Not necessarily, since there isn't simply "radiation".  SO2 might preferentially reflect wavelengths that are useless for solar panels.  This isn't all that unlikely, since solar panels use quite a narrow part of the spectrum anyway.  (But I don't have exact data to prove this either way.)

Yes, give or take different effects at different wavelengths. As you note elsewhere, reduced sunlight is also an issue for agriculture. A key question is the size of this effect relative to other sources of variability.

This NREL map of mean annual PV solar radiation indicates that, at equal latitudes across the southern U.S., the energy available for photovoltaic systems varies by something like 40% because of variations in clouds, haze, and so forth. Photosynthesis should see a similar variation.

A rough estimate of the reduction in sunlight needed to offset warming of 1°C can be made by noting that 1°C is about 1/300 of the absolute temperature. Because of the T^4 scaling of thermal radiation with temperature, this is equivalent (keeping albedo and thermal emissivity constant) to a change of about 4/300 in the input and output radiated power -- about 1.3%. From a photovoltaic power perspective, this would be equivalent to reducing PV cells from (for example) 30% to 29.6% efficiency.

From this rough calculation, it seems that weather variability is a much larger effect.

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