The study looked at how the urban heat island in Hangzhou, China affects air pollution by using a computer model. They found that the urban heat island caused a decrease in pollution near the ground but an increase higher up in the atmosphere. This was mainly due to the way the heat island affects air circulation. The movement of air from the city to higher altitudes brought pollution with it, leading to higher levels of certain types of aerosols. Specifically, the urban heat island effect led to an increase in secondary inorganic aerosols, like ammonium nitrate, in the lower free troposphere.