ASSESSMENT

An Air-Quality Strategy

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Air quality in the Sierra Nevada is highly variable in qualityexcellent much of the time and in many places, seriously degraded at other times and places. Many early writers extolled the quality of the air, and in the early twentieth century the Sierra Nevada was even the site of sanatoriums. Yet the Sierra Nevada was typically quite smoky in the summers as many small fires burned for months until the rains extinguished them each fall. There are two distinct aspects of air-quality issues in the Sierra. The first relates to state and federal ambient air quality standards (ozone, particulate mass, visibility reduction), which are periodically violated in the Sierra Nevada. The second relates to air-quality impacts not subject to ambient-air standards (acid deposition, transport of air toxics, eutrophication of Lake Tahoe), which have a more ecological than human health focus.
At present, the most important deleterious impacts are closely tied to the efficient wind transport of air pollutants from the Central Valley of California into the western slopes of the Sierra Nevada up to elevations of 6,000 feet or more. This transport is strong in summer, weak or absent in winter, severe in the southern reaches, and more modest north of Sacramento, where mountain slopes are more gentle. Of these pollutants, ozone has the best documented and most important effects, especially in its connection to serious injury to Jeffrey and ponderosa pines. Fine-particulate sulfates, nitrates, and smoke are also transported by the same winds, especially between April and October, and sharply reduce visibility. Other components of valley air, including nitrates, pesticides, and herbicides, are also efficiently transported into the mountains and deposited on vegetation and in watersheds, often with poorly understood but potentially significant effects. For example, the suggestion that valley air-quality changes may be a factor in the precipitous decline of some amphibians since the late 1960s needs further investigation.
Degradation of air quality is one of the difficult questions raised by proposals for increased use of prescribed fire both to control high levels of forest fuels and to restore the functional role of fire. There is good documentation on degradation of air quality in massive uncontrolled fires. There is much less data on the effect of prescribed fires on a rangewide basis, and smoke from such events is difficult to detect in the detailed fine particulate mass records since 1988. Most information comes from local measurements taken at such fires and the visual effect of smoke. While quantities of smoke from prescribed fires are usually much smaller than from wildfires, they can, under exceptionally unfavorable conditions, also approximate wildfire levels. However, only very rarely does either type of smoke exceed the federal 24-hour fine-particulate mass standard.
High-elevation towns of modest population can still generate very high levels of fine particles in winter smoke, with levels higher than are typically seen even in the largest urban areas of California. Rather surprisingly, there is a rough equality between the mass of fine particles seen in winter urbanized areas and that seen near downwind of massive forest fires. Both of these can greatly exceed state and even federal 24-hour particulate mass (PM-10) standards. Lake Tahoe has sharply reduced water clarity and increased algae, some of which is tied to local and/or transported atmospheric air pollutants such as nitrates. Other typically urban air pollutants, such as carbon monoxide, have been high enough to warrant creation of special air standards to protect human respiration at these high-altitude sites.
The rapid desiccation of eastern Sierra Nevada lakes, Mono and Owens Lakes, has resulted in dust storms that in most years generate the highest 24-hour fine-dust levels in the United States. Much of this dust is transported into the Sierra Nevada and the White and Inyo Mountains, the latter being the home of the ancient bristlecone pines.
On the other hand, acid rain and snow are not as much a problem as in the eastern United States. No permanently acidified lakes or streams occur in the Sierra Nevada, although pulses of acidity can occur during spring snowmelt and during occasional summer thunderstorms in southern California deserts. In the winter, over much of the nonurbanized Sierra Nevada, levels of some human-origin pollutants such as sulfates are extremely low, mimicking even those of the high-altitude world baseline station on Mauna Loa in Hawaii.
In this section, we will examine a few of the most important topics concerning air quality in the Sierra Nevada, especially those aspects that may be improved or degraded by future human decisions.