Rapid Nighttime Darkening of Biomass Burning Brown Carbon by Nitrate Radicals Is Suppressed by Prior Daytime Photochemical Aging

Brown carbon (BrC) carbonaceous aerosol affects climate through its ability to absorb light. Here, we investigate in an environmental chamber the changes to the optical properties of water-soluble biomass burning organic aerosol (BBOA) particles that arise via exposure to gas-phase NO₃ radicals, as occurs at night in the atmosphere. Low mixing ratios (1–2 ppt) of NO₃ lead to absorption enhancement by a factor of 2 at 375 nm via extremely rapid processing, on the time scale of 15 min, with the aging occurring faster and more extensively at a lower relative humidity (10 ± 3%) than at higher values (50 ± 10%). Prior daytime aging processes of the BBOA material lead to suppressed absorption enhancement at 375 nm by subsequent NO₃ oxidation. In particular, samples with 2 h of aqueous OH radical oxidation or 3 h of ultraviolet (UV) light exposure displayed a decrease of 52 and 32% of absorption at 375 nm, respectively, relative to no prior aging, indicating competitive mechanisms and common reactive entities within the BBOA. Even longer prior aqueous OH exposure largely removed the NO₃ absorption enhancement. Lastly, UV exposure after NO₃ aging led to absorption photoenhancement from 375 to 625 nm but at a slower rate than without prior NO₃ exposure. These results point to strong diurnal effects in the optical aging of BBOA particles, with the darkening that will rapidly occur via nighttime NO₃ exposure strongly modulated by prior photochemical processing in the preceding daytime.