Atmospheric Deposition of Air Toxics

Matt F. SimcikDivision of Environmental and Occupational Health

School of Public Health

University of Minnesota

Why should we care about Atmospheric Deposition of Air

Toxics?

from EPA website

AtmosphericDeposition is the only sourceto remote areassuch as the National Parks

The Atmospheric System

Source(s) Transport Deposition

Combustion

Volatilization

Reaction/Loss

Transformation

Gas-Particle Partitioning

Cloud Scavenging

OH Radical

Photolysis

Wet

DryParticleGas

Gas

Particle

SourcesCombustion

Volatilization

Produces both gas and particle phase toxics

PCDDs/PCDFs

PAHs

O

O OCl ClCl Cl

Cl ClCl Cl

Produces only gas phase toxics

PCBs DDTCl

Cl

Cl

Cl

CCl 3

ClCl

CH

Transformations

Gas-Particle Partitioning

Physi-chem icalsorptionAd

sorptioninto Liqu id-like M atrixAb

Surfaces:M ineralE lem ental CarbonIce

Liquid-like M atrices: Organic Carbon Water

Controls fate of contaminantCg Cp

from Simcik 2001

Governed by vaporpressure of toxic and amount of particles in air

TransformationsCloud Scavenging

C

C

C

C

C

CHenry’s

Law

Air-icepartitioning

or

Absorptioninto organic film

impaction

Browniandiffusion

interceptionimpaction

interceptionfiltration

Brownian d iffusion

aq

p s,rain

g s

org

from Simcik 2001

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

Dry Particle Deposition

Dry deposition flux = Concentration on particles times deposition velocity.Deposition velocity is a function of particle sizeTypical value of 0.2 cm/s is often used.

from EPA website

0.0001

0.001

0.01

0.1

1

10

100

V d (cm sec )

0 .001 0.01 0.1 1 10 100Partic le D iam eter ( m )

BrownianDiffusion

GravitySedim entationInertial forces

-1

Dry Gas Deposition to Terrestrial Surfaces

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.

Cg p

g

C

CBCF

Wet Deposition

Wet deposition is best done as total deposition (dissolved and particle)

from EPA website

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?

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 Henry’s Law

Air SamplingHI-Vol Air Sampler

Filte r

A dsorbent

This provides an operationally defined gas and particle phase

Typical 24 hour volumes of 720 m3

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

Historical Deposition

C C

C C

buria l

g p

d p

A ccum u la tion

2000

1970

1940

1910

Da

teLake sediments have often been used to investigatethe historical deposition of toxic contaminants.