| title | author | beamer_color_theme | beamer_header | classoption | date | description | header-includes | infojs_opt | keywords | language | macro | startup | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Atmospheric chemistry with a focus on ozone and hands-on modeling |
|
beaver |
\usefonttheme{professionalfonts} |
ignorenonframetext,presentation,smallest |
LCLUC Workshop, July 2017 |
Notes prepared for the 2017 LCLUC Training, Chiang Mai. See http://lcluc.umd.edu/meetings/lcluc-sari-international-regional-science-meeting-southsoutheast-asia |
|
view:t toc:f ltoc:f mouse:underline buttons:1 path:http://orgmode.org/org-info.js |
Training Atmospheric Chemistry Ozone Box modeling |
en |
BEAMERMODE presentation |
beamer |
-
Introduce concepts of atmospheric chemistry
- Today it's all about ozone
- Primary/secondary pollutants
- Emission/deposition
- Photochemistry
-
Run first numerical simulation of a chemical system
- Simple photochemical system
-
Code is available here
-
You can clone the code using git via
git clone git@gitlab.com:ptg21/LCLUC_presentation.git
-
My goal is to introduce atmospheric chemistry with a focus on tropospheric ozone and other secondary pollutants.
-
I won't discuss the chemistry in detail but will summarise the relevant reactions. It gets complex towards the end.
-
The goal is to use these reactions to study how ozone levels respond to other pollutants.
-
Our focus is on rates of production of ozone during the day.
-
For the purpose of this course, everything is a pollutant.
-
Mostly think about processes in terms of their characteristic timescales
- How fast is ozone formed?
- How fast is transport out of the planetary boundary layer?
- How does this compare with transport times?
-
What are the important species?
- Ozone
- NO
2 - Aldehydes
- Oxidants such as OH, NO
3 - Key species such as O^1^D
![Figure 2: []{#dickie_ridge}'Trees cause more pollution than automobiles do' - Ronald Reagan, 1981](/paultgriffiths/LCLUC_box_model_training/blob/master/figures/dickie_ridge.png "dickie_ridge")
Pollutant Concentration Lifetime / yr
CH4 1700 ppbv 10
H2 500 ppbv 4
CO 40-200 ppbv 0.2
O3 20-120 ppbv 0.05
OH 0.1 pptv 0.1s
1 ppbv = 10^-9^ 1 pptv = 10^-12^
Pollutant Low Moderate UFSG Unhealthy
Ozone 0-54 55-70 71-85 86-105
NO2 0-53 54-100 101-360 186-304
CO 0-4.4 4.5-9.4 9.5-12.4 12.5-15.4
Levels are in ppbv
Primary Emitted directly into the atmosphere (usually at the surface)
- Nitric oxide, NO
- Volatile organic compounds such as methane, CO
- Biogenic VOCs suc as isoprene, terpenes, formaldehyde (HCHO)
- Anthropogenic VOCs such as benzene, gasoline
- Primary aerosol such as soot
- SO_2
Secondary Made in the atmosphere by 5.1
- Ozone, O
3 - NO
2 - Formaldehyde (HCHO)
Considering the atmosphere as a whole, or some air-mass within in it, we could write an equation describing the rate of change ('tendency') of a species.
Prognostic equation for species X, with concentration
\vspace{-0.1in}
\begin{eqnarray*} \frac{dx}{dt} &=& R -k x \end{eqnarray*}
where R is the rate of emission of X and k is a constant
We now have a first-order linear differential equation, which can be solved to give
\vspace{-0.1in}
\begin{eqnarray*} x(t) &=& \frac{R}{k_1}\big(1-\exp (-k_1 t)\big) \end{eqnarray*}
System has a characteristic time,
-
Basic points
- Rate is defined as change in concentration per unit time
- Natural unit of concentration in air quality modelling:
- concentration: molecules per cm^3^ gas so units are cm$^{-3}$
- rate: cm$^{-3}$ s$^{-1}$
- Law of Mass Action - Double the concentration = Double the rate
-
NO + O
3= NO2+ O2- The rate of change of NO can be expressed as
\vspace{-0.1in}
\begin{eqnarray*} \frac{d [NO]}{dt} &=& -k_1[NO][O_3] \end{eqnarray*}
- Similarly,
$\frac{d[NO_2]}{dt} = k_1[NO][O_3]$
- Molecules absorb photons and the chemical bonds are broken - photolysis
\vspace{-0.1in}
\begin{eqnarray*} \mathrm{NO}_2 + hv \rightarrow \mathrm{NO} + \mathrm{O} \end{eqnarray*}
- Rate of photolysis depends on number of photons of the correct wavelength.
\vspace{-0.1in}
\begin{eqnarray*} \frac{d[\mathrm{NO}_2]}{dt} &=& - J [\mathrm{NO}_2] \end{eqnarray*}
J depends on molecule and flux of photons (hence: time of day, lat, lon, cloud cover). Units of J are s^-1^
Using the reactions already given,
\vspace{-0.1in}
\begin{eqnarray*} \mathrm{NO} + \mathrm{O}_3 & \rightarrow & \mathrm{NO}_2 + \mathrm{O}_2\ \mathrm{NO}_2 + hv &\rightarrow& \mathrm{NO} + \mathrm{O}\ \mathrm{O}_2 + \mathrm{O} &\rightarrow & \mathrm{O}_3\ \end{eqnarray*}
\vspace{-0.15in}
we can write rates of change for each species
\vspace{-0.1in}
\begin{eqnarray*} \frac{d[\mathrm{NO}_2]}{dt} &=& - J_1 [\mathrm{NO}_2] + k_3\mathrm{[NO]}\mathrm{[O}_3]\ \frac{d[\mathrm{NO]}}{dt} &=& J_1 [\mathrm{NO}_2] - k_3\mathrm{[NO]} \mathrm{[O}_3] \ \frac{d\mathrm{[O]}}{dt} &=& - k_2 [\mathrm{O}][\mathrm{O}_2] + J_1 [\mathrm{NO}_2] \ \frac{d\mathrm{[O}_3]}{dt} &=& k_2 [\mathrm{O}][\mathrm{O}_2] - k_3 \mathrm{[NO]} \mathrm{[O}_3] \end{eqnarray*}
A set of coupled differential equations results!
What is our mechanism going to do?
[columns]
[column=0.5]
- We can see that NO and ozone make NO2
- NO2 makes NO and O, and O makes O3
- so NO2 regenerates the NO and O3
- This is an active equilibrium - NO and NO2 interconvert, consuming/releasing ozone as they do so.
[column=0.5]
{ width=60% }
[/columns]
As we shall see in L2, this equilibrium is crucial.
-
So we expect our equations to solve to an equilibrium with zero net rate of change
-
There exists a wealth of literature on the solution of these stiff differential equations (lifetimes of each species vary by many orders of magnitude, resulting in small timesteps).
-
In our example, the lifetime of O is very short, set by k_2[O2], while that of NO2 is determined by J and can be much longer.
-
Step forward our numerical ('box') model...
[columns]
[column=0.5]
- Box models represent a single representative area of the atmosphere.
- Notionally 1cm^3 in volume
- Can be connected to the ground via emission/deposition.
- Could also be chosen to represent the free troposphere.
- Need to supply photolysis rates, emissions
[column=0.5]
{width=150%}
comment: 'image downloaded from here [http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap3-1.gif']
[/columns]
[columns]
[column=0.5]
- Box models need a chemical mechanism.
- The literature can supply these, or you can write your own.
- You then code up the mechanism as a differential for each species, in terms of other species' concentrations and other inputs.
[column=0.5]
[/columns]
[columns]
[column=0.5]
- Implementation in the language of your choice
- You need an integrator for the differential equations.
- There are good ones already implemented, so don't write your own!
- Typically you supply initial conditions, C0, functions for the tendency of each species,$f$, a timestep (dt) and an end point (tend).
[column=0.5]
{width=80%}
[/columns]
-
Getting started
- Open RStudio or R
- Look at \tt kinetics-box-model-pss.R
in the src folder.
- What do equations describe?
- What do you expect to happen?
-
Any Pythonistas in the audience?
-
Run the simulation
-
source("kinetics-box-model-pss.R") -
Do the results make sense?
- If so: get a coffee!
- If not: shout out!
-
Coffee break