This week we're in the field with a major scientific project that's investigating ozone and other air pollutants in the Philadelphia area. As part of the project, researchers took to the air twice a day from Northeast Philadelphia airport aboard a six-seat Piper Aztec airplane, which has been modified to hold an instrument rack. The principal investigator for the research flights is Professor Bruce Doddridge from the Meteorology and Chemical Physics Departments at the University of Maryland.

With the instruments it carries, this research aircraft can detect polluted air as it's forming and follow it as it moves. Several inlet pipes are mounted on the plane, so air samples can be collected in flight and the air's temperature and humidity measured. Also measured are the levels of various pollutants, including ozone, carbon monoxide, and tiny particulate matter. Other instruments measure how sunlight is interacting with liquid and solid particles in the air, essentially putting numbers to what we loosely call "hazy" skies.

A typical research flight lasts two and a half hours, at times flying horizontally, at other times flying vertically in a spiral pattern to measure properties of the air above a particular location. Even on days that appear pollution-free from the ground, an airborne observatory like this often tells a different story. It can detect thin layers of relatively dirty air that probably formed somewhere else and blew this way with the wind.