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United States Research Triangle Park, NC 27711

Environmental Protection

Agency

Research and Development EPA/600/S-99/004 March 1999

Community Summary

Lower Rio Grande Valley Transboundary Air

Pollution Project (TAPP)

Shaibal Mukerjee, Douglas S. Shadwick, Kirk E. Dean, Linda Y.

Carmichael, Jon J. Bowser, Larry J. Purdue, and V. Ross Highsmith

The Lower Rio Grande Valley

Transboundary Air Pollution Project (TAPP) was done to determine if movement of air pollutants across the U.S.-Mexican

border was occurring in the

Lower Rio Grande Valley (hereinafter called "the Valley") and,

if so, the extent. The study was

conducted from March 1996 to

March 1997 by the U.S. Environmental Protection Agency (EPA)

with the Texas Natural Resource

Conservation Commission

(TNRCC). The study objectives

were to:

a) Determine the air quality

within the Valley and establish a

baseline of air quality data for

future reference.

b) Determine the movement of air

pollutants across the U.S.-Mexican border.

c) Define what, if any, additional

studies might be needed.

Monitoring was done at three

fixed sites located very close to

the U.S.-Mexican border.

Inhalable particulate matter (PM),

chemical elements, volatile organic compounds (VOCs),

polycyclic aromatic hydrocarbons (PAHs), and pesticides

were measured. Short-term fine

inhalable particle sampling was

conducted to estimate potential

episodes of emissions. Meteorological measurements (weather

data) were also collected.

Based on the daily and near

real-time monitoring conducted

over the year, the study data

suggest that the air quality in the

Brownsville area of the Valley

was good. The reason for this

was that the vast majority of air

pollutants were lower than or

comparable to published reference values and data collected

in other areas. During the study

period, movement of air pollution

plumes across the border did not

appear to cause noticeable deterioration of air quality. The dominance of sea breezes from the

Gulf of Mexico was largely responsible for the relatively clean

air conditions observed. A full

assessment of possible crossborder movement of air pollution
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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 Community 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 back for

how to obtain additional information).

Introduction 

In 1996, the U.S.-Mexico Border XXI Program was developed

as a five-year binational plan to

address 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 monitoring approach used was based on

findings from the Lower Rio

Grande Valley Environmental

Scoping Study (LRGVESS). The

LRGVESS results were

presented earlier as a community

report and also published in a

Special Issue of a scientific journal, Environment International

(Volume 23, Number 5, 1997).

One component of the LRGVESS

was ambient (outdoor) air sampling at a fixed monitoring site

near the U.S. border with Mexico.

A lesson learned from the

LRGVESS was that more information was needed concerning

potential exposures to air contaminants from cross-border activities. The TAPP was developed to determine whether or not

movement of air pollution across

the border occurs in the Valley

and, if so, to what extent.

The study involved ambient

air sampling for one year at three

locations in or near the Valley city

of Brownsville, Texas. Air

measurements and meteorological data similar to those collected

in the LRGVESS were acquired at

each TAPP site. Fine inhalable

particle concentration data were

collected at each site on shortterm intervals to assess any potential episodic emissions. Precipitation measurements were also

performed.

The resulting data were compared to EPA standards such as

the U.S. National Ambient Air

Quality Standards (NAAQS), as

well as comparison values for air

pollutants developed by TNRCC

known as Effects Screening Levels (ESLs). Comparisons were

also made with data collected in

other areas. Wind sector analyses and chemical tracer analyses

were performed to determine the

potential extent of the movement

of air pollutants during the sampling period.

Procedure

Figure 1 shows the location of

the fixed ambient air sampling

sites. The proximity of these sites

to the border provided an opportunity to determine potential movement of air pollutants across the

border. Sources close to each

site were also noted.

Air sampling included: 1) the

mass of fine inhalable particles,

particles less than 2.5 micrometers in diameter (abbreviated as

PM2.5; a micrometer is one-thousandth of a millimeter or one

millionth of a meter), 2) the mass

of coarse inhalable particles, particles with diameters ranging from

2.5 to 10 micrometers (PM2.5-10), 3)

the chemical elements in PM2.5

and PM2.5-10, 4) elemental carbon,

5) volatile organic compounds

(VOCs), 6) polycyclic aromatic

hydrocarbons (PAHs), and 7)

pesticides. Meteorological parameters were also measured at

each site.

Particles were measured

since they can be produced by

many sources such as wind blown

dust, garbage incineration, road

construction, agricultural practices

including tilling, burning and fertilizer/pesticide applications, industrial operations, and burning of

diesel fuel and gasoline in trucks

and motor vehicles. PM2.5 was

measured because these particles have a greater potential for

deeper penetration into the lung

than the larger particles. VOCs

were measured since they are

associated with gasoline and

other petroleum products, industrial and household chemicals,

and even plants. PAHs and carbon (and VOCS) 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) measurements were also performed.

Most of the air pollutants were

sampled for 24 hours each day.

Additional continuous sampling

was conducted for inhalable fine

particles on a real-time basis.

The resulting 1-hour averages

were examined to determine

short-term variations in particle

emissions associated with any air

pollution episodes or other documented activities.

Results and Discussion

Overall, the air pollution levels

measured in the TAPP were low

and air quality in the Valley was
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considered good. Concentrations

for the majority of the environmental measurements were similar to or lower than data from

samples collected in other areas

at the same time. Elevated concentrations were observed for

some of the VOCs, but these only

slightly elevated levels were most

likely associated with automotive

emissions in the Brownsville area.

The TAPP results were compared with a variety of environmental standards (ESLs, NAAQS,

etc.). Actual numerical data are

presented in the Project Summary and Project Report. The

vast majority of air pollutants

measured in this study were well

below these reference standards.

In a few random instances, one

or two of the pollutant

measurements met or slightly exceeded a reference value. These

higher values were, in general,

observed from a single sampling

period or a small number of

samples and are most likely associated with uncertainties due to

inherent variability of the data or

the presence of a local, shortterm source or activity. For example, of the nearly 2600 particle

and VOC measurements 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, we

would caution over-interpreting

these pollutants with their ESLs.

ESLs are used for screening

purposes and are not ambient air

standards. If measured air pollutant levels of a certain constituent

do not exceed its ESL, adverse

effects are not expected. 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. Conversely, 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 used as

“fingerprints” or tracers for specific

sources. This knowledge was

applied in this study. The high

coarse particle chloride levels are

most likely associated with the

presence of sea salts. A

dominant Gulf of Mexico influence

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

measured VOCs (benzene,

methanol, 2-nitropropane, and

methylene chloride) are commonly

found in solvents or emitted into

the air by many sources (for

example, benzene is also found in

automobile emissions). Since measurements of emissions from

sources on both sides of the

border were not performed, it was

not possible to identify a specific

source associated with these VOC

emissions. The inability to

measure air quality on the

Mexican side and to measure

actual emissions from all major

sources in the Valley limited the

ability to fully assess possible

transboundary air pollution

impacts.

The highest levels of PM2.5

and several environmental

species (silver, methylene

chloride, vinyl acetate, etc.) were

measured during periods when

the wind came from the southeast

direction. It is possible that these

maximum levels came from a

combination of sources such as

sources in Mexico or more

immediate sources in Texas. The

study data could not be used to

distinguish between these two

possible sources. The

predominance of daily winds from

the southeast and the higher wind

velocities from this direction

provided a greater chance of

detecting a single random event

(i.e., an emission) from these

areas.

Of the five highest methanol

values, three came from the south

and two came from the north. One

high value for methylene chloride

occurred from the southeast (at

Site 2). There are probable

sources for these VOCs in close

proximity to the monitoring site on

both sides of the border. The

maximum values for benzene (at

Site 1) and 2-nitro-propane (at

Site 2) came from the north.

The EPA has recently made

PM2.5 a criteria air pollutant.

Criteria air pollutants are a group

of very common air pollutants

regulated by EPA. The PM2.5 from

this study can only be indirectly

compared with the revised

National Ambient Air Quality

Standard (NAAQS) for PM2.5

because of differences in the

sampling methods. Briefly, the

revised NAAQS for PM2.5 mass is

the three-year average of annual

average PM2.5, spatially averaged
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across an area (not yet defined).

According to the EPA, the level of

the three-year spatially averaged

value should not exceed 15

μg/m3

. For the three sites the

highest average of daily PM2.5

values was 10.4 μg/m3 at Site 1.

Thus, the annual means

calculated for the individual TAPP

sites were well below the NAAQS

limit for PM2.5.

Hourly averages of PM2.5

indicated that the highest levels

were coming from the south and

southeast. The hourly and daily

PM2.5 data were compared and

examined with local pollution

events. It was found that these

identified emission events had

only minimal influence at the sites

since the levels and patterns of

hourly PM2.5 data at all three sites

were similar. The similar daily

pattern at all sites indicated that

regional influences, such as dusts

or automobile traffic may have

been the dominant factors

influencing short-term pollution

levels.

Conclusions

1. Overall air quality in the

Brownsville area of the

Valley was good. Any

movement of air pollution

across the border 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 Valley.

3. The vast majority of air

pollution levels were lower

or comparable to published standards and

monitoring data (shown in

Project Report) at other

urban and agricultural

rural areas in Texas and

elsewhere. The very few

observations of higher

pollutant concentrations

are attributed to

experimental error or to

random, local source

emissions and activity

rather than any regional

phenomena or persistent

movement of air pollution

plumes from the border.

4. Some high short-term

observations may have

been influenced by

cross-border movement

of air pollution from the

south and southeast.

Local emissions in the

immediate location of the

sites could have also influenced these higher

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

exposure from air

pollution in the Valley

would require a multimedia monitoring

program similar to the

LRGVESS.

6. The TAPP provides a

baseline to assess future

air quality conditions in

the Valley. While the

study data do not

support the immediate

need for monitoring, a

multi-year effort of air

pollution and emission

source monitoring is

necessary to assess

long-term trends in air

quality. TNRCC is

continuing to monitor air

quality at Site 1 and other

sites in the Valley to

determine such trends.
Shaibal Mukerjee and V. Ross Highsmith with U.S. Environmental

Protection Agency (MD-47), Research Triangle Park, NC 27711;

Douglas S. Shadwick with ManTech Environmental Technology, Inc.,

Research Triangle Park, NC 27709; Kirk E. Dean with Texas Natural

Resource Conservation Commission, Austin, TX 78711-3087; Linda Y.

Carmichael, Jon J. Bowser, and Larry J. Purdue with QST

Environmental, Durham, NC 27713

 The complete project report and project summary entitled "Lower Rio

Grande Valley Transboundary Air Pollution Project (TAPP)" is

available at:

National Exposure Research Laboratory (MD-47)

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711