Alpha Jet

The Picarro G2401m is a commerical instrument that measures CO2, CH4, CO, and H2O. The analyzer is based on Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS), a time-based measurement utilizing a near-infrared laser to measure a spectral signature of the molecule. Gas is circulated in an optical measurement cavity with an effective path length of up to 20 kilometers. A patented, high-precision wavelength monitor makes certain that only the spectral feature of interest is being monitored, greatly reducing the analyzer’s sensitivity to interfering gas species, and enabling ultra-trace gas concentration measurements even if there are other gases present. As a result, the analyzer maintains high linearity, precision, and accuracy over changing environmental conditions with minimal calibration required.

The measurement software of the NOAA Picarro has been modified to have a shorter measurement interval (~1.2 seconds instead of ~2.4 seconds) by reducing the number of scans of the CO spectroscopic peak and therefore yielding a less-precise CO measurement (1σ on 1-2 second measurements is ~9 ppb instead of ~4 ppb). The instrument was also modified to have a lower cell pressure set point (80 torr instead of 140 torr) to allow it to operate across the full pressure altitude range of the DC8 without requiring upstream pressurization of the sample stream.

Alpha Jet (O₃) Ozone instrument details

Measurements of ozone (O₃) mixing ratios are performed using a commercial O₃ monitor (2B Technologies Inc., model 205 (http://www.twobtech.com/model_205.htm)) based on ultraviolet (UV) absorption techniques and modified for flight worthiness. The dual-beam instrument uses two detection cells to simultaneously measure UV light intensity differences between O₃-scrubbed air and un-scrubbed air to give precise measurements of O₃. The monitor has been modified by upgrading the pressure sensor and pump to allow measurements at high altitudes, including a lamp heater to improve the stability of the UV source, and the addition of heaters, temperature controllers and vibration isolators to control the monitor’s physical environment.

Ozone inlet

The air intake is through Teflon tubing (perfluroalkoxy-polymer, PFA) with a backward-facing inlet positioned on the underside of the instrument wing pod. Air is delivered through a 5 µm PTFE (polytetrafluroethylene) membrane filter to remove fine particles prior to analysis.

Ozone instrument calibrations:

The O₃ monitor has undergone thorough instrument testing in the laboratory to determine the precision, linearity and overall accuracy. Eight-point calibration tests (ranging from 0 – 300 ppbv) are typically performed before and after each flight using an O₃ calibration source (2B Technologies, model 306 referenced to the WMO scale). The calibration of all 2B Technologies Ozone Calibration Sources is traceable to NIST through an unbroken chain of comparisons and is sent back to the vendor annually for calibration. Calibrations in a pressure- and temperature-controlled environmental chamber have also been carried out using the O₃ calibration source over the pressure range 200 - 800 mbar and temperature range -15 to +25 ⁰C; typical pressure and temperature ranges observed in the wing-mounted instrument pod during flight.

The Meteorological Measurement System (MMS) is a state-of-the-art instrument for measuring accurate, high resolution in situ airborne state parameters (pressure, temperature, turbulence index, and the 3-dimensional wind vector). These key measurements enable our understanding of atmospheric dynamics, chemistry and microphysical processes. The MMS is used to investigate atmospheric mesoscale (gravity and mountain lee waves) and microscale (turbulence) phenomena. An accurate characterization of the turbulence phenomenon is important for the understanding of dynamic processes in the atmosphere, such as the behavior of buoyant plumes within cirrus clouds, diffusions of chemical species within wake vortices generated by jet aircraft, and microphysical processes in breaking gravity waves. Accurate temperature and pressure data are needed to evaluate chemical reaction rates as well as to determine accurate mixing ratios. Accurate wind field data establish a detailed relationship with the various constituents and the measured wind also verifies numerical models used to evaluate air mass origin. Since the MMS provides quality information on atmospheric state variables, MMS data have been extensively used by many investigators to process and interpret the in situ experiments aboard the same aircraft.

The 2D-S Stereo Probe is an optical imaging instrument that obtains stereo cloud particle images and concentrations using linear array shadowing. Two diode laser beams cross at right angles and illuminate two linear 128-photodiode arrays. The lasers are single-mode, temperature-stabilized, fiber-coupled diode lasers operating at 45 mW. The optical paths are arbitrarily labeled the “vertical” and “horizontal” probe channels, but the verticality of each channel actually depends on how the probe is oriented on an aircraft. The imaging optical system is based on a Keplerian telescope design having a (theoretical) primary system magnification of 5X, which results in a theoretical effective size of (42.5 µm + 15 µm)/5 = 11.5 µm. However, actual lenses and arrays have tolerances, so it is preferable to measure the actual effective pixel size by dropping several thousands of glass beads with known diameters through the object plane of the optics system.