Yields of HONO 2 and HOONO Products from the Reaction of HO 2 and NO Using Pulsed Laser Photolysis and Mid-Infrared Cavity-Ringdown Spectroscopy.

The reaction of HO 2 with NO is one of the most important steps in radical cycling throughout the stratosphere and troposphere. Previous literature experimental work revealed a small yield of nitric acid (HONO 2 ) directly from HO 2 + NO. Atmospheric models previously treated HO 2 + NO as radical recycling, but inclusion of this terminating step had large effects on atmospheric oxidative capacity and the concentrations of HONO 2 and ozone (O 3 ), among others. Here, the yield of HONO 2 , φ HONO 2 , from the reaction of HO 2 + NO was investigated in a flow tube reactor using mid-IR pulsed-cavity ringdown spectroscopy. HO 2 , produced by pulsed laser photolysis of Cl 2 in the presence of methanol, reacted with NO in a buffer gas mixture of N 2 and CO between 300 and 700 Torr at 278 and 300 K. HONO 2 and its weakly bound isomer HOONO were directly detected by their v 1 absorption bands in the mid-IR region. CO was used to suppress HONO 2 produced from OH + NO 2 and exploit a chemical amplification scheme, converting OH back to HO 2 . Under the experimental conditions described here, no evidence for the formation of either HONO 2 or HOONO was observed from HO 2 + NO. Using a comprehensive chemical model, constrained by observed secondary reaction products, all HONO 2 detected in the system could be accounted for by OH + NO 2 . At 700 ± 14 Torr and 300 ± 3 K, φ HONO 2 = 0.00 ± 0.11% (2σ) with an upper limit of 0.11%. If all of the observed HONO 2 was attributed to the HO 2 + NO reaction, φ HONO 2 = 0.13 ± 0.07% with an upper limit of 0.20%. At 278 ± 2 K and 718 ± 14 Torr, we determine an upper limit, φ HONO 2 ≤ 0.37%. Our measurements are significantly lower than those previously reported, lying outside of the uncertainty of the current experimental and recommended literature values.