Measurements of Aerosol Parameters  Aboard the DC-8
during the Transport and Chemical Evolution over the Pacific Mission

  Bruce E. Anderson, Principal Investigator
Wesley R. Cofer, III, Co-Investigator
Edward L. Winstead, Co-Investigator

Chemistry and Dynamics Branch
Atmospheric Sciences Competency
NASA Langley Research Center

We will provide in situ measurements of aerosol size distribution, volatility, and optical characteristics aboard the NASA DC-8 during the Transport and Chemical Evolution over the Pacific (TRACE-P) mission.  The instrument suite will include: 1) an ultrafine  condensation nuclei counter (CNC) to determine the number of particles between 0.003 and 0.012 mm (ultrafine CN) 2) a pair of standard CNCs, one with a heated inlet, to measure the number and volatility of aerosols > 0.012 mm; 2) a scanning mobility particle spectrometer (SMPS) coupled to a CNC to obtain 90-second resolution size distributions for 0.010 to 0.250 mm diameter particles; 3) A PMS passive cavity aerosol spectrometer probe (PCASP) for obtaining dry particle size distributions over the 0.1 to 3.0 mm size-diameter range; 4) A PMS forward scattering spectrometer probe (FSSP-300) to determine particle size distributions over the 0.35 to 3.0 mm size-diameter range 5) a 3-wavelength integrating nephelometer for measuring wavelength-dependent aerosol scattering; 6) an aethelometer for determining total aerosol (i.e., black carbon) absorption; and 7) a cloud, aerosol, and precipitation spectrometer (CAPS) to provide improved particle size information over the 0.3 to 1550 mm  size range along with particle asphericity and cloud liquid water content.   The parameters that will be archived are listed in Table 1 below.  The PCASP, FSSP, and CAPS are designed for aircraft use and will be mounted in the existing aerosol canisters that attach to the DC-8 wingtips.  The FSSP-300 and CAPS have open sample cavities and thus avoid many of the problems associated with anisokinetic sampling that are particularly serious for super micron-sized aerosols.  The PCASP has an enclosed sample cavity into which samples are drawn through a short (about 4 cm) length of heated capillary tubing.  Our calculations suggest that, because the probe is used to size fine aerosols that are much less sensitive to inertial and gravitational effects, this sample extraction technique minimally disturbs the size and number density of the ambient aerosol.   All probes will be calibrated prior to the missions using aerosols of known refractive index and size.  The CNCs, SMPS, nephelometer, and aethelometer are mounted within the aircraft cabin.  Sample air for these instruments will be drawn in through two window-mounted inlets: a scoop-type inlet that shunts particles larger the ~1 mm and an isokinetic, forward-facing inlet that efficiently transmits coarse aerosols.  The scoop inlet has been installed onboard the DC-8 for several previous missions and will deliver samples to the CNCs and SMPS systems.  Its design minimizes problems with precipitation particle shattering which permits collecting interstitial samples within clouds.  Sample air enters though this probe, passes in front of a small sample extraction orifice located within a manifold mounted on the side of the rack, and is subsequently dumped overboard through a rear-facing tube mounted on the aircraft window plate.  Air withdrawn though the sample orifice is diluted with a concentric flow of filtered air to maintain the sensor systems at a constant pressure of 160 Torr regardless of aircraft altitude. 

Table 1.  List of parameters to be archived.