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.