AEROSOL RESEARCH GROUP

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CN Counter


CNC   FCAS   FCAS III   N-MASS   MACS   LTI

Instrument Description:

1) Instrument Function. 

The ER-2 CNC counts particles in the approximate diameter range from 0.006 m to 2 m.  The instrument operates by exposing the articles to saturated Flourinert vapor at 28 C and then cooling the sample in a condenser at 5 C.  The supersaturation of the vapor increases as it is cooled and the vapor condenses on the particles causing them to grow to sizes which are easily detected.  The resulting droplets are passed through a laser beam and the scattered light is detected.  Individual particles are counted (Wilson et al., 1983) and are referred to as condensation nuclei (CN).  Two CN Counters are provided in the instrument.  One counts the particles after sampling from the atmosphere and the second counts particles that have survived heating to 192C.  Lab experiments show that pure sulfuric acid particles smaller than 0.05 mm are volatilized in the heater.  The heated channel detects when small particles are volatile and permits speculation about the composition.  The CNC II contains an impactor collector which permits the collection of particles on electron microscope grids for later analysis.  The collector consists of a two stages.  In the first stage the pressure of the sample is reduced by a factor of two without loosing particles by impaction on walls.  The second stage consists of a thin plate impactor which collect efficiently even at small Reynolds numbers.  The system collects particles as small as 0.02 m at WB-57F cruise altitudes.  As many as 25 samples can be collected in a flight.

2) Utility of the Data.

 In the vicinity of the tropopause, the mixing ratios of CN show strong gradients with latitude and altitude.  Thus they can be useful tracers in transport experiments (Wilson, et al. 1991).  The heated and unheated CNC concentrations have provided important information concerning emissions from aircraft (Fahey et al., 1995) and have provided important information concerning new particle formation near the tropical tropopause (Brock et al., 1993).  Aerosol composition also shows strong gradients near the tropopause (Sheridan et al., 1994) and will be very informative concerning rocket emissions.

3) Instrument Characteristics.

Accuracy.
  The instrument has been calibrated with monodisperse aerosol carrying a single charge.  The ER-2 CNC and the electrometer agree to within 15%.  Sampling errors may increase the uncertainty but a variety of comparisons suggests that total uncertainties are near 15%.

Precision.
  The precision equals 1/ÖN where N is the number of particles counted.  In many instances near the tropopause it is better than 3% at a one Hertz data rate.  At cruise altitude, the precision may reach 15%.  If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time
:  Data are acquired at 4 Hz.  However the response time depends upon the precision required to detect the change in question.  Small changes may require longer times to detect.

Weight:
Approximately 68 lbs.

References:

Brock, C. A., P. Hamill, J. C. Wilson, H. H. Jonsson, K. R. Chan,"Particle formation in the upper tropical troposphere: a source of nuclei for the stratospheric aerosol," Science, 270, 1650-1653, (1995).

 Fahey, D. W., E. R. Keim, K. A. Boering, C. A. Brock, J. C. Wilson, H. H. Jonsson, S.  Anthony, T. F. Hanisco, P. O. Wennberg, R. C. Miake-Lye, R. J. Salawitch, N. Louisnard, E. L. Woodbridge, R. S. Gao, S. G. Donnelly, R. C. Wamsley, L. A. Del Negro, S. Solomon, B. C. Daube, S. C. Wofsy, C. R. Webster, R. D. May, K. K. Kelly, M. Loewenstein, J. R. Podolske, K. R. Chan, Emission Measurements of the Concorde Supersonic Aircraft in the Lower Stratosphere, Science, 270,70-74(1995).

Sheridan, P. J., Brock, C. A., Wilson J. C., “Aerosol particles in the upper troposphere and lower stratosphere: Elemental composition and morphology of individual particles in northern midlatitudes” Geophysical Research Letters, 21, 2587-2590, 1994.

Wilson J.C., Blackshear E.D., Hyun J.H., "The Function and Response of an Improved Stratospheric Condensation Nucleus Counter," Journal of  Geophysical Research: Oceans and Atmospheres 88:6781-6785, 1983.

Wilson, J. C., Lai,W. T., Smith, S. D. "Measurements of condensation nuclei above the jet stream: evidence for cross jet transport by waves and new particle formation at high altitudes", Journal of Geophysical Research, 96:17,415-17,423, 1991

 

  Updated August 9, 2011

Contact: jwilson@du.edu