Transport into the Northern Hemisphere lowermost stratosphere revealed by in situ tracer measurements

The Lightweight Airborne Chromatograph Experiment (LACE) has made in situ m easurements of several long-lived trace gases in the upper troposphere and lower to middle stratosphere as part of the Observations of the Middle Stra tosphere (OMS) balloon program. The tracers measured by LACE include severa l photolytic species (CFC-11, CFC-12, and halon-1211) as well as SF6. LACE measurements of these long-lived tracers as well as nearly simultaneous mea surements of water vapor and CO2 are used to investigate transport into the lowermost stratosphere, a region where few in situ measurements exist. The measured photolytic species and water vapor are used in a simple mass bala nce calculation to estimate the mixture of tropospheric and overworld (thet a > 380 K) air in the lowermost stratosphere. In the northern midlatitudes during September 1996, most of the air in the lowermost stratosphere sample d at the flight location (34.5 degrees N) was transported quasi-isentropica lly from the troposphere. Measurements from both a May 1998 midlatitude fli ght and a June 1997 high-latitude flight (64.5 degrees N) revealed the air sampled in the lowermost stratosphere to be dominated by downward advection from the overworld. Atmospheric SF6 and CO2 can uniquely reveal timescales and spatial scales of transport due to these species' large growth rates a nd subsequent latitudinal surface and free tropospheric gradients. Measurem ents in the lowermost stratosphere from the September northern midlatitude flight coupled with surface measurements of these species revealed a transp ort timescale of no more than 1.5 months from the surface to the lowermost stratosphere. The SF6 and CO2 mixing ratios were also consistent with mostl y Northern Hemisphere tropospheric air in the lowermost stratosphere. These results point out the usefulness of high-resolution in situ measurements o f long-lived tracers to help determine timescales and spatial scales of tra nsport in the region of the upper troposphere and lowermost stratosphere.