effect, the presence of aerosols leads to clouds with more but smaller particles; such clouds are more reflective and therefore have a negative radiative forcing. These smaller cloud droplets can also decrease the precipitation efficiency and prolong cloud lifetime; this is known as the second indirect aerosol effect. The so-called semidirect aerosol effect occurs when absorption of solar radiation by soot leads to an evaporation of cloud droplets. A number of research avenues hold promise for improving understanding of indirect and semidirect aerosol effects and for better constraining estimates of their magnitude. These include fundamental research on the physical and chemical composition of aerosols, aerosol activation, cloud microphysics, cloud dynamics, and subgrid-scale variability in relative humidity and vertical velocity.

PRIORITY RECOMMENDATION:

Improve understanding and parameterizations of the indirect aerosol radiative and nonradiative effects in general circulation models using process models, laboratory measurements, field campaigns, and satellite measurements.

Better Quantify the Direct Radiative Effects of Aerosols

Aerosols have direct radiative effects in that they scatter and absorb both shortwave and longwave radiation. Knowledge of direct radiative forcing of aerosols is limited to a large extent by uncertainty about the global distributions and mixing states of aerosols. Mixing states have major implications on aerosol optical properties that are not well understood and are difficult to parameterize in climate models. Small-scale variability of humidity and temperature, which has a major impact on aerosol optical properties, is also difficult to represent in models. Mechanisms of aerosol production are not understood, so the effects of future changes in emissions and climate are highly uncertain. Removal of aerosols from the atmosphere occurs mainly by wet deposition, but model parameterizations of this process are highly uncertain and rudimentary in their coupling to the hydrological cycle.

PRIORITY RECOMMENDATIONS:

Improve representation in global models of aerosol microphysics, growth, reactivity, and processes for their removal from the atmosphere through laboratory studies, field campaigns, and process models.

Better characterize the sources and the physical, chemical, and optical properties of carbonaceous and dust aerosols.



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