Abstract
Abstract. Organic aerosol (OA) has been considered as one of the most important uncertainties in climate modeling due to the complexity in presenting its chemical production and depletion mechanisms. To better understand the capability of climate models and probe into the associated uncertainties in simulating OA, we evaluate the Community Earth System Model version 2.1 (CESM2.1) configured with the Community Atmosphere Model version 6 (CAM6) with comprehensive tropospheric and stratospheric chemistry representation (CAM6-Chem) through a long-term simulation (1988–2019) with observations collected from multiple datasets in the United States. We find that CESM generally reproduces the interannual variation and seasonal cycle of OA mass concentration at surface layer with a correlation of 0.40 compared to ground observations and systematically overestimates (69 %) in summer and underestimates (−19 %) in winter. Through a series of sensitivity simulations, we reveal that modeling bias is primarily related to the dominant fraction of monoterpene-formed secondary organic aerosol (SOA), and a strong positive correlation of 0.67 is found between monoterpene emission and modeling bias in the eastern US during summer. In terms of vertical profile, the model prominently underestimates OA and monoterpene concentrations by 37 %–99 % and 82 %–99 %, respectively, in the upper air (> 500 m) as validated against aircraft observations. Our study suggests that the current volatility basis set (VBS) scheme applied in CESM might be parameterized with monoterpene SOA yields that are too high, which subsequently results in strong SOA production near the emission source area. We also find that the model has difficulty in reproducing the decreasing trend of surface OA in the southeastern US probably because of employing pure gas VBS to represent isoprene SOA which is in reality mainly formed through multiphase chemistry; thus, the influence of aerosol acidity and sulfate particle change on isoprene SOA formation has not been fully considered in the model. This study reveals the urgent need to improve the SOA modeling in climate models.
Highlights
As one of the most important contributors (20 %–90 %) to total fine atmospheric particles (Kanakidou et al, 2004), organic aerosol (OA) plays an important role in the climate system by affecting the radiation budget (Ghan et al, 2012)
Through the validation against surface measurements and flight campaigns over the United States (US), we have found that CESM2.1 is able to capture interannual and seasonal variation of surface OA concentrations with a correlation coefficient of 0.41, but it systematically overestimates surface OA concentration in summer by 68.78 %
Larger summertime bias is found over the eastern US where biogenic VOCs (BVOCs) emissions are more intensive than the western US
Summary
As one of the most important contributors (20 %–90 %) to total fine atmospheric particles (Kanakidou et al, 2004), organic aerosol (OA) plays an important role in the climate system by affecting the radiation budget (Ghan et al, 2012). Secondary organic aerosol (SOA) formed via the oxidation of volatile organic compounds (VOCs) (Hallquist et al, 2009; Tsigaridis et al, 2014; Shrivastava et al, 2017). The radiative forcing effect of OA has been assessed with climate models through tremendous efforts during the past decades (Ghan et al, 2012; Myhre et al, 2013; Sporre et al, 2020; Chen and Gettelman, 2016), yet the limited capability of climate models in terms of simulating the productions and depletions of OA introduces large uncertainties. As OA loading and properties of aerosols varied, the estimated radiative forcing of OA ranged from −0.06 to −0.01 W m−2 among the 16 participating models (Myhre et al, 2013), revealing the fundamental uncertainty of OA simulation
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