How to measure Ozone
There are basically three characteristics of atmospheric ozone that are routinely measured and reported by ground and satellite monitoring systems, namely:
(a) Surface ozone;
(b) Total ozone;
(c) The vertical profile of ozone.
Surface ozone expresses the concentration of local ozone in the layer a few metres above the ground at a particular site on the Earth’s surface. Surface ozone measurements are commonly given in units of partial pressure or mixing ratio (by either mass or volume).
Total ozone refers to the total amount of ozone contained in a vertical column in the atmosphere above the ground extending from the Earth’s surface to the upper edge of the atmosphere. Commonly used units of total ozone are (a) column thickness of a layer of pure ozone at standard temperature and pressure (STP) and (b) vertical column density (number of molecules per area). Total ozone is measured with remote sensing using ground-based or satellite instruments that measure irradiances in the UU absorption band of ozone between 300 and 340 nm. Measurements are made from the ground using the direct sun, direct moon and zenith sky irradiances, and from space by measuring the solar UV radiation scattered back to space by the Earth’s atmosphere.
The vertical profile of ozone expresses ozone concentration as a function of height or ambient pressure. The amount of ozone at each height or pressure level in the atmosphere is commonly expressed as partial pressure, mixing ratio or local concentration (number density). The integral of the ozone profile from the ground to the top of the atmosphere is the total column amount of ozone.
Ground-based instruments for measuring total ozone
Ground-based remote-sensing instruments which measure the intensity of UV light at wavelengths in the absorption spectrum of ozone can be used to determine total ozone by differential optical absorption spectroscopy (DOAS) techniques. The most commonly used ground-based instruments in the ozone network of the WMO GAW Programme are the Dobson (Dobson, 1957 (3); WMO, 1980 (4)) and Brewer (Kerr, McElroy and Olafson, 1980 (5); Wardle and others, 1987 (6)) ozone spectrophotometers, and the M-124 filter ozonemeter (Gushchin, Sokolenko and Kovalyev, 1985 (7)). Other measuring instruments have been developed but their use has been limited to special experimental applications rather than to routine monitoring and data reporting.
The most accurate and the best-defined method for determining total ozone is to measure direct solar radiation from the ground at UV wavebands between 305 and 340 nm. The method comes from the lambert-Beer law that defines the direct spectral irradiance I0λreaching the Earth’s surface at wavelength λ after attenuation by column amounts by particular atmospheric constituents Xi:
where I 0λis a constant identified as the reading of Iλ by the instrument if it is located above the atmosphere (the extraterrestrial constant), α λi are the laboratory-measured extinction coefficients of the attenuating species, and μi are the ratios of the slant paths of the beam through the layers of the absorbing/scattering species to the vertical paths (relative optical air masses). If a spectrophotometer measures spectral irradiances Iλ at several wavelengths λi with different ozone absorptions, the influences of other atmospheric attenuators (mainly aerosols) can be eliminated by linear combinations of equation 1.
A general relation for the calculation of total ozone from direct sun observations XDS can be determined by:
where F is the combination of log (I λi), F0 is the combination of log (I0λi), the constants for the instrument, α and β,are the differential absorption and scattering coefficients of ozone in pure air, μm are the relative optical air masses of the ozone layer and the whole atmosphere, respectively.
In equation 2, the value F comes from measurements taken by the instrument, F0 is the calibration constant of the spectrophotometer, α and β are laboratory-determined values, and μm are calculated for the time and the geographical location of the measurement from astronomical relationships. Direct sun measurements are limited to daylight hours at times when the direct solar beam is not obscured by clouds or other obstacles for a period of at least 2 min (Dobson) or 5 min (Brewer). The solar zenith angles suitable for observations differ for particular types of spectrophotometers and wavelengths used for the measurements but usually do not exceed 72° for the Dobson and M-124 filter instruments and 75° for the Brewer spectrophotometer. While the Dobson spectrophotometer measures relative ratios of spectral irradiances at three wavelength pairs (A: 305.5/325.4; C: 311.5/332.4; D: 317.6/339.8 nm), the Brewer spectrophotometer registers spectral irradiances (photo counts) at five operational wavelengths (306.3, 310.1, 313.5, 316.8 and 320.1 nm). The M-124 filter instrument measures at 302 and 326 nm with the spectral band pass of 20 nm. Details on modification of the relation (equation 2) for particular types of instruments and their application for the processing of total ozone observations can be found in the references section of (8) or in the relevant GAW publication (WMO, 2003 (9)).
The Brewer spectrophotometer measures UV irradiances at several wavelengths that make it possible to calculate the total column amount of sulphur dioxide in the atmosphere using an equation similar to eq. 2. The relevant equation is developed by other linear combinations of differential absorption and scattering coefficients and spectral irradiances Ii. For processing total sulphur dioxide measurements, an extra-terrestrial constant for sulphur dioxide has to be defined in the calibration of the instrument.
Extracted from material of CIMO guide, Part I. Measurement of Meteorological Variables, chapter 16: Measurement of Ozone (8)
The geometry for the path of sunlight passing through the ozone layer in the Earth’s atmosphere is illustrated in the figure above. The solar irradiance (Iλ) at wavelength λ measured at the Earth’s surface is given by the following:
log (Iλ) = log (I0λ) – αλΧμ – α′λΧ′μ′ – βλm – δλ sec (θ)
I0 is the irradiance outside the Earth’s atmosphere (extra-terrestrial value) at wavelength λ;
αλ is the ozone absorption coefficient at wavelength λ (nm);
Χ is the total amount of column ozone in the atmosphere (m at STP);
μ is the ratio of the slant path of the beam through the ozone layer to the vertical path — the optical air mass of the ozone layer;
α′λ is the sulphur dioxide absorption coefficient at wavelength λ (nm);
X′ is the total column amount of sulphur dioxide in the atmosphere (m at STP);
μ′ is the ratio of the slant path of the beam through the sulphur dioxide layer to the vertical path — the optical air mass of the sulphur dioxide layer;
βλ is the Rayleigh molecular scattering coefficient of the air at wavelength λ;
m is the ratio of the slant path of the beam through the whole atmosphere to the
vertical path — the optical air mass of the whole atmosphere;
δλ is the particulate aerosol scattering coefficient at wavelength λ;
θ is the apparent solar zenith angle.
In practice, an accurate measurement of ozone cannot be made by measuring the irradiance at one wavelength because it is difficult to maintain the absolute sensitivity of an instrument over a long period. Also, particulate scattering due to aerosols and thin clouds significantly affects the amount of transmitted irradiance.
It is therefore necessary to measure the irradiances at more than one wavelength and to determine total ozone by techniques of differential optical absorption spectroscopy (DOAS). Measurements of irradiances made at N wavelengths are expressed by N equations of the form given in equation 3 with different values for I0λ , αλ , α′λ , βλ and δλ. These N equations may be linearly combined to give the following:
Σ wλ log (Iλ) = Σ wλ log (Ioλ) – (Σ wλαλ) Xμ – (Σ wλα′λ) X′μ′ – (Σ wλβλ) m – (Σ wλδλ) sec (θ)
where Σ represents the summation from 1 to N and wλ is a set of N weighting values, one for each wavelength.
The weighting values at each wavelength (wλ) are selected to minimize the effects of other atmospheric constituents, mainly aerosols. Weighting values for the Dobson AD measurement reduce the effects of haze. The effect of sulphur dioxide on the Dobson ozone measurement is ignored, although the presence of sulphur dioxide adds about 1 per cent false ozone for some stations. The weighting values for the Brewer total ozone measurement minimize the effects caused by aerosols and sulphur dioxide. The wavelengths for the Dobson AD and Brewer standard measurements with the appropriate values of wλ are given in the following table.
If the effects of sulphur dioxide and haze are neglected, equation 4 can be rewritten in the following form:
F + β m = F0 – α X μ
where: F = Σwλ log (Iλ)
F0 = Σwλ log (I0λ)
β = Σwλ β λ
α = Σwλ α λ
It follows from equation 5 that the value for total ozone is given by the following:
X = (F0 − F − βm) / αμ
Here, the term F is measured, F0 is a calibration constant which is equal to the value of F outside
the Earth’s atmosphere (the extra-terrestrial constant for the instrument), and βm and αμ are values which are calculated.
In order to determine the amount of total ozone, it is necessary to know F0, a value which is unique for each instrument. This constant is determined for most field instruments by direct intercomparison with the primary standard or secondary reference instruments.
Table 1: Wavelengths and the effective weighting values used for Dobson and Brewer standard ozone
|Dobson AD Measurement||Brewer Standard Measurement|
Extracted from material of CIMO guide, Part I. Measurement of Meteorological Variables, chapter 16: Measurement of Ozone, Annex 16.B (10)
- Staehelin, J., A. Renaud, J. Bader, R. McPeters, P. Viatte, B. Hoegger, V. Bugnion, M. Giroud and H. Schill, 1998: Total ozone series at Arosa, Switzerland: Homogenization and data comparison. Journal of Geophysical Research, 103, D5, pp. 5827–5842.
- Data from: temis.nl
- Dobson, G.M.B., 1957: Observer’s handbook for the ozone spectrophotometer. Annals of the International Geophysical Year, 5, pp. 46–89.
- World Meteorological Organization, 1980: Operations Handbook – Ozone Observations with a Dobson Spectrophotometer (W.D. Komhyr). WMO Global Ozone Research and Monitoring Project Report No. 6, Geneva.
- Kerr, J.B., C.T. McElroy and R.A. Olafson, 1981: Measurements of ozone with the Brewer ozone spectrophotometer. Proceedings of the Quadrennial Ozone Symposium (J. London, ed.) (Boulder, Colorado, August 1980), pp. 74–79.
- Wardle, D.I., W.F.J. Evans, H. Fast, A.J. Forester, G.S. Henderson, J.B. Kerr and R.K.R. Vupputuri, 1987: Stratospheric Ozone Science in Canada. An Agenda for Research and Monitoring. Internal Report of the Atmospheric Environment Service No. ARD‑87‑3, Toronto.
- Gushchin, G.P., S.A. Sokolenko and V.A. Kovalyev, 1985: Total-ozone measuring instruments at the USSR station network. Atmospheric Ozone (C.S. Zerefos and A. Ghazi, eds), Reidel, Dordrecht, pp. 543–546.
- CIMO guide, Part I. Measurement of Meteorological Variables, chapter 16: Measurement of Ozone: http://www.wmo.int/pages/prog/www/IMOP/publications/CIMO-Guide/Ed2008Up2010/Part-I/WMO8_Ed2008_PartI_Ch16_Up2010_en.pdf
- World Meteorological Organization, 2003: Comparison of Total Ozone Measurements of Dobson and Brewer Spectrophotometers and Recommended Transfer Functions (J. Staehelin, J. Kerr, R. Evans and K. Vanicek). Global Atmosphere Watch Report No. 149, WMO/ TD‑ 1147, Geneva.
- CIMO guide, Part I. Measurement of Meteorological Variables, chapter 16: Measurement of Ozone, Annex 16.B: http://www.wmo.int/pages/prog/www/IMOP/publications/CIMO-Guide/Ed2008Up2010/Part-I/WMO8_Ed2008_PartI_Ch16_Up2010_en.pdf