Nitric acid, tropospheric reaction with

The major tropospheric loss process of NOx involves the formation of nitric acid via reaction of OH radicals with N02 or by heterogeneous/ho-mogeneous hydrolysis of N205, with nitric acid being removed from the troposphere mainly by dry and wet deposition. 

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. 

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). 

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. 

The oxidation of nitric oxide in small concentrations in the troposphere by dioxygen is very slow. As shown in Scheme 21, nitric oxide is oxidized to nitrogen dioxide (ti/2 several days) either by HOO- radicals or by ozone. Reaction of nitrogen dioxide with hydroxide radicals forms nitric acid (ti/2 several days). Rain washes nitric acid out, thus acidic rain is formed. 

Extrapolation to the K/T boundary requires consideration of the time scales of acid deposition. Nitric acid formation occurs rapidly by aqueous phase reaction of NO and NO2 with liquid water produced by tlie incident K/T bolide on both impact and infall of ejecta. For tlie quantities of NO produced by the K/T impact ( 10 5 moles), conversion to HNO3 occurred wiUiin days, assuming sufficient liquid water was available in the posl-K/T atmosphere. The nitric acid will form an acid rain of pH 0 for a liquid water content of 1 g/m (typical of tropospheric clouds) but will contain enough protons to weather only 3 x 10 moles of Sr, for Sr/(Ta -0.003 in soil and bedrock minerals. Sulfuric acid formation occurred on a time scale of years [7] due to the slow rate of gas phase SO2 oxidation. Spread evenly over 10 years, 10 moles of SO2 produced a global acid rain of pH —4, and released —3 x 10 moles of Sr. 

NO2 in the troposphere reacts with hydroxyl radicals (from reaction 8.10, via reactions 8.2, 8.6, and 8.10) to form nitric acid ... 

The oxidation of N02 eventually leads to the formation of nitric acid and aerosol nitrate, which are deposited at the earth surface. The relevant oxidation pathways are indicated in Fig. 9-6. The following discussion deals first with observations of reaction intermediates then with tropospheric abundances of N02, PAN, and HN03/ aerosol nitrate, and finally with the budget of nitrogen oxides and their oxidation products in the troposphere. 

The reaction of OH radical with nitric acid (HONO2, HNO3) in the stratosphere is important as it reproduces active nitrogen from the reservoir molecule HONO2 in the NOx cycle. Although in the troposphere, water-soluble nitric acid is mainly removed by wet deposition into cloud and fog, and dry deposition on earth s surface, the OH reaction as well as photolysis are also important as removal processes and as active nitrogen regenerating process in the upper troposphere where clouds are not abundant. 

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