Methyl peroxy nitrate (CH₃O₂NO₂) is a non-acyl peroxy nitrate that is important for photochemistry at low temperatures characteristic of the upper troposphere. We report the first measurements of CH₃O₂NO₂, which we achieved through a new aircraft inlet configuration, combined with thermal-dissociation laser-induced fluorescence (TD-LIF) detection of NO₂, and describe the accuracy, specificity, and interferences to CH₃O₂NO₂ measurements. CH₃O₂NO₂ is predicted to be a ubiquitous interference to upper-tropospheric NO₂ measurements. We describe an experimental strategy for obtaining NO₂ observations free of the CH₃O₂NO₂ interference. Using these new methods, we made observations during two recent aircraft campaigns: the Deep Convective Clouds and Chemistry (DC-3) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) experiments. The CH₃O₂NO₂ measurements we report have a detection limit (S/N = 2) of 15 pptv at 1 min averaging on a background of 200 pptv NO₂ and an accuracy of ±40%. Observations are used to constrain the interference of pernitric acid (HO₂NO₂) to the CH₃O₂NO₂ measurements, as HO₂NO₂ partially decomposes (~11%) along with CH₃O₂NO₂ in the heated CH₃O₂NO₂ channel used to detect CH₃O₂NO₂.
