OPS
Ozone Profile Simulation
In order to simulate vertical ozone profiles dynamically in time as well as to achieve reproducible ozone concentrations, a separate gas mixing system is installed to provide up to 4 ozone sensors plus UV-photometer (OPM) with regulated ozone concentrations. The scheme of the so called ozone profile simulator (OPS) is presented in Figure 1.
Figure 1: Scheme of the set up of the ozone profile simulator (OPS)
Ozone is photolytically generated by UV-irradiation in a zero grade air flow through a quartz glass (Suprasil) tube using a low pressure Hg-lamp. Via the photodissociation of oxygen molecules at a wavelength of 185 nm and the additional reaction of oxygen atoms with oxygen molecules ozone is formed at high concentration levels of 0.1-2 % in a constant air mass flow of 20 cm3/min, pressurized at 4.0 Bar, through the quartz glass cell (volume » 40 cm3). In order to vary the mixing ratio of ozone between 10 and 10000 ppbv the highly ozone concentrated air flow is dynamically diluted by a multiple staged mixing with zero grade air flows. All air flows are regulated by mass flow controllers.
At a first dilution stage the high concentrate ozone/air flow FOPS,1 is mixed with zero grade air flow FOPS,2 (0-2000 cm3/min) at 2.0 Bar total pressure, the excess vented by a valve. A part of the resulting diluted ozone air flow,F OPS,3 (0-2000 cm3/min), is fed into a second dilution stage where it is mixed with two zero grade air flows, FOPS,4 (0-2000 cm3/min) and FOPS,5 (0-10000 cm3/min), at test room pressure conditions. The resulting air flow, FOPS,S (FOPS,S= FOPS,3 + FOPS,4 + FOPS,5), with diluted ozone is fed into the manifold, located in the test room of the simulation chamber, to supply the ozone sensors plus UV-photometer with controlled ozone mixing ratios. The relation between the diluted and undiluted mixing ratio of ozone, cO3,OPS and cO3,UnDil respectively, as a function of the mass flow rates, F OPS,1, FOPS,2, FOPS,3, FOPS,4 and FOPS,5 is given by:
| [1] |
Figure 2: View inside testroom of environmental simulation chamber with setup to test 4 ozone sondes. Detailed view of manifold at outlet of ozone profile simulator (OPS) to supply 4 ozone sondes plus UV-photometer with controlled ozone concentrations mixed in air.
The manifold shown in Figure 2 consists of a spherical glass vessel with a volume of about 150 cm3 with radially arranged connections to the individual ozone sensors and the UV-photometer with the inlet of the simulated ozone air flow FOPS,S being in the center of the manifold. Excess amounts of air are exhausted via an additional tube into the test room such that the manifold is kept to the test-room pressure conditions in order to prevent the ozone sensors from over-pressure effects. The latter effects would increase the mass flow rate through the ozone sensors and thus yield an overestimate of the actual ozone concentrations measured by the test ozone-sensors.
The volume flow rate of the simulated ozone air flow FOPS,S is kept constant within a range of 10-12 l/min which is sufficient to provide four ozone sensors (maximum 4 x 250 ml/min) and the photometer (maximum 8 l/min) yet limiting the exhaust flow into the test room such avoiding over-pressure in the manifold as well as a large excess of air flow which would affect the pressure regulation of the chamber.
During the entire simulation the undiluted ozone mixing ratio cO3,UnDil is kept constant by keeping the conditions for the ozone production such as pressure, UV-light and mass flow within the quartz tube at constant levels. Just prior to each simulation cO3,UnDil is adjusted to typical prescribed surface pressure , temperature and ozone conditions.
On the basis of the prescribed simulation profiles of ozone mixing ratio, pressure and temperature plus the experimental boundary conditions of a constant mixing ratio of ozone in the undiluted flow FOPS,1 and a constant sample volume flow rate FOPS,S the mass flow rates of FOPS,2, FOPS,3, FOPS,4 and FOPS,5 are calculated according equation 1, optimized and set by computer control.
Figure 3: Panel A: Prescribed and actual observed ozone profile simulated at typical mid latitude conditions according US-Standard Atmosphere (1976) for 40-50oN with a tropopause height of about 12 km. Panel B: Actual mass flow rates of the ozone profile simulator (OPS) as function of the simulated altitude.
An example of the simulation of a vertical ozone profile is presented in Figure 3 which shows a vertical profile of the prescribed ozone partial pressure as calculated from the experimental conditions (equation 1) as well as the actual profile of ozone as measured with the UV-photometer. It is seen that the actual simulated ozone profile tracks the prescribed profile very well, within an agreement of 5-10 %, in the tropospheric as well as in the stratospheric part of the simulation. The combined operation of the five mass flow controllers (FC-1,2,3,4&5) during the simulation of a vertical profile of ozone is illustrated in Figure 3-B showing the actual mass flow rates, FOPS,1, FOPS,2, FOPS,3, FOPS,4 and FOPS,5 as a function of the actual simulated height. A sensitivity analysis of equation 1 in combination with the actual mass flow rates, presented in Figure 7-B, have shown that the simulated ozone mixing ratio is most sensitive by the settings of the flow controllers FC-3, -4 & -5 of the second dilution stage. Regarding the limited accuracy and reproducibility of mass flow controllers if operated in the lower part of their measuring range it is clear that in the stratospheric part where the mass flows are small the simulated profile of ozone is becoming sensitive to small variations of the different flows. Therefore, it is likely that the fluctuations of ozone observed by the photometer in the stratospheric part (see Figure 3-B) are caused by the small variations of the air flows through FC-3, FC-4 and particularly FC-5.
References
U.S. Standard Atmosphere, NOAA, NASA, USAF, U.S. Government Printing Office, Washington, D.C., 1976.