Parameters Controlling Aerosol Forcing

D daylight fraction

S0 solar constant

Tat atmospheric  transmission

Ac cloud fraction

Rs surface albedo

Source: Haywood and Shine (1995)

DF average aerosol forcing at top of  atmosphere (TOA)

d aerosol optical depth

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Why measure Aerosol Single-Scattering Albedo?

• One of the key aerosol parameters necessary to derive local estimates of direct aerosol radiative forcing.

Define “Forcing”

• Define forcing: an externally imposed perturbation to the radiative balance of the earth. Implies need to separate anthropogenic and natural contributions.

• This equation represents an annual mean forcing at TOA, instantaneous forcings can be much larger. Surface forcings can also be much larger.

• This equation (or close variant) being used in current models of aerosol forcing, e.g., IPCC estimates.

• Corresponding equation for GHG much simpler, as radiative properties of all CO2 molecules are identical. Not so for aerosols.

• B depends on zenith angle, can’t measure directly. So we measure a surrogate instead. Both are integral properties of angular scattering function.