On rainy days there is a strong urin smell on trains in Tokyo. I suspect this smell comes from ammonia. On Wikipedia I read:

Ammonia and ammonium salts are also found in small quantities in rainwater […] Ammonium salts are found distributed through fertile soil and in seawater.

Both is valid for Tokyo. In rainy season especially there is a lot of rain. And Tokyo is located at the sea.

Air conditioners work by drying the air. Tokyo trains make extensive use of air conditioning. Could this process intensify or make the ammonia to fall out and thus create the strong urine smell? What other possibility is there for the strong smell?

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    $\begingroup$ I'll provide an alternative and obvious hypothesis: in other cities with subway systems I've been in, some people are known to take the liberty of relieving themselves in the subways (and urine is the least offensive smell). This is particularly true when homelessness is rampant, public restrooms are a rare commodity, and/or cold or rainy weather is not uncommon. $\endgroup$ – Buck Thorn Jun 24 '19 at 16:01
  • $\begingroup$ You might buttress your hypothesis by determining what airborne concentration of the offensive substance would be required and see if an oceanic source could account for it. You might want to check on geology SE if necessary. $\endgroup$ – Buck Thorn Jun 24 '19 at 16:07
  • $\begingroup$ Oh, in addition to people in need for a restroom there are other (perhaps more obvuously) possible sources: animals, including pets (dogs), rats and (maybe?) birds. $\endgroup$ – Buck Thorn Jun 24 '19 at 16:17
  • $\begingroup$ Ammonia is in the air, from sources Buck mentioned above, and when the condensed water+ammonia gets warm again in the sewage, it releases the ammonia again. It's trapped. $\endgroup$ – Karl Jun 24 '19 at 22:55
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    $\begingroup$ We can rule out animals or passengers. First this is Japan, there's free toilets on any station. Secondly this smell is general in any car on most lines. Even if there would be people who pee in the train. They'll hardly sprinkle all cars in Tokyo 😅 $\endgroup$ – steros Jun 24 '19 at 23:31
  • First, human sweat contains urea, so it doesn't require incontinent commuters. Despite constant cleaning, this effluvia accumulates in the rail cars.

  • Second, air conditioners precipitate water, entrained dust and volatile organic compounds, making an ideal environment for bacteria, fungi and protists to swim around, excreting their wastes. Microbial volatile organic compounds are none too pleasant, and may have the smell of urine or ammonia (though the waste products of yeast in baking bread can be quite appealing).

  • Third, there are probably birds in and around the train stations. Birds produce uric acid, which is rapidly converted to ammonia by fecal flora.

  • Fourth, in older drainage systems, rain water and waste water may be mixed. Given a high water level, wastes may be closer to the surface... or overflow.

  • $\begingroup$ Thank you for your answer. The problem I have with it is. That one day prior when it hot and people sweat much more. There is no such smell. It seems somehow related to the rainy weather condition. It changes rapidly from one day to another. $\endgroup$ – steros Jun 24 '19 at 23:29
  • $\begingroup$ The production of ammonia need water for hydrolysis. Water is more available at higher humidity due hygroscopicity. Additionally, at higher humidity, more water gets adsorbed ( and ammonia dissolved in it ) on aerial dust microparticles, leading to higher content of ammonia on nasal sensors. $\endgroup$ – Poutnik Jun 25 '19 at 6:47

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