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.

Sensitivity of Ambient Atmospheric Formaldehyde and Ozone to Precursor Species and Source Types Across the United States.

Formaldehyde (HCHO) is an important air pollutant from both an atmospheric chemistry and human health standpoint. This study uses an instrumented photochemical Air Quality Model, CMAQ-DDM, to identify the sensitivity of HCHO concentrations across the United States (U.S.) to major source types and hydrocarbon speciation. In July, biogenic sources of hydrocarbons contribute the most (92% of total hydrocarbon sensitivity), split between isoprene and other alkenes. Among anthropogenic sources, mobile sources of hydrocarbons and nitrogen oxides (NO x) dominate. In January, HCHO is more sensitive to anthropogenic hydrocarbons than biogenic sources, especially mobile sources and residential wood combustion (36% of national hydrocarbon sensitivity). While ozone (O3) is three times more sensitive to NO x than hydrocarbons across most areas of the U.S., HCHO is six times more sensitive to hydrocarbons than NO x, largely due to sensitivity to biogenic precursors and the importance of low-NO x chemistry. In winter, both HCHO and O3 show negative sensitivity to NO x (increases with the removal of NO x), although O3 increases are larger. Relative sensitivities do not change substantially across different regions of the country.

Technical challenges involved in implementation of VOC reactivity-based control of ozone.

Controlling VOC emissions on the basis of their individual contribution to ozone formation has been subject to extensive discussion and research in past years, and the concept has gained some acceptance in the air pollution community for certain product categories and industrial operations. Despite its potential to decrease ozone formation, there are some technical challenges that still remain before we can confidently apply the concept of reactivity in the most beneficial manner to reduce ozone concentrations. The goal of this paper is to (1) assess how existing science in this area supports the use of reactivity, particularly, the maximum incremental reactivity, for VOC control under a national policy application and (2) identify where uncertainties exist that could affect such a policy. Box model and air quality model results are used to show that there are ways to describe a chemical's reactivity that are relatively robust across large geographic areas. Modeling results also indicate that the choice of metric is important in determining the potential benefits and detriments of a reactivity-based emission control policy.