Based on many-year round-the-clock measurements at the Aerosol Station of the IAO SB RAS of the scattering coefficient of the dry base of submicron aerosol and Black Carbon (BC) concentration in particles, the influence of forest fire smokes on the variability of the BC fraction in the near-ground aerosol have been analyzed. Stable regularity has been revealed that BC concentration (1-4.5%) in smokes of remote forest fires is lower than in the background aerosol. This causes a decrease of its values, when smoke mists invade into the atmosphere over the measurement site (P-effect). It was found from the in situ measurements and laboratory experiments that this effect is stable in our region being caused by the decisive contribution of the pyrolysis of forest combustible materials to the formation of the smoke mist particles. It is proposed to take the BC fraction in the submicron aerosol as an information index (P-criterion) for distinguishing the state of the ground atmospheric layer affected by the forest fires far removed from the observation site.