Phase Behavior of Hydrocarbon-like Primary Organic Aerosol and Secondary Organic Aerosol Proxies Based on Their Elemental Oxygen-to-Carbon Ratio

Abstract

A large fraction of atmospheric aerosols can be characterized as primary organic aerosol (POA) and secondary organic aerosol (SOA). Knowledge of the phase behavior, that is, the number and type of phases within internal POA + SOA mixtures, is crucial to predict their effect on climate and air quality. For example, if POA and SOA form a single phase, POA will enhance the formation of SOA by providing organic mass to absorb SOA precursors. Using microscopy, we studied the phase behavior of mixtures of SOA proxies and hydrocarbon-like POA proxies at relative humidity (RH) values of 90%, 45%, and below 5%. Internal mixtures of POA and SOA almost always formed two phases if the elemental oxygen-to-carbon ratio (O/C) of the POA was less than 0.11, which encompasses a large fraction of atmospheric hydrocarbon-like POA from fossil fuel combustion. SOA proxies mixed with POA proxies having 0.11 ≤ O/C ≤ 0.29 mostly resulted in particles with one liquid phase. However, two liquid phases were also observed, depending on the type of SOA and POA surrogates, and an increase in phase-separated particles was observed when increasing the RH in this O/C range. The results have implications for predicting atmospheric SOA formation and policy strategies to reduce SOA in urban environments.

Phase Behavior of Internal Mixtures of Hydrocarbon-like Primary Organic Aerosol and Secondary Aerosol Based on Their Differences in Oxygen-to-Carbon Ratios

“…63−67 Recently, we used microscopy techniques to investigate the phase behavior of internal mixtures of hydrocarbon-like POA proxies and single-component SOA proxies. 68 We showed that two liquid phases were almost always observed when POA proxies having O/C < 0.11 were mixed with SOA proxies having 0.3 ≤ O/C ≤ 1.0. In the same study, we also found that one or two liquid phases were observed when POA proxies with 0.11 ≤ O/C ≤ 0.29 were mixed with the same SOA proxies depending on the type of the proxies.…”

“…We used 11 POA proxies and examined the phase behavior of each proxy when mixed with 7 different SOA materials generated in environmental chambers, resulting in 77 unique types of internal POA + SOA mixtures studied. 68 The O/C ratios of the POA materials ranged from 0 to 0.29, covering the O/C range observed for atmospheric HOA. 20 The O/C ratios of SOA ranged from 0.37 to 1.08, covering a large fraction of the O/C range observed for atmospheric SOA (0.32−1.32).…”

“…Environmental Science & Technology pubs.acs.org/est Article internally mixed POA + SOA particles. 68 In Figure S9, we have replotted the data from our previous study and furthermore indicated the regions where mostly one-and two-phase POA + SOA particles were observed (red and blue hashed regions). Overall, the general trends observed in our previous studies are consistent with the general trends observed in the current study.…”

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…25,158 Additional information about chemical speciation is necessary to determine morphology for particles with intermediate O : C values, 25,158 and these guidelines do have exceptions. 157,159,160…”

“…25,158 Additional information about chemical speciation is necessary to determine morphology for particles with intermediate O : C values, 25,158 and these guidelines do have exceptions. 157,159,160 Phase separation is common for aqueous organic aerosol in the atmosphere. 158 Lee et al 161 generated nascent SSA in a wave ume using seawater from the California coast, and they found that the core-shell morphology accounted for 31.2% of all particles detected in the 100 nm to 1.8 mm size range.…”