Evaluation of Biogenic Organic Aerosols in the Amazon Rainforest Using WRF‐Chem with MOSAIC

Biogenic secondary organic aerosol (BSOA) accounts for 70%–80% of submicron aerosol over the Amazon rainforest. However, uncertainties with regard to the chemical formation pathway, mass budget, and radiative impact of BSOA in the region still remain high. To address the issue, we used a regional chemistry transport model (WRF-Chem-MOSAIC) to evaluate the budget of BSOA in the Amazon forest during the wet season (March) of 2014 when the air mass was clean from influences from anthropogenic and biomass burning emissions. We found that the default model underestimated the surface concentration of submicron organic aerosol (OA) by 57% at a ground measurement site (T3). We subsequently updated the WRF-Chem-MOSAIC model by including SOA formation from isoprene epoxydiols (IEPOX-SOA) and SOA aging schemes. The updated model reproduced the observed submicron OA concentration well with an overestimation of only 2.5%. With the updated model, we will show that IEPOX-SOA accounts for 23% of surface submicron OA averaged over the entire Amazon rainforest. The modeled surface SOA (smaller than 2.5 μm in diameter) concentration in the Amazon region was about a factor of 14 larger than the global mean. Finally, a sensitivity study was conducted which showed that convective plumes can remove more than 40% of submicron OA below 5 km in the Amazon region. Overall, our study highlights the importance of IEPOX chemistry as well as the convective removal processes in the global SOA budget.