Observations of nitrous acid in an urban atmosphere by differential optical absorption

Abstract

In the past decade, increasing attention has been paid to the role of nitrous acid (HONO) in the polluted troposphere, primarily as an initiator of photochemical air pollution¹ through its photodissociation by solar UV radiation into hydroxyl radicals and NO. The OH radical may subsequently attack organics starting a chain photoxidation which leads to the production of O₃, peroxyacetyl nitrate and many other secondary pollutants. Measurements of the concentration of HONO, especially in the early morning, are needed to establish the initial conditions to be used in computer kinetic models of photochemical oxidant formation^2–4. From laboratory studies, it has been suggested that nitrous acid may also be a precursor to the possible formation of nitrosamines by reaction with simple secondary and tertiary amines in urban air⁵, or in vivo following inhalation. We report here a series of observations, at Riverside and Claremont, California, of the gradual buildup of HONO during the night and its rapid decay after sunrise. These, we believe, represent the first unequivocal measurements of HONO reported for a major urban air basin impacted by photochemical air pollution.