Abstract
Abstract. Long-term trends of total column ozone (TCO), assessments of stratospheric ozone recovery, and satellite validation are underpinned by a reliance on daily best representative values from Brewer spectrophotometers and other ground-based ozone instruments. In turn reporting of these daily total column ozone values to the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) has traditionally been predicated upon a simple choice between direct sun (DS) and zenith sky (ZS) observations. For mid- and high-latitude monitoring sites impacted by cloud cover we discuss the potential deficiencies of this approach in terms of its rejection of otherwise valid observations and capability to evenly sample throughout the day. A new methodology is proposed that makes full use of all valid direct sun and zenith sky observations, accounting for unevenly spaced observations and their relative uncertainty, to calculate an improved estimate of the daily mean total column ozone. It is demonstrated that this method can increase the number of contributing observations by a factor of 2.5, increases the sampled time span, and reduces the spread of the representative time by half. The largest improvements in the daily mean estimate are seen on days with the smallest number of contributing direct sun observations. No effect on longer-term trends is detected, though for the sample data analysed we observe a mean increase of 2.8 DU (0.82 %) with respect to the traditional direct sun vs. zenith sky average choice. To complement the new calculation of a best representative value of total column ozone and separate its uncertainty from the spread of observations, we also propose reporting its standard error rather than the standard deviation, together with measures of the full range of values observed.
Highlights
Global ground-based monitoring of total column ozone (TCO) relies on the international network of Brewer spectrophotometers since they were first developed in the 1980s (Kerr et al, 1981), which has expanded the number of sites and measurement possibilities from their still-operating predecessor instrument, the Dobson spectrophotometer
We propose, describe, and assess a new methodology for determining a more representative best daily value of total column ozone from Brewer spectrophotometer observations
This method overcomes the limitations of making the traditional choice between a possibly small number of direct sun measurements and zenith sky measurements
Summary
Global ground-based monitoring of total column ozone (TCO) relies on the international network of Brewer spectrophotometers since they were first developed in the 1980s (Kerr et al, 1981), which has expanded the number of sites and measurement possibilities from their still-operating predecessor instrument, the Dobson spectrophotometer Together these networks provide validation of satellite-retrieved total column ozone as well as instantaneous point measurements that have value for near-real-time low-ozone alerts, when sited near population centres, as inputs to radiative transfer models at ultraviolet wavelengths, and critically underpin the monitoring requirement of The Vienna Convention for the Protection of the Ozone Layer, 1985. There are recent indications that, for the first time since the treaty was enacted and chlorofluorocarbons and other depleting substances were banned, the ozone layer is showing signs of recovery (WMO, 2014, and references therein) These and related trend analyses, use daily mean TCO values as their starting point. We only consider the choice between DS and ZS observations
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