Evolution of the absorptivity and hygroscopicity of carbonaceous aerosol particles: From brown carbon to nanoplastics

Aerosol particles influence climate directly, through aerosol-radiation interactions, and indirectly, through aerosol-cloud interactions. The magnitude and direction of the radiative forcing resulting from these combined interactions is uncertain, in large part due to the complex evolution of carbonaceous aerosol, including biomass burning aerosol and, an emerging contaminant, nanoplastics. Here, the evolution in the absorptivity and hygroscopicity – properties governing the direct and indirect effects, respectively – of biomass burning aerosol and nanoplastics was investigated. First, the evolution of representative biomass burning aerosol was determined. Secondary light absorbing organic aerosol, or brown carbon, was generated from the aqueous OH oxidation of phenolic compounds, including phenol, catechol, and pyrogallol – abundant emissions of biomass burning through the thermal decomposition of lignin – and its light absorption and water uptake were monitored during aging using UV-vis spectroscopy and hygroscopic tandem differential mobility analysis. No systematic trend in hygroscopicity with functionalization of the precursor was observed. Furthermore, hygroscopicity was steady with further aging, while absorptivity ultimately decreased, suggesting broadly that the radiative forcing of brown carbon may change more through the direct effect than the indirect effect. Next, the evolution of representative nanoplastics was determined. Initially 100, 200, and 250 nm nanoplastic aerosol was generated from suspensions of polystyrene size standards and coated with secondary organic aerosol from the ozonolysis of α-pinene in a smog chamber. At low relative humidity, when the organic aerosol was viscous, non-spherical, partially-engulfed morphology was observed, and these coated particles were hygroscopic. Finally, nanoplastics were generated by passing filaments of different colors and materials through the heated nozzle of a 3D printer, collected on fiber filters, and exposed to simulated sunlight as well as ozone. The absorptivity of all samples decreased during 5 d of aging through irradiation or oxidation, but the extent of this whitening varied significantly. After 5 d, the absorptivity of each representative nanoplastic may be taken to decrease by at least 50% through these combined processes. The variability in this whitening of nanoplastics further highlights the complexity of representing carbonaceous aerosol in chemical transport models and motivates future work to quantify the evolution of this emerging contaminant.