The global modeling initiative (GMI) science team is developing a three-dimensional chemistry and transport model (CTM) for use in assessment of the atmospheric effects of aviation. This model must be documented, be validated against observations, use a realistic atmospheric circulation, and contain numerical transport and photochemical modules representing atmospheric processes. The model must retain computational efficiency for multiple scenarios and sensitivity studies. To meet these requirements, a facility model concept was developed in which the different components of the CTM are evaluated separately. The assessment of the impact on the stratosphere of the exhaust of supersonic aircraft will depend strongly on the meteorological fields used by the CTM. Three data sets for the stratosphere were considered: the National Center for Atmospheric Research Community Climate Model (CCM2), the Goddard Earth Observing System data assimilation system, and the Goddard Institute for Space Studies general circulation model. Objective criteria were developed to identify the data set that provides the best representation of the stratosphere. Simulations of gases with simple chemical control were chosen to test various aspects of model transport. The data sets were evaluated and graded on their performance on these tests. The CCM2 meteorological data set has the highest score and was selected for GMI. This objective model evaluation establishes a physical basis for interpretation of differences between models and observations. Further, the method provides a quantitative basis for defining model errors, for discriminating between different models, and for ready reevaluation of improved models. This will lead to higher confidence in assessment calculations. Copyright 1999 by the American Geophysical Union.

We use aircraft observations obtained during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission to examine the distributions and source attributions of O 3 and NO y in the Arctic and sub-Arctic region. Using a number of marker tracers, we distinguish various air masses from the background troposphere and examine their contributions to NO x, O 3, and O 3 production in the Arctic troposphere. The background Arctic troposphere has a mean O 3 of ∼60 ppbv and NO x of ∼25 pptv throughout spring and summer with CO decreasing from ∼145 ppbv in spring to ∼100 ppbv in summer. These observed mixing ratios are not notably different from the values measured during the 1988 ABLE-3A and the 2002 TOPSE field campaigns despite the significant changes in emissions and stratospheric ozone layer in the past two decades that influence Arctic tropospheric composition. Air masses associated with stratosphere-troposphere exchange are present throughout the mid and upper troposphere during spring and summer. These air masses, with mean O 3 concentrations of 140-160 ppbv, are significant direct sources of O 3 in the Arctic troposphere. In addition, air of stratospheric origin displays net O 3 formation in the Arctic due to its sustainable, high NO x (75 pptv in spring and 110 pptv in summer) and NO y (∼800 pptv in spring and ∼1100 pptv in summer). The air masses influenced by the stratosphere sampled during ARCTAS-B also show conversion of HNO 3 to PAN. This active production of PAN is the result of increased degradation of ethane in the stratosphere-troposphere mixed air mass to form CH 3CHO, followed by subsequent formation of PAN under high NO x conditions. These findings imply that an adequate representation of stratospheric NO y input, in addition to stratospheric O 3 influx, is essential to accurately simulate tropospheric Arctic O 3, NO x and PAN in chemistry transport models. Plumes influenced by recent anthropogenic and biomass burning emissions observed during ARCTAS show highly elevated levels of hydrocarbons and NO y (mostly in the form of NO x and PAN), but do not contain O 3 higher than that in the Arctic tropospheric background except some aged biomass burning plumes sampled during spring. Convection and/or lightning influences are negligible sources of O 3 in the Arctic troposphere but can have significant impacts in the upper troposphere in the continental sub-Arctic during summer. © 2011 Author(s).