Unless you are far far away (like NZ or Aus) you are better off dying instantly. Even far away, and even if there is no nuclear winter sort of result, the radiation and effects are likely get you as well. Volcanic dust can be pretty widespread (Krakatoa is likely bigger than all the nukes we have) but it's far more localized unlike a general nuclear exchange, plus the dust is likely to be both radioactive and contain all sorts of nasty volatilized chemicals.