Atmospheric black carbon (BC) warms Earth's climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E(abs)) and mixing state are reported for two California regions. The observed E(abs) is small-6% on average at 532 nm-and increases weakly with photochemical aging. The E(abs) is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E(abs) for BC are possible.

Optical feedback cavity ring-down spectroscopy (OF-CRDS) using a continuous wave distributed feedback diode laser at 1650 nm has been used to measure extinction of light by samples of monodisperse spherical aerosol particles <1 mum in diameter. The OF-CRDS method allows measurements of low levels of extinction of incident light to be made at repetition rates of 1 kHz or greater. A statistical model is proposed to describe the linear relationship between the extinction coefficient (alpha) and its variance (Var(alpha)). Application of this model to experimental measurements of Var(alpha) for a range of alpha values typically below approximately 1 x 10(-6) cm(-1) allows extinction cross-sections for the aerosol particles to be obtained without need for knowledge of the particle number density. Samples of polystyrene spheres with diameters of 400, 500, 600, and 700 nm were used to test the model by comparing extinction cross-sections determined from the experiment with the predictions of Mie theory calculations. Fitting of ring-down decay traces exhibiting amplitude noise to extract cavity ring-down times introduces additional quadratic and higher order polynomial dependencies of the variance that become significant for larger particle number densities and thus extinction coefficients (typically for alpha > 1 x 10(-6) cm(-1) under our experimental conditions). Aggregation of particles at larger number densities is suggested as a further source of variance in the measurements. Extinction cross-sections are severely underestimated if the measurements are made too rapidly to sample uncorrelated distributions of particle numbers and positions.

Jacobson argues that our statement that "many climate models may overestimate warming by BC" has not been demonstrated. Jacobson challenges our results on the basis that we have misinterpreted some model results, omitted optical focusing under high relative humidity conditions and by involatile components, and because our measurements consist of only two locations over short atmospheric time periods. We address each of these arguments, acknowledging important issues and clarifying some misconceptions, and stand by our observations. We acknowledge that Jacobson identified one detail in our experimental technique that places an additional constraint on the interpretation of our observations and reduces somewhat the potential consequences of the stated implications. In Cappa et al.(1), we explicitly compared observations of ambient black carbon (BC) particle absorption enhancements (E abs ) and average mixing states with observationally constrained Mie theory predictions to establish whether core-shell (CS) Mie theory accurately reproduces the observed E abs . Such comparisons are necessary because the ability of theoretical methods to accurately predict BC light absorption depends not only on particle mixing state (i.e., the extent to which BC is internally mixed with other components) but also on particle morphology (i.e., the physical arrangement of the BC with respect to the other components within a given particle). CS Mie theory assumes that internally mixed BC exists as spherical "cores" surrounded concentrically by non-BC "shell" material. Many, although not all, climate models have adopted CS Mie theory to simulate BC optical properties of internal mixtures (2). In (1), we addressed whether or not observations support this morphology assumption, and we ultimately concluded that (i) CS Mie theory did not accurately reproduce observed E abs , (ii) observed E abs for "thickly coated" and "aged" particles was surprisingly small, and (iii) laboratory observations can give E abs that are consistent with CS Mie theory and that are substantially larger than our field observations. These observations suggest that consideration of mixing state is a necessary, but not sufficient, criterion for establishing whether CS Mie theory is appropriate for use within climate models. The implication of our observations is that many climate models may overestimate the warming influence of BC particles, as many consider internal mixing and use CS Mie theory. Our conclusions and the corollary implication have been challenged by Jacobson (3).Jacobson (3) implies that through misinterpretation of the internal processes of some climate models and omissions of several factors or elements, our conclusions about "model error" are invalid. The stated omissions include: (i) our observations were made over short atmospheric times (up to 20 hours of aging) and thereby were "not completely aged"; (ii) our measurements were experimentally controlled under low relative humidity (RH) conditions and thus do not include high RH observations; (iii...

Cavity ring-down spectroscopy using a fiber-coupled continuous wave distributed feedback laser at a wavelength of 1520 nm has been used to measure extinction of light by samples of nearly monodisperse aerosol particles <1 μm in diameter. A model is tested for the analysis of the sample extinction that is based on the Poisson statistics of the number of particles within the intracavity laser beam: variances of measured extinction are used to derive values of the scattering cross section for size-selected aerosol particles, without need for knowledge of the particle number density or sample length. Experimental parameters that influence the performance of the CRD system and the application and limitations of the statistical model are examined in detail. Determinations are reported of the scattering cross sections for polystyrene spheres (PSSs), sodium chloride, and ammonium sulfate, and, for particles greater than 500 nm in diameter, are shown to be in agreement with the corresponding values calculated using Mie theory or Discrete Dipole Approximation methods. For smaller particles, the experimentally derived values of the scattering cross section are larger than the theoretical predictions, and transmission of a small fraction of larger particles into the cavity is argued to be responsible for this discrepancy. The effects of cubic structure on the determination of optical extinction efficiencies of sodium chloride aerosol particles are examined. Values are reported for the real components of the refractive indices at 1520 nm of PSS, sodium chloride, and ammonium sulfate aerosol particles.

Diode laser cavity ring-down spectroscopy is a versatile method for quantitative determination of trace atmospheric constituents. Examples include measurement of mixing ratios of small organic compounds, isotopologue-specific spectroscopy, and optical extinction by atmospheric aerosol particles.