Transcript Week 2 Slides

This Week
READING: Chapter 3 of text
Announcements
Problem Set 1 due Mon Oct 8.
Office hours will be
Describing and Predicting Change
• Mass Balance: Sources - Sinks
• Lifetime / Residence Time
• Steady – State
•Models
Today: Describing change composition
Goal of Atmospheric Chemistry
Concept 1: Mass Balance
Concept 2: Lifetime
Primary Goal of Atmospheric Chemistry
To describe the change in the atmospheric concentration
of chemicals as a function of time and location.
Ozone, NO2, and NO near Nashville, TN
O3
N X  r , t  100
t
NO2
NO
Mixing Ratio (ppbv)
80
60
40
20
0
171
171.5
172
172.5
173
Day of Year (mid June 1999)
173.5
174
CO2 Mixing Ratio (ppm) at Mauna Loa
CO2 Rate of Change
Give at least two rates
of change for CO2
390
380
370
Monthly Mean CO2 Mixing Ratio
Annual (running) Mean CO2 Mixing Ratio
What do we learn
about the sources and
removal of atmospheric
CO2 based on this
graph/analysis?
360
350
340
330
320
310
1950
1960
1970
1980
Year
1990
2000
2010
Factors Affecting d[X]/dt in a Box
Emissions
Anthropogenic
Transport
Biogenic
Natural
X
Flux out or Flux in
Chemistry
Deposition
O2
X
O3
O
XO
O2
X
X Wet
X
Dry
Atmospheric Lifetimes
Lifetime = Amount
Removal Rate
X
B
Z
A
Chemical lifetime/s
X
X
X
Transport lifetime
Deposition lifetime
Sink-specific lifetimes allow determination of the importance
of a particular process for controlling the fate of a species
Questions
1. CO2 is lost from the atmosphere by photosynthesis and physical
dissolution into the oceans. Photosynthesis by the biosphere
leads to the uptake of ~ 60 Pg C/yr of atmospheric CO2. The
oceans take up CO2 at about the same rate. Based on these
values, what are the sink-specific and overall lifetimes of CO2?
What does this calculation suggest about “fixing global warming”?
2. Fossil fuel burning and deforestation are the major
anthropogenic sources of CO2 to the atmosphere. Given that CO2
was ~ constant before the industrial revolution, it appears these
are the dominant sources of new CO2. Together, they add 8 Pg
C/yr of CO2. Given the measured atmospheric growth rate of
CO2 we determined last class (2ppm/yr), derive a second
estimate of the atmospheric lifetime of CO2.
3. Shouldn’t 1 and 2 give the same result?
Steady-State: When is it the case?
dm
S
 kt
 S  km  m(t )  m(0)e  (1  e  kt )
dt
k
Steady state
solution
(dm/dt = 0)
Initial condition m(0)
Characteristic time t = 1/k for
• reaching steady state
• decay of initial condition
Today: Models
Reading: Chapter 3 in text
One-box Models
Multi-box Models
Moving the Box Model
Model Development and Application Loop
DEFINE PROBLEM
USE MODEL
make hypotheses or predictions
Urban air pollution
Stratospheric Ozone Depletion
Atmosphere-Ocean interactions
DESIGN MODEL
(make simplifications)
TEST MODEL
with observations
Models are simplified representations of reality. Observations
of the real system are required to test the model.
One Box Model
Chemical
production
Inflow Fin
Chemical
loss
P
Outflow Fout
L
X
D
E
Deposition
Emission
sources
 sinks
 Fin  E  P
mass balance:
Atmospheric “box”;
spatial distribution of X
within box is not resolved
 Fout  L  D
dm
  sources - sinks
dt
lifetime: t x 
mx
 Sinks

mx
totallossrate
Column Model: A Moving Box
Typically temperature inversion
defines “mixing depth”
Emission Flux E (amnt/cm2/s)
If X has first order loss, then
in column moving across city
d[ X ] E k

 [X ]
dx
Uh U
[X]
0
L
x
Questions
1.
Choose the most appropriate modeling strategy for the following
problems (1-box, 2-box, n-box, plume/column model):
a. exchange of a uniformly mixed greenhouse gas
between the stratosphere and troposphere
b. production of ozone downwind of an urban area
c. the vertical and horizontally resolved abundance of a
reactive emission like CO.
2. Suppose operators of a 1-box model of Seattle’s urban “air shed”
predicted that the concentration of pollutant emitted downtown
was going to rise to a unhealthy level in the U-District. Should you
believe them, why or why not?