Simple systems for calibrating and auditing SO2 monitors at remote sites

Described are compact, lightweight, rugged, and simple-to-operate pollutant generating systems that can be used to calibrate and to audit SO₂ and possibly other ambient air monitors located at inaccessible sites. The field and laboratory experiments done during the development of these systems are also described. The laboratory studies indicated that the systems could also be used for NO₂, but the field tests were inconclusive. The portion of the systems used to generate the dilution air is sufficiently inexpensive and modular that many networks could dedicate one zero (dilution) air system to each field site. The systems use a diaphragm pump, a cartridge containing Purafil/silica gel, a cartridge containing silica gel, a flow measuring system, and a low output permeation tube to generate pollutant concentrations between 1 and 30 ppbv with an accuracy (for SO₂) of better than 2 ppbv. To avoid using a temperature control device such as a bath or oven, the output of the permeation tube as a function of temperature is determined in the laboratory. At the field site, the temperature of the dilution air flowing over the permeation device and the barometric pressure are used to determine the output of the permeation device.