Sun Position and Observer Position for Method 9

Sun Position

It is important to have the sun at your back when performing visible emission observations. Method 9 specifies that the sun must be oriented in a 140° sector to the observer's back. This includes both the vertical and horizontal sector, resulting in a 140° area shaped like a cone where the sun can reside.

If the sun is behind the plume (i.e., in front of you), the plume can become more luminous than the background and creates a high bias or positive error in your opacity reading. This is due to the effect from the forward scattering of light.

Forward Scattering of Light +More

Observer Position

The opacity reader should observe the following when performing observations:

• The observer should stand at a distance from the stack that is about three times the height of the stack.
• The observer's line of sight (LOS) shall be approximately perpendicular to the track of the plume. This places the observer in a position that causes them to look up from the horizon by about 18°.

There are several things to consider when determining the observer position; details are covered in the Performing Observations section.