Influence of Particle Physical State on the Uptake of Medium-Sized Organic Molecules

The uptake of medium-sized levoglucosan and 2,4-dinitrophenol to organic particles produced by α-pinene ozonolysis and to ammonium sulfate particles was studied from 10% to >95% relative humidity (RH). For aqueous sulfate particles, the water-normalized gas-particle partitioning coefficient of levoglucosan decreased from (1.0 ± 0.1) × 10^-3 to (0.2 ± 0.1) × 10^-3 (ng μg^-1)_particle/(ng m^-3)_gas from 40% to >95% RH, suggestive of a salting-in mechanism between levoglucosan and ionic ammonium sulfate solutions. For the organic particles, the levoglucosan partitioning coefficient increased from 10% to 40% RH and became invariant at (2.0 ± 0.4) × 10^-3 (ng μg^-1)/(ng m^-3) above 40% RH. A kinetic limitation on uptake below 40% RH was implied, compared to a thermodynamic regime above 40% RH. The estimated diffusivity was 10^-19±0.05 m² s^-1 at 40% RH. By comparison, the uptake of 2,4-dinitrophenol onto the organic particles was below detection limit, implying an upper limit on the partitioning coefficient of 6.8 × 10^-6 (ng μg^-1)/(ng m^-3) at 80% RH. The results highlight that the 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.