Field intercomparison of continuous Ambient FRM and FEM NO₂ Instruments in the Athabasca Oil Sands Region, Alberta, Canada and the Potential Impact on Ambient regulatory compliance

The Canadian Federal Government promulgated new and lower NO₂ Ambient Air Quality Standards (CAAQS) that went into effect in 2020 with additional decreases scheduled for 2025. The new hourly and annual NO₂ CAAQS are 60 and 17 ppb, respectively, and the 2025 hourly and annual CAAQS are 42 and 12 ppb, respectively. The province of Alberta has also promulgated Ambient Air Quality Objectives (AAAQO) for NO₂ currently set to 159 and 24 ppb on an hourly and annual basis, respectively. The Wood Buffalo Environmental Association (WBEA) in northeastern Alberta, Canada monitors NO₂ at 21 community and industrial sites throughout the Alberta Oil Sands Region (AOSR), for regulatory compliance using Thermo-Environmental (TEI) Model 42i Federal Reference Method (FRM) designated NO-NO₂-NOx analyzers. The 42i measures NO directly via NO-O₃ chemiluminescence, and NOx following the reduction of oxidized nitrogen to NO by a heated internal molybdenum converter. The difference between the NOx and NO channels is reported as NO₂. This study presents the results of a three year (2018-2021) WBEA comparison of four continuous NO₂ analyzers: TEI 42i FRM; the API Model T500U cavity attenuated phase shift (CAPS) Federal Equivalent Method (FEM); a total reactive odd nitrogen analyzer (TEI Model 42i-Y); and a TEI 42i equipped with an external photolytic converter. The study showed that NO₂ data from all analyzers were highly correlated and in general agreement, with r² values (versus the CAPS) ranging from 0.990 - 0.997 and slopes ranging from 0.933 - 0.992. Mean NO₂ concentrations over the study period ranged from 7.2-7.5 ppb. Differences between the TEI 42i, TEI 42i-Y, PhoNO, relative to the CAPS were all positive and highly significant (p < 0.0001), based upon non-parametric tests. The potential impact from the selection of different FRM/FEM measurement methods on current and future Canadian 2025 regulatory compliance in the region is evaluated.

The study objective was to compare/evaluate different regulatory NO2 measurement techniques from the perspective of a regional monitoring authority in a routine network operational context. Though relatively small, reported NO2 differences between the instruments resulted in significant differences with respect to regulatory compliance triggers, particularly hourly standards that are based on daily extreme value statistics (e.g., 99th percentiles). For example, mean hourly NO2 △ differences only ranged from 0.02 – 0.26 ppb over the study period but resulted in 2-3 ppb differences in the 3-year hourly CAAQS metrics. These differences could affect overall regulatory CAAQS and LARP compliance (management level) at some monitoring sites as observed in the 2019 annual and 2020 hourly LARP trigger exceedances.