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

CAROLYN LIU-KANG, Laura-Helena Rivellini, Jonathan Abbatt, University of Toronto

     Abstract Number: 205
     Working Group: Carbonaceous Aerosols

Abstract
Brown carbon (BrC) is a type of carbonaceous aerosol, primarily emitted from biomass burning sources such as wildfires. These aerosol particles have significant implications on climate by their ability to absorb light, potentially contributing to warming effects. Due to the expected increase of wildfire activity in the upcoming years, this source of particulate emission is also expected to increase. Oxidative and photochemical aging, however, can change brown carbon’s light absorption properties as well as its chemical composition. Therefore, significant uncertainties remain when estimating BrC’s contribution to climate effects.

Here, we focused on nighttime oxidative aging by NO₃ radicals of water-soluble wood smoke particles. We also investigated how diurnal effects, such as daytime aging by OH radical oxidation and UV light exposure, can affect the extent of subsequent NO₃ oxidation. We generated in-lab wood smoke by smoldering pine wood, before aerosolizing the aqueous extract into a chamber. Online monitoring of the changes in the optical properties, chemical composition, and size distributions was done with, respectively, an aethalometer, an aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS). Our results indicate that NO₃ radical oxidation leads to rapid absorption enhancement of our BrC aerosols in both the UV and visible with very low NO₃ mixing ratios. In addition, prior aging by daytime processes inhibits the extent of this nighttime aging process, indicating competitive mechanisms and common reactive entities within the BrC material.