a few thousand kilometers. The highly visible haze that persists in all the industrialized regions of the world is composed mainly of sulfates and organic compounds from emissions of sulfur dioxide (SO2); organic gases (e.g., terpenes); and organic matter and soot (carbon black) from biomass burning. The flux of SO2 emissions has increased exponentially over the past century to 65 to 80 teragrams (1 Tg = 1012 g) per year, mainly from the smelting of metal ores and the burning of fossil fuels, which has led to increased emissions of greenhouse gases, aerosol particles, and aerosol precursor gases as well.

The ocean is a significant source of natural tropospheric aerosols. Air-sea exchange of particulate matter contributes to the global cycles of carbon, nitrogen, and sulfur aerosols (an example of the last-mentioned is the dimethyl sulfide (DMS) produced by phytoplankton). Ocean water and sea salt are transferred to the atmosphere through air bubbles at the sea surface. As the water evaporates, the salt is left suspended in the atmosphere. Haywood et al. (1999) suggest that naturally occurring sea salt is the leading aerosol contributor to the global-mean clear-sky radiation balance over oceans. Other significant sources of natural tropospheric aerosols are volcanic eruptions and windblown dust from arid and semiarid regions.

While the direct and indirect radiative effects of sulfates are important, other tropospheric aerosols may contribute significantly to the global radiative balance. Among these are carbonaceous compounds and mineral dust. Carbonaceous compounds are present in the atmosphere in the form of elemental carbon (EC) or organic carbon (OC). A significant portion of ambient EC is soot directly emitted as a product of incomplete fossil fuel combustion. An important property of EC is its large share of the imaginary part of the refractive index at visible wavelengths. This property makes it a very good absorber of shortwave radiation and could decrease the single-scattering albedo of an aerosol to below the critical point, causing the aerosol to have a net heating effect instead of a cooling effect. Simple radiative-transfer calculations using a box model show that EC/SO4 ratios of 0.05 and 0.10 result in a positive forcing (heating) of +0.03 and +0.34 Wm−2, respectively. In comparison, the direct sulfate forcing has been estimated at -0.43 Wm−2 for the northern hemisphere.

OC is either directly emitted (primary OC) or formed in the atmosphere (secondary OC) by the condensation of volatile organic carbons (VOCs). The largest sources of anthropogenic organic carbon include biomass burning, dust from paved roads, industrial emissions, and combustion for domestic purposes (e.g., cooking of food and burning of wood in stoves and fireplaces). Because a number of secondary and primary forms of OC are hygroscopic and have size distributions and optical properties similar to those of sulfate particles, these OC particles are likely to force the climate as much as, or even more than, sulfate particles.

The biggest obstacle to determining the effect of these particles for use in climate models is the lack of well-defined, spectrally resolved refractive indices for determining fundamental optical properties (single-scattering albedo, asymmetry factor, and extinction efficiency). The refractive indices of these particles have not been well characterized because ambient OC is made up of more than 300 different compounds. As a result, the composition is highly variable from particle to particle depending on location, source, and meteorological conditions. Based on our current knowledge, any estimate of a set of optical properties for all OCs would entail great uncertainty. Penner et al. (1994) estimate up to −0.8 Wm−2(direct and indirect) forcing as a result of anthropogenic organic aerosols produced by biomass burning. There is evidence that organic aerosols play a key role in cloud nucleation and thus are responsible for a significant share of cloud albedo enhancement in regions affected by anthropogenic pollutants. Based on measurements made on El Yunque peak in eastern Puerto Rico, 37 percent (by number) of the total CCN were found to be sulfate particles and the remaining 63 percent were OC. Some OC particles are strongly hydrophilic and readily act as CCN. Others may be intrinsically inactive as nuclei but become active by the condensation of a thin coating of sulfuric acid.

Mineral dust absorbs and scatters solar radiation and absorbs terrestrial (infrared) radiation. Although there has been considerable interest in sulfate aerosols over the last two decades, our knowledge of the distributions, global burdens, and effects on climate change of elemental carbon, organic carbon, and mineral dust is meager compared to our knowledge of sulfate aerosols.

Although tropospheric aerosols are chemically complex and may be strongly influenced by local emissions, one persistent feature, worldwide, is the strong presence of sulfate. It is difficult to calculate the effect of tropospheric aerosols on Earth's climate because data are lacking for many places around the world and there is no clear understanding of the processes that link gas emissions with particle formation and growth. All the estimates



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