| Instrument: | High-Sensitivity Fast-Response CO2 Analyzer |
| Principal Investigator: | Steven C. Wofsy |
| Co-Investigators: | Arlyn E. Andrews, Bruce C. Daube, Jr. |
| Organization: | Department of Earth and Planetary Sciences Harvard University 20 Oxford Street Cambridge, MA 02138 |
Measurement Description: The fast response CO2 instrument measures CO2 concentrations in situ using the light source, gas cells, and solid-state detector from a modified nondispersive infrared CO2 analyzer (Li-cor, Inc., Lincoln, NE); these components are stabilized along the detection axis, vibrationally-isolated, and housed in a temperature-controlled pressure vessel. Stratospheric air enters a forward-facing inlet and is compressed by a Teflon diaphragm pump. The CO2 mixing ratio of air flowing through the sample gas cell is determined by measuring absorption at 4.26 relative to a reference gas of known concentration. In-flight calibrations are performed by replacing the air sample with reference gas every 10 minutes, with a low-span and a high-span gas every 20 minutes, and with a long-term primary standard every 2 hours. The long-term standard is used sparingly and serves as a check of the flight-to-flight accuracy and precision of the measurements, augmented by ground-based calibrations before and after flights. Sample and reference gas pressures and temperatures are controlled to 320 ±0.2 Torr and 36.5 ±0.2°C, respectively. Flow through the sample cell is maintained at 200 sccm, corresponding to a flush time of about 1.5 seconds. The instrument bandpass is 10 Hz and data are acquired at 4 Hz; reported data are generally median-filtered into 2-second intervals from the 4 Hz data. Measurements begin on ascent at 225 mbar (11 km) and end on descent at 290 mbar (9.5 km), often providing data for the upper troposphere. Interference from water vapor due to absorption and pressure-broadening is negligible for sampling conditions; dilution by H2O corresponds to <0.03 ppmv (equivalent CO2) for H2O <85 ppmv.
Since its completion in November 1992, the ER-2 instrument has measured CO2 on the SPADE, ASHOE/MAESA, STRAT and POLARIS payloads. Data from these missions have shown that CO2 is a useful tracer of stratospheric dynamics because of its unique time-varying behavior: the dominant source of stratospheric CO2 is transport from the troposphere where CO2 concentrations vary on a time-scale of months, due to seasonal photosynthetic activity in the biosphere, and on a time-scale of years, due to fossil fuel combustion. The stratospheric measurements of CO2 to date, in conjunction with simultaneous measurements of other species on the ER-2 such as N2O and H2O, have allowed us to observe the seasonal, annual, and interannual changes in tropospheric CO2 propagate into the lower stratosphere, provid qualitative and quantitative estimates of transport rates into and within the lower stratosphere. CO2 observations from the ER-2 and high-altitude balloons during SOLVE will provide insight into the origin of air in the Arctic polar vortex and the rate of descent within the vortex, helping to deconvolve the contributions of transport and chemistry to observed ozone loss.
| Accuracy: | 0.1 ppm |
| Precision: | 0.05 ppm |
| Weight: | 115 lbs |
| Power: | 300 W typical, 450 W maximum |
| Response Time: | 2 second |
| Location on ER-2: | Left wing pod |