A NUMERICAL EVALUATION OF CHEMICAL INTERFERENCES IN THE MEASUREMENT OF AMBIENT NITROGEN-DIOXIDE BY PASSIVE DIFFUSION SAMPLERS

Passive diffusion samplers containing tri-ethanolamine (TEA) as adsorb ent are widely deployed for measuring NO2 concentrations in urban and rural air. In this study, a one-dimensional model of the sampler incor porating diffusive transport and chemical reaction has shown that NO2 concentrations are systematically overestimated as a result of chemica l reactions in the tube, dominated by the reaction of NO with O-3 to g ive NO2. The contribution of PAN and other potential interferents is s hown to be small for British conditions. The extent of overestimation depends on the instantaneous [NO]/[NO2] ratio and on the relative conc entrations of NO and O-3. In rural air, where [NO] < [NO2], [NO] much less than [O-3] and total NOx(NO + NO2) concentrations are typically l ess than 5 ppbV (parts in 10(9) by volume), the model shows that diffu sion tubes effectively measure NOx rather than NO2 concentrations, alt hough in practice the difference between NO2 and NOx concentrations is likely to be within the measurement uncertainty. In urban air, howeve r, close to sources of NO, where average [NO] is of a similar magnitud e to [NO2], a large overestimate of NO2 is made. Using hourly measured gas concentration data for a summer month in Edinburgh, the model pre dicted 28% more NO2 captured by the diffusion tube than measured as NO 2 by a continuous monitor. The overestimate at rural sites, and at the urban site in winter, was between 8 and 14%. The systematic error of within-tube chemistry is independent of, and acts in the same directio n as, the possible overestimation caused by an effective shortening of diffusion path length at high wind speeds and may be one explanation for diffusion sampler overestimations previously attributed to wind ef fects. There appears to be no simple method for retrospectively correc ting NO2 measurement data obtained using diffusion tubes since the ove restimate is not simply related to the average [NO]/[NO2] nor [NO]/[O- 3] ratios, but reflects the dynamical behaviour throughout the samplin g period. Although passive diffusion samplers may give reliable concen tration data for NOx in rural air, they cannot be used with confidence in polluted urban air to estimate NO2 concentrations. (C) 1997 Elsevi er Science Ltd.