AERI Instrument
General Description
The AERI instrument is an interferometer which measures downward traveling infrared radiation in discrete wavelengths (less than one wavenumber resolution) from 3-25 µm. Additional details about the instrument are below. For information about some of the science done with the instrument, read the "Science" section.
Details
The AERI instrument, shown in Fig 1, is an advanced version of the high spectral resolution interferometer sounder (HIS) designed and fabricated at the University of Wisconsin (Revercomb et al. 1988) to measure upwelling infrared radiances from an aircraft. The AERI is a fully automated ground-based passive infrared interferometer that measures downwelling atmospheric radiance from 3.3 - 18.2 mm (550 - 3000 cm-1) at less than 10-minute temporal resolution with a spectral resolution of one wavenumber. Careful attention to calibration results in an absolute calibration accuracy of better than 1% of the ambient. The AERI instrument foreoptics consist of a scene mirror and two calibration blackbodies. These blackbodies are essential to provide a well known stable hot and ambient temperature reference for calibration of the downwelling skyview radiances. A typical measurement cycle consists of a three-minute sky dwell period followed by two-minute dwell periods for each of the blackbodies. While the interferometer acquires an uncalibrated spectrum every two seconds, averaging reduces the radiometric noise in the measurements. The temperature of one of the blackbodies is fixed at 60 oC, while the other fluctuates with the ambient temperature.
Figure 1: The AERI instrument deployed at the DOE ARM SGP CART site.
Both of the AERI calibration reference sources are high emissivity blackbody cavities containing highly accurate temperature sensors. Calibration error analysis shows that for an instrument that must operate within an ambient atmospheric environment, the extrapolation of the hot-ambient calibration to the coldest ambient scene temperatures has a comparable accuracy to a calibration that makes use of a stable cold target (e.g. liquid nitrogen). This is because the temperature and emissivity uncertainty in reference cavities operated at or above ambient temperature can be made much smaller than those typically operated below the dewpoint temperature.
Since the AERI system performs a self-calibration every 10 minutes, before and after each sky view, any temperature drifts in the ambient blackbody or the internal instrument temperature are accurately accounted for. One of the advantages of using an ambient calibration point is that much of the emission the AERI measures is radiating from the atmosphere near the environmental ambient temperature. This means that the emission from near the surface is measured very precisely with the AERI instrument.
This hot/ambient approach greatly simplifies the operations of the instrument by removing the requirement for large amounts of liquid nitrogen to provide a cold calibration source (Revercomb et al. 1988). However, since the detector (a sandwiched HgCdTe/InSb detector, providing sensitivity for 5.5 - 18.2 mm and 3.3 - 5.5 mm in channels 1 and 2, respectively) requires cooling, a mechanical Stirling cooler has been employed. An example of an AERI observed spectrum from channels 1 and 2 is shown in Fig 2. The highlighted radiance regions indicate where the temperature (red) and water vapor (blue) profiles are derived using the physical retrieval algorithm.
Figure 2: An example of an AERI infrared radiance measurement at one wavenumber resolution. The colored regions indicate the part of the spectrum in which temperature (red) and water vapor (blue) profiles are derived using the AERI physical retrieval algorithm.
Presentations
- AERI Instrument Radiometric Calibration
- F. A. Best, H. E. Revercomb, D. D. LaPorte, R. O. Knuteson, and W. L. Smith