The Arctic is a fragile ecosystem that is particularly sensitive to changes in the climate system and to the influence of anthropogenic pollution. We are using the GEOS-Chem global three-dimensional chemical transport model and its adjoint to exploit the new satellite measurements, together with existing surface measurements, to quantify the impact of intercontinental transport of pollution on the chemistry of the Arctic troposphere. In particular, we seek to better constrain the budget of ozone (O₃) and nitrogen oxides (NO_x) in the Canadian Arctic to obtain an improved understanding of the factor controlling the seasonal dependence of the O₃ and NO_x budgets.