About 90 % of all ozone is found in a layer high up in the stratosphere. Detecting trends in this layer and the atmosphere’s total ozone content requires confidence in the characteristics of monitoring instruments, assurance in their performance and robust calibration methods to ensure data accuracy. Precise measurements over many years will be needed to detect ozone recovery processes, which are expected to occur after restrictions in the use of chlorine and bromine compounds were imposed by the 1987 Montreal Protocol.

Established networks for monitoring total column ozone, which is the ozone in an imaginary column between ground level and the top of the atmosphere, rely on bulky, ageing, or proprietary instruments. Each network operates one or more master instruments to which all other network instruments are referred to ensure measurements inter-relate. However different instrument types yield small, but significant, differences in output data, hampering this process and impeding the merging of results from different networks, which is critical for reliably detecting future ozone layer recovery.

This project evaluated differences between the reference Dobson and Brewer spectrophotometers that are used by networks, generated corrections to improve their performance and developed tools to ensure greater accuracy in the field. Building on the developments from the previous EMRP project Traceability for surface spectral solar UV.

The project:

• Investigated and characterised Dobson and Brewer reference spectrophotometers at NMI facilities and identified causes of measurement uncertainty. This has produced greater comparability between networks
• Developed and validated a tuneable radiation source (TuPS) for use in confirming the performance and operational stability of Dobson spectrophotometers
• Generated new reference data sets and uncertainty budgets for ozone measurements made at network monitoring station
• Developed a prototype high-resolution array spectroradiometer system and evaluated its ability to measure total column ozone in the atmosphere.

This project has provided greater certainty for networks measuring total column ozone using Dobson and Brewer spectrophotometers. By introducing increased agreement between data sets from different instruments, and tools to confirm on-site performance, users of ozone data will be able to have increased confidence in ground-based measurements of atmospheric ozone. These developments offer new capabilities for spotting trends in the state of the Earth’s protective ozone layer, which over the next decades is expected to begin to replenish as strategies introduced under the Vienna Convention and Montreal Protocol begin to take effect.

EMPIR project 19ENV04 MAPP builds on this work.