The direct and indirect effects of airborne particles (aerosols) count among the greatest sources of uncertainty in our understanding of climate change. The uncertainty for direct radiative forcing (i.e., effects on solar radiation) is about a factor of two, and that for indirect forcing is even larger.1 Consequently, researchers have applied a variety of approaches to furthering knowledge in the field.
Owing to advances in measurement over the past two decades, satellite aerosol observations have been increasingly used to estimate the direct radiative effect of total aerosols worldwide. Yet satellite observations alone remain insufficient to determine the influence of specific components. As a result, estimates reported in the literature are mainly based on global climate models, which have their own uncertainties.
We have proposed a two-step approach to deriving component aerosol direct radiative forcing.2 Our method combines data on satellite Clouds and Earth's Radiant Energy System (CERES) solar wavelength (SW) radiative flux and Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical thickness (AOT) at 0.55μm with the AOT fractions of the component aerosols from the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model. This approach enables us to derive the global top-of-atmosphere aerosol direct radiative effect (ADRE) by component in cloud-free conditions. Here, ADRE refers to the impact of scattering and absorption of aerosol particles on SW radiative fluxes. In observational studies, it is also defined as the difference between these fluxes in the absence and presence of aerosols.
Figure 1. Global distribution of the 2001 annual mean top-of-atmosphere aerosol direct radiative effect (ADRE) for anthropogenic (left) and natural (right) aerosol components.
We based our work on two premises. First, current satellite measurements of total AOT and the corresponding ADRE are more accurate than calculations from global climate models. Second, global AOTs for detailed aerosol components (such as sea salt, dust, sulfate, and black and organic carbons) are only available from global models. Based on the first premise, in the initial step of our approach we derived ADRE for total aerosols using satellite CERES SW radiative fluxes and MODIS total AOT.3 In a second step, we partitioned total ADRE into individual aerosol components, including black carbon (BC), organic carbon (OC), sulfate (SU), dust (DU), sea salt (SS), anthropogenic aerosol (AN), and natural aerosol (NA) using the AOT fractions of these components relative to the total aerosols derived from the GOCART model according to our second premise.
Initially, we developed the approach for global oceans and then extended it to land areas.4 We validated our results by comparing the ADRE computation with calculations from the Fu-Liou radiative transfer model at globally distributed Aerosol Robotic Network (AERONET) sites. As model inputs, we used aerosol optical properties obtained by AERONET and surface reflectance from MODIS observations. The validation supported the extension of the two-step approach to land areas.
Figure 1 shows the global distribution of annual mean ADRE from anthropogenic (human-caused) and natural aerosol components for 2001. A relatively strong negative effect of anthropogenic aerosols is seen primarily over eastern China, the US, Southeast Asia, the Indian subcontinent, Europe, central South America, and central-south Africa, which is consistent with enhanced loading due both to industrial pollution and biomass burning in these regions. A major negative ADRE from natural aerosols can be found over the west coast of Africa, the Arabian Sea, and northwestern and northeastern China caused by mineral dust and, at latitudes higher than 45° in both hemispheres, by sea salts. The distributions for more specific components (BC, OC, SU, DU, and SS) are provided elsewhere.2
The estimates of global annual mean energy values of ADRE for clear-sky conditions are +0.3±0.2, −1.0±0.6, −2.3±0.7, −1.6±0.5, −2.2±0.6, −2.4±0.8, −4.5±1.2, and −6.8±1.7 (W m−2) for BC, OC, SU, DU, SS, AN, NA, and total aerosols, respectively. The all-sky values of component ADRE are about 42% of their clear-sky counterparts. Our results corroborate the conclusion by Bellouin and co-workers that “current model estimates of the anthropogenic ADRE are too weak.”5 Thus, the consensus AN value (−0.5±0.4W m−2) in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change1 may also be somewhat underestimated since it was mainly based on global model calculations. However, further study is needed to definitively resolve the differences among model- and observation-based estimates. Our next efforts will focus on identifying the causes of regional differences between our estimates and those of model-based studies.
Tom X.-P. Zhao
National Climatic Data Center
National Oceanic and Atmospheric Administration/
National Environmental Satellite, Data, and
Information Service (NOAA/NESDIS)
Tom Zhao is a physical scientist in the Remote Sensing and Application Division of NOAA/NESDIS. His research interests include remote sensing of aerosols and trace gases, modeling of aerosol microphysics and trace gas chemistry, and studies of air quality and climate effects of atmospheric constituents.
1. The Intergovernmental Panel on Climate Change, Climate Change 2007: The Physical Science Basis, Cambridge University Press, New York, 2007.
2. X.-P. Zhao, N. G. Loeb, I. Laszlo, M. Zhou, Global component aerosol direct radiative effect at the top of atmosphere, Int'l J. Remote Sens. 32
, no. 3, pp. 633-655, 2011. doi:10.1080/01431161.2010.517790
3. N. G. Loeb, N. Manalo-Smith, Top-of-atmosphere direct radiative effect of aerosols over global oceans from merged CERES and MODIS observations, J. Climate 18,
no. 17, pp. 3506-3526, 2005. doi:10.1175/JCLI3504.1
4. X.-P. Zhao, H. Yu, I. Laszlo, M. Chin, W. C. Conant, Derivation of component aerosol direct radiative forcing at the top of atmosphere for clear-sky oceans, J. Quant. Spectrosc. Radiat. 109
, no. 7, pp. 1162-1186, 2008. doi:10.1016/j.jqsrt.2007.10.006
5. N. Bellouin, O. Boucher, J. Haywood, S. Reddy, Global estimate of aerosol direct radiative forcing from satellite measurements, Nature 438
, pp. 1138-1141, 2005. doi:10.1038/nature04348