Visual Determination of the Opacity of Emissions from Stationary Sources – USEPA Method 9

USEPA Method 9 involves the visual determination of the opacity of emissions from stationary sources by qualified observers.  The method includes procedures for the training and certification of observers and procedures to be used in the field for determination of plume opacity.
 
To receive certification as a qualified observer, a candidate must be tested and demonstrate the ability to assign opacity readings in 5 percent increments to 25 different black plumes and 25 different white plumes, with an error not to exceed 15 percent opacity on any one reading and average error not to exceed 7.5 percent opacity in each category.  The certification is valid for a 6-month period, at which time the qualification procedure must be repeated in order to retain certification.
 
The appearance of a plume as viewed by an observer depends upon a number of variables, some of which may be controllable in the field.  The method includes specific criteria applicable to these variables.  Variables which can be controlled to an extent to which they no longer exert a significant influence upon plume appearance include:

• angle of the observer with respect to the plume;
• angle of the observer with respect to the sun;
• point of observation of attached and detached steam plume;
• angle of the observer with respect to a plume emitted from a rectangular stack with a large length to width ratio.

The observer records the following information for visual determination of the opacity of emissions:

• position
• field records
• observations
• data reduction

Two types of plumes may exist: attached steam plumes and detached steam plumes.  Attached steam plumes are observed when condensed water vapor is present within the plume as it emerges from the emission outlet.  Observations are made beyond the point in the plume at which condensed water vapor is no longer visible.  The observer records the approximate distance from the emission outlet to the point in the plume at which the observations are made.
 
Detached steam plumes are observed when water vapor in the plume condenses and becomes visible at a distinct distance from the emission outlet.  The opacity of emissions should be evaluated at the emission outlet prior to the condensation of water vapor and the formation of the steam plume.