<p><strong>Abstract.</strong> The hydroxyl radical oxidation of α-pinene under high NO_x conditions was studied in a photochemical reaction chamber to investigate organic nitrate (RONO₂) production and partitioning between the gas and particle phases. We report an organic nitrate yield of 26 ± 7% from the oxidation of this monoterpene in the presence of nitric oxide (NO). However, the organic nitrate yield was found to be highly dependent on both chamber relative humidity

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... H) and seed aerosol acidity, likely as a result of particle phase hydrolysis. The particle phase loss of organic nitrates perturbs the gas-particle equilibrium within the system, leading to decreased RONO₂ yields in both the gas and particle phases at elevated RH and an apparent non-equilibrium partitioning mechanism. This resulted in smaller apparent partition coefficients of the total organic nitrate species under high chamber RH. The hydrolysis of particle phase organic nitrates at low chamber relative humidity in this study implies that aerosol partitioning of organic nitrates may be an important sink for atmospheric NO_x and may have a significant impact on regional air quality.</p>