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Improvements to the characterization of organic nitrogen chemistry and deposition in CMAQ
Schwede, D. AND D. Luecken. Improvements to the characterization of organic nitrogen chemistry and deposition in CMAQ. National Atmospheric Deposition Program Fall 2014 Scientific Symposium, Chapel Hill, NC, October 27 - 29, 2014.
The National Exposure Research Laboratory (NERL) Atmospheric Modeling and Analysis Division (AMAD) conducts research in support of EPA mission to protect human health and the environment. AMAD research program is engaged in developing and evaluating predictive atmospheric models on all spatial and temporal scales for forecasting the air quality and for assessing changes in air quality and air pollutant exposures, as affected by changes in ecosystem management and regulatory decisions. AMAD is responsible for providing a sound scientific and technical basis for regulatory policies based on air quality models to improve ambient air quality. The models developed by AMAD are being used by EPA, NOAA, and the air pollution community in understanding and forecasting not only the magnitude of the air pollution problem, but also in developing emission control policies and regulations for air quality improvements.
Excess atmospheric nitrogen deposition can cause significant harmful effects to ecosystems. Organic nitrogen deposition can be an important contributor to the total nitrogen budget, contributing 10-30%, however there are large uncertainties in the chemistry and deposition of these compounds. Organic nitrogen comprises thousands of different types of molecules, with a corresponding large range of physical and chemical properties. For example, the reaction rate of common organic nitrates, which controls the chemical lifetime in the atmosphere, can vary from 15 minutes to 5 days depending on the structure of the compound. Even more dramatically, the Henry’s Law constant of different organic nitrogen compounds can vary over 4 orders of magnitude depending on the presence of polar functional groups, affecting its lifetime in the presence of clouds, rain and fog as well as dry deposition. Current chemical mechanisms and deposition modules used in air quality models assign organic nitrates to only one or two “representative” compounds. We modified the Carbon Bond (CB05) chemical mechanism and wet and dry deposition modules in the Community Multiscale Air Quality (CMAQ) model to provide an improved treatment of gaseous organic nitrate chemistry and deposition. To evaluate the model improvements, we compare model results against measured wet deposition values of total, inorganic, and organic nitrogen. Additionally, we examine source regions of air masses to investigate missing sources of organic nitrogen in the model such as amines.