An overview of aircraft observations from the Pacific Dust Experiment campaign

Fourteen research flights were conducted in the Pacific Dust Experiment (PACDEX) during April and May 2007 to sample pollution and dust outbreaks from east Asia as they traveled across the northern Pacific Ocean into North America and interacted with maritime storms. Significant concentrations of black carbon (BC, consisting of soot and other light-absorbing particles measured with a soot photometer 2 instrument) and dust were observed both in the west and east Pacific Ocean from Asian plumes of dust and pollution. BC particles were observed through much of the troposphere, but the major finding is that the percentage of these particles compared with the total number of accumulation mode particles increased significantly (by a factor of 2 - 4) with increasing altitude, with peak values occurring between 5 and 10 km. Dust plumes had only a small impact on total cloud condensation nuclei at the sampling supersaturations but did exhibit high concentrations of ice nuclei (IN). IN concentrations in dust plumes exceeded typical tropospheric values by 4 - 20 times and were similar to previous studies in the Saharan aerosol layer when differences in the number concentrations of dust are accounted for. Enhanced IN concentrations were found in the upper troposphere off the coast of North America, providing a first direct validation of the transport of high-IN-containing dust layers near the tropopause entering the North American continent from distant sources. A source-specific chemical transport model was used to predict dust and other aerosols during PACDEX. The model was able to predict several features of the in situ observations, including the general altitudes where BC was found and a peak in the ratio of BC to sulfate between 5 and 10 km.