United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-011 May 1983
SERA Project Summary
Studies in Air Quality
Meteorology at North Carolina
State University
Gerald F. Watson, Allen J. Riordan, Walter J. Saucier, and Ted L Tsui
This report summarizes eight studies
in diverse areas of air quality meteor-
ology resulting from a cooperative re-
search program involving the graduate
students and faculty of the atmospheric
sciences program of North Carolina
State University and the staff and facil-
ities of the EPA Meteorology and Assess-
ment Division.
Meteorological analysis of the St.
Louis RAPS data has shown that: (1)
The urban heat island strength during
the day amounts to about 0.5°C and
varies little with wind speed, cloud
cover, or season. The nocturnal heat
island, however, is highly responsive to
all three factors as well as to anthro-
pogenic heat sources. (2) Profiles of
ozone above the urban nocturnal sur-
face inversion are highly variable and
apparently related to details of the wind
structure. (3) Evaluation of the energy
budget over concrete, blacktop, and
soil surfaces suggests improvements in
the parameterization of the surface heat
flux in air quality models.
Studies of atmospheric visibility and
suspended particulates reveal that: (1)
Air transparency nation-wide has de-
clined significantly between 1955 and
1972. The annual cycle in visibility is
inversely related to that in relative
humidity. (2) The relative contributions
of haze, smoke, and dust to lowering
visibilities in eastern Texas between
1949 and 1968 are determined. (3)
Most sulfate concentration variability
at fixed sites is due to the vagaries in
long-range transport rather than to local
conditions of temperature, humidity, or
insolation.
Studies of the boundary layer meso-
scale wind structure over the Appala-
chian Mountains indicate that: (1) A pro-
nounced low-level jet with significant
diurnal variability can form even during
a period of air stagnation. (2) The stan-
dard 850 mb-level winds can be used to
estimate surface winds in complex
terrain given the valley-ridge orienta-
tion and time of day.
This project summary was developed
by EPA's Environmental Sciences Re-
search Laboratory. Research Triangle
Park, NC. to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
The proximity of the atmospheric sci-
ences program at North Carolina State
University (NCSU) to the Meteorology and
Assessment Division and other EPA facil-
ities has engendered over the past several
years professional and scientific coopera-
tion among personnel of the two organiza-
tions. NCSU graduate students have bene-
fitted through work at the EPA laboratory
and helpful guidance in thesis research
by EPA staff. It was proposed that this
on-going cooperative research effort be
enhanced through a formal program
which focused the joint effort of NCSU
students and faculty, and EPA staff,
toward solving specific problems of the
EPA in air quality meteorology.
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This report contains results of this
cooperative effort in the form of sum-
maries of eight research theses on diverse
topics. Three investigations are con-
cerned broadly with the meteorological
analysis of the St. Louis Regional Air
Pollution Study (RAPS) data. The topics
include the climatology of the St. Louis
heat island, the problem of ozone vari-
ability in the urban boundary layer, and
the analysis of the urban surface-energy
budget in an effort to improve parameter-
ization of the surface heat fluxes in air
quality models.
Three more investigations examine the
impact of suspended particulates as
manifest in the generally deteriorating
transparency of the air, both regionally
and nationally.
The final two investigations discuss
aspects of wind structure over the Appa-
lachian Mountains The implications of
this wind structure to pollutant transport
and dispersal in complex terrain is also
considered.
Research Investigations
Climatological Aspects of the
St. Louis Urban Heat Island
Joseph E. Steigerwald, G. F.
Watson, and J. K. S. Ching
The St. Louis urban heat island is
studied using one year of temperature
data (1976). Fluctuation in the strength of
the urban heat island due to changes in
cloud cover, wind speed, and season are
explored.
The heat island strength varies diur-
nally and seasonally and is largest on fall
evenings and weakest on summer morn-
ings. It is also very strong under low wind
speed and clear sky conditions, with the
wind speed exerting more control on the
nocturnal heat island strength than cloud
cover. The daytime heat island, the noc-
turnal heat island, and the transition
periods are examined in detail. The
nocturnal heat island and the transition
periods vary greatly with changes in cloud
cover, wind speed, and season; the day-
time heat island varies little with changes
in these factors.
Summers' (1965) heat island model
was used to evaluate the contribution of
the anthropogenic heat output to the
urban heat island. The results were
compared with results derived from an
analysis of the St. Louis Heat Emission
Inventory. The emission data comparison
was inconclusive.
A Study of the Vertical
Distribution of Ozone and the
Variability of the Wind Field
Above a Nocturnal Radiation
Inversion
Donald R. Hood and A. J. Riordan
The vertical distribution of ozone prior
to the morning breakdown of the radiation
inversion is related to the changes in the
nocturnal wind field and the trajectory of
the layer containing the ozone. Helicopter
and hourly pilot-balloon data collected on
five days during August 1976, in St.
Louis, Missouri, were analyzed.
The ozone was assumed to be uniformly
mixed throughout the boundary layer at
sunset the previous evening. The study
indicates that the similarities and differ-
ences between the vertical distribution of
ozone assumed at sunset and observed
the following morning are related to the
variability of the nocturnal wind field.
Analysis of height-time cross sections
of the nocturnal wind field indicated that
the winds exhibited both vertical and
temporal variability above an observation
point. Most of the temporal variability in
the nocturnal wind field occurs between
scheduled radiosonde launch times and
will not be completely detected. There-
fore, the trajectory computations will not
entirely represent the changes which
occur. The study also indicates that due to
the vertical variability of the wind field,
more representative trajectories can be
obtained by computing the trajectories for
several thin layers versus computing a
single trajectory for a thick layer.
Temporal Variation in the
Surface Energy Budget
Components for Three
Land Use Patterns
Dennis C. Doll, G. F. Watson, and
J. K. S. Ching
Special St. Louis RAPS data were used
to examine the diurnal variation of the net
radiation and heat flux components of the
surface energy budget for a two-month
spring time period for blacktop, concrete,
and soil surfaces. Sensible and latent
heat fluxes were analyzed from measured
and calculated net radiation and ground
heat fluxes for each surface.
Ground heat fluxfor concrete and black-
top was determined for different water
contents of the underlying soil layer. A
lower water content in the soil layer
reduces the magnitude of the total ground
flux throughout the blacktop and concrete
layer and soil sublayer. An accurate
determination of soil water content ap-
pears necessary in any ground heat flux
evaluation.
The sum of sensible and latent fluxes
for the soil surface was found to be nearly
twice as large as that for the blacktop
layer and approximately three times larger
than for the concrete surface during the
daytime period of the diurnal cycle. Latent
heating appears to be an important
component of the surface energy balance
for the soil.
Hourly average net radiation and
ground heat flux values resulted in an
improvement of a parameterization tech-
nique based on the ground heat flux to net
radiation ratio (G/Fn). Diurnal variation
in G/Fn for each surface is distinctly
different. For concrete and blacktop, G/Fn
is constant at night but variable during
the day. For soil, G/Fn changes through-
out the diurnal period.
Trends in Atmospheric Visibility
Across the United States from
1955 to 1972
Jerrold S. Foster, T. L. Tsui, and G.
C. Holzworth
Regular daytime synoptic observations
from 14 stations across the United States
but excluding those with relative humidity
greater than or equal to 90% and those
with precipitation or fog present, were
used to determine the trend in visibility
from 1955 to 1972.
Comparison of statistics at the begin-
ning and the end of the period, and linear
regression of the monthly mean visibil-
ities were used to discover relationships
between visibility and other meteorologi-
cal variables such as wind speed, wind
direction, and relative humidity. Spectrum
and cross-spectrum analysis of the time
series of visibility and relative humidity
were also performed.
The results of this study indicate: (1) a
deterioration of the visibility at all 14
stations, (2) a bimodal (six and twelve
month) oscillation in the annual cycle of
visibility, and (3) an inverse correlation of
relative humidity with the annual cycle of
visibility.
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A Visibility Study in the
Eastern Half of Texas from
1949 to 1968
Alfonse J. Mazurowski, A. J.
Riordan, and G. C. Holzworth
Hourly weather observations for times
0900-1500 CST were used to examine
visibility trends, obstructions to vision,
and the effects of different parameters on
visibility for eight stations in the eastern
half of Texas from 1949 to 1968.
Observations with any obstruction to
vision, along with the individual obstruc-
tions by haze, smoke, and dust, were
examined to determine their effect on
visibility. Mean visibility categorized by
wind direction was employed to investi-
gate the influence of winds from industrial
areas and cities on visibility at airports.
Trend analyses of mean visibility data,
both annual and seasonal, were per-
formed using mean ridits and shifts in
frequency from one visibility range to
another. The effects on visibility of popula-
tion growth and distance of stations from
the Gulf of Mexico were also analyzed.
The main results of this study are: (1) a
deteriorating trend in annual visibilities
was apparent for seven of the eight
stations, (2) smoke was a major factor in
the reduction of visibility in Houston, (3)
smoke appeared to be a result of local
transport of air polution while haze and
dust seem to be associated with long-
range transport processes, (4) daily resul-
tant winds from nearby industrial areas
produced the lowest mean visibilities, (5)
mean visibility increased significantly as
the distance of stations from the Gulf of
Mexico increased.
Synoptic-Scale Variability in
Atmospheric Suspended
Sulfate Concentrations
Brian W. Galusha, G. F. Watson,
and G. C. Holzworth
The spatial variation in atmospheric
suspended sulfate concentrations is stud-
ied for evidence of meteorologically linked
sulfate transformation and transporta-
tion. Low frequency (every 12-14 days)
data from 41 National Air Surveillance
Network stations as well as higher fre-
quency (every 2-3 days) data available
from two special studies are examined.
Variations in high frequency data with
wind direction for St. Louis, Missouri, are
ompared to local and regional sources of
precursor pollutants for two one-year
periods; January-December 1969 and
April 1975-March 1976. Spatial varia-
tions in low frequency sulfate concentra-
tions during the spring season (March-
May) for the six-year period 1969-1974
are compared with synoptic weather and
wind circulation patterns. These compari-
sons indicate that regional and local
sulfate transport can largely account for
large-scale sulfate variations. Sulfate
transformation due to humidity, tempera-
ture, and sunlight intensity do not ade-
quately explain observed variations, but
may be of secondary importance.
The existence of regionally high con-
centrations of sulfate in the northeastern
United States and of a general summer-
time peak in sulfate values is confirmed.
Diurnal Variation of Wind
Profiles Across Mountainous
Terrain During an Air
Stagnation Period
Julius A. Jackson, W. J. Saucier,
and W. D. Bach
The diurnal variation of wind profiles
across mountainous terrain during an air
stagnation period was evaluated for seven
days in the summer of 1957. The study
was conducted across the north-central
Appalachian Mountains, an area of heavy
pollution concentration. The study was
divided into easterly (16-18 July, 1957)
and westerly (19-22 July, 1957) flow
across the mountains.
Examination over the seven days
showed a diurnal variation in boundary
layer winds on the eastern side of the
mountain range with a maximum ampli-
tude of about 3 to 4 m sec"1 at 1000-1500
m MSL in both the easterly and westerly
flows. On the western side of the moun-
tain range, a diurnal variation with a
maximum amplitude of about 4m sec~1 at
600-1100 m MSL occurred in both flows.
This oscillation in the lower levels
showed the presence of a low-level jet,
which was unexpected in that this study
was conducted during an air stagnation
period. The low-level jet in the easterly
flow across the mountains reaches a
maximum wind speed at approximately
0600 GMT at about 300 m above ground
level. In the westerly flow, the low-level
jet occurs at approximately 1200 GMT at
600-800 m above the ground. This low-
level jet is due to an inertial-type oscilla-
tion driven by the diurnal variation of the
frictional forces aided by thermal forcing.
A Mesoscale Analysis of Air
Flow in Complex Terrain
Christofer Maxwell, W. J. Saucier,
T. L. Tsui, and G. C. Holzworth
Analyses of wind data are presented
which examine the coupling of the surface
and 850mb wind directions in a region of
complex terrain. The data were grouped
based on periods when the 850mb wind
directions were parallel or perpendicular
to the major valley axis. The diurnal and
pressure gradient influences were also
examined.
The analyses show that the surface and
850mb wind directions agree best when
the 850mb winds are about parallel to the
valley axis. The agreements are best
during daytime periods when the 850mb
wind speeds are strong. In addition, wind
measurement sites in valleys agreed
better with the 850mb winds than did
sites located on ridges. When the 850mb
winds were perpendicular to the valley
axis, the wind measurement sites on
ridges tended to agree better with the
850mb winds than did the valley sites.
Further analyses show that 30m tower
wind sensors can be discriminately used
to predict the direction of power plant
plume travel in complex terrain regions.
The tower sensors are unreliable during
nighttime hours and are best during day-
time hours when the 850mb winds are
parallel to the valley axis.
Conclusions
Conclusions based on the eight re-
search theses summarized m this report
and worthy of special note are listed here
by major topic.
Meteorological analysis of the St. Louis
RAPS data:
1. The urban heat island strength during
the day amounts to about 0.5°C and
varies little with wind speed, cloud
cover, or season. The nocturnal heat
island, however, is highly responsive
to all three factors, as well as to
anthropogenic heat sources.
2. Variability in ozone profiles above the
urban nocturnal surface inversion is
significant and appears to be related
to a similar variability in wind profiles.
The relevant wind variability occurs
on space and time scales unresolv-
able by the conventional radiosonde
network.
3. Evaluation of the ratio of subsurface
heat flux to net radiation over con-
crete, blacktop, and soil surfaces has
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shown that this ratio can be better
parameterized in air quality models
by incorporating its dependence on
surface type and on day versus night
conditions
Atmospheric visibility and suspended
participates
4. Visibility at 14 sites throughout the
United States has declined signifi-
cantly between 1955 and 1972. The
annual cycle in atmospheric transpar-
ency is inversely related to that in
relative humidity.
5 The relative contribution of haze,
smoke, and dust to decreasing visibil-
ities at several sites in eastern Texas
during the period 1949 to 1968 are
identified. The haze contribution is
part of a regional-scale problem,
while the smoke contribution is asso-
ciated with local sources and the
wind direction.
6. Most sulfate concentration variability
at particular sites appears explained
by long-range transport as dependent
on the wind pattern, rather than by
local conditions of temperature, hu-
midity, or insolation
Boundary layer wind structure in complex
terrain:
7. The unexpected occurrence of a
nocturnal low-level jet over the north-
central Appalachian Mountains dur-
ing a period of air stagnation has
significant implications for regional
pollutant transport.
8 Winds at the 850mb level can be
useful in describing near-surface
winds in mountainous terrain de-
pending on valley-ridge orientation
and time of day. This relationship
provides a simple parameterization
of winds in complex terrain for anti-
cipating pollutant dispersal
Recommendations
Results of the work described in this
report should, in many cases, be con-
sidered preliminary. Specific recommen-
dations for further research work include:
1. Use of the full St. Louis RAPS data-
set (1974-77) to strengthen the con-
clusions of the study of the urban
heat island based on 1976 data. In
particular, the role of rural soil mois-
ture and of urban anthropogenic heat
sources to the diurnal and seasonal
heat island strength need further
investigation.
2. Use of the full RAPS data-set to
extend the two-month study period
of energy budget evaluations for
various types of surfaces. The ulti-
mate goal of formulating an improved
parameterization of the surface heat
flux in air quality models which
properly accounts for seasonal vari-
ability and the urban environment
seems achievable with an extension
of this study.
Further regional assessments of
trends in atmospheric visibility, like
that in eastern Texas, is one means
of detecting the environmental im-
pact of mass migrations of the U.S.
populous as, for example, the appar-
ent shift in population from the north-
east to the "Sun Belt" states.
Further study of suspended sulfates
and their relationship to source areas
and meteorological variables. How-
ever, major revelations would only
seem possible with increased samp-
ling resolution over that provided by
the present National Air Surveillance
Network, as well as improved means
of paniculate collection and chemical
analysis.
Additional studies of the mesoscale
boundary layer wind structure in
complex terrain with emphasis or
consequent pollutant transport anc
dispersal over such regions.
G. F. Watson, A. J, Riordan, W. J. Saucier, and T. L Tsui are with North Carolina
State University, Raleigh, NC 27650.
Jason Ching is the EPA Project Officer (see below).
The complete report, entitled "Studies in Air Quality Meteorology at North
Carolina State University," (Order No. PB 83-181 743; Cost: $ 13.00, subject to
change J will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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