Abstract
Abstract. Aerosol optical properties were measured by NOAA's Airborne Aerosol Observatory over Bondville, Illinois, during more than two years using a light aircraft. Measured properties included total light scattering, backscattering, and absorption, while calculated parameters included aerosol optical depth (AOD), Ångström exponent, single-scattering albedo, hemispheric backscatter fraction, asymmetry parameter, and submicrometer mode fraction of scattering. The in-situ aircraft measurements are compared here with AERONET measurements and retrievals of the aerosol optical properties at the same location, although it is difficult to verify the AERONET retrieval algorithm at a site that is not highly polluted. The comparison reveals discrepancies between the aerosol properties retrieved from AERONET and from in-situ aircraft measurements. These discrepancies are smaller for the AOD, while the biggest discrepancies are for the other derived aerosol properties. Possible sources of discrepancy between the AOD measured by AERONET and the one calculated from the in-situ aircraft measurements are investigated. The largest portion of the AOD discrepancy is likely due to an incorrect adjustment to ambient RH of the scattering coefficient. Another significant part (along with uncertain nephelometer truncation corrections) may come from the possibility that there might be less aerosol below the lowest flight altitude or that the aircraft inlet excludes aerosol particles larger than 5–7 μm diameter.
Highlights
The comparison shows good correlation for both fits used (R2 ≈ 0.80 for the standard linear regression not forced through zero and R2 ≈ 0.72 when the line is forced through zero), the AErosol RObotic NETwork (AERONET) aerosol optical depth (AOD) values are higher than the calculated Aerosol Observatory (AAO) AOD by nearly a factor of 1.5 on average, based on the slope of the line forced through zero
The comparison showed discrepancies between aerosol properties retrieved from AERONET and those retrieved from in-situ aircraft measurements
The comparison of the AOD measured by the AERONET sun photometer with that calculated from the AAO aircraft measurements showed good correlation (R2 ≈ 0.79), but the AERONET AOD values were between 27 % and 47 % higher than the calculated AAO AOD
Summary
The key aerosol optical properties needed to estimate the aerosol radiative forcing of climate are the relative amounts of light scattering and absorption by the aerosol, the fraction of the incident solar radiation that is scattered upward to space by the suspended particles (upscatter fraction), and the aerosol optical depth. The knowledge of these properties along with estimates of solar, atmospheric, and surface properties permits calculation of cloud-free sky direct aerosol radiative forcing effects (Haywood and Shine, 1995). Far fewer in-situ vertical profile measurements of the entire suite of aerosol optical
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