Evolution of organic carbon in the laboratory oxidation of biomass-burningemissions

Biomass burning (BB) is a major source of reactive organic carboninto the atmosphere. Once in the atmosphere, these organic BB emissions, inboth the gas and particle phases, are subject to atmospheric oxidation,though the nature and impact of the chemical transformations are notcurrently well constrained. Here we describe experiments carried out as partof the FIREX FireLab campaign, in which smoke from the combustion of fuelstypical of the western United States was sampled into an environmental chamber andexposed to high concentrations of OH, to simulate the equivalent of up to2 d of atmospheric oxidation. The evolution of the organic mixture wasmonitored using three real-time time-of-flight mass spectrometricinstruments (a proton transfer reaction mass spectrometer, an iodidechemical ionization mass spectrometer, and an aerosol mass spectrometer),providing measurements of both individual species and ensemble properties ofthe mixture. The combined measurements from these instruments achieve areasonable degree of carbon closure (within 15 %-35 %), indicating that mostof the reactive organic carbon is measured by these instruments. Consistentwith our previous studies of the oxidation of individual organic species,atmospheric oxidation of the complex organic mixture leads to the formationof species that on average are smaller and more oxidized than those in theunoxidized emissions. In addition, the comparison of mass spectra from thedifferent fuels indicates that the oxidative evolution of BB emissionsproceeds largely independent of fuel type, with different fresh smokemixtures ultimately converging into a common, aged distribution of gas-phasecompounds. This distribution is characterized by high concentrations ofseveral small, volatile oxygenates, formed from fragmentation reactions, aswell as a complex pool of many minor oxidized species and secondary organicaerosol, likely formed via functionalization processes.

Automated summary

Biomass burning is a significant source of reactive organic carbon in the atmosphere. Experiments conducted as part of the FIREX FireLab campaign reveal that the atmospheric oxidation of organic carbon from biomass burning leads to the formation of smaller and more oxidized species. The oxidative evolution of biomass burning emissions appears to be independent of fuel type, resulting in a common aged distribution of gas-phase compounds consisting of small volatile oxygenates and various minor oxidized species and secondary organic aerosols.