Relative Content of Black Carbon in Submicron Aerosol as a Sign of the Effect of Forest Fire Smokes
| Panchenko, Mikhail | Institute of Atmospheric Optics |
| Kozlov, Valerii | Institute of Atmospheric optics |
The events of biomass burning are often observed in the regions containing vast forest tracts and peatbog fires. These processes lead to essential transformation of the optical properties and the aerosol composition of air cause by powerful emission of aerosol and BC (soot). Search and justification of the informative aerosol signs providing for revealing and selection of the effects of forest and peatbog fire smokes far from the site of observation is of interest.
The 8-year series of the data on the scattering coefficient of the dry matter of aerosol particles at the wavelength of 0.52 micron (nephelometer) and the mass concentration of BC (aethalometer) is obtained at the aerosol monitoring station of IAO. The peculiarities of the variability of the relative content of soot in aerosol particles (P=Ms/Ma, where Ma is the mass concentration of submicron aerosol, and Ms is the mass concentration of soot) are analyzed in the paper. It is also necessary for estimation of the aerosol single scattering albedo. Main attention was paid for the study of the possibility of the use of this parameter as an informative sign for estimation of the effect of forest fire smokes on the near-ground aerosol composition.
On the whole, individual values vary in the wide range from 0.5 to 15-20%. Daily mean values P in warm season under conditions of the clean atmosphere are about 5-10% at the concentrations of aerosol and soot, in average, 10-20 micrograms per cubic meter and 1-2 micrograms per cubic meter, respectively. Coming of air masses enriched with smoke to the region of observation leads to the significant increase of the aerosol and soot concentrations. Daily mean concentrations of dry submicron aerosol and soot in smokes can reach 450 micrograms per cubic meter (aerosol) and 10 micrograms per cubic meter (soot). The analysis has shown that the aerosol content in smoke haze increases in average by 10 times, but the increase of the concentration of soot is in average only 3 times. As a result, the decrease of the relative content of soot in particles is observed under the effect of smoke. The values P at moderate smoke can decrease to 3-4%, while in dense smoke haze (with meteorological visibility range lower than 1-2 km observed in October 1997) it decrease to 1-1.5%. Evidently, the obtained low estimate corresponds to the “true” value P in a dense smoke plume.
The obtained results are evidence of unambiguity of the effect of smokes on the dynamics of P and make it possible to recommend this parameter as an informative sign of revealing and selection of forest fires far from the site of observation. As the estimates show, the proposed informative sign is in agreement with the satellite data, and is now used when forming the arrays of data free of the effect of forest and peatbog fires.
We explain the revealed effect of the decrease of the relative content of soot by the fact that the process of pyrolysis of forest combustible materials makes the principal contribution into the aerosol mass at formation forest fire smokes. Investigations carried out earlier in the big aerosol chamber showed that weakly absorbing resinous particles are generated at the pyrolysis of wood, while strongly absorbing soot particles are produced at combustion with flame. This result is caused by different degree of oxidation of gaseous products at different regimes of burning.
The work was supported in part by Russian Foundation for Basic Research (grant No. 03–05–64787).
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