Abstract
Abstract. A highly portable ozone (O3) calibration source that can serve as a U.S. EPA level 4 transfer standard for the calibration of ozone analyzers is described and evaluated with respect to analytical figures of merit and effects of ambient pressure and humidity. Reproducible mixing ratios of ozone are produced by the photolysis of oxygen in O3-scrubbed ambient air by UV light at 184.9 nm light from a low-pressure mercury lamp. By maintaining a constant volumetric flow rate (thus constant residence time within the photolysis chamber), the mixing ratio produced is independent of both pressure and temperature and can be varied by varying the lamp intensity. Pulse width modulation of the lamp with feedback from a photodiode monitoring the 253.7 nm emission line is used to maintain target ozone mixing ratios in the range 30–1000 ppb. In order to provide a constant ratio of intensities at 253.7 and 184.9 nm, the photolysis chamber containing the lamp is regulated at a temperature of 40 ∘C. The resulting O3 calibrator has a response time for step changes in output ozone mixing ratio of < 30 s and precision (σp) of 0.4 % of the output mixing ratio for 10 s measurements (e.g., σp=±0.4 ppb for 100 ppb of O3). Ambient humidity was found to affect the output mixing ratio of ozone primarily by dilution of the oxygen precursor. This potential humidity interference could be up to a few percent in extreme cases but is effectively removed by varying the lamp intensity to compensate for the reduced oxygen concentration based on feedback from a humidity sensor.
Highlights
Ozone (O3) is a key constituent throughout the atmosphere
The response of the calibration source is faster considering that it is convolved with the Model 202 Ozone Monitor, which outputs the average of the two most recent 10 s measurements
The 2B Technologies Model 306 Ozone Calibration Source is capable of producing ozone in ozone-scrubbed ambient air with accuracy and precision better than 1 ppb in the range 30–100 ppb ozone or 1 % in the range 100–1000 ppb
Summary
Ozone (O3) is a key constituent throughout the atmosphere. Because of its adverse health effects, ozone is one of six criteria pollutants designated by the U.S Environmental Protection Agency (U.S EPA, 2018). In the stratosphere ozone is continuously formed in the photolysis of oxygen by UV light having wavelengths less than 242 nm. The ozone produced absorbs UV light, protecting the Earth from harmful UV-B radiation in the wavelength range 280–320 nm. Monitoring of the protective ozone layer is done by use of ground-based spectroscopic methods (Gotz, et al, 1934; Stone et al, 2015) along with balloon-launched ozonesondes (Komhyr, 1969), occasional aircraft measurements and satellites
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