Nonmethane hydrocarbon mixing ratios in continental outflow air from eastern North America: Export of ozone precursors to Bermuda

High surface ozone levels at Bermuda during springtime are associated with the transport of large scare frontal systems from the U.S. east coast over the North Atlantic. An objective of the AEROCE III study was to adopt a meteorologically informed sampling strategy of the chemical characteristics of air masses both in advance and behind eastward progressing cold fronts in order to differentiate between vertical sources of these elevated tropospheric ozone signals. Concentrations of hydrocarbons in air sampled over the eastern US and in offshore flights varied in a complex manner with sample altitude and prevailing meteorological situation. In several aircraft flights, the presence at altitude of distinct layers of air of elevated concentrations of NMHC's attested to the dynamic mixing of lower and upper air masses associated with springtime frontal activity. Layers of mid-tropospheric air of high O-3 (140 ppbv) and low background NMHC mixing ratios (1.44 ppbv ethane, 0.034 ppbv propene, 0.247 ppbv propane, 0.034 ppbv isobutene, 0.041 ppbv n-butane, 0.063 ppbv benzene, 0.038 ppbv toluene) mixing ratios were indicative of descending, stratospherically influenced air on a flight to the east of Norfolk, VA on April 24 (alt 4600 m). Layers of high O-3 (60-70 ppbv) and elevated NMHC concentrations (1.88 ppbv ethane, 0.092 ppbv propene, 0.398 ppbv propane, 0.063 ppbv isobutene, 0.075 ppbv n-butane, 0.106 ppbv benzene, 0.102 ppbv toluene) observed on a flight to the west of Bermuda on April 28 (alt. 4100m) indicated upwardly lifting convective activity of continentally sourced air masses: A meteorologically informed sampling strategy employed in the study was valuable m providing a framework with which to both optimize NMHC sampling decisions and to provide a context with which to interpret their observed mixing ratios.