Rethinking the global secondary organic aerosol (SOA) budget: stronger production, faster removal, shorter lifetime

摘要

Recent laboratory studies suggest that secondary organicaerosol (SOA) formation rates are higher than assumed in current models.There is also evidence that SOA removal by dry and wet deposition occursmore efficiently than some current models suggest and that photolysis andheterogeneous oxidation may be important (but currently ignored) SOA sinks.Here, we have updated the global GEOS-Chem model to include this newinformation on formation (i.e., wall-corrected yields and emissions ofsemi-volatile and intermediate volatility organic compounds) and on removalprocesses (photolysis and heterogeneous oxidation). We compare simulated SOAfrom various model configurations against ground, aircraft and satellitemeasurements to assess the extent to which these improved representations ofSOA formation and removal processes are consistent with observedcharacteristics of the SOA distribution. The updated model presents a moredynamic picture of the life cycle of atmospheric SOA, with production rates3.9 times higher and sinks a factor of 3.6 more efficient than in the basemodel. In particular, the updated model predicts larger SOA concentrationsin the boundary layer and lower concentrations in the upper troposphere,leading to better agreement with surface and aircraft measurements oforganic aerosol compared to the base model. Our analysis thus suggests thatthe long-standing discrepancy in model predictions of the vertical SOAdistribution can now be resolved, at least in part, by a stronger source andstronger sinks leading to a shorter lifetime. The predicted global SOAburden in the updated model is 0.88 Tg and the corresponding directradiative effect at top of the atmosphere is −0.33 W m, which iscomparable to recent model estimates constrained by observations. Theupdated model predicts a population-weighed global mean surface SOAconcentration that is a factor of 2 higher than in the base model,suggesting the need for a reanalysis of the contribution of SOA to PMpollution-related human health effects. The potential importance of ourestimates highlights the need for more extensive field and laboratorystudies focused on characterizing organic aerosol removal mechanisms andrates.