TIME AND TEMPERATURE DEPENDENCES OF FRACTIONAL HCL ABUNDANCES FROM AIRBORNE DATA IN THE SOUTHERN-HEMISPHERE DURING 1994

Measurements of HCl and CH4 taken by the aircraft laser infrared absor ption spectrometer (ALIAS) on the ER-2 high-altitude research aircraft during the Southern Hemisphere winter of 1994 have been used to exami ne the abundance of HCl as a fraction of total inorganic chlorine. The fractional abundance of HCl shows a threshold behaviour as a function of temperature history; on a 10 day timescale, the abundance dropped sharply in those air parcels experiencing a temperature < 195 K, but l ittle or no change was seen in parcels which stayed warmer than this t emperature. The behaviour mirrors well the temperature behaviour calcu lated for the transformation of HCl into reactive forms (Cl-2, HOCl) f rom laboratory studies of sulfate aerosols and polar stratospheric clo uds. During the course of the winter, the fractional abundance of HCl outside the vortex decreased from its values in late May by about a th ird, while inside it dropped to near zero by early August. Some recove ry was evident in October. Examples of the peel-off of low-HCl air equ atorward of the wind maximum were evident in early June. Meteorologica l trajectories are used to show, in a case study of a flight in early August, that air parcels which experienced temperatures of < 195 K, an d as a result had low fractional HCl abundances, did so largely polewa rd of the maximum in the polar night jet stream. Encountering temperat ures of < 195 K during the previous 10 days was a necessary and suffic ient condition for the transformation of HCl into reactive forms by he terogeneous reactions. The trajectories further showed that air arrivi ng from sub-tropical latitudes had higher fractional HCl abundances th an the air in the middle latitudes, and much higher fractions than the air at high latitudes. The resulting picture is one in which the frac tional abundance of HCl in air at mid latitudes was the result of mixi ng of air from sub-tropical latitudes with air mainly from polward of the jet stream core which has experienced temperatures < 195 K. The se nsitivity of the fractional abundance of HCl to the assumption that no HCl enters the stratosphere via the tropical tropopause is examined i n the light of an observed profile near the equator with a volume frac tion of 0.4 ppb HCl, zero ClO and tropospheric mixing ratios of CFCs a t the tropical tropopause.