Radiative Forcing by Anthropogenic Aerosols: Sources and Impacts
MICHAEL H. BERGIN
Georgia Institute of Technology
China is the most populous and fastest growing nation in the world. In addition, China’s gross domestic product is growing by ~10 percent per year (Freemantle, 1998). This extraordinary economic growth is associated with increases in anthropogenic pollutants, such as aerosols and ozone precursors, from industrial sources that will increase dramatically in the future. Sulfur emissions (from the burning of coal, which currently supplies ~75 percent of China’s total energy) are expected to double by 2010 (Daniel, 1994; Wolf and Hidy, 1997). The impact of anthropogenic pollutants, particularly aerosols, on the environment in this part of the world remains uncertain, however, because we have only a few measurements of the chemical, physical, and radiative properties of aerosols on relevant temporal and spatial scales.
A recent study of temperature trends in China over the past ~40 years shows that the temperature over a relatively large area of the country has increased by ~0.2–0.4°C, which is attributable to a concomitant increase in greenhouse gases (Li et al., 1995). At the same time, the temperature over a large part of southern China, extending from Sichuan Province to the Yangtze delta region, dropped by ~0.2–0.4°C. The cooling trend has been evident since the mid-1970s, which coincides with the start of economic reforms and intense industrialization in the area and has been accompanied by significant decreases in visual range, attributed to increases in anthropogenic aerosol loading. Li et al. (1995) suggest that the cooling is due to the scattering of shortwave radiation out of the Earth’s atmosphere during clear-sky conditions; this is known as “direct,” shortwave, aerosol, radiative forcing.
These results are consistent with the model estimates of Chameides et al.
(1999) suggesting that anthropogenic sulfur and carbonaceous aerosols may reduce the surface irradiance in these regions by as much as 30 percent, which may result in a decrease in crop yield. Modeling results based on projections of future sulfur emissions also predict cooling (Engardt and Rodhe, 1998; Wolf and Hidy, 1997).
China is also a significant source of black carbon (also called elemental carbon) aerosols from incomplete combustion processes. Black carbon absorbs solar radiation, which results in warming of the atmosphere. Using a global climate model to study the influence of black carbon emissions in China on both regional and global climate, Menon et al. (2002) found that increases in black carbon emissions correlated roughly with observed severe weather patterns (e.g., heavy rainfall and droughts) and influenced regional temperatures as far away as North America.
“Indirect” forcing of climate by aerosols emitted in China may also affect global climate. The indirect effects are attributable to changes in the optical properties of clouds associated with the presence of anthropogenic aerosol particles. Although much of China has a characteristic haziness, not much is known about the sources of aerosols and their impact on radiation balance, climate, and plant growth.
CONCENTRATIONS AND SOURCES OF AEROSOLS IN CHINA
Although little information is available on aerosol radiative properties in China, several studies report extremely high mass concentrations of aerosols in urban areas. Measurements of total suspended-particulate (TSP) mass made over a one-year period in several major cities averaged ~300 µg/m3; major sources include windblown dust and fossil fuel combustion (Hashimoto et al., 1994). This value is roughly four times the maximum allowable in the United States, according to the national ambient air quality standard (NAAQS) for TSP, which is 75 µg/m3. Waldman et al. (1991) report values of aerosol particle mass with diameters of less than 2.5 µm (PM2.5) over a two-week intensive sampling period of 139 µg/m3 (with a range of 54 to 207 µg/m3) in Wuhan, an industrialized city in central China. Particles of this size may not only accumulate in the lungs, but may also scatter and absorb light in visible wavelengths; the mean value is greater by more than a factor of 2 than the proposed U.S. PM2.5 NAAQS 24-hour average value of 50 µg/m3 (Waggoner and Weiss, 1980). Based on a study by Waldman et al. (1991), the contributions of sulfate, nitrate, and carbonaceous aerosol to PM2.5 are ~30 percent, 20 percent, and 50 percent, respectively. A recent year-long monitoring study in Beijing also found relatively high values of PM2.5, with a reported annual mean concentration of ~120 µg/m3 (Cao et al., 2003). The results showed that organic carbon, sulfate, and nitrate collectively account for ~60 percent of the fine particle mass in Beijing. Measurements made at several stations in the Pearl River delta region during January and February of
2001 found a mean PM2.5 concentration of 70 µg/m3, with nearly 40 percent of the fine particle mass composed of organic compounds (Cao et al., 2003).
Figure 1 shows the chemical composition of aerosols impacted by different sources in two locations, Beijing in June 1999 (Bergin et al., 2001) and Linan (in the Yangtze delta approximately 200 km southwest of Shanghai) in November 1999 (Xu et al., 2002). At both locations, the PM2.5 concentrations exceeded both the daily and annual mean U.S. NAAQS values. The chemical composition of fine particulate matter was dominated by carbonaceous aerosol and sulfate. The
organic carbon has a variety of sources, including diesel and automobile exhaust, the burning of biomass, cooking, the combustion of coal, and biogenic emissions from plants (Schauer et al., 1996). Mobile sources, as well as the burning of coal and biomass, are the main contributors to the organic carbon concentrations in Beijing. In addition, the effects of coal burning are reflected in the relatively large contribution of sulfate (14 percent) to the PM2.5. A characteristic of the Beijing aerosol is the presence of crustal species that come from windblown dust. Arid regions of China, particularly the Gobi and Takla Makan deserts, are upwind sources of dust for much of northeastern China (Zhang et al., 2002). Dust storms are particularly intense in the spring; dust from these storms has been detected as far away as Greenland (Bory et al., 2003) and North America (McKendry et al., 2001).
A surprising finding is the relatively high PM2.5 concentrations in the Yangtze delta region, because the sampling site is located in an agricultural area that does not have a great deal of industrial activity. Despite this, the mean PM2.5 concentration is 90 µg/m3, which is as high as concentrations measured in many urban areas of China. The most revealing difference between the chemical compositions in Linan and Beijing is the dominance of particulate organic carbon, which accounts for nearly half of the aerosol mass in the Yangtze delta. Elemental carbon, a product of incomplete combustion, is also comparatively high in the Yangtze Delta.
Although the sources of carbonaceous aerosol are not definitively known, the measurements in the Yangtze delta were made during the fall rice harvest, when the burning of crop residue is widespread throughout the region. Therefore, it would appear that the carbonaceous component of the aerosol in the Yangtze delta is primarily linked with the burning of biomass. In Beijing, particulate organic compounds are more likely from many different sources.
Although dust from the arid regions of China has been known to reach the Yangtze delta and more southerly regions of China, the contribution of dust to fine particulate mass is generally negligible in the measurements shown in Figure 1. The relatively high sulfate concentrations in the Yangtze delta, which account for ~24 percent of the PM2.5 mass, are probably from a combination of regional sources and the long-range transport of air masses carrying particulates from regions to the west, such as Sichuan Province. Thus, the rural region of the Yangtze delta, one of the most important agricultural areas in China, appears to have relatively poor air quality as a result of both local emissions and the transport of polluted air from industrialized regions.
Overall, many of the studies suggest extremely high aerosol loadings in urban and some rural areas of China, with primary anthropogenic contributions to fine particulate mass of sulfate and carbonaceous compounds. Streets et al. (2003), who have compiled emissions inventories in Asia for gaseous and primary aerosol precursors, found that anthropogenic sulfur dioxide (SO2) emissions, which are responsible for most of the sulfate in particulate matter in
China, come primarily from coal burning for power generation (50 percent) and industrial purposes (36 percent); a smaller, but still significant contribution comes from the domestic use of coal. A fraction of industrial SO2 emissions are linked to the generation of electricity by industry. The emission inventories estimated by Streets et al. (2003) are within several percent of independent estimates made by the Chinese government (SEPA, 2000).
Streets et al. also estimate that black carbon in China is dominated by the residential burning of coal and biofuels, which together account for ~75 percent of all emissions; in urban areas, however, black carbon emissions may be mostly from mobile sources (i.e., diesel and gasoline vehicles). Emissions of organic carbon are currently poorly understood. Streets et al. (2003) estimate the sources to be dominated by residential fuel combustion (~76 percent) and the burning of biomass (22 percent). The burning of coal for power generation cannot be ruled out, however, as a significant source of organic carbon, particularly in urban regions.
One must be very cautious about inferring the sources of carbonaceous aerosols based on current emissions inventories. Streets et al. set an uncertainty level of ±450 percent for organic carbon emissions. Data from two recent field experiments, the Asian Pacific Regional Aerosol Characterization Experiment (ACE-ASIA) and Transport and Chemical Evolution Over the Pacific (TRACE-P), that focus partly on concentrations of particulate matter and related precursors in East Asia, will soon be available. The results of these two studies should shed additional light on the chemical composition, as well as the effects, of aerosols on the radiation balance in the vicinity of China.
RADIATIVE FORCING BY AEROSOLS
Although several studies report the chemical and physical properties of aerosols, very few measurements on pertinent temporal and spatial scales are available on the radiative properties that govern the scattering and absorption of solar radiation by aerosol particles. As a result, estimates of the influence of aerosols on climate must be based on model estimates of aerosol loading and properties. Figure 2 (based on a model by Chameides et al., 1999) shows the estimated annual mean aerosol optical depth at a wavelength of 550 nanometers (i.e., the vertical profile of the aerosol light-extinction coefficient) based on SO2 emissions and the assumption that organic carbon and sulfate contribute similarly to fine particulate mass throughout China. Although the model estimates probably reflect the impact of SO2 emissions on aerosol loadings, they only very roughly reflect the influence of particulate organic carbon, because neither elemental carbon nor organic carbon emissions are taken into account directly. Nor does the model take into account the contribution of dust to annual aerosol loading, which, as discussed above, can account for a significant fraction of the fine particulate mass, particularly during the spring season in much of northern China. Even
though the results in Figure 2 are only rough estimates of the annual aerosol optical depth throughout the model domain, they agree qualitatively with actual measurements, which are also shown.
Relatively high aerosol loadings can be seen throughout China, with a “hot spot” centered in the Sichuan region and extending east over the Yangtze delta. The model results clearly indicate that upwind sources influence aerosol loadings in the Yangtze delta. Based on these data, we can also estimate the reduction in surface irradiance due to light extinction by aerosols (Figure 3). Surface irradiance is reduced annually by nearly 30 percent in the vicinity of the Sichuan region, with values of 10 to 15 percent throughout much of eastern China. Xu et al. (2003) combined these data with atmospheric radiative transfer modeling, which shows an annual mean top-of-the-atmosphere, direct aerosol radiative forcing of approximately –12 W/m2 in the Yangtze delta. This value is nearly an order of magnitude greater than the global mean value suggested by the Intergovernmental Panel on Climate Change (IPCC, 2001). Thus, direct scattering and absorption by aerosols can have a dramatic effect on radiation balances at the top of the atmosphere, as well as at the surface of the Earth.
Given these extremely high aerosol loadings, aerosols probably also have an
indirect effect on the radiation balance. The indirect effect, which results from the modification by aerosols of cloud albedo, amount, and lifetime, is extremely difficult to quantify because of our current lack of understanding of the processes that govern aerosol-cloud interactions. In fact, the uncertainty in estimates of indirect aerosol effects (from 0 to –2 W/m2) dominates the overall uncertainty in estimates of anthropogenic climate forcing (IPCC, 2001).
When Chameides et al. (2002) compared model estimates of aerosol loading over China and East Asia with satellite-derived cloud properties, including optical depth and amount, they found a surprisingly strong correlation (r2 ~ 0.6). In other words, regions of East Asia with high aerosol loadings also generally have more reflective clouds. Based on these findings, the indirect radiative forcing is estimated to be approximately 1.5 times the direct radiative forcing. This would mean, for instance, that, based on the direct estimates of Xu et al. (2002) (discussed above), the indirect aerosol radiative forcing in the Yangtze delta region is roughly –18 W/m2. Thus, the combined aerosol radiative forcing over much of China impacted by aerosols, taken as the sum of the direct and indirect effects, may be from –30 to –40 W/m2. These values are much more than an order of magnitude greater than the overall annual mean radiative
forcing (including greenhouse gases and aerosols) of a few W/m2 estimated by IPCC. Thus, we can conclude that the emissions of aerosols and their precursors in China are having a significant influence on the radiation balance of Earth and very likely significant climatic impacts in East Asia and perhaps globally (Menon et al., 2003).
INFLUENCE OF AEROSOLS ON CROPS
Because aerosols influence the balance of surface radiation, they may also influence plant growth by changing the amount of photosynthetically available radiation (PAR, i.e., radiation between 400 nm and 700 nm). Figure 4 shows the change in net PAR (NPAR) reaching the surface (NPAR is the difference between downward and upward PAR) as a function of aerosol optical depth at 500 nm (τ500) measured in the Yangtze delta region during November 1999. NPAR and aerosol optical depth are both normalized to account for the change in
solar zenith angle (SZA). Figure 4 shows that the amount of PAR reaching the surface decreases as aerosol loading increases. The slope of the line, commonly called the direct aerosol radiative forcing efficiency, indicates a decrease in NPAR of ~74 W/m2 for each unit of aerosol optical depth. Based on Figure 4 and measurements of aerosol properties, Xu et al. (2003) estimate that aerosols decrease the amount of PAR reaching the surface by ~15 to 20 percent; this is relatively consistent with the model estimates presented in Figure 3.
The attenuation of solar radiation by aerosols results in a decrease in the amount of direct solar radiation that reaches the surface, and, at the same time, an increase in diffuse radiation. In a plant canopy, an increase in atmospheric aerosol loading results in a decrease in PAR illuminating leaves that are normally sunlit and an increase in PAR for leaves that are shaded. Thus, as the aerosol optical depth increases, the relative amount of diffuse (and total) radiation reaching a plant canopy generally increases. Therefore, a decrease in surface PAR does not necessarily result in a proportionate decrease in plant growth. This is illustrated in Figure 5, which shows estimates of the change in rice yield as a function of aerosol loading for meteorological conditions similar to those of the Yangtze delta.
The estimates in Figure 5 are based on a coupled atmospheric radiative
transfer-crop model described by Greenwald et al. (2003). The lines represent three separate scenarios of radiation use efficiency (RUE, i.e., the mass of carbon fixed by a plant per MJ PAR absorbed) based on the relative amount of diffuse-to-total PAR reaching the plant canopy. The bottom (dashed) line represents no increase in RUE as a function of aerosol optical depth (and increasing diffuse-to-total radiation ratio). The top line assumes a maximum increase in RUE of 100 percent with increasing diffuse radiation. The gray area shown in the middle is the most likely expected crop yield, based on limited knowledge of rice crop response as a function of diffuse fraction and aerosol optical depth values in China. The estimates suggest that the decrease in crop yield ranges from a few percent to nearly 10 percent. These are rough estimates, of course, because of uncertainties in the aerosol loadings, crop response to changes in radiation, nutrient and water stresses, and meteorology. Nevertheless, the results suggest that a decrease in the sources of aerosols may lead to an increase in crop yield.
Another possible influence on plant growth is the deposition of particulate matter on leaves. In addition to damage to the leaves from acidity, aerosol deposits can scatter and absorb radiation, thus reducing PAR for photosynthesis. Particles that are insoluble in water pose the greatest threat because they cannot be easily washed from leaves by precipitation; thus, they accumulate over time. Measurements in the Yangtze delta suggest that a significant fraction of fine particulate matter (~30 to 40 percent) is not soluble in water. Given the relatively large concentration of fine particulate matter observed at many locations in China, the attenuation of PAR by deposits of particles may significantly decrease available PAR and hence plant photosynthesis. The deposition of dust particles, which are primarily insoluble in water, also decreases PAR in regions of China that experience high dust loading.
Figure 6 shows estimates of the decrease in PAR available for photosynthesis caused by dry deposition (EXPAR), based on measurements in the Yangtze delta of elemental carbon and water-insoluble aerosol mass concentration (Bergin et al., 2001). The combined scattering and absorption of deposited aerosols can account for a nearly 30 percent decrease in the amount of available PAR reaching leaves over a two-month period. The attenuation of PAR at the leaf surface is the result of absorption by elemental carbon and scattering by water-insoluble carbonaceous aerosols. Thus, in addition to attenuation of PAR by atmospheric aerosols, particles deposited on leaves may also influence photosynthesis and plant growth.
Atmospheric particulate matter in China is very likely having a wide range of impacts, including damage to human health, reduced visibility, modified climate, and decreased plant growth. Based on recent measurements, the main contributors to fine particulate matter are sulfate and organic compounds. Sulfate originates
primarily from the burning of coal for electricity and industrial purposes, with lesser contributions from residential sources. Based on measurements in both urban and rural locations, organic compounds appear to dominate the fine particulate mass. The sources of these compounds are currently not well understood but probably include diesel and automobile emissions, the burning of coal and biomass, and cooking. Clearly, future research must address the sources of these compounds. In relatively arid regions, dust can also be a significant fraction of the aerosol loading.
Anthropogenic aerosols have been found to have a profound influence on the local, regional, and even global radiation balance. Model estimates show that aerosols may be decreasing the radiation reaching the surface by as much as 30 percent in many parts of China. This decrease is associated with the scattering and absorption of light by anthropogenic aerosols. Recent model estimates suggest that the aerosol loadings over China may be influencing weather not only in China, but also elsewhere around the globe. In rough agreement with model
estimates, measurements in an agricultural region of the Yangtze delta indicate a 15 to 20 percent reduction in the amount of PAR reaching the surface, which may reduce crop production by as much as 10 percent in the region. The relatively high concentrations of fine particulate matter in the Yangtze delta may also result in the deposition of particulate matter on leaves, which may also affect plant growth by scattering and absorbing radiation. Overall, aerosols appear to have a negative influence on crop growth, and perhaps plant growth, in many regions of China.
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