NASA - National Aeronautics and Space Administration

The continuous flow diffusion chambers are oriented for vertical flow through an annular space. They are constructed of two cylindrical, thin, ebonized copper walls that are separated by approximately 1.1 cm. The walls of the CFDC are force-cooled either by circulating coolant through copper tubing coils surrounding the outer wall and inside the inner wall (laboratory CFDC) or by using these same coolant coils as evaporators for refrigeration compressor units (aircraft CFDC). In operation, the walls are coated with ice, achieved by flooding the chamber with water. An inlet manifold directs sample air containing aerosol particles into the center of a laminar flow field where the sample is surrounded on either side by particle-free sheath air (or N2). By varying the set temperatures of the two walls, the warm wall provides a vapor source to the cold wall so that water vapor and temperature fields are created. These fields and airflow determine the conditions of exposure for the aerosols during their typical 5 to 20 s residence time in the CFDC. Ice particles grow to relatively large sizes compared to aerosol particles and are distinguished from them using an optical particle counter (0.4 to 20 mm) at the base of the CFDC.

The aircraft CFDC transitions to a hydrphobic warm wall surface in the lower third of the device so that liquid water drops formed at RH>100% will evaporate, leaving only ice crystals as large particles. The only other physical differences between the two devices is the fact that the laboratory CFDC is approximately 50% longer, providing additional ice crystal growth time at ambient lab pressures and the laboratory device has associated equipment for aerosol generation and preconditioning.

An impactor is sometimes used following the optical counter to collect ice crystals onto specialized transmission electron microscope (TEM) grids for analysis of the residual particles. Calculations of air flow, temperature, and humidity are made assuming steady-state conditions (Rogers, 1988). The temperature and supersaturation range are determined by wall temperatures and air flow.

The RICE is a magnetostrictive oscillation probe with a sensing cylinder 6.35 mm in diameter and 2.54 cm in length. Ice buildup on the sensing cylinder causes the frequency of oscillation to change, which can be related to the rate of ice accretion and hence the cloud liquid water content (LWC). When approximately 0.5 mm of ice has accumulated, a heater melts the ice, which is shed into the air stream. The heater cycle is approximately 5 s, and the cylinder normally requires an additional 5–10 s to cool down to a temperature where it can begin accreting ice again.

PTR-MS is a chemical ionization mass spectrometry technique that allows for fast measurements of organic trace gases. In combination with the CHARON inlet, it measures the organic composition of submicrometer aerosol particles.

The DFGAS instrument utilizes a room temperature infrared (IR) laser source based upon non-linear difference frequency generation (DFG) in the measurement of CH2O.

Mid-IR laser light is generated in the DFG system by mixing the output of two near-IR room temperature laser sources (one at 1562-nm and the other at 1083-nm) in a periodically poled lithium niobate (PPLN) non-linear wavelength conversion crystal. The mid-IR difference frequency at 2831.6 cm-1 (3.53-μm) is generated at the PPLN output and directed through a multipass astigmatic Herriott cell (100-m pathlength using ~ 4-liter sampling volume) and ultimately onto IR detectors employing a number of optical elements. A portion of the IR beam is split off by a special beam splitter (BS) before the multipass cell and focused onto an Amplitude Modulation Detector (AMD) to capture and remove optical noise from various components in the difference frequency generation process. A third detection channel from light emanating out the back of the beam splitter is directed through a low pressure CH2O reference cell and onto a reference detector (RD) for locking the center of the wavelength scan to the absorption line center. The mid-IR DFG output is simultaneously scanned and modulated over the CH2O absorption feature, and the second harmonic signals at twice the modulation frequency from the 3 detectors are processed using a computer lock-in amplifier [Weibring et al. [2006].

Ambient air is continuously drawn through a heated rear-facing inlet at flow rates around 9 standard liters per minute (slm), through a pressure controller, and through the multipass Herriott cell maintained at a constant pressure around 50-Torr. Ambient measurements are acquired in 1-second increments for time periods as long as 60 to 120-seconds (to be determined during the campaign), and this will be followed by 15-seconds of background zero air acquisition, using an onboard CH2O scrubbing unit. The zero air is added back to the inlet a few centimeters from the tip at flow rates ~ 2 to 3 slm higher than the cell flow. This frequent zeroing procedure very effectively captures and removes optical noise as well as residual outgassing from inlet line and cell contaminants. Retrieved CH2O mixing ratios are determined for each 1-second ambient spectrum by fitting to a reference spectrum, obtained by introducing high concentration calibration standards (~ 3 to 7-ppbv) from an onboard permeation calibration system into the inlet approximately every hour. The calibration outputs for the two permeation tubes employed are determined before and after the field campaign using multiple means, including direct absorption employing the Beer-Lambert Law relationship. The 1-second ambient CH2O results can be further averaged into longer time intervals for improved precision. However, in all cases the 1-second results are retained. This flexibility allows one to further study pollution plumes with high temporal resolution, and at the same time study more temporally constant background CH2O levels in the upper troposphere using longer integration times.