Sherwood (2002) provides evidence for the hypothesis that observed doubling of stratospheric water vapor since 1950 in part may be due to aerosol from tropical biomass burning. Excessive amounts of atmospheric particles due to biomass burning lead to reduced ice-crystal size, since more particles are competing for the same amount of vapor for condensation. These smaller ice crystals are carried to the stratosphere in towering tropical cumulus clouds where they lead to moistening.
Reference
Sherwood, S., 2002: A microphysical connection among biomass burning, cumulus clouds, and stratospheric moisture. Science, 295, 1272-1275.