You will now clip the layers to the ca_outline layer as you are only interested in mapping the ozone levels within the state of California. This will produce a more appealing map.

To do so, right-click Layers and click Properties. Click the Data Frame tab. Check the Enable check box in the Clip to Shape area. Click Specify Shape. Click Outline of Features. Click the Layer drop-down arrow and click ca_outline. Click OK on the Data Frame Clipping dialog box. Click OK to close the Data Frame Properties dialog box.