Rapid uplift of nonmethane hydrocarbons in a cold front over central Europe

The vertical distribution of 21 C2-C7 nonmethane hydrocarbons (NMHCs) has been determined in planetary boundary layer (PBL) and free tropospheric (FT) air over central Europe under a range of meteorological conditions. High-frequency whole air sampling was conducted on board the U.K. Meteorological Office C-130 Hercules aircraft during the European Export of Precursors and Ozone by Long-Range Transport (EXPORT) experiment in August 2000. When vertical transport by large-scale flow or convection was weak, the expected large concentration gradient between the PBL and FT was observed for all short and medium lifetime hydrocarbons (e.g., average iso-butane, PBL 100 pptV, FT 6 pptV). During periods of strong convective activity associated with the passage of a cold front, a rapid uplift of reactive carbon from the boundary layer to the mid free troposphere was observed. Using changing ratios of hydrocarbons with different atmospheric lifetimes, a timescale for transport during this event was determined. Hydrocarbon lifetime measurements suggest that in certain regions of the system, it is convective transport embedded within the cold front rather than larger-scale advection along the warm conveyor belt that is dominant in transporting ozone precursors into the free troposphere.