Abstract
Abstract. Accurate long-term monitoring of total ozone is one of the most important requirements for identifying possible natural or anthropogenic changes in the composition of the stratosphere. For this purpose, the NDACC (Network for the Detection of Atmospheric Composition Change) UV-visible Working Group has made recommendations for improving and homogenizing the retrieval of total ozone columns from twilight zenith-sky visible spectrometers. These instruments, deployed all over the world in about 35 stations, allow measuring total ozone twice daily with limited sensitivity to stratospheric temperature and cloud cover. The NDACC recommendations address both the DOAS spectral parameters and the calculation of air mass factors (AMF) needed for the conversion of O3 slant column densities into vertical column amounts. The most important improvement is the use of O3 AMF look-up tables calculated using the TOMS V8 (TV8) O3 profile climatology, that allows accounting for the dependence of the O3 AMF on the seasonal and latitudinal variations of the O3 vertical distribution. To investigate their impact on the retrieved ozone columns, the recommendations have been applied to measurements from the NDACC/SAOZ (Système d'Analyse par Observation Zénithale) network. The revised SAOZ ozone data from eight stations deployed at all latitudes have been compared to TOMS, GOME-GDP4, SCIAMACHY-TOSOMI, SCIAMACHY-OL3, OMI-TOMS, and OMI-DOAS satellite overpass observations, as well as to those of collocated Dobson and Brewer instruments at Observatoire de Haute Provence (44° N, 5.5° E) and Sodankyla (67° N, 27° E), respectively. A significantly better agreement is obtained between SAOZ and correlative reference ground-based measurements after applying the new O3 AMFs. However, systematic seasonal differences between SAOZ and satellite instruments remain. These are shown to mainly originate from (i) a possible problem in the satellite retrieval algorithms in dealing with the temperature dependence of the ozone cross-sections in the UV and the solar zenith angle (SZA) dependence, (ii) zonal modulations and seasonal variations of tropospheric ozone columns not accounted for in the TV8 profile climatology, and (iii) uncertainty on the stratospheric ozone profiles at high latitude in the winter in the TV8 climatology. For those measurements mostly sensitive to stratospheric temperature like TOMS, OMI-TOMS, Dobson and Brewer, or to SZA like SCIAMACHY-TOSOMI, the application of temperature and SZA corrections results in the almost complete removal of the seasonal difference with SAOZ, improving significantly the consistency between all ground-based and satellite total ozone observations.
Highlights
Within NDACC, the UV-visible network consists of more than 35 certified UV-visible spectrometers operating from pole to pole and providing time-series of O3 and NO2 total columns made publicly available on the network web site. These data have been compared to Dobson and Brewer ground-based (Kyro, 1993; Høiskar et al, 1997; Van Roozendael et al, 1998), and satellite measurements (e.g., Lambert et al, 1999), showing significant biases as well as systematic seasonal variations in the difference attributed to cross-sections and solar zenith angle (SZA) dependencies in the UV measurements, and the lack of seasonal variation in the air mass factors (AMF) used to derive total ozone columns from twilight zenith-sky UV-vis Slant column densities (SCDs)
The first took place in Lauder (45◦ S, 170◦ E) in New Zealand in 1992 (Hofmann et al, 1995), and was followed by several others in Camborne (50◦ N, 5◦ W) in the UK in 1994 (Vaughan et al, 1997), at the Observatoire de Haute Provence (OHP; 44◦ N, 6◦ E) in France in 1996 (Roscoe et al, 1999), in Andøya (69◦ N, 16◦ E) in Norway in 2003 (Vandaele et al, 2005), and more recently in Cabauw (52◦ N, 5◦ E) in the Netherlands in 2009, as part of the CINDI campaign (Roscoe et al, 2010). Despite this effort of cross evaluations, it has been recognized that the O3 data sets still suffer from residual inconsistencies mainly due to (1) differences in the Differential Optical Absorption Spectroscopy (DOAS) settings, in particular the ozone absorption cross sections used for the various instruments and (2) a lack of homogeneity in the AMFs applied to O3 slant columns for their conversion into vertical columns
The main change with respect to the settings utilized so far, is the use of O3 AMF look-up tables (LUTs) based on the TOMS V8 O3 profile climatology, which allows accounting for the dependence of the AMF on the latitudinal and seasonal variations of the ozone vertical profile
Summary
For more than two decades, stratospheric ozone and related trace gases such as NO2, BrO, and OClO have been monitored at a number of stations belonging to the Network for the Detection of Atmospheric Composition Change (NDACC) using ground-based zenith-sky UV-visible absorption spectrometers (e.g., Pommereau and Goutail, 1988; Solomon et al, 1989; McKenzie, et al, 1991; Kreher et al, 1997; Richter et al, 1999; Van Roozendael et al, 1998; Struthers et al, 2004; Hendrick et al, 2008). Within NDACC, the UV-visible network consists of more than 35 certified UV-visible spectrometers operating from pole to pole and providing time-series of O3 and NO2 total columns made publicly available on the network web site (http://www.ndacc.org) These data have been compared to Dobson and Brewer ground-based (Kyro, 1993; Høiskar et al, 1997; Van Roozendael et al, 1998), and satellite measurements (e.g., Lambert et al, 1999), showing significant biases as well as systematic seasonal variations in the difference attributed to cross-sections and SZA dependencies in the UV measurements, and the lack of seasonal variation in the AMFs used to derive total ozone columns from twilight zenith-sky UV-vis SCDs. Data evaluation and quality assessment procedures, which are under the responsibility of the NDACC UV-visible Working Group (WG), are essential for ensuring the quality of these data sets on a long-term basis.
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