Anomalous Aerosol Extinction or the "Blue Moon Effect" observed at the ENA ARM site

Description

The ENA AOS (Aerosol Observing System) started sampling on October 3, 2013 on Graciosa Island in the Azores. An anomalous aerosol extinction effect has been observed several times; this occurs when larger sea salt particles are transported to AOS site due to high winds. The effect is evident in the humidigraph; with the observed red (700nm) aerosol scattering cross-section being greater than the green(550 nm) and blue (450nm). This is explained by the theoretical Mie Scattering for sea salt aerosol particles in the size ranges from 0.8 to 1.2 microns. Evidence that these particles are indeed sea salt is provided by the "wet" nephelometer of the humidigraph; which is modulated in relative humidity from 40 to 70 percent. The aerosol particles in the wet nephelometer are observed to grow in size with increasing humidity consistent with sea salt particles, but their Angstrom coefficient decreases below zero, due the predicted Mie scattering, again consistent with Mie scattering theory. The "blue moon" will be explained by anomalous aerosol extinction effect due to larger aerosol particles present in the atmospheric during volcanic and fire events.