Volume and surface area size distribution, water mass and model fitting of GCE/CASE/WATOX marine aerosols

Abstract

As a part of the Global Change Expedition/Coordinated Air‐Sea Experiment/Western Atlantic Ocean Experiment (GCE/CASE/WATOX), size distributions of marine aerosols were measured at two altitudes of about 2750 and 150 m above sea level (asl) over the size range 0.1 ∼ 32 μm. Lognormal fitting was applied to the corrected aerosol size spectra to determine the volume and surface area size distributions of the CASE‐WATOX marine aerosols. Each aerosol size distribution was fitted with three lognormal distributions representing fine‐, large‐, and giant‐particle modes. Water volume fraction and dry particle size of each aerosol size distribution were also calculated using empirical formulas for particle size as a function of relative humidity and particle type. Because of the increased influence from anthropogenic sources in the continental United States, higher aerosol volume concentrations were observed in the fine‐particle mode near‐shore off the east coast; 2.11 and 3.63 μm3 cm−3 for free troposphere (FT) and marine boundary layer (MBL), compared with the open‐sea Bermuda area values; 0.13 and 0.74 μm3 cm−3 for FT and MBL. The large‐particle mode exhibits the least variations in volume distributions between the east coast and open‐sea Bermuda area, having a volume geometric median diameter (VGMD) between 1.4 and 1.6 μm and a geometric standard deviation between 1.57 and 1.68. For the giant‐particle mode, larger VGMD and volume concentrations were observed for marine aerosols nearshore off the east coast than in the open‐sea Bermuda area because of higher relative humidity and higher surface wind speed conditions. Wet VGMD and aerosol water volume concentrations at 15 m asl ship level were determined by extrapolating from those obtained by analysis of the CASE‐WATOX aircraft aerosol data. Abundance of aerosol water in the MBL serves as an important pathway for heterogeneous conversion of SO2 in sea salt aerosol particles.