Weather forecast 20 13 12

In foggy days, there are many floating objects in the air. These floating objects are contaminated with water vapor in the clouds. When sunlight hits these floating objects, they will be refracted and form the illusion of the sun. Nanjing was heavily polluted that day, and the primary pollutant was PM2.5, which refers to particles with a diameter of 2.5μm or less and 2.5× 10-6m. Single PM2.5 is hidden in the floating dust in the air and easily inhaled into people's lungs.

So the answer is: refraction; 2.5× 10-6.