New, Experimentally Based, Secondary Organic Aerosol Paradigm Removes Discrepancies between Models and Data

Description

Significant fraction of atmospheric aerosols, which affect climate and health, are comprised of secondary organic aerosols (SOA). A number of resent studies that utilized independent methods for SOA characterization have shown SOA particles to be highly viscous semi solids and nearly non-volatile, rather than, as the models assume, equilibrated rapidly mixing and evaporating solutions. However, doubts as to whether these laboratory results are pertinent to atmospheric conditions remain. Here we present analysis of the detailed spatial distributions of organic aerosol (OA) loadings observed during the Mexico City 2006 MILARGO field campaign. We compare OA loadings measured repeatedly at the same locations and find that the patterns of OA spatial distributions are in close agreement with predictions by our newly constructed model, which treats SOA as non-volatile semisolids, and preclude presumptions of gas-particle equilibrium that form the basis of commonly accepted models. Field data provide direct evidence that the time scale, on which particles must respond by evaporation to maintain equilibrium, is orders of magnitude faster than measured evaporation rates. Moreover, the experimental data and model simulations indicate that in this new paradigm, loadings significantly increase due to well understood mechanism, reducing persistent discrepancies between measured organic aerosol loadings and model predictions, bringing them to within levels consistent with uncertainties in measurements and emissions. SOA loadings predicted by our newly developed comprehensive 3-D model that treats SOA as non-volatile semi-solid and includes fragmentation and multi-generation chemistry are in good agreement with measurements over the entire 3-D experimental domain over the Mexico City Plateau.