Welsh Journals

To consider the effect of the power station further a more sensitive method of analysis has been devised which involves a comparison of S02 concentrations at pairs of sites situated respectively upwind and downwind of the power station chimney on days of steady wind. Any extra S02 in the air due to the power station would then show up as increased S02 concentrations downwind. The wind direction needed to place each site downwind of the chimney was determined, and then an admissible range of variation of wind directions, ± 1 0 ° in most cases, was assigned to each pair of sites. As the daily change over of the sampling instrument varied from site to site within range 10.00-15.00 h., only the remaining 18 hourly wind directions were used. Whenever a site was found to be downwind of the chimney for at least 8 hours out of these 18 hours, the difference in S02 concentration between the downwind and upwind site was calculated. All the occasions when this occurred were then considered and the average increase due to passing the power station was calculated and compared with the change that occurred on similar days before the power station was operating. The statistical significance of the difference between these two means was then determined using the Student t test. Altogether ten site pairs were investigated, using the pre-commissioning data of 1969 and the post-commissioning data of 1971-72. The results of this analysis are given in Table 2, which shows that in passing the power station site, in 4 cases the pollution was increased and in 6 it was reduced below the changes found before the power station was operating. This is just the kind of result that would be expected from chance alone, with the power station absent. Clearly the power station had not increased the S02 downwind appreciably. Table 2. Comparison of SO2 concentration differences (injjg m ) between upwind and downwind sites in 1969 (Y,) and 1971-72 (f2) (DEGREES OF FREEDOM, d.f. = NUMBER OF OBSERVATIONS 2). Increase due to passing Power Station site Site 1969 1971-72 Significance pair* 71 72 t d.f level (%) 10-13 -23.0 -23.5 -0.12 36 NS 13-10 43.5 30.1 -2.40 42 -98 12-18 -13.4 -1.9 2.19 32 95 18-12 7.3 -1.9 -2.82 37 -99 13-17 15.8 4.0 -1.82 21 NS 17-13 -10.3 -5.5 0.94 30 NS 13-19 18.2 14.5 -0.83 27 NS 19-13 -20.0 -10.9 1.76 33 NS 12-10 35.5 30.0 -0.99 43 NS 18-7 1.0 1.3 0.07 29 NS First site upwind NS =not significant The two most significant changes were reductions during the post-commissioning period. The increase by 11.5pg m-3 noted at the site pair 12-18 (Table 2) was only significant at the 95% confidence level and could easily be due to chance in a sequence