Atmospheric Deposition of Air Toxics

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Atmospheric Deposition of Air Toxics

• Matt F. Simcik
• Division of Environmental and Occupational Health
• School of Public Health
• University of Minnesota

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Why should we care about Atmospheric Deposition
of Air Toxics?
Atmospheric Deposition is the only source to
remote areas such as the National Parks
from EPA website
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The Atmospheric System
Source(s)
Transport
Deposition
Transformation
Wet
Combustion
Volatilization
Gas
Gas-Particle Partitioning
Particle
Dry
Cloud Scavenging
Gas
Reaction/Loss
Particle
OH Radical
Photolysis
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Sources
Combustion
Produces both gas and particle phase toxics
PCDDs/PCDFs
PAHs
Volatilization
Produces only gas phase toxics
DDT
PCBs
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Transformations
Gas-Particle Partitioning
Cg
Cp
Controls fate of contaminant
Governed by vapor pressure of toxic and amount
of particles in air
from Simcik 2001
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Transformations
Cloud Scavenging
from Simcik 2001
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Reaction/Loss

• Reaction with the OH radical is predominant loss
  mechanism for most air toxics
• Most reactions occur fastest in the gas phase
• Reaction products can be more toxic than parent
  compounds

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Dry Particle Deposition
from EPA website
Dry deposition flux Concentration on particles
times deposition velocity. Deposition velocity is
a function of particle size Typical value of 0.2
cm/s is often used.
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Dry Gas Deposition to Terrestrial Surfaces
Cg
Living plants, soils, and detritus represent a
huge amount of surface area available for
sorption of gas phase toxics. In particular,
waxy, organic surfaces can take up a large amount
of lipophilic contaminants.
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Wet Deposition
from EPA website
Wet deposition is best done as total deposition
(dissolved and particle)
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Questions that need to be answered

• What are the current atmospheric loadings of air
  toxics to the National Parks?
• How do loadings compare to other areas?
• What is the historical deposition of air toxics
  to the National Parks?
• Are there any adverse effects of atmospheric
  deposition of air toxics?

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Loadings Estimation
Loading Direct Method Indirect Method
Wet Deposition Precip Collector Snow Collection N/A
Dry Particle Deposition Surrogate Surfaces Calculation from particle size distribution
Dry Gas Deposition N/A Calculation from BCF and Henrys Law
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Air Sampling
HI-Vol Air Sampler
This provides an operationally defined gas and
particle phase
Typical 24 hour volumes of 720 m3
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Alternatives to HI-VOL Samplers
Method Advantage Disadvantage
SPMD Easy Passive Concentrations difficult to determine Gas phase only
Lichens or other plant material Easy Passive Species differences Concentrations difficult to determine Integrates all deposition processes
Surrogate Surface Passive Must be baby-sat Deposition to surrogate surface Sorption of gases
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Historical Deposition
Lake sediments have often been used to
investigate the historical deposition of toxic
contaminants.