By: Dr. Gerardo Manuel Mejía Velásquez. **
In recent years there
has been a growing interest in the impact that man-made
emissions have and will be having on air quality.
This impact may already be seen locally and regionally,
and it is now becoming important on a worldwide basis.
Society is becoming more and more interested in finding
out about the effects that pollutants emitted by certain
industries can have and the impact they may have on
air quality in cities when combined with emissions
from other industries and vehicles. Large cities
of the world are currently having significant air
quality problems, and others have a high probability
for problems in the near future, especially those
that are located in areas of rapid industrial and
commercial growth.
Pollutants such as
nitrogen oxides (NOx), hydrocarbons (HC), particles
under 10 microns (PM10), sulfur dioxide (SO2)
and carbon monoxide (CO) emitted in large cities have
an impact on the health of the population by forming
ozone, secondary particles and other photochemical
oxidants. They are also transported in the atmosphere
and can start having a regional impact by forming
secondary aerosols and acids, which have adverse effects
on visibility and ecosystems. Furthermore, depending
on the amount of time they remain in the atmosphere,
the impacts of some kinds of chemicals which are not
considered to be pollutants on a short term, can start
having an adverse effect globally. This is the case
of CO2, which can influence the greenhouse
effect when it accumulates in the atmosphere, and
chlorofluorocarbons (CFCs), which play an important
role in the declining concentration of ozone in the
stratosphere.
Air quality problems
are identified in the air of cities when there are
monitoring stations. However, impacts on a regional
level are difficult to estimate because there is little
information on how they become transported to rural
areas. It is in these areas where they finally become
deposited in the ground or surface waters as particles
or acids. To properly measure these impacts, there
is a need to have more information on the location
of sources and quantities emitted by them, as well
as sufficient meteorological data to enable a proper
study of the transport of pollutants in the atmosphere.
Pollutants in the atmosphere
are introduced by different industrial, agricultural,
transportation and trade and service activities, as
well as by nature itself. There are some products
that due to their low degradability are considered
to be atmospheric pollutants and their use has been
eliminated in some countries. Criteria pollutants
are those that are typically found in the urban atmosphere
(NO2, CO, PM10, SO2 and ozone),
in addition to HCs and NO. These pollutants are directly
emitted by fixed, area or mobile sources, or they
are formed by reactions in the atmosphere. Data for
emissions from man-made sources reported by the National
Environmental Institute for Mexico City, Guadalajara
and Monterrey show that the most significant man-made
source is transportation with approximately 85 to
90% of emissions and industry and services with 10
to 15%. These percentages do not consider particles
emitted by natural sources, which can be a significant
source in some cities such as Monterrey. One important
issue is that vehicle emissions data are very uncertain,
because they are mobile sources and their emissions
are hard to estimate.
In the atmosphere,
pollutants are transported and dispersed depending
on wind, topography and land use conditions. Under
these conditions, the amounts of different pollutants
emitted reach certain levels of concentration in the
atmosphere, or else form other pollutants by reacting,
which can be hazardous to health depending on people’s
length of exposure. There are currently emission
standards for industry, and in some cities there are
vehicle emissions inspection programs. What is intended
with these standards is to have acceptable air quality
for the population of a city; that is, for air quality
standards to be complied with. However, many times
standards are not complied with, so it is necessary
to define emissions levels that can be handled by
an airshed in such a fashion that these air quality
standards will be complied with, it being understood
that an airshed is a region that due to its topographical
and meteorological conditions includes a group of
emissions sources, and its areas of influence. In
addition, because meteorological conditions are specific
for each airshed, permissible emissions levels for
each source will be different for them, which represents
an additional problem in developing and evaluating
environmental policies.
Other compounds such
as CFCs were introduced in 1930 as safe refrigerants
because they were non-toxic and non-inflammable, and
their impact on air quality in the troposphere was
not considered to be important. Years later an evaluation
was made of the impact these compounds have when reacting
with ozone in the stratosphere, enabling a higher
amount of ultraviolet radiation to reach the ground
with an adverse effect on exposed population and ecosystems.
In 1986 the Vienna Convention was held for protecting
the ozone layer, which led to a worldwide effort to
mitigate the effects of CFCs with the signing of the
Montreal Protocol in 1987. Chemical compounds such
as CO2 and methane (CH4), which
are not considered to be criteria pollutants in an
urban atmosphere, can be considered to be long term
pollutants, for when they accumulate in the atmosphere
they influence the greenhouse effect and very slowly
heat the atmosphere, which can cause important changes
in meteorological conditions on the planet. This
is getting attention worldwide to define strategies
for decreasing CO2 emissions, such as the
recently held Kyoto meeting.
In finding solutions
for air pollution problems locally and globally, technical,
social, economic and political issues must be considered.
The private sector, government, academia and society
must work together in developing strategies for prevention
and control of air pollution. The best strategies
must not only be focused on compliance with air quality
standards, but on decreasing and if possible eliminating
the hazard to the population from acute and chronic
exposure to pollutants in the atmosphere. We generally
think that anti-pollution technologies that have been
developed may in some way eliminate the problem.
Although great progress has been made, it is important
to realize that there is no process or technology
that may be capable of enabling us to have zero emissions
in production processes or transportation, for many
control strategies involve exporting pollution to
other areas or environments.
Cities or regions currently
having an air pollution problem are facing a control
problem, the solution for which is very costly and
requires several years for appreciable progress to
be seen. On the other hand, cities or regions with
problems in the beginning stages or that may emerge
in the future, can use the option of preventing the
problem, which requires a comprehensive urban, economic
and environmental development plan. The search for
solutions to existing air quality problems and prevention
for air quality require a proper balance of living
and protection standards for ecosystems within the
framework of sustainable development.
Industry has made a
significant effort to decrease its emissions, while
the government has been concerned about seeking proper
economic and social development and trying to link
this with care for the environment. On the other
hand, universities have devoted themselves to studying
the problem in an effort to support government decisions
and industries, and educating the public at large
on care for the environment. On its part, society
should actively participate in this process, because
it is the population that is mainly affected by air
quality conditions.
Our next articles in
this series will deal with issues having to do with
the impacts of pollutants on health and ecosystems,
the urban and industrial pollution problem, and strategies
for prevention and control of air pollution.
* Article
first published by Calidad Ambiental, Vol.
V, N. 3, May-Jun 2000, p. 10.
**
Dr. Gerardo Manuel Mejía Velásquez has a bachelor
degree in Chemical Engineering by the Autonomous University
of San Luis Potosí. In 1984 he received a master degree
in Engineering with specialization in Process Systems
by the Monterrey Technological Institute, and in 1992
he received a doctorate degree by the Chemical Engineering
Department of Texas A&M University. At the moment
he works as coordinator of the Environmental Modeling
Laboratory at the Environmental Quality Center of
the Monterrey TEC. He has served as a professor at
the Autonomous University of San Luis Potosí and at
the Monterrey TEC, campus of San Luis Potosí. Dr.
Gerardo Mejía has carried out research projects on
air quality in collaboration with professors of the
A&M Texas University, Houston, Carnegie-Mellon
and Utah. Nowadays, he carries out a sabbatical year
at the MIT, United States.
