Nitrous acid, troposphere

As discussed in Chapter 1, nitrous acid is important in tropospheric chemistry because it photolyzes to form OH ... 

Historically, the major acids believed to contribute to acid deposition in the troposphere have been sulfuric and nitric acids, formed by the oxidation in air of S02 and oxides of nitrogen, respectively. However, there is an increasing recognition that organic acids may contribute significantly to the total acid burden and indeed may represent the major acidic species even in polluted urban environments. In addition, since nitrous acid (HONO) is formed whenever N02 and water are present (see Chapter 7.B.3), its contribution to the total acidity, particularly to indoor air environments, has become of interest and concern. 

Hausmann, M., J. Rudolf, and U. Platt, Spectroscopic Measurement of Bromine Oxide, Ozone and Nitrous Acid in Alert, in The Tropospheric Chemistry of Ozone in the Polar Regions NATO ASI Series I Global Environmental Change (H. Niki and K. H. Becker, Eds.), Vol. 17, pp. 189-203, 1993. 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO" are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO", particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO" in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

Nitrous acid, HONO, is considered to be an important species [2] in the photochemistry of the troposphere since it is a source of hydroxyl radical [97]. 

The atmospheric lifetime of nitrous acid with respect to OH radicals is about 2 days. However, photolysis is the most important loss process of HONO during the daytime with an overhead Sun, its lifetime from photodecomposition is only about 10 min. see table IX-M-1. The reaction with OH can play a significant role during nighttime if the OH radical is produced. HONO can be an important source of OH radicals in the troposphere see section IX-H-1. 

The near ultraviolet absorption bands of nitrous acid, like all nitrites, involves an n n transition, and the absorption bands overlap well the envelope of actinic flux present in the troposphere see figure IX-H-2. The photodecomposition of HONO is efficient in generating OH radicals in the troposphere. Two primary processes have been proposed to rationalize the observed photodecomposition of HONO ... 

The temperature and density structure of the troposphere, along with the concentrations of major constituents, are well documented and altitude profiles have been measured over a wide range of seasons and latitudes for the minor species water, carbon dioxide, and ozone. A few profiles are available for carbon monoxide, nitrous oxide, methane, and molecular hydrogen, while only surface or low-altitude measurements have been made for nitric oxide, nitrogen dioxide, ammonia, sulfur dioxide, hydrogen sulfide, and nonmethane hydrocarbons. No direct measurements of nitric acid and formaldehyde are available, though indirect information does exist. The concentrations of a number of other important species, such as peroxides and oxy and peroxy radicals, have never been determined. Therefore, while considerable information concerning trace constituent concentrations is available, the picture is far from complete. 

The last step in the current manufacture of adipic acid involves oxidation by nitric acid, which results in the formation of nitrous oxide (N2O) that is released into the atmosphere. Given that N2O has no tropospheric sinks, it can rise to the stratosphere and be a factor in the destruction of the ozone layer. It also acts as a greenhouse gas (see Section 8.4.1). 

Globally, the oxides of nitrogen, NO (nitric oxide), NO2 (nitrogen oxide), and N2O (nitrous oxide), are key species involved in the chemistry of the troposphere and stratosphere. NO and N2O are produced mostly by microbial soil activity, whereas biomass burning is also an important source of NO. Nitric oxide is a species involved in the photochemical production of ozone in the troposphere, is involved in the chemical produaion of nitric acid, and is an important component of acid precipitation. Nitrous oxide plays a key role in stratospheric ozone depletion and is an important greenhouse gas, with a global warming potential more than 200 times that of CO2. 

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