The Characterization of Functional Groups at the Interface of Laboratory Flame Soot and sampled Combustion Nanoparticle

International audienceThe combustion conditions determine to a large extent the bulk and interfacial properties of the resulting combustion aerosol (soot). The complex chemical and physical nature of combustion aerosol in general precludes the complete chemical characterization of both bulk and interfacial components using analytical chemistry. Our vision is that the bulk composition reveals the flame conditions whereas the interface reflects aging and controls the interaction with solid surfaces (adhesion to supports) as well as with biological membranes (health effects, oxidative stress induced by particles, etc.). We have therefore developed a soft interrogation technique of the interfacial functional groups using heterogeneous chemistry of probe gases of differing reactivity. This procedure represents a minor perturbation of the aerosol particle compared to bulk analysis.In order to chemically characterize the surface of aerosol particles in terms of functional groups present on the particulate surface up to six probe gases (N((CH3)3), NH2OH, CF3COOH, HCl, NO2, O3) were selected to probe combustion aerosol (amorphous carbon, flame soot, Diesel engine soot) and sampled aerosol from several bus depots using heterogeneous reactions. Briefly, the method rests upon measuring the rate of uptake of a probe gas under molecular flow conditions in the presence of deposited aerosol particles. The gas uptake to saturation of the probes was measured in a Knudsen flow reactor and expressed as a density of surface functional groups on a particular aerosol whose size distribution was measured. The probe gases taken up by the sample addressed acidic (carboxylic) and basic (oxides such as pyrones and N-heterocycles) sites, carbonyl (R1-C(O)-R2) and oxidizable (olefinic, certain -OH) groups. The used probes were able to characterize the surface of the particles in terms of the oxidation state (oxidized, partially oxidized, oxidizable or reduced) and olefinic or surface-adsorbed, therefore reactive, polycyclic aromatic compounds. The limit of detection was generally well below 1% of a formal molecular monolayer. With few exceptions most investigated aerosol samples and nanometric materials (soot) interacted with all probe gases which points to the coexistence of different functional groups on the same aerosol surface such as acidic and basic groups. Generally, there were significant differences in surface group density: Printex 60 amorphous carbon had the lowest density of surface functional groups throughout, whereas Diesel soot recovered from a Diesel particulate filter had the largest. The presence of basic oxides on carbonaceous aerosol particles was inferred from the ratio of uptakes of CF3COOH and HCl owing to the larger stability of the acetate compared to the chloride counterion in the resulting pyrylium salt. Both soots generated from a rich and a lean hexane diffusion flame had a large density of oxidizable groups similar to amorphous carbon FS 101