Abstract
Abstract. A new solid-phase scrubber for use in conventional ozone (O3) photometers was investigated as a means of reducing interferences from other UV-absorbing species and water vapor. It was found that when heated to 100–130 °C, a tubular graphite scrubber efficiently removed up to 500 ppb ozone and ozone monitors using the heated graphite scrubber were found to be less susceptible to interferences from water vapor, mercury vapor, and aromatic volatile organic compounds (VOCs) compared to conventional metal oxide scrubbers. Ambient measurements from a graphite scrubber-equipped photometer and a co-located Federal equivalent method (FEM) ozone analyzer showed excellent agreement over 38 days of measurements and indicated no loss in the scrubber's ability to remove ozone when operated at 130 °C. The use of a heated graphite scrubber was found to reduce the interference from mercury vapor to ≤ 3 % of that obtained using a packed-bed Hopcalite scrubber. For a series of substituted aromatic compounds (ranging in volatility and absorption cross section at 253.7 nm), the graphite scrubber was observed to consistently exhibit reduced levels of interference, typically by factors of 2.5 to 20 less than with Hopcalite. Conventional solid-phase scrubbers also exhibited complex VOC adsorption and desorption characteristics that were dependent upon the relative humidity (RH), volatility of the VOC, and the available surface area of the scrubber. This complex behavior involving humidity is avoided by use of a heated graphite scrubber. These results suggest that heated graphite scrubbers could be substituted in most ozone photometers as a means of reducing interferences from other UV-absorbing species found in the atmosphere. This could be particularly important in ozone monitoring for compliance with the United States (U.S.) Clean Air Act or for use in VOC-rich environments such as in smog chambers and monitoring indoor air quality.
Highlights
Ozone (O3) in the lower atmosphere is produced by a complex set of photochemical reactions involving oxides of nitrogen and volatile organic compounds (VOCs)
In this study we present a means of greatly reducing the interferences of mercury, UV-absorbing VOCs, and water vapor in ozone measurements by replacing conventional metal oxide internal ozone scrubbers with a heated graphite scrubber (Birks et al, 2016)
Our results suggest that replacement of conventional scrubbers in commercially available ozone photometers with heated graphite scrubbers could increase their accuracy by reducing potential interferences
Summary
Ozone (O3) in the lower atmosphere is produced by a complex set of photochemical reactions involving oxides of nitrogen and volatile organic compounds (VOCs). Average concentrations of interfering species found in outdoor air would normally be expected to cause errors of only a few ppb at most, but this is well within measurement limits of UV ozone photometers when averaged over the 1 h time frame typical for compliance monitoring. Indoor ozone concentrations are typically 20–80 % of that measured outdoors (U.S.EPA, 2016); interfering species such as VOCs and mercury can be at much higher concentrations indoors from attached garages, cooking emissions, unvented heaters, the use of paints, cleaners, and outgassing from building components This places even more stringent constraints on ozone monitors to reduce biases in these indoor environments. In this study we present a means of greatly reducing the interferences of mercury, UV-absorbing VOCs, and water vapor in ozone measurements by replacing conventional metal oxide internal ozone scrubbers with a heated graphite scrubber (Birks et al, 2016). Our results suggest that replacement of conventional scrubbers in commercially available ozone photometers with heated graphite scrubbers could increase their accuracy by reducing potential interferences
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