Sunlight transmission through desert dust and marine aerosols: Diffuse light corrections to Sun photometry and pyrheliometry

Abstract

[1] Desert dust and marine aerosols are receiving increased scientific attention because of their prevalence on intercontinental scales and their potentially large effects on Earth radiation, climate, other aerosols, clouds, and precipitation. The relatively large size of dust and marine aerosol particles produces scattering phase functions that are strongly forward peaked. Hence Sun photometry and pyrheliometry of these aerosols are more subject to diffuse light errors than is the case for smaller aerosols. We quantify these diffuse light effects for common Sun photometer and pyrheliometer fields of view (FOV), using data on dust and marine aerosols from (1) Aerosol Robotic Network (AERONET) measurements of sky radiance and solar beam transmission and (2) in situ measurements of aerosol layer size distribution and chemical composition. Accounting for particle nonsphericity is important when deriving dust size distribution from both AERONET and in situ aerodynamic measurements. We obtain correction factors that can be applied to Sun photometer or pyrheliometer results for aerosol optical depth (AOD) or direct beam transmission. The corrections are negligible (less than $1% of AOD) for Sun photometers with narrow FOV (half-angle h < $1°), but they can be as large as 10% of AOD at 354 nm wavelength for Sun photometers with h = 1.85°. For pyrheliometers (which can have h up to $2.8°), corrections can be as large as 16% at 354 nm. AOD correction factors are well correlated with AOD wavelength dependence (hence Å ngström exponent). We provide best fit equations for determining correction factors from Å ngström exponents of uncorrected AOD spectra, and we demonstrate their application to vertical profiles of multiwavelength AOD.

Retrieval of aerosol optical depth in the visible range with a Brewer spectrophotometer in Athens

“…However, this is a minor issue in the visible range and only accounts for few percentage points when the largest aerosol particles (e.g. desert dust) are to be measured, as calculated by Russell et al (2004).…”

“…Furthermore, the measuring spectral range of MkIV Brewers nicely overlaps with the operating wavelengths of other sun photometers, thus avoiding the need of extrapolations in order to compare different kind of instruments. Finally, measuring AOD in the blue band is, in principle, simpler than in the UV, since several instrumental issues, such as the effects of spectral stray light, finite bandwidth (Slusser et al, 2000) and diffuse radiation entering the field of view (FOV) due to the aerosol forward scattering peak (Russell et al, 2004), are reduced at larger wavelengths. Therefore, a better understanding of AOD retrieval in the visible range may be a first step and a useful test bench towards improving AOD estimates at smaller (and more demanding) wavelengths.…”

Airborne sun photometer PLASMA: concept, measurements, comparison of aerosol extinction vertical profile with lidar

“…This relatively small FOV ensures that the instrument is less sensitive to atmospheric scattering into the FOV than instruments with broader FOV. The effect of atmospheric forward scattering on direct solar irradiance measurements increases with particle size, aerosol optical depth and instrument FOV; it has been shown that the impact is negligible (less than 1 % of AOD) for FOV smaller than 2 • (Russell et al, 2004). A more recent study (Sinyuk et al, 2012) confirms that in most cases for a CIMEL-like instrument, this effect can be neglected excepted for heavy dust loadings where the relative error may reach ∼ 1 % for an AOD of 3.5.…”

Atmospheric Properties from the 2006 Niamey Deployment and Climate Simulation with a Geodesic Grid Coupled Climate Model Third Quarter 2008

“…The MFRSR AODs were corrected to match the Cimel AODs following a method similar to that described in Russell et al (2004). This allows the construction of a corrected data set that extends throughout the AMF deployment period, rather than just the limited time period when the Cimel was operational.…”

“…To correct the MFRSR AODs, we devise a scheme similar to that presented in Russell et al (2004), for which the correction depends on the Angstrom exponent as a proxy for the aerosol size. For large Angstrom exponents (smaller particles), the correction is small, and the magnitude of the correction increases as the Angstrom exponent decreases (larger particles).…”