Enhanced Organic Nitrate Formation from Peroxy Radicals in the Condensed Phase

Organic alkoxy (RO) and peroxy (RO₂) radicals are key intermediates in multiphase atmospheric oxidation chemistry, though most of the study of their chemistry has focused on the gas phase. To better understand how radical chemistry may vary across different phases, we examine the chemistry of a model system, the 1-pentoxy radical, in three phases: the aqueous phase, the condensed organic phase, and the gas phase. In each phase, we generate the 1-pentoxy radical from the photolysis of n-pentyl nitrite, run the chemistry under conditions in which RO₂ radicals react with NO, and detect the products in real time using an ammonium chemical ionization mass spectrometer (NH₄⁺ CIMS). The condensed-phase chemistry shows an increase in formation of organic nitrate (RONO₂) from the downstream RO₂+NO reaction, which is attributed to potential collisional and solvent-cage stabilization of the RO₂–NO complex. We further observe an enhancement in the yield of carbonyl relative to hydroxy carbonyl products in the condensed phase, indicating changes to RO radical kinetics. The different branching ratios in the condensed phase impact the product volatility distribution as well as HO_x-NO_x chemistry, and may have implications for nitrate formation, aqueous aerosol formation, and radical cycling within atmospheric particles and droplets.