Sulfur dioxide is most commonly measured in a regulatory and public health context by pulsed uv fluorescence: air drawn into the instrument is passed into a reduced pressure region where it is subject to a 214 nm UV irradiation, commonly produced from a xenon lamp, leading to the excitation and subsequent 220-42 nm fluorescence of SO 2 molecules within the d through a filtered PMT Po sample, which is detecte include tential difficulties with the measurement quenching of the excited so, with water vapor, avoided through the use of a diffusion dryer the instrument inlet line, and interference signals from other species, 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 of the order of 0.1 ppb detection limit for a 1 min averaging time a level that is now rarely exceeded in many developed nations