Markus Petters,
PhD candidate, University of Wyoming
Defense
Laboratory
studies that constrain uncertainties in the calibration of two static thermal
gradient diffusion cloud concentration nuclei (CCN) instruments are presented.
The relative uncertainty in CCN concentration is determined to be between 10 to
30%, depending on the applied chamber supersaturation.
The latter was inferred by challenging the instruments with particles of known
composition made monodisperse in a differential
mobility analyzer (DMA) at two relative humidities
(73% and 10%). Results show that the effect of test particle asphericity or degree of compactness are not
responsible for a significant bias in the experimental assessment of applied
chamber supersaturation.
CCN
concentrations observed during the second Dynamics and Chemistry of Marine
Stratocumulus experiment (DYCOMS-II) are also presented. A formalism that combines atmospheric
variability and measurement uncertainties associated with the fitted CCN
activation spectra is developed. Observed CCN data are compared with
predictions based on aerosol size spectra and chemical composition
measurements.
Predictions
and observations are in agreement within the uncertainties imposed by both
measurement error and computational assumptions, with the exception of
measurements made at the effective supersaturation s
= 0.1%. Sensitivity studies
indicate that the discrepancy at s = 0.1% could be explained by assuming that
the particles were internally mixed with a soluble mass fraction of 0.25, or
that the passive cavity aerosol size spectrometer (PCASP) instrument was not
completely removing chemically bonded water.
A
case study analyzing Pockets of Open Cells (POCs)
during the second DYCOMS-II research flight is also presented. POCs are elucidated based on an aerosol/cloud microphysical
hypothesis. If this hypothesis is correct, pristine marine boundary layer
clouds may be more susceptible to aerosol indirect effects than thought previously. Also analyzed are Aitken
mode particles that are likely to be the result of recent a homogeneous
particle nucleation event that occurred within the marine boundary layer and
appeared to be associated with POCs. These particles
were not effective cloud nuclei and are thought to be non-hygroscopic.