Improving the representation of secondary organic aerosol (SOA) in the MOZART-4 global chemical transport model
Abstract. The secondary organic aerosol (SOA) module in the Model for Ozone and Related chemical Tracers, version 4 (MOZART-4) has been updated by replacing existing two-product (2p) parameters with those obtained from two-product volatility basis set (2p-VBS) fits, and by treating SOA formation from the following volatile organic compounds (VOCs): isoprene, propene and lumped alkenes. Strong seasonal and spatial variations in global SOA distributions were demonstrated, with significant differences in the predicted concentrations between the base-case and updated model versions. The base-case MOZART-4 predicted annual average SOA of 0.36 ± 0.50 μg m−3 in South America, 0.31 ± 0.38 μg m−3 in Indonesia, 0.09 ± 0.05 μg m−3 in the USA, and 0.12 ± 0.07 μg m−3 in Europe. Concentrations from the updated versions of the model showed a~marked increase in annual average SOA. Using the updated set of parameters alone (MZ4-v1) increased annual average SOA by ~8%, ~16%, ~56%, and ~108% from the base-case in South America, Indonesia, USA, and Europe, respectively. Treatment of additional parent VOCs (MZ4-v2) resulted in an even more dramatic increase of ~178–406% in annual average SOA for these regions over the base-case. The increases in predicted SOA concentrations further resulted in increases in corresponding SOA contributions to annual average total aerosol optical depth (AOD) by <1% for MZ4-v1 and ~1–6% for MZ4-v2. Estimated global SOA production was ~6.6 Tg yr−1 and ~19.1 Tg yr−1 with corresponding burdens of ~0.24 Tg and ~0.59 Tg using MZ4-v1 and MZ4-v2, respectively. The SOA budgets predicted in the current study fall well within reported ranges for similar modeling studies, 6.7 to 96 Tg yr−1, but are lower than recently reported observationally-constrained values, 50 to 380 Tg yr−1. With MZ4-v2, simulated SOA concentrations at the surface were also in reasonable agreement with comparable modeling studies and observations. Concentrations of estimated organic aerosol (OA) at the surface, however, showed under-prediction in Europe and over-prediction in the Amazonian regions and Malaysian Borneo during certain months of the year. Overall, the updated version of MOZART-4, MZ4-v2, showed consistently better skill in predicting SOA and OA levels and spatial distributions as compared with unmodified MOZART-4. The MZ4-v2 updates may be particularly important when MOZART-4 output is used to generate boundary conditions for regional air quality simulations that require more accurate representation of SOA concentrations and distributions.