Overview of the G-1 Aircraft Measurements during the VAMOS Ocean-Cloud-Atmosphere-Land Study - Regional Experiment (VOCALS-REx) (Invited Speaker)

The DOE G-1 aircraft made seventeen flights in clouds and clear-air associated with clouds during the 2008 VOCALS program conducted over coastal waters off northern Chile from October 15 through November 15, 2008. Aerosol mass concentrations were generally low overall and exhibited a strong longitudinal gradient with average mass-concentrations near the coast much higher (2.1 µg/m3) than well off shore (0.7 µg/m3). Sulfate was the dominant constituent comprising in most instances over 60% of the aerosol mass. The nitrate concentration was generally very low, and was associated with sea-salt particles which showed a rather uniform distribution longitudinally. On average the aerosol was very acidic with only ~25% of the acidity neutralized by ammonia. From various lines of evidence, the aerosol sulfate particles appeared to be externally mixed with modified sea-salt particles. In general below-cloud aerosol size spectra were bimodal exhibiting a Hoppel minimum, the position of which moved to smaller particle diameters with distance from the coast.

Cloud droplet number concentrations (CDNC) ranged from 200-400 cm-3 near the coast to values that were as low as 50 cm-3 well offshore; this decrease in the CDNC was associated with an increase in droplet diameter. Both the CDNC and size were strongly related to the expected way to the longitudinal gradient in the below cloud aerosol concentration. Consistent with the second indirect aerosol effect the decrease in droplet size with increasing aerosol concentration was associated with decreased drizzle concentration. It may be concluded that the longitudinal gradient in both aerosol and cloud properties is most likely due to anthropogenic aerosols and aerosol precursors originating from continental sources.