Mass fraction of black carbon in submicron aerosol as an indicator of influence of smoke from remote forest fires in Siberia

Based on round-the-clock measurements in 1997–2005 in West Siberia at the Aerosol Station (Tomsk) of the directed scattering coefficient of the dry matter of submicron aerosol and black carbon (BC) mass concentration in particles, the influence of forest fire smoke on the variability of the BC mass fraction P in the near-ground aerosol has been analyzed. Stable regularity has revealed that BC fraction (1–4.5%) in smoke of remote forest fires is lower than in the background aerosol, i.e. they are more weakly absorbing. This causes a decrease of P values, when smoke plumes 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 (smoldering) of forest combustible materials to the formation of the smoke plume particles. The intrusion of forest fire smoke in the region of observation leads to an increase of the single-scattering albedo of dry submicron aerosol above 0.95. It means that the radiative-climatic effect of forest fire smoke is cooling. It is proposed to take the BC mass fraction in the dry submicron aerosol as an information index (P-criterion) for distinguishing the states of the near-ground air layer affected by the forest fires far removed from the observation site.