Time-tagged Photon Detection for Measuring Air Pollutants

Air pollution is one of the biggest environmental concerns worldwide due to its damaging effects on ecosystems, the economy and human health. One harmful air pollutant is nitrogen dioxide (NO2) which, in the UK, mainly emanates from vehicle emissions. NO2 affects the respiratory system and is particularly problematic for children, the elderly and people with existing health conditions, such as asthma. Accurate monitoring of air pollutants is therefore vital in order to: 1) assess which locations are affected, 2) monitor the efficacy of pollution controls, and 3) inform policy makers of ways to control and/or mitigate the effects of poor air quality. This thesis develops a novel “time-tagged” spectroscopic instrument to directly quantify NO2 at the concentrations present in ambient air (but with wider applications for also measuring other trace gases). By recording time- and wavelength-resolved information about light passing through a ring-down cavity, the new time-tagged approach combines advantages from two established techniques. Firstly, like in broadband cavity enhanced absorption spectroscopy (BBCEAS), the sample’s absorption spectrum is measured over an extended bandwidth (here 440 – 470 nm), enabling NO2 to be identified and fitted by its unique absorption features. Secondly, measurement of the light’s phase delay, like in phase shift cavity ring-down spectroscopy (PSCRDS), provides the absolute measure of the cavity mirrors’ reflectivity otherwise lacking in conventional BBCEAS. The time-tagged instrument achieved a detection limit of 3.5 ppbV (1 in 9 minutes) in laboratory experiments using NO2 diluted in nitrogen. The instrument also demonstrated strong agreement with a commercial CAPS NO2 detector: a linear correlation plot of the two datasets yielded a gradient close to 1:1 (1.013  0.008) and a zero offset (−0.02 ± 0.6 ppbV). Typical concentrations of NO2 measured by the time-tagged instrument in ambient air were 19.2 ± 4.1 ppbV.