Cost-efficient techniques for urban- and background measurements of SO2 and NO2

Urban and background concentrations of SO2 and NO2 have been measured in Sweden for the last 30 yr. During this time the measuring techniques have continuously been improved. Today very cost-efficient, sensitive and accurate techniques are used to measure concentrations on a monthly and on a daily basis. Background as well as urban concentrations are measured on a monthly basis using diffusive samplers. Diffusive (passive) samplers have also been used extensively in workplace environments. When exposing diffusive samplers to outdoor air it is important that the wind does not affect the sampling rate by creating turbulent diffusion inside the sampler, and thereby increasing the sampling rate in an unpredictable way. This wind effect was first solved for a long tube sampler, aimed only for urban measurements. Later, a shorter and wider sampler was developed with a measuring range for monthly sampling of about 0.1–200 μg m-3 for SO2 and 0.1–400 μg m-3 for NO2 i.e. sensitive enough for background measurements but still satisfactory in town centres. The samplers can be stored at room temperature prior to as well as after sampling. 24 h average concentrations can be determined using an automatic sampler (active sampling). Comparison between diffusive and active techniques have been made under realistic conditions (routine monitoring) in both background and urban air. The two techniques agree within ca. ±15% which today is reasonable for routine monitoring.