Evaluation of CESM2 Secondary Organic Aerosol(SOA) modeling and Contributions from Monoterpene Generated SOA in United States

Abstract:

Organic aerosols (OA) have been considered as one of the most important uncertainties in climate modeling due to its complicated chemical production and depletion mechanisms. To better understand the capability of climate models and probe into the uncertainties associated with climate model predicted OA, we evaluated the latest version of Community Earth System Model (CESM) v2.1 with observations from multiple datasets in the United States. We found that CESM can generally reproduce the inter-annual variation and seasonal cycle of OA mass concentration at surface layer, but systematically overestimate (by 71%) in summer and underestimate (by -8%) in winter. Surface OA concentration in US was also found to gradually decrease during the past 32 years (1988-2019) due to substantial decrease of anthropogenic emission. Flight campaign measurements were collected to reveal that underestimation of OA concentration in the upper air was also predominant. 15 sensitivity experiments were performed to investigate the main contributor of total SOA based on Volatility Basis Set (VBS) scheme applied in CESM and showed the model bias in summer and winter was related to the large fraction of monoterpene derived SOA. This study reveals the urgent need to improve the SOA modeling in climate models and a rethink of the OA climate effects estimated by previous models which may contain large uncertainties.