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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. 

Abstract. Photoacoustic spectroscopy (PAS) has become a popular technique for measuringabsorption of light by atmospheric aerosols in both the laboratory andfield campaigns. It has low detection limits, measures suspended aerosols,and is insensitive to scattering. But PAS requires rigorous calibration to beapplied quantitatively. Often, a PAS instrument is either filled with a gasof known concentration and absorption cross section, such that the absorptionin the cell can be calculated from the product of the two, or the absorptionis measured independently with a technique such as cavity ring-downspectroscopy. Then, the PAS signal can be regressed upon the known absorptionto determine a calibration slope that reflects the sensitivity constant ofthe cell and microphone. Ozone has been used for calibrating PAS instrumentsdue to its well-known UV–visible absorption spectrum and the ease with whichit can be generated. However, it is known to photodissociate up toapproximately 1120nm via the O₃ + $h\mathit{\nu }(\>\mathrm{1.1}\mathrm{eV})\to {\mathrm{O}}_{\mathrm{2}}{(}^{\mathrm{3}}{\mathrm{\Sigma }}_g^{-})$ + O(³P) pathway, which is likely tolead to inaccuracies in aerosol measurements. Two recent studies haveinvestigated the use of O₃ for PAS calibration but have reachedseemingly contradictory conclusions with one finding that it results in asensitivity that is a factor of 2 low and the other concluding that it isaccurate. The present work is meant to add to this discussion by exploringthe extent to which O₃ photodissociates in the PAS cell and the rolethat the identity of the bath gas plays in determining the PAS sensitivity.We find a 5% loss in PAS signal attributable to photodissociation at 532nmin N₂ but no loss in a 5% mixture of O₂ in N₂.Furthermore, we discovered a dramatic increase of more than a factor of 2in the PAS sensitivity as we increased the O₂ fraction in the bathgas, which reached an asymptote near 100% O₂ that nearly matched thesensitivity measured with both NO₂ and nigrosin particles. Weinterpret this dependence with a kinetic model that suggests the reason forthe observed results is a more efficient transfer of energy from excitedO₃ to O₂ than to N₂ by a factor of 22–55 depending onexcitation wavelength. Notably, the two prior studies on this topic useddifferent bath gas compositions, and although the results presented here donot fully resolve the differences in their results, they may at leastpartially explain them.