NO_x instrument intercomparison for laboratory biomass burning source studies and urban ambient measurements in...

Understanding nitrogen oxides (NO_x = NO + NO₂) measurement techniques is important as air-quality standards become more stringent, important sources change, and instrumentation develops. NO_x observations are compared in two environments: source testing from the combustion of Southwestern biomass fuels, and urban, ambient NO_x. The latter occurred in the urban core of Albuquerque, NM, at an EPA NCORE site during February-March 2017, a relatively clean photochemical environment with ozone (O₃) <60 ppb for all but 6 hr. We compare two techniques used to measure NO_x in biomass smoke during biomass burning source testing: light absorption at 405 nm and a traditional chemiluminescence monitor. Two additional oxides of nitrogen techniques were added in urban measurements: a cavity attenuated phase shift instrument for direct NO₂, and the NO_y chemiluminescence instrument (conversion of NO_y to NO by molybdenum catalyst). We find agreement similar to laboratory standards for NO_x, NO₂, and NO comparing all four instruments (R² > 0.97, slopes between 0.95 and 1.01, intercepts < 2 ppb for 1-hr averages) in the slowly varying ambient setting. Little evidence for significant interferences in NO₂ measurements was observed in comparing techniques in late-winter urban Albuquerque. This was also confirmed by negligible NO_z contributions as measured with an NO_y instrument. For the rapidly varying (1-min) higher NO_x concentrations in biomass smoke source testing, larger variability characterized chemiluminescence and absorption instruments. Differences between the two instruments were both positive and negative and occurred for total NO_x, NO, and NO₂. Nonetheless, integrating the NO_x signals over an entire burn experiment and comparing 95 combustion experiments, showed little evidence for large systematic influences of possible interfering species biasing the methods. For concentrations of <2 ppm, a comparison of burn integrated NOx, NO₂, and NO yielded slopes of 0.94 to 0.96, R² of 0.83 to 0.93, and intercepts of 8 to 25 ppb. We attribute the latter, at least in part, to significant noise particularly at low NO_x concentrations, resulting from short averaging times during highly dynamic lab burns. Discrepancies between instruments as indicated by the intercepts urge caution with oxides of nitrogen measurements at concentrations <50 ppb for rapidly changing conditions. Implications: Multiple NO_x measurement methods were employed to measure NO_x concentrations at an EPA NCORE site in Albuquerque, NM, and in smoke produced by the combustion of Southwestern biomass fuels. Agreement shown during intercomparison of these NO_x techniques indicated little evidence of significant interfering species biasing the methods in these two environments. Instrument agreement is important to understand for accurately characterizing ambient NO_x conditions in a range of environments.