Cloud and albedo effects in tropospheric NO2 estimations using zenith observations
Oleg Postylyakov(1), Alex Elokhov(1) and Viktor Ivanov(1)
(1) Institute of Atmospheric Physics, RAS, Pyzhevsky per.3, Moscow, 119017, Russian Federation
Measurements of the NO2 total content and vertical distribution performed by ground-based spectrometers are used for validation of satellite instruments . Better understanding of effects of specific observational conditions on retrieved NO2 products contributes in development of more accurate methods of the comparison of the remote observations having the different averaging kernels of their products.
Ground-based stations use the spectral measurements of the scattered solar radiation, which comes from the zenith during twilight, for retrieval of the vertical distribution of NO2 in the stratosphere . The background tropospheric NO2 content insignificantly influences on stratospheric NO2 retrieval errors both in cloudy and cloudiness conditions in view of its smallness. However, the zenith observations performed in urban areas may detect the tropospheric NO2 because of the near-surface layer with typically enhanced NO2 content. Such urban enhanced NO2 may also affects stratospheric NO2 retrieval.
An approach to the estimation of the lower-tropospheric integral NO2 content, basing on the combination of the daytime and twilight ground-based observations of the zenith spectral radiance, is under investigation. The estimation is based on the assumption that urban NO2 is localized in a near-surface layer with height of a several hundred meters. The accuracy of the method depends on possible variations of the weighting functions of the inverse problem in this layer. Using radiative transfer model MCC++ we performed error analysis of the method with special emphasis to influence of clouds and the surface albedo.
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