Soot (Black Carbon)

Soot is the black particulate matter formed by the incomplete combustion of hydrocarbons — coal, diesel, biomass, and wood among them. It is not pure carbon: soot agglomerates are nanoparticle clusters (roughly 6-30 nanometers each) coated with polycyclic aromatic hydrocarbons (PAHs), sulfuric acid, metal oxides, and minerals. The strongly light-absorbing, refractory elemental-carbon fraction is called black carbon, the component responsible for soot's characteristic darkness and most of its climate impact. Because the finest particles are small enough to lodge deep in the lungs, soot is a primary source of PM2.5 air pollution. Exposure is linked to asthma, chronic lung disease, and coronary artery disease, and its PAHs are classified as known human carcinogens. Reductions in black carbon emissions are estimated to prevent hundreds of thousands to several million premature deaths each year. On the climate, black carbon ranks as one of the largest human contributors to warming after carbon dioxide. It warms three ways: airborne particles directly absorb sunlight and heat the surrounding atmosphere; deposited on snow and ice it darkens the surface, lowering albedo and accelerating melt (Arctic and Himalayan glacier warming of roughly 0.5-0.6 degrees C has been attributed to it); and it perturbs cloud behavior. Unlike CO2, black carbon persists only days to weeks, so cutting emissions yields fast climate benefits. Soot also plays the central role in Nuclear Winter: Scientific Models, Uncertainties, and Scale Requirements. Mass urban firestorms after a nuclear exchange would inject black carbon high into the atmosphere, where its strong sunlight absorption heats the surrounding air and makes it buoyant. This 'self-lofting' carries the soot into the stratosphere — above the rain that would normally wash it out — where it can linger for years, blocking sunlight and cooling the surface. Models of a full US-Russia war estimate on the order of 150-180 teragrams of stratospheric soot, the variable that determines the severity and duration of the cooling described in Nuclear Winter: Geographic Distribution and Weapon Quantity.