G2.28   Uncertainty in a priori profile shape for AMF calculations for zenith sky ozone retrievals

Gap detailed description

AMFs are required to convert the measured O3 slant columns into vertical columns with O3 and pressure/temperature a priori profiles being key input parameters for the AMF calculations. AMF uncertainties for zenith-sky twilight O3 retrievals are dominated by errors on a priori profile shape effects. There is a lack of an adequate database of tropospheric O3 in particular and in regions where tropospheric or stratospheric O3 contents deviate from the climatological values, uncertainties of several percent can be introduced in total column O3 retrievals. Apart from uncertainties in the O3a priori profiles, further sources of uncertainty are based on uncertainties in the aerosol and cloud information used. There is also a lack of harmonization of the AMF calculation methods, which can introduce inconsistencies between the data sets measured at different locations within e.g. the NDACC network.

This gap is to be addressed within GAIA-CLIM.

Activities within GAIA-CLIM related to this gap

In-depth uncertainty analysis of the AMF calculations.

Gap remedy(s)

Two remedies are listed.

Remedy #1

Specific remedy proposed

Improve climatological databases of a priori O3 profiles, with particular emphasis on tropospheric O3. Test the quality/suitability of the databases of O3 profiles through a comparison with ozonesonde profiles at a selection of stations.

Measurable outcome of success

If we can show that the updated and improved O3 database, when used as a priori for the O3 AMF calculations, leads to a smaller uncertainty in the calculation of O3 AMFs then we know that we have succeeded.

Achievable outcomes

Technological / organizational viability:  medium.

Differences between AMFs are causing the largest discrepancies between the NDACC O3 datasets and to reduce these discrepancies, the use of standardized and further improved O3a priori data that account for the latitudinal and seasonal dependencies of the O3 vertical profile will make a substantial contribution.

Indicative cost estimate: low (<1 million)


Improving the climatological databases of a priori O3 profiles will improve the accuracy of the a priori data used within the respective RT model to calculate the AMFs and hence to improve the overall accuracy of the measured total O3 column retrieved from zenith sky UV-visible measurements.


Marked improvement and results expected within the GAIA-CLIM time period.

Remedy #2

Specific remedy proposed

Standardize AMF calculation methods and databases of a-priori information used in AMF calculations.

Differences between AMFs can cause discernible discrepancies between the O3 data sets. For example, some NDACC UV-visible groups use their own individual DOAS settings and O3 AMFs calculated with different RTMs and sets of O3, pressure and temperature profiles as input data, and with or without latitudinal and seasonal variations.  The objective of the recommendations formulated by the NDACC UV-visible WG previously was thus to reduce these discrepancies through the use of standardized DOAS settings and O3 AMF look-up tables that account for the latitudinal and seasonal dependencies of the O3 vertical profile (see Hendrick et al., 2011). These tables will be reviewed and updated within GAIA-CLIM and promoted to be used to homogenise the O3 total column data measured at different locations.

Measurable outcome of success

Determine the difference between standardized AMFs and individually calculated ones and, in turn, the difference in the calculated vertical O3 columns. If the standardized AMF lead to smaller uncertainties in the total column O3 datasets we know that the remedy was successful. 

Achievable outcomes

Technological / organizational viability: medium.

Indicative cost estimate: low (<1 million).


Standardized AMFs will improve the overall accuracy of the measured total O3 column retrieved from zenith sky UV-visible measurements.


1-2 years.

Gap risks to non-resolution

Identified future risk / impact

Probability of occurrence if gap not remedied

Downstream impacts on ability to deliver high quality services to science / industry / society

AMFs used by different groups are not standardized


O3 measurements provided by different groups are not homogenized and will likely show some unknown bias from site to site or group to group

Work package: