Autumn 07: ATMS/CHEM 458
Global Atmospheric Chemistry
MWF 10:30 – 11:20
Tu 10:30 – 12:20
610 ATG Building
Course Goals
This class will provide an overview of atmospheric chemistry
and the fundamental underpinnings so that you will be able to:
•Describe the workings of the atmosphere as a
chemical reactor
•Explain several important atmospheric
phenomena from the molecular to the global scale
•Critically evaluate and participate in public
discussions of air pollution and climate change
Course Related Activities
(see course website for more information)
Lectures/Discussions
Lectures are for you, not me. Please interact!
3 in-class debates:
– national air quality standards
- funding studies of strat ozone chemistry?
- geo-engineering of climate?
Problem Sets
Are certainly doable, but require some thought. Never wait until
the last minute.
Projects
Choose a topic from class for further investigation, write a 10 – 15
pg report and give a 15 minute presentation to the class.
-further guided analysis of data provided in class
-run and maintain O3 and CO instruments during quarter
-propose a research question or review current literature
This Week
READING: Chapter 1 - 2 of text
Tasks: Find a computer on which you can use Matlab. If you
don’t have easy (free) access, let me know and I’ll set you up.
A problem set will be handed out on Friday, due the
following week which suggests the use of Matlab.
• Goals, Topic Overview
•Atmospheric Physical and Chemical Properties
• Fundamentals
Goal of Atmospheric Chemistry
To develop a detailed understanding of the chemical and
physical processes which control the amounts and spatial
and temporal distributions of atmospheric constituents.
Why?
The atmosphere plays a critical role in
Earth’s energy balance (climate)
Protects/Sustains life at the surface
Couples land,oceans, equator and poles
Human activity changes its composition
Atmosphere in the Earth System
Clouds
Aerosols, O3
Atmosphere-Biosphere
Exchange
The Atmosphere Moves
CO Movie
How Do We Begin?
Describe the general physical characteristics
mass, temperature, vertical extent, motions
Determine the major and minor components
describe absolute and relative amounts
Develop a physical-chemical framework to:
predict how a species evolves in time and space
Apply this framework to answer:
Why is Earth’s atmosphere mainly N2, O2, H2O, and CO2?
How does composition help explain Earth’s climate?
How and where are humans affecting this composition?
Urban Smog Problem
• Global chemistry and
climate implications
• Complex regulatory
issues
Houston, TX
Stratospheric Ozone Depletion
Climate – Chemistry Connections
Does composition change
drive climate change, or
vise versa, or both?
Spatial and Temporal Scales of Change
Gases trapped in ice show
changes over millennial
and annual timescales.
Ozone, NO2, and NO near Nashville, TN
O3
100
NO2
NO
Mixing Ratio (ppbv)
80
60
Chemical change occurs on
time scales ranging from
<1 second to >millennia
40
20
0
171
171.5
172
172.5
173
Day of Year (mid June 1999)
173.5
174
In-class activity
Let’s start thinking about the atmosphere…
Break into a couple groups, and determine how to:
1. Calculate the absolute amount of a specific gas
(e.g. CO2) that resides in the atmosphere if you
know its relative amount (e.g. 0.0004%)
2. Calculate the mass of the entire atmosphere.
This is not an aptitude test! The goal is to get you
thinking about the atmosphere, period.
Describing Amounts
The atmosphere contains gases (mostly) and some
liquids/solids – aerosols and clouds.
All gases can
be described
by ideal gas law
Aerosols and
clouds need:
Px = (nx/V)RT
Ptotal = (Px)
Size, Number, Composition, and
Phase State
mass and volume
concentrations
Mass ma of the Atmosphere
Mean surface pressure:
984 hPa
Radius of Earth:
6378 km
4 R PS
18
ma 
 5.2  10 kg
g
2
Total number of moles of air in atmosphere:
ma
Na 
 1.8  1020 moles
Ma
Today
• Atmospheric structure
•Vertical profiles of pressure and temperature
•A slightly closer look at composition
•H2O phases
•Aerosol particles
•Working with PV=nRT
Atmospheric Structure
Vertical profiles of pressure and temperature (means for 30oN, March)
Stratopause
Everest ~ 9 km
Tropopause
Average Composition as Mixing Ratios
Trace
gases
GAS
MIXING
RATIO
(dry air)
[mol mol-1]
Nitrogen (N2)
0.78
Oxygen (O2)
0.21
Argon (Ar)
0.0093
Carbon dioxide
(CO2)
365x10-6
Neon (Ne)
18x10-6
Ozone (O3)
(0.01-10)x10-6
Helium (He)
5.2x10-6
Methane (CH4)
1.7x10-6
Krypton (Kr)
1.1x10-6
Mixing Ratio is a mole fraction
(Moles X/Total Moles)
• Air also contains variable H2O
vapor (10-6-10-2 mol mol-1) and
aerosol particles
• Trace gas mixing ratio units:
1 ppmv = 1x10-6 mol mol-1
1 ppbv = 1x10-9 mol mol-1
1 pptv = 1x10-12 mol mol-1
Related Measures of Composition
Mixing Ratio
CX 
moles of X
total moles of air
Number Density
NX 
# molecules of X
unit volume of air
•Constant w.r.t. changes in air density
proper measure for
• calculation of reaction rates
• optical properties of atmosphere
NX and CX are related by the ideal gas law:
N X  N air C X 
N Avag Pair
RT
CX
Also define the mass concentration (g cm-3 of air):
M X N X ( g / mol )(molec / cm3 )
mass of X
X 


unit volume of air
N Avag
(molec / mol )
Not to be confused with the density of a substance (g cm-3 of substance)
H2O Phase Diagram
PH2O,SAT(T)
Saturation Vapor Pressure
Amount of water vapor volume
of air can hold.
Determines when condensed
phase H2O can exist
Formation of condensed phase
H2O often leads to removal by
precipitation
Relative humidity (%) = 100(PH2O/PH2O,SAT)
Dew point: Temperature Td such that PH2O = PH2O,SAT(Td)
Visibility Reduction by Aerosols (Haze)
•
clean day
moderately polluted day
Acadia National Park (Northeastern Maine)
http://www.hazecam.net/
Typical U.S. Aerosol Size Distributions
Maxima are most common sizes
volume frequency
Fresh
urban
Aged
urban
rural
remote
Warneck [1999]
Annual Mean Particulate Matter (PM)
Concentrations at U.S. Sites, 1995-2000
PM2.5 (aerosol particles < 2.5 mm diameter)
EPA particulate matter
assessment document
(NARSTO), 2003
U.S. air quality standard:
PM2.5 = 15 mg m-3
(annual mean)
Red circles indicate
sites in violation of the
standard
STANDARD IS EXPRESSED AS A MASS CONCENTRATION
Total Mass of Particles PER UNIT VOLUME AIR
Chemical Composition of PM2.5
(EPA/NARSTO PM
ASSESSMENT, 2003)
Sulfate
Esther (1995-99)
Egbert (1994-99)
4.6 ug m -3
8.9 ug m -3
Nitrate
Toronto (1997-99)
12.3 ug m -3
Ammonium
Black carbon
Abbotsford (1994-95)
Organic carbon
7.8 ug m -3
Soil
Other
St. A ndrews (1994-97)
5.3 ug m -3
Fresno (1988-89)
39.2 ug m -3
Quaker City OH (1999)
12.4 ug m -3
Kern Wildlife Refuge (1988-89)
23.3 ug m -3
Los Angeles (1995-96)
Arendstville PA (1999)
10.4 ug m -3
Mexico City Netzahualcoyotl (1997)
55.4 ug m -3
Washington DC (1996-99)
14.5 ug m -3
30.3 ug m -3
Colorado Plateau (1996-99)
3.0 ug m -3
Mexico City - Pedregal (1997)
24.6 ug m -3
Yorkville (1999)
14.7 ug m -3
Atlanta (1999)
19.2 ug m -3
Questions
1. How many molecules of air are in 1 cm3 of this room?
2. The mixing ratio of CO2 is currently ~ 380 ppm throughout
the atmosphere, sketch its partial pressure versus altitude.
3. A typical ozone number density at the surface is 1x1012
molec/cm3, while at 30 km it is 5x1012 molec/cm3. Where is
its mixing ratio highest?
4. The tropics are clearly warmer on average than the poles.
If the vertically averaged temperature is higher in the
tropics than at the poles, how will the pressure at some
altitude compare between the equator and poles?
QUESTIONS
1.
Oxygen has a constant mixing ratio in the
atmosphere. How does its number
density measured in surface air vary
between day and night?
2. Give a rough order of magnitude for the
number of molecules present in a typical 1
micrometer aerosol particle.
3. Does it make sense to talk about the
mixing ratio of aerosol particles in air?
To express the concentration of soot
aerosol in units of ppbv?
This Week—Transport Timescales
READING: Chapter 4 of text
•Tropical and Mid-latitude Circulation
•Vertical transport and stability: Buoyancy
Today&Tomorrow: General Circulation
Global Heat Engine
Coriolis Effect
Hadley Circulation
Upper level westerlies and surface trades
Convergence/Divergence
Uplift/Subsidence
Making Air Move
Pressure Gradient Force
Fluid will move from high to low pressure
Fluid with
horizontal
pressure
gradient
Applies to horizontal (parallel to earth’s surface)
and vertical (perpendicular to earth’s surface)
Buoyant Forces
Recall ideal gas law: density  1/T
buoyancy: when pressure gradient force in
vertical direction not equal to force due to gravity
When an air parcel’s density is lower/higher than
the surrounding air, it will rise/sink
The Engine’s Heat Source
Latitudinal gradient
in solar insolation
The equator receives a greater solar radiation energy
flux (J/m2/s) than the poles
Isobars (Constant Pressure Lines)
Suggest Eq. To pole flow
Alt.
PGF
The picture is roughly symmetric
for the Southern Hemisphere (SH)
Early Picture of General Circulation
Based on Hadley’s 1735 paper:
“Tropopause”
divergence
subsidence
Uplift (convection)
subsidence
L
H
H
convergence
90
45
0
-45
-90
Cold high lats
Warm Tropics Cold high lats
higher pressure Lower pressure higher pressure
Hadley Circulation (in 1735)
COLD
HOT
COLD
Explains:
Intertropical
Convergence Zone
(ITCZ)
Wet Tropics
Dry Poles
But it has some
problems.
Saturation Vapor Pressure
Amount of water vapor volume
of air can hold.
Determines when condensed
phase H2O can exist
Formation of condensed phase
H2O often leads to removal by
precipitation
Relative humidity (%) = 100(PH2O/PH2O,SAT)
Dew point: Temperature Td such that PH2O = PH2O,SAT(Td)
Intertropical Convergence Zone (ITCZ)
Surface warming by intense solar radiation leads to
warm air rising, creating surface low pressure.
Convergence and uplift leads to saturation/condensation
Today’s ITCZ
Moist Deep Convection—Where?
divergence
Questions
1. Subsidence of air leads to compression, which causes an air
parcel to warm. This will lead to:
a) rainy weather
b) dry weather
c) lower pressure at the surface
2. Large scale subsidence tends to occur near 30oN/S as a result
of the Hadley circulation. This subsidence exacerbates air
quality problems in cities like Los Angeles, Houston, Athens,
Cairo, Shanghai, etc.
a) True
b) False
Hadley Didn’t Know About Coriolis Effect
Earth rotates from West to East (an
Easterly direction)
The equator is rotating with a larger
velocity than higher latitudes
Air (or any object) on Earth always
has an easterly velocity component.
Object moving northward from the
equator has easterly velocity greater
than the Earth at northern latitudes
coriolis effect movie
Coriolis Effect: Modifies Hadley Circulation
Questions
1. Suppose Earth were a cylinder. Where would there be a
coriolis effect?
A
B
a)
b)
c)
d)
A only
B only
C only
A, B, and C
C
2. In the stratosphere over Antarctica during polar winter, air
cools and descends causing air in the midlatitude stratosphere
to head poleward. What direction does it end up going?
Surface Winds
In the tropics air flows towards the ITCZ, impacted by
Coriolis effect and friction. These are the “trades”
Subsidence leads to surface highs and divergence
subsidence
subsidence
Mean Horizontal Transport Times
1-2 months
2 weeks
1-2 months
1 year
Vertical Transport
Importance to composition
Buoyancy
Stability and Instability
Vertical Transport: Overview
1. Vertical transport critical for air quality and the vertical
distribution of surface emitted species (CO2, PM, H2O,
etc).
2. Determined by temperature differences between air
parcels and their surroundings.
LA Smog/Fog
Cumulonimbus
Vertical Transport: Buoyancy
Buoyancy refers to the density of an object in a fluid,
relative to the density of that fluid.
PGF
Fluid (f)
Fg
PGF
object
(ob)
Fbuoyancy = PGF - Fg
Fg
If obj > fluid, Fgobj will be > PGF, and so object will sink.
If obj < fluid, PGF > Fgobj and so object will rise.
Vertical Transport: Buoyancy
Key Points
1. Buoyant forces in atmosphere driven by temperature
differences (which lead to density differences).
2. Solar heating and “latent heat” of condensation drive
temperature differences between a parcel of air and
the surrounding atmosphere.
3. Warmer air rises, but as it rises, it cools due to
expansion. Colder air sinks, but as it sinks, it warms due
to compression.
4. Must compare “parcel” T to that of “surrounding”
atmosphere to know extent/likelihood of vertical mixing
Adiabatic Lapse Rate
Rising air parcel does expansion
work on surroundings.
Work done on surroundings
causes loss of internal energy
Internal energy of ideal gas
directly related to T
Stratopause
??
 dT  = 9.8 K/km


dz

 DRY
 dT 

6.5 K/km <   < 9.8 K/km
 dz actual
Tropopause
Vertical Transport: Stability/Instability
How do vertical motions get started and stopped?
Conduction: air warmed
by contact w/surface.
Z
Warm parcels become
buoyant or are lifted
mechanically to where
they are buoyant.
0 km
Compare Tparcel to TATM at
a given (z) to determine
sign of buoyancy
~ 2 km
Tparcel follows dry adiabatic
slope from expansion work
Tatm
T
Slope of Tatm(z) greater than dry adiabatic
lapse rate because of surface heating
Vertical Transport: Stability/Instability
 dT

 dz
 > 9.8 K/km: UNSTABLE w.r.t.

 ATM
vertical motion (well mixed)
 dT  < 9.8 K/km: STABLE w.r.t. vertical


 dz  ATM
motion (vertical mixing
suppressed)
~ 2 km
0 km
Dry adiabatic slope
from expansion work
T
-Slope greater than dry adiabatic lapse rate
because of surface heatingUNSTABLE
Temperature Structure and Air Quality
Daily cycle of surface heating and cooling
and the depth of the “well mixed” layer
z
Subsidence
inversion
MIDDAY
Temperature inversions cap
mixing depth  IMPORTANT FOR
1 km
POLLUTANT CONCENTRATIONS
Mixing
depth
NIGHT
0
MORNING
T
NIGHT
MORNING AFTERNOON
Typical Timescales for Vertical Mixing
tropopause
10 km
5 km
1 week
1 day
months
10 years
2 km
“planetary
boundary layer”
0 km
Question
1. If it only takes months for air at the surface to mix to
the top of the troposphere (tropopause), why does it
then take 10 years for air in the troposphere to cross
into the stratosphere?
2. Due to respiration, CO2 is emitted into the atmosphere
by plants and soil microbes throughout the night. At
what time of day would you expect CO2 concentrations
to maximize at the ground and why?