Development of a passive sampling technique for the determination of nitrogen dioxide and sulphur dioxide in ambient air

Since conventional badge-type samplers depend entirely on an empirical calibration against gas mixtures and use of the constant determined for calculations, a system based on the use of two badge-type passive samplers of different lengths has been studied for the determination of NO2 or SO2. The samplers make use of a filter membrane barrier to damp out eddy motions and promote molecular diffusion from the ambient air to the trapping medium. The badges have a large ratio of cross-sectional area to length of diffusion in order to increase sensitivity. Diffusion theory applied to this configuration shows that the concentration results are independent of wind-flow velocity. The samplers have been evaluated at different levels of relative humidity ranging from 0% to 90%. Batches of samplers were exposed for 48 h side-by-side with samplers exposed for consecutive periods of 24 h over the same interval. The results, expressed as a ratio (R) between the 48 h and the consecutive 24 h periods showed that the analytes collected (nitrite and sulphate) are stable (R=0.98+/-0.05 for nitrite and R=1.08+/-0.09 for sulphate). The detection limit attained (5 ppb of NO2 and 10 ppb of SO2 for a 24 h sampling) is adequate for the determination in urban and rural areas. The accuracy of the proposed method, expressed as percent relative error, when compared to a standard denuder technique for NO2 and SO2, was found to be better than about +/-10% with coefficient of variation of 5.5% and 6.5%, respectively.