I wonder if a radiant heat source beside the sensor, maybe also somehow focused in the same area, could get rid of the condensed water.

The problem is not condensed water. This can be solved with heating.

It's that fog is being detected as a particle. This distorts the measured values.

Fog is condensed water droplets suspended in the air. Radiant heat , especially if it's optically focused, could remove them in the area where the PM sensor is looking.