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Reactive uptake of ozone onto thin films of brown carbon and azo dyes
Summary
Researchers used a coated-wall flow-tube to investigate ozone uptake onto biomass burning organic aerosol (BBOA) films from eastern red cedar, finding that 16 hours of UV irradiation increased visible-wavelength absorption (warming effect) while decreasing reactive ozone uptake through oligomerization-induced viscosity increases, with relative humidity effects further demonstrating how phase separation can slow atmospheric oxidation.
Once carbonaceous aerosols are emitted or formed in the atmosphere, they are subjected to aging through irradiation and oxidation. The variable evolution during aging dictates the climate impacts of these species. Using a coated-wall flow-tube, we investigated the uptake of ozone onto thin films of whole, water-soluble, and waterinsoluble polarity-fractionated biomass burning organic aerosol (BBOA), generated from eastern red cedar sapwood. We explored how ultraviolet (UV) irradiation affected the uptake of ozone for both whole and polarity-fractionated films and also how relative humidity (RH) affected it specifically for the polarity fractions. After 16 hours of irradiation, equivalent to three days of irradiation in the atmosphere, the mass absorption coefficient at visible wavelengths of the whole BBOA increased, associated with an increased warming effect on climate. UV irradiation significantly decreased the reactive uptake of ozone for all BBOA films investigated, as a result of oligomerization leading to increased viscosity, associated with an increased lifetime with respect to oxidation. Effects of RH on the polarity-fractionated films highlight how liquid-liquid phase separation to form core-shell particles could also hinder oxidation. Together, these laboratory observations demonstrate important processes dictating the fate and impact of BBOA in the atmosphere. Nano- and microplastics are an emerging contaminant. Like many commercial products, these plastics are typically colored by azo dyes. A crucial first step for understanding their direct effect on climate, through the absorption and scattering of solar radiation, is learning how susceptible they are to oxidation. We demonstrated appreciable uptake of ozone onto three commercial azo dyes. The uptake coefficients were (2.0 ± 0.5) x 10⁻⁷ for sunset yellow, (2.7 ± 0.6) x 10⁻⁷ for amaranth, and (3.2 ± 0.3) x 10⁻⁷ for tartrazine at 80% RH and 100 ppb of ozone. Reactive uptake increased with increasing RH, due to plasticization, and decreased with increasing initial ozone mixing ratio, due to depletion of unreacted dye at the surface. These observations demonstrate that azo dyes in nanoplastics may react with gas-phase ozone, motivating further studies of how the optical properties of nanoplastics change during their residence time in the atmosphere.