Ultraviolet Irradiation Can Increase the Light Absorption and Viscosity of Primary Brown Carbon from Biomass Burning

Abstract

The light absorption of brown carbon (BrC) constituents of biomass burning organic aerosol (BBOA) changes in the atmosphere, in part due to multiphase oxidation. For example, ozonolysis leads to the whitening of primary BrC constituents. Irradiation can also change the properties of BrC. Here, we investigate the interplay between irradiation and multiphase processing by measuring the reactive uptake of ozone to thin films of BBOA before and after exposure to UV radiation in a photoreactor. Thin films were prepared from the lower volatility fraction of BBOA from eastern red cedar, a species associated with wildfires and prescribed fires in the southern Great Plains, United States. Irradiation increased the mass absorption coefficient of the BrC at near-UV and visible wavelengths. It also significantly decreased the reactive uptake of ozone, which was attributed to increased viscosity of the BBOA material. These changes in absorptivity and viscosity are consistent with results of mass spectrometry and volatility tandem differential mobility analysis, which show that high-molecular-weight species constitute a greater fraction of the total mass after irradiation. Our results may have significant implications on the warming effect of BrC, since UV irradiation here both darkens this BBOA material and makes it more resistant to multiphase processing and whitening by ozone under the conditions investigated.

Biomass burning organic aerosol (BBOA) from wildfires has multiple phases and is more viscous than laboratory generated BBOA

“…Current atmospheric models that treat biomass burning organic aerosols typically consider them as single-phased particles. 37,[47][48][49] The results herein show that BBOA, at least in regions where forests primarily consist of pine trees, should be treated as having two separate phases. Depending on the degree of internal/external mixing of organic aerosol in forest fire plumes, these two phases may coexist and be separated within individual particles.…”

“…45,46 Despite the above, models simulating the chemistry, transport, and climatic impacts of wildfire BBOA typically assume that BBOA comprises only a single phase. 37,[47][48][49] Our understanding of the viscosity and phase behaviour of BBOA remains incomplete, in part, because previous studies have focused on samples generated in the laboratory and not collected from the real atmosphere. The composition of BBOA in the real atmosphere may be different than the composition of BBOA in the laboratory for several reasons including atmospheric aging of BBOA, higher degrees of dilution, and burning conditions (e.g.…”

“…These factors could lead to atmospheric BBOA having a very different viscosity than what is assumed based on laboratory results, and phase behaviour may differ as well. 49,50 Particularly, phase separation has been explained as being driven by differences in polarity, approximated by the oxygen-to-carbon ratio (O:C). [51][52][53][54][55][56] Atmospheric aging increases O:C, 58,59 so atmospheric aging of particles could change their phase behaviour.…”

Enhanced Light Absorption and Elevated Viscosity of Atmospheric Brown Carbon through Evaporation of Volatile Components

“…Moreover, the expected increase in mixing the time scales of organic compounds in solid-like particles due to hindered bulk diffusion coefficients affects atmospheric multiphase reaction chemistry . More rigid, viscous aerosols undergo slower photochemical transformations and ozonolysis, , contributing to their chemical inertness as they disperse away from the emission source. This inertness can further amplify their absorbance contribution in radiative forcing processes, making them a crucial factor in understanding and modeling Earth’s climate system.…”

“…For example, high-molecular-weight BrC compounds have been observed to be less susceptible to photodegradation than lower molecular weight compounds. , Another study discovered that photolyzed BrC species featured higher oxidation state values and lower molecular weights, demonstrating photooxidation aging mechanisms during long-range transport . Recent laboratory studies have also reported changes in the composition and reactivity of BrC mixtures resulting from the evaporation of their components. − However, establishing a predictive understanding of BrC’s molecular composition and light absorption properties is yet incomplete, necessitating additional comprehensive studies. Notably, even trace amounts of strongly absorbing chromophores can contribute significantly to the overall optical properties of the bulk BrC material. ,,,,, Therefore, elucidating this intricate relationship requires the application of complementary multimodal analytical techniques that can measure both component-specific and bulk BrC properties.…”