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OCR for page 72
Contributions of Topography'
Meteorology' and Human Activity
to Carbon Monoxide Concentrations
INTRODUCTION
Topography, meteorology, and human activity contribute to high carbon
monoxide (CO) concentrations in some areas that exceed the National
Ambient Air Quality Standards (NAAQS). Despite the decline in national
ambient CO concentrations, maintaining the 8-hour standard of 9 parts per
million (ppm) has been a particular challenge for some locations (see Table
1-1~. Even when attainment of the standard has been achieved, there re-
mains a vulnerability to future exceedances. Expressed mathematically,
there is a nonzero probability of nonattainment in a future year.
After a more detailed discussion of topography and meteorology, this
chapter will discuss the seasonal, weekly, and diurnal patterns in CO con-
centrations measured in some CO problem areas. These patterns help
describe some of the physical and human factors contributing to the CO
problem in these locations. The chapter discusses vulnerability to future
exceedances, including a brief description of statistical approaches. The
chapter concludes with illustrative examples of the factors contributing to
the CO problems in CaTexico, California; Lynwood, California; Fairbanks,
Alaska; Las Vegas, Nevada; and Denver, Colorado.
72
OCR for page 73
Contributions of Topography, Meteorology, and Human Activity 73
The committee identified four factors that contribute to the difficulties
that the cities listed in Table 1-1 have had in meeting the NAAQS for CO:
1. Unfavorable topography. Low lying areas surrounded by higher
elevations on three or more sides are vulnerable to CO buildup.
2. Unfavorable meteorology. Stagnant winter conditions character-
ized by ground level temperature inversions (see definition below) and low
windspeeds inhibit vertical mixing of CO.
3. Significant local CO emissions.
4. High concentrations of CO transportedirom nearby areas.
Higher elevations with lower air densities tend to have higher CO con-
centrations (in ppm) for a given emission flux.i Denver's average air den-
sity is 85% of that at sea level. Lower oxygen density can increase CO
emissions rates in older vehicles. Topography also can affect meteorologi-
cal conditions in a variety of ways, as described below.
Meteorology can influence pollutant concentrations through its effects
on atmospheric mixing height, windspeeds and wind direction, and atmo-
spheric water content (humidity). Humidity is a factor because dry climates
and higher elevations tend to have lower total water columns overhead.
Because water vapor is an important greenhouse gas (infrared radiation
from the earth's surface is absorbedby wafer molecules end reradiated beck
down, warming the surface), reduced water vapor allows infrared radiation
to pass into space, producing ground level temperature inversions after
sundowns and lower mixing heights. These lower mixing heights, com-
bined with high evening traffic emissions, can lead to pollution buildup
near the ground. It is noteworthy that all of the cities listed in Table 1-1,
except Birmingham, are west of the Mississippi River, where the air tends
to be drier. Two are in Alaska, where high latitudes and low winter temper-
atures result in reduced solar heating at midday and atmospheric conditions
are typically dry.
~ _ _ ~
Considering CO emissions as an area source, the emission flux is the mass of
CO produced per square kilometer per hour.
2This radiation into space is the reason temperatures drop so rapidly at night in
the desert and in the mountains.
OCR for page 74
74 Managing CO in Meteorological and Topographical Problem Areas
METEOROLOGY AND TOPOGRAPHY
Background
The ease with which air can mix vertically to disperse pollutants de-
pends critically on how the air temperature changes with altitude. Warmer
air is less dense (more buoyant) and tends to rises and coot as the pressure
decreases and volume expands. Tfthe vertical temperature profile decreases
by 1°C/100 m (the adiabatic lapse rate) or more, the air mixes freely as
warmer air from below moves upward. If the temperature decreases more
slowly than 1°C/100 m, or increases with altitude (called an inversion),4
vertical mixing is inhibited. The faster the air temperature increases with
altitude during an inversion, the more strongly mixing is resisted.
Inversion Types
There are several atmospheric processes that can form inversions, as
illustrated in Figures 2-1 and 2-2. Cooling of the air near the ground as a
result of infrared radiation into space after sunset can create a surface-based
inversion, like that shown in Figure 2-la, and can produce a thermody-
namically stable layer, which tends to trap pollution near the ground. Hori-
zontal advection of warm air creates a high-altitude inversion and can simi-
larly increase the stable temperature stratification aloft (Figure 2-l[b]~.
Figure 2-~(c) shows the situation with both surface-based end high altitude
inversions. In a subsidence inversion (Figure 2-2), a surface-based inver-
sion can be strengthened by warm air that sinks and is warmed further as
a result of compression. Each of the inversion types reduces the atmo-
sphere's ability to mix through the inversion level, allowing pollution gen-
erated below the inversion level to accumulate.
3This is the principle on which hot air balloons operate. Vehicle exhaust from
a tailpipe is typically 50-100°C warmer than the surrounding air; however, it is
rapidly diluted and cooled as it is mixed into the ambient air.
4An "inversion" in meteorology is defied as "a departure from the usual
decrease or increase with altitude of the value of an atmospheric property" (Geer
1996~. The term is generally used to refer to a situation where temperature shows
an increase with altitude rather than the usual decrease.
OCR for page 75
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OCR for page 76
76 Managing CO in Meteorological and Topographical Problem Areas
1 000—
500—
· _
-
surface
\ ~ .
\ Advectiarl of
\ warm air aloft
\~- ''..
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+10
Temperature (Celsius)
+20
FIGURE 2-2 Schematic of how an existing surface-based inversion (solid lined
can be strengthened by subsidence (dashed lined or by advection of warm air
aloft (dotted line).
Recirculation
Atmospheric flow eddies can recirculate the air one or more times.
When pollution is emitted into these circulations, pollutant concentrations
can increase over time. Figure 2-3 illustrates such a recirculation pattern
in a trapping valley.
Sea and land breezes represent an additional cause of atmospheric
recirculation (Segal and Pielke 19811. As shown in the modeling study of
Eastman et al. (1995), at least 70% of pollution recirculates with the sum-
mer Lake Michigan sea breeze. These results mirror the observations of
Lyons et al. (1995~. That study shows that Gaussian-type models fait to
replicate the recirculation and the complex dispersion patterns that result
when spatial variations in sensible heat fluxes exist at the surface (Pielke
end Uliasz 1993~.
OCR for page 77
Contributions of Topography, Meteorology, and Human Activity 77
200 m -
WARM AIR
COLD AIR
Resurface
/
/
Ten~erature
FIGURE 2-3 Schematic of a trapping valley. The temperature profile in the
valley is shown on the right.
Stagnation
When air does not move significantly over tens of hours or more, the
atmosphere is said to be stagnant. Stagnation can occur because of weak
winds andlor the trapping of air (see Figure 2-3) When the atrno sphere is
stagnant, emitted pollution can accumulate over time. Simple box models,
such as those discussed in Pielke et al. (1991) and in Appendix C, can be
used to estimate pollution buildup associated with stagnation. Figure 2-3
illustrates air stagnation in a trapping valley.
Influence of Topography on Meteorological Conditions
Pielke (2002) discusses the influence of terrain on atmospheric condi-
tions under strong and weak large-scale winds. The following discussion
illustrates that the direction of movement of air is actually quite compli-
cated in complex terrain. In contrast to flat terrain, the wind flow in valleys
can go in almost any direction, depending on the relative importance ofthe
different forcing mechanisms. These mechanisms will affect the dispersion
of CO. Under strong flow, for instance, large upward and downward mo-
tions are produced, which can enhance pollution dispersion. Under weak
OCR for page 78
78 Managing CO in Meteorological and Topographical Problem Areas
flow, mountain-valley winds that are a result of diurnal warming and cool-
ing of the terrain surface can occur (Figure 2-44. These local winds trans-
port and disperse pollution, although recirculation can also occur.
Mountain-valley winds can occur on relatively small scales (in which case
they are called upsIope and drainage flows) or on larger scales (where
broad ascent and descent patterns occur). The resulting wind flows can be
quite complex.
Figure 2-5 illustrates the differences in the diurnal variation of the
valley wind direction as a function of the wind direction above the valley
(the geostrophic wind) for four distinct mechanisms that can control the
wind direction. According to Pielke (2002), these four physical mecha-
nisms operate as follows: (1) thermally driven winds are independent ofthe
above-valley winds and are controlled by locally developed valley pressure
gradients; (2) downward momentum transport ofthe above valley winds (as
is associated with a deep convective boundary layer) produces similar wind
directions at all levels; (3) forced channeling occurs when the valley flow
alignment is dependent on whether the above-valley flow has a net flow
down- or up-valley; and (4) pressure-driven channeling (which is out of
phase with forced channeling) occurs when the winds in the valley respond
only to large-scale horizontal pressure, not to the winds that occur above
the valley. Without terrain, the airflow would be nearly parallel to the
isobars. With other local flows involved (such as sea and land breezes),
wind flow is more complex (Pielke 2002~.
Large-Scale Meteorological and Climatological Events and
Their Impact on Attainment
Local air quality can be affected by large-scale meteorological and
cTimatological events. CO exceedances may have patterns that are related
to the occurrence of synoptic-scare meteorological events or cTimatological
events, such as the El Nino Southern Oscillation (ENSO). Changes in the
frequency of large-scale events could affect a location's ability to come into
and maintain compliance with the NAAQS for CO. The committee ex-
plored the potential effects of large-scare meteorological and climatological
phenomena on local CO episodes in three cities: Lynwood, California;
Fairbanks, Alaska; and Denver, Colorado. it should be noted, however,
that the impact of climate and meteorological variability on air quality,
including CO and related pollutants, is an area requiring more research.
OCR for page 79
Contributions of Topography, Meteorology, and Human Activity 79
A
>
> ~ >
~ >
o 1
1 5 10 15 20 25
Time - 4.0 hours
6 r, 81 ~ ~ 1 1 ~ ~ ~ ~ ~ ~ Y Y '~ ~-~' ~ ~ ~ ~ ~ '~ ~ ~ 7 7 'r-
.', 8,`
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, , · ~ · ~ ~ ~ ~ ~ ~ ~ ~ ~ _ ~ _ _~— ~ ~ · —
1 ~1
1 5 10 15 20 25 30
Time - 16.0 hours
FIGURE 2-4 Two dimensional simulation of (a) nocturnal drainage flow and
(b) upslope flow with no prevailing synoptic flow, with an input condition typi-
cal of summer in midlatitudes. Source: Mahrer and Pielke 1977. Reprinted with
permission; copyright 1977, American Meteorological Society.
OCR for page 80
80 Managing CO in Meteorological and Topographical Problem Areas
NIGHT DAY
it,
TOTAL
' ~
, ~
'
. a, >. ~~—
f
,,, . ~ .
An. i ~ ~1 ,!
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... ..
i E S W N ~ £ ~ W ~
THERMALLY
DRIVEN
DOWNWARD
MOMENTUM
TRANSPORT
FORCED
CHANNELING
PRESSURE
DRIVEN
CHANNELING
FIGURE 2-5 Relationships between above-valley (geostrophic) and valley
wind directions for four possible forcing mechanisms: thermal forcing, down-
ward momentum transport, forced channeling, and pressure-driven channeling.
The valley is assumed to run from northeast to southwest. Source: Whiteman
and Doran 1993. Reprinted with permission; copyright 1993, American
Meteorological Society.
Lynwood, California
Lynwood's local air quality may be influenced by ENSO. Historically,
E] Nino recurs every 3-7 years when sea-surface temperatures (SSTs) in the
equatorial Pacific Ocean offthe South American coast become warmer than
normal. La Nina is essentially the opposite of El Nino, and exists when
cooler-than-usual ocean temperatures occur near the equator between South
America and the International Date Line.
During an El Nino, the months of October through March tend to be
wetter than usual in a swath extending from southern California eastward
across Arizona, southern Nevada, Utah, and New Mexico, and into Texas.
Almost all of the major flood episodes on main rivers in southern Califor-
nia have occurred during El Nino winters. During La Nina years, dry con-
OCR for page 81
Contributions of Topography, Meteorology, and Human Activity 81
ditions are produced on the equator in the Pacific Ocean. La Nina gener-
ally does not affect the United States as much as El Nino; however, strong
La Ninas have been linked to dry seasons in southern California. In
Lynwood, CO exceedances may tend to increase during strong La Nina
years when dry and stable atmospheric conditions are produced: Con-
versely, CO exceedances may tend to decrease during E] Nino years. Al-
though some studies have explored the effects of E! Nino on ozone levels
(e.g., Chandra et al. 1998), as of yet no studies have examined correlations
between ENSO and CO exceedances. Further research may be needed in
this area, including an assessment of how ENSO affects conditions that
control concentrations of the pollutants associated with CO (i.e., air tonics
and PM).
Fairbanks, Alaska
In Fairbanks, Alaska, all exceedances of the 8-hour CO standard from
1996 through 2001 occurred when a Tow-pressure system in or near the
Gulf of Alaska produced southeasterly geostrophic winds. These winds,
which travel over the Alaska Range, are associated with the counterclock-
wise geostrophic flow around the low-pressure system. One hypothesis for
the coincidence of CO exceedances with this synoptic-scare meteorological
event is that the warm-air advection aloft reinforces the radiative ground-
leve] inversion. The downward movement of air over Fairbanks also exerts
a stabilizing influence on inversions. It is not known, however, what frac-
tion of nonexceedance days has such meteorological conditions or how
many of the exceedance days before 1996 had these conditions. Nonethe-
less, the surface pressure gradient observed during all six exceedances from
1996 to 2001 must have some significance. However, further research over
a longer period of time is needed to better understand the relationship.
Denver, Colorado
In the past, CO exceedances in Denver, Colorado, have coincided with
the occurrence of lee troughs—lines of surface Tow pressure on the lee side
(the side that is sheltered from the wind) of a mountain range. The air
coming over the mountains sinks and warms and, at the same time, the
lowering of pressure at the surface along the foothills draws colder air from
OCR for page 82
82 Managing CO in Meteorological and Topographical Problem Areas
the plains and lowlands areas back towards the mountains. Thus, the air
between the surface and 100-300 m becomes colder as the air above be-
comes warmer, enhancing the inversion. Neff and King (1991) character-
ized lee trough history for the 1980s and early 1990s. Lee troughs often
occur several times each week during the winter months and are a key
precursor to high CO levels in Denver. However, the effect of lee troughs
on CO exceedances has not been studied since the mid-199Os because of
the decline in exceedances. The decline in CO exceedances is mainly due
to lower vehicle emissions, but Neff (2001) noted that there was also a
decline in the occurrence of lee troughs during the late 1 980s, which per-
haps reduced the frequency and the severity of conditions producing CO
exceedances. Future studies also should explore the association of lee
troughs, the Arctic Oscillation, and air quality. When cold-air arctic out-
breaks occur, they usually provide a snow cover, which strengthens the
ground-level inversion. Fewer arctic outbreaks over the Great Plains could
help decrease the potential for pollution episodes in Denver. Denver has
had a decade-Ion" period without the long-term snow cover and associated
light winds that tend to promote atmospheric stagnation, which can lead to
CO buildup. The lack of conditions conducive to high CO means less
susceptibility to CO exceedances.
TEMPORAL PATTERNS OF CO CONCENTRATIONS
CO concentrations show seasonal, weekly, and diurnal patterns reflect-
ing the temporal patterns in emissions and meteorology. Figures 1-1 and
1-2 show seasonal patterns in the numbers of days with exceedances ofthe
8-hour CO standard. Figures 2-6 and 2-7 also show patterns in the total
numbers of exceedance days by month for Lynwood, California, and Fair-
banks, Alaska, for periods of approximately 30 years. Lynwood exhibits
a very symmetrical pattern, with the maximum number of exceedance days
in December, when the winter solstice (shortest day, least solar radiation)
occurs. Fairbanks exhibits the maximum number of exceedances in Janu-
ary. The considerably greater numbers of exceedance days in Fairbanks in
January compared with November, and in February compared with Octo-
ber, are attributed to reduced cloud cover in the winter months compared
with the autumn months.5 Clear skies in January and February contribute
month.
Wee Figure 2-3 in the interim report (NRC 2002) for Fairbanks cloud cover by
OCR for page 89
Contributions of Topography, Meteorology, and Human Activity 89
Effects of Meteorology and Emissions
on Vulnerability to Future Exceedances
This section examines the effects that meteorology and emissions have
on the vulnerability of locations to future CO exceedances. Table 2-1 sum-
marizes the characteristics of the mean diurnal patterns shown in Figure 2-
9, indicating the time of the daily maximum at each site for the average
nonholiday weekday, the mean CO concentration at that time, and the stan-
dard deviation. The next-to-last column provides the coefficient of varia-
tion (COY) the ratio ofthe standard deviation to the mean for the daily
maximum; the largest COVs (~0.7) are shown in italics. The last column
gives the average CO concentrations for the 1999-2000 winter season,
including weekends and holidays.
The variability indicated in the COV column in Table 2-1 is the result
of variability in both emissions rates and meteorological factors. Traffic
volumes at 6:00 p.m. (hour 18) on significant roadways in Fairbanks,
Alaska, during nonholiday winter weekdays show little variation, with a
COV for traffic of only 0.08 ~ 0.01.9 Much greater variability is exhibited
in CO concentrations than in traffic. Figure 2-10 compares the variability
in daily average traffic on Cushman Avenue in Fairbanks with the variabil-
ity in daily (24-hour) average CO concentrations measured at the Post
Office monitor on the same road during the winter of 1999-2000. In each
case, the data are sorted into two categories: (1) nonholiday weekdays, and
(2) weekends and major holidays. Then the data are sorted by decreasing
daily average value within each of those categories. The mean daily aver-
age traffic for winter weekdays in Figure 2-10 was 616 vehicles per hour,
with a standard deviation of 55 (COV = 9.0%), end the mean daily weekday
average CO concentration was 2.2 ppm, with a standard deviation of 1.1
(COV = 50%~.10 These figures confirm that CO concentrations were con-
siderably more variable than traffic flows. The highest and second-highest
daily average CO values for that winter occurred on Tuesday, February 8,
2000, and on Monday, November 19, 1999. Both were exceedance days,
9This mean (0.08) and standard deviation (0.01) are based on an analysis of
traffic on three roads in downtown Fairbanks during winters of 1995-1996 through
2000-2001 . For the locations of counters and monitors see Figure 2-4 in the interim
report (NRC 2002~.
Patois ratio of 0.50 differs from the 0.81 that appears in Table 2-1 because the
former refers to a 24-hour mean and the latter to a 1-hour mean.
OCR for page 90
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OCR for page 91
Contributions of Topography, Meteorology, and Human Activity 91
Boo
700
400
300
200
100
(a)
Weekdays
Weekends and Holidays
o
7.0
6.0
1.0
(b)
E
~ 5.0
In
~ 4.0
o
~, 3.0
Weekdays
Weekends and Holidays
0.0
FIGURE 2-10 Daily average (a) traffic on Cushman Avenue and (b) CO
concentrations at the Post Office on Cushman in Fairbanks during the winter of
1999-2000. Values for weekdays are rank ordered, as are those for weekends
and major holidays (Thanksgiving, Christmas, New Year's Eve, and New Year's
Days). Days with data missing are indicated by missing bars. The daily average
traffic was obtained by diving the total traffic count each day by 24 hours.
with maximum 8-hour average CO values of 11.5 ppm and 11.2 ppm,
respectively. Figure 2-11 shows a scatter plot of the data used for Figure
2-10. The two exceedance days (the top two points) had traffic flows (600-
OCR for page 92
92 Managing CO in Meteorological and Topographical Problem Areas
700 vehicles per hour) that produced many much Tower average CO con-
centrations.
An exceedance that occurred in Fairbanks on Saturday, January 11,
~ 997, highlights the importance of meteorological factors. The maximum
S-hour average CO concentration that day was 13.3 ppm even though the
daily mean traffic on Cushman Avenue was only 437 vehicles per hour.
The daily average Tower inversion strength (measured between 3 and ~ 0 m
above the ground)was 18.6°C/100 m, the average windspeed was 0.8 MPH,
and the average temperature 5.5°F.~ Although traffic is not the only factor
determining CO emissions rates (cold-start and idling emissions are also
important in Fairbanks), such an exceedance indicates that meteorology
may be able to produce exceedances despite emissions reductions.
Two cities with the same average CO values in winter (e.g., Kalispell,
Montana, and Denver, Colorado) can differ greatly in their vulnerability to
future exceedances depending on their respective variability in CO concen-
trations. Of the two Anchorage sites, site 2 (Turnagain) had a lower aver-
age CO concentration in winter than did site ~ (~.2 and 2.0 ppm, respec-
tively), but its much greater variability makes site 2 more vulnerable; in
fact, the two most recent CO exceedances in Anchorage occurred at this
residential site (see Table 1-1~. Characterizing the non-Gaussian distribu-
tion of 8-hour average CO concentrations in a location might make it possi-
ble to predict the probability of future exceedances based on projected
emissions inventories. (Gaussian models are discussed in detail in Chapter
3 .) In addition, to adequately test the hypothesis that high variability in CO
concentrations helps explain the difficulty that some areas have had in
meeting the standard, the variability in CO concentrations in areas that met
the standard relatively easily should also be examined.
Assessment of Vulnerability
The variability in CO concentrations leads to difficulties in predicting
high CO episodes especially in geographical areas with unusually challeng-
i~The ranges ofthe daily averages for these three meteorological variables for
the November through February 1996-1997 winter season were as follows: lower
inversion strength, -4.3 to 18.6°C/100 m; windspeed, 0.7-3.9 MPH; and tempera-
ture, -43 to +33°F. Data were provided by Paul Rossow ofthe Fairbanks North Star
Borough.
OCR for page 93
Contributions of Topography, Meteorology, and Human Activity 93
~ ~ Weekends and Holidays ~ Weekdays
~ p7.0-
8 Q6.0_
~ dV 5 0 '
,04.0-
~3.0-
~ O 10 = ' ' ~ ·; · ·., ~ art; ~
Q go
.
.
-
aa i --at
0 100 200
_ 1 ' '
300 400 500
Average Traffic on Cushman (Vehicles/Hr)
600 700 800
FIGURE 2-11 Scatter plot of Fairbanks traffic and CO during Me winter of
1999-2000.
ing meteorological and topographical conditions. Variability in meteoro-
logical conditions, such as strong temperature inversions or winds blowing
from the direction of nearby communities with high levels of CO, contrib-
utes to these difficulties. The status of problem areas could fluctuate be-
tween attainment and nonattainment until further emissions reductions
provide an adequate safety margin. Areas that have achieved attainment
recently and do not yet have an adequate safety margin remain vulnerable
to high CO episodes. Nonattainment might occur sporadically under unfa-
vorable meteorological conditions, even when emissions rates remain at the
levels projected in the SIP.
Vulnerability can be expressed in terms of the probability of non-
attainment in a future year or in terms of the reciprocal of this probability,
which can be interpreted as the number of years that are likely to pass until
the next nonattainment year takes place. The latter is analogous to a design
condition in civil engineering, such as when a bridge is designed to with-
stand a once-every-hundred-year's flood. It is also analogous to the con-
cept in public health of the number needed to treat (NNT), defined as the
reciprocal ofthe probability for a categorical change in outcome (e.g., from
death to survival) for a randomly selected future patient.
Given its stochastic nature, vulnerability is determined by both the
central tendency (such as the median or mean) and the spread (such as the
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94 Managing CO in Meteorological and Topographical Problem Areas
interquartile range or standard deviation) of the air quality indicator (e.g.,
the annual second maximum nonoveriapping 8-hour average CO concentra-
tion). An area with a large spread might have a substantial probability for
nonattainment in future years even after achieving attainment for several
years. To reduce vulnerability, emissions reductions must extend beyond
the attainment threshold to provide an adequate safety margin.
ILLUSTRATIVE EXAMPLES
This last section provides five illustrative examples of locations that
have had problems meeting the NAAQS for CO. The roles of topography
and meteorology in concentrating CO and how those factors combine with
patterns of emissions to produce episodes of high CO concentrations are
briefly described. The committee realizes that the definition of a meteoro-
logical and topographical problem area might be somewhat arbitrary, be-
cause in all areas meteorology and topography are important factors in
producing, concentrating, dispersing, or eliminating all air pollutants. In
these examples, the committee primarily focuses on locations in the west-
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Contributions of Topography, Meteorology, and Human Activity 95
em United States that experience winter temperature inversions and have
topographical features contributing to the accumulation of CO.
Calexico, California
Calexico is located 125 miles (ml) east of San Diego in California's
Imperial Valley, on the border with Mexico. This small city's population
of 27,109 (in 2000) is similar to Fairbanks's (30,224) (U.S. Census Bureau
2000b).~2 However, it is across the border from Mexicali, a much larger
Mexican city with a population of about 750,000. Motor vehicles on the
MexicaTi side tend to be older and tend to have less sophisticated emissions
control equipment that sometimes is not functioning properly or has been
removed. In addition, Mexicali has no vehicle emissions inspection and
maintenance program. The number of exceedance days recorded since
1995 at a monitoring site in Calexico (59) is surpassed only by the number
recorded in Birmingham, Alabama. Although Calexico is a major border
crossing point (an estimated 2,16S,000 vehicles crossed the border from
Mexico in 1999 fCaTexico 19993), CO measurements in Mexicali indicate
that the problem is not due to Tong lines of idling vehicles at the border.
The committee initially thought that CO episodes in Calexico did not
fit the profile of locations whose problems were created by meteorology
and topography. However, monitoring of CO concentrations in Calexico
indicated that the movement of a large, CO-rich air mass northward across
the border is responsible for most of the CO pollution in CaTexico. This is
especially true at night in winter when windspeeds are low and ground-
leve] temperature inversions are strong. The situation is exacerbated by
Calexico's topographical location in a valley down-slope from Mexicali.
(The SaTton Sea, 30 mi to the north, is 235 feet below sea level.) Although
Calexico does not have confining topography that traps or accumulates CO,
its topography puts the city in the pathway of CO drifting across the border
from MexicaTi, and its meteorology prevents CO from dispersing vertically.
Further analysis is needed to assess the relative roles of cross-border trans-
port of vehicles operating in MexicaTi compared with vehicles idling at the
border in producing CO. To address the problem of cross-border air pollu-
Tithe population ofthe Fairbanks North Star Borough, with a much larger area
of about 7,000 square miles, was 82,840 in 2000 (U.S. Census Bureau 2002).
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96 Managing CO in Meteorological and Topographical Problem Areas
tion, the United States and Mexico have agreed to a new Border Air Quality
Strategy, announced by EPA on November 26, 2002 (EPA 20026~.
Lynwood, California
Lynwood is a community of about 64,000 located approximately 12 mi
east of the Pacific Ocean. It is south of downtown Los Angeles in the Los
Angeles Basin, which has a total population of over 1 ~ million. Lynwood
is a densely populated area with numerous freeways and highways, and
many high-emitting vehicles. There were 58 exceedance days during the
7-year period from 1995 to 2001.
Numerous studies of the CO problems in Lynwood, including those
conducted by the California Air Resources Board (Nininger 1991; Bowen
et al. 1996), have been undertaken to assess why CO concentrations are
higher in Lynwood than other parts ofthe Los Angeles area. These studies
are aimed to determine the relative contributions of local versus area
sources of CO and the roles of meteorology and topography on CO concen-
trations. Motor-vehicle emissions are clearly important. About half of
Lynwood's CO emissions come from just 10°/O of the light-duty vehicle
(LDV) fleet (Lawson et al. 1990~. Singer and Harley (1996, 2000) also
noted CO emissions rates of vehicles registered in the Lynwood area were
double those registered in higher income areas because of the prevalence
of older vehicles. Nininger (1991) concluded that the entire Lynwood area
is a CO hot spot, due not only to high vehicle emissions but also to Tower
windspeeds and mixing volumes that occur in surrounding areas. Bowen
et al. (1996) further qualified the role of meteorology and topography in
contributing to the high concentrations of CO in Lynwood. A strong,
surface-based inversion occurs in the Los Angeles area soon after sunset,
and the strength of the inversion appears to be greater near Lynwood. In
addition, the gradient of the terrain is smaller near Lynwood than at most
other locations in the area, resulting in weaker nocturnal drainage winds.
The study concluded that significant CO emissions originating in the
Lynwood area are added to an urban air mass with high CO concentrations
that is transported into the Lynwood area. These emissions sources com-
bine with stable nighttime meteorological conditions to create high CO
concentrations in Lynwood.
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Contributions of Topography, Meteorology, and Human Activity 97
Fairbanks, Alaska
Fairbanks, Alaska, is a small city in which topography, meteorology,
and emissions conspire to produce air pollution in winter (NRC 2002~. The
meteorological and topographical characteristics of Fairbanks's air pollu-
tion problems are discussed in Bowling (1984, 1986~. The city has apopu-
lation of about 30,000 and is located in central Alaska in the Fairbanks
North Star Borough, a sparsely inhabited area of over 7,000 square mi with
a total population less than 85,000. The city is a center for government,
education, and distribution for the northern part of Alaska.
Fairbanks is sheltered by hills to the west, north, and east and is situ-
ated on Tow ground near the confluence of the Chena and Tanana Rivers.
The terrain is open to the south, with the Alaska Range roughly 45 mi
away.
The meteorology is extreme continental arctic. Low winter tempera-
tures are combined with unusually strong ground-based inversions and low
windspeeds. The warmest point in a vertical temperature sounding is com-
monly more than a kilometer above the surface, and near-surface inversion
strengths often exceed 10°C/100 m. These factors greatly limit the amount
of air available to dilute and disperse CO and other pollutants.
Temperatures during the winter months are normally below 20°F bel-
ow the limits of federal guidelines for cold-start emissions—so engine
starting emissions can be substantially higher than normal. As tempera-
tures drop below 0°F, automobiles become increasingly difficult to start.
At temperatures below -20°F (not uncommon in Fairbanks during winter),
most people use preheating "plug-ins," because it is nearly impossible to
start a vehicle that has not been preheated. Engine preheating reduces cold-
start CO emissions, so high ambient CO levels are rare at those low temper-
atures. Encouraging the use of pretreating plug-ins at temperatures between
20 and 0°F, when unheated vehicles can be started but emit large amounts
of CO, is the centerpiece of the borough's strategies to reduce CO emis-
s~ons.
Las Vegas, Nevada
Las Vegas is a rapidly growing city in southern Nevada that had a
population of nearly 41 S,000 in 1999, up from about 260,000 in 1990 (U.S.
Census Bureau 2000c). Las Vegas is located in a valley surrounded by
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98 Managing CO in Meteorological and Topographical Problem Areas
mountains: the Spring Mountains to the west, the Pintwater, Desert, Sheep,
and Las Vegas Mountains to the north; Frenchman Mountain to the east;
and the McCullough and Big Spring Ranges to the south. Automobile and
truck traffic go to and through the city 24 hours a day via three major high-
ways. In late fall and throughout winter, coo] air drainage winds from the
adjacent desert hills flow into the city and pool there, resulting in a local
accumulation of CO. This pooling effect has resulted in a total of seven
exceedance days recorded at two monitoring sites since 1995. Population
growth is expected to continue, so future violations are a serious concern.
Las Vegas undertook a significant CO saturation study to help assess
the monitoring network and movement of CO (Ransel 20021. The study
extensively augmented the 14 permanent monitoring sites with 63 tempo-
rary fixed sites and mobile sampling at over 2,500 locations. The study
concluded that the current monitoring network captures the peaks and
extent of high CO. It also noted that a tongue of high CO appears to be
caused by nocturnal drainage flow that follows the Las Vegas Wash. The
study noted that, away from the urban core and effects from transport in the
drainage flows, CO levels are relatively low (Ranse! 2002~.
Denver, Colorado
The city of Denver has a population of 501,700 (recorded in 2000) and
is located in the South Platte River Valley, approximately 1 mi above sea
level. To the west of Denver is the Front Range of the Central Rocky
Mountains, with peaks above 14,000 feet. The Cheyenne Ridge (about 70
mi to the north) and the Palmer Divide (about 25 mi to the south) run east
to west and are 1,000-2,000 feet above the plain; they combine with the
Front Range to form a three-sided basin in which the city sits. Denver is
a major national rail center, and two major interstate highways cross the
city. This growing community had hundreds of CO exceedances in the
1970s and l980s. Since 1995, there have been only two. The decline is a
result of local controls (including wood-burning bans), technological im-
provements in motor vehicles and wood burning stoves, and favorable
winter weather patterns that have permitted better ventilation of pollutants.
However, the city remains vulnerable to future CO exceedances because of
its steady population growth.
The meteorological factors that contribute to elevated CO concentra-
tions in Denver include: persistent light winds at the surface, a ground-
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Contributions of Topography, Meteorology, and [Iuman Activity 99
level inversion, a lee trough along the foothills, snow cover, and warm air
advection aloft (Neff and King 1991; King 1991; Neff 2001; Reddy 20011.
Reddy (2001) associated elevated CO concentrations with winds at less
than 1 MPH and an effective mixed layer less than 25 to 50 m that lasts for
at least 3 hours. Neff (2001) also noted microclimatological factors in-
volved in producing exceedances at the most problematic CO monitor. He
noted that extensive shadowing by downtown buildings in the afternoon
may exacerbate the trapping of pollutants in the area surrounding the moni-
tor by prolonging the cooling ofthe surface during the winter (thus intensi-
fying the ground-level inversion) and by blocking winds that could disperse
pollutants.
Representative terms from entire chapter:
exceedance days
