"If brown clouds were eliminated overnight, this could trigger a rapid global temperature rise of as much as to 2 degrees C."

If you believe this, then perhaps all recorded warming of 20th Century could have resulted from dramatic reduction of pollution in the western societies.

But I think they are overstating the effects of this pollution upon temperatures.
Though I don't doubt that heavy smog in a local region could effect that local regions temperatures by 2 C.
For example in regards to Kuwait oil field burning:

"The response of the global climate system to
smoke from burning oil wells in Kuwait is investi-
gated in a series of numerical experiments using
a coupled atmosphere-ocean general circulation
model with an interactive soot transport model and
extended radiation scheme. The results show a
decrease in surface air temperature of ~4 °C in
the Gulf region. Outside this region the changes
are small and statistically insignificant. "
http://www.mi.uni-hamburg.de/fileadmin/files/forschung/theomet/docs/pdf/nature91.pdf

This smoke was so thick that there were nighttime like conditions on the ground during the day.
Later studies found no measurable affect upon global temperature from the oil fields burning.

Having a 2 C decrease in temperatures in a local region is not very significant, Urban Heat islands affects can cause 5 C or more increase in a local region. So it's quite possible that urban heat island effects globally have bigger effect upon temperature than this smog [or "Brown Clouds"]. Nor are these "Brown Clouds" a new development [something that began to happen in a time frame of less than 100 years- but is only a new development in the sense of monitoring them using satellites].