Air quality

Air quality has been linked to declining water clarity in Lake Tahoe through direct deposition of nitrogen, phosphorus and fine soil. Researchers have estimated that 55 percent of nitrogen and 27 percent of phosphorus enter the lake via atmospheric deposition. Additionally, atmospheric pollutants can affect forest health. For example, ambient ozone has produced adverse effects on forest health in California’s mountain regions, and during the summer season, Lake Tahoe’s ozone levels may be toxic to vegetation, negatively affecting tree health. Perhaps the best known factor that decreases air quality in the Tahoe Basin is smoke from wildfire. However, fire has always been a component of the basin’s forests, so smoke is a natural process. Human practices, however, have greatly modified forest conditions, so smoke levels are now very different than natural conditions in location, timing and intensity. Large fires have had major impacts on regional visibility and lake clarity. Even prescribed fire can create enough smoke to lower air quality to unhealthy levels, limiting the timing of prescribed burning to times when climatic conditions are right. The mountain terrain traps locally generated smoke near the ground each night and releases it during subsequent days. In pre-European times, there was likely a low intensity smoke haze over the lake each morning from May through October that would clear in the afternoon.

Researchers in the field; sketch by Ron Oden

In addition, man-made air pollutants can also have an impact on air quality, and subsequently affect the water clarity of Lake Tahoe. Researchers have determined that airborne pollutants from outside sources, such as San Francisco and the Sacramento Valley, are relatively minor components of air Resources pollution in the Tahoe Basin. Local sources are likely the key drivers of regional air quality. Some local sources, such as road dust, can be modeled using a Geographic Information System (GIS) and may be a significant source reducing lake clarity – particularly in the winter. Studded snow tires and traction materials (e.g., sand) may greatly increase the amount of particulates that are generated. Researchers are using monitoring and modeling approaches to develop an emissions inventory that details emissions for every source including wood smoke, cars and other sources. Because most of the lake’s air pollution is locally produced, an accurate accounting of pollution sources enables greater control over these sources.