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Four-stroke and two-stroke gasoline engines as well as diesel engines can all emit dense clouds of blue-tinged smoke under various transient loads or incorrectly tuned conditions.

Raleigh scattering - in this case probably from particulates of a limited range in size - strongly favors short wavelengths, so there is a possible physical explanation.

But I would like to know if there are any chemistry effects that could lead to or contribute to the slightly blue color sometimes seen in engine smoke under these conditions.

Could there be large "blue molecules" in the smoke? I don't think so; usually gases are transparent, though there are a few notable exceptions like molecular iodine and chlorine, but I'm not sure.

enter image description here

above: Iodine gas, from here

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above: Chlorine gas, from here

below: snapshot sitting on a bicycle, looking at adjacent scooter while holding breath and waiting for the light to change.

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below: screenshot from the YouTube video Big Engines Starting Up.

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    $\begingroup$ Comment and not answer because I do not know if one can see it (anyway I am oriented on Rayleigh scattering as everyone here) but for sure there is a blue molecule which has been detected in exhaust and is azulene. It is a very fascinating molecules under several points of view. For sure is found in ppm in diesel engines exhaust. Take it as an answer about their presence in smoke whatever its colour is. $\endgroup$
    – Alchimista
    Jul 14, 2017 at 19:27
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    $\begingroup$ @Alchimista I've narrowed the language of the question to "blue chemicals" (horrible term, sorry) to cover that aspect, and to try to stay on-topic. I also assume it's Rayleigh scattering, but I don't know, nor do I even know how to ask if it's usually from unburned oil droplets, or particles or agglomerations of reaction intermediates. Nice molecule though, and first time I've seen a blue mushroom without having eaten one first! $\endgroup$
    – uhoh
    Jul 14, 2017 at 19:33
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    $\begingroup$ Well azulene is a blue chemical and can be found in exhaust. My "whatever the colour is" refers to the smoke, to make clear that I am not sure if, when azulene is present in the exhaust, the latter looks bluish. I even do not think so and I think as well read that Rayleigh scattering plays a role. It can be observed also with smoke rising gently rising from a cigarette tip, when sometimes a blu and a white streams are seen. $\endgroup$
    – Alchimista
    Jul 14, 2017 at 22:22
  • $\begingroup$ Very interesting, thank you for that image, I might even remember noticing that about cigarette smoke, though it could be the power of suggestion. I'll look into it. $\endgroup$
    – uhoh
    Jul 15, 2017 at 2:32
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    $\begingroup$ :)) indeed. Another funny thing : camille tee is well known to be a good topic remedy for eyes against inflammation. Azulene is in the composition of over the count medication for that. But if handle a small vial of azulene, it is better to open it with goggles else you cry! Paracelsus principle at work! $\endgroup$
    – Alchimista
    Jul 20, 2017 at 10:50

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I can't really tell you the chemical difference but usually blue smoke indicates that oil is burned, too. Like some of the oil cycles get into contact with the engine and it is burned as well. And then probably heavier components of your gasoline once you start the cold engine. My Prof (lecture on Petrochemistry) once told us that there is indeed a difference between premium gasoline and the cheap one although the octane number could be similar. For the cheap ones they add a lot of stuff with a very low octane number and finally only a small amount compounds with higher octane numbers to get to 95. But it makes a difference in burning it. Maybe if the engine is still cold some of the "harder to combust" compounds don't burn that well and cause the blue color, too.

It's probably due to Rayleigh scattering as you mentioned. The sky is blue and clouds are white, the normal exhaust gases should be water and CO2 so similar to clouds and the blue particles are smaller rests which appear blue.

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  • $\begingroup$ I have a hunch that you are right. If there is a bluish tinge it may just say something about the size distribution of particles, but if it has to do with with larger particles and their index of refraction, that could be chemical. Larger particles with a higher index of refraction at short wavelength might produce a color, and that might be related to chemistry in some way, but that might be too complicated for an answer here. Thanks for your insights on combustion as well! $\endgroup$
    – uhoh
    Mar 28, 2017 at 13:43
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    $\begingroup$ The oil leaking into the engine and not being fully combusted is usually cited as the reason for the blue smoke. So you are exhausting oil droplets along with some steam; the oil droplets could be dispersed as a fine mist that facilitates Rayleigh scattering. $\endgroup$
    – J. Ari
    Mar 28, 2017 at 14:02
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Interesting question. Real life question often have an open ended answer. Let us make some assumptions, smoke particles originate from incomplete combustion, however the temperature in combustion is high enough to burn most organics. This eliminates organic blue molecules, even if they exist, they cannot exist in the gas phase at those temperatures.

Now there are couple of things which give a blue hue to the smoke. One is scattering depending on the angle of view (Rayleigh scattering, Mie scattering). Secondly, another point which we our discussion might be missing is fluorescence from aromatic hydrocarbons in partially combusted diesel oil. These molecule glow really pure blue in UV. All you need is trace of a fluorescent molecule. I feel that there is some contribution of blue fluorescence in the exhaust. A quick search shows there are plenty of aromatic hydrocarbons in diesel exhaust which are fluorescent. BTW, petrochemicals can be naturally fluorescent. See this beautiful picture here

http://www.dakotatechnologies.com/learn-more/intro-to-lif/overview

If you ever wear a white shirt and expose it to UV it glows blue thanks to the aromatic dyes added to modern detergents.

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  • $\begingroup$ hey I think we are on to something! Yes I'm familliar with bluing but I'd never thought about fluorescence in this kind of exhaust. It's theoretically testable but getting an engine blowing blue smoke and a bright UV light in a dark space together sounds like a bit of a challenge. I wonder if there's a better way to test this. Thanks for the insight! Any idea how far into the UV would be necessary? There are UV LEDs but they get more expensive the shorter you go in wavelength. $\endgroup$
    – uhoh
    Mar 16, 2019 at 15:58
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    $\begingroup$ Not very far UV is needed. If it fluorescing in the sunlight how much UV do we need? Instead of doing the experiment, search the literature along these directions. Ordinary black light, is good enough for detecting fluorescence. $\endgroup$
    – AChem
    Mar 17, 2019 at 1:02
  • $\begingroup$ I've just stumbled upon Diesel exhaust; specific chemicals which lists Fluoranthene Pyrene at several thousand PPB. A four-cycle gasoline engine in poor condition as well as a two-cycle engine will be burning oil and so could also have such compounds. Fluorescence could indeed be all or at least part of the "blue" in blue smoke from such engines! I think you've almost nailed it. $\endgroup$
    – uhoh
    Apr 2, 2019 at 4:12
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    $\begingroup$ It is a plausible hypothesis but I love the very phenomenon of fluorescence! $\endgroup$
    – AChem
    Apr 2, 2019 at 4:27
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The blue colour is due to the particle size. In his wounderful book Heys (Chemistry Experiments for Boys and Girls) he wrote of an interesting experiment which requires you to stand near a person smoking.

If you look at the colour of the smoke from the end of a ciggertte which is emitting smoke such as one being held in the person's hand but not in their mouth it will emit smoke with a blue-white colour.

But if you look at the smoke which comes out of their mouths when they exhale it is white. The reason is that the average particle size increases inside their lungs. Please keep in mind that colour can be caused by either a coloured chemical or a physical (optical) effect caused by things like particle size.

By the way, smoking is a disgusting and carcinogenic habit.

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  • $\begingroup$ Why does the average particle size increase? Are they growing, or is it just that the smallest particles get preferentially stuck inside the smoker? $\endgroup$
    – uhoh
    May 10, 2018 at 20:14
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    $\begingroup$ Well I am not sure exactly why the average particle increases, the problem with Heys is that he did not explain much about why thing happen in his book. It is just a book telling you how to do chemistry experiments. I think that the smaller particles could be lost in the lungs or the particles could join together with each other or adsorb water from the water vapour in the lungs to increase in size. $\endgroup$
    – Nuclear Chemist
    May 10, 2018 at 20:17
  • $\begingroup$ I wish there was some data on this, I'd love to see it (and be disgusted as you point out). I have a hunch this is the right answer, but I can't quite click accept yet... $\endgroup$
    – uhoh
    Mar 16, 2019 at 9:27

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