Photochemical smog formaldehyde

Three specific eye irritants have been identified in photochemical smog formaldehyde, acrolein and peroxyactyl nitrate (PAN). The possible reaction sequences are ... 

Besides the various other substances with toxic effects, polluted air may also contain aldehydes, which are formed as reaction products of hydrocarbon photooxidation. The reactions of aldehydes are not very rapid, so that they are accumulated in concentrations of about 0.2 ppm in the photochemical smog. Formaldehyde and acrolein are the most important compounds in this group. Their irritating nature obviously contributes to the increase of the smog smell and to its irritating effects on the eyes. According to estimates, formaldehyde represents a fraction of 50% out of all the aldehydes in the polluted air. Acrolein is more irritating than formaldehyde and forms about 5% of the aldehydes. 

The automobile pollutants in the atmosphere are exposed to intense sunlight, which yields photochemical oxidants. This phenomenon give rise to photochemical smog. Three specific eye irritants have been already identified in the photochemical smog— Formaldehyde, acrolein and peroxyactyl nitrate (PAN). The possible reaction sequence is as follows ... 

Benefits depend upon location. There is reason to beheve that the ratio of hydrocarbon emissions to NO has an influence on the degree of benefit from methanol substitution in reducing the formation of photochemical smog (69). Additionally, continued testing on methanol vehicles, particularly on vehicles which have accumulated a considerable number of miles, may show that some of the assumptions made in the Carnegie Mellon assessment are not vahd. Air quaUty benefits of methanol also depend on good catalyst performance, especially in controlling formaldehyde, over the entire useful life of the vehicle. 

Dodge, M. C., Formaldehyde Production in Photochemical Smog As Predicted by Three State-of-the-Science Chemical Oxidant Mechanisms, J. Geophys. Res., 95, 3635-3648 (1990). 

For NO t > 0-5 ppb (typical of urban and polluted rural sites in the eastern USA and Europe) Equations (3) and (4) represent the dominant reaction pathways for HO2 and RO2 radicals. In this case the rate of ozone formation is controlled largely by the rate of the initial reaction with hydrocarbons or CO (Equations (1) and (2)). Analogous reaction sequences lead to the formation of various other gas-phase components of photochemical smog (e.g., formaldehyde (HCHO) and PAN) and to the formation of organic aerosols. 

VOCs are generally low-molecular-weight aliphatic and aromatic hydrocarbons like alcohols, ketones, esters, and aldehydes.19 Typical VOCs include benzene, acetone, acetaldehyde, chloroform, toluene, methanol, and formaldehyde. These compounds are typically considered to be regulated pollutants, as they can cause photochemical smog and depletion of the ozone layer if they are released into the atmosphere. They are not normally produced in the combustion process, but they may be contained in the material that is being heated, such as in the case of a contaminated hazardous waste in a waste incinerator. In that case, the objective of the heating process is usually to volatilize the VOCs out of the waste and to combust them before they can be emitted to the atmosphere. 

Wet-chemical analysis and optical methods have been used to determine the abundance of formaldehyde in ambient air. Table 4-9 shows mixing ratios observed at various locations. The highest values are associated with urban air, especially under conditions of photochemical smog. Maximum values may then reach 70-100 ppbv. Mixing ratios in rural areas are of the order of a few ppbv, and still lower values are found in marine air masses. 

Because PAN is in thermal equilibrium with NO2 and the peroxyacetyl radical, it can act as a means of transporting these more reactive species over long distances. The NO2 released by thermal decomposition of PAN is photolyzed rapidly in the troposphere to form O3 by Reaction 19.1 and Reaction 19.2. Ozone is a criteria air pollutant and is a major health concern. Thus, the PANs play important roles as a chemical means of transporting key species such as NO2 and formaldehyde to remote locations. As such, PANs are globally important atmospheric molecules, as well as urban air pollutants. Since the original observation of PANs in Los Angeles photochemical smog, PANs have been measured in every corner of the world. 

Formaldehyde is directly emitted into the air from vehicles. It is released in trace amounts from pressed wood products such as particleboard and plywood paneling, from old sick bnildings, and from cotton and cotton-polyester fabrics with selected crosslink finishes. Formation of formaldehyde has been observed in some frozen gadoid fish due to enzymic decomposition of the additive trimethylamine oxide (Rehbein 1985). Its concentration can build up during frozen storage of fish (Leblanc and Leblanc 1988 Reece 1985). It occurs in the upper atmosphere, cloud, and fog it also forms in photochemical smog processes. 

Alcohol-based fuels for automobiles lead to the production of formaldehyde (CH2O) in exhaust gases. Formaldehyde undergoes photodissociation, which contributes to photochemical smog ... 

UV irradiation of the exhaust hydrocarbon-NOx mixture increases the formation of formaldehyde, PAN (peroxyacetyl nitrate), PBzN (peroxy-benzoyl nitrate), NO2, and ozone. These compounds are of toxicological interest and are important in smog formation. The presence or absence of Et4Pb in gasoline does not affect the photochemical reactivity of the exhaust hydrocarbons produced from gasoline (Heuss et al, 1974). 

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