Yields of HONO2 and HOONO Products from the Reaction of HO2 and NO Using Pulsed Laser Photolysis and Mid-Infrared Cavity-Ringdown Spectroscopy.

Abstract

The reaction of HO2 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 (HONO2) directly from HO2 + NO. Atmospheric models previously treated HO2 + NO as radical recycling, but inclusion of this terminating step had large effects on atmospheric oxidative capacity and the concentrations of HONO2 and ozone (O3), among others. Here, the yield of HONO2, φHONO2, from the reaction of HO2 + NO was investigated in a flow tube reactor using mid-IR pulsed-cavity ringdown spectroscopy. HO2, produced by pulsed laser photolysis of Cl2 in the presence of methanol, reacted with NO in a buffer gas mixture of N2 and CO between 300 and 700 Torr at 278 and 300 K. HONO2 and its weakly bound isomer HOONO were directly detected by their v1 absorption bands in the mid-IR region. CO was used to suppress HONO2 produced from OH + NO2 and exploit a chemical amplification scheme, converting OH back to HO2. Under the experimental conditions described here, no evidence for the formation of either HONO2 or HOONO was observed from HO2 + NO. Using a comprehensive chemical model, constrained by observed secondary reaction products, all HONO2 detected in the system could be accounted for by OH + NO2. At 700 ± 14 Torr and 300 ± 3 K, φHONO2 = 0.00 ± 0.11% (2σ) with an upper limit of 0.11%. If all of the observed HONO2 was attributed to the HO2 + NO reaction, φHONO2 = 0.13 ± 0.07% with an upper limit of 0.20%. At 278 ± 2 K and 718 ± 14 Torr, we determine an upper limit, φHONO2 ≤ 0.37%. Our measurements are significantly lower than those previously reported, lying outside of the uncertainty of the current experimental and recommended literature values.