Light absorption dynamics of brown carbon particles during wood combustion and pyrolysis

The light absorption dynamics of brown carbon (BrC) particles emitted during combustion or pyrolysis of pinewood are elucidated here using an integrated thermal incineration platform which enables pyrolysis of wood at controlled conditions. This platform is coupled with a variety of real-time aerosol instrumentation and time-integrated sampling systems. The BrC particles emitted from pinewood combustion contain about 80 % of condensed volatile organic compounds (VOCs), regardless of the O₂ concentration, [O₂]. Removing the condensed VOCs by thermal denuding reveals that BrC nanoparticles from wood pyrolysis ([O₂] = 0 vol%) have a bi-modal size distribution containing 95 % of nanoscale particles with a mean mobility diameter, d_m = 37 nm and 5 % of large particles with mean d_m = 107 nm. Increasing [O₂] from 0 to 20 vol%, increases the fraction of large BrC nanoparticles up to 29 % and decreases their mean d_m to 78 nm. In this regard, the average mass absorption cross-section, MAC, of BrC increases from 0.1 to 0.27 m²/g with increasing [O₂]. This indicates that the light absorption of BrC from wood combustion and pyrolysis is determined by the fraction of large particles with mean d_m = 78–107 nm. The BrC MAC measured here can be interfaced with global climate models to estimate the contribution of particulate emissions from biomass combustors and wildfires to global warming.