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Secondary organic aerosol (SOA) from acid-driven reactive uptake of isoprene epoxydiols (IEPOX) contributes up to 40% of organic aerosol (OA) mass in fine particulate matter. Previous work showed that IEPOX substantially converts particulate inorganic sulfates to surface-active organosulfates (OSs). This decreases aerosol acidity and creates a viscous organic-rich shell that poses as a diffusion barrier, inhibiting additional reactive uptake of IEPOX. To account for this "self-limiting" effect, we developed a phase-separation box model to evaluate parameterizations of IEPOX reactive uptake against time-resolved chamber measurements of IEPOX-SOA tracers, including 2-methyltetrols (2-MT) and methyltetrol sulfates (MTS), at ~ 50% relative humidity. The phase-separation model was most sensitive to the mass accommodation coefficient, IEPOX diffusivity in the organic shell, and ratio of the third-order reaction rate constants forming 2-MT and MTS ( k M T / k M T S ). In particular, k M T / k M T S had to be lower than 0.1 to bring model predictions of 2-MT and MTS in closer agreement with chamber measurements; prior studies reported values larger than 0.71. The model-derived rate constants favor more particulate MTS formation due to 2-MT likely off-gassing at ambient-relevant OA loadings. Incorporating this parametrization into chemical transport models is expected to predict lower IEPOX-SOA mass and volatility due to the predominance of OSs.

Atmospheric nitrate, including nitric acid (HNO 3 ), particulate nitrate (pNO 3 ), and organic nitrate (RONO 2 ), is a key atmosphere component with implications for air quality, nutrient deposition, and climate. However, accurately representing atmospheric nitrate concentrations within atmospheric chemistry models is a persistent challenge. A contributing factor to this challenge is the intricate chemical transformations involving HNO 3 formation, which can be difficult for models to replicate. Here, we present a novel model framework that utilizes the oxygen stable isotope anomaly (Δ 17 O) to quantitatively depict ozone (O 3 ) involvement in precursor nitrogen oxides N O x = N O + N O 2 photochemical cycling and HNO 3 formation. This framework has been integrated into the US EPA Community Multiscale Air Quality (CMAQ) modeling system to facilitate a comprehensive assessment of NO x oxidation and HNO 3 formation. In application across the northeastern US, the model Δ 17 O compares well with recently conducted diurnal Δ 17 O(NO 2 ) and spatiotemporal Δ 17 O(HNO 3 ) observations, with a root mean square error between model and observations of 2.6 ‰ for Δ 17 O(HNO 3 ). The model indicates the major formation pathways of annual HNO 3 production within the northeastern US are NO+OH (46 %), N 2 O 5 hydrolysis (34 %), and organic nitrate hydrolysis (12 %). This model can evaluate NO x chemistry in CMAQ in future air quality and deposition studies involving reactive nitrogen.