A necessary precondition for the transport of "high" concentrations of ozone or ozone precursors from a "source area" to a "receptor area" is a degree of "local" pollution accumulation in or near the source area. Key factors which influence local pollution accumulation include local emissions density and local meteorology (especially mixing depth and wind speed). During the 1960's and early 1970's, there was considerable interest in evaluating the long-term "climatological" potential for local pollution episodes. Many of these historical evaluations were conducted strictly from a meteorological perspective - i.e. without regard to emissions densities. As such, they can provide a useful complement to recent OTAG modeling and Air Quality Analysis results. In particular, they may provide useful insights into the following general OTAG AQA observations:
The airmass over a given "source" location does not
reside indefinitely over that location (fortunately), but moves
eventually over other "downwind" locations. "Airmass
transport" does not necessarily result in a substantial contribution
to downwind ozone concentrations, as physical and chemical processes
may diminish ozone and/or precursor (O&P) concentrations en
route between source and receptor. However, with the exception
of ozone formed en-route from source area precursors, there are
no other mechanisms by which concentrations of "transported"
ozone concentrations can increase during transport. If the initial
concentrations of O&P are "low" at the time the
airmass leaves the source region, the net effect of transport
would be to dilute the concentration at the receptor. Consequently,
some degree of build-up or accumulation of O&P in or near
a source region is a necessary pre-condition for any "substantial"
degree of transported ozone at a downwind receptor. Thus "substantial"
ozone transport can be conceptually viewed as a two-stage process:
1. local accumulation, followed by 2. airmass transport.
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Climatology vs. Episodes
Many of the recent OTAG Air Quality Analysis studies are based
on long-term (5-10 year) "climatological" analyses 1-5.
Over time, the seasonal patterns of ozone and meteorological conditions
exhibit reasonably consistent and predictable patterns from year
to year; however, short-term variations in ozone and meteorology
are statistically random with respect to time, and are unpredictable
beyond more than a few days in the future1. Specific meteorological
conditions associated with a single historical episode will never
occur again. Consequently, episodic modeling results provide a
limited basis for prediction of meteorologically-dependent aspects
of future conditions (for example: the duration of local stagnation/accumulation
in specific source regions, or the distance, direction or persistence
of subsequent downwind transport). Relationships derived from
long-term data sets provide a more robust basis for probabilistic
predictions of future conditions. In the absence of substantial
long-term changes in climate or emissions, the meteorology (and
ozone) over a 5-10 year period in the future may be probabalistically
anticipated to exhibit similar characteristics to a 5-10 year
period in the past.
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Consistencies and Inconsistencies
The historical "wisdom" on the nature of (and required
control strategies for reducing) ozone pollution has presumed
that ozone is a local pollutant, responsive primarily to changes
in ground-level VOC emissions in urban areas. A recent review
of (episodic) ozone modeling studies suggested that NOx emissions
controls in general, and stationary NOx controls in particular
are relatively ineffective at reducing ozone concentrations13.
In contrast, recent OTAG model results suggest that NOx reductions
are relatively much more effective than VOC reductions over broad
areas of the Eastern US for selected episodes. Other evaluations
suggest that the OTAG model results may overstate NOx control
benefits - for example through overestimates of biogenic VOC concentrations14
and/or artificially enhanced mixing, resulting from use of excessively
large (12 km) grid squares.15
One recurring pattern observed in several recent OTAG climatological
analyses 2,3,4 is that the Midwest (a region characterized by relatively
high emissions of NOx from stationary sources) appears to be an
important ozone source region - or is at least frequently and
persistently upwind - for high ozone concentrations in a variety
of surrounding receptor areas. In apparent paradox, the Midwest
itself does not experience a high incidence of exceedances of
the current, 1-hour ozone standard.2
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Why the Midwest?
That the Midwest is frequently upwind of high ozone concentrations
at a variety of downwind receptors is clear. However, specific
reasons or mechanisms by which emissions from this source region
might contribute to high ozone levels in other regions remain
unclear. In addition to high NOx emissions densities, other possible
explanations suggested in OTAG AQA discussions include:
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Historical Climatological Evaluations
During the 1960's and early 70's - when national air pollution
control efforts were just getting under way - there was also a
considerable interest in the evaluation of climatological conditions
associated with high pollution concentrations 6-12. Many of these
earlier climatological studies focused exclusively on the space
and time patterns of meteorological conditions conducive to the
formation of high pollutant concentrations - without regard to
actual areal emissions densities or reaction chemistry. They were
all based on long-term (5 to 30 year) meteorological data sets,
and evaluated spatial and temporal patterns of variables like:
frequency of low-level inversions and wind speeds 6, diffusion
conditions 7, mixing depth 8, stagnating anticyclones 9, and various
combinations of the above.10 Many of these "pollution-conducive"
meteorological factors were originally observed as common conditions
in a number of severe, localized (or regional) pollution episodes
in the 1940s, '50s and '60s (Donora, PA, 1948; London, 1952, '56,
'57, '62; NY City, 1953, '63, '66; etc.). In turn, the subsequent
identification of key meteorological factors led to development
of a routine, national-scale air pollution episode forecast program
in the 1960s.11
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Selected Results from Holzworth (1972) 12
Among the most comprehensive of the historical climatological
analyses was an EPA publication entitled "Mixing heights,
wind speeds, and potential for urban air pollution throughout
the contiguous United States" (Holzworth, 1972).12 The focus
was on identification of spatial and seasonal patterns of meteorological
conditions conducive to the occurance of local urban or community-scale
pollution episodes. Actual urban locations, emissions densities
and chemical reactions and inter-regional transport were not considered.
The estimated episode potential was for non-reactive ("slowly
reacting") pollutants. The meteorological calculations were
based primarily on a record of morning and afternoon mixing heights
and wind speeds from a national network of 62 upper air NWS sites
for the 5 year period of 1960 - 1964. Holzworth also presented
results of "number of forecast days of high meteorological
potential for air pollution in a five year period" - which
for the eastern US, was based on a 10 year (1960-1970) record
of predictions from the National Air Pollution Potential Forecast
Although the meteorological data sets are dated - in excess of
30 years old, there is no reason to assume that current or future
climatic conditions will differ substantially from the 1960s.
Because of the focus on potential for episodes of non-reactive
pollutants, the relevance to ozone episodes relates only to the
initial - local accumulation stage - of ozone and precursors,
and not to any subsequent influences of transport or removal processes.
Selected results are presented below in Figures 1 through 6. Holzworth's
original graphics were electronically scanned, cropped to focus
on the OTAG domain, and colored to add clarity.
Mean Summer morning mixing heights for the OTAG domain are displayed
in Figure 1. The most distinct feature of this plot is the relatively
high mixing heights (500 to > 1,000 meters) along the Atlantic
and Gulf coastal areas. There is a relatively steep gradient of
decreasing morning mixing heights moving inland from the moderating
influences of warm nocturnal ocean temperatures.
Lower morning mixing heights (< 500 meters) prevail throughout the interior of the OTAG domain, with the lowest mixing heights evident in the Northwest. The most easterly extent of relatively low morning mixing heights (< 400 meters) includes areas of the Southeast (GA, SC) and Midwest (KY, OH, WV).
By mid-afternoon, substantially higher mixing heights prevail
throughout the East (Figure 2), and are greatest - in excess of
2,000 meters - along the western edge of the domain. A broad region
of high mixing heights - 1,800 to 2,000 meters - is also evident
throughout much of the South and in a long South-to-Northeast
corridor aligned approximately with the Appalachian Mountains.
Lower mixing heights - less than 1,600 meters are evident in the
Northwest, and especially along the Atlantic and Gulf coasts.
These coastal areas (with relatively high morning mixing heights)
experience a limited (2X) growth in mixing height from morning
to afternoon; whereas some interior sections are characterized
by a greater than 5-fold expansion of mixing height as the day
Along with mixing depth (volume), wind speeds are also exert an
important influence on the potential for local pollutant accumulation.
For a given emission rate (grams/second) and mixing depth (meters),
a lower wind speed (meters/second) increases the quantity of pollutant
released into a given volume of air. Figure 3 shows mean summer
morning wind speeds, averaged through the morning mixed layer.
The highest morning wind speeds - from 5 to > 7 meters/second
- are typically found in western (plains) sections of the OTAG
domain. Lowest Summer morning wind speeds - less than 4 meters/second
- occur in a broad band just west of the Appalachians from the
central Gulf coast up through OH and PA. An area of minimum morning
wind speed (< 3 m/s) is located near the intersection of the
Mean Summer afternoon wind speeds (Figure 4) are typically 2 to
3 meters per second faster than morning conditions. Lowest afternoon
speeds (< 5 m/s) occur in a broad region extending from the
Gulf Coast up through the southeastern Midwest. Highest afternoon
wind speeds (> 7 m/s) tend to occur in Western and New England
sections of the OTAG region.
Various combinations of low mixing heights, low wind speeds and
their persistence, in the absence of precipitation, over 2 or
more days were observed during many of the most severe historical
pollution episodes. Projected occurrences of these conditions
formed the basis for forecasts issued by the National Air Pollution
Potential Forecast Program.11
Figure 5 displays a count of the
number of forecast days of high meteorological potential for air
pollution in a five year period from the National Air Pollution
Potential Forecast Program. Within the OTAG domain, these forecasted
episode frequencies range from less than 10 (< 2 days/year)
to greater than 40 (> 8 days/year). The highest projected episode
frequency includes an area just west of the Appalachian Mountains,
extending northward from central Georgia and Alabama up through
West Virginia and Ohio. The maximum forecasted episode frequency
occurs in an area near the intersection of the Ohio/Kentucky/West
The forecasts in Figure 5 are
based on annual data, and include a number of forecast days in
the Fall, Winter and Spring which are not relevant to ozone episodes.
Figures 1, 2,
and 6 are based on Summer-only
Holzworth also developed estimates of potential pollutant concentration
( ) as a function of a normalized emission rate (Q) for hypothetical
urban areas with a sizes of 10 and 100 km. These calculations
were derived through application of a simple dispersion model
with wind flows perpendicular to a series of line sources for
a distance of 10 or 100 km. Figure 6 displays upper quartile Summer
morning X/Q values for a 100 km city size. Also displayed (but
not color shaded) are similar (dashed) isopleths for a smaller
hypothetical city size of 10 km.
A common feature of Figure 5
and 6 is an area with high meteorological
potential for local pollution accumulation along the western edge
of the Appalachian Mountains, with the highest values in both
cases in an area near the intersection of the OH/KY/WV/PA borders.
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The Midwestern area identified in Figures 5 and 6 as having a
high (1960's) meteorological potential for local pollution episodes
is also characterized by very high (1990's) stationary source
NOx emissions densities (Figure 7).
The same general region is also identified by (1990's) backward
trajectory calculations4 as being persistently upwind of high
ozone concentrations at a variety of receptor sites throughout
the OTAG domain (Figure 8). However,
the Midwest itself does not currently experience frequent exceedances
of the 1-hour ozone standard (Figure 9).
How can this be?
The Holzworth (and other 1960's) climatological estimates of local
episode potential were developed for "slowly-reacting"
pollutants - which typically build to maximum concentration (under
low mixing heights and low wind speeds) in the early morning hours.
In the above Figures 1-6 from Holzworth, the Midwest is identified
as having a high potential for pollution episodes primarily in
the "morning" plots (Figures 1,
3 and 6).
Ozone and most ozone precursors are "quickly-reacting"
pollutants. In the absence of sunlight, precursors may build to
high local concentration during the early morning hours, but ozone
itself is predominantly an "afternoon" pollutant. Holzworth's
plot of upper quartile Summer morning X/Q concentrations (Figure 6)
indicates a high pollution potential in the Midwest. However,
a similar calculation of Summer afternoon X/Q values (not shown
here) projects afternoon X/Q concentrations which are an order
of magnitude lower than the morning values, and which exhibit
almost no spatial variation throughout the eastern US. Note also
from Figures 1 and 2
that while morning mixing heights in the Midwest are typically
quite low (300-400 meters), they increase to roughly 5 times this
height (< 1,800 meters) by mid afternoon (when they are typically
higher than in surrounding regions).
So while morning precursor concentrations may build to high concentration
in a shallow mixed layer, afternoon ozone production takes place
within a substantially expanded volume. This "ballooning"
of the Midwestern mixing depth may help explain the relative absence
of high 1-hour ozone concentrations, despite high precursor emissions
and low wind speeds. These factors may also help explain the persistence
of relatively high chronic concentrations (high 10th percentile
and high 8-hour averages), despite the absence of high 1-hour
concentrations. As the Midwestern mixing height descends rapidly
in the evening hours, a high fraction of the preceding day's accumulated
ozone is preserved aloft, protected from chemical and physical
destruction at/near the earth's surface. This deep layer or "reservoir"
of moderately, but chronically high nocturnal ozone aloft can
be subsequently subject to higher wind speeds which can transport
the "substantially" high ozone concentrations to other
The subsequent transport from this centrally-located region can
and does occur in many directions, as wind directions in the central
eastern US can be quite variable from one episode to another (but
may tend to be more uni-directional during a specific episode).
Figure 11 from Schichtel and
Husar (1997,3c shows transport vectors and residence times derived
from 1991-95 NGM meteorological data. The met data have been sorted
at each location on the map to reflect the average conditions
during high ozone events (the highest 10% of ozone concentrations
at each location). The color shaded "residence times"
are approximately equal to the inverse wind speed. The vectors
show the average wind direction, while the vector length reflects
the persistence of the wind direction (during high ozone events).
The very short vector length in the Midwest indicates varied wind
directions during high ozone events. Locations around the periphery
of the Midwest show vectors pointing outward from this central
location. Note also that the residence times show a spatial pattern
(from 1990's data during high ozone conditions) which is quite
similar to the (Figures 3 and
4, 1960's-based) wind speed plots
Thus, from a climatological perspective, wind speeds derived from
a 5-year 1960's data set appear similar to (and relevant to) wind
speeds during high ozone events during a 5-year period in the
1990's. Summer wind speeds are low in the South and Midwest. Wind
directions from the Midwest during high ozone events are variable
from episode to episode, with high ozone episodes in areas surrounding
the Midwest in all directions frequently associated with flows
from the Midwest.
Figure 12 shows similar transport
vectors and residence times based on NGM data during the July,
1991 OTAG model episode. Unlike Figure 11, it includes
periods and locations of relatively low ozone during model "ramp-up"
days, and on "off-peak" days during the model period.
Note, however, that residence times in the Midwest are generally
shorter for this episode than in the Figure 11 plot of "climatologically
defined episodes". Note also that the transport vectors from
the Midwest are longer (more uni-directionally persistent) for
the individual episode. Thus any directional implications of ozone
transport based on any individual episode may reflect only part
of the story, and should not be viewed as "predictive"
of future conditions.
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1. Porter, P. S., S.T. Rao, I. Zurbenko, E. Zalewsky, R.F. Henry and J.Y. Ku (1996) at
2. Husar, R. B. (1996a,b,c,d) at
3. Schichtel, B. and R. B. Husar (1996a,b,c) at
4. Poirot, R. and P. Wishinski (1996a,b,c,d) at
5. Rao, S. T. and E. Brankov (1997) Role of atmospheric transport on ozone concentrations at Whiteface Mt., presentation to OTAG AQA Workgroup, Washington, DC.
6. Hosler, C. R. (1961) "Low level inversion frequency in the contiguous United States", Mon. Weather Rev. 89: 319-339.
7. Hosler, C. R. (1964) "Climatological estimates of diffusion conditions in the United States", Nuclear Safety 5: 184-192.
8. Holzworth, G. C. (1964) "Estimates of mean maximum mixing depths in the contiguous United States". Mon. Weather Rev. 92: 235-242.
9. Korshover, J. (1967) "Climatology of stagnating anticyclones east of the Rocky Mountains, 1936- 1965", Public Health Service Publication No. 999-AP-34, Cincinnati, OH.
10. Holzworth, G.C. (1967) "Mixing depths, wind speeds, and air pollution potential for selected locations in the United States", J. Appl. Meteor. 6:1039-1044.
11. Gross, E. (1970) "The national air pollution potential forecast program", EESA Tech. Memo. WBTM NMC 47, National Meteorological Center, Suiteland, MD.
12. Holzworth, G.C. (1972) "Mixing heights, wind speeds, and potential for urban air pollution throughout the contiguous United States", Office of Air Prog. pub. AP-101,USEPA, RTP, NC.
13. Morris, R.E. (1995) "Review of Recent Ozone Measurement and Modeling Studies in the Eastern United States Nov 30 1995 Draft", ENVIRON Corp., Navato, CA 949455010.
14. Edgerton, E. S., (1996) Presentation to OTAG AQS Workgroup on: Comparison of isoprene concentrations from UAM-V and ambient measurements at selected SOS and NARSTO-NE sites.
15. Imhoff, R. E. (1996) "Preliminary proposal: evaluation of the effect of the use of a 12 kilometer grid on otag modeling results", at http://capita.wustl.edu/otag/reports/mod12km/ufgrid12.html
16. Dickerson, R., S. Kondragunta, G. Stenchikov and W. Ryan (1996)
"The impact of aerosols on photochemical smog formation",
Presented at the First NARSTO-NE Data Analysis Symposium and Workshop,
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Figure 1 Mean Summer Morning Mixing Height (m) 12
Figure 2 Mean Summer Afternoon Mixing Height. (m) 12
Figure 3 Mean Summer Morning Wind Speed (m/s)
Averaged through the Morning Mixed Layer 12
Figure 4 Mean Summer Afternoon Wind Speed (m/s)
Averaged through the Summer Afternoon Mixed Layer 12
Figure 5 Number of Forecast Days per 5 Years with
High Meteorological Potenential for Air Pollution 12
Figure 6 Upper Quartile Summer Morning X/Q Values
(normalized concentration) for a 100 km City Size 12
Figure 7 1991 Point Source NOx Emissions from EPA Interim Inventory
Figure 8 Areas with Deviations > 0.3 GSD Above Mean,
Averaged for 23 Trajectory Sites 4
Figure 9 Areas with High 1-Hour Ozone 2
Figure 10 Areas with High 8-Hour Ozone 2
Figure 11 Transport Vectors and Residence Times for
Upper 10% Ozone Days, Summers: 1991-95 3
Figure 12 Transport Vectors and Residence Times for
July 1991 OTAG Modeled Episode 3
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