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° cone-shaped area where the sun can reside.

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

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