Multipollutant modeling of ozone, reactive nitrogen and HAPs across the continental US with CMAQ-CB6.

The accuracy of atmospheric chemical mechanisms used in air quality models is critical for robustly predicting the production and decay of air pollutants and thus to develop strategies to reduce concentrations that are above levels harmful to humans and ecosystems. In this study we document, evaluate and analyze the implementation of the CB6r3 chemical mechanism used in the Community Multiscale Air Quality (CMAQ) model, including changes that have been to the standard version, and demonstrate the impact of this update on predictions. In general, CB6r3 slightly improves the predictions of ozone and oxides of nitrogen, while providing more consistency with current scientific understanding. Nitric acid is generally overpredicted in both winter and summer, and ongoing work continues to address this overprediction and update other aspects of the mechanism.

Simulations of Titan's brightness by a two-dimensional haze model.

We have used a 2-D microphysics model to study the effects of atmospheric motions on the albedo of Titan's thick haze layer. We compare our results to the observed variations of Titan's brightness with season and latitude. We use two wind fields; the first is a simple pole-to-pole Hadley cell that reverses twice a year. The second is based on the results of a preliminary Titan GCM. Seasonally varying wind fields, with horizontal velocities of about 1 cm sec-1 at optical depth unity, are capable of producing the observed change in geometric albedo of about 10% over the Titan year. Neither of the two wind fields can adequately reproduce the latitudinal distribution of reflectivity seen by Voyager. At visible wavelengths, where only haze opacity is important, upwelling produces darkening by increasing the particle size at optical depth unity. This is due to the suspension of larger particles as well as the lateral removal of smaller particles from the top of the atmosphere. At UV wavelengths and at 0.89 micrometers the albedo is determined by the competing effects of the gas the haze material. Gas is bright in the UV and dark at 0.89 micrometers. Haze transport at high altitudes controls the UV albedo and transport at low altitude controls the 0.89 micrometers albedo. Comparisons between the hemispheric contrast at UV, visible, and IR wavelengths can be diagnostic of the vertical structure of the wind field on Titan.