A modeling study of tropospheric ozone and its precursors in winter--spring arctic outflow.

摘要

In this study, the impact of winter-spring arctic outflow events on the budgets of tropospheric NO_x and ozone over the North Atlantic Ocean is determined. An updated version of the NCAR Master Mechanism, a detailed, explicit photochemical model, is applied to trajectories representative of arctic outflow for the months of January through May. Revisions were made to the gas-phase mechanism in small increments to determine the effect of each set of revisions on simulated levels of NO_x and O₃. The effect of individual revision subsets had a much larger effect than the net result of all the revisions, as the revision subsets have opposing effects and counteract each other. The peroxy radical revisions had the largest effect on the level of NO_x, increasing the average level by 24% over the original mechanism. For O₃, the photolysis revisions had the largest effect, decreasing the net rate of ozone production by 0.5 ppb/day, a 49% reduction. Therefore, it is important that the impact of revisions to individual (or a family of) reactions are clearly understood, so that possible bias in calculated results can be avoided.; During simulations of arctic outflow, peroxyacetyl nitrate (PAN) mixing ratios drop by up to 270 pptv, increasing NO_x levels up to 120 pptv. However, the released NO_x is quickly destroyed by efficient NO_x loss reactions leading to formation of HNO₃ and, to a lesser extent, nitroaromatic compounds. As a result of elevated NO_x O ₃ levels are increased. The net impact resulting from elevated levels of PANs in the winter-spring Arctic on the budget of O₃ over the North Atlantic is estimated to be less than 712 Gg O₃/month. While this source is similar in magnitude to estimates of ozone produced from aircraft emissions, both are small in comparison to the export of ozone from North America. In conclusion, while elevated levels of pollutants in arctic outflow increase the levels of NO_x and O₃ in the North Atlantic region, on a seasonal basis the impact on the budgets of NO_x and O₃ in this region is small.