Biomass burning emissions have substantially increased with continued warming and drying in the southwestern U.S., impacting air quality and atmospheric processes. To better quantify impacts of biomass burning aerosols, an extensive laboratory study of fresh smoke emissions was conducted at Los Alamos National Laboratory. Laboratory burn experiments with selected native and invasive southwestern U.S. fuels were used to elucidate the role of fuel type, chemical composition, and ignition method on the hygroscopicity of smoke. Here we focus on a custom controlled relative humidity (RH) nephelometry system using the direct measurement of aerosol light scattering with two nephelometers—one at dry conditions and one at a controlled high RH (RH ~ 85%). Aerosol hygroscopicity was highly variable with the enhancement in light scattering coefficient in the range of 1.02 < f(RH = 85%) < 2.1 and corresponding to the kappa parameter (κneph) ranging from ~0 to 0.18. Hygroscopicity is determined primarily by the fuel's inorganic ion content. For example, invasive halophytes with high inorganic salt content exhibit much greater water uptake than native coniferous species with low inorganic content. Combustion temperature and phase, flaming or smoldering, play a secondary role in the water uptake of smoke. High‐temperature ignition methods create flaming conditions that enhance hygroscopicity while lower‐temperature smoldering conditions diminish hygroscopicity. Our results construct an empirical relation between κneph and the inorganic content of the fuel and smoke to predict water uptake.

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Understanding nitrogen oxides (NO = NO + NO) measurement techniques is important as air-quality standards become more stringent, important sources change, and instrumentation develops. NO observations are compared in two environments: source testing from the combustion of Southwestern biomass fuels, and urban, ambient NO. 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 (O) <60 ppb for all but 6 hr. We compare two techniques used to measure NO 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 NO, and the NO chemiluminescence instrument (conversion of NO to NO by molybdenum catalyst). We find agreement similar to laboratory standards for NO, NO, and NO comparing all four instruments (R > 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 NO measurements was observed in comparing techniques in late-winter urban Albuquerque. This was also confirmed by negligible NO contributions as measured with an NO instrument. For the rapidly varying (1-min) higher NO 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, NO, and NO. Nonetheless, integrating the NO 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, NO, and NO yielded slopes of 0.94 to 0.96, R 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 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 measurement methods were employed to measure NO 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 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 conditions in a range of environments.