If all the unburnt carbon particles spread themselves over the snow in polar regions we could have global warming on steroids.

The London smogs were caused by temperature inversions. I used to be able to leave school early when the pea soupers were about.
If we believe government hype we have substituted coal particulates for diesel particulates(that cannot be easily seen, apart from a semicircular haze that can be seen when on motorways)
A comparison chart below
http://www.stovesonline.co.uk/fuel-CO2-emissions.html

https://www.ucsusa.org/clean-vehicles/vehicles-air-pollution-and-human-health/diesel-engines#.WmogYXnLjIU