Ozone measurements at Uccle
At Uccle, two different sets of ozone measurements are available: total ozone measurements with spectrophotometers and measurements of the vertical distribution of the ozone amounts with ozonesondes. We will shortly present those datasets here, and refer to the Instruments and observation techniques section for a description of the instruments.
Total ozone
The thickness of the ozone layer or total ozone content at Uccle is measured with specially designed instruments called spectrophotometers and is commonly expressed in Dobson Units (DU). A 0.01 mm thick layer of ozone at 0°C and at pressure of 1 atmosphere (equal to 1013.25 hPa) is defined as one DU. If the ozone layer over our region were compressed to 0°C and one atmosphere pressure, it would be about 3 mm thick.
The measurement principle by spectrophotometers is based on the differential absorption of solar UV light by ozone: the intensities of the solar light arriving at the Earth's surface at two (or more) wavelengths in the UV (between 280 and 330 nm) are measured and compared. Ozone is mostly absorbed by light at the shorter wavelengths. From the knowledge of the absorption spectrum of ozone and the calibration of the instrument, the total ozone content of the atmosphere, i.e. the thickness of the ozone layer, is derived.
At Uccle, total ozone measurements are available since mid-1971. These were obtained with a Dobson spectrophotometer (from mid-1971 to spring 2009), and two Brewer spectrophotometers (a single Brewer no. 16 since 1984, and a double Brewer no. 178 since September 2001). Recently, since March 2020, total ozone is also available from a Pandora instrument.
The thickness of the ozone layer varies with time. The figure below shows measurements of the ozone layer performed by RMI at Uccle. The blue and black lines represent the measured daily values for this year and last year, respectively. The orange line represents the mean values derived from all the measurements performed since 1971. 95% of all the observations are in the orange shaded area. The day-to-day variability can be as large as 20%, meaning that the thickness of the ozone layer can change by more than 20% in one day. This variability is larger in winter and spring.
In the figure, the annual variation of the total ozone can be clearly identified. On average, the ozone layer is denser in March and April and thinnest in October and November. This cycle is related to the variability of the general air circulation in the stratosphere during different seasons, which is responsible for the global distribution of ozone.
Vertical ozone distribution
Ozone in the atmosphere is not evenly vertically distributed. This vertical distribution of ozone is highly variable and it is important to study and understand it. Observations of the vertical distribution of ozone are performed with ozonesondes launched with weather balloons.
In the troposphere, below about 10 km altitude, the concentration of ozone molecules in the atmosphere is low except during some periods in summer when ozone is produced near the ground by atmospheric pollutants. Above the tropopause, which marks the transition between the troposphere and the stratosphere, the ozone concentration increases sharply and reaches its maximum in the middle stratosphere around 25 km altitude. An example of the vertical profiles measured by an ozone sounding is given below.
Result of the ozone sounding at 16 April 2012. The magenta line shows the vertical distribution of ozone. During an ozone sounding also temperature (°C, red), relative humidity (%, green), wind direction (°,cyan) and wind speed (m/s, blue) are recorded. The second temperature (°C, sand color) is the temperature inside the isolating box that contains the ozonesonde instrument. Full lines are the ascent profiles, dashed lines the profiles measured during the faster descent.
Validation of satellite ozone measurements
RMI validates the GOME-2 and IASI satellite ozone profile products within the context of the EUMETSAT Atmospheric Composition SAF. For this validation work, the satellite retrieved ozone profiles are compared with other ozone observations obtained with ozonesondes, in Uccle and around the world.