Problems regarding the tropospheric O₃ residual method and its interpretation in Fishman et al. (2003)

Abstract. In this article we will show that the Total Ozone Residuals (TOR) method as presented by Fishman et al. (2003) contains an erroneous assumption, due to which the TOR as presented in this article reflects mainly a tropospheric O₃ climatology (Logan, 1999), scaled to the layer between the surface and the tropopause height, rather than a satellite measured TOR. We will show that it is possible to obtain a tropospheric O₃  column that is very similar to what is being presented in Fishman et al. (2003), solely based on the Logan (1999) tropospheric O₃  climatology and an estimate for the tropopause heights without using satellite data. Furthermore, we will show that one of the interpretations of observed O₃  variability (high TOR values over Southeast Asia) is not the result of pollution, but primarily of tropopause height variations. We suggest adjusting the method in such a way that the final retrieved TOR product becomes independent of the actual values of the Logan (1999) O₃  climatology. The proposed adjustments are also more in line with the original methodology suggested by Fishman and Balok (1999) in a previous paper. We furthermore show that the assumption that the stratospheric O₃  column is nearly constant within a 5-day period is not always valid. This can introduce errors in the final TOR product, although we recognize that these errors may become small when averaging the TOR product over longer periods of time. However, we feel that a detailed study on this assumption is also needed.