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Atmos. Chem. Phys. Discuss., 10, 18315-18363, 2010
www.atmos-chem-phys-discuss.net/10/18315/2010/ doi:10.5194/acpd-10-18315-2010 © Author(s) 2010. This work is distributed under the Creative Commons Attribution 3.0 License. |
Airborne observation of aerosol optical depth during ARCTAS: vertical profiles, inter-comparison, fine-mode fraction and horizontal variability
1NASA Postdoctoral Program, NASA Ames Research Center, Moffett Field, CA, USA
2Bay Area Environmental Research Institute, Sonoma, CA, USA
3SRI International, Menlo Park, CA, USA
4NASA Ames Research Center, Moffett Field, CA, USA
5School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI, USA
6CARTEL, Université de Sherbrooke, Sherbrooke, Québec, Canada
7Marine Meteorology Division, Naval Research Laboratory, Monterey, CA, USA
8Physical Research Laboratory, Ahmedabad, India
9NASA Goddard Space Flight Center, Greenbelt, ML, USA
10Environment Canada, Toronto, Ontario, Canada
*now at: Aina Kai Environmental, LLC, Aiea, HI, USA
Abstract. We describe aerosol optical depth (AOD) measured during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, conducted in North America in April and June–July 2008, focusing on vertical profiles, inter-comparison with correlative observations, fine-mode fraction and horizontal variability. The AOD spectra spanning 354–2139 nm measured with the 14-channel Ames Airborne Tracking Sunphotometer (AATS-14) are generally less wavelength-dependent below 2 km (499-nm Angstrom exponent 1.4 ± 0.3) than in 2–4 km (1.6–1.8) for Alaska in April 2008. Together with concurrent aerosol mass spectrometry and black carbon incandescence measurements, this corroborates the hypothesis that Arctic haze in these layers originates mainly from anthropogenic emission and biomass burning, respectively. The spectra are within 3%+0.02 of the vertical integral of local visible-light scattering and absorption for two thirds of the 55 vertical profiles examined. The horizontal structure of smoke plumes in central Canada in June and July 2008 explains most outliers. The differences in mid-visible Angstrom exponent are <0.10 for 63% of the profiles with 499-nm AOD>0.1. The retrieved fine-mode fraction of AOD is mostly between 0.7 and 1.0, and its root mean square difference from column-integral submicron fraction (measured with nephelometers, absorption photometers and an impactor) is 0.12. These AOD measurements from the NASA P-3 aircraft, after compensation for below-aircraft light attenuation by vertical extrapolation, mostly fall within 0.02 of AERONET ground-based measurements for five overpass events. Evidently, the fresh local emission in Canada in June and July makes the horizontal distribution of AOD highly heterogeneous (standard deviation ~19% of the mean over 20 km) and random (autocorrelation r=0.37 across 20 km), in contrast to long-range transport to Alaska in April (std~2%, r=0.95). The variability observed over 6 km is noticeably smaller (std~9%, r=0.71). The decrease represents the reduction in collocation error that remote sensing can potentially achieve by improving resolution for ARCTAS Canada and similar environments.
Citation: Shinozuka, Y., Redemann, J., Livingston, J. M., Russell, P. B., Clarke, A. D., Howell, S. G., Freitag, S., O'Neill, N. T., Reid, E. A., Johnson, R., Ramachandran, S., McNaughton, C. S., Kapustin, V. N., Brekhovskikh, V., Holben, B. N., and McArthur, L. J. B.: Airborne observation of aerosol optical depth during ARCTAS: vertical profiles, inter-comparison, fine-mode fraction and horizontal variability, Atmos. Chem. Phys. Discuss., 10, 18315-18363, doi:10.5194/acpd-10-18315-2010, 2010.