# A Co nceptual Model for

Ozone Transport

Pr epared by:

Dr. R obert Hudson

Department of A tmospheric & Oceani c Science

University of Ma ryland

College Park, M D 20742

January 24, 2006

11. Introd uction

Locations downwind of urban areas with high ozone (O 3 ) levels and sub stantia l

ozone precursors can be subject to high ozone exposure because winds carry VOC s and

NO

x , as well as ozone itself, from their original sources. Ozone transport can occur at
ground level on surface winds, or ozone can be transported long distances on wind

currents above the atmospher ic mix ing la ye r. Whether the transported pollutants travel
long distances or short distances, the transport phenom enon makes ozone a regional
proble m. It can geographical l y separate the s ource of the pollution from the location of
high ozone levels and poor air qualit y.

For more than a quarter

of a century
, states in the eastern U nited States have been
designing and imp lem en ting programs to attain the air qualit y standard for ground level

ozone. Although much progres s has been made to improve air qualit y , man y areas have

y et to attain the EPA ozone standard. Progres s in attaining the health- based ozone

standard has been li mit ed b y ozone transport. Tradition al programs that primari l y focus

on controls in the vicinit y of the ozone standard violation are not adequate for man y

areas. It has becom e apparent that, to attain the standard, it is necessary to also develop

control programs that reduce ozone- forming pollutants that are emi tted man y m iles
upwind of the area of violation.

This report summ ar izes the current kno wledge of the meteoro logic al processes

that imp act ozone levels which occur in the Ozone Transport Region (OTR).

2. Tem perature inversions
An import ant elemen t in the production of severe ozone events is the abilit y of the

at mosphere through temper ature inversions to inhibit the mix ing processes which under

norma l conditions would lead to dilution of the em itt ed pollutants. One can identif y two
major classes of te mpera ture inversions, (1) nocturnal (radiative) and (2) s ubsidence.

2.1 Nocturnal inversions

Land surfaces are far more efficien t at radiating heat than the at mosphere above,

hence at night, the Earth’s surface cools more rapidl y than the air. That te mpera ture drop

is then convey ed to the lowe st hundred met ers of the at mosphere. The air above this lay er

2cools more slowly

, and a temper ature inversion forms. The inversion divides the

at mosphere into two la ye rs w hich do not mix. Below the inversion the s urface winds are

weak, and any pollutants em itt ed overnight accum ulat e. Above the inversion, winds

3continue through the night and can even beco me stronger as the inversion isolates the

winds from the friction of the rough s urface.

4In the morning, the sun warms the Earth’s surface, and conduction and convection

transfer heat up ward to warm the air near the surface. By about 10:00 – 11:00 am the

te mpera ture of the surface has risen sufficientl y to remov e the inversion. Air from above

5Altitude (m above sea level)

Te m p e rature (°C)

F igure 1. Temper ature profile take n over For t Me ade, MD , on J une 19, 2001and below the inversion can then mix freel y. D epending on whether the air above the

inversion is cleaner

or more polluted

than the air

at the surface, this mixing can either

lower or increase air pollution levels. Figure 1 s hows a temp erature profile taken over
Fort Meade, M D, on June 19, 2001. The nocturnal inversion is mark ed on the figure.

2.2 Sub sidence inversions

Severe ozone events are usuall y as sociated with high pre ssure s ys tem s. In the

upper atmosphere, the winds around a high pres sure sys tem move in a clockwise

direction. H owever at the ground, friction between the ground and the winds, turn the
winds away from the center of the sy ste m and divergence occurs. This divergence gives

rise to sub sidence in the at mosphere, which in turn causes the air to warm as it moves downward and is compressed. A s the warmer air meets the colder

air below, an inversion

is formed. A s ubsidence inversion is particularl y strong, since it is as sociated with this
large scale down ward mo tion of the atmospher e. The s ubsidence inversion caps pollution

at a higher

altitude in the at mosphere (ty pi ca ll y from 1200 to 2000 met ers, or about 4000

to 6500 feet), and it is far more difficult to remove than the nocturnal inversion. Hence

the sub sidence inversion lim its vertical mixing in the middl e of the day during an air
pollution episode. An exampl e of a subsidence inversion is also s hown in Figure 1.

3. Transport processes

3.1 Introd uction

The classic sy

nopti c weather

pattern associated with severe ozone episodes within

the OTR is sho wn in Figure 2. A slo w moving high pressure s yste m lies over the MidAtlantic States with an associated clockwise circulation. The winds are u suall y light. In

addition, a s discussed above, the high pres sure sys tem produces sub sidence, which in

turn induces a subsidence inversion, and cloud free s kies. H owever, the picture shown in
Figure 2 is true onl y at pressure levels above about 925 mb. Close to the surface the
Appalachian M ountain range interferes with this classic picture. First it acts as a phy si cal

barrier; secondl y it can induce local effects such as mountain and valle y breezes, the lee6H

F igure 2. S c hematic of

a ty pical we ather

pattern a ssociated w i th seve re oz one e pisodes side trough, and the low level j et, all of w hich can overco me the weak winds of the

s ynop tic sy ste m. The proximi t y of the OTR states to the Atlantic O cean also produces

another interferenc e, notabl y the s ea and land breezes. The combina tion of these local

effects can produce severe stagnation, and winds that do not follow the sy nopt ic flow,

indeed at man y ti mes the s urface flow can be in the opposite direction to that of the

s ynop tic flow. The surface flow pattern for an y da y depends on the relativ e speeds of the

winds induced by the local and sy nopt ic flow. It therefore depends on the po sition of the
high pres sure s ys tem

. Transport processes for the OTR can roughly be broken do wn into three levels,

surface, mid

and upper. In the following sections the winds as sociated with these three

levels will be discussed in more detail.

3.2 Sur face level w in ds

3.2.1 Land, sea, m ou nt ain and valley breeze

7In the Mid-Atlantic Region, land and sea breezes, and mounta in and valle y

breezes can have an impor tant influence on local air qualit y. These local winds are driven

b y a difference in te mperatur e that produces a difference in pressure. Figure 3 show s a

schem atic of the formation of a sea breeze. The sea breeze forms in the afternoon w hen

the land is considerabl y hotter than the ocean or ba y. Air then flows from the high
pressure over the ocean toward the low pre ssure over land. At night, the opposite ma y

happen as the land cools to below the ocean’s temper ature, and a land breeze blow s out to

sea. Since the nighttim e land and water temp erature differences are usuall y much smaller

than in the day

, the land breeze is weaker than the sea breeze. Sea breezes ty pi ca ll y only

penetrate a few kilo meters inland, since the y are driven by temper ature contrasts which
disappear inland. Air qualit y is affected by s ea and bay breezes in coastal cities because

the sea breeze can bring a cit y ’s polluted air back over the cit y. Figure 4 sho ws the

average 2000-2002 wind direction frequenc y for elevated 1 hour ozone in the vicinit y of

Maine’s Kennebec and Penobscot rivers. There is a clear maxi mu m of pollution in the

direction of the sea breeze.

Mountain and Valle y breezes are also driven by a te mperatur e contrast. In the

da yt i me the side of the mount ain will heat up more quickly than the valle y, and hence a

flow from the valle y to the mount ain re sults. At night this flow is reversed as the

mount ain side cools more quickl y than the valle y.

8 a) S e a B r ee ze b) Lan d Bre eze

F igure 3. I llust r ation of a se a bre eze and a land bre eze. ( Lutgens and Tar buc k , 2001)3.2.2 Ap palachian lee- si de troug h

The Appalachian lee side trough forms on the leeward (down wind) side of the
Appalachian M ountains. An example can be seen in Figure 5. In a sen se, it is the

da yt i me companion to the low level jet, since it forms under sim ilar, stagnant conditions;

however the mech anism for its format ion is different. The lee side trough forms when

9westerl

1011 F igure 4. Aver ag e 2000 – 2002 wind di r ec t ion fr eque ncy assoc i a ted with eleva t e d onehour oz one le vels i n Ma ine

F igure 5. 1200Z S ur fa ce a naly sis for 13 Aug ust 2003. The Appalac hian lee si de t r oug h ca n be obse rved in isobar s ove r Ma ry land, Virg inia, N orth C a rolina, and South C a rolinaAtlantic LLJ and its as sociated characteristi cs during the warm seasons of 2001 and

2002 using both wind-profiler data and the dail y real- tim e mode l forecast products

from the MM5 model. Figure 6 s hows wind profiler data for the period 10 A ugust,

2002 to 12 A ugust 2002, from the Fort Meade station. The red s ym bo ls (high wind s)

are at the position of the low level jet.

A case stud y

with three model sensitivit y simulat ions was performed to gain
insight into the three- dimension al structures and evolution of a LLJ and the

me chanis ms b y

which it developed. Results from the M M5 simula tions sho wed that

the Mid-Atlantic LLJ, ranged from 8 to 23 ms -1 , appeared at an average altitude of 670

m (2200 ft), and on 15 – 25 da ys of each month. A bout 90% of the 160 observed LLJ

events occurred between 0000 and 0600 LST; and about 60% had south to we sterl y

directions. Statisti call y , the real-ti me forecasts capture most of the LLJ events with

nearl y the right ti ming, intensit y, and altitud e although individual forecasts ma y not
correspond to ob servations.

For a selected s outhwesterl y LLJ case, both the ob servations and the control

simul ation exhibit

a pronounced diurnal cy cl e of horizontal winds in the lowest 1.5 km
(~5000 ft). The simul ation s hows that the Appalachians tend to produce a sloping

mix ed la ye r with northeasterl y therma l winds during the day ti m e and reversed therm al

winds after midnigh t. With additional therm al contrast effects associated with the
Chesapeake Ba y

and the Atlantic Ocean, the day ti m e low-level winds var y significant l y from the east coast to the mounta inous regions. The LLJ after midnigh t

tends to be peaked

preferential l y around 77.5°W near the middl e portion of the sloping

terrain and it decreases eastward as a re sult of the oppo site ther mal gradientacross the

coastline from the mountain- generated.

3.4 Up per level wi nd s

Theoreti cal and numeri cal model simulat ions have suggested for some time that

there is a strong regional component to urban air

qualit y in the northeastern United States
( Liu et al ., 1987; Sillman et

al ., 1990; McKeen et al ., 1990). The first aircraft

observations, undertaken by U MD (The Universit y of Mary la nd) with M DE A RMA

12(Mary la nd Departmen t of Energ y – Air and Radiation M anage ment Administra tion)

funding for limi ted periods in 1992 and 1993, were not made during extre me regional

13Figure6.Windprofilerdatafrom10August2002to 12August2002,fromtheFortMeade,MDstation

14ozone events. However, these results sugge sted that the ozone upwind of the Baltim ore

area was enhanced during higher than average ozone periods.

Since 1992 over 300 aircraft flights have been mad e to me asure vertical profiles

of ozone, the nitrogen oxides, carbon dioxide, sulfur dioxide, and more recentl y aerosol
particl es during high ozone episodes. These measurem en ts were made as part of the

Universit y of Mary la nd RAM MPP (Regional Atmospheric Measurement and Modeling

and P rediction Program) program under the s ponsorship of M DE- A RMA, M ARA MA
(Mid-Atlantic Region Air Manage ment A ssociation), VA DEQ (Virginia Departm ent of

Environ ment al Qualit y ), and N CDEQ (North Carolina Departm ent of Environmen tal

Qualit y)

. Figure 7 s hows the results of profiles taken over central Virginia on 15 Jul y
1995, at about 9:00 am on the last day of a four da y severe ozone episode. D uring this
episode, wind s m easured at Sterling, Virginia (I AD) in the 500-3000 m (1600-9800 ft)

la ye r, where ozone wa s at a max im um , were consistentl y from the west to the north. This
was particularl y true on 15 J ul y. There were no periods of stagnation or reversal of wind

direction during this period.

F igure 7. Alti tude pr ofiles for oz one, c ar

bon di ox ide , NO

y , a nd S O
2 taken on 15 J uly 1995

15An exam inat ion of the data in Figure 7 sho ws that the ozone mix ing ratio above

the boundary la ye r is much larger than that at the ground, peaking at about 1200 m (4000
ft). It should be noted that w hile both automobi les and power plants emi t NO

,

auto mobil es emi t CO but not SO 2 , w hile power plants emi t SO 2 but not CO. The C O
profile is not correlated

well with the ozone data, indicating that the source of the carbon

monoxid e is not from local sources (automob iles). The peak in the NO y profile at around

800 m (2600 ft) is due to ‘aged’ air, and is also unlikel y to come from local sources (auto mobil es). Finall y the peak in the S O 2 profile is also unlikel y to come from local

sources. Indeed the presence of the SO 2 leads to the conclusion that the air is com ing

from po wer plants west of the A ppalachian Mountains.

4. The ozone reservoir

4.1 Introd uction

Consider a ty

pi ca l da y, starting at

sun set, for a severe ozone event associated with

a high pres sure sy ste m. A s the temp erature of the earth drops, a nocturnal inversion

forms, isolating the surface from stronger winds only a few hundred feet overhead.

Ozone near the s urface cannot mix with ozone above and is destroy ed a s it reacts with the

Earth’s surface. In a city

, fresh NO

x emissions react with ozone, furtherreducing its

concentrat ion b y

NO

x titration, s o that b y

morning, very littl e ozone is left below the

nocturnal inversion. At this ti me, the nocturnal inversion is at its strongest, and winds at

the surface are ty

pi ca ll y calm.

Above the nocturnal inversion, the situation is quite different. Ozone, and its
precursors, both from the previous day ’s local emissions and from transport rema in
largel y intact. There are no surfaces to react with the ozone and a large re servoir of ozone

rem ains above the inversion. At mid- morn ing, when the nocturnal inversion breaks do wn,

the ozone and precursor s that were above the inversion can no w m ix with the air near the

surface. The result of this mix ing is a sudden change in ozone. Figure 8 show s med ian

ozone profiles for morn ing and afternoon flights from 1996 – 2003. One can clearl y see

the breakdown of the nocturnal inversion throughout the day

. Pollutants that do not react with the Earth’s surface a s readil y as ozone do not

show s uch a marked diurnal cy cl e, but the s am e mix ing mech anism also affects their

16concentrat ions. Nighttim e emissions of trace gases and particles can build up under the

nocturnal inversion, leading to higher levels in the earl y morning. In the summ ert im e, regional sulfate levels above the inversion can comb ine with local emissions as the

inversion breaks up. In the winter, with les s direct s unshine, it’s harder to break the

inversion, and local em issions can continue to build unless washed out b y precipit ation or dispersed b y

winds.

4.2 Contributions to t he ozone reservoir

Contributions to the ozone reservoir can come from two sources. The first is from

the residual ozone and precursors in the at mosphere at sun set. The second is from

transport of ozone and precursor s from outside of the local region. A n anal y sis of the

comp let e set of aircraft flights undertaken b y RAM MPP between 1992 and 2003 has

recentl y been mad e b y Taubman et al ., 2005 to identif y these outside sources. Initial l y,

the data were divided into morn ing and afternoon profiles to identif y diurnal patterns.

Littl e diurnal variation wa s observed in the carbon monoxide and sulfur dioxide profiles.

The ozone values were greater in the afternoon than the morning, w hile ozone in the 

### O 3 

### m e d i a n p r o f i l e s f

o r m or ni ng a n d a f

t e r no o n f l i g ht s ( 1 9 9 6

- 2 0 0 3 )

0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0 3 5 0 0

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

M e d i a n O 3 ( p p b ) w i t h 2 5 t h a n d 75 t h p e rce n t i l e sPressurederivedaltitude(m) m e d ia n

a m ( 2 5 4 )

m e d ia n
p m ( 2 9 3 )

q 2 5 a m

q 7 5 a m

q 2 5 p m

q 7 5 p m

AM P M

F igure 8. Median ozone prof il e s for morning a nd after noon flig ht s f rom 1996 – 2003

17lower free troposphere (i.e. above the boundar y level), w here long range transport is
possible, w as consistentl y ~55 ppbv. Transport patterns and source regions during

summ er ti me haze and ozone episodes were anal yz ed with a cluster anal ys is of back
trajector y data. Eight clusters were identified, which were then divided into morning and

afternoon profiles. Table 1 lists the characterist ics of each cluster. Figures 9 and 10 s how

the trajector y densities for each cluster. When the greatest trajector y densit y lay over the

northern Ohio River Valle y , which ha s large NO

x and sulfur dioxide sources, the result
were large ozone values, a large SO 2 /CO ratio, large, s cattering particles, and high

aerosol optical depth over the Mid-Atlantic U.S. In contrast, relativ el y clean conditions

over the Mid-Atlantic occurred when the greatest trajector y density la y over the s outhern

Ohio River Valle y and nearl y missed man y large NO

x and SO 2 sources. The greatest

afternoon ozone values occurred during periods of stagnation that w ere most conducive

to photochem ical production. The least pollution occurred when flow from the northnorthwest w as too fast for pollution to accu mul ate and when flow wa s from the north,

where there are few urban or industrial sources.
Ozone transport over several hundred kilo met ers into the Mid-Atlantic U .S. was

esti mat ed b y

calcula ting the ratio of the residual la ye r ozone in the up wind morning
profiles to the do wnwind afternoon boundar y lay er values. The greatest ozone transport
(69-82%) occurred when the maxim um trajector y density la y over the southern and

northern Ohio River Valle y (~59% of the total profiles). The least ozone transport (55-
58%) was as sociated with fast southwesterl y flow (~3% of the total profiles), clean

northerl y flow (~6% of the total profiles), and stagnant, polluted conditions (~27% of the

total profiles). The average ozone transport to the Baltim ore Washington Urban corridor

is 64%, if the background ozone is remov ed, then this value is lowered to 55%.
In Figures 9 and 10 s pecific s ource regions were identifi ed b y overla yi ng the
trajector y densit y plots on maps with the largest annual N O

x and S O 2 emi tters. The results

indicat e that the areas of maxi mu m tra jector y densit y together

with wind speed are

effective predictors of regional pollution and loadings. Additionall y , due to the
Lagrangian nature of the dataset, the regionall y transported contribution to the total

afternoon boundar y

la ye r colu mn ozone content in each cluster

could be quantified.

185.0 Conclusion

This report has summ ar ized current knowledge of the m eteorolog ica l processes

that imp act local ozone levels within the OTR. A conceptual model of transport within

the OTR can be divided into three principle components, transport aloft, transport b y the

low-level jet, and ground level transport. Transport aloft is domina ted b y the antic yc lo nic flow around a high pre ssure s ys tem
, w hich can lead to transport from States that

lie outside the OTR. Ground level transport is the result of interact ion between the

s ynop tic flow, and local effects, such as the sea breeze and the Appalachian lee side
trough. All three modes of transport depend on the location of the high pressure sy ste m.

19Figure 9.Trajectorydensity mapsfor a) cluster1, b) cluster2, c)cluster3, d) cluster4. The plotsweregeneratedusing a39linearinterpolationmethodbetween thetrajectoryend points.Theyindicate therelativedensity (%)of airparcelsoverthetotalareadescribed bythe spaghettiplots. Alsopictured arethe locationsof the top0.3%emittersannually ofNOx(diamonds)and SO2(crosses) inthe easternU.S. Takenfrom Figure4 ofTaubman etal., 2005.

20Figure 10.Trajectorydensity mapsfor e) cluster5, f) cluster6, g) cluster7, h) cluster8. The plotsweregeneratedusing a39linearinterpolationmethodbetween thetrajectoryend points.Theyindicate therelativedensity (%)of airparcelsoverthetotalareadescribed bythe spaghettiplots. Alsopictured arethe locationsof the top0.3%emittersannually ofNOx(diamonds)and SO2(crosses) inthe easternU.S. Takenfrom Figure4 ofTaubman etal., 2005.

21Table 1.

Cluster Description Region
1 Large ozone, large SO 2 /CO ratio, moderat e

northeasterl y flow - aged point source air N orthern Ohio river

valle y 2 Large ozone, large SO 2 /CO ratio, high wind

speeds than cluster

1- aged points ource air N orthern Ohio river

valle y, extending into

the Great lakes region
3 Stagnant conditions with light southerl y flow.

Smal l SO 2 /CO ratio Central Mid-Atlantic
region
4 Moderate southeasterl y flow, smal l pollution

loading. SO 2 /CO ratio is small – low point

sources. Southern Ohio river

valle y

5 Fairl y fast north- northwe sterl y flow, over the

Northern G reat Lakes. Transports little ozone

into the region N orthern Great lakes

into Mid-Atlantic region

6 Northwesterl y flow, but faster wind s peeds

than (2). Cros ses several large S O 2 and N O

x

sources. N orthern Ohio valle y

7 Least pollution of an y of the clusters. Flow is

out of the North. Cool, dry continental air N orthern states

8 Fast flow. Ver y few trajectories Texas

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