AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Evolution of the Light-absorption Properties of Brown Polycyclic Aromatic Hydrocarbons Due to Reaction with Nitrate Radicals
ZEZHEN CHENG, Khairallah Atwi, Anita Avery, Manjula Canagaratna, Philip Croteau, Edward Fortner, Jordan Krechmer, Francesca Majluf, Leah Williams, Zhenhong Yu, Douglas Worsnop, Andrew Lambe, Rawad Saleh, University of Georgia
Abstract Number: 510 Working Group: Carbonaceous Aerosol
Abstract We have previously shown that brown carbon (BrC) in combustion emissions exhibits a continuum of light-absorption properties that depend on combustion conditions and that polycyclic aromatic hydrocarbons (PAHs) constitute an important fraction of this BrC. Here, we investigate the evolution of these brown PAHs following oxidation by nitrate radicals (NO3).
We conducted toluene combustion experiments controlled at different conditions to produce PAHs with variable light-absorption properties. We exposed the PAHs to NO3 in an oxidation flow reactor, where the NO3 exposure varied from 0 to ~5×1013 molec cm-3 sec, equivalent to approximately 24 hours of atmospheric exposure. We performed measurements of the chemical composition, size distributions, and absorption coefficients of the fresh and aged aerosols and retrieved the imaginary part of the refractive indices (k) from the light-absorption and size distribution measurements.
Starting with light PAHs (k at 550 nm, k550 = 0.007), exposure to NO3 led to significant darkening (50% increase in k550) of the oxidation products at maximum exposure. However, exposure of dark PAHs (k550 = 0.08) to NO3 led to negligible change in light-absorption properties. This discrepancy may be attributed to two competing effects: 1) formation of oxidation products that lead to an increase in k550 (e.g., nitro-PAHs), and 2) formation of oxidation products that are less absorbing than the original compounds and therefore reduce the effective k550 of the oxidized aerosol as has been observed with OH oxidation. It appears that effect (1) is more prominent for the light PAHs, while the two effects have similar importance for the dark PAHs.
These results indicate that NO3-induced oxidation darkens some types of atmospheric BrC, which can counterbalance bleaching effects induced by OH oxidation. Furthermore, the light-absorption properties of different BrC components evolve differently in the atmosphere.