Optical path modifications due to tropospheric clouds: Implications for zenith sky measurements of stratospheric gases

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Abstract
Potential influences of tropospheric clouds on zenith scattered light differential optical absorption spectroscopy (ZSL‐DOAS) of stratospheric gases, such as O3, NO2, OClO, BrO, and other species are discussed. It is argued that compared to clear sky, clouds can occasionally considerably change the geometrical path lengths in the troposphere, and hence change the relative contribution of the troposphere to the total atmospheric trace gas absorption. Major reasons for the geometrical path enhancements due to clouds are seen in the photon diffusion in optically thick clouds and in the multiple reflections of light between layers and patches of clouds. In addition, optically thin clouds in the upper troposphere (e.g., As and Ci clouds) may decrease the apparent absorption due to stratospheric species, whereby the magnitude of that effect largely depends on the solar zenith angle and wavelength. This is explained by the quite different air mass factors for the zenith scattered light observed under clear sky and cloudy sky conditions. Although opposite in sign, these “cloud effects” can combine and cause significant errors in ground‐based zenith sky observations of stratospheric gases. It is demonstrated that the proposed mechanisms can explain the recently reported cloudy sky absorption changes detected for O3, O4, NO2, BrO, and H2O.

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