Uncertainties in total ozone amounts inferred from zenith sky observations: Implications for ozone trend analyses

Abstract

In an effort to determine ozone measurement uncertainties associated with zenith sky radiation observations, a preliminary study involving radiative transfer calculations has been conducted to simulate ozone inference from Dobson spectrophotometer observations. By using a comprehensive radiative transfer algorithm, a model atmosphere, and a suitable set of ozone absorption cross sections, we have simulated, for a specific ozone profile and an ozone amount of about 350 Dobson units, the effects of cloud optical depth, cloud altitude, vertical distribution of ozone, temperature profile, and surface albedo on the total amount of ozone inferred by the Dobson procedure from zenith sky observations. Our simulations indicate that significant uncertainties may arise in determining ozone amounts from measurements of zenith sky intensity. The implications of these uncertainties for ozone trend analyses are described, showing, for example, that an appreciable error could arise in deduced ozone trend should tropospheric ozone double in the foreseeable future but not be taken into account in total ozone data processing. The radiative transfer algorithm may, furthermore, be employed to construct charts for use at each Dobson instrument site in processing zenith sky ozone observations. Such charts are currently derived empirically by comparing direct Sun and zenith sky observations. The theoretically derived charts would, however, need on‐site correcting to account for instrumental effects. Construction and use of synthetic correction tables for any particular station may alleviate difficulties encountered in the creation of empirical correction tables, since it avoids problems related to (1) time lapse between measurements and (2) diffuse radiation from forward scattering in clouds influencing the direct Sun measurements.