On the Mechanism for Nitrate Formation via the Peroxy Radical + NO Reaction

Abstract

We present a master equation study of organic nitrate formation from the peroxy radical (RO2) + NO reaction. The mechanism is constrained by both quantum chemical calculations of the potential energy surface and existing yield data. This mechanism displays heretofore unrecognized features of the system, including distinct conformers of a critical peroxynitrite (ROONO) intermediate that do not interconvert and a dual falloff behavior driven by collisional stabilization in multiple wells. These features have significant implications for atmospheric chemistry; in particular, only a fraction of the ROONO intermediates may easily isomerize to nitrates, resulting in a limit to total nitrate production. Existing mechanisms, extrapolated to low temperature and high pressure, produce nitrate almost exclusively. As a consequence, hydrocarbon oxidation sequences based on these mechanisms do not propagate radical chemistry, which is inconsistent with available experimental data. To reproduce observed nitrate yields, we model a transition state from the ROONO intermediate to RONO2 that differs considerably from the few found in computational studies. Specifically, the data require that this transition state energy lie well below the energy of separated radical products (RO + NO2), while computational studies find the transition state at higher energies. A second feature of yield data is difficult to model; to enable collisional stabilization of C5 systems, as observed, we reduce the unimolecular decomposition rate constants from the ROONO intermediate by a factor that is at the far end of the plausible range. However, with these experimental constraints in place, the model successfully reproduces multiple features of existing data quantitatively, including both high- and low-pressure asymptotes to nitrate production as well as the observed shifting of pressure falloff curves with carbon number. Consequently, we present a new parametrization of nitrate yields, providing interpolation equivalent to existing parametrizations but dramatically improved extrapolation behavior.

Effect of NO<sub>x</sub> level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes

“…For both α-pinene and longifolene, no acidic nitrates are observed under low-NO x conditions, consistent with the lack of NO x and the prevailing RO 2 +HO 2 reaction in this case. Organic nitrate yield from the RO 2 +NO reaction increases with increasing carbon number (Atkinson et al, 1987;Carter and Atkinson, 1989;O'Brien et al, 1998;Arey et al, 2001;Aschmann et al, 2001;Zhang et al, 2004), and with the larger carbon skeleton the organic nitrates formed will be less volatile, this is consistent with the much wider array and larger quantities of acidic nitrates detected in longifolene photooxidation under high-NO x conditions compared to the α-pinene experiments. Hence for photooxidation of larger compounds such as sesquiterpenes, the nitrate formation channel may play an important role in SOA formation under high-NO x conditions.…”

Reaction of gas phase OH with unsaturated self-assembled monolayers and relevance to atmospheric organic oxidations

“…As seen in Scheme 1, the latter branch should form surface aldehydes so that one might have anticipated by analogy to gas phase chemistry that the surface carbonyl yield would be significantly larger than that of the organic nitrates. The larger yield of organic nitrates in the present studies relative to carbonyl yields may reflect enhanced stabilization of the (ROO-NO) 66 intermediates that form RONO 2 due to the densely packed monolayer. Alternatively, the packing of the monolayer may yield RO 2 radicals with structures that favor the trans-form of ROO-NO over the cis-, where the trans form has been proposed to be responsible for organic nitrate formation.…”

Detection of HO<sub>2</sub> by laser-induced fluorescence: calibration and interferences from RO<sub>2</sub> radicals

“…In addition to alkoxy radicals alkyl nitrates can be formed, but their yield is not well-known (Atkinson and Arey, 2003). Studies show that the yield is increasing with increasing number of C-atoms in the peroxy radical and with increasing pressure (Atkinson and Arey, 2003;Zhang et al, 2004). The lifetime of peroxy radicals is approximately 1 ms for conditions, at which the detection cell was operated in the field (0.4 mm nozzle, [NO] = 1.3 × 10 14 cm −3 ).…”