Dependence of PM on Elevation

1
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

A thick layer of polluted air trapped in a
valley Photograph from Ahrens D.C. (1994)
Meteorology Today. West Publishing Company,
Minneapolis/St. Paul
Contact Bret Schichtel, Bret_at_mecf.wustl.edu
2
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.

3
Influence of the Seasonal Variation in Mixing
Heights on the PM Elevation Dependence

• 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.

4
Vertical Profile of the Light Scattering
Coefficient in the Los Angeles Basin
Mean morning and afternoon summer light
scattering profiles

• The light scattering coefficient is largely
  dependent on particle concentrations between 0.1
  - 1 mm.
• The bscat 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 bscat on primary
  emissions.

5
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.

6
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.

7
Incorporating Barriers in Mapping PM10
Topography
Spatial estimation of PM10 using topography
Sierra Nevada
San Joaquin Valley
South Coast Basin
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.
Key reference Capita
8
Resource Links

• Workbook Table of Contents
• Comment and Feedback Page
• Applications / Reports
• Data sets used in the Applications
• Methods and tools used in the Applications