Search

In the Intergovernmental Panel on Climate Change Fourth Assessment Report, the direct aerosol effect is reported to have a radiative forcing estimate of -0.5 watt per square meter (W [m.sup.-2]), offsetting the warming from C[O.sub.2] by almost one-third. The uncertainty, however, ranges from -0.9 to -0.1 W [m.sup.-2], which is largely due to differences between estimates from global aerosol models and observation-based estimates, with the latter tending to have stronger (more negative) radiative forcing. This study demonstrates consistency between a global aerosol model and adjustment to an observation-based method, producing a global and annual mean radiative forcing that is weaker than -0.5 W [m.sup.-2], with a best estimate of -0.3 W [m.sup.-2]. The physical explanation for the earlier discrepancy is that the relative increase in anthropogenic black carbon (absorbing aerosols) is much larger than the overall increase in the anthropogenic abundance of aerosols.

Atmospheric aerosols play important roles in climate and atmospheric chemistry: They scatter sunlight, provide condensation nuclei for cloud droplets, and participate in heterogeneous chemical reactions. Two important aerosol species, sulfate and organic particles, have large natural biogenic sources that depend in a highly complex fashion on environmental and ecological parameters and therefore are prone to influence by global change. Reactions in and on sea-salt aerosol particles may have a strong influence on oxidation processes in the marine boundary layer through the production of halogen radicals, and reactions on mineral aerosols may significantly affect the cycles of nitrogen, sulfur, and atmospheric oxidants., Over the past decade, there has been intense interest concerning the role of aerosols in climate and atmospheric chemistry. The climatic effects of aerosols had already been recognized in the [...]

Atmospheric measurements and numerical calculations described here indicate that binary homogeneous nucleation of [H.sub.2][SO.sub.4]-[H.sub.2]O particles occurs in the upper tropical troposphere. Particle concentrations decrease with increasing altitude above the tropical tropopause as a result of coagulation during the upward air transport produced by stratospheric circulations. During the extended periods of time that volcanic eruptions do not strongly influence stratospheric particle number concentrations, particles formed in the upper tropical troposphere provide nuclei upon which oxidized sulfur gases condense in the stratosphere. This particle source, coupled with aerosol microphysical properties and atmospheric transport, governs the number concentration of particles in the lower tropical and mid-latitude stratosphere., The stratospheric aerosol layer, or Junge layer, is a fine mist of highly concentrated liquid sulfuric acid ([H.sub.2][SO.sub.4]) particles that envelops the Earth at altitudes from a few kilometers above [...]

Phase changes in stratospheric aerosols were studied by cooling a droplet of sulfuric acid ([H.sub.2]S[O.sub.4]) in the presence of nitric acid (HN[O.sub.3]) and water vapor. A sequence of solid phases was observed to form that followed Ostwald's rule for phase nucleation. For stratospheric partial pressures at temperatures between 193 and 195 kelvin, a metastable ternary [H.sub.2]S[O.sub.4]-HN[O.sub.3] hydrate, [H.sub.2]S[O.sub.4] [center dot] HN[O.sub.3] [center dot] 5[H.sub.2]O, formed in coexistence with binary [H.sub.2]S[O.sub.4] [center dot] k[H.sub.2]O hydrates (k = 2, 3, and 4) and then transformed to nitric acid dihydrate, HN[O.sub.3] [center dot] 2[H.sub.2]O, within a few hours. Metastable HN[O.sub.3] [center dot] 2[H.sub.2]O always formed before stable nitric acid trihydrate, HN[O.sub.3][center dot]3[H.sub.2]O, under stratospheric conditions and persisted for long periods. The formation of metastable phases provides a mechanism for differential particle growth and sedimentation of HN[O.sub.3] from the polar winter stratosphere., Type I polar stratospheric clouds (PSCs) form during winter at temperatures between 192 and 195 K, which is 2 to 4 K above the frost point for water ice (1-3). [...]

Tropospheric aerosols affect the radiative forcing of Earth's climate, but their variable concentrations complicate an understanding of their global influence. Model-based estimates of aerosol distributions helped reveal spatial patterns indicative of the presence of tropospheric aerosols in the satellite-observed clear-sky solar radiation budget over the world's oceans. The results show that, although geographical signatures due to both natural and anthropogenic aerosols are manifest in the satellite observations, the naturally occurring sea-salt is the leading aerosol contributor to the global-mean dear-sky radiation balance over oceans., Tropospheric aerosols (1, 2), which include sulfate (3, 4), black carbon (5, 6), organic carbon (6), mineral dust (7), and sea-salt (8, 9), affect Earth's radiation budget in a significant [...]

How do aerosol particles affect climate? This is one of the key questions that has to be answered if we are to understand how humans influence Earth's climate. The burning [...]

Last August, as the 98th Congress was struggling to conclude its work and leave town, John Quetsch, an acting assistant secretary of the Army, asked the House and Senate appropriations [...]