Heterogeneous Reaction of NO2 on Soot Surfaces and the Effect of Soot Aging on its Reactivity Leading to HONO Formation

Soot aerosols are known to be an important atmospheric constituent. The physical and chemical properties of soot allows it to act as a precursor of gas-surface heterogeneous reactions, providing active sites for the reduction and oxidation of trace species in the atmosphere, potentially affecting atmospheric composition. In this work the heterogeneous reaction of NO2 on soot leading to nitrous acid (HONO) formation was studied through a series of kinetic uptake experiments and HONO yield measurements. The soot was collected from a diffusion flame using propane and kerosene fuels using two different methods. A low pressure fast-flow reactor coupled to a Chemical Ionization Mass Spectrometer (CIMS) was used to monitor NO2 and HONO signals evolution using atmospheric-level NO2 concentration. HONO yields up to 100 percent were measured and NO2 uptake coefficients varying from 5.6x10-6 to 1.6x10-4 were obtained. Heating soot samples before exposure to NO2 increased HONO yield and the NO2 uptake coefficient on soot due to the removal of the organic fraction from the soot backbone unblocking active sites, which become accessible for the heterogeneous reaction. From the kinetic uptake curves and the effect observed in the HONO yield and NO2 uptake coefficient measurements by heating the soot samples, our results support a complex oxidation-reduction mechanism of reaction. This heterogeneous reaction mechanism involves a combination of competitive adsorptive and reductive centers on soot surface where NO2 is converted into HONO, and the presence of processes on soot where HONO can be decomposed producing other products. Atmospheric soot "aging" effect on the reactivity of soot toward NO2 and HONO yield was studied by coating the soot surface with glutaric acid, succinic acid, sulfuric acid, and pyrene. Glutaric and succinic acid increased both HONO yield and the NO2 uptake coefficients, while sulfuric acid decreased both. However, pyrene did not show any particular trend.