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Some days I can smell bad smelling exhaust 50 meters or more away from the bus stop. I think the air is usually moist those days.

Does exhaust gases linger longer if the air is moist near the ground? Is it smog I am smelling those days?

On morning 12 march it was about

  • Temperature: 4°C
  • Humidity:93%
  • Barometer:978 mbar
  • Wind:10 m/s

Looking at the data from earth.nullschool.net at a few hours before that morning I saw surface temperature of 5.1°C and 5.8°C at 1000hPa.

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  • $\begingroup$ Could you provide a day and general location where you experienced this? Without an idea of the atmosphere that day it would be speculation at most. $\endgroup$ – Wxboyajm Mar 13 at 14:12
  • $\begingroup$ @Wzboyajm are those data enough? I can download more data from the national metrological institute if it is needed. I think it smelled bad the morning I wrote the question... $\endgroup$ – Emil Mar 13 at 20:17
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You can see this post for a more detailed discussion, but the typical answer to your question is air stagnation. When the atmosphere is stable, there is little mixing, and the air can stagnate. Typically this coincides with low wind speeds and a low boundary layer. The boundary layer holds air near the surface like a blanket. The boundary layer collapses overnight, becoming shallow, and grows when the sun heats the surface of the Earth. So, in the morning, the shallow boundary layer allows less vertical mixing to occur.

You might be noticing that on cloudy days, when clouds are low, there is little wind and no solar heating. Air pollution has no place to go, so it sticks around.

Another aspect is that vehicles emit more emissions under cold-start conditions. This is because vehicles get cold overnight and take a while to get up to normal temperature when they startup in the morning. The cold vehicle will have a low combustion efficiency and catalytic converters won't work as well.

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    $\begingroup$ Is humidity at sueface also an effect from the vertical temperature gradient? Was I noticing a correlating effect rather than the cause? $\endgroup$ – Emil Mar 14 at 8:48
  • $\begingroup$ I would expect higher humidity on cloudy days. $\endgroup$ – farrenthorpe Mar 18 at 0:53
  • $\begingroup$ I don't know if this makes sense, but this is another explanation that I gave to myself when I noticed the same phenomenon: traffic pollution is largely composed of solid and liquid particles suspended in the air. These particles, even more if associated with high humidity, low pressure, and air stagnation, favor the formation of micro droplets (condensation). Droplets then deposit on clothes and skin (and through inhalation also in the olfactory bulbs???), so you can smell "exhaust gases" more easily than when they're free to dissolve in the air. $\endgroup$ – Nemesi Mar 18 at 9:33

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