Evaluation of Uncertainties and Introduction of Tools for Quantification of Bulk Particle-phase Organic Nitrates Using Real-time Aerosol Mass Spectrometry

DOUGLAS A. DAY, Benjamin A. Nault, Pedro Campuzano-Jost, Jose-Luis Jimenez, CIRES, University of Colorado, Boulder

     Abstract Number: 88
     Working Group: Instrumentation and Methods

Abstract
Organic nitrates and particle-phase organic nitrates (pRONO₂) play important roles in tropospheric chemistry, including radical budgets, ozone formation, SOA formation, and the reactive oxidized nitrogen budget. Over the last decade, the Aerodyne aerosol mass spectrometer (AMS), widely-used for measuring online aerosol composition, has been increasingly applied to quantify pRONO₂. An approach that relies on relative intensities of NO⁺ and NO₂⁺ ions in AMS spectra (“NO_x⁺ ion ratio method”), when using a standard vaporizer, is the most common method applied, and was recently supported by demonstration of a reliable way to estimate pRONO₂ NO_x⁺ ion ratios for complex ambient aerosol, the so-called “Ratio-of-Ratios” (RoR) method. However, little attention has focused on quantifying uncertainties for these methods. Therefore, we have systematically explored uncertainties associated with quantifying pRONO₂ (and NH₄NO₃) with the RoR method. Identified uncertainties are derived, quantified, and propagated to estimate overall uncertainty, relative importance of terms, and how they vary with averaging and aerosol composition. The analysis is extended to estimation of detection limits and unit mass resolution data. Overall uncertainties have substantial contributions from the ambient NO_x⁺ ratio measurement, precision of NH₄NO₃ NO_x⁺ ratio determination, RoR estimate of the pRONO₂ NO_x⁺ ratio, and accuracy of the AMS total nitrate concentrations. Effects on quantification from matrix and instrument memory effects are evaluated using laboratory-generated mixtures of nitrates with inorganic and organic aerosol as well as complex ambient mixtures. We conclude that such effects, if present in ambient aerosol, are likely small. We have developed a publicly-available, open-source software for calculation of concentrations, uncertainties, and detection limits for pRONO₂ and NH₄NO₃. Finally, thermal denuder measurements for several field datasets were used to explore the efficacy of the RoR method and quantify volatility of pRONO₂, which was similar to less-oxidized, oxidized organic aerosol PMF factors.