The evolution of the physicochemical properties of biomass burning organic aerosols

Biomass burning organic aerosol (BBOA), a major component of wildfire smoke, is an important contributor to air pollution and is predicted to become even more significant as wildfires increase in the future. To accurately model BBOA in the atmosphere and predict its impact on climate and health, we need to know physicochemical properties of BBOA such as viscosity and the number of phases. Furthermore, these properties could change as BBOA ages, but the aging of BBOA is poorly constrained. We show that aqueous extracts of laboratory-generated BBOA can become glassy in conditions like those in the upper troposphere, extending the lifetime of light-absorbing species in BBOA by slowing down their reactions with ozone, thereby increasing BBOA’s positive radiative effect. We then found that water/methanol extracts of field sampled wildfire BBOA in British Columbia were much more viscous than the laboratory-generated BBOA, with viscosities ≥ 10⁸ Pa s up to 50% relative humidity. All of the field sampled BBOA had two separated organic phases. To investigate how time in the atmosphere changes BBOA’s properties, lab-generated BBOA was aged with OH in an oxidative flow reactor, increasing its viscosity by ≥ 100,000 times with 5.2 days worth of OH exposure. The BBOA had two organic phases up to at least 8.6 equivalent days of aging. Evaporating 66% of non-polar BBOA can cause the same viscosity change, displaying the need to consider multiple aging mechanisms when predicting the evolution of BBOA. Finally, we used three types of mass spectrometers to relate viscosity changes from OH aging to the chemical composition, finding that the main chemical change is an increase in the average O/C and number of oxygens per molecule. Also, the right combination of high resolution mass spectrometer and parameterization can predict the viscosity of BBOA within one order of magnitude. Overall, this shows that BBOA has two phases and is often much more viscous than traditionally assumed. These properties can increase the lifetime of compounds in BBOA, affect particle growth rates, and influence how clouds form, which needs to be accounted for to accurately predict BBOA’s impact on health and climate change.