A Conceptual Model for 

Ozone Transport 

Prepared by:

Dr. Robert Hudson

Department of Atmospheric & Oceanic Science

University of Maryland

College Park, MD 20742

January 24, 2006

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1. Introduction

Locations downwind of urban areas with high ozone (O3) levels and substantial

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

NOx, 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 atmospheric mixing layer. Whether the transported pollutants travel

long distances or short distances, the transport phenomenon makes ozone a regional

problem. It can geographically separate the source of the pollution from the location of

high ozone levels and poor air quality.

For more than a quarter of a century, states in the eastern United States have been

designing and implementing programs to attain the air quality standard for ground level

ozone. Although much progress has been made to improve air quality, many areas have

yet to attain the EPA ozone standard. Progress in attaining the health-based ozone

standard has been limited by ozone transport. Traditional programs that primarily focus

on controls in the vicinity of the ozone standard violation are not adequate for many

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

control programs that reduce ozone-forming pollutants that are emitted many miles

upwind of the area of violation.

This report summarizes the current knowledge of the meteorological processes

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

2. Temperature inversions

An important element in the production of severe ozone events is the ability of the

atmosphere through temperature inversions to inhibit the mixing processes which under

normal conditions would lead to dilution of the emitted pollutants. One can identify two

major classes of temperature inversions, (1) nocturnal (radiative) and (2) subsidence.

2.1 Nocturnal inversions

Land surfaces are far more efficient at radiating heat than the atmosphere above,

hence at night, the Earth’s surface cools more rapidly than the air. That temperature drop

is then conveyed to the lowest hundred meters of the atmosphere. The air above this layer

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cools more slowly, and a temperature inversion forms. The inversion divides the

atmosphere into two layers which do not mix. Below the inversion the surface winds are

weak, and any pollutants emitted overnight accumulate. Above the inversion, winds

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continue through the night and can even become stronger as the inversion isolates the

winds from the friction of the rough surface.

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

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

temperature of the surface has risen sufficiently to remove the inversion. Air from above

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Altitude (m above sea level)

Temperature (°C)

Figure 1. Temperature profile taken over Fort Meade, MD, on June 19, 2001
and below the inversion can then mix freely. Depending 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 shows a temperature profile taken over

Fort Meade, MD, on June 19, 2001. The nocturnal inversion is marked on the figure. 

2.2 Subsidence inversions

Severe ozone events are usually associated with high pressure systems. In the

upper atmosphere, the winds around a high pressure system move in a clockwise

direction. However at the ground, friction between the ground and the winds, turn the

winds away from the center of the system and divergence occurs. This divergence gives

rise to subsidence in the atmosphere, which in turn causes the air to warm as it moves

downward and is compressed. As the warmer air meets the colder air below, an inversion

is formed. A subsidence inversion is particularly strong, since it is associated with this

large scale downward motion of the atmosphere. The subsidence inversion caps pollution

at a higher altitude in the atmosphere (typically from 1200 to 2000 meters, or about 4000

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

the subsidence inversion limits vertical mixing in the middle of the day during an air

pollution episode. An example of a subsidence inversion is also shown in Figure 1. 

3. Transport processes

3.1 Introduction

The classic synoptic weather pattern associated with severe ozone episodes within

the OTR is shown in Figure 2. A slow moving high pressure system lies over the MidAtlantic States with an associated clockwise circulation. The winds are usually light. In

addition, as discussed above, the high pressure system produces subsidence, which in

turn induces a subsidence inversion, and cloud free skies. However, the picture shown in

Figure 2 is true only at pressure levels above about 925 mb. Close to the surface the

Appalachian Mountain range interferes with this classic picture. First it acts as a physical

barrier; secondly it can induce local effects such as mountain and valley breezes, the lee6
H

Figure 2. Schematic of a typical weather pattern associated with severe ozone episodes

side trough, and the low level jet, all of which can overcome the weak winds of the

synoptic system. The proximity of the OTR states to the Atlantic Ocean also produces

another interference, notably the sea and land breezes. The combination of these local

effects can produce severe stagnation, and winds that do not follow the synoptic flow,

indeed at many times the surface flow can be in the opposite direction to that of the

synoptic flow. The surface flow pattern for any day depends on the relative speeds of the 

winds induced by the local and synoptic flow. It therefore depends on the position of the

high pressure system.

Transport processes for the OTR can roughly be broken down into three levels,

surface, mid and upper. In the following sections the winds associated with these three

levels will be discussed in more detail.

3.2 Surface level winds

3.2.1 Land, sea, mountain and valley breeze

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 In the Mid-Atlantic Region, land and sea breezes, and mountain and valley

breezes can have an important influence on local air quality. These local winds are driven

by a difference in temperature that produces a difference in pressure. Figure 3 shows a

schematic of the formation of a sea breeze. The sea breeze forms in the afternoon when

the land is considerably hotter than the ocean or bay. Air then flows from the high

pressure over the ocean toward the low pressure over land. At night, the opposite may

happen as the land cools to below the ocean’s temperature, and a land breeze blows out to

sea. Since the nighttime land and water temperature differences are usually much smaller

than in the day, the land breeze is weaker than the sea breeze. Sea breezes typically only

penetrate a few kilometers inland, since they are driven by temperature contrasts which

disappear inland. Air quality is affected by sea and bay breezes in coastal cities because

the sea breeze can bring a city’s polluted air back over the city. Figure 4 shows the

average 2000-2002 wind direction frequency for elevated 1 hour ozone in the vicinity of

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

direction of the sea breeze.

Mountain and Valley breezes are also driven by a temperature contrast. In the

daytime the side of the mountain will heat up more quickly than the valley, and hence a

flow from the valley to the mountain results. At night this flow is reversed as the

mountain side cools more quickly than the valley.

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a) Sea Breeze b) Land Breeze

Figure 3. Illustration of a sea breeze and a land breeze. (Lutgens and Tarbuck, 2001)
3.2.2 Appalachian lee-side trough

The Appalachian lee side trough forms on the leeward (downwind) side of the

Appalachian Mountains. An example can be seen in Figure 5. In a sense, it is the

daytime companion to the low level jet, since it forms under similar, stagnant conditions;

however the mechanism for its formation is different. The lee side trough forms when 

9
 westerl

10
11

Figure 4. Average 2000 – 2002 wind direction frequency associated with elevated onehour ozone levels in Maine

Figure 5. 1200Z Surface analysis for 13 August 2003. The Appalachian lee side trough can be 

observed in isobars over Maryland, Virginia, North Carolina, and South Carolina 
Atlantic LLJ and its associated characteristics during the warm seasons of 2001 and

2002 using both wind-profiler data and the daily real-time model forecast products

from the MM5 model. Figure 6 shows wind profiler data for the period 10 August,

2002 to 12 August 2002, from the Fort Meade station. The red symbols (high winds)

are at the position of the low level jet.

 A case study with three model sensitivity simulations was performed to gain

insight into the three-dimensional structures and evolution of a LLJ and the

mechanisms by which it developed. Results from the MM5 simulations showed that

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

m (2200 ft), and on 15 – 25 days of each month. About 90% of the 160 observed LLJ

events occurred between 0000 and 0600 LST; and about 60% had south to westerly

directions. Statistically, the real-time forecasts capture most of the LLJ events with

nearly the right timing, intensity, and altitude although individual forecasts may not

correspond to observations.

For a selected southwesterly LLJ case, both the observations and the control

simulation exhibit a pronounced diurnal cycle of horizontal winds in the lowest 1.5 km

(~5000 ft). The simulation shows that the Appalachians tend to produce a sloping

mixed layer with northeasterly thermal winds during the daytime and reversed thermal

winds after midnight. With additional thermal contrast effects associated with the

Chesapeake Bay and the Atlantic Ocean, the daytime low-level winds vary

significantly from the east coast to the mountainous regions. The LLJ after midnight

tends to be peaked preferentially around 77.5°W near the middle portion of the sloping

terrain and it decreases eastward as a result of the opposite thermal gradient across the

coastline from the mountain-generated.

3.4 Upper level winds

 Theoretical and numerical model simulations have suggested for some time that

there is a strong regional component to urban air quality in the northeastern United States

(Liu et al., 1987; Sillman et al., 1990; McKeen et al., 1990). The first aircraft

observations, undertaken by UMD (The University of Maryland) with MDE ARMA

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(Maryland Department of Energy – Air and Radiation Management Administration)

funding for limited periods in 1992 and 1993, were not made during extreme regional

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Figu

re 6.Wind profildaer 

ta

20ust Aug10from 

02 

20ust Augto 12 

02

deMeaFort the from , 

,

onstatiMD

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ozone events. However, these results suggested that the ozone upwind of the Baltimore

area was enhanced during higher than average ozone periods. 

Since 1992 over 300 aircraft flights have been made to measure vertical profiles

of ozone, the nitrogen oxides, carbon dioxide, sulfur dioxide, and more recently aerosol

particles during high ozone episodes. These measurements were made as part of the

University of Maryland RAMMPP (Regional Atmospheric Measurement and Modeling

and Prediction Program) program under the sponsorship of MDE-ARMA, MARAMA

(Mid-Atlantic Region Air Management Association), VADEQ (Virginia Department of

Environmental Quality), and NCDEQ (North Carolina Department of Environmental

Quality). Figure 7 shows the results of profiles taken over central Virginia on 15 July

1995, at about 9:00 am on the last day of a four day severe ozone episode. During this

episode, winds measured at Sterling, Virginia (IAD) in the 500-3000 m (1600-9800 ft)

layer, where ozone was at a maximum, were consistently from the west to the north. This

was particularly true on 15 July. There were no periods of stagnation or reversal of wind

direction during this period.

Figure 7. Altitude profiles for ozone, carbon dioxide, NOy, and SO2 taken on 15 July 1995 

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An examination of the data in Figure 7 shows that the ozone mixing ratio above

the boundary layer is much larger than that at the ground, peaking at about 1200 m (4000

ft). It should be noted that while both automobiles and power plants emit NO,

automobiles emit CO but not SO2, while power plants emit SO2 but not CO. The CO

profile is not correlated well with the ozone data, indicating that the source of the carbon

monoxide is not from local sources (automobiles). The peak in the NOy profile at around

800 m (2600 ft) is due to ‘aged’ air, and is also unlikely to come from local sources

(automobiles). Finally the peak in the SO2 profile is also unlikely to come from local

sources. Indeed the presence of the SO2 leads to the conclusion that the air is coming

from power plants west of the Appalachian Mountains.

4. The ozone reservoir

4.1 Introduction

Consider a typical day, starting at sunset, for a severe ozone event associated with

a high pressure system. As the temperature of the earth drops, a nocturnal inversion

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

Ozone near the surface cannot mix with ozone above and is destroyed as it reacts with the

Earth’s surface. In a city, fresh NOx emissions react with ozone, further reducing its

concentration by NOx titration, so that by morning, very little ozone is left below the

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

the surface are typically 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 remain

largely intact. There are no surfaces to react with the ozone and a large reservoir of ozone

remains above the inversion. At mid-morning, when the nocturnal inversion breaks down,

the ozone and precursors that were above the inversion can now mix with the air near the

surface. The result of this mixing is a sudden change in ozone. Figure 8 shows median

ozone profiles for morning and afternoon flights from 1996 – 2003. One can clearly see

the breakdown of the nocturnal inversion throughout the day.

Pollutants that do not react with the Earth’s surface as readily as ozone do not

show such a marked diurnal cycle, but the same mixing mechanism also affects their

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concentrations. Nighttime emissions of trace gases and particles can build up under the

nocturnal inversion, leading to higher levels in the early morning. In the summertime,

regional sulfate levels above the inversion can combine with local emissions as the

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

inversion, and local emissions can continue to build unless washed out by precipitation or

dispersed by winds.

4.2 Contributions to the ozone reservoir

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

the residual ozone and precursors in the atmosphere at sunset. The second is from

transport of ozone and precursors from outside of the local region. An analysis of the

complete set of aircraft flights undertaken by RAMMPP between 1992 and 2003 has

recently been made by Taubman et al., 2005 to identify these outside sources. Initially,

the data were divided into morning and afternoon profiles to identify diurnal patterns.

Little diurnal variation was observed in the carbon monoxide and sulfur dioxide profiles.

The ozone values were greater in the afternoon than the morning, while ozone in the

O3 median profiles for morning and afternoon flights

(1996 - 2003)

0

500

1000

1500

2000

2500

3000

3500

0 20 40 60 80 100 120

Median O3 (ppb) with 25

th

and 75th percentiles

Pressure

deri

ved

altitu

d

e (m)

median

am (254)

median

pm (293)

q25 am

q75 am

q25 pm

q75 pm

AM PM

Figure 8. Median ozone profiles for morning and afternoon flights from 1996 – 2003

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lower free troposphere (i.e. above the boundary level), where long range transport is

possible, was consistently ~55 ppbv. Transport patterns and source regions during

summertime haze and ozone episodes were analyzed with a cluster analysis of back

trajectory data. Eight clusters were identified, which were then divided into morning and

afternoon profiles. Table 1 lists the characteristics of each cluster. Figures 9 and 10 show

the trajectory densities for each cluster. When the greatest trajectory density lay over the

northern Ohio River Valley, which has large NOx and sulfur dioxide sources, the result

were large ozone values, a large SO2/CO ratio, large, scattering particles, and high

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

over the Mid-Atlantic occurred when the greatest trajectory density lay over the southern

Ohio River Valley and nearly missed many large NOx and SO2 sources. The greatest

afternoon ozone values occurred during periods of stagnation that were most conducive

to photochemical production. The least pollution occurred when flow from the northnorthwest was too fast for pollution to accumulate and when flow was from the north,

where there are few urban or industrial sources.

Ozone transport over several hundred kilometers into the Mid-Atlantic U.S. was

estimated by calculating the ratio of the residual layer ozone in the upwind morning

profiles to the downwind afternoon boundary layer values. The greatest ozone transport

(69-82%) occurred when the maximum trajectory density lay over the southern and

northern Ohio River Valley (~59% of the total profiles). The least ozone transport (55-

58%) was associated with fast southwesterly flow (~3% of the total profiles), clean

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

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

is 64%, if the background ozone is removed, then this value is lowered to 55%.

In Figures 9 and 10 specific source regions were identified by overlaying the

trajectory density plots on maps with the largest annual NOx and SO2 emitters. The results

indicate that the areas of maximum trajectory density together with wind speed are

effective predictors of regional pollution and loadings. Additionally, due to the

Lagrangian nature of the dataset, the regionally transported contribution to the total

afternoon boundary layer column ozone content in each cluster could be quantified.

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5.0 Conclusion

This report has summarized current knowledge of the meteorological processes

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

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

low-level jet, and ground level transport. Transport aloft is dominated by the anticyclonic flow around a high pressure system, which can lead to transport from States that

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

synoptic 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 system.

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Figure 9.

Trajectory density maps for a) cluster1, b) cluster2, c) cluster 3, d) clusterwer4. The plots 

e 

generated using a 39 linear interpolation betwemethod 

en the 

trajectory indiThey end points. 

cate the 

relativeof adensity (%)

ir parcels 

over the totalarea described by the spaghetti plots. Also pictured arethe locations emit0.3% of the top 

ters 

annually of (cand SO2 (diamonds) NOx 

rosses) in 

the eastern U.S. Taken T4 of from Figure 

aubman et 

al., 2005.

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Figure 10.

Trajectory density maps for e) cluster5, f) cluster 6, g) cluster7, h) clusterwer8. The plots 

e 

generated using a 39 linear interpolation betwemethod 

en the 

trajectory indiThey end points. 

cate the 

relativeof adensity (%)

ir parcels 

over the totalarea described by the spaghetti plots. Also pictured arethe locations emit0.3% of the top 

ters 

annually of (cand SO2 (diamonds) NOx 

rosses) in 

the eastern U.S. Taken T4 of from Figure 

aubman et 

al., 2005.

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Table 1. 

Cluster Description Region

1 Large ozone, large SO2/CO ratio, moderate

northeasterly flow - aged point source air

Northern Ohio river

valley

2 Large ozone, large SO2/CO ratio, high wind

speeds than cluster 1- aged point source air

Northern Ohio river

valley, extending into

the Great lakes region

3 Stagnant conditions with light southerly flow.

Small SO2/CO ratio

Central Mid-Atlantic

region

4 Moderate southeasterly flow, small pollution

loading. SO2/CO ratio is small – low point

sources.

Southern Ohio river

valley

5 Fairly fast north-northwesterly flow, over the

Northern Great Lakes. Transports little ozone

into the region 

Northern Great lakes

into Mid-Atlantic region

6 Northwesterly flow, but faster wind speeds

than (2). Crosses several large SO2 and NOx

sources.

Northern Ohio valley

7 Least pollution of any of the clusters. Flow is

out of the North. Cool, dry continental air

Northern states

8 Fast flow. Very few trajectories Texas

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