A simple sampler for surveillance of time integrated SO2 and NO2 concentration distributions has been developed. The sampler is inexpensive in use, simple to handle and have a good overall precision and accuracy. This method has been used in industrial areas, in urban areas and for studies of indoor/outdoor exposures.

A sensitive diffusion sampler for sulphur dioxide (SO2) and nitrogen dioxide (NO2) in ambient air has been used in several investigations to undertake a screening of the spatial concentration distribution.

The 24 h average NO2 concentration distribution measured in Oslo on 3 -4 February 1994 with 20 passive samplers show that the highest concentrations occurred along the main road systems and in central parts of down-town Oslo. This 24 h average distribution may change considerably from one day to the next depending on meteorological conditions in the Oslo airside.

The sampler was developed by the Swedish Environmental Research Institute (IVL) and has been used in several cases by NILU. The sampler includes an impregnated filter inside a small plastic tube. To avoid turbulent diffusion inside the sampler, the inlet is covered by a thin porous membrane filter. Gases are transported and collected by molecular diffusion. The uptake rate is only dependent upon the diffusion rate of the gas. The collection rate is 31 l/24h for SO2 and 36 l/24h for NO2. Also NH3 can be collected at a rate of 59 l/24h.

For SO2 the measuring ranges are approximately 0,1-80 ppb for a sampling period of one month. The corresponding range for NO2 is 0,02-40 ppb.The passive samplers are assembled and made ready for use at NILU. After exposure the samplers are usually returned to NILU where concentrations of SO2 are determined as sulphate by ion chromatography. NO2 and NH3 is determined by spectrophotometry.

Some examples of applications
Studies using passive sampling have been undertaken by NILU outdoor and indoor in Norway, Egypt, Russia and Venezuela. The sampling periods have varied from 1 day to one month dependent upon the general pollution levels.

The integrated passive sampling of SO2 and NO2 is well correlated to the available active sampling methods.

Several investigations have been performed to study the indoor to outdoor NO2 pollution levels. In urban locations in Oslo indoor NO2 concentrations were considerably lower and varied less than outdoor concentrations. At 40 µg/m3 ambient NO2 the indoor NO2 concentration was on the average about 10 µg/m3.

A three day integrated SO2 concentration distribution 4 km downwind from a large refinery show a SO2 plume passing across the sampler array. The plume width was slightly more than 1 km and the maximum SO2 -concentrations about 800 µg/m3.

A SO2 concentration screening performed with passive samplers in the Helwan industrial area located south of Cairo in Egypt indicated weekly average concentrations of 40 to 50 µg/m3 downwind from major industrial sites.

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