The CO2LAS instrument was jointly developed by JPL and Lockheed Martin Coherent Technologies under funding from the NASA Earth Science Technology Office Instrument Incubator Program.

The instrument uses three continuous-wave (c.w.) Th:Ho:YLF lasers, one of which is used as an absolute frequency reference and is locked to a carbon dioxide absorption line in an internal gas cell using a phase modulation spectroscopy scheme. The remaining two lasers are offset frequency locked from the reference laser to provide the online and offline beams that are propagated through the atmosphere. The online and offline beams are expanded to an eye-safe level and transmitted to the ground where they are reflected back to the instrument, collected by the receive optics and detected. The use of the offset frequency-locking scheme together with the absolute frequency reference enables the absolute frequency of the online and offline lasers to be held to within 200 kHz of the desired values. The CO2LAS transceiver uses separate co-axial transmit/receive paths for each of the on-line and off-line channels.

A Doppler frequency shift is induced between the outgoing and return signals by pointing the transmit beams slightly off nadir. This frequency offset, together with a polarization transmit/receive architecture, ensures the receive signals are separated from the transmit signals by both polarization and frequency. The nominal Doppler offset is 15 MHz but this will vary as the aircraft attitude changes. The return signals on each channel are digitized and stored during flight for post-processing. Throughput of the data collection system was increased from ~8% to >20% between 2006 and 2007.

In order to ensure the instrument remains stable, the output power and frequency of all three lasers are monitored. The output power values for the online and offline lasers are used in the determination of the on-line and off-line absorption as part of the LAS measurement. The output power value for the reference laser is used primarily as a laser health status to check the integrity of the CO2 line center lock.

The electronics for the CO2LAS are mounted in two racks that typically mount to the seat rails of the host aircraft. One rack contains the control electronics for the transceiver system, laser controller, frequency locking electronics and provides the user interface for the overall system.

The second rack houses the chiller that supplies the optical transceiver with coolant and the signal processor which receives housekeeping data from the electronics rack, and digitizes, stores and analyzes the lidar return signal. The CO2LAS uses a Gigabit Ethernet system to distribute data across the system and to other computers that can be connected into the gigabit hub located in the back of one of the racks.

Ozone is measured in a dual-beam ultraviolet (254 nm) absorption analyzer. Ambient air flows through one absorption cell while air scrubbed of ozone flows through an adjacent one. This allows continuous measurement of both background and absorption signals. Flows are switched between cells by a pair of solenoid valves, which permits monitoring of optical changes. Water vapor is detected with a tunable diode laser spectrometer designed and built by Randy May. This sensor employs a room-temperature near-infrared laser (single mode at about 1.37 microns) and second harmonic detection, rather than direct absorption. Unlike the JPL water instrument, this sensor has an internal absorption path, optimized for the mid-troposphere. Carbon dioxide is measured by its absorption in the infrared (4.25 microns) using a LiCor NDIR instrument. This is also a dual-cell device, in which the absorption caused by the ambient air sample is compared to that from a reference gas of known composition. Halocarbons are monitored with a custom-built gas chromatograph, using short, packed columns and small ovens, and HP micro-electron capture detectors. Ambient sample and standard will be run simultaneously on paired columns to reduce errors associated with drift in ECD response.

The FCAS II sizes particles in the approximate diameter range from 0.07 mm to 1 mm. Particles are sampled from the free stream with a near isokinetic sampler and are transported to the instrument. They are then passed through a laser beam and the light scattered by individual particles is measured. Particle size is related to the scattered light. The data reduction for the FCAS II takes into account the water which is evaporated from the particle in sampling and the effects of anisokinetic sampling (Jonsson et al., 1995).

The FCAS II and its predecessors have provided accurate aerosol size distribution measurements throughout the evolution of the volcanic cloud produced by the eruption of Mt. Pinatubo. (Wilson et al., 1993). Near co-incidences between FCAS II and SAGE II measurements show good agreement between optical extinctions calculated from FCAS size distributions and extinctions measured by SAGE II.

Accuracy: The instrument has been calibrated with monodisperse aerosol carrying a single charge. The FCAS III and the electrometer agree to within 10%. Sampling errors may increase the uncertainty but a variety of comparisons suggests that total uncertainties in aerosol surface are near 30% (Jonsson, et al., 1995).

Precision: The precision equals 1/ÖN where N is the number of particles counted. In many instances the precision on concentration measurements may reach 7% for 0.1 Hz data. If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time: Data are processed at 0.1 Hz. However, the response time depends upon the precision required to detect the change in question. Small changes may require longer times to detect. Plume measurements may be processed with 1 s resolution.

Weight: Approximately 50 lbs.

The CNC counts particles in the approximate diameter range from 0.006 m to 2 m. The instrument operates by exposing the articles to saturated Flourinert vapor at 28 C and then cooling the sample in a condenser at 5 C. The supersaturation of the vapor increases as it is cooled and the vapor condenses on the particles causing them to grow to sizes which are easily detected. The resulting droplets are passed through a laser beam and the scattered light is detected. Individual particles are counted and are referred to as condensation nuclei (CN). Two CN Counters are provided in the instrument. One counts the particles after sampling from the atmosphere and the second counts particles that have survived heating to 192C. Lab experiments show that pure sulfuric acid particles smaller than 0.05 mm are volatilized in the heater. The heated channel detects when small particles are volatile and permits speculation about the composition. The CNC II contains an impactor collector which permits the collection of particles on electron microscope grids for later analysis. The collector consists of a two stages. In the first stage the pressure of the sample is reduced by a factor of two without loosing particles by impaction on walls. The second stage consists of a thin plate impactor which collect efficiently even at small Reynolds numbers. The system collects particles as small as 0.02 m at WB-57 cruise altitudes. As many as 25 samples can be collected in a flight.

C-STAR measures precipitation, surface water and near ocean surface wind speed and direction.

CIP obtains cloud particle images using a 64-element photodiode array probe to generate 2-Dimensional images of particles from 25-1550 μm, as well as sizing in 1-Dimensional histogram form, and includes housekeeping data.

The Broadband Radiometers (BBR) consist of modified Kipp & Zonen CM-22 pyranometers (to measure solar irradiance) and CG-4 pyrgeometers (to measure IR irradiance) (see http://www.kippzonen.com/). The modifications to make these instruments more suitable for aircraft use include new instrument housings and amplification of the signal at the sensor. The instruments are run in current-loop mode to minimize the effects of noise in long signal cables. The housing is sealed and evacuated to prevent condensation or freezing inside the instrument. Each BBR has the following properties: Field-of-view: Hemispheric Temperature Range: -65C to +80C Estimated Accuracy: 3-5% Data Rate: 1Hz

The Charged-coupled device Actinic Flux Spectroradiometers (CAFS) instruments measure in situ down- and up-welling radiation and combine to provide 4 pi steradian actinic flux density spectra from 280 to 650 nm. The sampling resolution is ~0.8 nm with a full width at half maximum (FWHM) of 1.7 nm at 297 nm. From the measured flux, photolysis frequencies are calculated for ~40 important atmospheric trace gases including O3, NO2, HCHO, HONO and NO3 using a modified version of the NCAR Tropospheric Ultraviolet and Visible (TUV) radiative transfer model. The absolute spectral sensitivity of the instruments is determined in the laboratory with 1000 W NIST-traceable tungsten-halogen lamps with a wavelength dependent uncertainty of 3–5%. During deployments, spectral sensitivity is assessed with secondary calibration lamps while wavelength assignment is tracked with Hg line sources and comparisons to spectral features in the extraterrestrial flux. The optical collectors are characterized for angular and azimuthal response and the effective planar receptor distance. CAFS have an excellent legacy of performance on the NASA DC-8 and WB-57 platforms during atmospheric chemistry and satellite validation mission. These include AVE Houston 2004 and 2005, PAVE, CR-AVE, TC4, ARCTAS, DC3, SEAC4RS, KORUS-AQ, ATom and FIREX-AQ. For FIREX-AQ, upgraded electronics and cooling reduced noise and allowed for a decrease to 1 Hz acquisition.

The Airborne Synthetic Aperture Radar (AIRSAR) was an all-weather imaging tool able to penetrate through clouds and collect data at night. The longer wavelengths could also penetrate into the forest canopy and in extremely dry areas, through thin sand cover and dry snow pack. AIRSAR was designed and built by the Jet Propulsion Laboratory (JPL) which also manages the AIRSAR project. AIRSAR served as a NASA radar technology testbed for demonstrating new radar technology and acquiring data for the development of radar processing techniques and applications. As part of NASA’s Earth Science Enterprise, AIRSAR first flew in 1988, and flew its last mission in 2004.

The single mass analyzer CIMS (S-CIMS) was developed for use on NASA’s ER-2 aircraft. Its first measurements were made in 2000 (SOLVE, see photo). Subsequently, it has flown on the NASA DC-8 aircraft for INTEX-NA, DICE, TC4, ARCTAS, ATom, KORUS, FIREX, as well as on the NCAR C-130 during MILAGRO/INTEX-B. HNO3 is measured by selective ion chemical ionization via the fluoride transfer reaction: CF3O^- + HNO3 → HF • NO3^- + CF2O In addition to its fast reaction rate with HNO3, CF3O^- can be used to measure additional acids and nitrates as well as SO2 [Amelynck et al., 2000; Crounse et al., 2006; Huey et al., 1996]. We have further identified CF3O^- chemistry as useful for the measurement of less acidic species via clustering reactions [Crounse et al., 2006; Paulot et al., 2009a; Paulot et al., 2009b; St. Clair et al., 2010]: CF3O^- + HX → CF3O^- • HX where, e.g., HX = HCN, H2O2, CH3OOH, CH3C(O)OOH (PAA) The mass analyzer of the S-CIMS instrument was first upgraded from a quadrupole to a unit-mass resolution time-of-flight (ToF) analyzer. In 2023, the mass filter was again upgraded to an 1m flight path (~5000 deltaM/M).  The ToF admits the sample ion beam to the ion extractor, where a pulse of high voltage orthogonally deflects and accelerates the ions into the reflectron, which in turn redirects the ions toward the multichannel plate detector. Ions in the ToF follow a V-shaped from the extractor to detector, separating by mass as the smaller ions are accelerated to greater velocities by the high voltage pulse. The detector collects the ions as a function of time following each extractor pulse. The rapid-scan collection of the ToF guarantees a high temporal resolution (1 Hz or faster) and simultaneous data products from the S-CIMS instrument for all mass channels [Drewnick et al., 2005]. We have flown a tandem CIMS (TCIMS) instrument in addition to the SCIMS since INTEX-B (2006). The T-CIMS provides parent-daughter mass analysis, enabling measurement of compounds precluded from quantification by the S-CIMS due to mass interferences (e.g. MHP) or the presence of isobaric compounds (e.g. isoprene oxidation products) [Paulot et al., 2009b; St. Clair et al., 2010]. Calibrations of both CIMS instruments are performed in flight using isotopically-labeled reagents evolved from a gas cylinders or from a thermally-stabilized permeation tube oven [Washenfelder et al., 2003]. By using an isotopically labeled standard, the product ion signals are distinct from the natural analyte and calibration can be performed at any time without adversely affecting the ambient measurement.