Basicity and acidity promote hydrolysis of methyl nitrate in aqueous aerosols

The chemistry of organic nitrates (ONs), also known as alkyl nitrates (RONO₂), controls the lifetime of nitrogen oxides in continental areas, which in turn affects air quality and varies ozone concentration throughout the troposphere. ONs can be emitted to the troposphere from marine sources. Also, they can be produced in the atmosphere through addition of NO to peroxy radicals or through the reaction of NO₃ radicals with volatile organic compounds. Atmospheric ONs may subsequently undergo oxidation or photolysis, in both gas and aerosol phases, or hydrolysis in aqueous aerosols. Though some recent studies have believed acid-catalysis promotes hydrolysis of ONs, earlier studies have claimed that acids have no effect on ON hydrolysis, and that it is the hydroxyl ion that can improve the hydrolysis process. The limited number of experimental studies performed so far have left this conflict with no appropriate answer, as mechanistic insight and full kinetics details have been partially or completely missing for the studied ONs. We report the detailed mechanism of methyl nitrate hydrolysis in acidic, neutral and basic conditions, in addition to analyzing the degradation of methyl nitrate into formaldehyde and nitrous acid in the presence of water and hydronium ions. According to the potential energy surfaces obtained at the CCSD(T)/cc-pVDZ//ωB97X-D/def2-TZVP level of theory (including the SMD solvent model) along with the rate coefficients estimated using asymmetric Eckart tunneling-corrected transition state theory (TST), mediation of water molecules and hydronium ions hinders degradation of methyl nitrate into formaldehyde and nitrous acid and, in general, this decomposition reaction is kinetically unfavorable. Furthermore, neutral hydrolysis of methyl nitrate is extremely slow with pseudo-first order rate coefficients (k; 298 K and 1 atm) falling below 10^-27 s^-1. Similarly, hydrolysis of methyl nitrate by hydronium ions is observed to be extremely slow (k < 10^-27 s^-1). However, under acidic conditions, protonation of methyl nitrate is quite feasible with the protonation Gibbs free energy of -429.1 kJ mol^-1, at 298 K and 1 atm, and protonated methyl nitrate can hydrolyze into protonated methanol and nitric acid much faster relative to the hydronium ion-based and neutral hydrolysis (k = 3.83 s^-1). On the other hand, the hydroxyl ions generated under basic conditions can hydrolyze methyl nitrate readily to give methanol and nitric acid (k = 6.63 × 10³ s^-1), or formaldehyde, nitrate and water (k = 9.40 × 10⁶ s^-1). In addition, regardless of the limitation on the rate of solvent-phase chemical reactions by the rate of diffusion, basic hydrolysis can produce methoxy ions and nitric acid quite fast (k = 8.95 × 10⁹ s^-1). In other words, methyl nitrate hydrolysis is faster in basic aerosols (i.e. some marine aerosols) and, to a less extent, in highly acidic aqueous aerosols (e.g. haze and urban aerosols).