OPM
Ozone Reference: Dual Beam UV-Photometer
As ozone reference serves a fast response dual-beam UV-absorption photometer, developed by Proffitt and McLaughlin [1983] for the use on stratospheric balloons. The instrument has previously flown at several times during BOIC missions in 1983/1984 [Hilsenrath et al. 1986]. A scheme of the instrument is shown in Figure 1.
The instrument has two identical UV-absorption chambers (40 cm long and constructed from Teflon tubing), each alternating between reference mode (ozone free by scrubber) and sample mode. A four port valve alternates the scrubbed air between the two chambers, such that one chamber is in null mode while the other chamber is in sample mode or vice versa. The principle of the instrument of measuring the concentration of ozone in the air sample is based on the spectroscopic UV-absorption measurement of ozone at 254 nm wavelength in the sample chamber according Beer-Lambert absorption law:
| [1] |
where and Io (= zero mode) and It (=sample mode) are the lamp intensities at the detector when the chamber contains the sampled gas with and without removal of the ozone. L is the length of the absorption chamber, s O3 is the molecular absorption cross section of ozone at l =254 nm, and CS is the average concentration of ozone in the absorption chamber. Since L and s O3 are well known quantities, the transmittance R of the absorption chamber from the observed signal frequencies of the photo detector in sample and zero mode, Fsample and Fzero respectively is determined by
| [2] |
yields CS
| [3] |
The dual beam feature cancels the effects of lamp intensity fluctuations while the mode alternation compensates for mechanical changes and also provides continuous measurements. Details of the data reduction method are described by Proffitt and McLaughlin (1983). The instrument is an absolute measuring device with a fast response time of about 1 second at a sampling volume flow rate of about 8 l/min.
Figure 1: Scheme of the dual beam UV-absorption photometer [Proffitt and McLaughlin 1983]
The ozone UV-Photometer is installed in a separate vacuum vessel which is connected to the simulation chamber such that the UV-photometer is operated at the same pressure conditions as inside the test room. The air sample intake of the photometer is connected via a Teflon tube to the manifold of the ozone profile simulator which is located in the center of the test room.
The minimum detectable ozone concentrations are determined by the inherent noise of the photometer. Figure 2-A shows the fluctuations of the background of the photometer as a function of altitude. The results are presented as ozone equivalent and were obtained from flight simulations in the chamber by feeding the photometer with ozone free air. It is seen that the fluctuations of the background of the photometer are rather constant for altitudes between surface and 25 km. It is seen from Figure 2-B that the statistical distribution of the background of the photometer is closely following Gaussian statistics with a mean background of 0.005 mPa and a standard deviation of ±0.025 mPa. The minimum detectable levels of ozone of the photometer are around 0.025 mPa (@0.6x1010 molecules/cm3 of O3) with a virtually zero(£ 0.005 mPa of O3) background level. The precision of the ozone measurement made by the UV-photometer as reference is estimated to be better than ± 0.025 mPa of O3 (@ 0.6x1010 O3 molecules/cm3).
| Figure 2: | Background of dual beam ozone UV-photometer as ozone pressure equivalent.
|
| Left panel A: | Background as a function of altitude. |
| Right panel B: | Statistical distribution (histogram) of background. |
| Figure 2: | Background of dual beam ozone UV-photometer as ozone pressure equivalent. |
The absolute accuracy of the photometer is primarily determined by the uncertainty of the absorption path length, the uncertainty of the molecular absorption cross section of ozone and eventual losses of O3 within the instrument. The path length of 40.2 cm is known to better than ± 0.2% while the molecular absorption cross section of ozone of 1.147x10-17 molecules/cm2 [Vigroux, 1953] is considered to have an uncertainty smaller than ± 1.5% [Proffitt and McLaughlin 1983]. Ozone losses inside the instrument can in principle occur on the wall surfaces. In general relative wall losses of trace constituents in laminar flows varies as P-2/3 where P is the total gas pressure []. Based on laboratory measurements Proffitt estimated relative losses of ozone within the instrument to be smaller than 1% for air pressures at 25 hPa, 2% at 15 hPa and 4 % at 5 hPa. Therefore, it is obvious that, for air pressures corresponding to tropospheric and lower stratospheric conditions up to 25 km altitude, ozone losses within the photometer are negligible small and do not significantly affect the accuracy of the instrument. The overall accuracy of the ozone measurements made by the UV-Photometer as reference is therefore better than ± 2% for simulated altitudes up to 25 km while it declines to ± 3.5 % at 35 km altitude.
References
Hilsenrath, E., W. Attmannspacher, A. Bass, W. Evans, R. Hagemeyer, R.A. Barnes, W.Komhyr, K. Mauersberger, J. Mentall, M. Profitt, D. Robbins, S. Taylor, A. Torres and E. Weinstock, Results from the Balloon Ozone Intercomparison Campaign (BOIC), J. Geophys.Res. 91, 13137-13152, 1986.
Levich, V.G., Physicochemical Hydrodynamics, Prentice-Hall, Englewood Cliffs, NJ, 1962.
Proffitt, M.H., and R.J. McLaughlin, Fast response dual-beam UV-absorption photometer suitable for use on stratospheric balloons, Rev. Sci. Instrum., 54, 1719-1728, 1983.
Vigroux, E., Determination des coefficients moyen d'absorption de l'ozone, Ann. Phys., 8, 709-762, 1953.