Measurements and modeling of organic nitrate gases and aerosol: Influences on the lifetimes of NOx, ozone and aerosol

To address NOAA�??s goal of reducing uncertainties in the role of �??reactive nitrogen processes in the atmosphere as they relate to aerosol formation,�?� we propose research aimed at improving our understanding of how biogenic volatile organic carbon (BVOC) interacts with nitrogen oxides (NOx and NOy) to affect the spatial and temporal patterns of NOx, ozone and aerosol. In the U.S. we are moving from a regime where urban mobile sources and large power plants dominate the spatial pattern of NOx to one where agricultural sources are predominant. This is bringing about dramatic changes in the chemical regime affecting the reaction pathways that govern the interactions of NOx and BVOC. Recent research, with key contributions from our research group, has established that biogenic organic nitrate production is the dominant sink of NOx on the continents. This fact makes knowledge of the fate of biogenic organic nitrates much more important for an understanding of atmospheric chemistry. Evidence suggests that the ultimate fate of a significant fraction of RONO2 molecules includes hydrolysis in aerosol or clouds or oxidation to release NOx. At the same time a significant fraction of aerosol mass has been identified as organic nitrates. Deposition to the surface and through the stomata of plants are also likely important. We plan two lines of research to address the underlying uncertainties about these processes:

1) We propose to flesh out a comprehensive chemical mechanism for BVOC oxidation for use in WRF-CHEM and WRF-CMAQ and to use this mechanism in a series of model calculations and analyses to assess the role of organic nitrates in the chemistry of the atmosphere. The mechanism will conserve carbon and nitrogen and include production and oxidation of many individual anthropogenic and biogenic nitrates. It will have detailed representation of isoprene and monoterpene (more than one) chemistry. For all of these molecules, the mechanism will represent both gas and aerosol processes and deposition to surfaces and stomata. Our research will include optimization of chemical mechanisms, comparison to field observations of concentrations of total and individual gas and aerosol RONO2, comparison to field observations of correlations of RONO2 with other major co-products such as aerosol mass, or H2CO. One focus of the work will be comparison of calculations to in situ and space-based observations of nitrogen oxides and aerosol in wet and dry environments to assess the effects of clouds and humidity on the chemistry. Another focus will be an assessment of the range of uncertainty introduced by incomplete knowledge of boundary layer mixing during day and night. The role of nighttime chemistry is strongly mediated by boundary layer dynamics, both because of remnant effects of composition in the residual layer and because the extent of mixing of emissions of monoterpenes into the thin nocturnal layer near the surface determine the volume over which reactions take place.

2) The extent to which organic nitrates are removed by stomatal uptake vs. simple deposition to all surfaces is a largely unexplored aspect of RONO2 chemistry. We propose a series of laboratory experiments to observe the deposition of some example organic nitrates at the leaf level. Using tools we have developed for the study of leaf level NO2 emission and uptake, we will expose a suite of plants to specific organic nitrates and measure the role of surface and stomatal uptake on their deposition rates. These results will guide development of modeling to represent the two distinct deposition processes affecting RONO2 concentrations.