List eight major classes of primary outdoor pollutants

Outdoor air pollution is a complex mixture of several pollutants, the common sources of outdoor pollution are emissions caused by combustion processes from motor vehicles, solid fuel burning and industry. 
They major classes of outdoor pollutants include;
Carbon monoxide; carbon dioxide, sulfur dioxide, nitrogen oxides, VOC's, SPM's, photochemical oxidants, ozone and PANs.

When scientist examine data such as historical documents, they are using the "scientific methods"