In situ aerosol-size distributions and clear-column radiative closure during ACE-2

Author
Collins, D.R.
Jonsson, H.H.
Seinfeld, J.H.
Flagan, R.C.
Gasso, S.
Hegg, D.A.
Russell, P.B.
Schmid, B.
Livingston, J.M.
Ostrom, E.
Noone, K.J.
Date
2000Metadata
Show full item recordAbstract
As part of the second Aerosol Characterization Experiment (ACE-2) during June and July
of 1997, aerosol-size distributions were measured on board the CIRPAS Pelican aircraft
through the use of a DMA and 2 OPCs. During the campaign, the boundary-layer aerosol
typically possessed characteristics representative of a background marine aerosol or a
continentally influenced aerosol, while the free-tropospheric aerosol was characterized by
the presence or absence of a Saharan dust layer. A range of radiative closure comparisons
were made using the data obtained during vertical profiles flown on 4 missions. Of particular
interest here are the comparisons made between the optical properties as determined through
the use of measured aerosol-size distributions and those measured directly by an airborne
14-wavelength sunphotometer and 3 nephelometers. Variations in the relative humidity
associated with each of the direct measurements required consideration of the hygroscopic
properties of the aerosol for size-distribution-based calculations. Simultaneous comparison
with such a wide range of directly-measured optical parameters not only offers evidence of
the validity of the physicochemical description of the aerosol when closure is achieved, but
also provides insight into potential sources of error when some or all of the comparisons
result in disagreement. Agreement between the derived and directly-measured optical properties
varied for different measurements and for different cases. Averaged over the 4 case
studies, the derived extinction coefficient at 525 nm exceeded that measured by the sunphotometer
by 2.5% in the clean boundary layer, but underestimated measurements by 13%
during pollution events. For measurements within the free troposphere, the mean derived
extinction coefficient was 3.3% and 17% less than that measured by the sunphotometer
during dusty and non-dusty conditions, respectively. Likewise, averaged discrepancies
between the derived and measured scattering coefficient were −9.6%, +4.7%, +17%, and
−41% for measurements within the clean boundary layer, polluted boundary layer, free
troposphere with a dust layer, and free troposphere without a dust layer, respectively. Each
of these quantities, as well as the majority of the .100 individual comparisons from which
they were averaged, were within estimated uncertainties.
Description
Tellus (ACE-2 Special Issue), 52B, 498-525.
Rights
This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.Collections
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