AIR POLLUTION
PART A.
Background
Los Angeles is the undisputed air pollution capital of the United States.
Those who live here, or perhaps visit, are familiar with the hazy skies and
reduced visibility. And yet, most Californians do not know some of the
fundamental relationships between weather and air pollution. This set of
exercises introduces some of the concepts associated with air pollution
measurements and air pollution variability.
In this section we introduce the air pollution problem in Los Angeles,
including the various sources of pollution, the types and how they vary over the
L.A. Basin. We look at the geographic and climatic influences on smog levels.
In PART B, the student takes a sample of the particulate air pollution load
using a High-Volume air sampler.
From the weight of particles collected on a
filter paper and the volume of air sampled, the student calculates the
concentration of the total suspended particulates (TSP) in micrograms/cubic
meter. PART C deals with the temporal and spatial variations in air pollution
levels in the L.A. Basin. Seasonal and diurnal patterns are discussed and how
these levels vary from coastal locations toward the inland valleys. The
relationships between weather and air pollution are covered in PART D, with an
air pollution dispersion model used to show how pollution concentrations vary
due to changes in emission and weather factors in section E.
OBJECTIVES:
1. To become familiar with the main air pollutants and the
metric units used in their measurements.
2. To take a sample of particulate air pollution using a
High-Volume air sampler and to calculate its concentration.
3. To analyze air pollution data from the L.A. Basin monitoring network to view temporal and spatial variations.
4. To analyze air pollution and meteorological data to
find relationships between air pollution levels and
weather.
5. To better understand the photochemical process and its
impacts on air pollution levels.
6. To understand how dispersion models can be used to show
how variables effect air pollution concentrations
downwind.
Los Angeles sits in a basin surrounded by mountains to the north and east,
and the Pacific Ocean to the west and southwest. Its climate is considered
Mediterranean and semi-arid, with about 15" (380 mm) of rain concentrated in an
average 26 days during the winter (Nov.-Apr.). This mild climate and perceived
ideal location has been a magnet for people inflow and the accompanying sources
of pollution, with nearly 14 million people and 11 million cars, buses and
trucks. Despite having the world's toughest air pollution control program, L.A.
continues to lead the U.S. with the worst air pollution levels.
During the summer, the Pacific subtropical anticyclone (high pressure
center) moves to a more northerly position off the coast of California, with
weak horizontal winds and sinking, warm air that produces a persistent
inversion. The summer inversion minimizes vertical mixing, leading to greater
concentrations of pollutants in the shallow marine layer below. Under clear
skies of summer, locally-produced vapors react in sunlight to produce unhealthy
levels of photochemical smog.
Air pollution can be classified as gaseous pollutants or particulates.
The main gaseous pollutants monitored by the South Coast Air Quality Management
District (SCAQMD) are: carbon monoxide, hydrocarbons, sulfur oxides, nitrogen
oxides, and ozone. Table 1 shows concentration levels associated with air
quality standards. For example, the federal Clean Air standard for ozone is .12
ppm (parts per million) or 100 on the PSI (Pollution Standards Index). Above
this level, the air is considered unhealthy. A Health Advisory is issued at .15
ppm or above. A 1st Stage Episode (very unhealthy) is reached at .20 ppm, 2nd
Stage at .40 (hazardous), and 3rd Stage at .50 ppm and above. There has not
been a 3rd Stage Episode in L.A. since the 1970's.
Particulate air pollutants are collected on filter paper using a HighVolume air sampler, an instrument that draws ambient air through the filter.
The filter paper is weighed before the sample is taken and after a known period
of time (usually 24 hours). The difference in weight divided by the volume of
air sampled gives the particulate concentration in ug/m3 (micrograms per cubic
meter, where 1 microgram = 10-6 grams). The Hi-Vol is very efficient at
collecting particles at sizes of 0.1 um (micrometers = 10-6 meters). Presently,
the standard measurement for particulates is PM10, particulate matter of mean
diameter 10 um or less. Particles of smaller sizes have been connected to
respiratory ailments as well as reduced visibility. Therefore a SSI (size
selector inlet) is placed over the Hi-Vol to collect only those particles less
than 10 um.
Particulate matter is also analyzed for chemical composition. The main
constituents are sulfates, nitrates and carbons. Another source of particulate
aerosols (aerosols are generally those with sizes less than 1 um) are a result
of gas-to-particle conversion in the atmosphere. While the mechanisms for these
secondary particle sources are poorly understood, they do add a significant
amount to the particulate load. Figure 1 shows the proportional distribution of
particulates by chemical make-up for different L.A. Basin locations.
QUESTIONS.
1. Using Table 1, what would be the concentrations of the
following PSI values? Give the corresponding air quality
(good, moderate, etc...).
a. PSI = 250
ozone = _________ ppm; air quality = _____________
carbon monoxide = _______ ppm;
PM10 = ________ ug/m3
b. PSI = 80
ozone = ________ ppm; air quality = ____________
nitrogen dioxide = _________ ppm
PM10 = __________ ug/m3
2. Looking at Figure 1, compare the chemical composition of
particulates of coastal locations and inland valleys by
naming the top 3 constituents for each:
Coastal
Inland
1. ___________
___________
2. ___________
___________
3. ___________
___________
3. Briefly give reasons for the differences in question 2.
PART B. Taking an Air Sample
In this section, we will go through the procedures to take a 24-hour
sample of particulate air pollution using the High-Volume air sampler. The HiVol works like a big vacuum machine, with a pump that pulls air and its
contained particles through a filter paper. Since there is no size selection
inlet, the measurement will be the TSP, total suspended particulates.
1. Preparing the filter paper. The Hi-Vol uses an 8 1/2 by
10" fiber-glass filter paper, which captures at least 99%
of particulates passing through it. Since the weight of
the sample needs to be accurate, care must be taken not
to dirty the filter, nor lose some of the sample.
a. Using a precision weighing balance, weight the filter
to the nearest hundredth of a gram. Bend, but do not
fold the paper if needed.
weight before sampling: ____________ g
b. Place the filter paper in the Hi-Vol filter paper
holder (see Figure 2) by removing the top of the
holder, placing the paper so that it overlaps the
sampling area, then replacing and fastening the top of
the holder.
c. Switch on (plug in) the Hi-Vol and record the time and
date. The Hi-Vol needs to run approximately 24 h.
unless the instructor says otherwise.
Time: _________
Date: __________
2. Attach an air flow chart if the Hi-Vol has a place for
one on the front of the box. Measure the air flow meter
(push button) reading. After 24-hours, estimate the
average air flow. If neither measurement is available, assume an average flow of 60 ft3/min (or 1.7 m3/
min).
a. Average air flow from chart: _______ ft3/min
b. Air flow meter (start of sample) _________
(end of sample)
_________
Take the average of the 2 above readings: ________ (1)
3. Remove the filter paper, carefully folding in half and
placing in a large envelope to be reweighted. Record time
and date.
Time: ________
Date: ____________
4. Carefully weight the filter paper and subtract the presample weight.
Weight before sampling: ______________ g
Weight after sampling:
Net weight of sample:
______________
_______________
(2)
5. Calculate the TSP concentration. The concentration is
weight divided by volume of sample air.
a. Calculate the volume of air sampled
Volume = average airflow (ft3/min) x time (min.) x
(.028317 m3/ft3)
Volume = ___________ m3
(3)
b. Divide weight by volume, or (2)/(3)
TSP = __________ g/m3 or
changing to ug, multiply by 106 ug/g, then
TSP = __________ ug/m3
QUESTIONS:
1. If the concentration was PM10 instead of TSP, what would
be its PSI value? __________
2. What effect does location have on your reading? Would
the TSP level increase or decrease with greater elevation?
PART C. Los Angeles Air Pollution Variations
Air pollution levels vary considerably over the L.A. Basin. They also
vary seasonally and diurnally, changing from good to poor air quality sometimes
in a matter of hours, or as one travels from one area to another.
1. Temporal variations.
It is well known that L.A. smog is most prevalent in summer, while usually
minimal in winter. Figure 3 shows the seasonal variation in ozone for coastal
and inland locations. Note the differences in magnitude of ozone as you travel
away from the coast. Figure 4, which presents the monthly average temperatures
of downtown L.A., shows a similarity to the ozone pattern. Since ozone is a
result of photochemical reactions, and these reactions are faster at higher
temperatures, we would expect higher ozone when temperatures increase.
There is also large differences in smog levels over the 24-hour daily
cycle. Figure 5 shows a typical summer day reading of gaseous pollutants in
L.A. You can see that NO and hydrocarbons (HC) rise sharply with the morning
rush-hour traffic. As the day warms, photochemical reactions convert NO into
the more toxic NO2, nitrogen dioxide. Further reactions between hydrocarbons
and nitrogen dioxide in sunlight create ozone. Ozone peaks near noon when the
sun's rays are strongest. In the afternoon, as the sunlight diminishes, first
NO2, then ozone decrease. The late afternoon rush-hour produces another NO
peak, but without photochemical reactions, NO2 and ozone continue to decrease.
2. Spatial variations
Since cars are the main source of air pollution, the largest emission
sources occur in the heaviest traffic going into the downtown area, though urban
sprawl throughout the Basin helps distribute pollutants widely. Figures 6 and 7
indicates the annual average ozone and PM10 levels respectively for the L.A.
Basin. There is a similarity in patterns for both the photochemical oxidant,
ozone, and particulates.
QUESTIONS:
1. Why do coastal locations have significantly lower ozone
levels in summer than do inland locations? (Hint: sea
breezes are stronger in summer.)
2. Figure 8 shows the diurnal patterns for air pollution at
several Basin locations on Sept. 20, 1995. For the date
given, compare the times of ozone mazimum at Long Beach,
downtown L.A., Burbank and Riverside. How does the time
of maximum ozone change as distance from the coast
increases? Why?
3. Compare the levels of maximum ozone for the locations
given in question 2. How does the levels change with
distance inland?
4. If L.A. did not have mountains surrounding it, what would
Figures 6 and 7 look like?
PART D. Weather and Air Pollution
In Part C we saw that there are daily and seasonal cycles to air
pollution, just like there are cycles in weather and climate. During the sumer,
warmer temperatures and persistent inversions lead to higher air pollution
levels. Photochemical reactions add to the pollution load, when abundant
sunlight during the warmer, longer days of summer lead to secondary air
pollutants formation and hazy lowered visibility. Winter storms and more
variable and cooler winds mark the winter season with cleaner air. Spring and
fall are transitional seasons with short periods of higher pollution levels
during times of stagnant air and poor ventilation.
Given the large quantity of emissions in the L.A. Basin, especially from
cars, the smog concentrations are often determined by how well these emissions
and the ambient air is diluted or mixed in the horizontal and vertical
directions.
Horizontal ventilation is controlled by wind speed and direction. During
most warm days, a sea breeze starts up in the late morning blowing emissions
landward towards the inland valleys. Gentle at first, these onshore breezes may
strengthen by mid-afternoon. The sea breeze usually weakens by evening when
often a cool land breeze begins to reverse the flow back towards the coast. The
land breeze, weaker than its counterpart sea breeze, may return pollutants
across the Basin out to sea.
Daytime heating and night-time cooling also creates local mountain-valley
winds. During the day, warm air flows upslope, while cool, denser air flows
downslope at night. With the geography of L.A.'s coastline and surrounding
mountains, the sea breeze and upslope mountain winds combine during the day with
onshore breezes towards the valleys and upslope flow along the foothills and
mountains (see Figure 9). This combination produces a "chimney effect", pumping
air pollutants up through the inversion into the local mountains damaging
valuable forests. At night, the downslope flow and land breeze combine in a
sweeping down and offshore flow (Figure 10). When winds are light, the same
pollution "cloud" may be pushed across the Basin, first inland then back towards
the coast, for several days. However, the cloud, rather than dispersing,
accumulates the total emissions during that period making for an air pollution
episode.
Vertical ventilation is determined by how well surface air mixes with the
air above it. This depends on the vertical temperature profile of the
atmosphere, or its stability. During the day, as the sun heats the ground, warm
air parcels rise and are replaced by cooler parcels. The rising air "bubbles"
expand and cool, since air pressure decreases with height. Conversely, cool air
that sinks, is compressed by increasing air pressure and warms. The rates of
cooling and warming for vertical motion are the same, 1oC for every 100 meters
of vertical movement.
During winter storms, or cyclones, the surface pressure is lower and the
atmosphere is more unstable. That is, the temperature profile of the atmosphere
has a greater decrease of temperature that 1o per 100 meters. This allows
surface warm air parcels to rise freely, since their temperature remains warmer
than the surrounding atmosphere (see Figure 11). In summer, the Pacific High,
or anticyclone, dominates surface weather of L.A. and the atmosphere is more
stable. Temperatures decrease slower than 1oC per 100 m., and often even
increase with height, creating an inversion. In this case, rising surface air
parcels would become colder than the surrounding atmosphere, tending to sink or
remain near the surface (see Figure 12). With surface high pressure systems and
sinking air, especially with inversions, air pollutants are confined to a
shallow volume of air. Air pollution forecasters can calculate the atmosphere's
mixing depth, the vertical height that surface air parcels can freely mix with
the surrounding air, from the ascent of the daily instrumented weather balloon,
called a radiosonde, which measures and records the ambient air's temperature
profile. When there is an inversion, the mixing depth usually is the base of
the inversion. As the day warms and adds to the energy of rising air, the
inversion weakens and the mixing depth raises. Figure 13 indicates three
temperature profiles taken on separate days near L.A. International Airport
(LAX). Notice that the most stable case had the worst air quality.
QUESTIONS:
1. From Figures 9 and 10 and the spatial distribution of
smog (Figure 6), where would you expect the highest
levels of ozone at (a) 10 am; (b) noon; and (c) 3 pm?
2. Figure 14 shows the relationship between the 850 mb.
temperatures and ozone levels. 850 mb. pressure level is
found at about 1400 meters, near the summer average
height of the inversion or mixing depth. What season
would you expect the 850 mb. temperatures to be highest?
Lowest?
3. During Santa Ana winds, strong northeast flow (from the
NE) flow from the mountain passes into the L.A. Basin.
How would Santa Ana winds effect smog levels (a) east of
downtown L.A., and (b) west of downtown L.A.?
PART E. Dispersion Air Pollution Models
In Part D, we've learned that horizontal and vertical ventilation
determine how well pollutants are diluted in the atmosphere. In general,
stronger winds and a less stable environment (higher mixing depth) lead to good
ventilation and lower air pollution concentrations. Weak winds and greater
atmospheric stability, with lower mixing depth, result in poor dispersion and
higher air pollution levels. Pollution forecasters and air quality managers use
air pollution dispersion models to estimate concentrations of pollutants
downwind from emission sites. These models can be used to estimate dispersion
for point sources, like a smoke stack or other individual pollutor, such as a
car, a line source, such as highways or crop dusters, or area sources, like
industrial areas or cities. Let us first consider a point source.
A simple dispersion model is called the box model. Figure 15 shows that
the concentration of smoke belching from a stack depends on the emission rate,
or how much pollution is emitted per unit time, and the prevailing wind that
disperses it. We can see that for an emission rate of 6 units/sec., the
concentration for each meter distance downwind is 1/3 as great at 6 m/sec wind
speed as with 2 m/sec. The wind speed not only determines the dilution of the
emission, but also the travel time from source to a downwind target.
Of course wind is not steady. Often surface layers of air are affected by
irregular or turbulent winds. These turbulent motions help disperse pollutants
more rapidly than with smoother flow.
A very common model is called the Gaussian plume model. It is probably
the best available method for making calculations of pollution concentrations
based on atmospheric conditions. The model is based on the same normal curve
used in statistics, and assumes that pollutants are dispersed nearly randomly
into a normal Gaussian distribution, diluting in the x,y and z directions
continually downwind of its source (see Figure 16). Using physical laws and
some simplifying assumptions, the model calculates horizontal and vertical
mixing allowing the modeler to input emission and weather variables to calculate
pollution concentrations at any point in the x, y, z directions.
We will use a computerized version of the Gaussian plume model to show how
changing weather conditions effect downwind pollution concentrations. The
computer program is called AIRPOL, which runs on IBM compatible computers. Each
student will be provided with a diskette containing AIRPOL.EXE which runs the
program. The program asks questions that allows you to enter emission and
weather data needed to do the Gaussian calculations.
QUESTIONS:
Using the diskette provided containing the AIRPOL program follow the directions
below and answer the questions.
Place the diskette in the appropriate drive (a or b) and change to that
directory by typing a: or b:. To start the program, type airpol and answer the
questions by typing in your response.
1. For cloud cover, choose clear, type 1.
2. For month and day, type 7 and 15 for July 15th.
3. For terrain, choose urban-type 2.
4. For wind speed, enter 3 m/sec.
5. For day/night, choose day-1.
6. For air pressure, temperature data, enter N for no.
7. For inversion, choose Y (yes) and for height, type 300 m.
8. Type for top of stack as 15 m.
9. For source strength, type 100 g/sec. emission.
10. Type N (no) for fumigation.
11. The program calculates the stability class from A to F,
with A the most unstable, F the most stable.
12. The program now calculates either the maximum concentration downwind or the point concentration at some
given distance downwind (x), crosswind (y) and vertical
(z). Choose point concentration, type P.
13. For changing the wind speed at stack top, type N (no).
14. For downwind distance, x, type 200 m.
15. For crosswind, y, and vertical height, z, type 0 for
both.
16. The program calculates the horizontal and vertical
effective dispersion, Sy and Sz respectively, and XL,
the distance downwind (m) where vertical mixing becomes
effective, and the concentration of pollutants in ug/m3.
a. The concentration for the above choices = _______ .
b. Change x from 200 to 400 meters downwind. What is the
new concentration? ________
c. Change y from 0 to 50 m. What is the concentration?
__________
d. Change z from 0 to 100 m. Concentration = _________.
e. Change z to 600 m (above the inversion). Now the
concentration = __________.
f. Re-do the calculations and find the maximum concentration for the original choices. Concentration
= _________ .
g. Repeat (a) for NO INVERSION choice. Concentration
= _________ .
h. For (a) choices, change STACK HEIGHT from 15 to 30 m.
Concentration = __________.