Heterogeneous Chemistry of HO2NO2 in Liquid Sulfuric Acid

Derivation of Hydroperoxyl Radical Levels at an Urban Site via Measurement of Pernitric Acid by Iodide Chemical Ionization Mass Spectrometry

Hydroperoxyl radical (HO₂) is a key species to atmospheric chemistry. At warm temperatures, the HO₂ and NO₂ come to a rapid steady state with pernitric acid (HO₂NO₂). This paper presents the derivation of HO₂ from observations of HO₂NO₂ and NO₂ in metropolitan Atlanta, US, in winter 2014 and summer 2015. HO₂ was observed to have a diurnal cycle with morning concentrations suppressed by high NO from the traffic. At night, derived HO₂ levels were nonzero and exhibited correlations with O₃ and NO₃, consistent with previous studies that ozonolysis and oxidation by NO₃ are sources of nighttime HO₂. Measured and model calculated HO₂ were in reasonable agreement: Without the constraint of measured HO₂NO₂, the model reproduced HO₂ with a model-to-observed ratio (M/O) of 1.27 (r = 0.54) for winter, 2014, and 0.70 (r = 0.80) for summer, 2015. Adding measured HO₂NO₂ as a constraint, the model predicted HO₂ with M/O = 1.13 (r = 0.77) for winter 2014 and 0.90 (r = 0.97) for summer 2015. These results demonstrate the feasibility of deriving HO₂ from HO₂NO₂ measurements in warm regions where HO₂NO₂ has a short lifetime.