Aerosols, which are minute airborne particles, have a profound effect on the Earth's climate system, influencing the amount of sunlight that impinges on the surface, altering the properties of clouds, and impacting the hydrological system.

Aerosols scatter sunlight - a phenomenon readily observable in the reduced visibility of hazy days; consequently, aerosols reduce the amount of solar energy reaching the Earth's surface, thereby serving as climate cooling agents. Indeed, computer simulations of climate change which include a primitive representation of aerosols, have demonstrated that aerosols can "offset" a significant fraction of the warming due to accumulation of greenhouse gases in the atmosphere (see Figure 1). Such model findings, combined with theoretical expectations and field observations of aerosols and their impact, place research on the role of aerosols at the epicenter of efforts to detect climate change and to attribute its causes.

Figure 1. Simulated global annual mean warming from 1860 to 1990 allowing for increases in greenhouse gases only (dashed curve) and greenhouse gases and sulphate aerosols (solid curve), compared with observed changes over the same period. Reproduced from Figure 15, p.33 of "Climate Change 1995," The Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change, ed. J.T. Houghton et al, Cambridge University Press, 1996.

The chemical composition and optical properties of aerosols vary with their sources; for example, aerosols emitted into the air as urban industrial pollution influence the climate very differently from windblown desert dust or sea salt or biogenic aerosols. To further complicate the global picture, aerosols tend to remain in the air for only a few days to a week or so, resulting in extreme spatial and temporal variability over the surface of the Earth. As a consequence, measuring or describing "typical" properties of aerosols, no less quantifying their radiative impact on global and regional scales, is a daunting task. Yet this is precisely the challenge embraced by scientists at the NOAA laboratories, whose research includes world-wide measurements of aerosol properties (see Figure 2), and development of state-of-the-art models of their impacts on climate (see Figure 3).

Figure 2. Locations of NOAA field measurements of aerosol properties, showing the ship tracks of oceanographic cruises and the positions of NOAA ground sampling stations.

Figure 3. Modeled simulation by NOAA/GFDL scientists of satellite observations from the Earth Radiation Budget Experiment (ERBE) of reflected sunlight from the Earth under clear-sky conditions. Upper panel: difference between observations and simulation without inclusion of aerosols. Lower panel: difference with aerosols included in the simulation. From Haywood, J.M.,V. Ramaswamy, and B.J. Soden, Science 283, 1299-1303, 1999.

While the direct radiative cooling effect of aerosols has long been recognized, recent measurements have highlighted the importance of light absorption by aerosols, a phenomenon which cools the surface of the earth but heats the atmosphere. The net effect of aerosols on the Earth's radiation balance at the top of the atmosphere depends on the relative amounts of light scattered or absorbed by the particles, as well as the reflectivity of the surface below them. Highly absorbing aerosols, such as soot, are so highly efficient as light absorbers, that their net radiative impact (scattering plus absorption) usually results in a net warming of the climate system. Other commonly found aerosols, such as mineral dust or weakly absorbing organic species, exhibit a delicate balance between cooling (light scattering) and warming (light absorption) that varies with their precise and highly variable chemical composition, relative humidity, and proximity to clouds.

Recent precise measurements on polluted air masses flowing out of India have demonstrated that for the darkly colored aerosols typical of that region, fully two thirds of the sunlight that is blocked from reaching the ground is deposited into the lower atmosphere by aerosol light absorption. Aside from the direct radiative warming effect, these vast quantities of energy (which can exceed the accumulated impact of greenhouse gases by a factor of ten!), are adequate to alter convective activity in the atmosphere with potential to reduce rainfall. In addition, simulations of cloud cover have demonstrated the new finding that this localized warming of the atmosphere is adequate to "burn off" cloud cover, causing further radiative and hydrological feedbacks. Thus the climatic role of aerosols, because of their light absorption, has been shown to be far more profound, complex, and wide reaching than was believed only a year or two ago.

As a consequence, in order to assess the global impact of aerosols on climate, it is necessary to map their mean properties as functions of latitude, longitude, and altitude, as well as the variability of their properties with time (e.g., seasonal behavior due to meteorological and human patterns of activity). Since these measurements cannot be made with precision from space (indeed, aerosol light absorption cannot be directly measured from space at all), a concerted and ongoing program of in situ field measurements is required to elucidate this problem. Figures 4, 5, and 6, showing recent results of NOAA field research, demonstrate both the magnitude and the high variability of aerosol light absorption in space and time. Such characterization is essential to developing an understanding of the impact of aerosols on the climate system.

	Figure 4: Field data showing the high variability of aerosol light absorption coefficient with latitude and longitude, measured by NOAA/PMEL scientists aboard the NOAA Research Vessel Ron Brown during the Aerosols 99 and INDOEX (Indian Ocean Experiment) cruises. The aerosol light absorption coefficient (presented in all figures in units of Mm-1) describes the aerosol cross-sectional area for light absorption, with typical units of square meters of absorbing cross section per cubic meter of air. Measurements are made at a wavelength of 550nm. (Courtesy of P. Quinn and T. Bates, NOAA/PMEL.)
Figure 5: Field data showing the high variability of aerosol light-absorption (Mm-1) with altitude, measured by NOAA/CMDL scientists aboard the C-130 research aircraft during the INDOEX field campaign. Profile was measured over the Indian Ocean at approximately 6 degrees north latitude on February 16, 2000. (Courtesy of P. Sheridan and J. Ogren, NOAA/CMDL.)
	Figure 6: Field data showing the high variability of aerosol light-absorption (Mm-1) with time, measured by NOAA and University of Illinois scientists at the NOAA monitoring ground-station at Bondville, Illinois. (Courtesy of J. Ogren, NOAA/CMDL and M. Rood, UIUC)