Lake Tahoe Visibility Study

Abstract

Visibility monitoring and airborne particulate sampling in the Lake Tahoe Basin were used to document visual air quality levels and to assess the relative impacts of major contributing emission source categories. Visibility data were obtained by long path contrast and particle scattering techniques. Particles were sampled in two size ranges at three locations and were analyzed for mass concentration and composition of elements greater in atomic weight than Na. Statistical analysis showed fine particle concentration (particle diameter less than 2.5 ..mu..m) to be related to visibility. Measured elements plus the mass of material assumed to be associated with them accounted for only 20% of the fine particle concentration. The remaining 80% was assumed to be nitrates and carbonaceous materials, the latter associated with wood smoke, terpenes and transport from upwind areas. A model was developed to apportion all of the fine particle mass to source categories. The results of this effort were then used to determine an optical extinction budget by particle source. This study indicates 70% of the basin-wide visibility impact and 30% of the south Lake Tahoe visiblity impact are caused by natural and long range transported emissions. Residential wood smoke emissions are responsible for the majority ofmore » the remaining impact; at South Lake Tahoe automotive emissions are also significant.« less