Fig. 3.

Probability distribution of the OC/BC threshold above which emissions are no longer net warming, assuming an average geographic and altitudinal distribution for the emitted particles. Typical OC/BC values of different combustion sources from (3) are marked. Solid lines incorporate the snow albedo effect, assuming that it is caused by CC and BB in proportion to their BC emissions, whereas dashed lines exclude the snow albedo effect. For example, residential biofuels have a typical OC/BC ratio of 3.9; based on H, assuming an average geographic and altitudinal distribution, and including the snow albedo effect, there is a 78% chance they produce carbonaceous aerosols with a net warming effect. Although some sources (e.g., diesel engines using high sulfur fuel) coemit SO₂, the RF of the sulfate aerosol thereby formed will depend on the relative quantities of compounds emitted and whether the resulting aerosols are internally mixed (increasing RF) or externally mixed (leading the positive BC RF to be partially cancelled by the negative sulfate RF).