<p>Organic nitrate (RONO<sub>2</sub>) formation in the atmosphere represents a sink of NO<sub>x</sub> (NO<sub>x</sub> = NO + NO<sub>2</sub>) and termination of the NO<sub>x</sub>/HO<sub>x</sub> (HO<sub>x</sub> = HO<sub>2</sub> + OH) ozone formation and radical propagation cycles, can act as a NO<sub>x</sub> reservoir transporting reactive nitrogen, and contributes to secondary organic aerosol formation. While some fraction of RONO<sub>2</sub> is thought to reside in the particle phase, particle-phase organic nitrates (pRONO<sub>2</sub>) are infrequently measured and thus poorly understood. There is an increasing prevalence of aerosol mass spectrometer (AMS) instruments, which have shown promise for determining quantitative total organic nitrate functional group contribution to aerosols. A simple approach that relies on the relative intensities of NO<sup>+</sup> and NO<sub>2</sub><sup>+</sup> ions in the AMS spectrum, the calibrated NO<sub>x</sub><sup>+</sup> ratio for NH<sub>4</sub>NO<sub>3</sub>, and the inferred ratio for pRONO<sub>2</sub> has been proposed as a way to apportion the total nitrate signal to NH<sub>4</sub>NO<sub>3</sub> and pRONO<sub>2</sub>. 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<sub>x</sub><sup>+</sup> ratios measured for various pRONO<sub>2</sub> compounds and mixtures from multiple AMS instruments, groups, and laboratory and field measurements. We show that, in the absence of pRONO<sub>2</sub> standards, the pRONO<sub>2</sub> NO<sub>x</sub><sup>+</sup> ratio can be estimated using a ratio referenced to the calibrated NH<sub>4</sub>NO<sub>3</sub> ratio, a so-called <q>Ratio-of-Ratios</q> method (<em>RoR</em> = 2.75 ± 0.41). We systematically explore the basis for quantifying pRONO<sub>2</sub> (and NH<sub>4</sub>NO<sub>3</sub>) with the <em>RoR</em> 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 pRONO<sub>2</sub> and related (e.g., total gas + particle RONO<sub>2</sub>) measurements, generally showing good agreement/correlation. A broad survey of ground and aircraft AMS measurements shows a pervasive trend of higher fractional contribution of pRONO<sub>2</sub> to total nitrate with lower total nitrate concentrations, which generally corresponds to shifts from urban-influenced to rural/remote regions. Compared to ground campaigns, observations from all aircraft campaigns showed substantially lower pRONO<sub>2</sub> contributions at mid ranges of total nitrate (0.01–0.1 up to 2–5 μg m<sup>−3</sup>), suggesting that the balance of effects controlling NH<sub>4</sub>NO<sub>3</sub> and pRONO<sub>2</sub> formation and lifetimes — such as higher humidity, lower temperatures, greater dilution, different sources, higher particle acidity, and pRONO<sub>2</sub> hydrolysis (possibly accelerated by particle acidity) — favors lower pRONO<sub>2</sub> contributions for those environments and altitudes sampled.</p>