Improving the representation of secondary organic aerosol (SOA) in the MOZART-4 global chemical transport model

摘要

The secondary organic aerosol (SOA) module in the Model for Ozone andRelated Chemical Tracers, version 4 (MOZART-4) was updated by replacingexisting two-product (2p) parameters with those obtained from two-productvolatility basis set (2p-VBS) fits (MZ4-C1), and by treating SOA formationfrom the following additional volatile organic compounds (VOCs): isoprene,propene and lumped alkenes (MZ4-C2). Strong seasonal and spatial variationsin global SOA distributions were demonstrated, with significant differencesin the predicted concentrations between the base case and updated modelsimulations. Updates to the model resulted in significant increases inannual average SOA mass concentrations, particularly for the MZ4-C2simulation in which the additional SOA precursor VOCs were treated. Annualaverage SOA concentrations predicted by the MZ4-C2 simulation were1.00 ± 1.04 μg m?3 in South America, 1.57 ± 1.88 μg m?3 in Indonesia, 0.37 ± 0.27 μg m?3 in theUSA, and 0.47 ± 0.29 μg m?3 in Europe with correspondingincreases of 178, 406, 311 and 292% over the base-casesimulation, respectively, primarily due to inclusion of isoprene. Theincreases in predicted SOA mass concentrations resulted in correspondingincreases in SOA contributions to annual average total aerosol optical depth(AOD) by ~ 1–6%. Estimated global SOA production was 5.8, 6.6 and 19.1 Tg yr?1 with correspondingburdens of 0.22, 0.24 and 0.59 Tg for the base-case, MZ4-C1 and MZ4-C2simulations, respectively. The predicted SOA budgets fell well withinreported ranges for comparable modeling studies, 6.7 to 96 Tg yr?1, butwere lower than recently reported observationally constrained values, 50 to380 Tg yr?1. For MZ4-C2, simulated SOA concentrations at the surfacealso were in reasonable agreement with comparable modeling studies andobservations. Total organic aerosol (OA) mass concentrations at the surface,however, were slightly over-predicted in Europe, Amazonian regions andMalaysian Borneo (Southeast Asia) during certain months of the year, andunder-predicted in most sites in Asia; relative to those regions, the modelperformed better for sites in North America. Overall, with the inclusion ofadditional SOA precursors (MZ4-C2), namely isoprene, MOZART-4 showedconsistently better skill (NMB (normalized mean bias) of ?11 vs. ?26%) in predicting totalOA levels and spatial distributions of SOA as compared with unmodifiedMOZART-4. Treatment of SOA formation by these known precursors (isoprene,propene and lumped alkenes) may be particularly important when MOZART-4output is used to generate boundary conditions for regional air qualitysimulations that require more accurate representation of SOA concentrationsand distributions.