Ozone concentrations in the stratosphere

  • Image, Ozone concentrations.

    Ozone in the upper atmosphere (stratosphere) absorbs ultraviolet (UV) radiation from the sun and protects Earth from being exposed to harmful UV levels. Exposure to UV radiation has been linked to skin cancer. Monitoring variations in stratospheric ozone concentrations is important because of New Zealand’s high skin cancer rates.

    Our ozone layer is thickest during spring and thinnest during autumn. This means the ozone layer over New Zealand thins during summer, providing less protection from UV radiation at a time when Earth is closest to the sun. This variation is natural and not linked to the ozone hole, an area where the ozone layer is depleted by human-made chemicals, which lies over Antarctica and does not have a large effect on our ozone concentrations.

    We classified Ozone concentrations in the stratosphere as a case study.

    Key findings

    From November 1978 to February 2017, median monthly ozone concentrations measured at Lauder decreased slightly at a median rate of 0.35 Dobson units (DU) per year. This trend was assessed at the 95 percent confidence level.

    From 1979 to 2016:

    • The highest concentration measured at the Lauder site in Otago was in 1984 (480DU). Its lowest was in 2015 (215DU).
    • Long-term average daily ozone concentrations vary across the year by 106DU (29 percent) – from a minimum of 258DU in March to a maximum of 364DU in October.
    • On average, ozone concentrations vary day-to-day by around 15DU, which is approximately 5 percent of the average thickness of the ozone layer over New Zealand.
    • The annual average thickness of the ozone layer over New Zealand was 307DU, which is slightly thicker than the global average of 300DU (Liley & Mckenzie, 2007).

    Figure 1

    Note: Column ozone is a measure of the amount of ozone found in a vertical column extending to the top of the atmosphere.

    Figure 2

    Graph, daily average column ozone, 1979–2016.

    Note: Column ozone is a measure of the amount of ozone found in a vertical column extending to the top of the atmosphere.

    Definition and methodology

    Approximately 90 percent of ozone (O3) is found in the stratosphere, 20–25 kilometres above Earth, in what is known as the ozone layer. Ozone is produced when UV radiation from the sun meets oxygen molecules (O2). These ozone molecules then absorb UV rays before they reach the surface of the earth.

    The thickness of the ozone layer in a column of air is measured in Dobson units (DU). One DU represents the amount of ozone molecules needed to produce a 0.01mm layer of pure ozone. These measurements are made using ground-based spectrometers, balloons, and other airborne instruments, or satellites.

    NIWA supplied ozone data up to February 2017 in two forms, with different starting dates:

    • measurements made using a Dobson spectrophotometer (number 72), from 1987
    • data assimilated from satellite measurements recalibrated against the global Dobson network, from 1978.

    NIWA takes measurements using the Dobson spectrophotometer 72 under clear-sky, direct sunlight conditions at Lauder in Otago. There are gaps in the time series due to days with cloud, rain, or too much wind. However, over the whole period, each individual calendar day of the year was measured. This allows us to calculate statistics based on the day of the year.

    Ozone concentrations measured over New Zealand are not affected directly by the ozone hole, which lies over Antarctica. The ozone hole is an area where the ozone layer is less than 220 DU, caused mostly by ozone-depleting substances emitted by people. However, when the ozone hole breaks up in spring, it can send ‘plumes’ of ozone-depleted air over New Zealand. This briefly decreases the column ozone levels by approximately 5 percent (Ajtić et al, 2004), about the same amount as normal daily variation.

    Ozone also occurs at ground level, where it is linked to increased health risks, such as respiratory and cardiovascular diseases. While ground-level ozone can occur naturally it is also formed by chemical reactions involving sunlight, volatile organic compounds, and nitrogen oxides (the main source being vehicle emissions).

    Data quality

    We classified Ozone concentrations in the stratosphere as a case study


     Image, Partial relevance.  This case study is a partial measure of the ‘Atmospheric properties’ topic.


       The accuracy of the data source is of high quality.

    See Data quality information for more detail.


    Ajtić, J., Connor, B. J., Lawrence, B. N., Bodeker, G. E., Hoppel, K. W., Rosenfield, J. E., & Heuff, D. N. (2004). Dilution of the Antarctic ozone hole into southern midlatitudes, 1998–2000. Journal of Geophysical Research: Atmospheres, 109(D17).

    Liley, JB, & Mckenzie, RL (2007). Where on Earth has the highest UV? (PDF, 251 kB). Retrieved from www.niwa.co.nz.

    Archived pages

    See Ozone concentrations (archived October 2017).

    Updated 19 October 2017

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