Characterization of instrument-specific parameters of a cloud condensation nuclei counter
J. T.
(1)
Oberman
and T. M.
(1) Department of Chemical and Biological Engineering, University of Wisconsin – Madison
(2)
VanReken
(2) Department of Civil and Environmental Engineering, Washington State University, Pullman
A) Introduction
E) Results
The
largest
uncertainties
associated
with
anthropogenic climate change arise from the effects
of airborne particles (aerosols) on the optical
properties of clouds, as the relationship between the
two is not fully understood (IPCC 2007). Recently,
improved instruments such as the Continuous-Flow
Streamwise Thermal Gradient CCN Counter (CFSTGC)
have been developed to count the number of
particles that can act as cloud condensation nuclei
(CCN) as a function of supersaturation. Because of its
complexity, data from the CCN counter is best
analyzed with an instrument model. This work aims
to determine thermal resistance, an instrument
parameter used as model input.
• The thermal resistance of our CCN counter is
approximately 3.7 K/W.
• The fraction of doubly charged particles becomes
more significant at lower supersaturations.
• Behavior at supersaturations of .2% or lower is
different from that at higher supersaturations. The
instrument model may not apply in this range.
Figure 5 – Activation curves for three different days
• The choice of Köhler model formulation is influential
in the calculation of thermal resistance, on the order
of 30% relative difference for the formulations tested.
This is consistent with the findings of Rose et al 2008.
D) Analysis
1) Coincidence Correction – We corrected for
multiple particles passing simultaneously through the
detector of the CPC or CCN counter.
P0H20
PH20
Heat
Water
Vapor
External
Thermoelectric
coolers
3) Curve Fit – A cumulative Gaussian distribution
function, as recommended by Rose et al 2008, was fit
to each run to obtain the particle diameter at half
activation (the cut diameter).
Figure 3 - Our CFSTGC with the cover removed
Supersaturation Profile
C) Data Collection
Figure 1 – The operating principle behind the CFSTGC
B) Theory
A particle’s ability to act as a CCN depends on size
and composition. This relationship is quantified by
the Köhler equation:
 2ν wσ sol 
S = α w exp

 RTr 
2) Charge Correction – DMA operation assumes all
particles are singly charged or neutral, so multiply
charged particles required correction.
Temperature
Air
Flow
(1)
where S is the saturation ratio, αw is the activity of
water, νw is the partial molar volume of water, σsol is
the solution surface tension, R is the gas constant, T is
the temperature and r is the particle radius.
Thermal resistance can be calculated if the CCN
counter’s effective supersaturation (Seff) and external
temperature gradient are known. The external
gradient is set by the user, so our experiments were
designed to estimate Seff. An aerosol of known
composition was generated by an atomizer, size
selected by a Differential Mobility Analyzer (DMA)
and then split between a Condensation Particle
Counter (CPC), which counts total particles, and our
CCN counter. Five external gradients were selected to
bracket instrument supersaturations of atmospheric
relevance. For each gradient, the ratio of CCN to total
particles (the “activated fraction”) was measured
over a range of particle sizes. Activated fraction
increases with size in a characteristic sigmoidal trend;
these activation curves can be seen in Figure 5. Each
curve was used to compute Seff via Köhler theory. The
calculated supersaturations and measured external
gradients were fit to the empirical relationships
derived by Lance et al 2006 to estimate the thermal
resistance. Fifteen estimates, three at each gradient,
were made over three days. An arithmetic mean of
these estimates was taken to be the true thermal
resistance of the instrument.
Air
The CCN counter exposes aerosol to a known
supersaturation, then counts particles that “activate”
and grow rapidly to droplet size. Supersaturation is
developed in the instrument by applying an external
temperature gradient to a wetted column (Roberts
and Nenes 2005). However, heat loss across the walls
affects the supersaturation profile, and must be
included in instrument models as a function of the
thermal resistance of the walls. This key parameter
varies considerably from instrument to instrument.
To atmosphere
Figure 6 – Comparison of fit functions and charge
correction schemes for the calibration data
HEPA filter
Ammonium
sulfate
solution
Pressurized Sheath Flow
• The negative correlation between nominal
supersaturation and thermal resistance is not
physical. It may represent a fault in the methods
used or a problem with the model and should be
investigated.
G) References
Needle
Valve
Diffusion
dryer
F) Future Studies
•Any future field studies taking data at low
supersaturations (less than or equal to .2%) should
attempt to calibrate the instrument in the field rather
than use the model.
Dilution
System
HEPA filter
Atomizer
(generates
aerosol)
Figure 7 – The colors represent different approaches
to determining the critical supersaturation from the
calibration data using Equation 1. They differ in how
they estimate αw.
• The instrument model should be verified before it is
applied to field data.
Vacuum Pump
Flow controller
Figure 4 – The arrangement of instruments used in this experiment
To atmosphere
IPCC: Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change
(2007), edited by: Solomon, S., Qin, D., Manning, M., et al.,
Cambridge University Press.
Lance, S., J. Medina, J.N. Smith and A. Nenes (2006), Mapping the
operation of the DMT continuous flow CCN counter, Aerosol
Science and Technology, 40(4), 242-254.
Neutralizer
DMA (size selector)
Figure 2 – Activation behavior of a 50 nm particle
4) Köhler calculation – The cut diameter from the fit
was assumed to be the smallest activated particle,
meaning it had a critical supersaturation equal to Seff.
Our aerosol was of known composition, so we used
it’s size and density to estimate αw, νw and r in
Equation 1. By solving for the maximum of S, we
obtained the critical supersaturation of the particle
and therefore Seff.
CPC
(total particle
counter)
CFSTGC
(CCN counter)
Roberts, G. C. and A. Nenes (2005), A continuous-flow streamwise
thermal gradient CCN chamber for atmospheric measurments.
Aerosol Science and Technology, 39 (3), 206-221.
Rose, D., S.S. Gunthe, E. Mikhailov, G.P. Frank, U. Dusek, M.O.
Andreae, and U. Poschl (2008), Calibration and measurement
uncertainties of a continuous –flow cloud condensation nuclei
counter (DMT-CCNC): CCN activation of ammonium sulfate and
sodium chloride aerosol particles in theory and experiment,
Atmospheric Chemistry and Physics, 8(5), 1153-1179.
H) Acknowledgments
The authors would like to thank the National Science Foundation
for funding through the Research Experience for Undergraduates
program, grant number ATM-0754990