Latest revision as of 11:35, 15 May 2017

Abstract

Black carbon (BC) aerosols can strongly absorb solar radiation in very broad spectral wavebands, from the visible to the infrared. As a potential factor contributing to global warming, BC aerosols not only directly change the radiation balance of the earth-atmosphere system, but also indirectly affect global or regional climate by acting as cloud condensation nuclei or ice nuclei to alter cloud microphysical properties. Here, recent progresses in the studies of radiative forcing due to BC and its climate effects are reviewed. The uncertainties in current researches are discussed and some suggestions are provided for future investigations.

Keywords

black carbon ; radiative forcing ; climate effects

1. Introduction

Black carbon (BC), an important component of atmospheric aerosols, is produced from the incomplete combustion of hydrocarbon-containing materials, including fossil fuels, biofuels, and biomass. The BC aerosol concentration in the atmosphere has increased rapidly due to human use of coal, oil, and other fossil fuels, agricultural and biological burning, and vehicle exhaust emissions since the industrial revolution. Bond et al. [2004] analyzed global BC aerosol emissions in detail and calculated a current value of approximately 8.0 Tg C per year. Of this, they estimated that emissions from fossil fuel and biomass burning constituted 4.6 Tg C per year, while emissions from open burning equaled 3.3 Tg C per year. Ito et al. [2005] estimated the BC aerosol emission from global fossil fuel combustion to be 2.8 Tg C per year in 2000, representing a roughly threefold increase since the 1950s. Before 1950s, the largest BC aerosol emission sources were North America and western Europe, but now they are in tropical areas and developing countries in East Asia [ Bond et al., 2007 ].

BC aerosols account for a small proportion of the atmospheric total aerosols (usually several to ten-plus percent), and their concentration in the atmosphere is generally low. However, their impact on climate and the atmospheric environment should not be overlooked. BC aerosols can absorb solar radiation strongly from visible to infrared wavebands, thus increasing the solar radiation energy absorbed in the earth-atmosphere system. Hence BC aerosols have been regarded as a potential factor in global warming [ Hansen et al ., 1998  ; Zhang et al ., 2008  ;  Ramanathan and Carmichael, 2008 ].

The importance of research on BC aerosols is now widely recognized, and the emission sources, spatial and temporal distribution, and environmental and climate effects of BC aerosols are key subjects in global and regional atmospheric environment and climate change research.

2. Source strength distribution and BC aerosol clearing

There are two kinds of emission sources of BC aerosols: natural and anthropogenic. The natural emission sources include volcanic eruptions and forest fires. The anthropogenic sources mainly consist of the combustion of coal, oil, and other fossil fuels, biomass burning, and car exhaust emissions. Bond et al. [2007] calculated the global distribution of BC aerosol annual emission strength. As shown in Figure 1 , there are four strong BC sources: eastern China, western Europe, South America, and central Africa. Weaker sources of BC are also distributed in parts of southern North America and Australia. In China, a large consumer of coal, coal combustion is the main source of BC aerosol emissions. Another main source of BC aerosols is the burning of crop straw in rural areas of China. Cao et al. [2007] estimated that China emitted 1.50 Tg of BC in 2000, with larger emissions in the east than in the west. Emission clearly showed seasonal variations. Larger emissions in winter than in summer reflect heating fuel use in winter.

Figure 1. Global distribution of BC aerosol emission strength (unit: t per year) [ Bond et al., 2004 ]

The lifetime of BC aerosols in the atmosphere is generally a few days, much shorter than that of greenhouse gases. Hence the spatial distribution of BC aerosols is extremely uneven. There are three ways to remove BC from the atmosphere: dry, wet, and gravitational deposition. Of these, rainwater washing and cleaning (wet deposition) is the most important.

3. Radiative forcing due to BC aerosols

Following the infamous London smog events of the 1950s, scientific interest in BC aerosols gradually increased. In the 1980s, the special role that BC aerosols play in radiative forcing was recognized and their influence on climate began to be studied more deeply.

In recent years, a number of field experiments such as the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOE) [ Russell et al., 1999 ], the Aerosol Characterization Experiment (ACE-II) [ Raes et al., 2000 ], and the Indian Ocean Experiment (INDOEX) [ Ramanathan et al., 2001 ] have allowed for systematic study of the radiative forcing of BC aerosols. Unlike the greenhouse effect of CO₂ , which leads to a positive radiative forcing in the atmosphere and at the surface with moderate latitudinal gradients, BC has the opposite effect. BC adds energy to the atmosphere but reduces energy at the surface [ Zhang et al ., 2008  ;  Ramanathan and Carmichael, 2008 ; Wang et al., 2009 ]. By integrating the results of various studies, the IPCC [2007] concluded that direct radiative forcing by BC aerosols at the top of atmosphere (TOA) is (+ 0.24±0.14) W m⁻² . Ramanathan and Carmichael [2008] compared the global mean radiative forcing due to greenhouse gases, the direct radiative forcing due to BC aerosols, and the direct and indirect radiative forcing due to non-BC aerosols ( Fig. 2 ). The results showed that the total radiative forcing due to aerosols at the TOA was −1.4 W m⁻² . This forcing could offset 50% of the positive radiative forcing due to greenhouse gases. However, the direct radiative forcing due to BC aerosols was + 0.9 W m⁻² , larger than that due to any greenhouse gas except CO₂ . The radiative heating effect of BC aerosols on the whole atmosphere was + 2.6 W m⁻² , almost double that of all greenhouse gases. To date, studies have provided mixed results on BC aerosol radiative forcing. Furthermore, most climate models have not considered the internal mixing of BC with other aerosols, which could lower the radiative forcing by BC to the range of + 0.2 to + 0.4 W m⁻² [ Highwood et al., 2006  ; Koch et al., 2007 ]. However, a large number of observational studies have shown that BC aerosols can mix internally with sulfate, organic carbon, and other water-soluble aerosols. Such mixing would greatly change the optical properties of BC and increase its radiative forcing [ Chung and Seinfeld, 2002  ;  Sato et al ., 2003 ]. Although BC aerosols produce positive radiative forcing at the TOA, the overall direct radiative forcing due to aerosols in the atmosphere is negative. The IPCC [2007] summarized the results from various models and found that the total global annual average radiative forcing due to anthropogenic aerosols was about −0.4 W m⁻² , while the average value from several satellite observations was about −0.55 W m⁻² . Zhuang et al. [2009] also investigated the first indirect forcing of BC aerosol using a regional climate model coupled with a tropospheric atmosphere chemistry model, and indicated that the average indirect radiative forcing at the tropopause due to BC aerosol was −0.39 and −1.18 W m⁻² in January and July over China, respectively.

Figure 2. Comparison of the global mean radiative forcing due to greenhouse gases, BC, and non-BC aerosols (unit: W m−2 ) from the pre-industrial era to present [ Ramanathan and Carmichael, 2008 ]

Clouds are also an important factor that can affect the radiative forcing due to BC aerosols. When a cloud layer is above the BC aerosol layer, it can reflect part of the solar radiation before it reaches the aerosol layer and hence decrease the solar flux absorbed by the BC. This leads to reduced radiative forcing by BC aerosols at the TOA and the surface. However, when the cloud layer is below the BC aerosol layer, reflection of solar radiation by the cloud can cause secondary absorption of the solar radiation by the BC. This increases the positive radiative forcing of the BC at the TOA and decreases the negative radiative forcing at the surface. Zhang et al. [2009] and Wang et al. [2009] studied the effect of clouds on the direct radiative forcing due to BC aerosols and obtained a similar conclusion.

The solar radiative flux at the surface has an important influence on the surface runoff, surface latent heat, and vegetation cover; thus a clear understanding of radiative forcing due to BC aerosols at the surface is also very important. Any kind of aerosols, absorbing or scattering, will decrease the solar radiative flux arriving at the surface, thereby causing a negative radiative forcing there. While a few studies have indicated that aerosol extinction can cause global dimming [ Wild et al., 2005 ], it is still unclear whether global dimming is caused directly by aerosols. Further study on this subject is needed.

4. Effect of BC aerosols on climate

BC aerosols affect climate in three ways. First, they can directly absorb solar and infrared radiation, disturb the energy balance in the earth-atmosphere system, and directly affect climate. Second, BC, aerosols can mix with sulfate, organic carbon, and other water-soluble aerosols to become cloud condensation nuclei or directly act as ice nuclei, change the microphysical and radiative properties and lifetime of cloud, and indirectly affect the climate system. Finally, BC aerosols in a cloud layer can absorb solar radiation and heat the air of the cloud layer, thus directly cause cloud evaporation and reduction. This is known as the semi-direct effect of BC aerosols.

4.1. Effect on global climate

Because the concentration of BC aerosols is extremely uneven both spatially and temporally, climate models offer an effective tool for studying the global climate effects of BC. Chung and Seinfeld [2005] simulated the direct radiative forcing due to BC aerosols and their climate effects using the General Circulation Model II (GCM II) developed by the Goddard Institute for Space Studies. They showed that if the BC and sulfate aerosols were mixed externally, their radiative forcing would be + 0.33 W m⁻² , which would increase global average surface temperature by 0.2 °C. If the mixing was internal, the value would be + 0.6 W m⁻² , leading to a surface temperature increase of 0.37 ^° C. Both mixing types caused a precipitation increase from 0°−20°N and a decrease from 0° to 20°S, resulting in northward movement of the inter-tropical convergence zone (ITCZ). However, a simulation by Kristjansson et al. [2005] using the National Center for Atmospheric Research (NCAR) Community Climate Model 3 (CCM3) indicated that the total direct and indirect effects due to atmospheric aerosols were a global cooling of about −1.33 °C, and led to southward movement of the ITCZ. Gu et al. [2006] also showed that the overall direct effect of aerosols decreased the surface temperature and precipitation in the ITCZ. By considering the combined absorbing BC and scattering organic carbon (OC), Zhang et al. [2009] found that the carbonaceous aerosols would weaken the Hadley and polar circulations in the Northern Hemisphere. Studies using on-line coupled aerosol-climate models also showed that BC aerosols had an effect on atmospheric temperature, surface temperature, radiative flux, surface latent heat, and other weather parameters [ Wang , 2004 ]. Absorbing aerosols, mostly from BC, could also reduce (increase) mid- (low-) altitude cloud cover and hence increase (decrease) the radiative fluxes reaching the surface [ Allen and Sherwood , 2010 ]. Chen et al. [2010] showed that the BC aerosol takes an important effect on aerosol indirect effect, and the global cloud radiative forcing would be decreased if BC concentration is reduced. Responses of the surface and atmosphere to the radiation depend on the radiative forcing at the TOA. Ramanathan and Carmichael [2008] reported that although BC aerosols decreased the amount of solar radiation reaching the surface, they increased the temperature in the atmosphere and at the surface. Over most of the Northern Hemisphere, including the Arctic region, BC led to a warming from the surface to about 12 km altitude, by as much as 0.6 °C [ Ramanathan et al., 2007 ]. However, Penner et al. [2003] noted that the total radiative forcing due to BC aerosols depended on their altitude of injection. Higher altitude injections tended to enhance negative longwave forcing, increase the cloud cover in the lower atmosphere, and eventually decrease the temperature in the lower atmosphere. Figure 3 shows the precipitation trend from 1950 to 2002. Over these 50 years, the figure shows a fairly obvious characteristic of precipitation variations marked by decreases in rainfall over the Sahel region of Africa and the Indian monsoon areas, and the so-called “northern drought/southern flooding” phenomenon in China. Observational research demonstrated that natural variability and the aerosol forcing effect are the main factors that caused the aforementioned precipitation variations [ Chung and Ramanathan, 2007 ].

Figure 3. Precipitation trend from 1950 to 2002 (unit: mm d−1 ) [ Chung and Ramanathan, 2006 ]

4.2. Effect on the monsoon

BC aerosols can change the energy balance in the atmosphere and on the Earth’s surface, and alter atmospheric circulation patterns, and the water cycle, which will have major impacts on the monsoon. Incorporating the aerosol optical depth in the GISS GCM and manipulating single scattering albedos of the aerosols, Menon et al. [2002] studied the climate effects of the BC aerosols in the East Asian monsoon region. Their simulation showed that the phenomenon of “northern drought/southern flooding” that has often occurred in summer during the past 50 years in China may be related to BC aerosols. However, the findings of Zhang et al. [2009] contradicted those of Menon et al. [2002] . Taking the effects of both BC and OC into account, Zhang et al. [2009] suggested that carbonaceous aerosols, through direct and semi-direct effects, heat the atmosphere, reduce cloud cover and precipitation, and increase the solar radiation reaching the surface and thus the surface temperature in southern China. In their simulation, the opposite situation would occur in northern China (Fig. 4 ). Lau et al. [2005  ; 2006 ] concluded from observational data and climate modeling that the absorbing aerosols on the southern and northern sides of the Tibetan Plateau could strengthen the southwestern flow stream over the Bay of Bengal from the end of May to early June, which would increase precipitation and cause the onset of the South Asian summer monsoon. Chung and Ramanathan [2006] also reported that BC aerosols can heat the troposphere and enhance vertical convection over the North Indian Ocean and Indian subcontinent, causing an increase in precipitation. Wang et al. [2009] simulated the effect of BC aerosols on the Asian summer monsoon and found that the BC aerosol heating effect in this region caused an early onset of the summer monsoon in the Bay of Bengal and related coastal areas. This resulted in the early occurrence and enhancement of the South Asian summer monsoon, but a weakened the East Asian summer monsoon due to the effect on the surface pressure and the vertical motions. Also, as a result, the Western Pacific subtropical high extended northward and westward and the early summer rainy zone in east China moved northeastward. Liu et al. [2009] used GCM CAM3.0 to study the effect of BC direct forcing on East Asian monsoon. They showed that the direct effects of BC aerosols in China distinctly weakened the East Asian monsoons in both summer and winter seasons. Many studies have also shown that aerosols prevent solar radiation from reaching the surface and hence induce surface cooling, which then slows tropical water circulation and weakens the Asian monsoon [ Ramanathan et al., 2005 ].

Figure 4. Effects of carbonaceous aerosol on (a) total cloud cover (unit: %), (b) surface temperature (unit: K), and (c) total precipitation (unit: kg m−2 ) in summer in East Asia [ Zhang et al., 2009 ]

4.3. Effect on snow/ice

Many observational studies have shown that the heating effect of BC aerosols on the atmosphere has increased the temperature in the Himalayan region by around 1 °C and significantly reduced snow/ice cover since the 1950s [ Thompson et al ., 2003  ;  Barnett et al ., 2005 ]. The deposition of BC aerosol on snow/ice surfaces enhances the ability of these surfaces to absorb solar radiation, thus increases the surface temperature and further accelerates the melting of glaciers worldwide. Thus, BC aerosols make an important contribution to global warming by reducing the albedo of snow/ice surfaces [ Warren et al., 1985  ; Koch et al., 2009 ]. Rypdal et al. [2009] found that BC deposited on snow/ice induced strongly positive radiative forcing on the surface of snow/ice, and the forcing was coincidentally largest with snowmelt onset, triggering strong snow-albedo feedback. The simulation by Flanner et al. [2009] showed that BC in snow/ice caused an obvious increase of surface temperature over Eurasia and North America and snow/ice cover loss in springtime by using an AOGCM coupled with a snow, ice, and aerosol radiation model. Over the past few years, researchers from the Institute of the Tibetan Plateau, Chinese Academy of Sciences, have analyzed a large number of samples from glacier snow/ice pits and surface snow/ice in the Tibetan Plateau. The results indicated that many BC aerosols attached to snow/ice in winter and increased heat absorption, resulting in snow/ice melting in March and April. However, because the melting precedes the agricultural season, these scarcely available snow/ice water resources cannot be put into good use. Menon et al. [2009] quantified the impact of BC aerosols on snow/ice cover over the Indian subcontinental region, and showed that simulated snow/ice cover was decreased by 0.9% due to BC over the Himalayas from 1990 to 2000. The contribution of the enhanced Indian BC to this decline is 30%.

BC aerosols can also act as cloud condensation nuclei or ice nuclei, change the microphysical and radiative properties and lifetime of clouds and thus indirectly affect climate. However, relatively little research has examined this subject to date. The GISS climate model embedded with a detailed aerosol microphysical scheme, called MATRIX, was used to estimate the impact of microphysical processes involving BC [ Bauer et al., 2010 ]. They found that reducing BC emissions by 50% can lead to an increase of cloud droplet number concentration (CDNC) globally, thus reduce cloud cover and LWP, and make a positive aerosol indirect effect change of + 0.12 W m⁻² .

5. Uncertainties in BC aerosol research

First, factors such as the source emissions of BC aerosols, column content calculations, aerosol parameterization schemes, and radiative transfer and climate models used all greatly impact the estimation of BC aerosol radiative forcing and its climate effect. To quantitatively evaluate the contribution of BC aerosols to regional or global warming, we not only need reliable data of BC aerosols from various emission sources in different areas, but also want to know various optical parameters such as extinction factors, single scattering albedo, asymmetric factors, and other characteristics including the mixing processes and mixing states between BC and other aerosols. Much more practical work is required to accomplish these. Second, the parameterization scheme in GCMs is a source of uncertainty in the study of the climate effects of BC aerosols. Finally, the effect of BC aerosols on cloud condensation nuclei or ice nuclei is even more uncertain, and more research is urgently needed in this aspect.

6. Conclusions and discussion

In recent years, many international experts have proposed that BC emission be included in the greenhouse gas emission inventory. Zhang et al. [2009] pointed out that absorbing BC and scattering OC are emitted into the atmosphere at the same time. The impact of OC on climate should also be studied. Furthermore, conclusions on the effects of BC or carbonaceous aerosols on climate may differ depending on the climate models used. Thus, emphasis should also be placed on model research combined with the actual conditions of aerosol emissions and unique optical properties of aerosols in China, rather than using assumed optical model to reach the conclusion about the effects of BC aerosols on the climate change in China.

BC aerosols play a complex and yet important role in the climate effect of the atmospheric aerosols. Thus, research on BC aerosol radiative forcing and its climate effect is extremely important to regional and global climate changes, and possesses positive implications in protecting the atmospheric environment and human health. Indeed, because of the short lifetime of BC we may be able to quickly reduce global warming by controlling BC emissions.

In the authors’ opinion, the following aspects should be the key areas of future research on BC aerosols: improved source emission data acquisition and analysis; the study of the heterogeneous chemical reactions on aerosol surfaces and their mixing states with other aerosols; the impact of BC aerosols on the melting of glaciers and the related climate feedbacks; and the indirect effect of BC aerosols on climate. We can minimize uncertainties regarding the effects of BC aerosols on climate by combining observational data analyses and numerical simulations.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (2011CB403405 and 2010CB955608), the Public Meteorology Special Foundation of MOST (GYHY200906020).

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