From Mwn
Well, the reason why the increase in water vapor (humidity) is so important because most of the haze we see, especially on the east coast, is because of the chemical reactions that occur in the air between pollutants like Sulphur Dioxide (SO2), which originates from the burning of fossil fuels, other industrial processes, volcanic eruptions, and dah dum… wildfires. SO2 bonds with the oxygen (O) and hydrogen (H) in the water vapor and air to create various sulphur oxides (SOx) and can even precipitate as Sulphuric Acid (H2S04), if in liquid form. If less water is present, then as a solid in the form of ammonium salts, which if mixed with water can create Sulphuric Acid. Either way, they both act to scatter the light because the molecules are big compared to air and in different light spectrum, can appear differently. Sometimes, the bonds are created by oxidation reactions in which the sun helps the process along, a process called photo-oxidation.