Is this mist or haze?

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  • #26
chemisttree
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In short. Pollution is higher at 4am to 10am or early morning and this is official and occur everywhere? We tend to think pollution is greater at mid day as there are more and more cars.
Well, perhaps not everywhere. The diurnal variability of PM2.5 is a known thing. The layer that I referred to in my previous post is described as the “planetary boundary layer” (PBL) in the referenced study. How that varies during the day is discussed on pages 8-10 and it is noted that the maximum concentration of PM2.5 is seen at dawn and the early morning hours for most of India during this time of year. This is mainly due to the diurnal variation in the PBL, its expansion during the day diluting the particles. It’s not true everywhere though.
 
  • #27
russ_watters
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But pm2.5 particles being 2.5 microns or less dont gravitate, so why dont they diffuse to the atmosphere above? Im still trying to confirm whether haze or mist is major portion of it so i can decide whether to buy an 60 sq. mtr air purifier...

I asked all this so i can know whether to buy an air purifier. Should I? I live in 32th floor. Look at the city picture. The sky is big. Why dont pm2.5 particulate diffuse away into the rest of atmosphere instead of getting in my unit at 32th floor?
2.5micron particles do "gravitate". The smaller the particle the more it behaves like a gas, but there is no firm cutoff where they no longer settle. A quick google tells me that the half life of 1 micron particles in a 2.5m high clean room with turbulent air have a half life for settling of 12 hours. That's pretty slow, but it is enough to say they won't disperse without help from the weather. A calm morning means no help. And when an inversion layer traps them (as others said), it's even worse.

So I don't see a conundrum here; believe your particle counters and get an air purifier.
 
  • #28
chemisttree
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And don’t forget. Some of these particles are sulfate aerosols which are known to adsorb moisture in the air when the relative humidity is high. Kind of like an inverted jelly roll. When this happens the particle size grows and it is these water-swollen particles that contribute to the increased scattering of the shorter wavelength light at dawn. The blues from the early morning sky provides a uniform source and scattering is fairly pronounced. Later in the day when the RH drops, these particles dry out and shrink down in size. The smaller sizes aren’t efficient scatterers and the air appears to clear up. Of course you know you can still read them on your meter.
 
  • #29
berkeman
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Sounds like the OP has been getting good advice. This is a good time to close the thread. Thanks everybody for helping the OP out. :smile:
 

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