Atmospheric AerosolsBackground retrieved from

http://images.sciencedaily.com/2009/08/090801095810-large.jpg

Yatit ThakkerJake YapENV 6130 – Aerosol MechanicsApril 12, 2012

Outline

• Classification of Atmospheric Aerosols– Sources– Size Distributions– Growth/Removal Mechanisms – Nucleation, Accumulation, Coarse – Chemical Composition

• Health Effects• Global Effects• Visibility

Why do we care?

• Atmospheric aerosols (AA’s) affect global temperatures, health, pollution, and even the weather!

• What is the most important atmospheric aerosol that you encounter every day?

• What are some other reasons?

How can we describe atmospheric aerosols?

• Natural or anthropogenic• Solid or liquid• Primary or secondary– Primary aerosols enter directly into the

atmosphere.– Secondary aerosols are produced in the

atmosphere (ex: chemical reactions, condensation).

Atmospheric Aerosol Sources

http://aerosol.ees.ufl.edu

Do you think there are always more natural aerosols than anthropogenic? Why or why not?

Growth Mechanisms

• Condensation• Coagulation• Nucleation and fusing with water aerosols

Removal Mechanisms

• Settling• Impaction (against leaves, buildings, trees)• Washout – removal by larger particles or rain

and snow.

Where are these aerosols found?

http://frobinett.edublogs.org/files/2011/02/layers-of-the-atmosphere-29q693x.jpg

Layers of the Earth

• Troposphere (first 11 km)– Contains majority of aerosol mass (ex: soil, sea salt, power plant ash). First km contains ~80% of direct emissions.

• Stratosphere (11 – 50 km) – very low moisture, very low particle concentration.

• What is the primary mechanism of aerosol formation in the stratosphere?

Troposphere - What is clean air?

• Difficult question to answer, but defined as having an ambient aerosol concentration of 700#/cm3 or less, according to the textbook.

• How would ambient aerosol concentration vary with height in the troposphere?

Stratosphere

• Contains the Ozone Layer in the lower stratosphere

• Altitudes of 18-20 km is known as the Junge Layer, where Sulfuric Acid particles form and accumulate.

• SO3+H2O H2SO4

• Implications?

Urban Aerosols

Nuclei mode

• Short lifespans (rapid coagulation)– High # concentration near source

• dp < 0.1 µm (very small)• Sources: combustion, gas to particle

conversion• Acts as the nucleus for condensation.• http://aerosol.ees.ufl.edu/atmos_aerosol/sect

ion04-1.html

Accumulation Mode

• Coagulate more slowly• 0.1 μm < particle size < 2.5 μm• Longest lifespan• Originate from combustion, smog, and nuclei

coagulation.• Account for visibility effects of atmospheric

aerosols.• http://aerosol.ees.ufl.edu/atmos_aerosol/sect

ion04-2.html

Coarse Particles

• Originate from windblown dust, large salt particles, and agriculture/surface mining particles.

• dp> 2.5 µm• Lifetime of only few hours b/c of size• http://aerosol.ees.ufl.edu/atmos_aerosol/sect

ion04-3.html

Brief Overview

http://aerosol.ees.ufl.edu/atmos_aerosol/section05.html

Alternate Characterizations

Source: http://www.ess.uci.edu/~cmclinden/link/xx/node22.html

What it means

• Sulphates – Account for the submicron haze. Great spatial and temporal variability.

• Marine Aerosols – composed primarily of sea salt and ocean water picked up by winds.

• Cumulus Clouds – formed by condensation of water droplets through convection currents.

• Cirrus Clouds – composed of ice crystals and supercooled water droplets.

Visual Dynamic Interactions

Chemical Composition in the Atmosphere

• Fine Particles vs. Coarse Particles– Fine Particles: acidic and made up of sulfates,

ammonium compounds, hydrocarbons, elemental carbon (soot), toxic metals, and water in the atmosphere

– Coarse Particles: basic and contain most of the crustal materials and their oxides, such as silicon, iron, calcium, and aluminum, as well as large sea salt particles and vegetation debris

Chemical CompositionTABLE 3. Average composition of fine and coarse particles in µg/m3 at an urban and a rural site

Health Effects

• What are some possible health effects from atmospheric aerosols?

• Examples: Smog• London Smog Disaster• Estimated 4000 people died due to

exacerbated respiratory illnesses.• Gainesville Smog Disaster – Jan 29, 2012.

Gainesville Smog Disaster

PM related health diseases

What can PM do for you?

More on PM

• Long term exposure is much more deadly.

• Why do you think the EPA uses PM2.5 as a base standard for measuring health effects of atmospheric aerosols?

Global Effects-Ozone Depletion

• Why is Ozone important to us?

Aerosol Effects on Ozone

• Under cold Temp., nitric acid and water vapor condense to form polar stratospheric clouds – Surface of cloud particles act as a catalyst for

conversion of Cl compounds to atomic Cl

Ozone Depletion-Process

(1) Cl Compounds

(2) Cl + O3 → O2 + ClO

(3) ClO +hν → O + Cl

• Under which season does the ozone hole grow larger?• How can this process be enhanced?

AerosolsCl

Cl is recycled and process repeats

Enhancement

• Addition of atomic Chlorine into atmosphere– CFC’s• Sources: A/C cooling units, refrigerators, hair spray

• Volcanic eruptions– Increases stratospheric aerosols for additional

catalysis

Global Cooling• Scattering of incoming solar radiation–Natural aerosol property

• Increased cloud reflectivity–High # concentration of cloud droplets

• Known as “whitehouse effect”-all within the troposphere

• http://aerosol.ees.ufl.edu/atmos_aerosol/section08.html

Global Warming

• Aerosols can absorb the radiation from the sun and thus trap the heat– Ex: Carbon Black

Example of Global Cooling

• Mt. Pinatubo Eruption of 1991– Location: Philippines– Emitted 14-20 Tg of SO2

Warming or cooling?

• Global warming has heated the Earth by +1.25 W/m2.

• The Earth’s volcanoes have a major cooling effect (-2.7 W/m2)– (Hinds-numbers from book)

• Why isn’t the earth colder?!

Light Extinction in the Atmosphere

• Remember Bouguer’s Law!

• For Monodisperse Aerosol

• To which phase of the atmosphere is this measurement method most applicable?

Cairo, Egypt

Visual Range

• The visual range is how far an object can be seen when viewed against the horizon during Daylight.

• How could Atmospheric Aerosols have led to the extinction of dinosaurs?

Reflection