A Preliminary Examination of the Injection of Soot into the Stratosphere from Urban Firestorm Simulations

Abstract:

It has been suggested that even a limited nuclear exchange between India and Pakistan would lead to a significant loss of life---on the order of a billion people. This claim is based upon the belief that a large amount of aerosol or soot from burning fires would be injected into the stratosphere eventually leading to significant cooling of the troposphere via nuclear winter. While a large amount of soot will be injected into the atmosphere, the fraction of soot getting into the stratosphere plays a primary role in subsequent climatological impacts and is the basis for this preliminary study. To examine soot transport from an urban firestorm, a computational fluid dynamics (CFD) code in conjunction with a simplified combustion model was utilized in the current study. Note, while simplified, the combustion model is based upon extensive simulations using a more refined, but expensive, combustion model that can simulate the propagation of a fire front through an array of buildings and vegation. The simulations conducted using the CFD model described in the next sections are designed to represent the worst case scenario for an urban firestorm: sufficient winds to propagate the fire but not prevent vertical plume rise; no impact of atmospheric water vapor or moisture within the fuel on damping the fire; buildings are wood and completely burn; soot particles do not have a settling velocity; and a grid resolution that maximizes plume rise at the expense of entrainment (no turbulence model active). Given these assumptions and preliminary analysis of several simulations, the following are a few tentative but important findings: 1) worst-case scenario leads to an over-shooting plume into the lower stratosphere; 2) this over-shoot transports a small portion of the total soot content produced by the urban firestorm into the stratosphere; and 3) departures from this worst-case scenario will lead, in some cases, to a significant reduction in the amount of soot that can be injected into the lower stratosphere. Hence, our preliminary findings suggest that estimates regarding the amount of soot displaced into the lower stratosphere are overstated.