Molecular diffusion sampler, which does not require a power source and is produced at low cost, is good for simultaneous measurements at several places. We measured the concentration and distribution of SO
2 and NO
2 in Sapporo city in the summer and winter of 1988 using the molecular diffusion samplers re-modelled to tolerate the temperature below 0°c. Samplers were set up in 50 locations and included 6 air pollution monitoring stations where automatic measuring instruments were used by the standard method to adjust the molecular diffusion samplers. The calibration curve and the distribution of volume of gas sampled at each location showed the error range of measurements made by the molecular diffusion samplers was 6-8 %. The concentration maps of this measurement obtained at ground level showed high concentrated SO
2 around the center of the city and over the industrial areas, and NO
2 appeared along the main truck roads narrowly and highly concentrated. Regard ing the seasonal changes, SO
2 concentration scarcely changed but NO2 concentration in the whole area doubled in winter. This is believed to be caused by SO
2 and NO
2 emissions at different heights combined with the condition of boundary layer at nighttime in winter.
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