The O3_M41 sensor measures ozone concentration by selective UV absortion. The light
emitted by a low pressure mercury lamp with a principal spectral line at 253.7nm is
transmitted through a glas tube holding the ambiant air; light absorption is measured by
an optical system:
For 4 seconds time air (containing ozone) is pumped through the optical tube; let i be the
intensity of the UV light falling on the optical sensor. To get a zero reference signal
i0, the tube is now flushed for 4 seconds with fresh air sucked through an ozone absorbing
charcoal filter. The well known law of Beer- Lambert allows to compute the ozone
concentration from i and i0:
where c(O3) is given in ppm at STD (standard temperature and pressure). alpha*l is the
product of the specific absorption coefficient of the UV light at 253.7nm and the length
of the optical path; this product represents the calibration factor of the instrument.The
last two factors convert the measured concentration to an STP value.
In Luxembourg and many other countries, the ozone concentration is usually given in ug/m3
(micro-gram per cubic meter. At STP conditions:
1 ppb = 2.142ug/m3
The precision of the sensor is 1 ppb = 2.143ug/m3. To allow quick calibration checks, the
instrument has a built-in ozone generator which delivers ozone concentrations between 150
and 750ppb. The accuracy of this generator is 5%.
The sensor is mounted on top of the roof of a 4 storey building (height= 25m above
ground), on a small wall facing East. The inlet is at about 1.80m above the flat roof
(altitude = 215m above sea-level). No chimneys nor other active ventilation outlets are on
this roof. The inlet is protected from western wind and rain. The sensor works
continuously, one reading being made every minute. The average of 30 readings are stored
in the datalogger.
The Luxembourg Ministry of Environment (Division de l'Air et du Bruit, 1a Rue Auguste
Lumière, L-1950 Luxembourg) operates 4 other ozon sensors of the same model located at
Vianden, Luxembourg, Bonnevoie and Ellingen. The Vianden station is close to Meteo_LCD,
albeit at an higher altitude (ca. 360m). The O3 measurements of Meteo_LCD will allow to
study the relationship between UV radiation and ozone concentration.