COMPOSITION OF MARINE AIR OFFSHORE OF THE WESTERN UNITED-STATES

Hydrocarbon concentration data derived from gas chromatography of canister observations off the central coast of California from aircraft are statistically analyzed to determine hydrocarbon concentrations typical of clean marine air originating from the mid-Pacific. The sample set is divided in groups by height and by applying various concentration thresholds of specific hydrocarbons, halocarbons, and ozone to remove samples characteristic of urban areas. Clean marine air appears to be characterized by nonmethane hydrocarbon (NMHC) concentrations of 10 parts per billion Carbon (ppbC) and concentrations of carbon monoxide (CO), methane (CH4), Freon (F-12), ethane, ethylene, and acetylene. These hydrocarbon concentrations appear to be consistent with background estimates made from over the mid-Pacific. The carbon composition of the identified portion of the NMHC concentrations consists of 76% paraffins, 10% olefins, 11% aromatics, and 3% ethylene. The median ozone concentration of 60 ppb for clean samples is significantly larger than most remote observations made in the Northern Hemisphere troposphere during last summer.