Instead of just using the airbrush to get shadow effects, consider using the bump modes. I've included an image (with your picture in AR) to show you where these are. If you create another layer, set your opacity to less than 20%, and go to "soft light" you can then choose a dark color in the same tone as your lit area, and apply it in the shadow areas how ever you like: airbrush, paintbrush, pencil, art pen... whatever. If you go over it two or three ways in this manner, you can get pretty realistic lighting effects with ease.
Early to bed early to rise makes a man stupid and blind in the eyes - Orson Scott Card