The C-130 aerosol instrumentation is similar to that flown during the ACE-1 experiment
and some of the comments are provided in that context. It is planned during INDOEX that
flyby intercomparisons with ship and ground platforms will be carried out as in ACE-1.
CN counters (TSI 3025, 3010, 3760): A large number of these instruments were compared extensively at a pre-ACE-1 workshop in Seattle and agreed well with manufacturers performance figures. A paper from this workshop is in preparation. During ACE-1 field intercomparison of aircraft, ship and ground CN counter data were almost identical (R. Weber, ACE-1 workshop 3/97). These results suggest that another workshop is not necessary for these counters and that field intercomparisons will suffice.
Nephelometer (TSI): This instrument has also been extensively tested and results were published (T. Anderson). It is field calibrated with test gases and makes continuous internal reference measurements. ACE-1 field intercomparison data between platforms were very good and within measurement uncertainty. Results suggest that standard field gas calibration is adequate.
Internal Optical Particle counter (LAS-X): measures dry aerosol size - field calibrations possible using DMA and latex spheres. Major uncertainties are due to Mie oscillation and ambient refractive index. Data stored raw and can be converted based upon measured aerosol composition and refractive index. Expected to be accurate to about 20% but can provide relative compositional change information (eg. sulfate, soot) regardless of absolute calibration.
RDMA: This instrument is actually a calibration standard for aerosol size provided flows are know correctly. These are generally known better than 5%. does require correction of diffusive loss of particles at lower sizes. These can be determine in the laboratory and applied to field data.
The integral over the size distribution is also continuously comparable to the CN counters such that anomalous behavior is readily detected. Integral differences between ACE-1 platforms was less than 20% before corrections. Data from ACE-1 workshops and field data are adequate to define performance provided intercomparisons are made between platforms during INDOEX.
Wing Mounted Optical Probes (NCAR): These instruments measure ambient aerosol and calibration depends on refractive index used. Data from the experiment can be used to assess this in conjunction with the MASP that intended to measure refractive index. Standard calibration procedures will be followed for these instruments and some redundancy of instrumentation is planned (ie. additional PCASP from A. Andreae and additional FSSP from A. Clarke). A PVM-100 that provides liquid water and effective radius measurement is also included that has been calibrated and shown to provides a bound to integral droplet distributions.
Filter Pack and Impactor data- These data for Ion Chromatography are usually good to about +/- 5%.
Cloud Condensation Nucleus Counter: The widely recommended instrument (J. Hudson, DRI) employs in flight calibration with aerosol sourced from a DMA. A secondary system will also be flown if space is available (A. Andreae) but it does not have flight experience at this date). A CCN counter workshop has not been carried out to our knowledge perhaps due to the limited number of comparable specialized instruments that exist. A new design (R. Flagan) at CALTECH) is also being constructed. We would like to see a CCN intercomparison workshop prior to INDOEX but do not know if this is feasible or not. At this point we depend upon the calibration procedures and reputation of J. Hudson and his instrumentation.
CVI: The NCAR Counter Volume Impactor has been calibrated in previous studies but I am unfamiliar with operational in-field calibrations.
A key concern of any aircraft aerosol sampling platform is that of particle losses in inlets. We expect to use the C-130 with the Community Aerosol Inlet that was not fully operational during the ACE-1 program. In that experiment the observations to date suggest that it lost most aerosol in the boundary layer with dry sizes above 3µm (about 6µm wet). This should not be too serious for pollution aerosol with a peak in submicrometer mass but would result in underestimates of dust aerosol aspirated into the aircraft. However, large dust aerosol is usually dry meaning that probably up to 6µm particles will not be lost and is also of sizes that can be effectively detected by the wing probes. Moreover, an intercomparison experiment (CAINE2) is scheduled for the summer of 1997 where improvements will be implemented and characterized for the transmission of this inlet (B. Huebert, UH)
Cloud Radiometer and Lidar
MCR: The Multichannel Cloud Radiometer is a 7-channel scanning radiometer built by NASA GSFC and on loan to NCARıs RAF. The radiometer, as planned for INDOEX, will have channels at 0.63, 0.754, 0.756, 1.139, 1.626, 2.165 and 10.757 mm. It has an IFOV of 0.007 radians and a nadir view angle of ħ45. The radiometer will be used to remotely determine aerosol optical depths, scattering phase functions, single-scattering albedos, cloud optical depths and droplet radii. Some of these determinations, such as the scattering phase function, will require flight patterns which in combination with MCRıs scanning will provide multiple views of a scene from which effects due to aerosol concentrations are to be separated from those due to the scattering phase function. Cloud retrievals will use techniques like those used previously with the MCR by Nakajima et al (1991). Determination of the single scattering albedo will rely on differential absorption in and out of the oxygen A-band. The MCR is being refurbished to its original condition with the spectral channels noted above by ATD. Its calibration and characterization will be undertaken as part of the refurbishment. In the field, calibrations will be monitored by matching radiances collected with the MCR to those obtained with satellite sensors and with the TDDRs.
SABL: The Scanning Airborne Backscatter Lidar (SABL) is a doubled-Nd-YAG system capable of probing in the upward and downward directions from the C-130. It has output at 1.06 and 0.532 mm. It will be used to detect and characterize the vertical extent of aerosol layers, both above and below the C-130. For these layers, it will provide backscattered extinction and through the ratio of the returns at the two wavelengths, an index of aerosol sizes. It will also be used to obtain cloud top altitudes. SABL operated on the C-130 during ACE-1 and in a boundary layer, marine stratus experiment off the coast of California.