Comparative evaluation of nitrogen oxides and ozone passive diffusion tubes for exposure studies

Passive diffusion tubes are recognised as a cost-effective sampling method for characterising the spatial variability, as well as the seasonal and annual trends, of NO₂ concentrations in urban areas. In addition, NO_X and O₃ passive diffusion tubes have been developed and deployed in urban and rural areas. Despite their many advantages (e.g. low operational and analysis cost, small size and no need for power supply), they have certain limitations mainly related to their accuracy and precision. In particular, the absorbent solution used, the length of the exposure period, the exact location and use of protective devices, and other environmental conditions (e.g. wind, ambient temperature and relative humidity) may have a significant impact on the performance of passive diffusion tubes. The aim of this study is to evaluate the performance of co-located NO₂, NO_X and O₃ diffusion tubes in an urban environment. A one-year passive sampling campaign was carried out in Birmingham (UK) for this purpose. NO₂, NO_X and O₃ diffusion tubes (including triplicate sets of each) were co-located at one urban background and two roadside permanent air quality monitoring stations equipped with standard gas analysers. In addition, meteorological data, such as wind speed and direction, ambient temperature and relative humidity, were obtained during the same period of time. A thorough QA/QC procedure, including storage and laboratory blanks was followed throughout the campaign. The analysis of results showed a very good agreement of NO₂ passive samplers with co-located chemiluminescence analysers, but substantial underestimations of total NO_X levels by the diffusion tubes. The O₃ diffusion sampler appeared to marginally overestimate the automatic UV analyser results, especially during warm weather periods.