Nitrous Acid (HONO) as the Major Re-NOx-ification Intermediate Product in the Troposphere

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This project addresses the cycling of reactive nitrogen species in the troposphere, focusing on nitrous acid (HONO) as an intermediate product of re-NOx-ification, a process that converts less reactive species (nitric acid, nitrates) into the more reactive species nitric oxide (NO) and nitrogen dioxide (NO2), collectively referred to as NOx. In the atmosphere, NOx plays a critically important role in the production of oxidants, such as ozone and hydroxyl radicals (OH), through a photochemical cycle involving NO oxidation by peroxyl radicals and NO2 photolysis. NOx is removed from the atmosphere mainly through the reaction of NO2 with OH to form nitric acid (HNO3), which is considered to be photochemically unreactive and thus the end product of NOx. However, recent studies suggest that HNO3 can be converted back to NOx. Earlier work by this research group has suggested that HONO is a major product of HNO3 photolysis on surfaces, and may be the major intermediate in re-NOx-ification of the troposphere. In this project, three hypotheses will be tested:1. Organic compounds, such as light-absorbing aromatic compounds, may significantly enhance the HONO production rate from photolysis of HNO3 on surfaces and nitrate in aerosols, making HONO the major product of the photolytic process.2. Photolysis of HNO3/nitrate on surfaces, including vegetation, is a dominant daytime HONO source and a re-NOx-ification pathway in the rural atmospheric boundary layer.3. Photolysis of particulate nitrate in aerosols is a major daytime HONO source and a re-NOx-ification pathway in the troposphere above the boundary layer. These questions will be addressed using laboratory, airborne, and ground-based measurements. The laboratory investigations will examine the effect of various types of organic compounds on the rate and product distribution of HNO3/nitrate photolysis on surfaces and the rate and product distribution of photolysis of particulate nitrate collected on filters from ambient air. Aircraft-based measurements will establish HONO vertical profiles over land (forest and farmland) and Lake Michigan to examine the transport of HONO from the ground into the overlying atmosphere and the in situ production of HONO in the free troposphere above the boundary layer. Ground-based field measurements will quantify daytime HONO fluxes from the canopy surface and investigate the relationships between the HONO flux with HNO3 deposition and solar UV intensity. If the above hypotheses are supported by the results of this research, there will be significant implications for our understanding of atmospheric chemistry. A "recycled" source of NOx and HONO from photochemical re-NOx-ification of HNO3 on surfaces and of particulate nitrate in aerosols would cause the troposphere to be more photochemically reactive than previously realized, leading to higher production of photooxidants, such as ozone and OH radicals. This would be especially important in the boundary layer. This study will provide training opportunities for graduate, undergraduate and high school students in atmospheric chemistry, and will establish and foster research and educational collaborations among the faculty and students at three academic institutions. In addition, the PI will incorporate the research experience and results into the courses he is teaching.