Aerosol optical properties at Trinidad Head - ITCT and beyond

The far northern coast of California is believed to be a good location to monitor aerosols being transported to the US from across the Pacific Ocean. In mid-April 2002, NOAA/CMDL deployed a mobile aerosol system and began a year-long effort to measure aerosol optical properties at Trinidad Head (THD), California. The start of these aerosol measurements coincided with a month-long field program for the Intercontinental Transport and Chemical Transformations (ITCT) program, which was designed to investigate how atmospheric pollutants from one continent can influence air quality and regional and global climate on other continents. Although Trinidad Head is a remote coastal site, during the first six months of deployment, it was impacted by emissions from a variety of non-marine aerosol sources. During the ITCT program in April and May 2002, several incursions of Asian dust were observed at the site, while in August 2002, smoke from forest fires in southern Oregon was detected at THD. Additionally, air mass trajectories show occasional transport to the site from urban Northern California. While background light scattering and absorption (for diameter < 10 microns) at the site are approximately 20 Mm^-1 and 1 Mm^-1 respectively, dust events increased the scattering and absorption to a factor of four higher than the background amount and the intense smoke events at the site resulted in both scattering and absorption values an order of magnitude higher than the background values. The single scattering albedo is ~0.98 for background conditions and ~0.97 and ~0.94 respectively for the dust and smoke events observed at the site. Particle hygroscopicity decreased during the smoke and dust events. While the THD surface site is ideally located for investigating a variety of aerosol types, the surface-based measurements have also demonstrated the importance of measuring vertical profiles of aerosol properties. For example, during one dust event, the dust resided in a layer aloft which was detected by the column optical depth instrument but not by the in-situ instruments at the surface, while during another dust event both in-situ and column measurements indicated the presence of dust. Both events were consistent with predictions from an aerosol forecasting model (Navy Aerosol Analysis and Prediction System (NAAPS)). Long term aerosol measurements at THD will, in conjunction with radiation and chemistry measurements contribute to understanding the role of inter- and intra- continental transport of aerosol particles in climate forcing and air quality.