Hydrocarbons ozone formation

More precisely, the rate of ozone formation depends closely on the chemical nature of the hydrocarbons present in the atmosphere. A reactivity scale has been proposed by Lowi and Carter (1990) and is largely utilized today in ozone prediction models. Thus the values indicated in Table 5.26 express the potential ozone formation as O3 formed per gram of organic material initially present. The most reactive compounds are light olefins, cycloparaffins, substituted aromatic hydrocarbons notably the xylenes, formaldehyde and acetaldehyde. Inversely, normal or substituted paraffins. 

Chemical kinetic analysis of these simplified reactions allows net ozone formation to be directly related to hydrocarbon consumption by HO on a time-independent basis... 

The extent to which this occurs depends on a number of issues (Finlayson-Pitts and Pitts 1997), including the reactivity of the hydrocarbon that is itself a function of many factors. It has been proposed that the possibility of ozone formation is best described by a reactivity index of incremental hydrocarbon reactivity (Carter and Atkinson 1987, 1989) that combines the rate of formation of O3 with that of the reduction in the concentration of NO. The method has been applied, for example, to oxygenate additives to automobile fuel (Japar et al. 1991), while both anthropogenic compounds and naturally occurring hydrocarbons may be reactive. 

One of the considerations regarding the use of methanol as a fuel is that it emits higher amounts of formaldehyde, which is a contributor to ozone formation and a suspected carcinogen, compared to gasoline. Proponents of methanol dispute this, saying that one-third of the formaldehyde from vehicle emissions actually comes from the tailpipe, with the other two-thirds forming photochemically, once the emissions have escaped. They state that pure methanol vehicles produce only one tenth as much of the hydrocarbons that are photochemically converted to formaldehyde as do gasoline automobiles. 

For soybean-based biodiesel at this concentration, the estimated emission impacts for percent change in emissions of NO,, particular matter (PM), HC, and CO were +20%, -10.1%, -21.1%, and -11.0%, respectively (EPA, 2002). The use of blends of biodiesel and diesel oil are preferred in engines in order to avoid some problems related to the decrease of power and torque, and to the increase of NO, emissions (a contributing factor in the localized formation of smog and ozone) that occurs with an increase in the content of pure biodiesel in a blend. Emissions of all pollutants except NO appear to decrease when biodiesel is used. The use of biodiesel in a conventional diesel engine dramatically reduces the emissions of unbumed hydrocarbons, carbon dioxide, carbon monoxide, sulfates, polycyclic aromatic hydrocarbons, nitrated polycyclic aromatic hydrocarbons, ozone-forming hydrocarbons, and particulate matter. The net contribution of carbon dioxide from biomass combustion is small. 

No one pollutant can be blamed as the major cause of ozone formation. Replacing the more reactive hydrocarbons with less reactive ones would delay the formation of ozone, but would not prevent it. Reducing the NO, concentration seems to reduce the maximal oxidant concentrations observed, but the effect is nonlinear. Heavy injections of nitric oxide into the air can temporarily reduce the local ozone concentration, as often happens in urban centers, but additional oxidant formation can be expected later downwind. Although these effects can be understood qualitatively, it is not yet possible to make accurate predictions of oxidant formation, even in lalx)ratoty experiments. 

Haagen-Smit, A. J., and M. M. Fox, Photochemical Ozone Formation with Hydrocarbons and Automobile Exhaust, J. Air Pollut. Control Assoc., 4, 105-108, 136 (1954). 

The cyclic sequence of Reactions 5 and 6 can be accelerated by adding a species which promotes the formation of Co(III) from Co(II). In the oxidation of alkylaromatic hydrocarbons ozone (7), acetaldehyde (8), and methyl ethyl ketone (4) act as promoters in this way. 

Through the last sequence H02 is reformed to react with NO. The main point here is that nitrogen oxides are cycled through reactions R1 - R2, and therefore, this cycle will not limit the ozone forming potential. However, the formation of the other compound involved in the initial step in the ozone formation, H02, requires that CO be oxidised. The number of ozone molecules formed is therefore determined by the amount of CO present. H02 molecules can in a similar way be formed through the oxidation of CH4 and other hydrocarbons. The initial methane oxidation mainly through the reaction with OH ... 

Further oxidation leads to the formation of H02 and other peroxy radicals that participate in the ozone formation process. Similar products are formed when higher hydrocarbons are oxidised. 

Multi-component hydrocarbon standards to provide accurate calibration of instruments (generally gas chromatographs) used to monitor the concentrations of a wide range of volatile organic hydrocarbon compounds (VOCs) in ambient air. These standards currently contain 30 different hydrocarbon species that are important to photochemical ozone formation, with concentrations ranging down to a few parts per billion by molar value. They are disseminated widely in the United Kingdom and the rest of Europe as calibration standards, and as test mixtures for assessment of the quality of international ambient hydrocarbon measurements (often under the auspices of the European Commission - EC). 

This qualitative behavior is analogous to observations in the Los Angeles Basin. There hydrocarbon levels have been reduced without equivalent reduction in NO j, and presumably ozone formation in the vicinity of downtown Los Angeles has decreased while a similar decrease has not occurred in areas downwind where the reacting air mass has experienced a longer reaction time 72). 

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. 

The ozone formation process is almost always initiated by a reaction involving a primary hydrocarbon (abbreviated here as RH), other organic or CO with the OH radical. The reaction with OH (Equation (1)) removes a hydrogen atom from the hydrocarbon chain, which then acquires O2 from the atmosphere to form a radical with the form RO2. For example, methane (CH4) reacts with OH to form the RO2 radical CH3O2 propane (CsHg) reacts to form C3H7O2, etc. The equivalent reaction for CO (Equation (2)) forms HO2, a radical with many chemical similarities to the various RO2 radicals ... 

The largest sink for alkanes in the atmosphere is reaction with OH and NO3 radicals. The formation of photochemical smog is described in detail in (Chapter 9.11, Sillman). Mono-aromatic hydrocarbons react only slowly with O3 and NO3 radicals in the troposphere. The only important atmospheric processes for mono-aromatic hydrocarbons, and naphthalene and dinaphthalenes are reactions with OH radicals (Atkinson, 1990). The products of these reactions include aldehydes, cresols, and, in the presence of NO, benzylnitrates. Methane can be an important contributor to ozone formation, especially in the remote troposphere, as described in (Chapter 9.11, Sillman). 

In photochemical experiments conducted in sunlight and in concentrations of 3-methylheptane and nitrogen oxides approximating those known to occur during smog periods, ozone formation is proportional to the product of the hydrocarbon and nitrogen oxide concentrations. Similar conclusions can be drawn from the experiments with 2-butene (Figure 9). 

Inhibiting Action of Hydrocarbons on Ozone Formation by Silent Electrical Discharge... 

The reaction mechanism of ozone formation was studied to clarify the inhibiting action of hydrocarbons. The rate of ozone formation varies linearly with the hydrocarbon concentration, regardless of the type of hydrocarbon. 

When the oxygen concentration is constant, both the first and second terms in Equation 11 become constant. The rate of ozone formation, accordingly, can be expressed by the first-order function of the hydrocarbon concentration. 

Then the critical concentration of hydrocarbon for the perfect inhibition of ozone formation is... 

To identify Equation 11 experimentally, the rates of ozone formation from air were observed by changing the concentration of admixed hydrocarbons. The experiment was performed by using the apparatus shown in Figure 1. 

In Figure 2, the amounts of ozone formed during a 10-minute period are plotted against the concentrations of n-hexane, cyclohexane, and n-heptane. The rate of ozone formation changes linearly with the hydrocarbon concentration (Equation 11). And [RH]criticai was about 1.1 volume % (8.5 mm. of mercury) for air. 

Figure 2. Effect of hydrocarbon concentration on ozone formation...

---