110
Tropospheric Ozone
AOSC 637
Atmospheric Chemistry
R. Dickerson
OUTLINE
V. ATMOSPHERIC BUDGETS
V. A. The Ox Family
2. Tropospheric ozone
Finlayson-Pitts, Chapter 6, 14, & 16
Seinfeld, Chapter 4
Wayne, Chapter 5
WMO Atmospheric Ozone (2006)
NAS Report, Seinfeld, ed., 1991.
EPA ozone Criteria Document (2007)
http://cfpub.epa.gov/ncea/CFM/recordisplay.cfm?deid=149923
Ozone is a major pollutant. It does billions of dollars worth of damage to agricultural crops each year and is the principal culprit in photochemical smog. Ozone, however, exists throughout the troposphere and, as a major OH source and a greenhouse gas, plays a central role in many biogeochemical cycles. That photochemical processes produce and destroy stratospheric ozone have been recognized since the thirties, but the importance of photochemistry in tropospheric ozone went unrecognized until the seventies.
The classical view of tropospheric ozone was provided by Junge ( Tellus , 1962) who looked at all the available ozone observations from a handful of stations scattered over the globe. Free tropospheric concentrations appeared to be fairly uniform, but boundary layer concentrations were reduced. He also noticed a repeating annual cycle with spring maxima and fall minima.
Tropospheric ozone maxima lagged stratospheric maxima by about two months. From this he concluded that ozone is transported from the stratosphere into the troposphere where it is an essentially inert species, until it contacts the ground and is destroyed. The implied residence time varies from 0.6 to 6.0 months.
Levy ( Planet. Space Sci ., 1972) first suggested that radicals could influence the chemistry of the troposphere, and Crutzen ( Pageoph , 1973), shortly followed by Chameides and Walker ( J.
Geophys. Res.
, 1973), pointed out that these radical reactions could form ozone in the nonurban troposphere. Chameides and Walker's model predicted that the oxidation of methane (alone) in the presence of NOx would account for all the ozone in the troposphere and that ozone has a lifetime of about 1 day. Chatfield and Harrison ( J. Geophys. Res ., 1976) countered with data that show the diurnal variation of ozone in unpolluted sites is inconsistent with a purely photochemical

111 production mechanism and showed that meteorological arguments could explain most of the observed ozone trends described by Chameides and Walker.
To summarize, chemists found a possible major anthropogenic perturbation of a vital natural process. In their zeal to explain this problem some of the chemists completely neglected the physics of the atmosphere. This irritated some meteorologists, who point out that one can equally well interpret the observations in a purely meteorological context. With the dust settled, we can see that the physics of the atmosphere controls the day-to-day variations and the general spatial structure, but chemistry can perturb the natural state and cause long term trends. This paradigm recurs.
Logan ( J. Geophys. Res ., 10463, 1985) and Crutzen (Tropospheric Ozone: An Overview, in
Tropospheric Ozone, pp. 3-32, ed. I.S.A. Isaksen, NATO ASI Series, Reidel Pub. Co., 1988),
Sillman et al., JGR, 1990, present good reviews of a clear picture of global tropospheric chemistry.
Logan's conclusions were that atmospheric O
3
is increasing near the surface in Europe and in the central and eastern U.S. The increase has been 20 to 100% since 1940's. The best overall summary is the EPA Criteria Document (2007).
Surface: summer maximum in Northern Hemisphere rural areas. Summer minimum in Northern
Hemisphere remote areas and in Southern Hemisphere
Mid troposphere: similar to surface
Upper troposphere: spring maximum everywhere

112
Note Northern Hemisphere and Southern Hemisphere are out of phase
Hypothesis: The summer maximum at the surface is caused by in situ photochemistry, while the spring maximum in the upper troposphere is caused by transport.
Measurements - free troposphere

Hemisphere asymmetry: Northern Hemisphere >> Southern hemisphere

Minimum about 10 o
S not ITCZ.

Maximum 40 o
N - why?

In Northern Hemisphere downward transport is greater, but so is surface destruction - there is more land in the Northern Hemisphere.
Tropospheric Ozone Photochemistry
Carbon monoxide oxidation in a clean environment:
(1) O
3
+ h
 
O
2
+ O(
1
D)
(2) O(
1
D) + H
2
O

2OH
(3) OH + O
3

HO
2
+ O
2
(4) HO
2
+ O
3

2O
2
+ OH
-----------------------------------------
(3+4) 2O
3

3O
2
NET
Carbon monoxide oxidation in a dirty (polluted) environment:
(3') OH + CO

H + CO
2
(4') H + O
2
+ M

HO
2
+ M
(5') HO
2
+ NO

NO
2
+ OH
(6') NO
2
+ h
 
NO + O

113
(7') O + O
2
+ M

O
3
+ M
-------------------------------------------------
(3'-7') CO + 2 O
2

CO
2
+ O
3
NET
Methane oxidation in a polluted environment:
CH
4

OH

CH
3

H
2
O
CH
3

O
2

M

CH
3
O
2

M
CH
3
O
2

NO

NO
2

CH
3
O
CH
3
O

O
2

H
2
CO

HO
2
HO
2

NO

NO
2

OH
NO
2
 h

NO

O
O

O
2

M

O
3

M

CH
4

4O
2
 h

2O
3

H
2
CO

H
2
O NET
What happens to the formaldehyde?
2H
2
CO
 h

H
2

CO

HCO

H
H

O
2

M

HO
2

M
HCO

O
2

HO
2

CO

2H
2
CO

2O
2

2CO

2HO
2

H
2
NET
This is two ozone molecules per formaldehyde. The grand total for methane is four O
3
produced!
Methane is a good model for all alkanes, but by itself reacts too slowly to form much ozone locally, it is however important on a global scale. The net production of ozone requires converting of NO to NO
2
without consuming O
3
.

114
The key is [NO]. If the remote troposphere contains sufficient NO then smog chemistry will occur on a regional or global scale. The test for net photochemical ozone production vs. destruction is the relative strength of R5’ to R4. In lower troposphere [NO] must be > ~10 ppt to have net O
3 production.
Ethane is more interesting:
C
2
H
6
+ •OH → C
2
H
5
• + H
2
O
C
2
H
5
• + O
2
+ M → C
2
H
5
OO• + M
C
2
H
5
OO• + NO → C
2
H
5
O• + NO
2
But this is not always the pathway. Ethane can make methyl hydrogen peroxide:
C
2
H
5
OO• + HR → C
C
2
H
5
OOH + h

→ C
2
2
H
5
OOH + •R
H
5
O• + •OH
Or ethyl nitrate.
C
2
H
5
OO• + NO + M → C
2
H
5
ONO
2
+ M
Alkyl nitrates are NOx reservoir species that extend the effective lifetime of NOx.
Qualitative Budget
Sources: stratosphere transport; local photochemistry where [NO] > ~10 ppt
Sinks: surface deposition; local photochemistry where [NO] < ~10 ppt
Summary of quality of global ozone data:

Distribution - good

Sources (stratosphere) - fair

Sources (troposphere) - good

Sinks (stratosphere) - good

Sinks (troposphere) - fair

Sink (surface) - good
Needs:

115

More free troposphere measurements (0 - 15 km)

Flights near tropopause folding events

Tropospheric [NOx] and sources and sinks, esp. developing world.

OH and HO
2
measurements

OH and HO
2
kinetics

CO and NMHC measurements
Without an improved understanding of ozone, our knowledge of most other gases in the troposphere will be handicapped because ozone is a major source of OH.
Quantitative Budget
1) Burden
Mean global concentration [O
3
] ~ 6x10
11
cm
-3
Tropospheric column density:
[O
3
] x H o
= 6x10
11
x 8x10
5
x 0.75 = 3x10
17
molec cm
-2
NOTE: ¾ of atmosphere in troposphere.
2) Production
Stratosphere - troposphere exchange (diffusion)
~1x10 11 molec cm -2 s -1
But there are also tropopause folding events. On the cyclone (L) side of the jet the tropopause sinks and deforms, carrying stratosphere air down into the troposphere.

116
The fraction of total global troposphere O
3
from tropopause folds remains uncertain.
3) Destruction
Dry deposition - depends on the surface:
Loss term =

S i
V i
Where S i is the global surface area of vegetation type i and V i
is the deposition velocity to vegetation type i
SURFACE V (cm/s) AREA(%)
-------------------------------------------
GRASS 0.8 8%
FOREST 0.4 18%
OCEAN 0.06 74%
Deposition velocities are highly uncertain, but the mean global value is about:

117
0.18 cm/s
From the mean column ozone content calculated above,
Loss ( O
3
) = [O
3
] V = 1.1x10
11
molec cm
-2
s
-1
Production and destruction about equal - good!
Lifetime = burden/rate prod.
= burden/rate loss
= 3x10
17
/ 1x10
11
= 3x10
6
s ~ 1 month
Looks great, but the uncertainties are substantial. Steady state requires that production equal loss, but even if the sources and sinks terms you have found match there may still be other terms out there. In the case of ozone, the chemical terms went unrecognized for ten years.