Effects of Marine Organic Aerosol as Sources of Ice Nucleating Particles on Mixed-Phase Clouds at High Latitudes

Description

Mixed-phase clouds are frequently observed in the Arctic and Antarctic and over the Southern Ocean, and have important impacts on the surface energy budget and regional climate. Marine organic aerosol (MOA), a natural source of aerosol emitted over 70% of Earth’s surface, may significantly modify the properties and radiative forcing of mixed-phase clouds. However, the importance of MOA as a source of ice nucleating particles (INPs) in comparison with mineral dust, and its effects as cloud condensational nuclei (CCN) and INPs on mixed-phase clouds remain an open question. In this study, we implemented MOA as a new aerosol species into the Community Atmosphere Model version 6 (CAM6), the atmosphere component of the Community Earth System Model version 2 (CESM2). MOA emission is based on the OCEANFILMS (Burrows et al., 2014) scheme, which models the relationship between emitted sea spray aerosol chemistry and ocean biogeochemistry. The contribution of sea spray aerosol to INP concentrations is calculated based on the active site density parameterization for one component of marine INPs (McCluskey et al., 2018). Model experiments are conducted to examine the role of MOA as a source of INPs and its impacts on mixed-phase clouds. Modeled INP concentrations are compared with observations from the DOE ARM’s MARCUS campaign over the Southern Ocean and M-PACE and ISDAC campaigns at North Slope of Alaska, and from other observations (e.g., Mace Head Station, SOCRATES, CAPRICORN). Model results indicate that INP concentrations of sea spray aerosol vary with time and geographic location with the maximum contribution within the boundary layer over the Southern Ocean, where dust has a limited influence. By affecting the cloud water content and droplet effective radius of mixed-phase clouds, MOA exerts a shortwave cloud forcing of -4.2 W m–2 over the Southern Ocean in the summer. The separate CCN and INP effects of MOA are further quantified. These findings highlight the vital importance of Earth System Models to consider the MOA as an important component of biogeochemistry, hydrological cycle, and climate change.