Articles | Volume 8, issue 1
Research article
22 Jan 2015
Research article |  | 22 Jan 2015

Tropospheric ozone and ozone profiles retrieved from GOME-2 and their validation

G. M. Miles, R. Siddans, B. J. Kerridge, B. G. Latter, and N. A. D. Richards

Abstract. This paper describes and assesses the performance of the RAL (Rutherford Appleton Laboratory) ozone profile retrieval scheme for the Global Ozone Monitoring Experiment 2 (GOME-2) with a focus on tropospheric ozone. Developments to the scheme since its application to GOME-1 measurements are outlined. These include the approaches developed to account sufficiently for UV radiometric degradation in the Hartley band and for inadequacies in knowledge of instrumental parameters in the Huggins bands to achieve the high-precision spectral fit required to extract information on tropospheric ozone.

The assessment includes a validation against ozonesondes (sondes) sampled worldwide over 2 years (2007–2008). Standard deviations of the ensemble with respect to the sondes are considerably lower for the retrieved profiles than for the a priori, with the exception of the lowest subcolumn. Once retrieval vertical smoothing (averaging kernels) has been applied to the sonde profiles there is a retrieval bias of 6% (1.5 DU) in the lower troposphere, with smaller biases in the subcolumns above. The bias in the troposphere varies with latitude. The retrieval underestimates lower tropospheric ozone in the Southern Hemisphere (SH) (15–20% or ~ 1–3 DU) and overestimates it in the Northern Hemisphere (NH) (10% or 2 DU).

The ability of the retrieval to reflect the geographical distribution of lower tropospheric ozone, globally (rather than just ozonesonde launch sites) is demonstrated by comparison with the chemistry transport model TOMCAT. For a monthly mean of cloud-cleared GOME-2 pixels, a correlation of 0.66 is found between the retrieval and TOMCAT sampled accordingly, with a bias of 0.7 Dobson Units. GOME-2 estimates higher concentrations in NH pollution centres but lower ozone in the Southern Ocean and South Pacific, which is consistent with the comparison to ozonesondes.

Short summary
This work provides a description and validation of significantly updated algorithm for the retrieval of atmospheric ozone profiles, with a focus on the sensitivity to ozone in the lower troposphere. The satellite-derived ozone profiles are validated against ozonesondes globally, and achieves an average bias of 6% in the lower troposphere. The global distribution is also compared to the ozone distribution from a chemistry transport model, with an average agreement of less than 2 Dobson units.