102 Cardwell Hall
Combustion-generated particulate matter containing black carbon (BC) and brown carbon is a major component of air pollution with known health risks and adverse environmental impacts. Notably, polycyclic aromatic hydrocarbons (PAHs) found in them and associated with carcinogenic and mutagenic effects.
The structural properties of generated aerosols shed insight into the formation of soot aggregates from large young particles. The consistent generation of PM samples with repeatable nanostructure and chemical composition is achieved in a laminar diffusion inverted gravity flame reactor (IGFR). Here, the Fuel dilution by Ar as well as reagent preheat, is used to control the flame temperature. The colder flames result in lower PM yields; however, their organic carbon and PM PAH content increases significantly. Temperature thresholds for PM transition from low to high organic carbon content were characterized based on the maximum flame temperature and highest soot luminosity region temperature. Low-temperature flames produce relatively large 100nm-300nm particles with isotropic or multiple core structures, which may support a soot maturation pathway where young particle evolves into a mature fractal aggregate via an internal nucleation route. During the process, these large amorphous particles can form internal voids as the particle loses mass due to pyrolysis or oxidation. Transmission electron microscopy (TEM) shows that young soot aggregates contain a higher fraction of shorter fringes and highly curved aromatics, which agrees with their higher organic carbon content. Increasing the flame temperature reduces the curvature of polycyclic aromatic hydrocarbons (PAHs) and allows for more efficient layer stacking, as indicated by a higher percent of stacked fringes. For these gaseous fuels, carbonization appears to be primarily a function of the flame temperature and independent of the fuel composition.
The colder flames produce PM with a high concentration of higher molecular weight (MW>226), which are not present in the flames that reach soot maturation threshold temperatures. Though we characterized the temperature effect in simple laminar diffusion flames, generally, the curved PAHs that form young soot particles will transition to graphitic structures forming mature soot aggregates, i.e., black carbon. Principal component regression of the PAH data in IGFR samples correlates to GCMS data from wood smoke and diesel exhaust. The lower temperature samples also exhibit a brown color, which is characterized by high OC fraction, often found in low-temperature flames, such as biomass combustion due to the lower heating value of the fuel (high moisture content) and the complex (endothermic) surface chemistry reaction. The increase in particle structure is the key to a reduction in particle reactivity, changes in particle optical properties, and reduction in the active sites on the soot surface. The current work can aid modeling efforts in soot formation, guide the design of combustion systems, and estimate environmental and health effects based on the reactivity of combustion generated particulate matter.