Journal of Geophyshysical Research, Vol. 111, No. D5, D05S09The article of record as published may be located at http://dx.doi.org/10.1029/2005JD006250In May 2003, a Twin Otter airplane, equipped with instruments for making in situ
measurements of aerosol optical properties, was deployed during the Atmospheric
Radiation Measurements (ARM) Program’s Aerosol Intensive Operational Period in
Oklahoma. Several of the Twin Otter flights were flown in formation with an instrumented
light aircraft (Cessna 172XP) that makes routine in situ aerosol profile flights over the site.
This paper presents comparisons of measured scattering coefficients at 467 nm, 530 nm,
and 675 nm between identical commercial nephelometers aboard each aircraft. Overall,
the agreement between the two nephelometers decreases with longer wavelength. During
the majority of the flights, the Twin Otter flew with a diffuser inlet while the Cessna had a
1 mm impactor, allowing for an estimation of the fine mode fraction aloft. The fine mode
fraction aloft was then compared to the results of a ground-based nephelometer.
Comparisons are also provided in which both nephelometers operated with identical 1 mm
impactors. These scattering coefficient comparisons are favorable at the longer
wavelengths (i.e., 530 nm and 675 nm), yet differed by approximately 30% at 467 nm.
Mie scattering calculations were performed using size distribution measurements, made
during the level flight legs. Results are also presented from Cadenza, a new
continuous wave cavity ring-down (CW-CRD) instrument, which compared favorably
(i.e., agreed within 2%) with data from other instruments aboard the Twin Otter. With
this paper, we highlight the significant implications of coarse mode (larger than 1 mm)
aerosol aloft with respect to aerosol optical properties
