Monitoring of benzene, toluene and p-xylene in urban air with differential optical absorption spectroscopy technique

Instruments based on the differential optical absorption spectroscopy (DOAS) technique are used for monitoring air pollutants in urban areas all over the world. In Göteborg (Sweden) the DOAS technique has been used for several years for continuous monitoring of aromatic hydrocarbons and other traffic-related air pollutants. In the present study, measurements of benzene, toluene and p-xylene performed by DOAS have been evaluated. In an intercomparison study, adsorbent sampling and gas chromatography were used for measurements at the same sites as the DOAS instruments. The evaluation illustrated that the quality of the DOAS measurements was unsatisfactory even at concentrations exceeding the recommended lower limits. The proportions of benzene, toluene and p-xylene are usually uniform in vehicle polluted urban air. A significant deviation from the expected proportions may be used as an indication that the performance of a DOAS instrument should be checked.