Diesel Demons

Pollution has actually reduced. Few people can be unaware of the latest campaign of the Green Party against diesel cars. The claim seems to be that diesel cars are polluting our cities to death, and absolutely must be got rid of.  Yet, if you stop to think about it, other classes of diesel vehicle have been in use for a very long time.

That said, it's also easy to forget that the Greens are themselves responsible for the boom in diesel car sales, having promoted diesels as a 'lower carbon' form of transport, and therefore a way of fighting climate change. On the strength of this, excise duty on diesels was reduced in order to promote sales.

Now, the Greens want everyone who bought a diesel to scrap it, and buy an electric car. Leaving aside the issue of deliberate wastemaking this raises, how come the sudden about-face on diesels, and on what evidence is it based?

It is rather strange. After all, if environment science advisors were to promote a fuel type as producing less 'carbon pollution' you'd think they would first check to see if it had any other, perhaps more serious, pollution issues.

You'd think.

It gets even more puzzling if you check the figures for air pollution in the UK and in Germany.

The two main pollutants that diesels are accused of creating, are oxides of nitrogen and particulates, especially those in the PM2.5 class. Official figures indicate that there has been a steady and substantial reduction in emissions of oxides of nitrogen from all classes of transport since 1990.

It's an increase, Scotty, but not an increase as we know it... Moreover, the reduction in NOx emissions from passenger cars is slightly greater than that from HGVs, in spite of HGVs having been diesel for many decades, and passenger diesels having gained in popularity since 1990. Which is the reverse of what you would expect if diesels were so bad.

Even some official reports make written statements that don't seem to match the data on the same page. For example this page makes condemnatory statements about diesel cars in relation to NOx emissions, as in:

 1990-2010  Passenger Cars Switching from petrol to diesel cars  Increase in emissions 

Yet, the graph on the same page indicates a huge reduction in the NOx emissions from passenger cars over the same period mentioned.

It's hard to reconcile the rhetoric with the figures. Now, I may not be the best graph interpreter in the world, but I don't understand how that can be an increase. It's a decrease. Does the statement refer to something not shown in the graph.. or... what? Clarification needed.

I believe the main reason for the reduction was the introduction of exhaust catalyst technology. The bell shape of the curve is exactly what you would expect from a new pollution reduction technology; an initial rapid reduction as uptake gains pace and the technology is developed further, then a levelling-out once the technology has become mature and is in use across the board. Thus, nothing unusual about it, and nothing here to indicate the rise of a new pollution source.

Meanwhile, PM2.5 emissions from vehicle exhausts make up only a small proportion of total sources. These have also shown a moderate overall reduction.

Even during the 'diesel car boom' era of the last decade or so, there has been no significant spike in levels of either.

"Between 1990 and 2014, the transport sector reduced significantly emissions of certain air pollutants: carbon monoxide (CO) and non-methane volatile organic compounds (NMVOCs) (both around 83 %), nitrogen oxides (NOx) (39 %), sulphur oxides (SOx) (42 %) and particulate matter (37 % in the case of PM2.5 and 31 % for PM10)." - European Environment Agency.

Now, nobody is saying that any level of pollution is acceptable, or that we can afford to rest on our laurels over the improvements we've made.

However, there is something about this Green Party anti diesel car campaign which does not add up.

For a start it is a wastemaking exercise, not unlike the American gas guzzler sales of the 60's where everyone was encouraged to change their car every year, creating mountains of scrap metal. Such an exercise cannot possibly be good for the environment. Why on earth is an environmental party engaging in such tactics?

Then again, maybe the scientists got it wrong about diesels being good for the planet.. but wait a minute, the official figures say they aren't exactly bad for it, anyway. Air pollution has reduced, not increased, during the diesel era.

In view of the massive costs to the public and to the exchequer of this campaign, not to mention the costs to the environment of creating another monster scrap heap of perfectly serviceable vehicles, I reckon that some explanations are called for. 

Only 22% of total from transport, and since 1990 a huge.. er.. well, actually a moderate decrease.











Emissions of NOx and PM have substantially reduced since 1990. Source: DEFRA.

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