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.

Argus is a two channel, tunable diode laser instrument set up for the simultaneous, in situ measurement of CO (carbon monoxide), N2O (nitrous oxide) and CH4 (methane) in the troposphere and lower stratosphere. The instrument measures 40 x 30 x 30 cm and weighs 21 kg. An auxiliary, in-flight calibration system has dimensions 42 x 26 x 34 cm and weighs 17 kg.

The instrument is an absorption spectrometer operating in rapid scan, secondharmonic mode using frequency-modulated tunable lead-salt diode lasers emitting in the mid-infrared. Spectra are co-added for two seconds and are stored on a solid state disk for later analysis. The diode laser infrared beam is shaped by two anti-refection coated lenses into an f/40 beam focused at the entrance aperture of a multi-pass Herriott cell. The Herriott cell is common to both optical channels and is a modified astigmatic cell (New Focus Inc., Santa Clara, California).

The aspherical mirrors are coated with protected silver for optimal infrared reflectivity. The cell is set up for a 182-pass state for a total path of 36m. The pass number can be confirmed by visual spot pattern verification on the mirrors observed through the glass cell body when the cell is illuminated with a visible laser beam. However, instrument calibration is always carried out using calibrated gas standards with the Argus instrument operating at its infrared design wavelengths, 3.3 and 4.7 micrometers respectively for CH4 and CO detection. The electronic processing of the second harmonic spectra is done by standard phase sensitive amplifier techniques with demodulation occurring at twice the laser modulation frequency of 40 kHz. To optimize the secondharmonic signal amplitude in a changing ambient pressure environment the laser modulation amplitude is updated every 2 seconds to its optimal theoretical value based upon the measured pressure in the Herriott cell.

The NOAA chemical ionization mass spectrometer (CIMS) instrument was developed for high-precision measurements of gaseous nitric acid (HNO3) specifically under high- and variable-humidity conditions in the boundary layer. The instrument’s background signals (i.e., signals detected when HNO3-free air is measured), which depend on the humidity and HNO3 concentration of the sample air, are the most important factor affecting the limit of detection (LOD). A new system to provide HNO3-free air without changing both the humidity and the pressure of the sampled air was developed to measure the background level accurately. The detection limit was about 23 parts per trillion by volume (pptv) for 50-s averages. Field tests, including an intercomparison with the diffusion scrubber technique, were carried out at a surface site in Tokyo, Japan, in October 2003 and June 2004. A comparison between the measured concentrations of HNO3 and particulate nitrate indicated that the interference from particulate nitrate was not detectable (i.e., less than about 1%). The intercomparison indicated that the two independent measurements of HNO3 agreed to within the combined uncertainties of these measurements.