Operational Status:
requestable, ready for deployment (but some problems are undergoing further study in 2012 in the case of the water-based CN counter). TSI-3010 and TSI-3760A are functional but no long supported by the manufacturer, so are approaching being obsolete.
General Description:
- Photograph of the WCPC-LP water-based CN counter:
- “Condensation nuclei” are particles that serve as condensation centers when exposed to high supersaturations of some condensible vapor like butanol or water. They are normally detected by exposing the airstream to such a supersaturated vapor and then detecting the resulting droplets after they grow to sizes where they are readily counted by, e.g., an optical counter. The supersaturation used is typically about 300% for either butanol or water.
- The primary condensation-nucleus counter used on the NSF/NCAR GV is a modified version of the TSI 3786 Ultra-Fine Water-Based Condensation Nucleus Counter, with modifications made by Aerosol Dynamics, Inc., and Quant. (See the photograph above) The modifications were primarily to lower the temperature in the region where droplets grow on condensation nuclei, which was necessary because the 60 C growth temperature of the standard 3786 is the boiling point when the pressure is about 200 mb, and the GV flies at lower pressure levels. Other changes were made to the flow control, flow rates, pumps, and water injection scheme to adapt to the large altitude range of the GV. One substantial advantage of this instrument over other CN counters is that it does not depend on butanol as the operating fluid and so does not require handling a flammable gas around the aircraft or flight with a flammable substance.
Particles flow continuously through a small internal chamber where they are counted, and concentrations are then determined by dividing by the volume vlow rate. RAF data processing then corrects the measured concentration to ambient air density (not to standard conditions). The threshold particle size detected by the water-based CN counter is about 5 nm, becoming larger at low pressure but remaining below the ultra-fine size range (< 10 nm) at pressures as low as 150 mb. The butanol instruments sense particles larger than about 20 nm. The instrument also is relatively insensitive to coincidence losses, continuing to perform with coincidence losses < 10% up to concentrations around 100,000 cm-3. Tubing losses can be significant for small particles, so size-dependent and pressure-dependent corrections may be needed unless the lines can be kept very short (not more than a few m).
Sources of CN in the atmosphere include combustion by-products (soot), wind-generated dust, sea spray salt, gas-to-particle reactions, residues from in-cloud aqueous chemical reactions, etc. Atmospheric removal of CN occurs primarily through growth to larger sizes by coagulation with other particles, gravitational or inertial settling, and (dominantly) scavenging by clouds. Typical concentrations of CN are ~100 cm-3 in clean background air and ~5,000 cm-3 in cities, with peak values > 50,000 cm-3.
- Photograph of the WCPC-LP water-based CN counter:
Measurement Characteristics:
- Overall estimate of uncertainty:
- Response time: about 1 s
- Precision:
- Other measurement characteristics (comments on signal/noise, bias limits, etc):
