London/Los Angeles Smog Flashcards
London Smog
Combination of extended fog periods and high concentration of smoke from coal burning for residential heating and other needs
- It takes place during the winter with little or no sunlight
- High concentration of particulate matter emitted from
fuel combustion and industrial processes - Stagnant air allows the concentration of these pollutants
to reach dangerous levels - It reduces visibility and generates problems to human
health
Los Angeles Smog
Los Angeles Smog = Photochemical smog:
A number of gaseous pollutants emitted by cars, industry, and other human activities mix together in the atmosphere and start reacting chemically, sparked by energy from the sun
They produce secondary pollutants (O3)
These secondary pollutants could be more dangerous than the initial pollutants (Primary
pollutants)
Primary Pollutants: are emitted directly into the atmosphere from various sources(HC, NOx, CO, SO2, particles)
Secondary Pollutants: they are produced in the atmosphere from reactions of the primary pollutants (O3, particulate matter)
Los Angeles Smog effects
It reduces visibility
It harms human health and welfare
It creates a variety of human respiratory illnesses
It damages various construction materials
It damages vegetation
How to control the Air Quality?
There is a relationship between air pollution and
human health.
Some solutions to reduce air pollutants emissions:
- Reduce automobile emissions
Change engine design
Change fuel
Expand public transportation
Expand exhaust catalysts - Control industrial emissions
- Introduce new solvents for paints and industrial processes
We have to know:
1. How pollutants accumulate in the atmosphere
2. How they interact and react chemically
3. What are the meteorological conditions affecting the
formation of secondary pollutants
Conditions for Photochemical Smog Formation:
- Combination of NOx, hydrocarbons and sunlight
- Climatological or geographical conditions must be such that the reactive compounds and products accumulate near earth’s surface, rather than dissipating into higher reaches of the atmosphere where they would become diluted and do little harm
These conditions often prevail in valleys or other areas where little
air mixing occurs, particularly under thermal inversion conditions
Thermal Inversion - Temperature Inversion
Air pollution episodes are usually accompanied by a meteorological
phenomenon called a temperature inversion
Under normal conditions the temperature of the lower atmosphere falls
with increasing altitude, and there is good vertical
mixing within a height of about 1000 m.
Under inversion conditions a layer is formed above the ground where
the temperature actually increases with altitude
* This layer of warm air acts as a lid that prevents the
mixing of air below with the air above
* The presence of a temperature inversion restricts the
dilution of pollutants and sets the stage for the buildup
of high concentrations of pollutants during an air
pollution episode
Inversion height:
The altitude at the bottom of the inversion layer
For typical air pollution episodes it is around 500 m
In the inversion layer, cooler air below cannot mix with warmer
air above
What Meteorological Conditions cause the Thermal Inversion?
- High pressure systems
Air is falling from higher altitudes of the
atmosphere - Low wind speeds
Pollutants stay longer over urban areas and
accumulate in the atmosphere - Clear skies
- Sunshine
- Warm conditions
How is it resolved
When a new weather front brings cooler, cleaner air over the area
the episodes end
* Low pressure system
Air rises rather than fall
This upward motion leads to expansion and cooling air
The stable inversion layer also breaks up, allowing
greater mixing and dilution of polluted air with cleaner
air
Urban Ozone Levels
- The daily max ozone concentration occurs between noon and 5 p.m.
In most central or downtown urban areas - Concentration of O
3 decreases during the night and often becomes
zero at ground level - For locations downwind of these urban centers, peaks in ozone are
observed even during the night
There are two daily peaks:
1. First peak: in the early afternoon and is due to O3 produced
locally
2. Second peak: in the evening due to O3 transported from other
locations
- Ozone is both a local and regional problem
Ozone is both produced from local emissions and transported long
distances from cities upwind of an affected area
FORMATION OF OZONE IN URBAN AREA
Ozone is formed in the atmosphere as follows:
O2 + O ———–> O3 (Reaction 1)
^There is in the atmosphere
NO2 + (sunlight) ———–> NO + O (Reaction 2)
^From combustion of fuels in automobiles, powerplants
NO + O3 ———–> NO2 + O2 (Reaction 3)
The faster Reactions 1 and 2 are, the more ozone will be produced.
But the faster Reaction 3 is, the lower the ozone levels
Effect of VOC Concentration on Ozone Formation
VOCs: are mostly hydrocarbons from human activities
VOCs: serve as the fuel for the formation of O3
Increasing the concentration of VOCs you increase the
formation of O3
Ozone Isopleth Diagrams
Ozone Isopleth Diagram: is a diagram which shows the dependence
of O
3 production on the availability of
VOC and NO
x
Isopleth: a line of constant concentration
Isopleth diagrams differ from one urban region to another because of
differences in source emissions, meteorology, and other
factors
Ozone isopleth diagrams are useful to design effective emission
control strategies for regions where O3 levels exceed air
quality standards
Ridge line in ozone isopleth
Ridge Line: identifies the max O3 concentration that can be achieved
at a given VOC level, allowing the NOx level to vary
The rigid line separates the diagram into two parts:
- The region below the ridge line: NOx-limited regime
- The ozone concentration is relatively insensitive to
changes in VOC levels but responds readily to
reductions in NOx
- There are plenty of hydrocarbons but not enough NOx
for the production of O3 - The region above the ridge line: VOC-limited regime
- The O3 concentration is sensitive to reductions in VOC
concentrations; but decreases in NOx concentration
cause an increase in ozone levels
