Lapse rate describes the decreasing of pressure with height. As a plume of smoke rises, the pressure around it decreases, causing it to expand and cool. As smoke rises and cools as a result of this lapse rate, it can sink to lower levels affect sensitive areas.
This is for the usual tropospheric case where temperature decreases with height requires that pressure decrease with elevation, in agreement with the hydrostatic equation.
Source:Ferguson, Sue A. 2001. Smoke Management Meteorology. A chapter in Smoke Management Guide. by Achtemeier, Gary, Brenner, James D., Core, John E., Ferguson, Sue A., Hardy, Colin C., Hermann, Sharon M., Jackson, Bill, Lahm, Peter, Leenhouts, Bill, Leuschen, Tom, Mutch, Robert E., Ottmar, Roger D., Peterson, Janice L., Reinhardt, Timothy R., Seamon, Paula, and Wade, Dale. A Publication of the National Wildfire Coordinating Group.