Oxygenated VOCs

Oxygenated VOCs include methanol, ethanol, 2-propanol and acetone, and in general the comparative ease of destruction is ... 

There is a variety of processes that act to remove a trace gas from either the troposphere or the stratosphere. For oxygenated VOCs the main tropospheric loss occurs via gas phase oxidation reactions involving OH, O3, NO and Cl radicals, and photolysis. However, the hydroxyl radical is the most important oxidizing species in the global troposphere [16-19]. As a result of its role in initiating the majority of oxidation reaction chains, the OH radical is the primary cleansing agent for the lower atmosphere and has been called the "tropospheric vacuum cleaner" [20]. The dominant production cycle for tropospheric OH involves the reaction of 0( D), produced from the photolysis of O3, with H2O ... 

The reactions of oxygenated VOCs with the OH radical proceed mainly by H-atom abstraction from C-H and, to a much lesser extent, from O-H bonds, or by addition to the carbon atoms of any C=C bonds [19,22]. Many of these reactions exhibit apparent negative activation energies, especially at temperatures below room temperature, i.e. the rate constants are observed to decrease with increasing temperature... 

TABLE 12.8 Binding energies (kcal/mol) of oxygenated VOCs -I- OH radical complexes, obtained from electronic energies -t- ZPE corrections... 

In summary, the utility of quantum chemical calculations to elucidate the detailed mechanisms of OH radical reactions with oxygenated VOCs has been proven. The importance of including reactant complexes in such modeling, to obtain accurate values of the rate coefficients, has also been shown. The best results are those obtained when it is assumed that such complexes are in their vibrational ground state. The relative site reactivity of the studied compounds towards OH radicals has been shown to be strongly influenced by intramolecular hydrogen-bond-like interactions that arise in the transition states. 

Since the main tropospheric sink of oxygenated VOCs seems to be their reactions with OH radicals, at least at daytimes, the mechanistic and kinetic information discussed in this work is relevant to fully understand the tropospheric chemistry of such compounds, as well as their subsequent fate. Hopefully, the large amount of experimental and theoretical work that has been revisited here, which has been devoted to chemical reactions of environmental significance, could contribute in some extent to act in the right directions and prevent more damage to our atmosphere. 

Kinetic studies have been performed on the individual steps occurring in the NO3 and OH initiated oxidation of VOCs. The studied reactions include essentially reactions of NO3 with alkenes, di-alkenes and dimethyl sulfide (DMS), reactions of NO3 with intermediate peroxy radicals (HO2, CH3O2, C2H5O2) and reactions of OH with methane and oxygenated VOCs (ethers, alcohols). The rate constants for these reactions have been measured, and mechanistic information has been determined. The experimental methods used were discharge-flow reactors coupled with mass spectrometry, electron paramagnetic resonance (EPR), laser-induced fluorescence (LIF) analysis and the laser photolysis associated with LIF analysis. The discharge-flow LIF and laser photolysis LIF experiments have been especially developed for these studies. 

Reactions of OH with oxygenated VOCs (ethers, alcohols)... 

Fig. 4 Arrhenius plot for the reactions of OH with the studied oxygenated VOCs Achievements...

Filella, I., Penuelas, J., Seco, R. (2009) Short-chained oxygenated VOC emissions in Pinus halepensis in response to changes in water availability. Acta Physiologiae Plantarum, 31, 311-318. 

Alcohols (methanol, ethanol, 2-propanol, and -propanol), aldehydes (formaldehyde), ketones (acetone), and esters (ethylacetate) are considered oxygenated VOC environmental pollutants and, therefore, their emissions are under a permanent control [44]. 

RCO 7000 is preferred for exhaust streams containing mainly oxygenated compounds such as acetone or butyl acetate. This catalyst can destroy these oxygenated VOCs at 350°C. However, it is not very active for aromatics and would need temperatures higher than 850°C to completely deshoy them. Additionally, since it is a base metal catalyst, RCO 7000 is not effective for CO and has been found to form CO when used with streams containing methyl ethyl ketone. 

Good Conversion Aromatics Aromatics Oxygenated VOCs... 

Fairty Good Conversion Oxygenated VOCs Aromatics... 

Aromatic VOCs toluene, xylene, benzene, ethylbenzene, etc. Oxygenated VOCs ketones, acetates, etc. 

Total oxidation of oxygenated VOCs over Mn oxide-based catalysts... 

In spite of their lower combustion activity with respect to noble metal-based catalysts, base metal-based catalysts, such as Mn0x/Al203, are commercially used for the catalytic combustion of oxygenated VOCs. Manganese-based catalysts, such as unsupported Mu304, as well as Mn oxides supported on carriers such as alu-... 

T Karl, A Guenther, A Jordan, R Fall, W Lindinge. Eddy covariance measurement of biogenic oxygenated VOC emissions from hay harvesting. Atmospher Environ 35 491-494, 2000. 

From the description above, some of the limitations with GC and its variants become clear. It is not a universal technique, since the choice of trap and column will affect the sensitivity and accuracy towards certain classes of compounds. In particular, oxygenated VOCs are more of a problem than hydrocarbons for GC analysis. The principal problem with oxygenated VOCs in GC stems from their polarity, which requires polar or semipolar column materials to achieve sufficient compound separation. Unfortunately, these columns also easily degrade when they come into contact with water, and therefore stringent efforts must be made to avoid any moisture. A particularly serious matter in GC analysis is the speed of measurement which, because of the need for sample collection and also some degree of pre-concentration, is often limited to a single measurement every few minutes, at... 

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