Urban Air Pollution and Forests: Resources at Risk in the Mexico City Air Basin

411 Pages·2002·8.99 MB·English

by  Guadalupe de la Lanza Espino

Ecological Studies, Vol. 156
Analysis and Synthesis
Edited by
M.M. Caldwell, Logan, USA
G. Heldmaier, Marburg, Germany
O.L. Lange, Wtirzburg, Germany
H.A. Mooney, Stanford, USA
E.-D. Schulze, Jena, Germany
U. Sommer, Kiel, Germany
Ecological Studies
Volumes published since 1995 are listed at the end of this book.
Springer
New York
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Tokyo
Mark E. Fenn L.I. de Bauer
Tomas Hernandez-Tejeda
Editors
Urban Air Pollution
and Forests
Resources at Risk in the
Mexico City Air Basin
Foreword by Mario J. Molina
With 97 Illustrations, lOin Full Color
" Springer
Mark E. Fenn L.1. de Bauer
USDA Forest Service Especialidad de Agrometeorologfa
Pacific Southwest Research Station Instituto de Recursos N aturales
Riverside, CA 92507, USA Colegio de Postgraduados
[email protected] CP 56230 Montecillo, Mexico
[email protected] [email protected]
Tomâs Hernândez-Tejeda
SAGARPA
Instituto Nacional de Investigaciones
Forestales, Agrfcolas y Pecuarias (INIFAP)
Centro Nacional de Investigacion
Disciplinaria en Conservacion y
Mejoramiento de Ecosistemas Forestales
(CENID-COMEF)
CP 04110 Mexico City, Mexico
[email protected]
[email protected]
Cover illustration: Modified from Figure 6.2, showing, in the upper portion, the location of air-quality
monitoring stations in the different sectors of the Mexico City Metropolitan Area (RAMA, Red
Automâtica de Monitoreo Atmosferico or Automated Network for Atmospheric Monitoring), and the
main forested areas in the Basin. Courtesy of Alfonso Salas-Cruz and Ricardo Torres-Jardon, both of
the Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico.
Library of Congress Cataloging-in-Publication Data
Urban air pollution and forests: resources at risk in the Mexico City Air Basin / editors,
Mark E. Fenn, L.1. de Bauer, Tomâs Hernândez-Tejeda.
p. cm.-(Ecological studies; v. 156)
Includes bibliographical references.
ISBN 978-1-4684-9573-7 ISBN 978-0-387-22520-3 (eBook)
DOI 10.1007/978-0-387-22520-3
1. Trees-Etfect of air pollution on-Mexico-Mexico City Metropolitan Area. 2. Forest
ecology-Mexico-Mexico City Metropolitan Area. 3. Air-Pollution-Environmental
aspects-Mexico-Mexico City Metropolitan Area. 1. Fenn, Mark E. II. Bauer, Ma. de
Lourdes de la 1. de. III. Hernândez-Tejeda, Tomâs. IV. Series.
SB745 .U73 2002
634.9'6 I 9'097253-dc2 I 2001042989
Printed on acid-free paper.
© 2002 Springer-Verlag New York, lnc.
Softcover reprint ofthe hardcover 1s t edition 2002
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Foreword
At present, roughly half of the world's population lives in urban centers. There
are now more than 20 cities with a population of over 10 million inhabitants,
compared to less than 5 about 50 years ago. This tendency toward urbanization
is expected to continue, particularly in the developing world. A consequence
of this growing trend is that millions of people are being exposed to harmful
levels of urban air pollutants caused mainly by emissions from motor vehicles
and from industrial and domestic activities involving the combustion of fossil
fuels.
The driving force for the design and implementation of emission control strate
gies aimed at improving air quality has been the protection of the health of the
population in urban centers. There are, however, other consequences of the pres
ence of air pollutants besides the direct effect on human health. Reduced visibil
ity, damage to monuments and buildings, and many other such consequences
indirectly affect our quality of life. Another set of consequences involves damage
to ecological systems. In fact, the nature of "photochemical smog" was first
uncovered in the 1950s in connection with observations of its harmful effects on
crops and plants in the vicinity of Los Angeles.
The potential damage to ecological systems caused by air pollution is an issue
that is particularly worrisome if one considers the current growing population and
urbanization trends. The degradation of air quality is no longer just a local, urban
problem; it is beginning to acquire regional and even global proportions, and to
affect large portions of the Northern Hemisphere.
v
VI Foreword
The focus of this book is the impact of air pollution on forests in the Mexico
City Air Basin and its vicinity. The chapters are written by leading experts in the
field, and the book begins with a few introductory chapters on environmental and
sociological aspects of the air pollution problem in the Mexico City Air Basin.
Next are several chapters on the air pollution trends and control strategies and
the ecological effects. The book ends with two chapters summarizing the impacts
of multiple air pollutants on forests and an overall perspective on the problem.
This book provides an invaluable service, not only by calling attention to the
harmful effects of air pollution on ecological systems around Mexico City but
also by documenting the existing knowledge on the topic. The information
presented in the book should contribute to the overall assessment of the air-quality
problem and to the design of improved strategic control options to protect these
important natural resources.
It is a pleasure for me to write the Foreword, and I congratulate the editors and
the authors of the book for their important contributions.
Mario 1. Molina
Preface
Mexico City has been the cultural, social, economic, religious, and political center
of Mexico since pre-Hispanic times. However, the rapid increase in population
in this megacity during the latter half of the twentieth century has led to unusu
ally severe pressure on the limited natural resources of the Basin of Mexico. The
current population within the Basin of Mexico of more than 18 million puts a
severe strain on the capacity of the government and public utilities to provide
services to the expanding population. Not surprisingly, this has caused severe
environmental and ecological problems for the Basin and adjacent areas, includ
ing the chronic occurrence of air pollution concentrations that are unhealthy for
plants, animals, and humans. For example, recent studies in forested watersheds
near Los Angeles, California, and downwind of Mexico City demonstrate that the
quality of drinking water extracted from these watersheds is affected by air pol
lution. Chronic deposition of atmospheric nitrogen, the main source of which is
the use of fossil fuels, results in elevated nitrate levels from springs and streams.
Furthermore, preliminary evidence suggests that the accumulation of lead in
watersheds to the south and southwest of Mexico City can result in lead export
in drainage waters. The severity and geographic extent of this problem is largely
unknown at this time. These and other studies illustrate that urban air pollution
impacts natural resources and the services provided by ecosystems within the
geographic zone of influence.
We wish to insert a brief explanation here of the terminology commonly used
to describe this unique region. The most common terms are the Basin of Mexico
VII
Vlll Preface
or the Valley of Mexico. Although the Basin of Mexico is the most correct term
when referring to the larger area encompassed by the mountain ranges, these
terms have often been used interchangeably. Some have even referred to this
geographic region as the Valley of the Basin of Mexico or the Mexico City Basin.
In some chapters of this volume, the term Mexico Cit.v Air Basin is used in
reference to the ground-level atmosphere of the Basin.
Severe air pollution is a world-famous problem in the Mexico City Air Basin
as a result of a combination of factors: over 3 million vehicles, including 1.4
million pre-199l models; numerous domestic, industrial, commercial, and natural
(e.g., fugitive dust) sources of air pollutants; abundant solar radiation that pro
vides the energy for smog-producing photochemical reactions; thermal atmos
pheric inversions; a basin surrounded by volcanic mountain ranges; and frequent
stagnant air masses. Atmospheric inversions and the mountain ranges circum
venting the Basin prevent the transport of atmospheric pollutants from the Basin.
The drainage of several lakes that formerly covered much of the Valley of Mexico
has resulted in severe dust pollution during certain times of the year, in spite of
continuous efforts by the government over the last 40 years to alleviate this
problem.
Several commonalities between Mexico City and Los Angeles, California,
link these North American urban regions. Both have a strong Spanish
American/Mexican heritage, both are exposed to severe photochemical smog
as mentioned above, both experience atmospheric inversions, and both are
surrounded by mountains inhabited by a dominant ozone-sensitive pine species
exhibiting classic symptoms of ozone injury. Also, as mentioned above, in both
areas, watersheds exposed to urban smog are exporting high levels of nitrate,
although levels rarely exceed the drinking water standard. Atmospheric particu
late concentrations commonly reach unhealthy levels in both areas, although
extreme exposures are more common in the Mexico City Air Basin.
Increased tree mortality has also been documented in the most polluted forest
stands downwind of these two metropolitan regions, following the period of
major urban growth. Considerable evidence demonstrates a prominent role for
air pollution in both cases. However, tree mortality was much more rapid and
dramatic in the case of the decline of sacred fir in the Desierto de los Leones
National Park near Mexico City, as entire stands were decimated. These dead
zones became known as "cemeteries." Needless to say, the sudden death of thou
sands of fir trees in the highly valued Desierto de los Leones National Park in
the 1980s was alarming to many and greatly increased public awareness of the
impacts of air pollution on forests within the Basin. The causal mechanisms
leading to the rapid onset of sacred fir decline in the 1980s and the continuing
decline of the species are not definitively known, but the more dramatic appear
ance of the syndrome in the Mexican forests is exacerbated by the fact that fir
decline generally occurs in pure fir stands. In contrast, increased mortality of
ozone-stressed ponderosa pine trees in the San Bernardino Mountains east of Los
Angeles occurred in mixed conifer stands, in which ponderosa pine was one
species within a mix of four to five major overstory species.
Preface IX
Differences between the Los Angeles and Mexico City regions include the fact
that ocean breezes carry pollutant loads to mountains downwind of Los Angeles,
but there is no nearby ocean in the Mexico City Air Basin. The predominant wind
pattern in the Mexico City Air Basin is to the southwest, which results in severe
pollution exposure to the highly valued forests located in the south southwestern
portion of the Basin. Rainfall patterns also differ between the two urban areas,
the prolonged summer drought in the Los Angeles Basin contrasting with the
summer rainy season in the Mexico City Air Basin. In Mexico City, sulfur dioxide
emissions have decreased in recent years as a result of reductions in the sulfur
content of fuels, yet sulfur deposition to forests in the southwest of the Basin
remains high. This is in contrast with the Los Angeles Basin where sulfur pollu
tants are minimal compared to nitrogen oxides and ozone. In both regions, lead
emissions have decreased dramatically with the introduction of unleaded fuels,
although this is a much more recent trend in the Mexico City Air Basin.
Ozone concentrations and nitrogen oxide emissions have steadily decreased in
recent years in the Los Angeles Basin, although greater reductions are needed.
Occurrences of emergency levels of ozone have recently decreased in Mexico
City, but ozone levels still remain dangerously high throughout much of the year.
However, we are encouraged by an increasing recognition by the public and gov
ernment officials of Mexico City of the importance of improving air quality. They
have reacted by enacting and enforcing stricter regulatory controls. Only in this
way can the concentrations of all criteria air pollutants begin to approach levels
that are within the established air-quality standards on a consistent basis. The
trend for future impacts of air pollutants on forests and other natural resources in
the Basin of Mexico largely depends on the effectiveness of future emissions
control programs and the level of public support for these measures.
Studies of the impacts of air pollutants in forests and crops in the Mexico City
Air Basin have been carried out with the allocation of minimal financial resources
to this area of study. The first published studies took place in the early 1970s
mainly in the form of biomonitoring, that is, using established bioindicator plant
species and field surveys to document the occurrence of phytotoxic levels of air
pollutants in the urban and montane regions of the Basin. Largely, through the
cooperation of researchers from various institutions in Mexico, the United States,
Canada, and Europe, and Mexican students studying abroad, the scientific inves
tigation of the effects of air pollution on forests and vegetation of the Mexico
City Air Basin has slowly advanced over the past 30 years. Prominent interna
tional scientists who have contributed to air pollution studies on forests and crops
in the Mexico City Air Basin, whose roles in advancing the science we wish to
acknowledge, include Sagar Krupa, William J. Manning, Paul Miller, John Skelly,
and others.
The accumulated body of work regarding the effects of air pollution on forests
and vegetation in the Mexico City Air Basin has been independently published
in a myriad of disparate sources, many of which are available to only a very select
audience (e.g., relatively unknown graduate theses, government reports, scien
tific proceedings papers, articles, and papers spread throughout various Mexican
x Preface
and international journals). It is the aim of this volume to summarize and
synthesize this work, to bring this information together into one readily available
source, and to guide future researchers in filling many of the important gaps in
our understanding of the impacts of air pollution on the natural resources of the
Basin of Mexico. It is also our hope that this book will further highlight and raise
awareness of the environmental and ecological problems occurring in the region
affected by one of the world's largest metropolitan centers.
Mark E. Fenn
L.I. de Bauer
Tomas Hernandez-Tejeda