In the News

Special Report: Lake Tahoe Air Quality
date posted: February 20 2003

Meteorology and Air Quality in the Lake Tahoe Airshed

There are two meteorological regimes that affect air quality in the Lake Tahoe airshed. First, Lake Tahoe lies in a depression between the crests of the Sierra Nevada and Carson ranges. The bowl shape of the Basin has significant implications on air quality in the Region. Locally generated air pollutants are often trapped in the bowl by the Basin’s frequent inversions. These inversions limit the volume of air that can be mixed, which in turn allows pollutants to accumulate to elevated concentrations. Inversions most frequently occur during the winter months in Tahoe, when a layer of wood smoke from residential heating can be seen over the Lake on many mornings, but they can occur throughout the year.

A second common meteorological event is atmospheric transport of pollutants from the Sacramento Valley and Bay Area. The location of Lake Tahoe directly to the east of the Sierra Nevada crest allows the prevailing westerly winds combined with local mountain up-slope winds, to bring air from the populated regions west of the Sierra into the Tahoe Basin. This pattern is usually strongest in summer, beginning in April, and usually ceases in late October (Cahill et al., 1987; Lake Tahoe Air Quality Research Scoping Document, TRPA, 2000). Contact Jennifer Quashnick, TRPA, (775) 588-4547, for more information.

Thermal Inversions

A “thermal inversion” occurs when cold air is near the ground and there is a layer of warmer air above it.  This happens because the cold air near the earth’s surface, which is called stable air, is more dense than the warm air above it, so the colder air resists rising. Examples of thermal inversions include:

* Low level, or surface inversions, usually take place on a clear night, when the earth's surface radiates heat rapidly.  If the air is clear, the ground, and the air directly above it, can be cooler than the air at higher altitudes. 

* Another type of surface inversion takes place at night in valleys, when cold, dense air flows downslope under the influence of gravity, draining off the slopes and uplands, and into the valleys.  The air in the valley bottoms is colder than the air above. Summer inversions in Tahoe are generally a result of this situation. 

* Other types of inversions may also develop under various conditions.

Air Pollution From Automobiles and Snowmobiles

Mobile sources of pollution, such as automobiles, are significant sources of pollution in the Tahoe Basin. These sources emit carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons (HCs), particulate matter (PM), and other pollutants. Most of these constituents impact not just human health, but also lake clarity. Traffic in the Basin is a result of both the local residential population, which increases slowly due to local restrictions on new development, and the tourist population, which has a significant, and sometimes overwhelming, impact on local traffic. The Tahoe Regional Planning Agency has adopted standards for transportation as it relates to air quality called “threshold indicators.”

One indicator is traffic flow (as opposed to stop-and-go traffic), which can impact CO emissions. Another indicator is vehicle miles traveled (VMT), which impacts all pollutant concentrations, plus re-entrained road dust, which can make its way into the Lake, further reducing lake clarity. While transportation improvements have resulted in improved traffic flow (specific to an intersection in South Lake Tahoe), TRPA’s “2001 Threshold Evaluation Report” indicates that Basin-wide VMT has increased. Since there are no limits on the number of vehicles entering the Basin, and development to the north, east and west of Lake Tahoe has skyrocketed in recent years, the number of nonresidents driving into the Basin is expected to increase and to further impact Basin-wide VMT.

Although snowmobiles are not included in VMT calculations, they also emit a significant amount of pollution into the atmosphere. Since snowmobiles do not have the strict emission controls that most vehicles require, emissions per mile for a snowmobile are much higher than for a car. Recent estimates suggest that emissions from just seven hours of snowmobile use (a two-stroke engine) are comparable to emissions produced from driving a newer model car 100,000 miles (CARB 1999).

While snowmobile use has not resulted in exceeding any health standards in the Basin, pollutant levels in the immediate vicinity of snowmobile paths may be above established health standards and pose a health risk (Ingersoll 1999). Also, while the impacts of snowmobile use on Lake Tahoe have not been directly studied, research outside of the Basin suggests that some pollutants may deposit onto the snow, thus ending up in the Lake when the snow melts and drains to the Lake. The impacts of this to lake clarity are unknown, but this kind of deposition may have a localized impact on aquatic life. Contact Jennifer Quashnick, TRPA, (775) 588-4547, for more information.

Keeping Warm and Breathing Clearly - Air Pollution From Wood Stoves

During cold weather, Lake Tahoe residents fire up wood stoves and stoke fireplaces to stay warm. Only dry, seasoned wood should be burned because fresh wood (such as leftover Christmas trees) does not burn as well. It can also emit more toxic air pollution than dry wood. Andrew Goodrich, director of air quality, Washoe District Health Department warns “pine needles and branches have a large amount of sap and other oils in them, which are released into the air when they are burned.” Also, colored wrapping paper should not be burned. “When you burn paper that has been treated with special inks, you may be releasing toxics into the air we all breathe,” explains Goodrich. “And if the paper has any type of metal in it, it’s even worse.”

Another way to help keep the winter air clean is to use EPA-approved wood stoves. Old, less efficient wood stoves, such as those installed in the 1980s or earlier, not only require more wood to generate the same amount of heat as a newer, more efficient stove, but also release significantly higher amounts of pollution into the air. Due to the excessive pollution emitted by the older wood heaters in the region, the TRPA adopted a wood heater retrofit program in 1993. This program requires that all wood heaters in any home sold, transferred or conveyed in the Tahoe Basin must meet certain emission standards for particulates. For example, if a home contains a nonconforming wood heater, it must be replaced with a conforming unit prior to sale. Pellet and gas stoves are popular, clean alternatives to wood heaters and can heat homes just as well. Wood smoke from residential heating poses a significant health risk since the smoke is often trapped at the bottom of the Basin due to winter inversions, resulting in higher concentrations of air pollution at the ground surface.