Using measurements from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, we show that methyl peroxy nitrate (CH<sub>3</sub>O<sub>2</sub>NO<sub>2</sub>) is present in concentrations of ~5–15 pptv in the springtime arctic upper troposphere. We investigate the regional and global effects of CH<sub>3</sub>O<sub>2</sub>NO<sub>2</sub> by including its chemistry in the GEOS-Chem 3-D global chemical transport model. We find that at temperatures below 240 K inclusion of CH<sub>3</sub>O<sub>2</sub>NO<sub>2</sub> chemistry results in decreases of up to ~20 % in NO<sub>x</sub>, ~20 % in N<sub>2</sub>O<sub>5</sub>, ~5 % in HNO<sub>3</sub>, ~2 % in ozone, and increases in methyl hydrogen peroxide of up to ~14 %. Larger changes are observed in biomass burning plumes lofted to high altitude. Additionally, by sequestering NO<sub>x</sub> at low temperatures, CH<sub>3</sub>O<sub>2</sub>NO<sub>2</sub> decreases the cycling of HO<sub>2</sub> to OH, resulting in a larger upper tropospheric HO<sub>2</sub> to OH ratio. These results may impact some estimates of lightning NO<sub>x</sub> sources as well as help explain differences between models and measurements of upper tropospheric composition.