively. Under supersaturated conditions (0.86%), the critical diameter (D_d) of each system was reported and compared relative to the pure compounds. Under a supersaturated environment, the presence of EtOH was shown to increase the water uptake efficiency of partial and low solubility particles, IPTA and TPTA, but not the water-soluble PTA particles. In supersaturated environments, the intrinsic solubility of aerosol particles governs its water uptake behavior. Since PTA exhibits a high-water solubility, the presence of EtOH exerted little to no influence on the dissolution of PTA particles. Hence, EtOH had no impact on the water uptake of PTA particles. This is signified by the minimal change in critical diameter (∼0.1–2 nm) between the PTA systems that contained EtOH compared with the pure PTA particles. Conversely, the addition of EtOH to IPTA and TPTA aerosols enhanced the dissolution of these partial/low-water solubility particles, as a result, increasing their water uptake affinity. The addition of ethanol to IPTA and TPTA particles resulted in a decrease in critical diameter by ∼6–8.2 and ∼16–20.1 nm, respectively. Our subsaturated results show that EtOH has the opposite effect relative to our supersaturated results. Under a subsaturated environment, the water uptake affinity of PTA particles was enhanced by the presence of EtOH and diminished for IPTA and TPTA particles. The water uptake of PTA was enhanced with the presence of aqueous ethanol (G_f increased from ∼1.02 to ∼1.23), while IPTA and TPTA particles showed reduction in size (G_f decreased from ∼1.01 to ∼0.95). This is attributed to the morphological properties of the particles supported by TEM images and shape factor measurements. Hence, atmospheric organic solvents can readily coat aerosol particles and play a major role in the water uptake properties under sub- and super-saturated conditions.