Overestimation of urban nitrogen dioxide by passive diffusion tubes:: a comparative exposure and model study

A detailed comparative trial of passive diffusion tubes (PDT) for measurement of NO2 in urban air has been undertaken in Edinburgh, UK. Acrylic, foil-wrapped and quartz tubes were exposed in parallel for 1-week and 4-week periods at three urban sites equipped with continuous analysers for NO, NOx and O-3. Standard acrylic PDTs significantly overestimated NO2 concentrations relative to chemiluminescence analysers, by an average of 27% over all sites for 1-week exposures. No significant difference was observed between standard and foil-wrapped acrylic tubes (both UV blocking). The mean ratio between quartz (UV transmitting) tubes and chemiluminescence analysers was 1.06. Quartz PDT data suggest a tendency for in situ photolysis to offset (but in a non-quantifiable way) the effect of chemical overestimation. The 4-week exposures yielded systematically lower NO2 concentration than average NO2 from four sequential 1-week exposures over the same period. The reduction in the apparent NO2 sampling rate with time most likely arises from in situ photolysis of trapped NO2. Hourly NO1, NO and O-3 data for 20 1-week exposures were used as input to a numerical model of diffusion tube operation incorporating chemical reaction between co-diffusing NO and O-3 within the tube. The mean calculated overestimation of 22% for NO2 from the PDT model simulations is close to the average difference between acrylic PDT and analyser NO2 concentrations (24% for the same exposure periods), showing that within-tube chemistry can account for observed discrepancies in NO2 measurement between the two techniques. Overestimation by PDT generally increased as average NO2/NOx ratios decreased. Accurate quantitative correction of PDT measurements is not possible. Nevertheless, PDT NO2 concentrations were correlated with both analyser NO2 and NOx suggesting that acrylic PDTs retain a qualitative measure of NO2 and NOx variation at a particular urban location. (C) 1999 Elsevier Science Ltd. All rights reserved.