Abstract
An attempt was made to clarify the atmospheric conditions associated with high SO2 pollution, and the diurnal changes of the relation between atmospheric conditions and air pollution were discussed regarding its regional difference in a multiple-source situation. Mean hourly SO2 measurements at nine stations in Sendai (Fig. 1) were analyzed by a statistical method.
In order to evaluate not only the factors having effects upon the day-to-day variation in concentration but also the diurnal and regional change patterns of the factors, daily SO2 data were examined at each hour by using Factor Analysis. Contributions of main factors, that is, “first-factor” and “second-factor”, are shown in Fig. 3. Because of high contribution throughout the day, the day-to-day variation in the suburban area is generally explained by the “first-factor”. On the other hand, in the built-up area, a major source area, the variation in the daytime is mainly connected with the “second-factor”, though the night-time variation is caused by the “first-factor”. In the built-up area, therefore, the atmospheric condition which causes air pollution at night is considered to be different from that which causes it in the daytime.
High concentrations resulting from the “first-factor” occur in the suburban area through-out the day and in the built-up area at night under a migratory anticyclone, whereas those from the “second-factor” occur in the daytime in the built-up area under windy conditions when winter-monsoon prevails (Fig. 4).
Fig. 5 shows mean concentrations classified by wind speed at two selected stations. When it is windy, the concentration at Station-4 in the built-up area increases extremely by day, but it decreases by night. On the other hand, the concentration at Station-1 in the suburban area increases with decreasing wind speed throughout the day. According to Fig. 6, showing the high-pollution appearance classified both by wind speed and lapse rate (between Station-A (H: 38m) and Station-B (H: 220m)), high concentrations at night tend to occur under calm and stable conditions at the both stations.
Therefore, the “first-factor” corresponds to the meteorological conditions for stagnation of pollutants, while the “second-factor” corresponds to conditions for “gale-pollution”. Pollutants are generally much diluted by the strong wind, and concentration decreases with the distance from the sources. However, strong wind tends to be accompanied with extraodinary high concentrations due to exposure of undiluted plume in the vicinity of active sources, where the effect of strong wind is in opposition to its diluting action.
Thus, the relationship between concentration and meteorological condition not only varies from place to place in a small area but also changes diurnally.
