Does the reaction of HO₂ with NO produce HONO₂ and HOONO

Abstract

HO_x (HO₂ and OH) and NO_x (NO₂ and NO) radicals are key intermediates in chem. throughout the atm.; the HO_x and NO_x cycles catalyze ozone depletion in the stratosphere and ozone and photochem. smog prodn. in the troposphere. Interconversion within the HO_x family and within the NO_x family happens continuously, partially through the reaction of HO₂ and NO to form OH and NO₂. Since these radicals are continually recycled, even a small branching yield of nitric acid (HONO₂) from the reaction of HO₂ with NO would impact radical concns. predicted in the troposphere and stratosphere, by cumulatively sequestering radicals in a stable reservoir species. Butkovskaya et al. obsd. a small yield of HONO₂ from the reaction of HO₂ with NO (0.5 ˆat 1 atm and 298 K) in a turbulent flow reactor using Chem.-Ionization Mass Spectrometry. We investigated this reaction by an alternative method: detecting the HONO₂ - as well as its weakly bound isomer HOONO - with Pulsed-Cavity Ringdown spectroscopy. HO₂ radicals were produced by Pulsed Laser Photolysis of Cl₂ in a slow flow cell, in the presence of methanol or formaldehyde. Addn. of 700 torr of CO prevented unwanted HONO₂ formation from the reaction of OH and NO₂. Our expts. provide a complementary approach, allowing detection of products spectroscopically on short time-scales in the absence of any wall reactions. Our results raise doubts about the magnitude of the HONO₂ yields obsd. by Butovskaya et al.