ARB Research Seminar
This page updated June 19, 2013
Heterogeneous Reactions of Oxides of Nitrogen in the Atmosphere: Is it Magic?
Barbara J. Finlayson-Pitts, Ph.D., Department of Chemistry, University of California, Irvine
November 07, 2001
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA
Also presented in El Monte on December 5, 2001
Oxides of nitrogen are a key component of photochemical air pollution, acid deposition and particles in the atmosphere. Their gas phase reactions are reasonably well understood and represented in air quality models used to predict the degree of control of VOC and NOₓ needed to achieve air quality standards.
However, it has been known for at least 50 years that some reactions of oxides of nitrogen known to be slow in the gas phase are enhanced in the presence of surfaces. For example, the reaction of gaseous NO₂ with water vapor to generate nitrous and nitric acids is unimportant in the absence of surfaces.
2 NO₂ + H₂O -------- > HONO + HNO₃ (1)
However, when a surface is present, the reaction proceeds at a measurable rate. In laboratory studies of such reactions, the surfaces are those of the reactors used in the experiments. In urban air, there are myriad surfaces, particularly in the boundary layer; these include not only airborne particles, but also soil, buildings, plants, highways etc.
Available laboratory data suggest that reaction (1) is likely a major source of HONO in polluted urban areas. Indeed, under many early morning conditions in such regions, HONO is the major free radical source that initiates the chain chemistry leading to the formation of ozone, nitric acid, particles and a host of other secondary pollutants.
Despite the importance of such heterogeneous NOₓ chemistry, very little is known about the kinetics and mechanisms on a molecular level. Reactions such as (1) above are frequently shown with "surface" over the arrow; while this may sometimes be interpreted as indicative of a mechanism, our lack of understanding of why the surface is needed and what its role is puts such chemistry almost into the realm of "magic."
Because of these difficulties, heterogeneous NOₓ chemistry cannot currently be accurately represented in airshed models. Clearly, understanding chemistry on surfaces at the molecular level is critical for removing the "magic" and allowing quantitative assessments of the impacts of such chemistry on control strategies to be carried out.
Results of recent studies in my laboratory on heterogeneous NOₓ chemistry will be presented, and the current status of our understanding of heterogeneous chemistry assessed. This includes the long-recognized reaction (1) as well as the newly discovered reaction of HNO₃ on surfaces with gaseous NO. The implications for the chemistry of polluted urban areas and control strategy development will be discussed.
Barbara J. Finlayson-Pitts is Professor of Chemistry at the University of California, Irvine. Her current research efforts focus on understanding reactions at the air-water interface and in thin water films found on surfaces when water vapor is present. This chemistry involves both the reactions of oxides of nitrogen as well as of sea salt particles. She serves on the editorial boards of several journals, including Atmospheric Environment and the Journal of Physical Chemistry, and on a number of panels, including the NASA Panel for Data Evaluation. Professor Finlayson-Pitts is a Fellow of the American Association for the Advancement of Science and was awarded the Orange County Section of the American Chemical Society's 'Service Through Chemistry' Award. She is coauthor, with J. N. Pitts Jr., of two books in the field: Atmospheric Chemistry: Fundamentals and Experimental Techniques (Wiley, 1986) and Chemistry of the Upper and Lower Atmosphere: Theory, Experiments and Applications (Academic, 2000).