Many things are carried around in the city's air. Solid particles suspended in the air are a form of air pollution. Scientists form explanations from observations, previous experiences and conversations with others who are investigating the same question. When scientists have different explanations for the same observations, making more observations may help resolve the differences.
Choices have consequences, some more serious than others.
Have groups of students construct a number of particle collectors. Tape plastic wrap as flat as possible over one of the window holes, securing all edges. Overlay the other part of the frame for strength and support, then tape the edges closed. Spread a THIN layer of Vaseline over the plastic wrap window. The idea is to see the dirt, not the Vaseline layer.