(160e) Stratocumulus Cloud Water Properties Off the California Coast During the 2011 E-PEACE Field Campaign
This study reports on cloud water chemical and pH measurements off the California coast during the July-August 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). Eighty two cloud water samples were collected by the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter in boundary layer stratocumulus clouds impacted to varying degrees by marine-derived emissions, ship exhaust, and continental emissions. Cloud water pH ranged between 2.92 and 7.58, with an average of 4.46. Peak pH values were observed north of San Francisco, simultaneous with the highest concentrations of Si, B, and Cs. One case flight shows how air mass trajectories transported over land covered with vegetation and soil resulted in the highest cloud water pH (~ 7.5) and Si concentration (air equivalent of 4.39 µg m-3) during E-PEACE. These results point to an important role for basic particle types (e.g. soil dust and fly ash) originating from continental sources in impacting stratocumulus clouds in the study region. The lowest pH values were observed south of San Francisco in an area with extensive ship traffic. These samples exhibited the highest concentrations of sulfate, nitrate, V, Fe, Al, P, Cd, Ti, Sb, and Mn, which are linked largely to cargo and tanker ships. Marine biogenic emissions were responsible for the close association between a number of species including chloride, methanesulfonate, Na, Se, K, Rb, Sr, and I. The highest levels of Fe, Mn, Ti, and P were observed in the area of most extensive ship traffic rather than near the northern part of the region impacted by continental sources, suggestive of the role of ship emissions in providing a source for these ocean micronutrients and aqueous-phase reaction catalysts to stratocumulus cloud drops.