On the morning of December 4, 2013, hazy weather caused two suns, one large and one small, to appear in the sky over Nanjing, which lasted for about one minute. The two suns are actually called

On hazy days, there are a lot of floating objects in the air. These floating objects are stained with water vapor in the clouds. When the sun's rays hit these floating objects, they are refracted to form the sun dog phenomenon. Nanjing was severely polluted that day. The primary pollutant was PM2.5. PM2.5 refers to particulate matter in the atmosphere with a diameter less than or equal to 2.5μm (2.5×10-6m). A single PM2.5 is hidden in the floating dust in the air and is easily Harmful if inhaled into human lungs.

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