This RAPID project involves performing online single-particle mass spectrometry (SP-MS) measurements of the chemical composition and mixing state of biomass particles during the fourth Fire Lab At Missoula Experiment (FLAME IV). FLAME IV affords a unique and important opportunity to better understand the evolution of biomass smoke more completely than is possible in the Carnegie-Mellon University (CMU) laboratory, by taking advantage of the collaborative simultaneous measurements conducted by a wide range of scientists. The SP-MS will determine how the aerosol mixing state evolves as the biomass smoke is photochemically aged in a novel twin environmental chamber system. The chambers will be filled with combustion smoke generated from a variety of globally relevant biomass fuels, and diluted to atmospheric concentrations. Atmospheric photochemistry will be simulated in the chambers for several hours using photo-simulator lights and the optional addition of oxidants such as hydroxyl radical and nitrogen oxides. The twin chamber design will provide an unperturbed control aerosol system to which aerosols in the perturbed chamber can be compared at any time during the multi-hour long experiments. Through tandem experiments with FLAME IV participants from Colorado State University, the SP-MS will also determine the chemical composition of ice nuclei present in fresh and aged biomass burning smoke. A novel suite of gas and aerosol instruments will be used to thoroughly characterize the biomass aerosol prior to, during, and after simulated photochemistry. The instrument package includes a High-Resolution Time-Of-Flight Aerosol Mass Spectrometer, Proton-Transfer-Reaction Mass Spectrometer, and a new single-particle mass spectrometer. This will be the first deployment of this new Laser Ablation Aerosol Particle Time-Of-Flight (LAAP-TOF) mass spectrometer in any collaborative experimental or field campaign. The LAAP-TOF will allow better understanding to be gained of the variations in individual particle composition and mixing state for the different fuels tested, how this mixing state evolves during photo-oxidation, and what role this mixing state might play in the partitioning of organic carbon between the gas and particle phases.
The research cuts across multiple disciplines including chemistry, combustion, air quality engineering, atmospheric science, and climate change. Thus, this research will have immediate impacts on a wide range of interdisciplinary fields including forest management, atmospheric chemistry, cloud microphysics, and biogeochemistry. The project will compose a significant portion of at least two graduate student Ph.D. dissertations, and the research of one post-doctoral associate. The collaborative and multi-disciplinary nature of the FLAME IV experiment will provide our participants with the unique opportunity to work alongside a wider range of scientists than otherwise possible during their research at CMU.
The burning of wood (biomass burning) for residential heating and cooking, or during forest fires, is a major global source of gaseous and particulate pollutants in the atmosphere. These emissions impair health, reduce visibility, and alter the planet's climate, in ways we only poorly understand. We studied the emissions from the burning of numerous different types of wood during the FLAME-4 campaign at the Missoula Fire Sciences Lab to better understand the impact of biomass burning on the environment. We simulated the atmospheric aging that wood smoke experiences during transport through the atmosphere, by exposing the smoke to different oxidants and to artificial sunlight in dual smog chambers. In some cases this simulated photochemical aging resulted in the formation of a significant amount of additional particulate matter over was present into the original smoke. In other experiments this aging caused the total amount of particulate matter to decrease. This variation is attributed to differences between the type of wood fuel burnt, and the conditions it was burned with (e.g. hot flaming burn versus less hot smoldering burn). It is important to understand how the smoke evolves through chemical aging in the atmosphere to properly assess the full impact that biomass burning has on air pollution and its environmental impacts. Biomass burning, along with other types of combustion, emits light-absorbing black carbon (or soot) particles into the atmosphere, along with non-absorbing organic carbon and inorganic compounds. Light-absorbing particles such as black carbon are important because they absorb incoming solar radiation and cause the planet to warm, offsetting the overall cooling effect caused by other sources of particulate matter that tends to reflect sunlight, and increase the brightness of clouds. We found that biomass burning smoke also exhibits a significant amount of light absorption that is caused not by black carbon, but by brown carbon. Brown carbon is a chemically undefined organic carbon substance that absorbs light strongly at the shorter visible and ultraviolet wavelengths, and does not have the graphitic chemical structure of black carbon. In smoke produced from a large number of different fuels, the amount of light-absorbing brown carbon correlated well with the amount of black carbon present. This suggests that black and brown carbon have similar origins or precursors, such as aromatic compounds that become fused together during combustion to produce graphitic soot. The additional light absorption caused by brown carbon must be accounted for to properly assess the contributions biomass burning makes to radiative forcing and climate change. It is important to distinguish between light absorption by brown versus black carbon as they have very different light absorption characteristics, as well as different physical and chemical properties and resulting behavior in the atmosphere. The ability to accurately predict the amount of brown carbon from the more easily measured black carbon that we have reported for the first time here will allow us to more easily assess the impact that biomass burning makes to light absorption and atmospheric warming. These are two important but poorly constrained aspects of how anthropogenic activities are causing global climate change.
