sulfur dioxide is most commonly measured in a regulatory and public health context by pulsed UV into rescence air draw into the instrument is passed reduced pressure region where it is subject to a 214 nm UV irradiation, commonly produced from a zinc or xenon lamp, leading to the excitation and subsequent 220-420 nm fluorescence of SO, molecules within the sample, which is detected through a filtered PMT Po tential difficulties with the measurement include quenching of the excited SO2 with water vapor, avoided through the use of a diffusion dryer on the instrument line, and interference signals from other which can be excited/fluoresc at similar wavelengths, in particular, aromatic hydrocarbons; this is circumvented by scrubbing of the hydrocarbon species using a trap or semipermeable membrane. Typical instrument perfor mance is order of 0 ppb detection limit for a l min of the exceeded averaging time a level that is now rarely in many developed nations