Several reported NO 2 measurements are based on the inversion of absorption curves as measured from balloon platforms during sunrise or sunset. This measurement method takes advantage of the long pathlengths which result when the suns' rays pass tangentially through the atmospheric layers below the balloon. The tangent ray measurement technique may be affected by variations of NO 2 concentrations at twilight since NO is being converted to NO 2 during sunset and NO 2 is being photolyzed to NO during sunrise. The effect of NO 2 changes during twilight on tangent ray NO 2 measurements has been quantitatively analyzed. The correction factors were found to be small.
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Kerr et al. (1977) studied this question.
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