Total column ozone (TCO) monitoring with the Dobson spectrophotometer no. 84 have been carried out at Belsk (51°50′, 20°47′), Poland, since 23 March 1963. TCO observations were made for various combinations of double wavelength pairs (AD, CD, CC′) and instrument settings (direct Sun, zenith blue, and zenith cloudy). In total, results of 115,736 manual observations were recorded in the period 1963–2019. The following metrics of the intraday TCO variability are examined: standard deviation divided by the mean value and the difference between the daily maximum and minimum divided by the mean value. The mean value, standard deviation, and 5th–95th percentile range for the intraday changes of the metrics are {1.6%, 0.8%, 2.5%} and {4.3%, 2.3%, 7.3%,}, respectively. To examine interday TCO variability, one-day changes of the metrics and the daily mean TCO are analysed. The corresponding statistics for one-day change of TCO are {−0.2%, 6.9%, 22.6%}. The short-term TCO variability changed only slightly (if ever) since the beginning of the ozone observations at Belsk.

Anthropogenic changes in the stratospheric ozone layer have been the subject of scientific and public interest for almost half a century. The possible thinning of the ozone layer was suggested in the early 1970s by the 1995 laureates of the Noble Prize in Chemistry: Paul J. Crutzen, Mario J. Molina, and F. Sherwood Roland (Crutzen,

In response to the threat of ozone destruction, the Montreal Protocol (MP) was signed in 1987, which established restrictions on the production of the most ozone depleting substances (ODS) (e.g. chlorofluorocarbons – CFC) containing chlorine and bromine, which are involved in the catalytic destruction of the stratospheric ozone shield against the UV radiation. Ground-based monitoring of the ozone layer with Dobson spectrophotometers played a decisive role in the discovery of the Antarctic ozone hole and led to the identification (in the beginning of 1990s) of significant long-term ozone depletion in winter and spring over extratropical regions (Staehelin et al.,

The MP implementation and its further amendments resulted in a turnaround of the ODS concentration in the stratosphere in the late 1990s (middle latitudes) and ∼2000 (high latitudes) (WMO,

Determining the onset of ozone regeneration in response to ODS changes has been the subject of extensive scientific debate based on the results of the statistical and chemistry-climate models (Reinsel et al., 2005; Harris et al.,

Surprisingly the ozone depletion still exists in the lower stratosphere in the Northern Hemisphere midlatitudes (Ball et al.,

Usually, the ozone observations were averaged on the monthly basis and used in long-term variability studies. This paper is an extension of our earlier paper (Krzyścin et al.,

TCO observations have been carried out at Belsk (the Central Geophysical Observatory of the Institute of Geophysics Belsk, Polish Academy of Sciences) since March 1963 using the Dobson spectrophotometer No. 84. The TCO retrieval follows the technique of the differential optical absorption spectroscopy applied to a wavelength pair with strong and weak ozone absorption in the UV range. The following pairs are used: A (305.5 and 325.0 nm), C (311.5 and 332.4 nm), and D (317.5 and 339.9 nm). Different combinations of double wavelengths pairs (AD, CD) and observation settings, direct sun (DS), zenith blue (ZB), and zenith cloudy (ZC), have been applied to achieve the highest possible accuracy of the TCO observation under given atmospheric conditions (Degórska et al.,

Selection of the wavelength pairs and the observation settings follow the World Meteorological Organization (WMO) recommendations implemented in the global ozone observing network (Dobson,

Numerous trend analyses were carried out using the TCO monthly means, averaging the daily representatives of TCO at Belsk, derived from manual observations by the Dobson spectrophotometer

The Belsk’s daily TCO representatives have been archived at the World Ozone and Ultraviolet Data Centre (WOUDC) (

For comparison purposes, concurrent TCO daily values are taken from the Belsk’s overpasses with Solar Backscattered UV (SBUV) instruments (1970–2019) on various satellite platforms: Nimbus 4 and 7, National Oceanic and Atmospheric Administration (NOAA) 9, 11, 14, 16–19, and Suomi National Polar-orbiting Partnership (SNPP). The data are taken from SBUV Merged Ozone Data Set ver.8.6 available at

The short-term TCO variability is discussed using metrics based on the standard statistics of the intraday TCO measurements: daily mean (

These indices are calculated for days with at least three measurements per day. Each metrics is examined in five categories of the TCO observations: the entire set, DS&ZB, AD&DS, ZC, and DS. The ranking (best to worst) of the TCO observation accuracy by the Dobson spectrophotometer is as follows: AD&DS, DS, DS&ZB, and ZC (Dobson,

In addition, the relative one-day change of TCO, _{X}, is used as a metrics of the interday changes of TCO:
_{X}(_{X} (_{ALL} is for all observations, _{AD&DS} for AD&DS subset, and so on. In case of TCO from SBUV Merged Ozone Data Set, the metrics is denoted as _{SBUV}.

Standard statistics (mean value, standard deviation, median, and 5th–95th percentile range) are calculated based on the daily values of the metrics defined by the formulas (1), (2), and (3). Moreover, the two-sample Kolmogorov–Smirnov (KS) test is applied to examine differences between statistical distributions of the metrics for three possible combinations of the data subsamples, i.e. 1963–1979 versus 1980–1999, 1963–1979 versus 2000–2019, and 1980–1999 versus 2000–2019. The two-sample KS test is a general nonparametric method for comparing two samples, based on difference in shape of the empirical cumulative distribution functions of two compared samples. This test has the advantage of analyzing cumulative distribution functions without checking the significance of differences between selected statistical parameters, such as means or medians, which requires assuming the distribution type.

Yearly number of the intraday Dobson spectrophotometer measurements in the period 1963–2019: the entire set (DS&ZB&ZC) - black curve, the DS subset – red curve, and the ZC subset – blue curve.

The yearly frequency of the DS (red curve) and ZC (blue curve) observations in the period 1963–2019.

The yearly means and their smoothed profile (by the lowess filter) of total column ozone taken from various subsets of the Dobson spectrophotometer measurements at Belsk in the period 1963–2019: the ZC subset (the yearly data and the smooth curve in blue), the entire set (black), the DS&ZB subset (red), and the entire set from WOUDC daily means (magenta).

The long-term TCO variability patterns (

For several days a month, there were no DS and ZB observations due to the cloud cover. In this case, the monthly means used different numbers of the daily means. The increasing frequency of observations also means more daily values to be used in the monthly averaging, resulting in better consistency between the long-term ZC and non-ZC (DS&ZB) profiles.

The correlation coefficient between the TCO monthly means based on DS&ZB and ZC observations is 0.96 and the regression line of the DS&ZB on the ZC monthly means is

The monthly means of total column ozone from the DS&ZB measurements versus the corresponding means from the ZC measurements: the linear regression fit – blue line, the smoothed curve by the lowess filter – red line, and the diagonal of the square (the perfect agreement line) – the dashed black line.

The statistical parameters of CV and RR are presented in

^{th}-95^{th} range from 9.43 PP to 10.8 PP (

A marginally statistically significant difference (with probability ∼0.05) is found in the comparison of one-day changes of CV between the 1980–1999 and 2000–2019 subperiods using the entire sample of measurements. In this case, standard deviation decreases from 1.11 PP to 1.06 PP, and the 5th–95th range decreases from 3.63 PP to 3.4 PP (

The two-sample KS test applied to the relative one-day change of TCO (see _{X} variables) shows significant difference between samples for only two cases (out of total 15 cases), when the sample from 1963 to 1979 containing all available Dobson measurements was compared with the sample from 1980–1999 to 2000–2019. The SBUV samples of one-day change of TCO have insignificant differences in the comparisons of three subperiod pairs, 1963–1979 & 1980–1999, 1963–1979 & 2000–2019, and 1980–1999 & 2000-2019.

Nowadays, increasing variability of the climate, which is possibly related to the anthropogenic forcing in recent decades, is a widely discussed issue (Nicholls and Alexander,

The yearly means (1963–2019) and their smoothed pattern derived from the daily values of the intraday metrics of the short-term total column ozone variability: coefficient of variability (blue) and the relative range (red).

The two-sample KS test applied to the intraday (next day minus current day value) changes of the metrics also reveal a change in the short-term TCO variability but for a limited number of cases (4 cases out of total 48,

A statistically significant change is revealed by the comparison of one-day changes in RR between the 1963–1979 and 2000–2019 subset for the entire set of observations and for the ZC subset (

Therefore, this analysis does not categorically confirm an increase of the variability of the short-term (with time scale up to one-day) fluctuations in TCO at Belsk. The observed significant increase in the variability of the intraday metrics of the TCO fluctuations is probably strongly affected by increasing frequency of the ozone observations at Belsk.

TCO measurements are of the highest quality when using the AD double wavelength pairs for DS measurements (Komhyr,

The DS&ZB observations are not always possible at Belsk because of numerous cloudy days. The long-term variability of TCO derived only from the DS&ZB subset of observations showed agreement with the long-term pattern based on the entire data sets but it was worse in the period with a limited number of the Dobson observations (

Cloudless or overcast conditions over Belsk are related to specific air masses advection over the observing site. For example, cloudless days appear frequently during advection of the cold air from the north and north/east in winter and highly variable cloudiness appears during advection of the wet Atlantic air masses in summer. Therefore, the monthly TCO averages should include the whole spectrum of the atmospheric masses appearing over the site.

Envisaged limitation of the manual TCO observations to only ones with the highest accuracy rank, because of expected diminished interest in the ozone monitoring in the trendless era of the stratospheric ozone, which seems to begin at Belsk in the early 2000s (

The main findings of the paper are summarized as follows:

assumption of the constant TCO value throughout the day is frequently invalid

the short-term TCO variability (with time scale up to ∼1 day) changed only slightly (if ever) since the beginning of the ozone observations at Belsk

for trend estimates, all types of the ozone observations should be carried out in higher latitude stations, not only these with the higher accuracy rank

It is expected, that the intraday TCO data base will be very useful when new ozone absorption coefficients are officially recommended by WMO for the global network of the ozone monitoring as expected from recent papers (Redondas et al.,

This work was supported by the Ministry of Science and Higher Education of Poland under Grant number 3841/E-41/S/2020; and the Chief Inspectorate of Environmental Protection, Poland under Grant number GIOŚ/19/2021/DMŚ/NFOŚ.

No potential conflict of interest was reported by the authors

The dataset used in this article is available on the PANGAEA repository (Rajewska-Więch et al.,

Statistical characteristics of intraday TCO variability: Mean value (Mean), standard deviation (SD), and Span (difference between the daily maximum and minimum TCO value).

All values are in Dobson units (DU). DS, ZB, and ZC denote direct sun, zenith blue, and zenith cloudy class of the Dobson spectrophotometer observations, respectively. AD means the observation class (with the highest accuracy) using the double wavelength pairs: A (305.5 nm, 325.0 nm) and D (317.5 nm, 339.9 nm).

Statistical characteristics of the coefficient of variation, according formula (1), of the intraday TCO values.

Mean, SD, median, and [range] denote average value, standard deviation, median, and the 5th–95th percentile range. All values are percentages.

The same as

Probability of the same distribution for the intraday metrics in the subperiod pairs by the two-sample Kolmogorov–Smirnov test.

The results are for the entire set of observation (ALL) and various combinations of the Dobson spectrophotometer settings (DS&ZB, AD&DS, ZC, and DS).

(…)* marks cases with the probability less than 0.05.

CV, coefficient of variability; RR, relative range.

Statistical characteristics of one-day change of the coefficient variability.

The same notation as in

The same as

The same as

_{ALL}

_{DS&ZB}

_{AD&DS}

_{ZC}

_{DS}

_{SBUV}

_{ALL}

_{DS&ZB}

_{AD&DS}

_{ZC}

_{DS}

_{SBUV}

_{ALL}

_{DS&ZB}

_{AD&DS}

_{ZC}

_{DS}

_{SBUV}

The values are in percentages.

Probability of the same distribution for the one-day change of the intraday metrics (CV, RR, and _{X}) in the subperiod pairs by the two-sample Kolmogorov–Smirnov test applied to the entire set of the Dobson measurements and various combinations of the instrument settings.

_{ALL}

_{SBUV}

_{DS&ZB}

_{AD&DS}

_{ZC}

_{DS}

_{SBUV} shows the probability based on the comparisons of the satellite data over Belsk (SBUV Merged Ozone Data Set).

Marks cases with the probability less than 0.05.

_{X,} relative one-day change of TCO for subset X in %.

_{2}concentrations increase extreme event risk in a 1.5 °C world