Validation of ACE-FTS version 5.2 ozone data with ozonesonde measurements

Abstract. Two decades of ACE-FTS version 5.2 (v5.2) ozone data (2004–2023) are evaluated with ozonesonde data from across the globe. The biases between the ACE-FTS and ozonesonde measurements are first estimated by analysing coincident data pairs. A second approach is taken for the validation by comparing the ACE-FTS and ozonesonde monthly mean time series, with the former generated by sampling the ACE-FTS data within latitude/longitude boxes (i.e., ±5°/±30°) surrounding the stations and calculating the monthly averages. The biases, correlations, variation patterns and the mean states of the two time series are compared. The biases estimated in this way exhibit more consistent and smoother features than using the coincident pair method. The ACE-FTS and ozonesonde monthly mean time series are highly correlated and exhibit similar variation patterns in the lower stratosphere at all latitudes. The ACE-FTS instrument drifts for each station are assessed in terms of the long-term linear trends relative to ozonesondes, which, although highly stable, may have their own minor changes with time. The ACE-FTS ozone profiles exhibit in general high biases in the stratosphere, increasing with altitude up to ~10 % at around 30 km, and have local maximum differences with ozonesonde profiles at the tropopause heights. The ACE-FTS instrument drifts are generally insignificant overall in the stratosphere with high variation between the stations. Averaging the individual station instrument drifts within several latitude bands results in small insignificant drifts of within ±1 % dec^-1 in the northern mid- to high latitudes, and the southern high latitudes, and a small positive insignificant drift of 0–3 % dec^-1 in the tropics and southern mid-latitudes with overall uncertainties at 2–3 % dec^-1 (2σ level) in the low stratosphere. In the troposphere, the average ACE-FTS instrument drifts vary with altitude and exhibit large drifts between -10 and +10 % dec^-1 with uncertainties of 10 % dec^-1.