Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity

Abstract

The mechanistic pathway by which high relative humidity (RH) affects gas–particle partitioning remains poorly understood, although many studies report increased secondary organic aerosol (SOA) yields at high RH. Here, we use real-time, molecular measurements of both the gas and particle phase to provide a mechanistic understanding of the effect of RH on the partitioning of biogenic oxidized organic molecules (from α-pinene and isoprene) at low temperatures (243 and 263 K) at the CLOUD chamber at CERN. We observe increases in SOA mass of 45 and 85% with increasing RH from 10–20 to 60–80% at 243 and 263 K, respectively, and attribute it to the increased partitioning of semi-volatile compounds. At 263 K, we measure an increase of a factor 2–4 in the concentration of C10H16O2–3, while the particle-phase concentrations of low-volatility species, such as C10H16O6–8, remain almost constant. This results in a substantial shift in the chemical composition and volatility distribution toward less oxygenated and more volatile species at higher RH (e.g., at 263 K, O/C ratio = 0.55 and 0.40, at RH = 10 and 80%, respectively). By modeling particle growth using an aerosol growth model, which accounts for kinetic limitations, we can explain the enhancement in the semi-volatile fraction through the complementary effect of decreased compound activity and increased bulk-phase diffusivity. Our results highlight the importance of particle water content as a diluting agent and a plasticizer for organic aerosol growth.

The effect of temperature and relative humidity on secondary organic aerosol formation from ozonolysis of Δ3-carene

“…The increase in cis-3-caric acid could potentially be linked to enhanced partitioning of cis-3-caric acid as the bulk diffusivity increases and the viscosity decreases with increasing RH. 49 Surprisingly, we observe that the mass fraction of cis-3-caronic acid found in the particle phase at 80% RH is lower than the mass fraction of cis-3-caronic acid at 0% RH. Surdu et al 49 discussed the effect of water on the gas-particle partitioning of semi-volatiles in detail.…”

“…Later studies investigated the temperature effects on HOM formation and found that it decreases with decreasing temperature both in the range from −15 to 20 °C 45 and from −50 to 25 °C. 46 Surdu et al 49 investigated the effect of relative humidity on SOA yield and chemical composition from ozonolysis of α-pinene at −10 and −30 °C. They observed an increase in the SOA yield with increasing relative humidity and attributed this enhancement to increased partitioning of semi-volatile organics.…”

Predicting Atmospheric Water-Soluble Organic Mass Reversibly Partitioned to Aerosol Liquid Water in the Eastern United States

“…The trend in Figure S6b shows that the evaporated WSOM mass increases as the WSOM concentration increases during all three summers. In the summer of 2015, the average amount of evaporated WSOM increased considerably from 0.17 ± 0.17 μg m –3 to 1.53 ± 1.07 μg m –3 as the concentration of WSOM increased from <2 μg m –3 to >10.5 μg m –3 . In the summer of 2016, the average amount of evaporated WSOM was 0.61 ± 0.34 μg m –3 at WSOM < 2 μg m –3 and 3.36 ± 2.23 μg m –3 at WSOM > 10.5 μg m –3 (Table S3).…”

“…In the summer of 2015, the average amount of evaporated WSOM increased considerably from 0.17 ± 0.17 μg m −3 to 1.53 ± 1.07 μg m −3 as the concentration of WSOM increased from <2 μg m −3 to >10.5 μg m −3 . 16 In the summer of 2016, the average amount of evaporated WSOM was 0.61 ± 0.34 μg m −3 at WSOM < 2 μg m −3 and 3.36 ± 2.23 μg m −3 at WSOM > 10.5 μg m −3 (Table S3). The lowest increases were observed in summer 2017 from 0.11 ± 0.10 μg m −3 to 0.90 ± 0.72 μg m −3 in the same WSOM concentration range (Table S3).…”

“…This phenomenon, in turn, leads to changes in both the composition and mass concentration of organic matter in the atmosphere. Multiple studies have observed the importance of this multiphase chemistry on the formation of SOA (e.g., refs − ).…”