[Mechanism]
The nuclear winter scenario assumes that if 100 or more city firestorms follow the nuclear explosions of a nuclear war and the firestorms loft large enough amounts of sooty smoke into the upper troposphere and lower stratosphere, soot lifted by the conveyor belt offered by the pyrocumulonimbus clouds that form during a firestorm. At 10–15 kilometres (6–9 miles) above the Earth's surface, the absorption of sunlight could further heat the smoke, lifting some, or all of it, into the stratosphere, where the smoke could persist for years, if there is no rain to wash it out. This aerosol of particles could block out much of the sun's light from reaching the surface, with this causing surface temperatures to drop drastically, and with that, it is predicted surface air temperatures would be akin to, or colder than, a given region's winter, for years on end. |
Aerosol removal timescale The exact timescale for how long this smoke remains, and thus how severely this smoke affects the climate once it reaches the stratosphere, is dependent on both chemical and physical removal processes. The physical removal mechanisms affecting the timescale of smoke particle removal are how quickly the aerosol particles coagulate, and fall out of the atmosphere via dry deposition, and to a slower degree, the time it takes for solar radiation pressure to move the particles to a lower level in the atmosphere. Whether by coagulation or radiation pressure, once the aerosol of smoke particles are at this lower atmospheric level cloud seeding can begin, permitting precipitation to wash the smoke aerosol out of the atmosphere by the wet deposition mechanism. The chemical processes that affect the removal are dependent on the ability of atmospheric chemistry to oxidize the smoke, via reactions with oxidative species such as ozone and nitrogen oxides, both of which are found at all levels of the atmosphere. Historical data on residence times of aerosols, albeit a different mixture of aerosols, in this case stratospheric sulfur aerosols and volcanic ash, from megavolcano eruptions, appear to be in the 1-2 year time scale. Aerosol atmosphere interactions are still poorly understood. |