NO ₂ reduction to HONO by small α-hydroxycarbonyls: a laboratory investigation relevant to nighttime production of atmospheric HONO

This work presents a gas-phase experimental study on the reduction of NO₂ (nitrogen dioxide) to HONO (nitrous acid) by two atmospherically significant volatile organic compounds (VOCs), namely, glycolaldehyde (Gla) and hydroxyacetone (HAc), under a simulated tropospheric condition. FTIR spectroscopic probing reveals that HONO is the only gaseous reduced product of NO₂ in each reaction. The measured data indicate that the reactions in both cases occur in a 2 : 1 stoichiometry with respect to NO₂ and Gla/HAc. The oxidized coproducts of the hydroxycarbonyls are expected to be glyoxal and methylglyoxal, and the reactions are energetically favorable from the thermochemical viewpoint according to the predictions of the electronic structure theories. In each case, the overall reaction occurs as a 3rd order process, and the measured rate constant for the HAc-NO₂ reaction at 30 °C is 2.5 × 10^-35 cm⁶ molecule^-2 s^-1 and the same for Gla-NO₂ reaction is 0.42 × 10^-35 cm⁶ molecule^-2 s^-1. Temperature variation studies show that the reaction rates are increased distinctly with the rise in temperature, and the estimated Arrhenius activation energies of the reactions with Gla and HAc are 47 and 58 kJ mol^-1, respectively. The surface-to-volume ratio variation of the reaction cell indicates that the reactions occur predominantly in the gas phase. A mechanism of the reaction is proposed whose feasibility is verified by IRC calculations. A brief discussion about the atmospheric significance of the findings is presented, making comparison of NO₂ reactions with other VOCs.