The model would need to account for at least two different sets of physical properties exhibited by two or more different chemicals, instead of just one set of properties for a single chemical. It would need to adjust for changes over time in the proportion of each chemical in the mixture whenever mixture components differing in volatility escape at different rates during a release. When some chemicals are accidentally released, either as mixtures or alone, they can react together or with other chemicals in the environment. Reactions can result in the formation of completely new chemicals. In such cases, the chemicals dispersing downwind may be completely different from the chemicals that originally escaped from a container.
Reactions can also change the temperature of the escaping pollutant.
At Richmond, a variety of chemical reactions may have occurred. An especially important one would have been the reaction of sulfur trioxide with water to form sulfuric acid. When sulfur trioxide within a vapor cloud reacts with water in the atmosphere, the reactions can heat up the cloud and cause it to be more buoyant than we would otherwise expect. That's what may have happened during the Richmond incident. People who saw the cloud as it first formed reported that it began to roll along the ground like a heavy gas.