Assessment

AN AIR-QUALITY STRATEGY

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Goals

Three primary goals for an air-quality strategy for the Sierra Nevada are:

1. Reduce ozone levels and associated impacts.

2. Reduce fine-particle pollution and associated impacts.

3. Minimize smoke levels while maximizing the beneficial use of fire.

Possible Solutions

Ozone

Through rigid enforcement of the current California state standard of 0.09 ppm ozone, peak hourly rate, only modest damage would be expected for plant species known to be sensitive. Note that acceptance of the federal standard of 0.12 ppm, peak hourly rate, would not result in elimination of vegetation damage. New technology has been adopted and allows for identification of grossly emitting vehicles as they drive along the highway; removing these vehicles from service provides an effective means of significantly reducing emissions. In addition, reformulated gasolines now coming into use further reduce emissions of ozone precursors.
The dramatic decline of peak ozone concentrations that have been seen in recent years in areas like Los Angeles with high vehicular densities are not being seen in the Central Valley. Thus, it has become evident that confounding valley sources must exist. Most likely is the intense biological activity associated with agriculture and its accompanying emissions of hydrocarbon and ozone precursor gases. Also, regulatory controls have to date been somewhat less stringent in the Central Valley than in Los Angeles. The federal and particularly the state air-quality agencies are beginning to pursue ozone control measures associated with these apparent sources.

Fine-Particle Pollution

The Clean Air Act of 1977, extended by the amendments of 1990, mandates the mitigation of human sources of fine-particle pollution insofar as they degrade visibility in Class I areas such as Yosemite National Park. This can be accomplished by enforced limitations of upwind emissions of sulfur in the Bay Area and San Joaquin Valley, especially the oil refineries and chemical plants near the Carquinez Strait; continued efforts to control oxides of nitrogens, and tighter controls on or elimination of all agricultural burning during summer months. These measures would result in sharply improved visibility and the accompanying reduction of fine-particle deposition.

Smoke

Increasing by a factor of five the annual acreage of Sierra Nevada forests burned by surface-burning controlled burns and prescribed natural fire would reduce overall pollution from smoke. Burning would be concentrated in spring (mid-April through mid-June) and fall (mid-September through mid-November) to avoid coinciding with peak summer levels of smoke originating in the Central Valley. The increase in local and subregional smoke associated with prescribed burns must be traded off against the large regional smoke plumes of the wildfires that can be expected without increased prescribed burning.
Smoke originating from residential areas within the Sierra Nevada can be reduced by burn and no-burn days, highly efficient woodstoves, and changes in fuel from local pine to dried fruitwoods. Even more beneficial is an increasing transition from woodstoves of all kinds to natural gas, when available.

Implications

Meeting the air-quality goals has three principal implications:

1. Evidence indicates that if peak hourly ozone values remain below 0.09 ppm, injury to Jeffrey pine, ponderosa pine, and other sensitive species would be decreased.

2. The economic values associated with tourism would be enhanced by higher scenic visibility. Deposition of potentially harmful pollutants on vegetation, soils, and hydrologic systems would be reduced.

3. Comparing data from the 1992 Cleveland fire in the Eldorado National Forest with calculations for optimizing a fivefold increase in the annual controlled burn acreages for this same forest, indications are that there would be drastically lower levels of regional particle loading achieved by the application of human prescribed fire. Levels would be even less than the average daily winter levels typical for the woodstove smoke component at mountain communities like Truckee.

Although the particulate pollution levels from this strategy meet state and federal standards and would not greatly increase particulate smoke in towns downwind, it is not true that there would be no impacts. Fires, besides being unsightly, can carry allergens to susceptible human populations, with accompanying short-term respiratory impacts. This effect limits the amount of material that can be burned at any one time and place and should caution application of prescribed fire that would overlap with winter smoke problems already typical of some urban areas.
The proposed new federal fine-particulate standard, conceived as an annual average based upon a 2.5 micrometer cut point (PM-2.5), would put additional pressures on mountain urban communities to control winter smoke levels. The standard would actually favor this strategy, because the additional smoke from prescribed fires would not be exacerbating already elevated urban levels.
Spatial trends for air-quality concerns have been noted from the northern to the southern end of the Sierra Nevada. Trends for west to east transport of Central Valley and Bay Area pollutants are also clearly mapped, with many sources displaying clear signatures to the sophisticated monitoring apparatus already in place. Continued and enhanced monitoring will provide ample opportunity for gauging and interpreting the success of reducing Bay Area sources of fine particles, valley sources of biomass burning and ozone generation, and smoke from prescribed fire versus wildfire in the Sierra Nevada itself. A reversal of current trends would be obvious.