Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of black-carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottom-up inventory methods are 7500 Gg yr−1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m−2 with 90% uncertainty bounds of (+0.08, +1.27) W m−2. Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m−2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. When the principal effects of short-lived co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil fuel and biofuel) have an industrial-era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short-lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all short-lived species from black-carbon-rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2). The uncertainties in net climate forcing from black-carbon-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co-emitted organic carbon. In prioritizing potential black-carbon mitigation actions, non-science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near-term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black-carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.

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Bounding the role of black carbon in the climate system: A scientific assessment

A primary component of the observed, recent climate change is the radiative forcing from increased concentrations of long-lived greenhouse gases (LLGHGs). Effective simulation of anthropogenic climate change by general circulation models (GCMs) is strongly dependent on the accurate representation of radiative processes associated with water vapor, ozone and LLGHGs. In the context of the increasing application of the Atmospheric and Environmental Research, Inc. (AER) radiation models within the GCM community, their capability to calculate longwave and shortwave radiative forcing for clear sky scenarios previously examined by the radiative transfer model intercomparison project (RTMIP) is presented. Forcing calculations with the AER line-by-line (LBL) models are very consistent with the RTMIP line-by-line results in the longwave and shortwave. The AER broadband models, in all but one case, calculate longwave forcings within a range of -0.20 to 0.23 W m{sup -2} of LBL calculations and shortwave forcings within a range of -0.16 to 0.38 W m{sup -2} of LBL results. These models also perform well at the surface, which RTMIP identified as a level at which GCM radiation models have particular difficulty reproducing LBL fluxes. Heating profile perturbations calculated by the broadband models generally reproduce high-resolution calculations within a few hundredths K d{sup -1} in the troposphere and within 0.15 K d{sup -1} in the peak stratospheric heating near 1 hPa. In most cases, the AER broadband models provide radiative forcing results that are in closer agreement with high 20 resolution calculations than the GCM radiation codes examined by RTMIP, which supports the application of the AER models to climate change research.

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Radiative forcing by long‐lived greenhouse gases: Calculations with the AER radiative transfer models

The software package OPAC (Optical Properties of Aerosols and Clouds) is described. It easily provides optical properties in the solar and terrestrial spectral range of atmospheric particulate matter. Microphysical and optical properties of six water clouds, three ice clouds, and 10 aerosol components, which are considered as typical cases, are stored as ASCII files. The optical properties are the extinction, scattering, and absorption coefficients, the single scattering albedo, the asymmetry parameter, and the phase function. They are calculated on the basis of the microphysical data (size distribution and spectral refractive index) under the assumption of spherical particles in case of aerosols and cloud droplets and assuming hexagonal columns in case of cirrus clouds. Data are given for up to 61 wavelengths between 0.25 and 40 μm and up to eight values of the relative humidity. The software package also allows calculation of derived optical properties like mass extinction coefficients and Angstrom coefficients. Real aerosol in the atmosphere always is a mixture of different components. Thus, in OPAC it is made possible to get optical properties of any mixtures of the basic components and to calculate optical depths on the base of exponential aerosol height profiles. Typical mixtures of aerosol components as well as typical height profiles are proposed as default values, but mixtures and profiles for the description of individual cases may also be achieved simply.

Optical Properties of Aerosols and Clouds: The Software Package OPAC

Description of the NCAR Community Atmosphere Model (CAM 3.0)

The problem of deriving a complete set of aerosol optical properties from Sun and sky radiance measurements is discussed. Algorithm development is focused on improving aerosol retrievals by means of including a detailed statistical optimization of the influence of noise in the inversion procedure. The methodological aspects of such an optimization are discussed in detail and revised according to both modern findings in inversion theory and practical experience in remote sensing. Accordingly, the proposed inversion algorithm is built on the principles of statistical estimation: the spectral radiances and various a priori constraints on aerosol characteristics are considered as multisource data that are known with predetermined accuracy. The inversion is designed as a search for the best fit of all input data by a theoretical model that takes into account the different levels of accuracy of the fitted data. The algorithm allows a choice of normal or lognormal noise assumptions. The multivariable fitting is implemented by a stable numerical procedure combining matrix inversion and univariant relaxation. The theoretical inversion scheme has been realized in the advanced algorithm retrieving aerosol size distribution together with complex refractive index from the spectral measurements of direct and diffuse radiation. The aerosol particles are modeled as homogeneous spheres. The atmospheric radiative transfer modeling is implemented with well-established publicly available radiative transfer codes. The retrieved refractive indices can be wavelength dependent; however, the extended smoothness constraints are applied to its spectral dependence (and indirectly through smoothness constraints on retrieved size distributions). The positive effects of statistical optimization on the retrieval results as well as the importance of applying a priori constraints are discussed in detail for the retrieval of both aerosol size distribution and complex refractive index. The developed algorithm is adapted for the retrieval of aerosol properties from measurements made by ground-based Sun-sky scanning radiometers used in the Aerosol Robotic Network (AERONET). The results of numerical tests together with examples of experimental data inversions are presented.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JD900282

A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements

The transfer of monochromatic radiation in a scattering, absorbing, and emitting plane-parallel medium with a specified bidirectional reflectivity at the lower boundary is considered. The equations and boundary conditions are summarized. The numerical implementation of the theory is discussed with attention given to the reliable and efficient computation of eigenvalues and eigenvectors. Ways of avoiding fatal overflows and ill-conditioning in the matrix inversion needed to determine the integration constants are also presented.

Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media.

A radiation model is constructed that includes radiative interactions with atmospheric gases as well as parameterized treatments of scattering and absorption/emission by cloud droplets and haze particles. A unified treatment of solar and terrestrial radiation is obtained by using identical cloud and haze parameterization procedure for the shortwave and longwave region. The influence of the relative humidity of the haze particles is also considered. Snow conditions of the arctic region are simulated in terms of snow grain sizes and soot contamination in the surface layers. Data from the Arctic Stratus Cloud Experiment collected in 1980 are used for model comparisons and sensitivity studies under cloudy and hazy sky conditions. During the arctic summer, stratus clouds are a persistent feature and decrease the downward flux at the surface by about 130–200 W m−2. Arctic haze in the summertime is important if it is above the cloud layer or in air with low relative humidity, and it decreases the downwa...

Radiative Energy Budget in the Cloudy and Hazy Arctic

The scattering phase matrices for finite hexagonal cylinders oriented randomly in space were computed by superposing the scattered intensities of Fraunhofer diffracted rays and geometrical optics rays. However, the effect of interference is considered when the optical path lengths for two rays, split by some obstacle and scattered in the same direction, are equal to each other. Single models (c/a = 2.5 and 0.4) for a hexagonal column and a plate, resembling the corresponding crystals in atmospheric clouds, are used in the computations. Our results showed different values for the phase matrix elements, P33 and P44, from those obtained previously by Cai and Liou. The backscattering linear depolarization ratios and the asymmetry factor for hexagonal plates oriented horizontally were then computed. The backscattering linear depolarization ratios exceeded 1.0 at certain orientations. Within the limitation of the use of single-crystal models for a hexagonal column and a plate, the results appear to agree well with most field and laboratory observations.

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Scattering phase matrix for hexagonal ice crystals computed from ray optics.

Observations of the physical properties of Arctic status clouds (ASC) over the Beaufort Sea area were made by the NCAR Electra aircraft during June 1980. The cloud morphology and microstructure observed during these experiments are described here. Arctic stratus clouds were formed under various meteorological conditions, but not when the axis of the Beaufort Sea ridge was zonal and the airflow into the region was continental. The mean drop diameter in clouds observed under all conditions remained near 10 μm, while the mean liquid water content (LWC) was characteristic of the air mass forming the clouds and essentially determined by the mean drop concentration. Clouds showed considerable horizontal homogeneity but significant vertical changes occurred within them. The vertical profiles of LWC show that the values generally increased with height, as a result of an increase in droplet size rather than concentration. The drop size distribution near the base was monomodal, characteristic of condensati...

Physical Characteristics of Arctic Stratus Clouds

Air samples collected at several heights above the Arctic Ocean contained at least two types of bacteria and five different fungal spores which could elevate the freezing temperature of water droplets It was observed that the freezing temperature increased logarithmically with the number of cells per drop up to a maximum temperature dependent upon species Bacteria were more effective than fungal spores but, unlike the spores, they lost their activity under laboratory growth conditions

Maximum concentration of nearly 10 microbial cells per liter occurs in clouds and 1 per liter in air outside clouds However, numbers and varieties of microbial cells in excess of those found near sea ice surface level were observed up to 7 km

Kolf Jayaweera

Papers

Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media.

Radiative Energy Budget in the Cloudy and Hazy Arctic

Scattering phase matrix for hexagonal ice crystals computed from ray optics.

Physical Characteristics of Arctic Stratus Clouds

Investigations on biogenic ice nuclei in the Arctic atmosphere