Amendments to the Washington Clean Air Act in 1991 required modifications to the Smoke Management Plan. The major change was that participation in the program was no longer voluntary (except for Indian tribes).

Particulate matter, or visible smoke, is regulated through the ambient air quality standard for Total Suspended Particulates (TSP). The standards for TSP cover a range of particles from 0 to 50 microns. Particulate matter of this size can remain suspended in the atmosphere for periods of a few seconds to several months. Suspended particulate matter is that portion of the TSP (<10 microns) which is transported large distances downwind and can have the greatest impact on air quality.

The EPA recommended that TSP, as an indicator of particulate matter, be replaced for primary and secondary standards by standards which include only those particles less than 10 microns (PM-10). The PM-10 standards focus on particles less than 10 microns because particles of this size are inhalable and can therefore affect human health (for comparison, a human hair is 70 microns).

Ninety percent (90%) of smoke particles from prescribed fires, or slash burns, are less than 10 microns, easily breathable and also good scatters of light. Eighty two percent (82%) or smoke particles from prescribed fires are less than 1 micron. Particles of this size can be trapped in alveoli in the lungs.

Fires can be classified into three main categories:

Prescribed Fires

These are fires ignited by land managers to achieve specific resource management objectives such as 1) site preparation for replanting, 2) aesthetic enhancement, 3) controlling undesirable species, 4) perpetuating fire dependent plant and animal species, 5) controlling insects and forest diseases, 6) improving wildlife habitat, and 7) wildfire hazard reduction.

Prescribed Natural Fires

These fires are naturally ignited fires (lightning or person-caused) but allowed to burn if they meet certain predetermined management objectives.

Wildfires

Wildfires are natural or person-caused fires that do not meet predetermined management objectives and are therefore actively suppressed.

Smoke management programs have three main objectives. They are :

1. Identify and avoid smoke sensitive areas

2. Reduce emissions

3. Disperse and dilute smoke before it reaches smoke sensitive areas.

The key to an effective smoke management program lies in the land manager's ability to combine favorable weather conditions with a variety of prescribed fire techniques to keep smoke emissions to a minimum.

Land managers use three techniques to manage smoke emissions from prescribed fires. They are:

Avoidance - Conduct prescribed burns on days when smoke intrusions into smoke sensitive areas is highly unlikely, i.e., burn on days when the winds will carry smoke away from smoke sensitive areas.

Dispersion/Dilution - Dispersion refers to processes within the atmosphere which mix and transport pollutants away from the source. Dispersion depends on three characteristics of the atmosphere; atmospheric stability, mixing heights, and transport winds. Burning during periods of good vertical dispersion or burning at slower rates will dilute smoke through greater volumes of air.

Emission reduction - Effective firing techniques and the proper scheduling of burns can minimize smoke output per unit area burned. Effective firing techniques include 1) backing fires which tend to minimize the inefficient smoldering phase, 2) burning when the duff and larger fuels are too wet to burn, producing less emissions, 3) increased utilization, i.e., remove more slash material from the burn site, 4) mop up as soon as possible to minimize the smoldering phase of the fire, 5) expand the burning season, 6) keep soil out of dozer piles and windrows, and 7) cover piles to keep fuels dry.

Local winds such as valley/slope winds or land/sea breezes are also important considerations in all smoke management forecasts. Local winds often become the transport winds, especially when winds above the fire become light, or at night when radiational cooling causes a surface-based inversion to form in the boundary layer, resulting in very low mixing heights.

Local downslope/downvalley winds can also cause residual smoke to drain downvalley into populated areas overnight. This is especially important for prescribed fires in the mopup or smoldering stage, since emissions from a smoldering fires are twice that of a flaming fire.

Atmospheric stability (a measure of the tendency for mixing in the vertical) determines the volume of air through which smoke will mix and therefore, it's concentration in the atmosphere. Most NWS offices forecast mixing heights which are defined as the height above the surface through which relatively vigorous mixing will take place in the vertical due to convection. Mixing heights are based on the intercept of a lifted parcel of air with the upper air sounding. Some offices forecast a Ventilation Index which is the product of the mixing height and the transport wind speed. A few NWS offices - mainly those in the Southern Region - forecast Pasquill Stability Categories which combine surface winds speeds with solar insolation and cloud cover to arrive at six stability categories ranging from A (extremely unstable) to F (moderately stable).