The complete tube is positioned at the monitoring site, vertically oriented with the open end facing downward, and left for a period of weeks (see comment later), during which time NO2 diffuses into the tube and reacts with TEA forming a nitrite. Following exposure, the tube is analyzed by colorimetry : generally, solutions of sulf-anilamide in orthophosphoric acid and napthyl ethylene diamine dihydrochloride (NEDA) are added to form azo dye, the concentration of which is measured by its absorption at approximately 540 nm. The resulting nitrite level in the diffusion tube absorbent is related to the average ambient NO2 levels during exposure via the tube dimensions and appropriate (temperature-dependent) diffusion coefficient for NO, in air