1. Atmospheric Processes and Composition:
Cambridge Centre
for Climate
Science
Atmospheric Processes and Composition:
Simulating gas phase composition in an ESM.
Alexander T. Archibald et al.
Centre for Atmospheric Science st E2SCMS Earth System Modelling Summer School
Sunday 3rd June 2012, Kos, Greece. 1 1

2. Talk outline:
Summary of some of the important physical features of the atmosphere.
Simulating gas phase chemical processes in an Earth System Model.
Important problems concerning the gas phase composition of the lower and middle atmosphere.
No real discussion on aerosols.

3. Human activities have changed the composition of the atmosphere.

4. Changes in components of radiative forcing (PI-PD).

5. Climate Forcing and Ecosystem Effects: Is Radiative Forcing the “right” metric?
Effect on terrestrial carbon budget:
12.8 – 5.6% – 14.4 % – % – %
(Huntingford et al., 2011 Phil. Trans. Royal. Soc. A.)

6. Thermal structure of the atmosphere.
The “ignore-osphere”
Heating by ozone absorption in stratosphere
Heating largely from below at Earth’s surface
convection, overturning

7. NOVA 8 balloon flight by CU Spaceflight - 33 km
The sun as the driver for atmospheric composition-climate interactions.
NOVA 8 balloon flight by CU Spaceflight - 33 km

8. The sun as the driver for atmospheric composition-climate interactions.
Gases can:
Absorb radiation (re-emit or break apart)
Scatter radiation

9. Atmospheric composition.
Fraction of gas does change when it is:
(a) chemically active - e.g. O3
(b) physical or biological source or sink - e.g. CO2
Well mixed (fraction of an inert gas does not change - e.g. N2, O2)
More than 99% below 50 km
Units in atmospheric chemistry:
mixing ratio (ppm = 1/million, ppb = 1/billion , ppt = 1/trillion)
concentration (moles, molecules cm-3)

10. Simulating gas phase chemical processes in an Earth System Model.
Continuity equation
d[X]/dt = transport + chemistry
chemistry = production – loss
transport = transport in - transport out
Transport in
Transport out
chemical change

11. Chemical time constants.
Timescale required to reach equilibrium (or for decay) First order reaction: X  products
(rate of change) d[X]/dt = -k[X]
(integrate) [X]t = [X]o exp(-kt) The timescale is C = 1/k
(k = rate constant)

12. Dynamical time constants.
Strict definition: d : time for [X] to change by 1/e by advection alone More practically, time for constituent to be transported a characteristic distance, L e.g. scale height for vertical transport For example, a constituent in the stratosphere with a scale height H = 5 km, moving vertically at 1 mm.s-1, the time scale for vertical transport would be:
vert = 5103/10-3 s = 5106 s ~ several months
Stratosphere zonal (E-W) direction: order of days to weeks meridional (N-S) direction: months Troposphere vertical and horizontal dynamical timescales are of the order of days to weeks.

13. Chemistry vs dynamics.
chem << dyn Photochemically controlled region. transport << chemistry Chemical steady state a good approximation  Ox (secs) or NO2 (minutes) chem >> dyn Dynamically controlled region. transport >> chemistry Chemistry is not important in determining structure. Changes occur primarily as a result of air movement  CH4 (C ~ 10 years)  O3 in the low stratosphere (except polar regions). chem ~ dyn Both chemistry and dynamics are important. transport ~ chemistry

14. Lifetimes and advection scales.

15. Building a chemical mechanism.
The first thing to do is be clear with what question do you want to ask?
The chemical mechanism needs to represent the way in which a chemical compound is converted in the atmosphere.
[NOx] > 0.1 ppb < [NOx]
Contains (hopefully!) stoichiometric equations and information to calculate rates of change (rate constants).

16. An example mechanism for methane oxidation (on paper).
Use of appropriate information from laboratory experiments and from atmospheric observations allow us to build up reaction “mechanisms” for a range of compounds.

17. An example mechanism for methane oxidation (for the model).
CH4 + OH -> CH3 + H2O (k1 = 1E-15)
CH3 + O2 + M -> CH3O2 + M (k2 = 1E+2)
CH3O2 + NO -> CH3O + NO2 (k3 = 1E-11)
CH3O + O2 -> HCHO + HO2 (k4 = 1E+1)
NO2 + “hv” -> NO + O (j5 = 1E-2)
O + O2 + M -> O3 + M (k6 = 2E-12)
O3 + “hv” -> O* + O (j6 = 1E-3)
O* + H2O -> OH + OH (k8 = 2E-10)
CH4 + OH -> HCHO + HO2 + H2O (ktotal = 1E-15)

18. Ozone: A not so radical history.
Christian Schönbein ( )
Levels of ozone in the troposphere have changed since the PI.
Three oxygen atoms (trioxygen)
From the Greek – to smell

19. Challenges concerning the composition of the lower and middle atmosphere.
Antarctic ozone hole
Arctic ozone hole

20. Ozone: A key ingredient for life.

21. Ozone: A key ingredient for life, but…
Damage to crops in EU ~ €2.87 billion
Cost to health ~ €11.9 billion!

22. An ‘oxygen-only’ stratospheric chemistry scheme.
O2 + h O + O J1  240nm
O + O2 + M  O3 + M k2
O3 + h  O + O J3  1100nm
O + O3  O2 + O2 k4
d([O] + [O3])/dt = 2 J2 [O2] k4 [O] [O3]
[O] + [O3] is ‘odd-oxygen’ or [Ox]
By late 1960s, realised that Chapman scheme predicts too much ozone.
(Chapman (1930) chemistry)

23. Radical catalysed loss of ozone.
O + O3  O2 + O2 (very slow)
X + O3  XO + O2
XO + O  X + O2
Net: O + O3  2O2
NO, NO2 (NOx) from N2O; OH, HO2 (HOx) from H2O, CH4
Cl and ClO (ClOx), breakdown products of the CFCs, increased rapidly after the 1950s.

24. VOC Traditional understanding of VOC oxidation chemistry.
In the troposphere O3 is formed from a complex interplay between reactions of VOC and NOx:
NO2 + hn  NO + O(3P)
O(3P) + O2 (+M)  O3 (+M)
NO2 NO OH
HO2
carbonyl product(s)
VOC
RO2 RO O2 NO
NO2
Peroxy radical
Oxy radical 24

25. Simulating ozone chemistry in the Met Office Unified Model with UKCA.
Dynamics:
Non-hydrostatic model.
Horizontal res. 2.5°×3.75°
60 vertical levels extending to 84 km.
Chemistry:
76 Chemical tracers.
264 photochemical reactions.
Photolysis calculated online.
Emissions:
Eight chemical species are emitted in the model.
Results from an example simulation for the present day.

26. Surface ozone distribution.
High near areas of pollution
(ppb)
Low where no sources

27. Zonal mean ozone distribution.
Highest in tropical region.
Why?
(ppm)

28. Total column ozone distribution.
Spring time maximum.
Why is the tropics low?
Polar ozone depletion in the stratosphere

29. Climate change effects on ozone.
Predicted changes in stratospheric O3 from 2x CO2. Increased stratospheric O3 and a predicted more rapid circulation will in turn lead to increased tropospheric O3.

30. Surface ozone: Sensitivity to model resolution.
N96 (1.87ºx1.25º ~150km) N320 (0.55ºx0.37º ~40km)
N96 (1.8x1.2 ~ 150km)
N320 (0.5x0.3) ~40km

31. Summary.
Gas phase composition can be readily included in an ESM and is nothing more complicated than a series of coupled differential equations.
As with any process, increasing complexity needs to be driven by a clear scientific question.