Which atmospheric processing of biomass burning organic aerosol produces the most singlet oxygen: photochemical aging or dilution?

Atmospheric organic aerosols containing chromophores undergo excitation upon absorbing visible and UV light. These chromophores are integral components of brown carbon (BrC), predominantly originating from incomplete combustion sources such as forest fires, biomass burning, and cooking. When exposed to light BrC can act as photo-sensitizers, generating reactive oxygen species (ROS), including singlet oxygen (¹O₂). ¹O₂ is a competitive ROS species within atmospheric aerosols and can be produced in diverse environmental matrices, including cloud water, fog water, rainwater, and particulate matter extracts. However, the relative sources and sinks of ¹O₂ depend on the chromophores present in the biomass burning organic aerosols (BBOA), and the chemical composition of these chromophores evolves during atmospheric processing with unknown implications for ¹O₂ production. This study aims to quantify the impact of atmospheric aging, including dilution and photochemical processing, on the ability of BBOA to sensitize ¹O₂.

To assess this goal, we combusted different biomass samples, i.e. straw, cow dung, beechwood, and plastic in the PSI smog chamber. On quartz filters we collected the primary organic aerosols generated after the burning of each sample from a holding tank. Next, we aged the BBOA via two different pathways: UV aging and dilution. First, BBOA was collected after photooxidation treatment using a Potential Aerosol Mass (PAM) chamber. Second, the BBOA was passed through a heated diffusion dryer to simulate dilution of the plume. Our method involves extracting filters with acetonitrile to obtain the non-soluble fraction of BBOA, in which we expect to find the most effective sensitizers for ¹O₂. In these solvent extracts, we added furfuryl alcohol as a ¹O₂ probe and exposed the extracts to UVA light in a photochemical reactor. We measured the pseudo-first order kinetics of ¹O₂ to calculate singlet oxygen quantum yield and steady-state concentration.

The results show an increase in quantum yield when the photooxidation process occurs, i.e. of about 3% for beechwood samples. This outcome suggests that aged chromophores through indirect photochemistry are more effective sensitizers. Remarkably, we observed a decrease in quantum yield of BBOA due to dilution, of about 1% and 3% for straw and beechwood respectively. This result might imply that the most effective chromophores are volatile and partitioning to the gas phase during dilution, with important implications for evolving BBOA plumes. When changing the burning fuel, this trend always appeared showing a possible change in the quantity and quality of the chromophores present. Moreover, ¹O₂ quantum yield and steady-state concentrations differ within the type of fuel, such as beechwood showing higher values compared to straw, highlighting the importance of analyzing different biomass burning organic materials. Our experimental results offer insights into how different atmospheric processing can impact the production of ¹O₂, useful for the development of a global model that encompasses both chromophores and ¹O₂ production.