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 O3. 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 mostlikely arises from in situ photolysis of trapped NO2. Hourly NO2, NO and O3 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 O3 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.