I agree, and the US indices of PM2.5 and PM10 show a plateau...

I agree, and the US indices of PM2.5 and PM10 show a plateau over the past 10 years. (PM2.5 significant health risk)
So China is clearly the major contributor currently,"And Beijing is nowhere near the worst – it ranked 75th worst for air quality out of 149 cities in China listed at the weekend, with the worst reading in the northern city of Harbin." http://www.independent.co.uk/news/world/asia/china-plans-emergency-measures-to-control-worsening-air-pollution-8459248.html

This prompted me to check out the cold snap in Europe and the Atlantic Meridional Overturning Circulation AOMC:
https://www2.ucar.edu/atmosnews/features/8711/words-about-warming-matter
NCAR UCAR, Revisiting a landmark IPCC statement notes the following chilling prediction
"Wigley stresses the importance of considering aerosols side by side with greenhouse gases, not only because of their countervailing effects on climate, but also because of the stark differences in how greenhouse gas and aerosol influences evolve over time.

“The offset from aerosols will only last as long as the emissions of sulfur dioxide continue,” notes Wigley. In contrast, carbon dioxide is far more long lived, and as a result, “the warming influence of greenhouse gases will continue for many centuries.”
"

In simple terms IMHO SO2, dust and soot produced currently has been masking the overall increase in temperature. In effect like a hypothesised nuclear winter or major asteroid strike has done previously to the earths weather.