A balloon-sonde gas sampling system for full-column trace gas profiling

Description

In situ measurements of the vertical structure of CO2 and other greenhouse gases (GHG) are essential for evaluation of GHG remote sensing, errors in inverse model GHG emission studies, and GCM representation of GHG radiative forcing. Current in situ measurements at the ARM Facility include airborne measurements of CO2 and flask sampling for other species (e.g., CH4, N2O, SF6, and halo carbons). However, this work is currently limited to altitudes up to 5 km (50% of the column), and occasional flights of opportunity have only reached ~ 12 km (90% of the column). Here, we describe a balloon-sonde sampling system, termed an AirCore, that collects atmospheric samples from the stratosphere (> 20 km) to the Earth's surface, and describe preliminary planning for balloon-sonde flights at the Southern Great Plains Atmospheric Science Research Facility. The AirCore described here is a 150-m-long stainless steel tube, open at one end and closed at the other, that relies on positive changes in ambient pressure for passive sampling of the atmosphere. The AirCore evacuates while ascending to a high altitude and collects a sample of the ambient air as it descends. It is sealed upon recovery and measured with a continuous analyzer for trace gas mole fraction. Measurements of CO2 and CH4 mole fractions in laboratory tests indicate a repeatability and accuracy of better than 0.05 ppm for CO2 and 0.4 ppb for CH4 under a variety of conditions. Comparisons of AirCore data with continuous in situ and flask data in aircraft field tests indicate average absolute differences of 0.3 ppm and 5 ppb for CO2 and CH4, respectively, with no apparent bias. Accounting for molecular diffusion and flow-induced mixing, the expected measurement resolution for CO2 and CH4 is 110 m at sea level and 260 m at 8000 m above sea level after three hours of storage. As part of planning for flights in the ARM SGP area, we analyze balloon-sonde trajectories and identify prospective launch locations that facilitate recovery of the Aircore package.