Characterization of reactive nitrogen in the global upper troposphere using recent and historical commercial and research aircraft campaigns and GEOS-Chem

Abstract. Reactive nitrogen (NO_y) in the upper troposphere (UT; ~8–12 km) influences global climate, air quality, and tropospheric oxidants, but this is informed by limited knowledge of the relative contribution of individual NO_y components in this undersampled layer. Here we use sporadic NASA DC-8 aircraft campaign observations, after screening for plumes and stratospheric influence, to characterise UT NO_y composition and evaluate current knowledge of UT NO_y as simulated with the GEOS-Chem model. Use of DC-8 data follows confirmation that these sporadic data reproduce NO_y seasonality from routine commercial aircraft observations (2003–2019), supporting use of DC-8 data to characterize UT NO_y. We find that peroxyacetyl nitrate (PAN) dominates UT NO_y (30–64 % of NO_y), followed by nitrogen oxides (NO_x ≡ NO + NO₂) (6–18 %), peroxynitric acid (HNO₄) (6–13 %), and nitric acid (HNO₃) (7–11 %). Methyl peroxy nitrate (MPN) makes an outsized contribution to NO_y (24 %) over the Southeast US relative to the other regions sampled (2–7 %). GEOS-Chem, sampled along DC-8 flights, exhibits much weaker seasonality than DC-8, underestimating summer and spring NO_y and overestimating winter and autumn NO_y. The model consistently overestimates peroxypropionyl nitrate (PPN) by up to 16 pptv and underestimates NO₂ by 6–36 pptv, as the model is missing PPN photolysis. An ~80 pptv (20-fold) underestimate in modelled MPN over the Southeast US results from uncertainties in processes that sustain MPN production as air ages. Our findings highlight that greater understanding of UT NO_y is critically needed to determine its role in the nitrogen cycle, air pollution, climate, and abundance of oxidants.