A systematic re-evaluation of methods for quantification of bulk particle-phase organic nitrates using real-time aerosol mass spectrometry

Organic nitrate (RONO2 ) formation in the atmosphere represents a sink of NOx (NOx = NO + NO2 ) and termination of the NOx /HOx (HOx = HO2 + OH) ozone formation and radical propagation cycles, can act as a NOx reservoir transporting reactive nitrogen, and contributes to secondary organic aerosol formation. While some fraction of RONO2 is thought to reside in the particle phase, particlephase organic nitrates (pRONO2 ) are infrequently measured and thus poorly understood. There is an increasing prevalence of aerosol mass spectrometer (AMS) instruments, which have shown promise for determining the quantitative total organic nitrate functional group contribution to aerosols. A simple approach that relies on the relative intensities of NO+ and NO+ 2 ions in the AMS spectrum, the calibrated NO+ x ratio for NH4 NO3 , and the inferred ratio for pRONO2 has been proposed as a way to apportion the total nitrate signal to NH4 NO3 and pRONO2 . This method is increasingly being applied to field and laboratory data. However, the methods applied have been largely inconsistent and poorly characterized, and, therefore, a detailed evaluation is timely. Here, we compile an extensive survey of NO+ x ratios measured for various pRONO2 compounds and mixtures from multiple AMS instruments, groups, and laboratory and field measurements. All data and analysis presented here are for use with the standard AMS vaporizer. We show that, in the absence of pRONO2 standards, the pRONO2 NO+ x ratio can be estimated using a ratio referenced to the calibrated NH4 NO3 ratio, a so-called “Ratioof-Ratios” method (RoR = 2.75 ± 0.41). We systematically explore the basis for quantifying pRONO2 (and NH4 NO3 ) with the RoR method using ground and aircraft field measurements conducted over a large range of conditions. The method is compared to another AMS method (positive matrix factorization, PMF) and other pRONO2 and related (e.g.,