As you can see in the next image, I build a forest (read square) to bring the pollution down to 0. But in the calculation screen you can see that the forest is not listet an the city have a total pollution of 1.

In my next example I tried to create a city with negativ pollution to compensate the pollution of other cities. With this strategy the forests (red squares) also doesn't count in the calculation. 

To reproduce this issue build one building with pollution >0 and after that build one forest.

My expected solution ist, that a forest counts correctly as -10 pollution. Furthermore it should be possible to build a city with negativ pollution.

Thanks for checking.