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

该项目涉及对流层中活性氮物质的循环，重点是亚硝酸（HONO）作为再NOx化的中间产物，这是一个将活性较低的物质（硝酸，硝酸盐）转化为活性较高的物质一氧化氮（NO）和二氧化氮（NO2）的过程，统称为NOx。在大气中，氮氧化物在通过光化学循环产生氧化剂（如臭氧和羟基自由基（OH））中起着至关重要的作用，该光化学循环涉及过氧化氢自由基和NO2光解的NO氧化。NOx主要通过NO2与OH反应形成硝酸（HNO3）从大气中去除，硝酸被认为是光化学不反应的，因此是NOx的最终产物。然而，最近的研究表明，HNO3可以转化回NOx。该研究小组的早期工作表明，HONO是HNO3在表面上光解的主要产物，可能是对流层再NOx化的主要中间体。 在本研究中，我们将检验三个假设：1.有机化合物，如吸光芳香族化合物，可显著提高表面HNO3和气溶胶中硝酸盐光解产生HONO的速率，使HONO成为光解过程的主要产物。2. HNO3/硝酸盐在地表（包括植被）上的光解作用是白天HONO的主要来源，也是农村大气边界层中NOx再化的途径. 气溶胶中硝酸盐颗粒的光解是白天HONO的主要来源，也是对流层边界层以上NOx再生成的途径。这些问题将通过实验室、空中和地面测量来解决。实验室调查将检查各种类型的有机化合物对表面HNO3/硝酸盐光解的速率和产物分布的影响，以及过滤器从环境空气中收集的颗粒硝酸盐的光解速率和产物分布。基于飞机的测量将建立陆地（森林和农田）和密歇根湖上空的HONO垂直剖面图，以研究HONO从地面到上覆大气的传输以及HONO在边界层上方的自由对流层中的原位产生。基于地面的实地测量将量化白天从冠层表面的HONO通量，并调查HONO通量与HNO3沉积和太阳紫外线强度之间的关系。如果上述假设得到这项研究结果的支持，将对我们理解大气化学产生重大影响。来自表面上HNO3和气溶胶中颗粒硝酸盐的光化学再NOx化的NOx和HONO的"再循环"源将导致对流层比以前认识到的更具光化学反应性，导致产生更多的光氧化剂，如臭氧和OH自由基。这在边界层中尤其重要。 这项研究将为研究生、本科生和高中生提供大气化学方面的培训机会，并将在三个学术机构的教师和学生之间建立和促进研究和教育合作。此外，PI将把研究经验和结果纳入他所教授的课程。

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.