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Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC’s radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally overestimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial overestimation in absorption by the total particle population, with greater heterogeneity associated with larger model–measurement differences. We show that accounting for these two effects—variability in per-particle composition and deviations from the core-shell approximation—reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Furthermore, our consistent model framework provides a path forward for improving predictions of BC’s radiative effect on climate. 

Knowledge of the viscosity of particles containing secondary organic material (SOM) is useful for predicting reaction rates and diffusion in SOM particles. In this study we investigate the viscosity of SOM particles as a function of relative humidity and SOM particle mass concentration, during SOM synthesis. The SOM was generated via the ozonolysis of α-pinene at < 5 % relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (τ_{exp, flow}) of SOM after poking the material with a needle. In the first set of experiments, we show that τ_{exp, flow} increased by a factor of 3600 as the RH increased from < 0.5 RH to 50 % RH, for SOM with a production mass concentration of 121 µg m⁻³. Based on simulations, the viscosities of the particles were between 6  ×  10⁵ and 5  ×  10⁷ Pa s at < 0.5 % RH and between 3  ×  10² and 9  ×  10³ Pa s at 50 % RH. In the second set of experiments we show that under dry conditions τ_{exp, flow} decreased by a factor of 45 as the production mass concentration increased from 121 to 14 000 µg m⁻³. From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14 000 µg m⁻³ was determined to be between 4  ×  10⁴ and 1.5  ×  10⁶ Pa s compared to between 6  ×  10⁵ and 5  ×  10⁷ Pa s for SOM with a production mass concentration of 121 µg m⁻³. The results can be rationalized by a dependence of the chemical composition of SOM on production conditions. These results emphasize the shifting characteristics of SOM, not just with RH and precursor type, but also with the production conditions, and suggest that production mass concentration and the RH at which the viscosity was determined should be considered both when comparing laboratory results and when extrapolating these results to the atmosphere.