Glowacki-AT207-L1 - University of Bristol

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Nov 16, 2013 (3 years and 11 months ago)

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Atmospheric chemistry

Lecture 1:



Introduction & Overview



Structure of the atmosphere



Atmospheric Transport

Dr. David Glowacki

University of
Bristol,UK

david.r.glowacki@bristol.ac.uk

Our goals in these lectures…


Atmospheric Chemistry is
fascinating because it is spans a
range of fascinating subjects


In one week, I hope to:


give you an overview of
atmospheric chemistry


teach you some of the key
principles


provide you sufficient
background to understand
the details of two key arctic
atmospheric phenomena:

(1)
arctic haze

(2)
Polar ozone holes

Useful Reading Materials…


Daniel Jacob,
Introduction to Atmospheric Chemistry,
1999, available on the web at


http://acmg.seas.harvard.edu/publications/jacobbook/index.html


Seinfeld & Pandis,

Atmospheric Chemistry And Physics:
From Air Pollution to Climate Change


Progress and Problems in Atmospheric Chemistry
,
edited by John R. Barker


G Marston, “Atmospheric Chemistry”,

Annu. Rep. Prog.
Chem., Sect. C, 1999, 95, p 235
-
276


G.E. Shaw, “The Arctic Haze Phenomenon”
, Bull. Am.
Met. Soc.
, 1995, 76(12), p 2403
-
2413


P.S. Monks, “Gas phase radical chemistry in the
troposphere”,
Chem Soc. Reviews
, 2005, p 2

21


Where I live…

What I do…


I work on the frontier where chemistry
meets theoretical physics


I use the mathematical tools of quantum &
classical mechanics to understand what
molecules do


Most of my research involves
massively

parallel computers


A lot of what I do concerns how to make
more accurate approximations to solving
the full quantum mechanical equations


Lots of applications:


Atmospheric chemistry


Combustion


Materials Science


Biochemistry




My background…


During my PhD, I did atmospheric
chemistry experiments:


Instrument design


Laser spectroscopy


Optics


Chemical kinetics


http://www.chem.leeds.ac.uk/HIRAC/

Before my PhD…


MA in religion and Cultural Theory at the University of Manchester:


Undergraduate degree at the University of Pennsylvania in Philadelphia:


Major in Chemistry with lots of work in math, physics, and Humanities
subjects


That’s where I met Mark Hermanson


We worked together to teach an environmental chemistry class


Originally from Milwaukee, WI

Our Plan for Today’s Lecture


The general structure of the atmosphere


Vertical Mixing in the Atmosphere


Variation of Pressure with altitude


Variation of Temperature with altitude


Horizontal Mixing in the Atmosphere


Coriolis Forces



Hadley Circulation


Atmospheric Temperature and pressure
variations

z


Heating by
exothermic
photochemical
reactions


Convective heating from
surface. Absorption of
IR (and some VIS
-
UV)
radiation from the sun

Vertical Mixing Processes

Variation of pressure with Altitude: The
hydrostatic equation


Consider a column of air at altitude
z



A cross section of the air has width
dz
,


It has two opposing forces:


Upward direction:
[
P
(
z
)
-
P
(
z+dz
)]
A


Downward direction:

-
ρgAdz


If the air parcel is in equilibrium, then:

[
P
(
z
)
-
P
(
z+dz
)]
A

=
-
ρgAdz


[
P
(
z
)
-
P
(
z+dz
)]

=
-
ρgdz


z

P
(
z+dz
)

P
(
z
)

dz

Rewriting gives the hydrostatic equation:

-
ρgAdz

[
P
(
z
)
-
P
(
z+dz
)]
A

Combining the hydrostatic equation with the
Ideal gas Law: the Barometric equation


Rearranging and integrating we
obtain the
Barometric equation
:



Ideal gas law tells us that


PV=nRT



The Density of a gas,
ρ
, may thus
be written as:


r

= m(n/V) = m(P/RT)


where m is the molecular weight of
the gas


Plugging this expression for density into the
hydrostatic formula gives:

z

P
(
z+dz
)

P
(
z
)

dz

-
ρgAdz

[
P
(
z
)
-
P
(
z+dz
)]
A

Properties of the barometric equation


H
s

is termed the
scale
height


It is the altitude over which the
pressure falls by a factor of 1/e (
0.37)
{hint: you can see this by setting
z

=
H
s
}


The Barometric equation written above:



-

Assumes T is constant (remember that T actually depends on
z!)



-

May be compared with a Boltzmann distribution



-

Has an average
M
air

= 28.8 g mol
-
1



-

H
s

= 6 km for T = 210 K; and ~8,5 km for T = 290 K.



-

Species with a smaller molecular mass would have a larger


scale height; however, because of turbulent mixing, this


separation is not important in the troposphere and stratosphere

A simple application of the barometric
equation: sea breeze


Fluids flow from regions of
high

density (pressure) to
low density (pressure)

Mass conservation

Variation of Temperature with altitude: the
dry adabiatic lapse rate


1
st

law of Thermodyamics
(Conservation of Energy)



Δ Heat


Δ work

(pressure


volume)



Δ System
Energy



Δ enthalpy



The air parcel doesn’t
exchange heat with the
surroundings (adiabatic
process)



Tells how much energy we
have to put into the system to
change its temperature


From the hydrostatic

equation

(dp/dz=
-
ρg)


Dry
Adiabatic
Lapse Rate


The adiabatic lapse rate


As air parcels rise, they expand and cool


On earth,
g
and
C
p

combine to give






G
d

~ 9.8 K
km
-
1


The actual atmospheric temperature
gradient,


is defined as:





=
-
(dT/dz)
atm


The adiabatic lapse rate may be less
than or equal to



This affects vertical mixing, giving rise to
either stable or unstable conditions

Adiabatic lapse

Stable Atmosphere


Convective Atmosphere


If
G
d
>


the atmosphere is
stable

& little mixing occurs


As a rising air packet
A

expands, it
cools faster than the surroundings


At the same pressure,
T
A
(z+dz
) <
T
ATM
(
z+dz
), making
A

cooler and
denser (
r 
P/T
) than its
surroundings, slowing its rise


If
G
d
<


the atmosphere is
unstable
& convection occurs.


As a rising air packet
A

expands, it
cools slower than the surroundings


T
A
(z+dz
) >
T
ATM
(
z+dz
), making
A

warmer & less dense than its
surroundings, accelerating its rise

(slope Λ)


Little vertical mixing

Fast vertical mixing
-

convective


(slope = Γ
d
)

(slope = Γ
d
)

(slope Λ)


Γ
d

is constant

Λ changes depending on conditions

The Planetary Boundary Layer


The subsidence inversion creates stability & inhibits mixing, often
leading to bad pollution build
-
up in large cities


Planetary Boundary Height = 500


3000 m.


Mixing near the surface is always fast because of turbulence




The Planetary Boundary Layer: diurnal
variations


During the day, the earth heats the
surface layer by conduction and then
convection mixes the region above in
the
convective mixed layer.
There is
usually a small T inversion (dT/dz >0)
above this which marks the top of the
BL. This slows transfer from the BL to
free troposphere (FT). Traps pollutants.


Night


surface cools, dT/dz > 0 in
surface layer


surface inversion.
Confines pollutants to surface layer.


Can get extreme inversions in the
surface layer in winter that can lead to
severe pollution episodes. High build
up of pollutants.



Vertical Mixing


Average atmospheric lapse rate is 6.5 K km
-
1
, giving moderately stable
conditions


Turbulence, most important near the surface, increases mixing


Solar heating also makes the atmospheric unstable & increases mixing
(accounts for different mixing between night and day)


Water vapor and clouds complicate all these things


The stratospheric Temperature inversion significantly limits vertical
mixing between the troposphere & Stratosphere, limiting transport of
many ground level VOCs to the stratosphere (The polar regions are
special though!)


Tropospheric/stratospheric mixing times are on the order of years!


The Temperature profile of the stratosphere means it is much more
stable than the troposphere






Atmospheric Transport


Random motion


mixing


Molecular diffusion


Molecular diffusion is slow, diffusion coefficient D ~ 2x10
-
5

m
2

s
-
1


Average distance travelled in one dimension in time t is ~

(2Dt)


Molecular diffusion more important at very high altitudes & low
pressures


Air Parcel diffusion


In the troposphere, eddy diffusion of air parcels is more important
with a diffusion coefficient K
z

~ 20 m
2

s
-
1



Takes ~ month for vertical mixing (~10 km). This has implications
for short and long
-
lived species.


Directed motion


Advection


winds & geostrophic flow


Occurs on a number of different scales


Local (e.g. offshore winds & sea breeze


see earlier)


Regional (weather events)


Global (Hadley circulation)

Horizontal Mixing Processes

Global circulation


Hadley Cells


Intertropical conversion zone (ITCZ)


rapid vertical transport
near the equator.


To a first approximation, horizontal mixing within the atmosphere is
well described as sea breeze circulation driven by the T difference
between the hot equator and cold polar regions


Hadley circulation model developed in the 18
th

Century


Coriolis Forces


Longitudinal winds are well described by a coupling of Hadley type
circulation to Coriolis forces


What is a Coriolis force?

2d example

3d example

http://www.youtube.com/watch?v=BPNLZyBNPTE&feature=related

http://www.youtube.com/watch?v=Wda7azMvabE&NR=1

Coriolis Forces & Hadley Cells

Geostrophic Flow: A balance of Coriolis
Forces & Pressure Gradients

The theoretical flow that
would result if the system
was no more complicated
than Coriolis forces and

parallel isobars

The general circulation: Hadley Cells
coupled to Coriolis Forces

ITCZ

High pressure latitudes

(location of major deserts)

Trade Winds

(easterlies)

Westerlies

Trade Winds

(easterlies)

High pressure latitudes

(dry areas)

Stronger westerlies

Roaring 40s & Screaming 50s

(less friction)

Polar high pressure region

Polar high pressure region

Ferrel cell

Horizontal transport timescales

Summary


Atmospheric chemistry depends on atmospheric structure
& transport dynamics


Some simple physics gives us basic insight into some of
the principles that determinate atmospheric structure and
transport dynamics


The barometric equation describes the relationship
between pressure & altitude


The adiabatic lapse rate helps us understand the
atmospheric vertical T dependence, and vertical
transport


To a first approximation, global circulation may be thought
of as sets of sea breeze cells coupled to Coriolis forces


Mixing processes are coupled to chemical change, which
we will learn about tomorrow

Some Questions to Consider


Using your knowledge of (1) the adiabatic lapse rate and (2) the
temperature profiles of the troposphere and stratosphere, explain why
vertical mixing in the stratosphere is much slower than in the
troposphere


Using (1) a diagram involving adiabatic lapse temperature profiles
and (2) your knowledge of stability/instability, explain the origins of the
planetary boundary layer


Should footballers worry about
Coriolis

forces when they are taking a
north to south 20m free kick that travels 50 km/hr? (hint: see Jacob
equation 4
-
3)


Based on the model of general circulation:


Why are the timescales for east west transport shorter than those
for north south transport?


Using your knowledge of Hadley transport, the Sea Breeze model,
and
Coriolis

forces:

1.
Explain why the Arctic is so windy

2.
What direction would you expect the wind to blow on the ground in
Longyearbyen
?