Physics and Chemistry of the Mesopause Region Physics and Chemistry of the Mesopause Region

Home , Mesopause

2001 CEDAR-SCOSTEP Longmont, Colorado June 17-22, 2001

Tutorial Lecture #6

by Anne Smith ACD/NCAR, USA

A. K. Smith NCAR

Processes involved in the coupling be tween composition, energy and dynamics

Importance of external forcing Outline

composition of the mesopause region: how it differs from other regions

impact of dynamics and energy on com position

impact of composition on energy and dy namics

example: role of CO2 in mesopause struc ture

example: photochemical destabilization of gravity waves Definitions

• mesopause: altitude of the temperature mini mum, which varies on seasonal and shorter time scales; the talk will consider the range 80-100 km.

• composition: mostly concerned with the variable part of the composition, making up <1% of the total

• mixing ratio: density of species/total density

• energetics: the thermal budget & the processes for converting between heat and other forms of energy

• vertical scale: for the models, log(pressure) scaled to give approximate altitude

Note: many of the illustrations are taken from numer ical models HRDI Mean Temperatures for July

160 140 -60 -40 -20 0 20 40 60 MSIS Mean Temperatures for July

100

80 160 70 140 -60 -40 -20 0 20 40 60 from Ortland et al., JGR, 1998

-80 -60 -40 -20 0 20 40 60 80 ^q I....I....I....I •!' inlunl'irliMilmiliiiiliiiiliiiil 110

THERMOSPHERE

• decrease in proportion of dominant molecules (O2, N2) • increase in several simple molecules (NO, CO)

• increasing dominance of atoms

• more excited states

• reactions can be slow because of low density

• ions and ion reactions are important

STRATOSPHERE & LOWER MESOSPHERE

• breakdown of larger molecules produced in the troposphere

• larger role for atoms than in the troposphere

• density is still high enough for 3-body reactions

• sharp daytime maximum of some atoms & com pounds (O, CI, H, OH, NO, etc.) • only one excited species (O^D)) plays a signifi cant role

• catalytic cycles of NOx, ClOx and HOx destroy ozone Which atoms <& molecules are present?

UPPER MESOSPHERE

• dominant molecules (O2, N2) still well mixed

• concentrations of NO, CO can be large

• excited states participate in chemistry (N(2D)) or energetics (e.g., 02(15I))

• a few tropospheric molecules remain: H2Of CO2

• increasing role for atoms, particularly O and H

• decreasing density affects 3-body reactions

• photochemically driven diurnal variations of most minor species

• catalytic cycle of HOx dominates ozone destruc tion

• ions (NO+) begin to have an effect on neutrals altitude (km)

05 CD CD o o o o O o o

1 i i t i 1 i i i i I i i i y I i i i i i i i i i i i i i i i i i |"-|~ 1 i i i i o

w \ - 3

: •-* : \ o O — \ i C H- 1—

- •

~ • - a - • w t—» • ^ • — o s z 1 o ^ o \ / o \ K ' ^"""^^ ^*^ \ ° s^*"^ - 1—»• \ x ' ^ P O z i—'• • 01 X ^>v^ \ ' j^ 0 // ~ \ «•»1 ~~ ~~ "* s S \ \ / i—' Z \ -v /\\ / ^„• z z s. ** ^ z O /\\ / f^ Z. N ^* / \\ / **^^ i N "**«^ 03 V ** «» *• "' N / ^-'S^**- \\ - -v*-**"^ x \\/ - \ viT-^^^ o " \ ~

1—' ~ ^ ->*c w — y^>^\ *"• z o =- V 1 C^"— \ \ ^^ \ ffi / / s \ X s •^ \ /> - •—^. \ jr _ h-* N. \ J-^ — O z 1 S>v *- "*s^ O) VS.s*^^^^j. "" ^^"^ ,» "* ^s?— »* ^^^"^

^~*~~ , l •*•"*— ^^^^ "' z O -' 1 ro ro X - X

ro I—1 E o z O = i i i i i i i i i i i i i i i i i i i 1 i i i i 1 i i i i 1 i i i i 1 i t i i = What are the important species?

1) from the point of view of controlling the composi tion:

• O: produced locally from O2 and O3, transported downward from thermosphere; increases strongly with altitude

• O3: produced from O+O2; rapidly destroyed dur ing the day

• H, OH, HO2: produced during the day from H2O, or transported downward from thermosphere (H)

2) from the point of view of measurements

• O3

• C02

• sodium and other metals

• excited species that emit: 02(15I), OH, etc. Trace Gas Mixing Ratio

•8 80

\ / \ 1 / • » ' / / \/ 60 I I I IMIll l/l I llllll 1 1 v I 1 1 1 mmiiI I 1 1 mml 1 1 "I"'! » ' 1mill 1 1 mint I I I Mill -12 10-n1 n-11 10-xu1 n-10 1Q-y1 n-9 10-a1 n-8 1Q-71 n-7 1Q-b1 n-6 10-d 10-* 1Q-j 1Q. 10 Which reactions occur?

photolysis, e.g: O3+/11/ -» O+O2

• sun angle

• solar flux

• column of absorbing gases

• weakly on temperature

2-body reactions, e.g: O+O3 -• O2+O2

• density of reactants

• temperature

3-body reactions, e.g: O+O+M -• O2+M

• density of reactants

• temperature

• density

*M is any atom or molecule Day/Night Differences

Daytime

• Sunlight splits molecules: O2, O3, H2O

• Absorption generates excited states of atoms and molecules that can react chemically or pass their energy along to another species

Nighttime

• at lower altitudes atoms & radicals recombine into more stable molecules (e.g., O -* O2, O3 and H, OH, H02 -• H20

• O and H persist at the mesopause & above

Transport

• dynamics is similar except for phase of the tides & processes affected by tides

• vertical transport can be very different because of gradients Global log(mixing ratio) of 0 at four local times

0 LT 6 LT

-nodel s;ep: 960 UT: 0.0 Doy: 196 nodel step: 960 UT: 0.0 Day. 196

12 LT 18 LT

rro>: -1.07 mo: -M.0 »ai: -1.05 mn: -14.0 model step: 960 UT: 0.0 Doy: 196 model step: 960 UT: 0.0 Doy: 196 Impact of anthropogenic gases on composi tion

• increased CO2 —>- cooling

• increased hydrocarbons ->• ozone loss

Impact of dynamics on composition

• winds advect species

• temperature affects reaction rates

• winds and temperature control propagation and dissipation of waves

Impact of energetics on composition

• photolysis affected by incident radiative flux

• atomic/molecular energy levels changed by ab sorption of radiation

• energetic particles can generate ionized or excited states

• temperature affects reaction rates temperature ABSOLUTE DIFFERENCE (K)

-4 10

100-

-2 _ --10

~3 a,

bfl 10U£ o

_-10'

90S 60S 30S Eq 30N 60N 90N latitudes (angular degrees) 03 relative difference (%)

E M a 60-

bfl G

90S 60S 30S Eq 30N 60N 90N latitudes (angular degrees) Diffusive transport molecular diffusion

• diffusive component reduces gradients

• "advective" component moves species differen tially

• effects on mesopause H, H2, CO2, temperature

• diffusion rate varies with temperature eddy diffusion

• caused by turbulence or by any unresolved dy namical scales

• important in the vertical because of strong mixing ratio gradients

• limited measurements

• mass mixing, i.e., all species (mixing ratio) and potential temperature have same diffusion coef ficient

• extremely variable due to intermittent dynamics Fig. 5. Comparison of measured and calculated CO mixing ratios

120

110 -

100 - E

Q> s 90 - --A--ISAMS •----EV18 80 •••••---- SL3-Mean TIME-GCM -GS

70 •' I I I I L L_j__i i i Ul i i—i—u 0 50 100 150 200 250 300 350 400 Volume mixing ratio (ppmv) UpeT-P*"-**-5•*"*Xt Advective transport

r variability gravity waves hours hours-seasonal tides hours days-seasonal planetary waves days weeks-seasonal mean circulation seasonal annual

General rules:

time scale of variability increases with spa tial scale

propagation of low frequency waves (plan etary and gravity waves) strongly affected by zonal wind SUNRISE/SUNSET ANOMALY

HALOE NO MARCH 199. 120p

t F / i- 11oF- r u I- 1

' /

— o 10 10 ~ 10 10 VOLUME MIXING RATIO

from Marsh & Russell, GRL, 2000 % Difference from Mean W 0600 HRS

-40 -20 0 20 40 -40 20 0 20 LATITUDE(DEG) LATITUDE(DEG)

% Difference from Mean W 1800 HRS Ml Hlkli iJiliU'i

LJ x

-40 -20 0 20 40 LATITUDE(DEG) fx (m/s/day) ; January

-0.001

0.010£

:-o.ioo

90S 60S 30S Eq 30N 60N 90N latitudes (angular degrees) (qui) amssaid o

Z Q

z o

Z o en

So o ,8

^ c

d C3

00 O

co I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I [ I ! I o OS o o O O o o (ui>i) Qptuppg d-§0| log-p altitude (km)

3 C/3

B3 P

o Pressure (mb) log-p altitude (km)

O m

CD 3

CD >-. P 3 H! 0\ CD ©

U3 o

C D- o

OQ W EL*0 p —.

O 03 o o

o z

CA O z

o Pressure (mb) Qualitative feature of the mean circulation

momentum from wave dissipation drives summer to winter flow

rising in summer, sinking in winter gives adiabatic forcing

IR radiation responds to local tempera ture

but ....

IR emissions are inefficient because of cold mesopause temperatures

stronger temperature response than would occur for comparable momentum forcing in a warmer region Impact of composition on energetics absorption of solar radiation depends on composition:

• O2: wavelengths 100-205 nm (Lyman-a, Schumann- Runge continuum, Schumann-Runge bands)

• O3: wavelengths 242-310 nm (Hartley band) absorbed energy goes into:

• breaking chemical bonds

• exciting vibrational or rotational levels of original absorber or its products

• converted immediately to heat S-R continuum S-R bands 200 < T^^ r

< 50

i. 50 100 150 200 250 300 WAVELENGTH (nm)

altitude at which flux is attenuated by 1/e

from Brasseur &i Solomon, Aeronomy of the Middle Atmosphere What happens to the energy of photolysis?

energy used to break bonds is "stored" in the products and can be converted to heat when they react

heat-releasing reactions can occur hours or days after and far away from original absorption

Reaction o + o3 ->• o2 + o2 O + O + M-* 02 + M * O + OH -• H + 02 * O + H02 -> OH + 02 * H + 02 + M -• H02 + M * 0 + 02 + M-»03 + M * H + O3 ->• OH + 02

* reaction rate decreases with temperature Chemical Heating

Spring, Night Spring, Day Chemical Potential Heating (K/day) Chemicql Potential Heating (K/day)

X o \_ Q. Q. <

-90 -60 -30 o 30 60 90 -90 -60 -30 0 30 60 90 Latitude Latitude

from MIynczak and Solomon, JGR, 1993 What happens to the energy of excited states?

converted to heat:

Os + hv -• 02(}/\g) + O^D)

02(1A5) + 02 -»• 02 + 02 + heat transferred by collision:

O^D) + 02 -+ O + 02(15I9) radiated out of region

02(1A5) -+ 02 + hv

O2(151o) ->• 02 + hv reacts:

N(2D) + 02 -• NO + O Magnitude &l distribution of terms in the ther mal budget

diffusion

advection by the mean circulation

direct solar 02 heating*

direct solar O3 heating*

chemical heating*

infrared cooling

* minus fraction lost through emission (airglow) log-p altitude (km)

o O (S)

o oo

—• O

o P r-1- H- • o go

ca P c v: & o •j~. cjq ffl EL*0

—. a a> aq d o

•o log-p altitude (km)

p a- P a- p —' • o cr Q P r-t •—» •

OQ cr

O - 5 P

C P — log-p altitude (km)

o go

o GO o to sr o p i—K »—• •

CTQ

O GO P- P

era m

—.

CD o w

GO o

P 2

o Pressure (mb) log-p altitude (km)

O G>-> cr

P r-f- •— •

rjq - 5 P

c p >-* Chemical Heating (K/day) January

100-

0.001

E M

p 0.010i n 80 B

-0.100

90N latitudes (angular degrees) Iog-p altitude (km)

-J CO O o O o o

o I I I I I I I I I I I I I I I I I I I I I i i I I I I I I I I I I I I I I I GO

O GO

p a- >—• • p r-f- < CD o O

o >— • GO era. p s c p

9-CD ' w / 5 / p OQ W . / C 43 I P 8- - fl CD CD V) ". /

GO : , O _ \ 2 \

o SI

o Pressure (mb) EXAMPLE OF INTERACTION polar mesopause temperature determined by

adiabatic processes of wave-driven circu lation

heating & cooling (CO2 cooling rate; chem ical heating) upward motion (summer pole):

adiabatic cooling

high C02

enhancement to IR cooling log-p altitude (km)

CO vo O o 3 O O O

o i i i i i i i i i I i i i i i i i i i I I I I I I I I I I I I I I I I I I I I I I I l I l l GO

n o CO

p p

o Pressure (mb) log-p altitude (km)

so o GO

CD H P r-t

n CD

CD GO

CD HI CD

O CD

a- o GO s= CD

r-t- O ^

C O- o & 3 O 5 O i-t bo a o P

—. o o I

O -

_ _ _|

SO o I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I i i i i—r

o b o o •—• o o o Pressure (mb) IMPACT OF GLOBAL C02 STRUCTURE

positive feedback

enhances the cold summer, warm winter temperature differences

model results may underestimate magni tude because of neglect of molecular dif fusion Photochemical Destabilization of gravity waves

Air parcel trajectory in a propagating gravity wave

cold

warm

Composition and density

high O, total density low

lower O, higher total density midnight density of O and O3 gravity wave unstable growth rate 100

GrowthRate (10V) Density (cm')

solid: Kzz high; w=0 dashed: Kzz low; w=upward

from Xu et al. JASTP, in press variation in growth rate with wavelength

1x103 2x10 3x10" 4x103 Horizontal Wavelength (km ) cutoff for Kzz = 102m2/s

Photochemical destabilization more likely for:

• low temperature

low diffusion

sharp vertical gradients of O

long vertical, short horizontal wavelengths Composition

variability

— regular diurnal, seasonal and interan- nual time scales

— irregular variability due to forcing from above (energy) and below (waves)

feedback

— affects all scales of motion from mean circulation to individual gravity waves

— controls the conversion of energy to heat