On the export of reactive nitrogen from Asia: NO_x partitioning and effects on ozone

Abstract. The partitioning of reactive nitrogen (NO_y was measured over the remote North Pacific during spring 2006. Aircraft observations of NO, NO₂, total peroxy nitrates (ΣPNs), total alkyl and multi-functional nitrates (ΣANs) and nitric acid (HNO₃, made between 25° and 55° N, confirm a controlling role for peroxyacyl nitrates in NO_x production in aged Asian outflow. ΣPNs account for more than 60% of NO_y above 5 km, while thermal dissociation limits their contribution to less than 10% in the lower troposphere. Using simultaneous observations of NO_x, ΣPNs, ΣANs, HNO₃ and average wind speed, we calculate the flux of reactive nitrogen through the meridional plane of 150° W (between 20° and 55° N) to be 0.007 ± 0.002 Tg N day⁻¹, which provides an upper limit of 23 ± 6.5% on the transport efficiency of NO_y from East Asia. Observations of NO_x, and HO_x are used to constrain a 0-D photochemical box model for the calculation of net photochemical ozone production or tendency (Δ O₃) as a function of aircraft altitude and NO_x concentrations. The model analysis indicates that the photochemical environment of the lower troposphere (altitude < 6 km) over the north Pacific is one of net O₃ destruction, with an experimentally determined crossover point between net O₃ destruction and net O₃ production of 60 pptv NO_x. Qualitative indicators of integrated net O₃ production derived from simultaneous measurements of O₃ and light alkanes (Parrish et al., 1992), also indicate that the north Pacific is, on average, a region of net O₃ destruction.