The VCSEL hygrometer is an open-path, laser-based hygrometer that measures absolute concentration of water vapor (molecules per cm-3) at a rate of 25 Hz. The instrument is designed for operation throughout the troposphere and lower stratosphere. Two water vapor absorption lines are used: a “weak” line at 1853.37 nm for lower tropospheric mixing ratios and a “strong” line at 1854.03 nm for middle and upper tropospheric concentrations. VCSELs have a wide current tuning capability and can probe each line by changing the laser injection current with only slight adjustments to the laser temperature. Switching between the absorption lines generally occurs near a fractional absorption of 10-3, though a hysteresis is built in to prevent rapid switching near this transition (generally a factor of four in each direction). The thresholds for line switching changes slightly with temperature and pressure, but it generally is in the -15 to -25 C frost point range.

The University of Colorado Closed-path Laser Hygrometer, version 2 (CLH2) is an infrared absorption instrument designed to measure so-called “total water”, the sum of water vapor and particulate water. It is a second-generation sensor that derives from the original CLH and was developed for the NSF DC3 campaign in 2011 as an alternative to the NCAR CVI for measurements of cloudwater contents. It has flown on the NASA DC-8 and the NSF/NCAR G-V and C-130. The most recent campaign was NSF SOCRATES in 2018. CLH-2 uses a fiber-coupled tunable diode laser at 1.37 μm to measure by absorption the water vapor resulting from the evaporation of cloud particles. The spectrometer will be housed in a modified PMS canister and coupled to a heated forward-facing inlet. Sampling of particles is deliberately sub-isokinetic, which results in enhancements of particle mass relative to ambient by factors ranging between 30 and 70. Therefore, condensed water even in very thin clouds can be measured with high precision and accuracy.

The GV Scanning Mobility Particle Sizer (SMPS) measures the particle size distribution over the mobility diameter range of 3 to 500 nm (pressure-dependent). It consists of two components: an electrostatic classifier (EC) and a condensation particle counter (CPC). The EC samples aerosol-laden ambient air, places a well-defined charge distribution on the particles, and then selects a narrow range of particle “mobility diameter” (approx. equal to cross-sectional area-to-charge ratio) using a differential mobility analyzer (DMA). The selected diameter can be scanned by a time-varying high voltage applied to the DMA; following this particles are counted by the CPC. The total scan time and the number of counting intervals, the latter of which determines the number of diameter bins in the size distribution, are selected based on ambient particle concentrations and altitude. The raw data (particle counts over each counting interval as a function elapsed time during the linear diameter scan) is mathematically inverted during post-processing to obtain the particle size distribution.

The primary condensation nucleus counter used on the NSF/NCAR G-V is a modified version of the TSI 3786 Ultra-Fine Water-Based Condensation Nucleus Counter, with modifications made by Aerosol Dynamics, Inc., and Quant. The modifications were primarily to lower the temperature in the region where droplets grow on condensation nuclei, which was necessary because the 60 C growth temperature of the standard 3786 is the boiling point when the pressure is about 200 mb, and the GV flies well above this altitude. Other changes were made to the flow control, flow rates, pumps, and water injection scheme to adapt to the large altitude range of the G-V. One substantial advantage of this instrument over other CN counters is that it does not depend on butanol as the operating fluid and so does not require handling of a flammable gas around the aircraft or flight with a flammable substance.

The threshold particle size detected by the WCN is about 5 nm, becoming larger at low pressure but remaining below the ultra-fine size range (<10 nm) at pressures as low as 150 mb. The instrument also is relatively insensitive to coincidence losses, continuing to perform with coincidence losses <10% up to concentrations around 10⁵ cm-3. Tubing losses can be significant for small particles, so size-dependent and pressure-dependent corrections may be needed unless the lines can be kept very short (not more than a few m).

The Three-View Cloud Particle Imager (3V-CPI) measures the size, shape and concentration of water drops and ice particles in clouds. The probe is a combination of three imaging instruments. Two of them comprise a 2D-S (Two- Dimensional Stereo hydrometeor spectrometer), in which two high-resolution (about 9 mm resolution) 2D probes image particles as they pass through laser beams that are orthogonal to each other. If particles also lie in the intersection of the sensitive areas of the two beams, they are seen by both 2D probes. In that case, the third instrument, a Cloud Particle Imager (CPI), is triggered to take a high-resolution picture, via a briefly illuminated high-resolution imaging array. This image has a pixel size of about 2.3 µm and so provides very high resolution for determining shapes and habits of ice crystals. The probe is particularly suited to imaging such crystals, but also provides good detection of other hydrometeors including large cloud droplets, drizzle and small rain drops, and other precipitation particles.

The 2DC probe records images of hydrometeors that pass through its sample volume, and so provides measurements of ice or water drop concentration, their size distribution, and their shapes. It obtains these images by recording the status (illuminated or shadowed) of a 64-element photodiode array as the shadow of the hydrometeor passes over the array. Probes with 25 µm and 10 µm resolution are available; at 25 µm, the 64-element array provides a sample of about 8 L per 100 m of flight. Images of individual particles are recorded, usually with no loss except at very high concentrations. Special records containing these images in digital form are recorded as needed, so they will be interspersed with the standard periodically sampled records. The 2DC probe was originally manufactured by Particle Measuring Systems, Inc., but the electronics have been replaced with high-speed circuitry matched to the flight speed of the G-V, data transmission has been changed to USB-2, the photodiode array was replaced with one having twice as many elements and supporting faster response, and other changes were made to the optics and electronics of the G-V 2DCs.

Because the depth of field reduces to less than the distance between the arms that define the sample aperture for particle sizes less than about 125 µm, and because diffraction makes the sizes of such small particles hard to determine, the probe has limited ability to measure concentrations at sizes less than about 100 µm, even though it has resolution smaller than this. The array size and optics limit the largest size that can be imaged fully to 1600 µm for the 25-µm-resolution probe. The probe also has been shown to measure falsely high concentrations as a result of shattering (Korolev et al., 2011), so new tips have been installed that reduce but do not eliminate the effects of shattering.

TOGA measures volatile organic compounds (VOCs). TOGA was deployed with an Agilent quadrupole mass spectrometer from 2006 through 2018. Since 2019, the TOGA has been equipped with a TOFWERK high-resolution time-of-flight (HR-TOF) mass spectrometer detector (TOGA-TOF). Specific data will be obtained for radical precursors, tracers of anthropogenic and biogenic activities, tracers of urban and biomass combustion emissions, tracers of ocean emissions, products of oxidative processing, precursors to aerosol formation, and compounds important for aerosol modification and transformation. TOGA measures a wide range of VOCs with high sensitivity (low to sub-ppt), frequency (2 minutes or better), accuracy (20% or better), and precision (<3%). Over 100 species are routinely measured throughout the troposphere and lower stratosphere from the surface to 16 km or higher. See table for list of VOCs that have been quantified using TOGA and TOGA-TOF. The TOGA-TOF is contained in a dual extended HIAPER rack, weighs approximately 225 kg and consumes ~1 kW of power. The major components of the instrument are the inlet, cryogenic preconcentrator, gas chromatograph, time-of-flight mass spectrometer detector, zero air/calibration system, and the control/data acquisition system. All processes and data acquisition are computer controlled.