United States
Environmental Protection
Agency
Research Triangle Park, NC 27711
Research and Development EPA/600/SR-99/047
March 1999
EPA Project Summary
Lower Rio Grande Valley Transboundary Air
Pollution Project (TAPP)
Shaibal Mukerjee, Douglas S. Shadwick, Kirk E. Dean, Linda Y.
Carmichael, Jon J. Bowser, and Larry J. Purdue
The Lower Rio Grande Valley
Transboundary AirPollution Pro-
ject (TAPP) was conducted by
the U.S. Environmental Protec-
tion Agency (EPA), with the
Texas Natural Resource Conser-
vation Commission (TNRCC), to
determine if transboundary
transport of air pollutants was
occurring in the Lower Rio
Grande Valley (hereinafter called
"the Valley") and, if so, the
extent. Monitoring was
conducted for one year at three
fixed sites very close to the U.S.-
Mexican border. Inhalable
particles, chemical elements,
volatile organic compounds
(VOCs), polycyclic aromatic
hydrocarbons (PAHs), and
pesticides were measured. At
each site data was also collected
on short-term variations in the
concentration of fine inhalable
particles to assess episodic
emissions that may have
crossed the border.
Meteorological measurements
were also performed.
Overall air quality in the
Brownsville area of the Valley
was good; the vast majority of
air pollutants were lower than or
comparable to reference values
as well as to monitoring data co-
llected in other areas.
Transboundary transport of air
pollution plumes did not appear
to cause noticeable deterioration
of air quality. During the study
period, the dominance of sea
breezes from the Gulf of Mexico
was largely responsible for clean
air conditions in the Brownsville
area of the Valley. A full
assessment of possible impacts
from transboundary air pollution
was limited since air monitoring
was restricted to the U.S. side of
the border and emissions from
air pollution sources were not
measured. Still, the TAPP
establishes a baseline for future
air monitoring studies in the
Valley.
This Project Summary was
developed by EPA to announce
key findings of the research
project that is fully documented
in a separate report of the same
title (see Project Report ordering
information at back).
Introduction (Background)
In 1996, the U.S.-Mexico
Border XXI Program was
developed as a five-year
binational plan to address
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transboundary environmental
problems along the border. One
such project under the
Environmental Health Workgroup
of the Border XXI Program was
known as the Lower Rio Grande
Valley Transboundary Air
Pollution Project (TAPP). The air
sampling protocol was based on
results and analyses from the
Lower Rio Grande Valley Environ
mental Scoping Study
(LRGVESS) presented as a
community report and published
in a Special Issue of a scientific
journal, Environment International
(Volume 23, Number 5, 1997).
Among its multi-media and
residential sampling efforts, one
component in LRGVESS was air
sampling at a fixed site near the
border with Mexico. A lesson
learned from that study was that
more information was needed
concerning exposure to air
contaminants from cross-border
activities. To that end, the TAPP
was developed to determine whe-
ther or not transboundary
transport of air pollution occurs in
the Valley and, if so, to what
extent. The study involved air
sampling for one year in three
locations where transboundary air
pollution could be measured if it
were occurring. The sites were in
or near the Valley city of Browns-
ville, Texas. Air measurements
and meteorological data similar to
those collected in the LRGVESS
were acquired at each TAPP site.
At each site, data were also
obtained on short-term variations
in the concentration of fine inha-
lable particles to assess episodic
emissions. Precipitation
measurements were also
performed.
The data were compared to
TNRCC Effects Screening Levels
(ESLs) and to data collected in
other areas to assess general air
pollution impacts in and near
Brownsville. ESLs have been
defined as comparison values for
constituents in air and are not
ambient air standards. In addition
to other analyses detailed in the
Project Report, wind sector
analyses and chemical tracer
analyses were performed. These
were done to determine the
potential extent of transboundary
air transport of pollutants during
the sampling period and to
identify possible transboundary air
pollution sources.
Procedure (How the
Information Was Collected)
Figure 1 shows the location of
the air sampling sites. Although
the sites were primarily impacted
by nearby sources, their proximity
to the border provided an
opportunity to determine potential
transboundary transport of air
pollutants.
Air sampling included the
following: 1) mass of fine inhalable
particles less than 2.5
micrometers abbreviated as PM25
(a micrometer is one-thousandth
of a millimeter), 2) mass of coarse
inhalable particles of 2.5 to 10
micrometers (PM25_10), 3)
chemical elements in PM25 and
PM25_10, 4) particulate carbon, 5)
volatile organic compounds
(VOCs), 6) polycyclic aromatic
hydrocarbons (PAHs), and 7)
pesticides. Meteorological
parameters were also measured
at each site. Particles were me-
asured since they can be
produced by many sources such
as garbage incineration, road
construction, agricultural burning
and fertilizer/pesticide
applications, industrial operations,
and diesel fuel burning from truc-
ks. PM2 5 was measured because
these particles have a greater
potential to be lodged in the lung
than particles with a larger
diameter. VOCs were measured
since they are associated with
gasoline and other petroleum pro-
ducts, industrial chemicals, and
even plants. PAHs and carbon
were measured because they are
found in soot and emissions
associated with combustion
activities. Since farming is a
major activity in the Valley,
pesticides were also measured.
Precipitation (rainfall) me-
asurements for metals, PAHs,
and pesticides were also done.
While most of the air pollutants
were sampled for a 24-hour
period day-to-day, additional
sampling for fine inhalable
particles was done on a real-time
basis using 1-hour averages;
these measurements were
performed to determine short-
term variations in emission
impacts associated with air
pollution episodes or other
incidents.
Results and Discussion
(Summary and Possible
Explanations)
Overall, air pollution levels of
chemicals measured in the TAPP
were not unusually high nor of a
persistent nature (Table 1). This
indicated that air quality in the
Brownsville area of the Valley was
good. As discussed in the Project
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Report, concentrations for the
majority of air and precipitation
pollutants were similar to or lower
than data from samples collected
in other areas. Air pollutants
having higher concentrations than
data sampled elsewhere were
primarily VOCs from automobile
and gasoline pollution.
Interpretation of the results
was based upon how the
information collected related to
comparison values and/or
information collected in other
areas. As done in the LRGVESS,
the TAPP results were compared
with Effects Screening Levels
(ESLs) developed by the TNRCC.
ESLs are based on health effects
data, odor nuisance potential,
vegetation effects, or corrosion
effects. ESLs are used for
screening purposes and are not
regulatory standards. The vast
majority of air pollutants
measured did not exceed the
ESL; hence, adverse effects were
not expected. It should be noted
that an air pollutant level below
the ESL is based on
measurements during the
monitoring period and does not
mean that air pollution conditions
in the future will remain the same.
In a few random instances, one or
a couple of the pollutant
measurements met or slightly ex-
ceeded a reference value.
Besides ESLs, other comparative
data from exposure monitoring
studies were used. These are
also presented in Table 1;
additional comparative data are
presented in this study's project
report. Of the more than 250
pollutants measured, only seven
air pollutants had levels above the
ESLs (Table 1). Of the
approximately 2600 particle and
VOC samples taken in this study,
silver, 2-nitropropane, benzene,
methylene chloride, and vinyl
acetate exceeded the ESL only
once and are not expected to
result in any long-term adverse
effects. While acrolein and
methanol exceeded the ESL
more than once, the collection of
these pollutants using devices in
this study is difficult. Hence,
caution should be exercised
when comparing this data to the
ESLs. If an air pollutant
exceeded the ESL, it does not
necessarily mean there is a
problem, but rather is an
indication that further review is
warranted. Further review may
include additional sampling or
consideration of ambient levels in
the environment. As with all
comparison values, ESLs
undergo periodic review and
revision to insure that they are
based on current scientific
literature.
Based on previous air
pollution research, certain air
pollutants can be considered as
tracers for specific sources. This
knowledge was applied in this
study. The total particle chlorine
levels encountered were probably
associated with sea salts; the
dominance of sea influences from
the Gulf of Mexico was seen in
much of the data. Elevated sulfur
loadings were probably sulfates
emitted by the nearby Gulf of
Mexico, Laguna Madre and
possible coal combustion
sources.
Many of the elevated VOCs
(benzene, methanol, 2-nitr-
opropane, and methylene
chloride) are found in solvents or
can be emitted into the air by
many sources (for example,
benzene is also found in
automobile emissions). Since me-
asurements of emissions from
sources on both sides of the
border were not done, it was not
possible to identify a specific
source for VOCs. The inability to
measure air quality on the
Mexican side and to measure
actual emissions from all major
sources in the Valley, such as
vehicles and industry, limited the
ability to fully assess possible
transboundary air pollution
impacts.
The highest levels of silver
occurred at Site 1; the highest
levels of methylene chloride, and
vinyl acetate were at Site 2. The
maximum values for these
pollutants came from the
southeast direction. It is possible
that these maximum levels came
from man-made sources in Me-
xico or more immediate sources
in the U.S.; it is also possible that
higher wind velocities from this
direction may have affected air
pollutant levels. An additional
reason could have been due to a
greater chance of detecting a
random event (i.e., an emission)
from the southeast since winds
predominate from that direction.
Daily PM25 data were highest
from the southeast; approximately
50 percent of daily winds also
came from that direction. This
wind direction pattern was typical
for almost all of the fine and
coarse particle elements and
most of the VOC data.
Many of the detected
chemicals could have come from
either side of the border. For
example, of the five highest
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methanol values, three came from
the south and two came from the
north. The three high levels from
the south could have come from
Mexico; the two high levels from
the north could have come from
the U.S. One exceedance of the
ESL for methylene chloride
occurred from the southeast.
While transboundary influences
occurring at Site 2 from either
side of the border may be initially
deduced from these observations,
it should be noted that this site
was one block west-northwest of
a propane/butane filling station
and this may have influenced
some of the VOC data. The
maximum values for benzene (at
Site 1) and 2-nitropropane (at Site
2) came from the north.
The EPA has recently made
PM9
a criteria air pollutant.
Criteria air pollutants are a group
of very common air pollutants
regulated by EPA on the basis of
health effects. Although PM25
data from this study cannot be
directly compared with the revised
National Ambient Air Quality
Standard (NAAQS) for PM25
because of method and time-
span differences, the annual
average of the daily PM25 data
can still be used as an indication
of this comparison. Briefly, the
revised NAAQS for PM2 5 mass is
the three-year average of annual
average PM25, spatially averaged
across an area (not yet defined).
According to the EPA, the level of
the three-year spatially averaged
value should not exceed 15
ug/m3. For the three sites the
highest average of daily PM25
values was 10.37 ug/m3 (shown
as 10370 ng/m3 in Table 1) at Site
1. Thus, the annual means
calculated for individual TAPP
sites were at most only two-thirds
of the NAAQS limit for PM2 5. This
indicates that the Valley air,
during the study, was under the
level of concern for PM2 5.
Hourly averages of PM25
measured on a real-time basis
indicated highest levels coming
from the south and southeast. As
suggested for the daily PM2 5 data,
it is possible that these maximum
levels were the result of man-
made transboundary influences
as well as other factors such as
prevailing wind patterns
discussed earlier. Local pollution
events on certain days identified
by site operators were examined
with the hourly data. Although
emission events may have
occurred, it was found that these
identified emission events had a
minimal influence at the sites
since the levels and patterns of
hourly PM2 5 data at all sites were
similar. The similar daily pattern
at all three sites indicated that
regional influences, such as dusts
or automobile traffic may have
been dominant factors influencing
short-term pollution levels.
Conclusions and
Recommendations
1. Overall air quality in the
Brownsville area of the
Valley was good when
compared to other data.
Transboundary transport
of air pollution did not
appear to cause
noticeable deterioration
of air quality in the Valley.
2. The dominance of winds
from the Gulf of Mexico
was largely responsible
for the clean air
conditions in the
Brownsville air shed.
3. The vast majority of air
pollutants were generally
lower or comparable to
ESLs and monitoring data
(shown in Project Report)
at other urban and
agricultural rural areas in
Texas and elsewhere.
The few observations of
pollutants exceeding their
ESLs appeared to be
more the result of
randomness in the data
and/or local events than
regional phenomena or
transboundary plumes.
4. Short-term observations
may have been influe-
nced by transboundary
transport of air pollution
from the south and sout-
heast. However,
predominant wind flows
came from the southeast
and may have resulted in
a greater opportunity for
pollutants to come from
these directions.
Emission sources in the
immediate location of the
sites could have also in-
fluenced short-term
observations. Without the
ability to monitor air
quality on the Mexican
side and the inability to
measure emissions from
air pollution sources, it is
difficult to ascertain the
extent of this influence.
5. Any assessment of total
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exposure from air
pollution in the Valley
would require a multi-
media monitoring program
similar to the LRGVESS.
The TAPP provides a
baseline to assess future
air quality conditions in the
Valley. A multi-year
monitoring effort of air
quality and emission
sources is necessary to
assess trends in air qual-
ity. As shown in Figure 1,
TNRCC is continuing to
monitor air quality at Site 1
and other sites in the
Valley to determine such
trends.
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ShaibalMukerjee with U.S. Environmental Protection Agency (MD-
47), Research Triangle Park, NC 27711; Douglas S. Shadwickwith
ManTech Environmental Technology, Inc., Research Triangle Park,
NC 27709; K.E. Dean with Texas Natural Resource Conservation
Commission, Austin, TX 78711-3087; Linda Y. Carmichael, JonJ.
Bowser, and Larry J. Purdue with QST Environmental, Durham, NC
27713
Shaibal Mukerjee is the EPA Work Assignment Manager (see below)
The complete report entitled "Lower Rio Grande Valley
Transboundary Air Pollution Project (TAPP) "
(Order No.: PB99-146938; Cost: $51.00 subject to change) is
available from:
National Technical Information Service
Springfield, VA 22161
Telephone: 703-605-6000
The EPA Work Assignment Manager can be contacted at:
National Exposure Research Laboratory (MD-47)
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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