Dependence of PM on Elevation Background and Rationale Influence of the Seasonal Variation in Mixing Heights on the PM Elevation Dependence Vertical Profile of the Light Scattering Coefficient in the Los Angeles Basin Seasonal PM2.5 Dependence on Elevation in Appalachian Mountains Barrier Influence on PM Incorporating Barriers in Mapping PM10 Resource Links Contact: Bret Schichtel, A thick layer of polluted air trapped in a valley Photograph from Ahrens D.C. (1994) Meteorology Today. West Publishing Company, Minneapolis/St. Paul

Background and Rationale The PM2.5 elevation dependence is necessary information to determine the representativeness of a monitoring site to its surrounding areas. For example, a high elevation site outside the haze layer is not representative of the valley concentrations. PM2.5 dependence on elevation is the result of the limited extent and intensity of vertical mixing, source elevation, and changes in the chemical and physical removal processes with height. These causal factors vary both seasonally and diurnally and the PM2.5 dependence on elevation should also vary with season and time of day.

Influence of the Seasonal Variation in Mixing Heights on the PM Elevation Dependence Summer Winter During the summer, the afternoon mixing heights typically reach 1-3 km and PM is evenly distributed throughout this layer. The lower mixing heights during the winter evenly distribute the PM throughout the first several hundred meters. Above the mixing height the PM concentrations normally decrease with height.

Vertical Profile of the Light Scattering Coefficient in the Los Angeles Basin The light scattering coefficient is largely dependent on particle concentrations between  m. The b scat is fairly uniform through the mixing layer (1.5-2 km) and drops to low levels above the mixing layer. The morning and afternoon profiles are similar indicating weak dependence of b scat on primary emissions. Mean morning and afternoon summer light scattering profiles

Seasonal PM2.5 Dependence on Elevation in Appalachian Mountains During August, the PM2.5 is independent of height up to at least 1200 m above which it decreases. During January, the PM2.5 decreases about 50% from 300 to 800 m. It is then approximately constant up to 1200 m where the concentrations again decrease ~50% from 1200 to 1700 m.

Topographical Influence on PM Mountains can restrict the horizontal flow of particles while the mixing height restricts the vertical mixing of particles. Pollutants can be “trapped” in valleys depending upon the height of the surrounding mountains and the height of the mixed layer. When the mixed layer is lower than the mountain top site, the elevated site may have low concentrations. The analyst needs to know the physical and meteorological properties of the mountain sites in order to assess the data collected at that site.

Because topography can significantly affect PM concentrations, it should be considered in preparation of spatial contour maps of PM concentrations. As an example of the mountain-valley effect, the concentrations in the San Joaquin Valley and South Coast Basin are much higher than in the Sierra Nevada Mountains. Without the incorporation of topography, the higher concentrations would "spread" across neighboring mountains. Incorporating Barriers in Mapping PM 10 TopographySpatial estimation of PM 10 using topography Sierra Nevada San Joaquin Valley South Coast Basin Key reference: Capita

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