Transcript Lecture 11 - Air Pollution & Metereology ENV526

AIR POLLUTION
AND
METEOROLOGY
Dr.K. Subramaniam,
Senior Lecturer (Environmental Health and Safety )
METEOROLOGY OF AIR
POLLUTION
• Transport and dispersion
• Removal mechanisms
Important Aspects of Air Pollution
Meteorology
• Atmospheric Turbulence
• Scales of Atmospheric/Turbulent
Motion
• Plume Behavior
• Planetary Boundary Layer (PBL)
• Effects on Dispersion
• Applications
Meteorological Parameters
that Influence Air Pollution
• Turbulence
• Wind Speed and Direction
• Temperature
• Stability
• Mixing Height
Atmospheric Turbulence
• Responsible for dispersion/transport of
pollutants
• Refers to the apparently chaotic nature
of fluid motions (in this case,
atmospheric motions)
• Irregular, almost random fluctuations of
such parameters as:
i. velocity
ii. temperature
iii. scalar concentrations (pollutants)
Atmospheric Turbulence
Sources
• Mechanical Forcing
• Buoyant or Thermal
Forcing
Atmospheric Turbulence
(Sources)
• Mechanical Forcing:
i. Air flowing over irregular surface
ii. Change in horizontal wind speed with
height
• Factors Influencing Mechanical Forcing:
a) Speed of local winds
b) Roughness of terrain over which
wind is blowing
Adiabatic Lapse Rate
• It is the temperature profile of what
would happen to a parcel of air that is
raised or lowered vertically, and allowed
to cool or heat from expansion or
contraction with no exchange of energy
or heat.
Atmospheric Turbulence
(Sources)
• Buoyant Forcing (Thermal):
– Air rises or sinks based on temperature; heated
air becomes less dense & rises on its own;
cooled air becomes more dense & sinks
• Factors Affecting Buoyant Forcing
– “Stability” of the atmosphere
– Vertical temperature profile of the atmosphere
– Lapse Rate; specifically the Dry Adiabatic Lapse
Rate which is:
1oC/100m = 10oC/km = 5.4oF/1000 ft
Atmospheric Turbulence (Buoyant
Forcing)
DRY ADIABATIC PROCESS
Cooler Air
Cooler Air
Warmer Air
Ground
Atmospheric Turbulence (Buoyant
Forcing)
Unstable Conditions - Turbulence is produced
Cooler Air
Warmer Air
Cooler Air
Displaced warmer air
will now rise on its own
(Thermals; Thunderstorm updrafts)
Ground
Atmospheric Turbulence (Buoyant
Forcing)
Stable Conditions - Turbulence is suppressed
Warmer Air
Cooler Air
Warmer Air
Displaced cooler air
will sink back to starting point
Ground
Atmospheric Turbulence (Buoyant
Forcing)
Neutral Atmospheric
Conditions
Environment
Environment
Air Parcel
Ground
Planetary Boundary Layer (PBL)
• Top of the atmospheric boundary layer can be defined
as the lowest level in the atmosphere at which the
ground surface no longer influences the
meteorological parameters through turbulence transfer
of mass
• During day this corresponds to Mixing height (up to 3
km in height)
Processes include:
i. Roughness of terrain
ii. Obstructed flow
iii. Heat and energy transfer
The effect of boundary layer stability
on plume behavior
In a well-mixed turbulent boundary layer on a hot day (forced by
buoyancy), the turbulent eddies may be large and intense enough to
advert the whole plume down to the ground. This can result in extremely
high plume concentrations in the vicinity of the source.
The effect of boundary layer stability
on plume behavior
This is the kind of form assumed for a Gaussian plume, when
the boundary layer is well-mixed and turbulent eddies are
smaller than the plume scale. The plume forms a cone
The effect of boundary layer stability on
plume behavior
In a stable boundary layer, the plume spreads out horizontally at its level
of neutral buoyancy. Vertical motion is weak, so there is little upward
spread, but the plume forms a `fan' when viewed from above. The plume
is not well-mixed in the vertical, which implies relatively slow dilution, but
there are not likely to be high plume concentrations at the ground.
Unfortunately, this kind of plume may be the precursor to a `fumigation'
event if the inversion is subsequently mixed to ground level.
The effect of boundary layer stability
on plume behavior
At early evening, if a surface inversion is developing, vertical
motion may be inhibited below the plume while remaining
active above: the plume is diluted but does not reach the
ground. This is a favorable situation.
The effect of boundary layer stability
on plume behavior
There is a strong inversion restricting mixing above, and the plume is
mixed throughout the boundary layer. This can occur quite rapidly. For
example, after sunrise when the nocturnal inversion is being eroded from
below by buoyant eddies, plume-level air of high concentration may be
brought down to the surface over a wide area.
Effects of PBL Height on Stack Pollutant
Dispersion
PBL below stack top: little or no concentration of pollutants at the
surface
Horizontal Winds
PBL Top
PBL
Effects of PBL Height on Stack Pollutant
Dispersion
PBL Top
Buoyant
Turbulence
PBL
PBL well above stack top: decreased
concentrations of pollutants at the surface
Effects of PBL Height on Stack Pollutant
Dispersion
PBL just above stack top: increased concentrations of
pollutants at the surface
PBL Top
PBL
Buoyant
Turbulence
Temperature Profile in
Atmosphere
1. INVERSIONS
2. ATMOSPHERIC STABILITY
Effects of Stability on Stack Pollutant
Dispersion
Unstable Conditions: leads to greater dispersion of pollutants
PBL Top
PBL
Effects of Stability on Stack Pollutant
Dispersion
Stable conditions: lead to less dispersion of pollutants
PBL Top
PBL
Effects of Stability
(Ground Source Pollutant Dispersion)
Buoyant
Turbulence
XXX
Unstable Conditions: Lead to lower concentration of
pollutants at surface
Effects of Stability
(Ground Source Pollutant Dispersion)
Stable Conditions: Leads to greater concentration of
pollutants at surface
XXX
WIND SPEED AND DIRECTION
• Mesoscale circulation
• Large scale circulation
Mesoscale Circulations Affecting
Dispersion
Land-Sea Breeze: Daytime (Sea Breeze)
Upper Level Return Flow
Air Warmed over Land Expands
(Becomes Less Dense)
Air Cooled over Water Contracts
(Becomes More Dense)
Sea Breeze (arises due to density differences)
Warmer Land
Cooler Water
Reverses at Night as Water Remains Warmer than
Land to Make Land Breeze
Mesoscale Circulations Affecting
Dispersion
1. Mountain/Valley Winds
Day:
Night:
Warm
Mtn
Cool
Mtn
2. Urban/Heat Island (Night)
PBL Top
CITY
Large Scale Circulation
• Transboundary air pollution
• Acid deposition
• Ozone transport
Applications of Air Pollution
Meteorology
• Atmospheric Dispersion Modeling
• Study of Accidental Release of Hazardous Substances
Including Radioactive Nuclides
• Applications of air quality meteorology can be used for
dispersion modeling, i.e., predicting the path of the
pollutant concentration and for calculations of ground
sources, such as hazardous waste spills.
• Let’s first look at dispersion modeling.
Air Pollution Meteorology
• Meteorology very important factor in
developing strategies for air pollution
control
• State of the lower troposphere (PBL)
plays large role in dispersion of
pollutants and plumes:
– Mechanical Turbulence
– Buoyant Turbulence
– Circulation
Wind Speed and Direction
• The average ground level wind
speed is about 4.5 m/s.
–“Calm” wind is less than 0.5m/s
• Wind speed almost always
increases with height.
–ground friction slows lower level
winds
A Wind Rose
A Wind Rose
Wind Speed With Height
• Deacon’s power law:
u2 / u1 = (z2 / z1)p
where:
u1 is the wind speed at elevation z1
u2 is the wind speed at elevation z2
and p is an exponent that depends on stability
and ground characteristics
Note: Wind speed measured by the NWS is
usually obtained at z = 10 meters (z1)
Impact of Fixed Geographic
Features
• TERRAIN EFFECTS
•
•
•
•
Sea breeze
Valley wind
Drainage wind
Flow patterns due to topographical
features
Temperature Gradient
• Air temperature is not uniform with
altitude at a given location.
• Reasons:
a) heating by the ground
b) heating by the sun
c) cloud cover
d) evaporative cooling over the oceans
e) expansion of gases due to air
movement
Stability and Lapse Rate
• The lapse rate determines how readily
parcels of air move upward or
downward.
• In stable atmospheres = vertical
movement is opposed by the
temperature gradient
• In unstable atmospheres = vertical
movement is enhanced
• In neutral atmospheres = neither
Stability Classes
A = very unstable
B = moderately unstable
C = slightly unstable
D = neutral
E = slightly stable
F = stable
Why is stability important?
• Stability affects plume rise.
• Plume rise can be calculated using
information about the stack gases and
meteorology.
• Stability can effect the dispersion and
appearance of plumes being emitted
from stacks.
Inversions
• An inversion is a situation of
increasing temperature with height.
• Pre-dawn mornings have an
inversion that reached up to about
1000 ft (100m).
• Atmospheres within an inversion are
extremely stable, with damped
vertical mixing.
Surface Temperature Inversions:
a)Are very common
b)Are easy to recognize
c) Affect the dispersal of very small spray
droplets suspended in the air
d)Do not increase the amount of off-site
movement
e)Can increase the potential for offsite
affects & the distance at which affects
can be observed
Atmospheric Stability
i. Indicator of atmospheric turbulence
ii. Depends on static stability, thermal and
mechanical turbulence
iii. Unstable : Lapse rate > dry adiabatic lapse
rate
iv. Neutral : Lapse rate = dry adiabatic lapse rate
v. Stable : Lapse rate < dry adiabatic lapse rate
vi. Turner method: solar angle, cloud cover and
wind speed
IMPORTANCE OF METEOROLOGY
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Dispersion
Transport
Wind speed and direction
Temperature
Stability
Mixing height
Any questions?
Thank you…