Role of Aerosol Liquid Water in Secondary Organic Aerosol Formation from Volatile Organic Compounds

Abstract

A key mechanism for atmospheric secondary organic aerosol (SOA) formation occurs when oxidation products of volatile organic compounds condense onto pre-existing particles. Here, we examine effects of aerosol liquid water (ALW) on relative SOA yield and composition from α-pinene ozonolysis and the photooxidation of toluene and acetylene by OH. Reactions were conducted in a room-temperature flow tube under low-NO conditions in the presence of equivalent loadings of deliquesced (∼20 μg m ALW) or effloresced (∼0.2 μg m ALW) ammonium sulfate seeds at exactly the same relative humidity (RH = 70%) and state of wall conditioning. We found 13% and 19% enhancements in relative SOA yield for the α-pinene and toluene systems, respectively, when seeds were deliquesced rather than effloresced. The relative yield doubled in the acetylene system, and this enhancement was partially reversible upon drying the prepared SOA, which reduced the yield by 40% within a time scale of seconds. We attribute the high relative yield of acetylene SOA on deliquesced seeds to aqueous partitioning and particle-phase reactions of the photooxidation product glyoxal. The observed range of relative yields for α-pinene, toluene, and acetylene SOA on deliquesced and effloresced seeds suggests that ALW plays a complicated, system-dependent role in SOA formation.

Effect of relative humidity on the composition of secondary organic aerosol from the oxidation of toluene

“…This was attributed to the aqueous partitioning and subsequent reactions in ALW of smaller photooxidation products, such as glyoxal (Faust et al, 2017). Our results suggest that the increase in the SOA yield due to the ALW-supported chemistry is at least partly counteracted by the lower yield of oligomers under high-RH conditions.…”

“…In previous studies on the effect of RH on SOA yield from toluene in the presence of hygroscopic seed Kamens et al, 2011;Faust et al, 2017;Liu et al, 2017;Jia and Xu, 2017), an opposite effect was observed in which the SOA yield increased at high RH. This was attributed to the aqueous partitioning and subsequent reactions in ALW of smaller photooxidation products, such as glyoxal (Faust et al, 2017).…”

“…Most of these studies focused on the effect of RH on SOA yield (Edney et al, 2000;Zhou et al, 2011;Cocker III et al, 2001;Kamens et al, 2011;Cao and Jang, 2010;Faust et al, 2017;Liu et al, 2017;Jia and Xu, 2017) and not the chemical composition of SOA particles. The comparison between different experiments is complicated by the fact that some experiments are done in the presence of hygroscopic seed particles, where ALW may be playing a role; others are done with seed particles containing strong acids, which favor acid-catalyzed chemistry, and others use no seed particles.…”

“…The comparison between different experiments is complicated by the fact that some experiments are done in the presence of hygroscopic seed particles, where ALW may be playing a role; others are done with seed particles containing strong acids, which favor acid-catalyzed chemistry, and others use no seed particles. For toluene SOA produced in presence of hygroscopic seed particles, the yield is generally found to be larger under high-RH conditions Kamens et al, 2011;Faust et al, 2017;Liu et al, 2017;Jia and Xu, 2017) because additional organic compounds are produced by the aqueous photochemistry of small highly soluble compounds, such as glyoxal, partitioned in ALW. However, Cao and Jang (2010) observed a negative correlation between RH and SOA yield for low-NO x experiments; i.e., lower RH resulted in higher SOA yields.…”

Intermittent turbulence contributes to vertical diffusion of PM<sub>2.5</sub> in the North China Plain

“…3b and 3c for Case-2 verify this conclusion as well. Besides, an ambience with high relative humidity (RH) is favorable for the increase of PM2.5 concentration in the ABL through secondary formation by heterogeneous reactions (Quan et al, 2015;Wang et al, 2012;Faust et al, 2017) and hygroscopic growth (Engelhart et al, 2011;Petters and Kreidenweis, 2008). For Case-1, the RH during the CS keeps high with a mean value of 53% but sharply 25 falls into a very low level once entering the TS.…”