Change the Blending Mode of this layer from Normal to Color for better effect.

Now you make the image bit darker. Add another adjustment layer Curves on the top of the layers and set it as on the following picture.

On the following image you can see the adjustments you've made in this step.

Step 8 - Darkening the Sky

Sky on the image should be bit more darker to create more balanced feel. To fix that add new adjustment layer Curves and set it as on the following image.

You want to adjust only the upper part of the image. To do that first you need to grab the Paint Bucket Tool (G), pick black color and fill the layer mask of this adjustment layer with it.

Grab the Gradient Tool (G), pick white color and select Linear Gradient.

Fill the upper part of layer mask with this gradient. You can get the idea where to use the gradient on the following image. I highlighted it with red color.

Step 9 - Adding Smog

Another color that comes in hand with pollution (according to me at least) is green. In this step you'll add green smog on the horizon to add even more atmosphere. Add new empty layer on the top of all layers and name it SMOG. Grab the Gradient Tool (G) and this time select Reflected Gradient.

Pick some green color (I used #6c7345) and create gradient as on the picture below.

To blend it well with the rest of the image change its Blending Mode from Normal to Overlay and lower its Opacity on about 65%.

On the following photo you can see how your photo manipulation should look like so far.

Step 10 - Red Cars

I believe that the the bottom part of the image where streets of the city are is quite uninteresting right now. You'll add interest by creating red lights around the roads. It would look like there is lots of traffic down there. Add new empty layer on the top of all layers and name it CARS. Grab the Brush Tool (B), pick some soft round brush and lower its Opacity on about 40%. Pick some reddish colors and start painting over the roads. Look at the following picture to get the idea where you should paint.

To blend it with the rest of the image change Blend Mode of this layer from Normal to Overlay.

Step 11 - Darker Sky

Sky should be even more darker. You'll change it in this step. Add new adjustment layer Curves on the top of all layers and set it as on the following picture.

To adjust only sky and not the rest of the image grab the Paint Bucket Tool (B) and fill the the layer mask of this adjustment layer with solid black color. Grab the Gradient Tool (G), pick white color and make linear gradient from the upper part of the image to horizon similarly as you did in one of the previous steps. You can see where to put gradient on the following photo. That area is highlighted with red color.

After this step your photo manipulation should look similar as the picture below.

Step 12 - Lighter Center

In this step you'll make the center of the image lighter. It will create more dimension and drama in your photo manipulation. Human eye first looks at the bright parts of the image. You can use this fact to lead viewer's eye on the parts of the image you want. Add new adjustment layer Levels and set the Input Levels on 0; 1,00; 175.