Global sensitivity studies of the direct radiative forcing due to anthropogenic sulfate and black carbon aerosols

Abstract Large eddy simulations (LES) show that the presence of black carbon aerosols in marine boundary layers leads to a marked reduction of stratocumulus liquid water path (LWP) by heating the cloud layer and suppressing convection in the boundary layer. The reduction of LWP leads to a positive radiative forcing known as the semidirect effect. In this study LES results are compared with results from the National Center for Atmospheric Research (NCAR) Single-Column Community Climate Model (SCCM). The SCCM represents clouds and boundary layer processes through simple parameterization schemes that are typical of general circulation models (GCMs) used for climate experiments. In a case study in which black carbon aerosols were introduced in a stratocumulus-capped boundary layer the SCCM gave a semidirect aerosol radiative forcing that was a factor of 5 smaller than the value obtained from the LES. The cloud response to absorbing aerosols was underestimated because of the way that cloud cover and cloud radiative properties were parameterized in the SCCM. Furthermore, the SCCM gave a poor representation of processes, such as entrainment and boundary layer decoupling, that are crucial to determining stratocumulus LWP. This study shows that GCMs may not include all the physical processes necessary to adequately capture the semidirect aerosol effect. Previous GCM estimates of the semidirect effect that have incorporated simple cloud parameterizations should, therefore, be treated with some caution.

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Black carbon fractal morphology and short-wave radiative impact: a modelling study

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-23103-2011 ◽

2011 ◽

Vol 11 (8) ◽

pp. 23103-23137

Author(s):

M. Kahnert ◽

A. Devasthale

Keyword(s):

Optical Properties ◽

Fractal Dimension ◽

Black Carbon ◽

Radiative Forcing ◽

Homogeneous Sphere ◽

Radiative Impacts ◽

Radiative Impact ◽

Sphere Approximation ◽

Carbon Aerosols ◽

Black Carbon Aerosols

Abstract. We investigate the impact of the morphological properties of freshly emitted black carbon aerosols on optical properties and on radiative forcing. To this end, we model the optical properties of fractal black carbon aggregates by use of numerically exact solutions to Maxwell's equations within a spectral range from the UVC to the mid-IR. The results are coupled to radiative transfer computations, in which we consider six realistic case studies representing different atmospheric pollution conditions and surface albedos. The spectrally integrated radiative impacts of black carbon are compared for two different fractal morphologies, which brace the range of recently reported experimental observations of black carbon fractal structures. We also gauge our results by performing corresponding calculations based on the homogeneous sphere approximation, which is commonly employed in climate models. We find that at top of atmosphere the aggregate models yield radiative impacts that can be as much as 2 times higher than those based on the homogeneous sphere approximation. An aggregate model with a low fractal dimension can predict a radiative impact that is higher than that obtained with a high fractal dimension by a factor ranging between 1.1–1.6. Although the lower end of this scale seems like a rather small effect, a closer analysis reveals that the single scattering optical properties of more compact and more lacy aggregates differ considerably. In radiative flux computations there can be a partial cancellation due to the opposing effects of differences in the optical cross sections and asymmetry parameters. However, this cancellation effect can strongly depend on atmospheric conditions and is therefore quite unpredictable. We conclude that the fractal morphology of black carbon aerosols and their fractal parameters can have a profound impact on their radiative forcing effect, and that the use of the homogeneous sphere model introduces unacceptably high biases in radiative impact studies. We emphasise that there are other potentially important morphological features that have not been addressed in the present study, such as sintering and coating of freshly emitted black carbon by films of organic material.

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Black carbon aerosols over an urban region: Radiative forcing and climate impact

Journal of Geophysical Research Atmospheres ◽

10.1029/2009jd013560 ◽

2010 ◽

Vol 115 (D10) ◽

Cited By ~ 106

Author(s):

S. Ramachandran ◽

Sumita Kedia

Keyword(s):

Black Carbon ◽

Radiative Forcing ◽

Climate Impact ◽

Urban Region ◽

Carbon Aerosols ◽

Black Carbon Aerosols

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Observations of black carbon aerosols characteristics over an urban environment: Radiative forcing and related implications

The Science of The Total Environment ◽

10.1016/j.scitotenv.2017.06.082 ◽

2017 ◽

Vol 603-604 ◽

pp. 319-329 ◽

Cited By ~ 21

Author(s):

Samina Bibi ◽

Khan Alam ◽

Farrukh Chishtie ◽

Humera Bibi ◽

Said Rahman

Keyword(s):

Urban Environment ◽

Black Carbon ◽

Radiative Forcing ◽

Carbon Aerosols ◽

Black Carbon Aerosols

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Black carbon fractal morphology and short-wave radiative impact: a modelling study

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-11745-2011 ◽

2011 ◽

Vol 11 (22) ◽

pp. 11745-11759 ◽

Cited By ~ 57

Author(s):

M. Kahnert ◽

A. Devasthale

Keyword(s):

Optical Properties ◽

Fractal Dimension ◽

Black Carbon ◽

Radiative Forcing ◽

Homogeneous Sphere ◽

Radiative Impacts ◽

Radiative Impact ◽

Sphere Approximation ◽

Carbon Aerosols ◽

Black Carbon Aerosols

Abstract. We investigate the impact of the morphological properties of freshly emitted black carbon aerosols on optical properties and on radiative forcing. To this end, we model the optical properties of fractal black carbon aggregates by use of numerically exact solutions to Maxwell's equations within a spectral range from the UVC to the mid-IR. The results are coupled to radiative transfer computations, in which we consider six realistic case studies representing different atmospheric pollution conditions and surface albedos. The spectrally integrated radiative impacts of black carbon are compared for two different fractal morphologies, which brace the range of recently reported experimental observations of black carbon fractal structures. We also gauge our results by performing corresponding calculations based on the homogeneous sphere approximation, which is commonly employed in climate models. We find that at top of atmosphere the aggregate models yield radiative impacts that can be as much as 2 times higher than those based on the homogeneous sphere approximation. An aggregate model with a low fractal dimension can predict a radiative impact that is higher than that obtained with a high fractal dimension by a factor ranging between 1.1–1.6. Although the lower end of this scale seems like a rather small effect, a closer analysis reveals that the single scattering optical properties of more compact and more lacy aggregates differ considerably. In radiative flux computations there can be a partial cancellation due to the opposing effects of different error sources. However, this cancellation effect can strongly depend on atmospheric conditions and is therefore quite unpredictable. We conclude that the fractal morphology of black carbon aerosols and their fractal parameters can have a profound impact on their radiative forcing effect, and that the use of the homogeneous sphere model introduces unacceptably high biases in radiative impact studies. We emphasise that there are other potentially important morphological features that have not been addressed in the present study, such as sintering and coating of freshly emitted black carbon by films of organic material. Finally, we found that the spectral variation of the absorption cross section of black carbon significantly deviates from a simple 1/λ scaling law. We therefore discourage the use of single-wavelength absorption measurements in conjunction with a 1/λ scaling relation in broadband radiative forcing simulations of black carbon.

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Linearity of Climate Response to Increases in Black Carbon Aerosols

Journal of Climate ◽

10.1175/jcli-d-12-00715.1 ◽

2013 ◽

Vol 26 (20) ◽

pp. 8223-8237 ◽

Cited By ~ 17

Author(s):

Salil Mahajan ◽

Katherine J. Evans ◽

James J. Hack ◽

John E. Truesdale

Keyword(s):

Black Carbon ◽

Radiative Forcing ◽

Global Climate ◽

Temperature Response ◽

Ocean Model ◽

Climate Response ◽

Global Average ◽

High Level ◽

Carbon Aerosols ◽

Black Carbon Aerosols

Abstract The impacts of absorbing aerosols on global climate are not completely understood. This paper presents the results of idealized experiments conducted with the Community Atmosphere Model, version 4 (CAM4), coupled to a slab ocean model (CAM4–SOM) to simulate the climate response to increases in tropospheric black carbon aerosols (BC) by direct and semidirect effects. CAM4-SOM was forced with 0, 1×, 2×, 5×, and 10× an estimate of the present day concentration of BC while maintaining the estimated present day global spatial and vertical distribution. The top-of-atmosphere (TOA) radiative forcing of BC in these experiments is positive (warming) and increases linearly as the BC burden increases. The total semidirect effect for the 1 × BC experiment is positive but becomes increasingly negative for higher BC concentrations. The global-average surface temperature response is found to be a linear function of the TOA radiative forcing. The climate sensitivity to BC from these experiments is estimated to be 0.42 K W−1 m2 when the semidirect effects are accounted for and 0.22 K W−1 m2 with only the direct effects considered. Global-average precipitation decreases linearly as BC increases, with a precipitation sensitivity to atmospheric absorption of 0.4% W−1 m2. The hemispheric asymmetry of BC also causes an increase in southward cross-equatorial heat transport and a resulting northward shift of the intertropical convergence zone in the simulations at a rate of 4° PW−1. Global-average mid- and high-level clouds decrease, whereas the low-level clouds increase linearly with BC. The increase in marine stratocumulus cloud fraction over the southern tropical Atlantic is caused by increased BC-induced diabatic heating of the free troposphere.

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