Nitric acid, tropospheric

Air pollution (qv) problems are characteri2ed by their scale and the types of pollutants involved. Pollutants are classified as being either primary, that is emitted direcdy, or secondary, ie, formed in the atmosphere through chemical or physical processes. Examples of primary pollutants are carbon monoxide [630-08-0] (qv), CO, lead [7439-92-1] (qv), Pb, chlorofluorocarbons, and many toxic compounds. Notable secondary pollutants include o2one [10028-15-6] (qv), O, which is formed in the troposphere by reactions of nitrogen oxides (NO ) and reactive organic gases (ROG), and sulfuric and nitric acids. 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

Photolytic. Irradiation of vinyl chloride in the presence of nitrogen dioxide for 160 min produced formic acid, HCl, carbon monoxide, formaldehyde, ozone, and trace amounts of formyl chloride and nitric acid. In the presence of ozone, however, vinyl chloride photooxidized to carbon monoxide, formaldehyde, formic acid, and small amounts of HCl (Gay et al, 1976). Reported photooxidation products in the troposphere include hydrogen chloride and/or formyl chloride (U.S. EPA, 1985). In the presence of moisture, formyl chloride will decompose to carbon monoxide and HCl (Morrison and Boyd, 1971). Vinyl chloride reacts rapidly with OH radicals in the atmosphere. Based on a reaction rate of 6.6 x lO" cmVmolecule-sec, the estimated half-life for this reaction at 299 K is 1.5 d (Perry et al., 1977). Vinyl chloride reacts also with ozone and NO3 in the gas-phase. Sanhueza et al. (1976) reported a rate constant of 6.5 x 10 cmVmolecule-sec for the reaction with OH radicals in air at 295 K. Atkinson et al. (1988) reported a rate constant of 4.45 X 10cmVmolecule-sec for the reaction with NO3 radicals in air at 298 K. 

In the troposphere N205 is an important nighttime source of nitric acid through its rapid hydrolysis on wet surfaces and aerosol particles ... 

Acid rain arises from the oxidation of S02 and N02 in the troposphere to form sulfuric and nitric acids, as well as other species, which are subsequently deposited at the earth s surface, either in precipitation (wet deposition) or in dry form (dry deposition). The contribution of organic acids has also been recognized recently (see Chapter 8). These oxidation and deposition processes can occur over relatively short distances from the primary pollutant sources or at distances of a fOOO km or more. Thus both short-range and long-range transport must be considered. 

There are many different types of surfaces available for reactions in the atmosphere. In the stratosphere, these include ice crystals, some containing nitric acid, liquid sulfuric acid-water mixtures, and ternary solutions of nitric and sulfuric acids and water. In the troposphere, liquid particles containing sulfate, nitrate, organics, trace metals, and carbon are common. Sea... 

Laux, J. M., J. C. Hemminger, and B. J. Finlayson-Pitts, X-ray Photoelectron Spectroscopic Studies of the Heterogeneous Reaction of Gaseous Nitric Acid with Sodium Chloride Kinetics and Contribution to the Chemistry of the Marine Troposphere, Geophys. Res. Let., 21, 1623-1626 (1994). 

There is evidence from laboratory studies that heterogeneous reactions on sulfate particles may be important in the upper troposphere as well. For example, HCHO uptake into sulfuric acid solutions or ternary mixtures of sulfuric and nitric acids and water has been observed in laboratory studies (e.g., Tolbert et al., 1993 Jayne et al., 1996 Iraci and Tolbert, 1997). In sulfuric acid, the effective Henry s law constant at the low... 

Chatfield, R. B., Anomalous HN03/NOr Ratio of Remote Tropospheric Air Conversion of Nitric Acid to Formic Acid and NO, Geophys. Res. Lett., 21, 2705-2708 (1994). 

S. Madronich, HN03/N0, Ratio in the Remote Troposphere during MLOPEX 2 Evidence for Nitric Acid Reduction on Carbonaceous Aerosols Geophys. Res. Lett., 23, 2609-2612... 

Choi, W., and M.-T. Leu, Nitric Acid Uptake and Decomposition on Black Carbon (Soot) Surfaces Its Implications for the Upper Troposphere and Lower Stratosphere, J. Phys. Chem. A, 102, 7618-7630 (1998). 

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. 

Typical tropospheric concentrations of HN03. Given the difficulties in measuring atmospheric nitric acid,... 

Laaksonen, A., J. Hienola, M. Kulmala, and F. Arnold, Supercooled Cirrus Cloud Formation Modified by Nitric Acid Pollution of the Upper Troposphere, Geophys. Res. Lett., 24, 3009-3012 (1997). 

The residence times of SO2 and H2S04 in the troposphere are typically only a few days, but sulfuric acid aerosols reaching the stratosphere can be very persistent together with nitric acid, they provide the solid surfaces in polar stratospheric clouds on which reaction 8.9 and related processes occur heterogeneously. Indeed, studies suggest that NOx emissions of commercial supersonic aircraft in the lower stratosphere may pose less of a threat to the ozone layer than previously supposed however, the accompanying formation of sulfuric and nitric acid aerosols may exacerbate ozone loss by increasing the available catalytic surface area. 

Society is facing several crucial issues involving atmospheric chemistry, Species containing nitrogen are major players in each. In the troposphere, nitrogen species are catalysts in the photochemical cycles that form ozone, a major urban and rural pollutant, as well as other oxidants (references 1 and 2, and references cited therein), and they are involved in acid precipitation, both as one of the two major acids (nitric acid) and as a base (ammonia) (3, 4). In the stratosphere, where ozone acts as a shield for the... 

Although the nitric acid molecule is subject to various reactions and to photodissociation, nevertheless it remains, and it becomes the most important of the molecules containing NO (HN04, N205, NO3,. . . ) in the lower stratosphere. However, it cannot accumulate because it crosses the tropopause into the troposphere, where it rapidly disappears because of its solubility in water. Thus, if N20 is the source of the nitrogen oxides in the stratosphere, nitric acid is the sink that prevents their accumulation beyond certain limits. But it is now known that the sequence of reactions (20), (21), and (22) results in a lower concentration of stratospheric ozone than would be possible in a pure oxygen atmosphere. 

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). 

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