Atmospheric ozone measurement with an inexpensive and fully automated porous tube collector-colorimeter

The bleaching action of ozone on indigo and related compounds is well known. We describe sensitive automated instrumentation for measuring ambient ozone. Air is sampled around a porous polypropylene tube filled with a solution of indigotrisulfonate. Light transmission through the tube is measured. Light transmission increases as O3 diffuses through the membrane and bleaches the indigo. Evaporation of the solution, a function of the RH and the air temperature, can, however cause major errors. We solve this problem by adding an O3-inert dye that absorbs at a different wavelength. Here we provide a new algorithm for this correction and show that this very inexpensive instrument package (controlled by a BASIC Stamp Microcontroller with an on-board data logger, total parts cost US$ 300) provides data highly comparable to commercial ozone monitors over an extended period. The instrument displays an LOD of 1.2 ppbv and a linear span up to 300 ppbv for a sampling time of 1 min. For a sampling time of 5 min, the respective values are 0.24 ppbv and 100 ppbv O3.