Troposphere oxidation

The evolution of the emissions of some atmospheric pollutants in Europe (EU-15) in the period 1990-1999 has been presented in the report of Goodwin and Mareckova (2002). The report includes acidifying pollutants (ammonia, sulfur dioxide, and nitrogen oxides), tropospheric ozone precursors, NMVOCs, carbon monoxide, and particulate matter... 

Like methane and nitrous oxide, tropospheric ozone is a natural greenhouse gas, but one which has a short tropospheric residence time. Ozone s bending vibration occurs at 14.2 pm, near that for CO2, and thus it does not contribute much to the enhancement of the greenhouse effect since atmospheric carbon dioxide already removes much of the outgoing light in this wavelength. 

PROBABLE FATE photolysis , aquatic photodissociation is precluded by volatilization, tropospheric photooxidation precludes stratospheric photodissociation, C-Cl bond can photolyze slowly oxidation tropospheric photooxidation by hydroxyl radicals is rapid and yields a variety of products the half-life due to photooxidation is 11 hrs in relatively clean air and <2 hrs in polluted air in water, photooxidation is unimportant hydrolysis to slow to be significant very... 

PROBABLE FATE photolysis not important except as photooxidation, C-Cl bond can photolyze slowly oxidation tropospheric photooxidation by hydroxyl radicals yields trichloro-acetyl chloride and phosgene, predominant fate, photooxidation half-life in air 16-160 days hydrolysis probably too slow to be important volatilization rapid volatilization is the primary transport process sorption too slow to compete with volatilization biological processes moderate potential for bioaccumulation, possible biodegradation by higher organisms... 

Water-soluble sdanols such as (1) were found to undergo successive oxidative demethylations with tropospheric ultraviolet irradiation in the presence of suitable chromophores, such as nitrogen oxides (516). The water-soluble methylated sdicones did not promote diatom (Nap cu/apelliculosd) growth but the demethylated photo products did. The sequence of sod-induced degradation of sdicones to water-soluble species such as (1), followed by light-induced conversion to sdicate, suggests a pathway, conceptually at least, for the mineralization of sdicones. 

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. 

The chemical transformations occurring in the atmosphere are best characterized as oxidation processes. Reactions involving compounds of carbon (C), nitrogen (N), and sulfur (S) are of most interest. The chemical processes in the troposphere involve oxidation of hydrocarbons, NO, and SO2 to... 

Ait Quality Criteria for Ozone and Related Photochemical Oxidants. Volume 1.3. Tropospheric Ozone and Its Precursors. Research Triangle Park, NC F.PA (1996). http //www.epa.gov.ncc.i/ ozone.htm. 

In addition to reactions with HO, tropospheric organic compounds may be oxidized by ozone (via ozonation of non-aromatic carbon/carbon double bonds, Atkinson 1990) and in some cases by reaction with nitrate radical, described below. Table I gives representative trace-gas removal rates for these three processes. In spite of these competing reactions, HO largely serves as... 

Oxidant Formation. The role of HO. in controlling the time-scale and severity of tropospheric oxidant pollution may be seen from the parameterization of O Brien and co-workers (75,76). The simplest possible mechanism for oxidant (Le. ozone, PAN, H2O2, etc.) formation consists simply of the reaction of an individual NNlHCj with HO. to convert the NMHCj to a generic product(s) PRODj, followed by removal of the product by HO. (PROD photolysis may be important, but is ignored here)... 

Certainly, photochemical air pollution is not merely a local problem. Indeed, spread of anthropogenic smog plumes away from urban centers results in regional scale oxidant problems, such as found in the NE United States and many southern States. Ozone production has also been connected with biomass burning in the tropics (79,80,81). Transport of large-scale tropospheric ozone plumes over large distances has been documented from satellite measurements of total atmospheric ozone (82,83,84), originally taken to study stratospheric ozone depletion. 

HO oxidation of CO is much faster than the reaction with methane, resulting in a mean CO lifetime of about two months, but considerably slower than reaction with the majority of the nonmethane hydrocarbons. Table I gives representative removal rates for a number of atmospheric organic compounds their atmospheric lifetimes are the reciprocals of these removal rates (see Equation E4, below). The reaction sequence R31, R13, R14, R15 constitutes one of many tropospheric chain reactions that use CO or hydrocarbons as fuel in the production of tropospheric ozone. These four reactions (if not diverted through other pathways) produce the net reaction... 

R8 is the simplest of a large suite of peroxyl radical combination reactions, generalized as R02 + H02 and R02 + R02 that generate poorly characterized radical and non-radical reaction products. Such reactions are of greatest significance in air with low nitric oxide concentration where the R02 species can reach elevated concentrations (95). The dependence of [H02 ] upon the tropospheric NO concentration is discussed below. 

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