Timescales of Secondary Organic Aerosols to Reach Equilibrium at Various Temperatures and Relative Humidities

Abstract

<p><strong>Abstract.</strong> Secondary organic aerosols (SOA) account for a substantial fraction of air particulate matter and SOA formation is often modeled assuming rapid establishment of gas-particle equilibrium. Here, we estimate the characteristic timescale for SOA to achieve gas−particle equilibrium under a wide range of temperatures and relative humidities using a state-of-the-art kinetic flux model. Equilibration timescales were calculated by varying particle phase state, size, mass loadings, and volatility of organic compounds. Model simulations suggest that the equilibration timescale for semi-volatile compounds is on the order of seconds or minutes for most conditions in the planetary boundary layer, but it can be longer than one hour if particles adopt glassy or amorphous solid states with high glass transition temperature at low relative humidity. In the free troposphere with lower temperatures it can be longer than hours or days even at moderate or relatively high RH due to kinetic limitations of bulk diffusion in highly viscous particles. The timescale of partitioning of low-volatile compounds is shorter compared to semi-volatile compounds, as it is largely determined by condensation sink due to very slow re-evaporation. These results provide critical insights into thermodynamic or kinetic treatments of SOA partitioning for accurate predictions of gas- and particle-phase concentrations of semi-volatile compounds in regional and global chemical transport models.</p>

Aerosol Mass and Optical Properties, Smoke Influence on O3, and High NO3 Production Rates in a Western U.S. City Impacted by Wildfires

“…One potential simple explanation is distance, in that the summer prescribed fire was closer to the Missoula valley than the wildfires impacting the valley during that same year and thus experienced less dilution‐driven evaporation. Additionally, lower surface temperatures (8–29°C) during the time of the prescribed fire impact, in comparison to temperatures during some of the wildfire impacts, may have been less conducive to PM evaporation (Li & Shiraiwa, 2019). The ~15% higher ΔPM 2.5 /ΔCO ratio for 2017 wildfire smoke in Missoula may reflect younger average smoke age (Selimovic et al, 2019).…”

Organic aerosol volatility and viscosity in North China Plain: Contrast between summer and winter

“…2017; Li and Shiraiwa, 2019;Liu et al, 2016). Figure 6 shows the 2D-VBS framework of O/C vs. log 10 C* and the correlation between T g,org and log 10 C*.…”

“…Overall, in winter of 2018 in Beijing, the OA occurs as solid with the predicted viscosity > 10 12 Pa s due to low ambient temperature and RH. The mixing time is larger than 10 3 hours, thus kinetically limited gas-particle partitioning may need to be considered when simulating SOA formation in winter in Beijing (Li and Shiraiwa, 2019;Maclean et al, 2017;Shiraiwa et al, 2011). The viscosity of OA varies from 10 2 to 10 6 Pa s in Beijing in summer and from 10 3 to 10 11 Pa s in Gucheng in winter, suggesting a semi-solid phase throughout the day.…”

Predictions of the glass transition temperature and viscosity of organic aerosols by volatility distributions

“…The SOA simulations applying the VBS framework have not included the effects of viscosity on SOA formation and evolution. When the gas-particle partitioning is in the bulk diffusion-limited regime, kinetic treatments of SOA partitioning may need to be applied (Perraud et al, 2012;Liu et al, 2016;Yli-Juuti et al, 2017;Li and Shiraiwa, 2019). Some chamber experiments probing the mixing timescales of SOA particles did not observe significant kinetic limitations at moderate and high RH under room temperature (Loza et al, 2013;Ye et al, 2018), warranting further investigations on the degree of kinetic limitations in ambient tropospheric conditions.…”