Thursday, 13 February 2003: 1:30 PM
Sea-salt size-distributions from breaking waves: implications for marine aerosol production and optical extinction measurements during SEAS
The Shoreline Environment Aerosol Study (SEAS) field experiment took place at Bellows Air Force Station (BAFSBAFS) on the east coast of Oahu, Hawaii between April 16 and May 1, 2000. SEAS brought together several investigators and new capabilities for assessing the marine aerosol physical, chemical and optical properties. Measurements were made at a coastal instrumented tower facing east into the prevailing trade winds and overlooking a lagoon region with a shallow reef area about 1km offshore. Our measurements during SEAS focused on the coastal aerosol size distribution and related optical measurements including aerosol light scattering, visibility and remote sensing of aerosol using lidar backscatter. Aerosol production from shoreline breaking waves and the more distant reef (ca. 1km) were characterized for dry sizes between 10nm and 10um both for their contribution to the marine aerosol population and their influence on near surface lidar extinction. Thermal volatility was used to extract the refractory sea-salt size distributions from the other constituents volatile at 360C. At 7m asl and 20m inland from the waters edge the concentration of sea salt nuclei number were often in the range 50 to 100 cm-3 above the background value of ca 250 cm-3. This number peak was near 30nm dry diameter but extended down to 10nm sizes and up to over 10um sizes, spanning nine order of magnitude in aerosol mass. Light scattering from breaking waves was dominated a few particles larger than 1um. These data indicate that sea-salt production contributes not only to aerosol mass and optical effects but also to nuclei mode particle number in remote regions. Separate optical closure studies quantified links between the size distribution and optical scattering measurements, visibility and extinction values for nearshore breaking waves and open ocean conditions. These data confirmed that extinction derived from coastal lidar measurements at (530nm) about 300-400m offshore were accurate to within the 25% uncertainty in calibration claimed for both background for breaking wave conditions.