Oh, say can you see?
From my perch on the North Shore, I can see mountain ridges beyond the south end of the lake, shining white and sharply etched against a cobalt sky. Whitecaps spread across the lake. It’s spring, and windy. The air is crystal clear.
In winter and during summer mornings, the air is often still. Tiny particles of smoke, dust, and sulfates accumulate, float in the air, and produce haze. During those windless, stagnant periods, we see a layer of haze obscuring the lower reaches of those mountains to the south.
Tiny haze particles absorb and scatter sunlight. Lighting is diminished, contrast is reduced and colors are muted. Our views are degraded.
Folks that lived and visited in the Tahoe Basin between the 30s and 60s, swear that visibility was fantastic in those days. Yet, Comstock era (1870 to 1900) travelers to the basin reported smoky skies from slash burning. Sheep and cattle herders added more smoke by burning pastures to revitalize them for the following year. Mark Twain’s famous quote about the air the angels breathe suggests that skies were crystal clear in the 1860s. But, Native Americans used fire to manipulate their landscape and that suggests a scene of smoky summer mornings instead of clear skies.
When TRPA folks decided to set environmental thresholds for visibility in 1980, they needed a basis for the standards. Without clear data from the past, they called on experts at the EPA. Researchers M. Pitchford and D. Allison responded in May 1981, installed instruments in the basin and monitored visibility until June 1982.
A teleradiometer and a camera, installed on the North Shore, were aimed at five targets around the basin. A nephelometer and aerosol sampler were installed on the South Shore and at Bliss State Park. Teleradiometers measure contrast between a target and its surroundings. Nephelometers measure the amount of light scattered from a light beam by small particles in the air. Aerosol samplers collect small particles for chemical analysis.
After evaluating the data, TRPA created the following visibility thresholds. For regional visibility, maintain a visual range of 103 miles at least 50 percent of the time and a visual range of 58 miles at least 90 percent of the time. For sub-regional visibility, maintain a visual range of 54 miles at least 50 percent of the time and a visual range of 16 miles at least 90 percent of the time. These ranges were all to be calculated from measurements of fine particle concentrations in air.
Regional visibility was defined as the overall visibility in the basin. Sub-regional visibility is characterized by the haze layer over developed areas. To increase sub-regional visibility, wood smoke emissions were to be reduced by 15 percent.
Suspended soil particles (from road dust) were to be reduced by 30 percent.
In 1989, TRPA established a state-of-the-art visibility monitoring system. To monitor sub-regional visibility, a more sophisticated nephelometer and aerosol sampler were installed in South Lake Tahoe.
Instruments installed on a meteorological tower measure wind speed and direction, temperature and humidity. Nephelometer data and analysis of collected particles are used to calculate visibility. A camera at South Lake Tahoe checks the view to the north.
To check on regional visibility, a duplicate station was established at Bliss State Park, 700 feet above the lake and 1/4 mile from the Desolation Wilderness. A transmissometer mounted at the Zephyr Cove Lookout monitors the intensity of a light source across the lake at Bliss State Park. Light intensity is related to visibility.
John Molenar, with Air Resources Specialists, Inc., correlated the 1981 measurements with current monitoring data to determine how visibility has changed over the past two decades.
And the results are intriguing. For example, there’s been a decrease in particle concentrations at South Lake Tahoe since 1981, but the current station is at a different location, so the observation can’t be quantified exactly. There’s a significant decrease in atmospheric lead concentrations, probably due to phasing lead out of gasoline.
Aerosol concentrations at Bliss are greatest in summer. The opposite is true at South Lake Tahoe. The heavy winter haze at South Lake Tahoe doesn’t affect the Bliss site. This indicates that the haze is locally generated. The summer increase at Bliss has been associated, by UC researchers, with winds from the west and sources in the Sacramento Valley and the Bay Area. Though nitrates and sulfates, classic air pollutants, are significant components of aerosols at both sites, the majority of the particles are made of carbon and organic materials.
Regional visibility didn’t change between 1981 and 1989. TRPA’s goal of reducing airborne soil particle concentrations by 30 percent is not being achieved. However, measurements show that visibility has improved over the past decade, and TRPA’s thresholds for visibility are being met. So, believe it or not, we can see better today than 20 years ago.
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