You could select the area and then using levels, curves or exposure make it darker; but you would have to blend the two areas together after as you will have a sharp line where your selection was. Even if you feather your selection you will still have a line.
If you have GMIC you could also try dehaze, not too strong as it will darken your photo.
If you have GMIC you could also try dehaze, not too strong as it will darken your photo.
