Sea-salt vertical profiles over the Southern and tropical Pacific oceans: Microphysics, optical properties, spatial variability, and variations with wind speed

About 100 vertical profiles of marine aerosol were contrasted between two remote regions over the Southern Ocean and the equatorial Pacific. Real-time thermal analysis of particles at near 40°C, 150°C, and 300°C resolved the volatile (mostly sulfate and organic) and refractory (mostly sea salt) size distributions continuously throughout two major aircraft campaigns. The average sea-salt contributions to aerosol optical depths (AOD) in the marine boundary layer over the Southern Ocean and tropical Pacific were 0.037 (63% of the total column AOD) and 0.022 (31%), respectively, while the volatile component contributed 0.011 (19%) and 0.038 (53%), respectively. This shows the large difference in relative importance of sea salt and volatile components for regions with different residence times and source strengths. The total column AOD is estimated to be 0.037, 0.043, and 0.090 for 0–4, 4–8, and 8–12 m s-1, respectively, over the Southern Ocean, and 0.060, 0.062, and 0.128 for 3–5, 5–7, and 7–9 m s-1, respectively, over the tropical Pacific. Wind speed was a good indicator of sea-salt concentrations on a campaign average basis but a poor indicator on a case-by-case basis. This highlights the important effect of removal, wind duration, and fetch on sea-salt variability. Variability in the marine aerosol was assessed statistically for 96 circular flight legs of about 60 km diameter. This mesoscale variability increased with altitude and resulted in variations in column AOD of about 25% over these spatial scales. Hence mesoscale variability is an important concern for satellite remote sensing, closure studies, and model validation even for this nominally homogeneous clean marine atmosphere.