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Comparison of Ozone Measurement Methods in Biomass Burning Smoke: An evaluation under field and laboratory conditions
Citation:
Long, R., A. Whitehill, A. Habel, S. Urbanski, H. Halliday, M. Colon, S. Kaushik, AND M. Landis. Comparison of Ozone Measurement Methods in Biomass Burning Smoke: An evaluation under field and laboratory conditions. Atmospheric Measurement Techniques. Copernicus Publications, Katlenburg-Lindau, Germany, 14(3):1783-1800, (2021).
Impact/Purpose:
Agency Research Drivers – Federal Reference Methods (FRMs) and Federal Equivalent Methods (FEMs) are designed to provide sufficient measurement accuracy, precision, specificity, and minimal measurement uncertainty to provide data suitable for monitoring EPA National Ambient Air Quality Standards (NAAQS) attainment, protecting public health, and to support exposure and health studies. For ozone, the most widely used FEM, based upon the Ultraviolet (UV) photometry, has been observed to suffer from a severe positive interference when operated in the presence of biomass burning smoke. This interference will result in the over estimation of ozone concentrations at State and Local Air Monitoring Station (SLAMS) and could potentially lead to false ozone NAAQS exceedances. As a result, States often seek exceptional event designations to disqualify high ozone readings during smoke impacted events. An accurate ozone monitoring method, free of any potential interferences, will enhance states abilities to accurately separate true ozone concentration from measurement artifacts. Under the Clean Air Act and as part of NAAQS implementation activities, FRMs and FEMS are continuously reviewed evaluated to ensure they meet all needed performance specifications and in order to provide States with the proper tools and information needed to address ozone NAAQS attainment/non-attainment issues. Research Approach – This research effort required a systematic ambient and laboratory-based evaluation of ozone monitoring methods in biomass burning smoke. The research was performed during a series of prescribed grassland fire burns in Kansas and Oregon during 2017. In addition, evaluations were performed during controlled burn chamber experiments at the U.S. Forest Service Rocky Mountain Fire Sciences Laboratory in Missoula Montana during 2018 and 2019. FRM and FEM ozone monitoring methods were compared to assess the capabilities for the accurate, monitoring of ozone in biomass smoke. • Results – The research effort described in this output resulted in the identification of a severe interference associated with multiple UV-photometric method instruments for measuring ozone in biomass burning smoke. It is hypothesized that this interference is associated with volatile organic compounds (VOCs) that are emitted freely during biomass combustion and that absorb in the same optical UV region as ozone. In addition, it was discovered that operation of UV-photometric instruments in heavy smoke events results in irreversible damage to instrument components that will adversely impact the measurement of ozone with the same instrument in non-smoke impact conditions. The recently promulgated nitrogen oxide (NO) chemiluminescence FRM for measuring ozone was confirmed to show no measurement interference in smoke and remains the most robust method for the ambient measurement of ozone. • Anticipated Impact – The results detailed in this output will provide State and local air monitoring agencies with the tools and knowledge to address ozone measurement challenges in areas frequently impact by wildland fire smoke. The observed large positive artifact in the UV-photometric method has the potential to result in false exceedances of the ozone NAAQS or in the disqualification of monitoring data through an exceptional event designation. The recently promulgated FRM for ozone based upon NO-chemiluminescence has been shown to be the free of interferences in smoke an provides States with the tools needed to accurately determine ambient ozone concentrations during smoke events. Accurate methods, such as the FRM will reduce the burden of developing and reviewing exceptional event request packages, data loss/disqualification, and provide States with tools adequately evaluate public exposure risks and provide accurate public health messaging during wildfire/smoke events.
Description:
In recent years wildland fires in the United States have had significant impacts on local and regional air quality and negative human health outomes. Although the primary health concerns from wildland fires come from fine particulate matter (PM2.5), large increases in ozone (O3) are also observed downwind of wildland fire plumes. Conditions generated in and around wildland fire plumes, including the presence of interferring chemical species, can make the accurate measurement of O3 concentrations using the ultraviolet (UV) photometric method challenging if not impossible. UV photometric method instruments are prone to interferences by volatile organic compounds (VOCs) that are present at high concentrations in wildland fire smoke. Four different O3 measurement methodologies were deployed in a mobile sampling platform downwind of active prescribed grassland fire lines in Kansas and Oregon and during controlled chamber burns at the United States Forest Service, Rocky Mountain Research Station Fire Sciences Laboratory in Missoula, Montana. We demonstrate that the Federal Reference Method (FRM) nitric oxide (NO) chemiluminescence monitors and Federal Equivalent Method (FEM) gas-phase (NO) chemical scrubber UV photometric O3 monitors are relatively interference-free, even in near-field combustion plumes. In contrast, FEM UV photometric O3 monitors using solid-phase catalytic scrubbers show positive artifacts that are positively correlated with carbon monoxide (CO) and total gas phase hydrocarbons (THC), two indicator species of biomass burning. Of the two catalytic scrubber UV photometric methods evaluated, the instruments that included a Nafion® tube dryer in the sample introduction system had artifacts an order of magnitude smaller than the instrument with no humidity correction. We hypothesize that Nafion-permeable VOCs (such as aromatic hydrocarbons) could be a significant source of interference for catalytic scrubber ultraviolet photometric O3 monitors, and that the inclusion of a Nafion® tube dryer assists with the mitigation of these interferences. The interference-free chemiluminescence FRM method is highly recommended for accurate measurements of O3 in wildland fire plume studies and at regulatory ambient montioring sites frequently impacted by wildland fire smoke.
