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 measureme nt 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 per iods at three urban sites equipped with continuous analysers for NO, NOx an d O-3. Standard acrylic PDTs significantly overestimated NO2 concentrations relative to chemiluminescence analysers, by an average of 27% over all sit es for 1-week exposures. No significant difference was observed between sta ndard and foil-wrapped acrylic tubes (both UV blocking). The mean ratio bet ween quartz (UV transmitting) tubes and chemiluminescence analysers was 1.0 6. 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-we ek exposures yielded systematically lower NO2 concentration than average NO 2 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 sit u photolysis of trapped NO2. Hourly NO1, NO and O-3 data for 20 1-week expo sures were used as input to a numerical model of diffusion tube operation i ncorporating chemical reaction between co-diffusing NO and O-3 within the t ube. The mean calculated overestimation of 22% for NO2 from the PDT model s imulations is close to the average difference between acrylic PDT and analy ser NO2 concentrations (24% for the same exposure periods), showing that wi thin-tube chemistry can account for observed discrepancies in NO2 measureme nt 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 cor related with both analyser NO2 and NOx suggesting that acrylic PDTs retain a qualitative measure of NO2 and NOx variation at a particular urban locati on. (C) 1999 Elsevier Science Ltd. All rights reserved.