References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-7-575-2007

On the variability of the Ring effect in the near ultraviolet: understanding the role of aerosols and multiple scattering

Langford

A. O.

¹ Schofield

¹ ³ ⁴ Daniel

J. S.

¹ Portmann

R. W.

¹ Melamed

M. L.

¹ ³ Miller

H. L.

¹ ³ Dutton

E. G.

² Solomon

NOAA Earth System Research Laboratory/Chemical Sciences Division, Boulder, Colorado

NOAA Earth System Research Laboratory/Global Monitoring Division, Boulder, Colorado

CIRES, University of Colorado, Boulder, Colorado

now at: Alfred-Wegener-Institute for Polar and Marine Research, Research Department Potsdam, Potsdam, Germany

31 01 2007

7 3 575 586

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

This article is available from http://www.atmos-chem-phys.net/7/575/2007/acp-7-575-2007.html

The full text article is available as a PDF file from http://www.atmos-chem-phys.net/7/575/2007/acp-7-575-2007.pdf

The "filling-in" (FI) of Fraunhofer lines, often referred to as the Ring effect, was examined using measurements of near ultraviolet sunlight scattered from the zenith sky above Boulder, Colorado during July and August 2005. The FI of the 344.1 nm Fe I line was directly determined by comparing direct sun and cloud-free zenith sky spectra recorded on the same day. The results, obtained over solar zenith angles (SZA) from 20° to 70°, are compared to the predictions of a simple rotational Raman Scattering (RRS) spectral model. The measured FI was found to be up to 70% greater than that predicted by first-order molecular scattering with a much stronger SZA dependence. Simultaneously measured aerosol optical depths and Monte Carlo calculations show that the combination of aerosol scattering and second-order molecular scattering can account for these differences, and potentially explain the contradictory SZA dependences in previously published measurements of FI. These two scattering processes also introduce a wavelength dependence to FI that complicates the fitting of diffuse sunlight observations in differential optical absorption spectroscopy (DOAS). A simple correction to improve DOAS retrievals by removing this wavelength dependence is described.

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