Uptake of Organic Molecules by Atmospheric Particles

Abstract

The uptake of molecules onto atmospheric particles can be influenced by particle viscosity and physical state because of effects on species diffusivity. Relative humidity is a key regulator. Herein, the uptake of gas-phase levoglucosan and 2,4-dinitrophenol was studied from 10% to >95% relative humidity (RH). The uptake was studied for organic particles produced by α-pinene ozonolysis as well as ammonium sulfate salt particles. Levoglucosan and 2,4-dinitrophenol are two medium-sized, atmospherically relevant organic molecules, both emitted to the atmosphere primarily by biomass burning. The results show that levoglucosan uptake depended strongly on particle physical state. For aqueous sulfate particles, uptake increased continuously for higher particle water content at higher RH, tripling from 40% to >95% RH. The implied effective Henry’s law constant changed from (1.2 ± 0.1) × 10^8 to (2.1 ± 0.2) × 10^8 M atm^-1 across this RH range. For the organic particles, levoglucosan uptake increased from 10% to 40% RH in conjunction with a decrease in viscosity and an increase in molecular diffusivity. The implication is a kinetic limit on uptake across this RH range. The estimated diffusivity at 40% RH was 10^(19±0.05) m^2 s^-1. The uptake saturated above 40% RH, suggesting a transition to a thermodynamic regime that regulated maximum uptake. The effective Henry’s law constant was (1.2 ± 0.1) × 10^8 M atm^-1. By comparison, the uptake of 2,4-dinitrophenol onto the organic particles was not significant across the studied RH range. The explanation may be the vapor pressure of 2,4-dinitrophenol was too high to favor significant uptake, suggesting an absence of strong effects on activity coefficients within the organic particle matrix. The effective Henry’s law constant should be less than 3.9 × 10^5 M atm^-1 at 80% to explain the observations. The results reported herein highlight that molecular uptake of gases onto particles can be regulated by both kinetic and thermodynamic factors, either of which can limit the uptake of medium-sized organic molecules by atmospherically relevant particles.