Wednesday, 30 September 2015

the oh! zone

Amid the scandal that has erupted over Volkswagen following the discovery that it has been cheating on tests of emissions from its diesel cars in the United States, reporting has been misleading and/or inaccurate. For example, there has been confusion over the nature of the pollutants emitted by diesel engines, and over the emissions that are being tested for.

I have not been able to confirm this, but it has been reported that the pollutants being tested for were NOx, an umbrella term for the three oxides of nitrogen: nitrous oxide, nitric oxide and nitrogen dioxide. Nitrogen is a fairly inert gas, which explains why it makes up almost 80 percent of the Earth’s atmosphere. And there are only two natural ways in which it can combine with oxygen to produce NOx or nitrate ions: during electric storms and by nitrogen-fixing bacteria on the roots of leguminous plants.

But there is a problem. The sparks that ignite the fuel/air mixture in a petrol engine create an electric storm in miniature, which leads to the conversion of some nitrogen to NOx. However, diesel engines do not have spark plugs, so I cannot see how oxides of nitrogen can be produced in such an environment.

Of course, I may be wrong, and diesel engines do produce NOx, but there has been no discussion of the nature of these pollutants in the mainstream media. Although nitrous oxide is a far more potent greenhouse gas than carbon dioxide, the real danger from these gases occurs in locations with copious amounts of sunshine and a lot of cars. The oxides of nitrogen and unburnt hydrocarbons are converted by ultraviolet radiation to peroxyacyl nitrates (PANs), which are severe eye and respiratory irritants and key ingredients in photochemical smog. This is a different kind of smog to that for which London was once famous, and unsurprisingly the type location is Los Angeles.

Four days ago, I came across an article on the BBC News website by the corporation’s environment correspondent Matt McGrath under the title ‘Concern over hidden diesel pollutant’. I thought that this article might be discussing PANs, but it turned out to be full of vagueness and inaccuracy, not the kind of thing I’d expect from a science journalist. Here is one particularly egregious piece of nonsense:
Researchers found that long-chain hydrocarbons are significantly under-reported in car manufacturer’s [sic] data.
These hydrocarbons are a key component of two of the worst air pollutants, ozone and particulate matter.
Oh really! The last time I checked, ozone was a form of molecular oxygen formed by three atoms instead of the two in the gas we breathe to stay alive. It is possible that long-chain hydrocarbons perform some kind of catalytic function in the production of ozone, but this is not what the quoted statement is suggesting. They are not ‘a key component’. Ozone contains neither carbon nor hydrogen, even in trace amounts.

The mistake is made again, in different words, later in the article:
But diesel also contains more complex, long-chain hydrocarbons, whose role in air pollution has been little understood until now.
They can form dangerous air pollutants, especially ozone and particulate matter, which are emitted into the air as unburned fuel or diesel vapour.
This re-statement of the error is made worse by the grammatical chaos of the second sentence. Leaving aside the annoying journalistic habit of making each sentence a new paragraph, the real problem in that second sentence is the relative clause. It is probably intended to qualify ‘dangerous air pollutants’, but I’ve reached this conclusion only because the statement would be untrue if the relative clause was intended to qualify ‘ozone and particulate matter’, which I have every right to expect it to do given its position in the sentence. And to cap it all off there is the pleonasm: is there really a difference between ‘unburned fuel’ and ‘diesel vapour’?

Of course, long-chain hydrocarbons are the main constituents of particulate matter. By the way, I do wonder how well the average person understands this phrase. Perhaps ‘soot’ could be used instead. At least it would help people to form a concrete image of the problem, which in turn might encourage more people to give up their cars. These are—regardless of their manufacturer or of the possible ubiquity of fiddled tests—the real problem.

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