The scientists studied how different chemicals in the air combine and form crystals. They made tiny particles in the lab and tested how they reacted to different conditions. They found that a mix of ammonium nitrate and ammonium sulphate doesn't always turn into crystals, even though people thought it did in the air. Adding soot doesn't help either. But when the mix has equal parts of both chemicals, it does form crystals. This is similar to what happens in the real world. The researchers concluded that in the air, particles are made of a mix of ammonium nitrate and ammonium sulphate.