Ronald G. Prinn, Gary Kleiman, Alexander A. Pszenny, Ashwini G. Deshpande, Xianqian Shi
Center for Global Change Science
MIT Rm. 54-1312
77 Massachusetts Ave
Cambridge, MA 02139
Depending on meteorological and oceanic conditions, we have identified two distinct regimes for relative and absolute concentrations of nonmethane hydrocarbons (NMHC) measured in surface air and seawater during the recent IGAC Aerosol Characterization Experiment (ACE-1). The (approximately hourly) measurements were made from the NOAA ship Discoverer in the Southern Ocean. Of particular note are two qualitatively different regimes of NMHC observations. The first regime, typified by observations from JD 329-332, consisted of highly correlated water and air concentrations with relatively high air concentrations for many of the NMHC species (approaching saturation). The second regime, typified by observations from JD 333 onwards, showed air and water measurements not as well correlated and air concentrations a few orders of magnitude less than saturated values. The conditions in the second regime are consistent with the majority of observations made by Donahue and Prinn (JGR 98, No. D9, pg. 16,915, 1993) during the 1990 SAGA III cruise, whereas the conditions in the first regime are more consistent with many of the observations made during the 1992 IGAC/MAGE cruise by Shi and Prinn (JGR, submitted). The presence of these two different regimes indicates that for at least short periods of time, near saturation conditions (typified by NMHC concentrations 10-100 times greater than background tropospheric values) can exist in the remote marine boundary layer. The hypothesis that we were seeing air of local oceanic origin during the period JD 329-332 is consistent with back-trajectories calculated using the Australian Bureau of Meteorology GASP model. These show that air on JD 329 at 100m altitude hadtraveled several thousand kilometers before arriving at the ship's position, whereas air on JD 330 had traveled only a few hundred kilometers away from it's oceanic origins. Radiosonde soundings from the Discoverer during this timeperiod confirm the existence of a stable inversion layer which would have prevented mixing with free tropospheric air during JD 329-332.