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| Titel |
Vertical profiles of aerosol optical properties over central Illinois and comparison with surface and satellite measurements |
| VerfasserIn |
P. J. Sheridan, E. Andrews, J. A. Ogren, J. L. Tackett, D. M. Winker |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 23 ; Nr. 12, no. 23 (2012-12-10), S.11695-11721 |
| Datensatznummer |
250011655
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| Publikation (Nr.) |
 copernicus.org/acp-12-11695-2012.pdf |
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| Zusammenfassung |
| Between June 2006 and September 2009, an instrumented light aircraft
measured over 400 vertical profiles of aerosol and trace gas properties over
eastern and central Illinois. The primary objectives of this program were to
(1) measure the in situ aerosol properties and determine their vertical and temporal
variability and (2) relate these aircraft measurements to concurrent surface
and satellite measurements. The primary profile location was within 15 km of
the NOAA/ESRL surface aerosol monitoring station near Bondville, Illinois.
Identical instruments at the surface and on the aircraft ensured that the
data from both platforms would be directly comparable and permitted a
determination of how representative surface aerosol properties were of the
lower column. Aircraft profiles were also conducted occasionally at two
other nearby locations to increase the frequency of A-Train satellite
underflights for the purpose of comparing in situ and satellite-retrieved aerosol
data. Measurements of aerosol properties conducted at low relative humidity
over the Bondville site compare well with the analogous surface aerosol data
and do not indicate any major sampling issues or that the aerosol is
radically different at the surface compared with the lowest flyby altitude
of ~ 240 m above ground level. Statistical analyses of the in situ
vertical profile data indicate that aerosol light scattering and absorption
(related to aerosol amount) decreases substantially with increasing
altitude. Parameters related to the nature of the aerosol (e.g.,
single-scattering albedo, Ångström exponent, etc.), however, are
relatively constant throughout the mixed layer, and do not vary as much as
the aerosol amount throughout the profile. While individual profiles often
showed more variability, the median in situ single-scattering albedo was 0.93–0.95
for all sampled altitudes. Several parameters (e.g., submicrometer
scattering fraction, hemispheric backscattering fraction, and scattering
Ångström exponent) suggest that the fraction of smaller particles in
the aerosol is larger near the surface than at high altitudes. The observed
dependence of scattering on size, wavelength, angular integration range, and
relative humidity, together with the spectral dependence of absorption, show
that the aerosol at higher altitudes is larger, less hygroscopic, and more
strongly absorbing at shorter wavelengths, suggesting an increased
contribution from dust or organic aerosols. The aerosol profiles show
significant differences among seasons. The largest amounts of aerosol (as
determined by median light extinction profile measurements) throughout most
of the sampled column were observed during summer, with the lowest amounts
in the winter and intermediate values in the spring and fall. The highest
three profile levels (3.1, 3.7, 4.6 km), however, showed larger median
extinction values in the spring, which could reflect long-range transport of
dust or smoke aerosols. The aerosols in the mixed layer were darkest (i.e.,
lowest single-scattering albedo) in the fall, in agreement with surface
measurements at Bondville and other continental sites in the US. In situ profiles
of aerosol radiative forcing efficiency showed little seasonal or vertical
variability. Underflights of the CALIPSO satellite show reasonable agreement
in a majority of retrieved profiles between aircraft-measured extinction at
532 nm (adjusted to ambient relative humidity) and CALIPSO-retrieved
extinction, and suggest that routine aircraft profiling programs can be used
to better understand and validate satellite retrieval algorithms. CALIPSO
tended to overestimate the aerosol extinction at this location in some
boundary layer flight segments when scattered or broken clouds were present,
which could be related to problems with CALIPSO cloud screening methods. The
in situ aircraft-collected aerosol data suggest extinction thresholds for the
likelihood of aerosol layers being detected by the CALIOP lidar. In this
study, aerosol layers with light extinction (532 nm) values > 50 Mm−1
were detected by CALIPSO ~ 95% of the time, while
aerosol layers with extinction values lower than 10 Mm−1 had a
detection efficiency of < 2%. For all collocated comparison
cases, a 50% probability of detection falls at an in situ extinction level of
20–25 Mm−1. These statistical data offer guidance as to the likelihood
of CALIPSO's ability to retrieve aerosol extinction at various locations
around the globe. |
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