The physics behind the effect is relatively simple. To form clouds, the water vapor needs cloud condensation nuclei to condense upon. Without such particles, one can reach and surpass 100% humidity without getting condensation (I often joke that it would feel like DC in summer... which of course is wrong since there is enough pollution there to have as many cloud condensation nuclei as the water vapor could wish).
In the regions with the larger particle density, the same amount of condensing water vapor now has more particles to condense upon. This means that the clouds are then formed from more numerous but smaller droplets. Overall, we therefore get a much higher surface to volume ratio. Since the volume is the same (the given amount of condensing vapor), we get a larger total reflecting surface area. These clouds are therefore whiter. They also tend to live longer because it is harder for those smaller drops to grow in size, and rain out. The net result is that the pollution wakes of ships can manifest themselves as a white streak in the clouds, as is apparent in the first figure.
Last week I was on such a polluting boat, and to my amazement, one could actually see the cloud of dark polluting particles linger behind the boat for miles. The ship definitely polluted like hell!
Incidentally, this phenomenon unequivocally demonstrates that by playing around with the density of cloud condensation nuclei, one can affect the albedo (reflectivity) of clouds, and with it, Earth's average albedo. This explains why cosmic rays, which appear to affect the formation of condensation nuclei, can affect Earth's climate.