ca. 2000: This commercial instrument has been modified to replace the commercial optical windows with highly polished (10-lambda) sapphire windows. This modification inhibits the condensation of water in micro-crevices during transitions between very humid and dry conditions, which has been observed to be a source of transient spikes (Kleindeinst 80’s or early 90’s). With the improved optical windows, this ozone instrument is not vulnerable to artifactual transient noise at sharp humidity interfaces. The time resolution of this measurement is 10 seconds. The detection limit is 1 ppbv with an uncertainty of ±5%.