Impact of Biomass Burning on Aerosol Size Distribution, Aerosol Optical Properties and Associated Radiative Forcing

The influence of biomass burning on aerosol size distributions, particle number and radiative forcing has been studied at a rural site in Spain. It has been found that air contaminated by aerosols from biomass burning presents four times the total number of particles registered in non-contaminated air. In the case of the smallest fraction of the fine mode, between 0.1 and 0.2 mu m, the increase soars to over seven times the total number of particles. An analysis of the evolution of the count mean diameter in the fine mode (CMDf) in the 8 daily measurements has revealed a decrease of over 25% in this parameter in the modified measurements when compared with measurements that were not contaminated by aerosols from biomass burning. In contrast, when the aerosol transport time is long, the increases detected in CMDf range between 15% and 100% when compared with measurements of air by non-aged aerosol from biomass burning. Shortwave radiative forcings have been calculated for these high loads of fine aerosols with GAME (Global Atmospheric Model) software. For the August event, the daytime average of surface radiative forcing is -66 (+/- 30) W/m(2), and at the top of the atmosphere the forcing is -32 (+/- 12) W/m(2). Induced daytime average of atmospheric radiative forcing reaches 34 (+/- 20) W/m(2). The study demonstrates that wildfires affect not only the number of particles and the size distribution, causing a clear increase in the number of aerosols in the atmosphere, but they are also responsible for altering the local radiative balance.