Potential role of intraparticle diffusion in dynamic partitioning of secondary organic aerosols T

Özet

Dynamic gas-particle partitioning for secondary organic aerosols (SOA) is modeled by incorporating all the three processes: external mass transfer from bulk air to particle surfaces, intraparticle diffusion (characterized by diffusion coefficient, D p ), and particle-phase reaction (characterized by first-order rate constant, k c ). A fully analytical solution to this model is provided to examine role of intraparticle diffusion. Based on the solution, a criterion is provided to judge whether intraparticle diffusion can be neglected or not with particle-phase reaction accounted for. Results indicate that taking particle-phase reaction into consideration allows a wider range to neglect intraparticle diffusion. Sensitivity analysis shows that for a fast reaction case (k c = 10 −1 /s), intraparticle diffusion can be reasonably neglected for D p higher than 10 −19 m 2 /s and gas-particle partition coefficient (K part ) higher than 10 9 . For a slow reaction case (k c = 10 −5 /s), intraparticle diffusion becomes negligible for D p higher than 10 −19 m 2 /s and K part higher than 10 11 . For 12 semi-volatile species used in a regional Community Multiscale Air Quality (CMAQ) model, D p is roughly approximated by their surrogates in various polymers. Species with relatively small K part and slow particle-phase reaction are more likely to be impacted by in- traparticle diffusion, besides small D p . Dynamic partitioning framework might be therefore necessary for SOA parameterization and photochemical models such as CMAQ.