What causes SPM contamination in cities?

A few years back I could clearly see the blue outline of the Nandi Hills (55 kms away) when I just looked to my left through the 11th floor window of my office in Brigade Road, Bangalore. Nowadays this is almost never. Rarely ( about once a month) I see a hazy grey out line if I look hard. Two days of rain followed by sun allows the sight of the hills for a few hours till the dust again draws a veil.

The blame is always laid on the nearly 75 lakhs motor vehicles emitting pollutants in Bangalore. 

We are all engineers. Most of us are aware of particles settling in air. Assuming that the dust is mostly silica with a density of 2.65 gms/cc and a particle size of 2.5 micrometers why not we attempt to see if there could be an alternative explanation for the high SPM levels in cities other than the fall guy of motor vehicle emissions?

Bangalore has been damp for a month now. The roads are damp. Intermittent drizzle and heavy rain has washed down SPM. Nandi Hills is again visible even through the mist. There is no reduction in the traffic all these days.

What I am getting at is that vehicular emissions has bourne the brunt of all the blame for SPM, while the culprit is probably the <2.5 micron dust kicked up by the corporation cleaners and kept up by the passing vehicles and wind. Now that the roads are damp this doesn’t happen.

@Kaustubh I was trying to provoke discussions with a few posts  like this. No takers.

I was hoping that SPM would generate some calculations and arguments for and against.

I calculated that siliceous dust of 2.5 micron size has a settling velocity of about 3 mm/sec. This is very small being about 100 m/hour.  With all the traffic around and the faintest of breezes it will not settle.

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