Atmospheric Aging of Biomass-Burning Aerosol: Oxidants, Oxidation Products, and Measurement Methods

We put forth in this work a multifaceted approach aimed at reducing the uncertainty regarding the effect of biomass-burning emissions on photochemical smog production and organic aerosol formation as a result of aging, includingphotochemical reactions and reaction with nocturnal oxidants. First, we simulated aging of authentic biomass-burning emissions from known fuels using a pair of Teflonsmog chambers. We measured the evolution of the organic aerosol within the chambers as they aged, and calculated the amount of new secondary organic aerosolthat was formed. The production of secondary organic aerosol could be largely predicted using extensive characterization of the major known gas-phase semivolatileorganic compounds also emitted by the burning, using two-dimensional gas chromatography and proton transfer reaction mass spectrometry. We also determined that inorganic compounds emitted in the biomassburningplume can have a significant effect on the atmospheric oxidant budget. Important nocturnal reservoirs of nitrogen oxides – nitryl chloride and dinitrogen pentoxide – were observed for the first time to form readily in the presence ofbiomass-burning emissions in dark smog chamber experiments. A positive correlation was observed between smoke emissions with high particulate chloridecontent and the proclivity to form nitryl chloride vapor. Acid displacement of hydrochloride acid gas by nitric acid proved to be an effective means of reducing particulate chloride, although the absorbed nitric acid did not appear to prevent the formation of nitryl chloride by the particulate chloride that remained in the particle phase. Lastly, we performed characterization experiments on two particle massspectrometers with the end goal of improving the accuracy of particle composition measurements using complex mixed aerosols generated in the lab, as proxies forrealistic atmospheric particles. We revealed that the soot particle aerosol mass spectrometer (SP-AMS) has a variable response factor to two important inert tracerspotassium and black carbon, which changes by as much as a factor of three as the particles grow in size. The laser desorption/ionization single particle mass spectrometer (LAAPTOF) proved to have reduced sensitivity to particles less than 160 nm in diameter, due to particle beam divergence prior to reaching the laser ablation region in the mass spectrometer. During smog chamber experiments thatcoated authentic biomass-burning emissions with α-pinene derived secondary organic aerosol, we measured a positive correlation between organic ion signal from the LAAPTOF and secondary organic mass per particle. This shows that despite the challenges associated with quantitative mass measurements from an instrument such as the LAAPTOF, the fundamental relationship between detected signal and analyse mass exists even for a complex sample like biomass-burning particles.