Both are true.
A 2012 study by Booth et al. indicated that increases in aerosol emissions experienced prior to the 1980s were linked to cooler Atlantic temperatures and persistent drought in Sahel Africa. Since the 1980s, aerosol emissions have fallen, in response to legislation to cut pollution. Aerosols are mostly produced by power stations, and from the burning of trees and crop waste.
A 2012 study by Allen, et al. suggested that black carbon (soot) and tropospheric (lower atmosphere) ozone, formed from the burning of fossil fuels, biofuels and plants, are warming the atmosphere in the Earth's mid latitudes (30° to 50° N), causing the tropical belt to become bigger.