In spite of the recent increasing interest in energy production from renewable sources, polluting hydrocarbon-fueled power plants will continue to provide most of the electricity to the world's population in the coming decades. This work studies the long term impact on the regional ambient air which can be attributable to three plants with different power outputs, fuel types, and stack heights. The study is carried out in an area with relatively flat topography and typical coastal meteorology. A dense air pollution monitoring network, operating for many years, makes this area a real life laboratory for studying the pollution routes, the impact of the sources at different directions and distances, and the effects of transition to cleaner fuel. The direct impact of each of the two large power plants on the ambient SO2 levels could be clearly detected in most of the monitoring stations at distances up to 40 km away. Interestingly, a relatively large impact can also be attributed to the indirect effect of emissions that are recirculated back to the region with the land breeze. The transition from using fuel oil to natural gas in one of the large power plants resulted in a dramatic reduction in the mean SO2 levels in all of the monitoring stations. The contribution of the industrial emissions to the ambient NO2 levels seems to be very modest relative to that from traffic. An analysis of the NO, NO2 and O3 records suggests that the highest mean NO2 concentrations, and a large proportion of the total NO2 encountered in the study area, are probably due to recirculated NOx emitted by traffic in a densely populated region north of it.