Evolution of the light-absorption properties of combustion brown carbon aerosols following reaction with nitrate radicals
Polycyclic aromatic hydrocarbons (PAHs) are important constituents of brown carbon (BrC) that are subject to atmospheric processing by gas-phase oxidants such as the hydroxyl radical (OH) and the nitrate radical (NO3). While OH oxidation of BrC has been investigated extensively, studies of NO3 oxidation are limited. Here, we generated BrC from the combustion of toluene containing a complex mixture of light-absorbing PAHs and investigated changes in their chemical composition and light-absorption properties following exposure to NO3 in an oxidation flow reactor. Three types of BrC were studied, with varying light-absorption properties that were classified in terms of the imaginary part of the refractive index (k) as: light BrC (k at 532 nm, k532 = 0.008), medium BrC (k532 = 0.026), and dark BrC (k532 = 0.091). Exposure to NO3 led to ∼30% increase in k532 of the light and medium BrC and ∼5% decrease in k532 of the dark BrC. This discrepancy is attributed to two competing effects: 1) addition of chromophoric functional groups by NO3-induced heterogeneous oxidation and 2) condensation of gas-phase PAH + NO3 oxidation products that were less-absorbing than the particulate PAHs. Analysis of the aerosol chemical evolution revealed that effect (2) was more important in the dark BrC experiments. We performed optical calculations to isolate effect (1) and showed that heterogeneous oxidation led to ∼50% increase in k532 for all the BrC types. These results indicate that NO3-induced heterogeneous oxidation darkens some types of atmospheric BrC, which can counterbalance bleaching effects induced by OH oxidation.
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