Modeling the effect of non-ideality, dynamic mass transfer and viscosity on SOA formation in a 3-D air quality model

摘要

In this study, assumptions (ideality and thermodynamicequilibrium) commonly made in three-dimensional (3-D) air quality models werereconsidered to evaluate their impacts on secondary organic aerosol (SOA)formation over Europe. To investigate the effects of non-ideality, dynamic mass transfer and aerosolviscosity on the SOA formation, the Secondary Organic Aerosol Processor(SOAP) model was implemented in the 3-D air quality model Polyphemus. Thisstudy presents the first 3-D modeling simulation which describes the impactof aerosol viscosity on the SOA formation. The model uses either theequilibrium approach or the dynamic approach with a method specially designedfor 3-D air quality models to efficiently solve particle-phase diffusion whenparticles are viscous. Sensitivity simulations using two organic aerosol models implemented inPolyphemus to represent mass transfer between gas and particle phases showthat the computation of the absorbing aerosol mass strongly influences theSOA formation. In particular, taking into account the concentrations ofinorganic aerosols and hydrophilic organic aerosols in the absorbing mass ofthe aqueous phase increases the average SOA concentration by 5 % and 6 %,respectively. However, inorganic aerosols influence the SOA formation notonly because they constitute an absorbing mass for hydrophilic SOA, but alsobecause they interact with organic compounds. Non-ideality (short-, medium- andlong-range interactions) was found to influence SOA concentrations by about30 %. Concerning the dynamic mass transfer for the SOA formation, if the viscosityof SOA is not taken into account and if ideality of aerosols is assumed, thedynamic approach is found to give generally similar results to theequilibrium approach (indicating that equilibrium is an efficient hypothesisfor inviscid and ideal aerosols). However, when a non-ideal aerosol isassumed, taking into account the dynamic mass transfer leads to a decrease ofconcentrations of the hydrophilic compounds (compared to equilibrium). Thisdecrease is due to differences in the values of activity coefficients, whichare different between values computed for bulk aerosols and those for eachsize section. This result indicates the importance of non-ideality on thedynamic evolution of SOA. For viscous aerosols, assuming a highly viscous organic phase leads to anincrease in SOA concentrations during daytime (by preventing the evaporationof the most volatile organic compounds). The partitioning of nonvolatilecompounds is not affected by viscosity, but the aging of more volatilecompounds (that leads to the formation of the less volatile compounds) slowsdown as the evaporation of those compounds is stopped due to the viscosity ofthe particle. These results imply that aerosol concentrations may deviatesignificantly from equilibrium as the gas–particle partitioning could behigher than predicted by equilibrium. Furthermore, although a compoundevaporates in the simulation using the equilibrium approach, the samecompound can condense in the simulation using the dynamic approach if theparticles are viscous. The results of this study emphasize the need for 3-D air quality models totake into account the effect of non-ideality on SOA formation and the effectof aerosol viscosity for the more volatile fraction of semi-volatile organiccompounds.