Surface ozone and its precursors at Summit, Greenland: comparison between observations and model simulations

Abstract. Recent studies have shown significant challenges for atmospheric models to simulate tropospheric ozone (O₃) and its precursors in the Arctic. In this study, ground-based data were combined with a global 3-D chemical transport model (GEOS-Chem) to examine the abundance and seasonal variations of O₃ and its precursors at Summit, Greenland (72.34° N, 38.29° W; 3212 m a.s.l.). Model simulations for atmospheric nitrogen oxides (NO_x), peroxyacetyl nitrate (PAN), ethane (C₂H₆), propane (C₃H₈), carbon monoxide (CO), and O₃ for the period July 2008–June 2010 were compared with observations. The model performed well in simulating certain species (such as CO and C₃H₈), but some significant discrepancies were identified for other species and further investigated. The model generally underestimated NO_x and PAN (by  ∼  50 and 30 %, respectively) for March–June. Likely contributing factors to the low bias include missing NO_x and PAN emissions from snowpack chemistry in the model. At the same time, the model overestimated NO_x mixing ratios by more than a factor of 2 in wintertime, with episodic NO_x mixing ratios up to 15 times higher than the typical NO_x levels at Summit. Further investigation showed that these simulated episodic NO_x spikes were always associated with transport events from Europe, but the exact cause remained unclear. The model systematically overestimated C₂H₆ mixing ratios by approximately 20 % relative to observations. This discrepancy can be resolved by decreasing anthropogenic C₂H₆ emissions over Asia and the US by  ∼ 20 %, from 5.4 to 4.4 Tg year⁻¹. GEOS-Chem was able to reproduce the seasonal variability of O₃ and its spring maximum. However, compared with observations, it underestimated surface O₃ by approximately 13 % (6.5 ppbv) from April to July. This low bias appeared to be driven by several factors including missing snowpack emissions of NO_x and nitrous acid in the model, the weak simulated stratosphere-to-troposphere exchange flux of O₃ over the summit, and the coarse model resolution.