AIR QUALITY MODELING WITH THE
PROBLEM
SOLVING ENVIRONMENT (PSE) PROGRAM
D. Dabdub & T. Demayo
April 1999
University of California,
Irvine
Air pollution is one of the most serious problems
that the world faces today. The last
few decades of rapid industrialization, urbanization, and economic development
in both industrialized and developing countries has lead to a worldwide
increase in pollutant emissions and a resultant decrease in air quality. Most of the air pollution is caused by the
combustion of fossil fuels in energy production, vehicular emissions, and
industrial activities.
In urban areas, air pollutants
contribute to the formation of smog.
There are two types of smog. The
first to be identified, with reports dating back to the 17th
century, was sulphurous smog. This smog
is characterized by high concentrations of sulphur compounds (i.e., SO2
and sulphates), resulting from the combustion of coal and high-sulphur content
fuels. The second type of smog is known
as photochemical smog. This type of
pollution first appeared in Los Angeles with the advent of huge fleets of
automobiles following WWII. Sunlight
and hot weather catalyze a complex series of reactions in which nitrogen oxides
(NOx) and volatile organic compounds (VOCs) react to form a
hazardous pool of photochemical oxidants.
Causing haze, eye irritation, respiratory problems, and plant damage,
these oxidants consist of ozone (O3), organic nitrates, organic
acids, oxidized hydrocarbons, and aerosols.
Today, photochemical smog occurs in numerous cities around the world,
such as Mexico City, Santiago de Chile, São Paulo, Tokyo, Paris, and Bangkok.
In order to help understand pollutant formation and
to develop optimal emissions control strategies, scientists, engineers, and
regulators have developed mathematical models of air pollution processes, such
as the Problem Solving Environment (PSE) program used in this exercise. The PSE, designed by K. Nguyen and D. Dabdub
for airshed modeling, enables teachers, students, and concerned citizens to
dynamically simulate and study air pollution patterns and to understand the
consequences of public policy on pollution control.
In the model, one can vary temperature, wind patterns, humidity, solar radiation, and primary pollutant emissions in California’s South Coast Air Basin. In addition, one can choose whether or not to include the various components of air quality models (i.e., diffusion, advection, area and point emissions, chemistry, aerosol dynamics, and deposition) in order to study their effects on the model’s air quality predictions. After running the simulation, the PSE will then generate over the course of one day animated colour contour plots of pollutant mixing ratios.
·
Review
the basics of photochemical smog formation (see “Primer” below).
·
Review
the basics of air quality models.
·
Divide
the class into groups of 3-4 people.
·
Run
the PSE demo and discuss the operation of the program.
·
Ask
each group to answer the following questions, using printouts of contour maps
and short written conclusions to backup their results:
1.
What
happens to ozone formation under the following conditions. Assign one condition to each group.
Overcast day (low ultraviolet radiation)
Hot day
Cold day
Dry day (low
relative humidity)
Humid day
d)
Which
regions of the basin would experience improved conditions? Worsened conditions? (i.e., who gains and
who pays?)
·
Answer
the above questions but for different photochemical oxidants.
The formation of tropospheric O3, one of the
principal constituents of photochemical smog, results from a complex series of
chemical reactions. The sequence starts
with the reaction of combustion-generated NO with residual ozone to form NO2:
NO + O3
à NO2 + O2
Energy radiated by bright
sunlight provides the key step in ozone production by photolyzing NO2
to produce O radicals. These radicals
then react with surrounding O2 molecules to create O3
NO2 + hn(l<400 nm) à O• + NO (1)
O• + O2 à O3 (2)
In addition to the reaction of NO with O3,
another very important pathway for NO to NO2 conversion occurs by
hydrocarbon oxidation. The presence of
radicals such as OH• results in the photo-oxidation of VOCs (represented here
by R) found in the troposphere, giving rise to complex peroxy radicals RO2·:
R + OH• à R·
R· + O2
à RO2·
These peroxy radicals react with NO in a
polluted environment to form:
RO2· + NO à RO· + NO2
More O3 is then generated by reactions
(1) and (2).
Commonly, the alkoxy radical
RO• reacts with O2 to form a carbonyl compound and HO2•:
RO· + O2 à HO2· + R=O
The hydroperoxyl radical can then react with NO to
regenerate OH• and produce another NO2 (and thus O3)
molecule through reactions (1) and (2):
HO2• + NO à NO2 + OH•