Airborne Ozone Measurements
We have a broad and long-term experience in the development of ozone (O3) instruments which led to the most accurate and fastest sensors existing worldwide. We apply two measurement techniques:
- UV photometry that is based on the light absorption of O3 at a wavelength of 250-260 nm
- chemiluminescence detection that is based on the chemiluminescence of O3 at the surface of an organic dye adsorbed on dry silica gel
On aircraft we ever use both techniques simultaneously in order to combine the high measurement accuracy of the UV photometer with the high measurement frequency of the chemiluminescence detector.
... the most accurate technique existing today. It is based on the strong absorption lines of O3 in the UV (Hartley band). As light source either a low-pressure mercury lamp emitting primarily at a wavelength of 253.7 nm or a UV LED emitting at 250-260 nm is used. The decrease in light intensity due to the presence of O3 can be described by the formula of Beer-Lambert:
I = I0 · e -s · l · n · c
with s: absorption cross sections of O3 at the light source wavelength
l: absorption length, in this case of the used cuvette (38 - 270 cm)
n: concentration of air molecules in the cuvette
I0: incident light intensity
c: mixing ratio ozone/air molecules
Our group runs two O3 calibration units (Ansyco KTO3M, UMEG reference standard that agrees with the Standard Reference Photometer of the WMO).
... the fastest technique existing today. The measurement frequency is 50 Hz, but is set onboard the CARIBIC aircraft to 10 Hz and on HALO to 12.5 Hz, which results in a high spatial resolution of merely 20-25 m at cruising altitude. This fast O3 sensor is linear but needs calibration because of variable sensitivity of the sensor plates and its aging. The UV photometer is used during post flight data processing for calibrating the sensor plate. This device is described in Zahn et al. (Atmos. Meas. Techn., 2012) and in the meantime is commercially via the company Enviscope.
It is based on the chemiluminescence of O3 (at λ = 450-500 nm) on the surface of an organic dye (Coumarin 47) adsorbed on silica gel powder deposited on a small, only 1 cm diameter aluminum disk. The chemiluminescence light is detected by a small channel photomultiplier.
Use onboard the CARIBIC passenger aircraft
Since May 2005 a completely home-made instrument that houses both an UV photometer and a chemiluminescence detector is installed in the CARIBIC container. The 1-sigma precion is ~0.3 ppbv at a measurement frequency of 4 s and 1 bar which corresponds to a measurable optical density of ~2 · 10-6.
A typical ozone cross-section during a flight from Frankfurt to Orlando (USA) at 10-12 km altitude in March 2009 is shown in Fig. 1a), the UV photometer in red and the chemiluminescence detector in blue. Below O3 mixing ratios of ~120 ppbv the flight occurred in the troposphere, above this threshold in the stratosphere. The correlation of the data measured by both instruments with a correlation factor R of 0.9999 is shown in Fig. 1b).
Fig. 1: a) Cross-section during a flight from Frankfurt to Orlando (USA) at 10-12 km altitude in March 2009. b) Correlation of the data measured by the chemiluminescence and the UV spectrometer spectrometer instrument.
Use onboard the HALO research aircraft
A new, very light-weight (14.5 kg) and accurate ozone instrument (again combining both techniques) was developed for the use on board the HALO aircraft (name FAIRO). As light source for the UV photometer a UV-LED is used which can nicely be controlled (in constrast to Hg lamps) to constant light emission (at λ = 250-260 nm). The 1-sigma precion is ~0.08 ppbv at a measurement frequency of 4 s and a cuvette pressure of 1 bar. The chemiluminescence detector shows a measurement frequency of 12.5 Hz and a high precision of ~0.05 ppbv (at 10 ppbv absolute, a measurement frequency of 5 Hz, and a pressure of 1 bar).
FAIRO was used during the first HALO campaigns TACTS and ESMVal in July/August 2012. Its performance was excellent during all 13 flights. As example, see the detection of gravity waves at ~13.7 km altitude during a flight from Germany to the Cape Verde Islands (Fig. 2).
|Fig. 2: Detection of gravity waves at ~13.7 km during a flight from Germany to the Cape Verde Islands|