While reading about ground-level air pollution, I was struck by the the section on VOC and NOx reduction. The concept of reaction dependence on particular compounds based on a given set of conditions is fairly standard, but the implications of the affects of NOx reduction in certain areas were quite interesting. In NOx-limiting regions, high concentrations of VOCs produce an abundance of HOO and ROO free radicals, which react with nitric oxide to produce the hydroxyl radical and nitrogen dioxide. Nitrogen dioxide then proceeds to undergo photolysis to produce atomic oxygen, which combines with molecular oxygen to form ozone. So, in this case, we see an increase in ground-level ozone due to the presence of nitric oxide molecules. Conversely, in VOC-limiting regions, the existence of NOx, specifically nitrogen dioxide, reacts with the hydroxyl radical to form nitric acid and effectively inhibits the production of ozone by decreasing the amount of OH radical available to react with VOCs. In this case, NOx actually help in the prevention of ozone formation.
VOC-limiting areas tend to be urban parts of the globe, which brings about somewhat of an environmental paradox: If reducing NOx levels in VOC-limiting regions, which tend to be urban areas, will increase the amount of ground-level ozone produced, how beneficial are catalytic converters? Do they not decrease the level of NOx produced from fuel emissions, which would imply that more of the hydroxyl radical in urban areas would be free to produce ozone by reacting with VOCs? It seems as though there is a trade off between decreasing the amount of NOx air pollution and preventing ground-level ozone production. Would it not be more beneficial to reduce VOCs in urban areas since those compounds are the ones the hydroxyl radical will react with if NOx levels are low? I found very similar postulates in a paper published in 2004 on the "weekend effect," which involves the effects of the lower levels of NOx due to decreased truck travel during weekends on ground-level ozone production.
http://www.aei.org/outlook/19746
It's an interesting topic of debate. Is our current environmental policy of NOx reduction in urban areas the best policy based on the chemistry we know occurs in VOC-limiting regions?
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Another thing to think about in the debate over the use of catalytic converters was brought up in this weekend's reading. On p.167 in the text book, it is noted that "there is evidence that methylated benzenes are demethylated in catalytic converters and that, as a consequence, additional benzene is emitted into the air under some operating conditions". Benzene is carcinogenic and is classified as a hazardous air pollutant and so it is dangerous is toluene and other methylated benzenes are being broken down into a more toxic form. Does the release of less NOx outweigh the increased emission of benzene?
ReplyDeleteJust linked this article on my facebook account. it’s a very interesting article for all.
ReplyDeleteCatalytic Converter