Download Air Pollution Meteorology

Air Pollution Meteorology
Prof Shirley Brooks and V Zungu (PhD…)
What is Stability?
Vertical motion of air parcels.
What is Atmospheric Stability
• The vertical movement of air molecules characterized by
certain basic conditions (Temperature) that determine the
general stability of the atmosphere.
• Downward motion – Adiabatic warming
• Upward motion – Adiabatic cooling
Stability & Movement
•A rock, like a parcel of air, that is in stable equilibrium will return to its
original position when pushed.
•If the rock instead departs in the direction of the push, it was in
unstable equilibrium.
Behavior of Rising and Sinking Air
•Rising air expands, using
energy to push out, which
slows and adiabatically cools
the air.
•A parcel of air may be forced
to rise or sink, and change
temperature relative to the
environmental air, which is
sampled using radiosonde
balloons.
•The radiosonde balloon
expands in size from
approximately 6 feet to a
diameter between 24 and 32 ft
before it bursts. The balloon
carries the instrument package
to an altitude of
approximately 25 mi (27-37
km) where the balloon bursts
(at a pressure of
approximately 10 mb).
= Exerted Pressure
Key Terms When Discussing Stability
• Adiabatic Process - is when an air parcel cools by expansion or warms
by compression with no exchange of heat from the the outside
environment.
• Dry Adiabatic Rate - the rate at which an “unsaturated” parcel is
cooled or warmed adiabatically (adiabatic process). The dry adiabatic
rate is 10°C per 1000 m or 5.5°F per 1000 ft and it remains constant.
• Moist Adiabatic Rate - the rate at which a “saturated” parcel is cooled
and warms with ascending or descending motion. This rate varies but it
is less than the dry adiabatic rate due to latent heating from condensation
offsetting the cooling. A commonly used value for the moist adiabatic
rate is 6°C per 1000 m or 3.3°F per 1000 ft. This rate is not an
adiabatic process due to latent heating.
• Environmental Lapse Rate - the rate at which ambient air temperature
decreases with height. This rate can vary as well and must be measured
by a radiosonde.
Lapse Rate
• Important characteristic of atmosphere is
ability to resist vertical motion: stability
• Affects ability to disperse pollutants
• When small volume of air is displaced upward
– Encounters lower pressure
– Expands to lower temperature
– Assume no heat transfers to surrounding
atmosphere
– Called adiabatic expansion
Adiabatic Expansion
To determine the change in temp. w/ elevation due to
adiabatic expansion
– Atmosphere considered a stationary column of air in a
gravitational field
– Gas is a dry ideal gas
– Ignoring friction and inertial effects
( dT/dz)adiabatic perfect gas = - (g M/ Cp)
•
•
•
•
•
T = temperature
z = vertical distance
g = acceleration due to gravity
M = molecular weight of air
Cp = heat capacity of the gas at constant pressure
Adiabatic Expansion
( dT/dz)adiabatic perfect gas = -0.0098°C/m
or
( dT/dz)adiabatic perfect gas = -5.4°F/ft
Change in Temp. with change in height
Lapse rate
• Lapse rate is the negative of temperature
gradient
• Dry adiabatic lapse rate =
Metric:
Γ = - 1°C/100m or
SI:
Γ = - 5.4°F/1000ft
Conti….
• Important is ability to resist vertical motion:
stability
• Comparison of Γ to actual environment lapse rate
indicates stability of atmosphere
• Degree of stability is a measure of the ability of
the atmosphere to disperse pollutants
Atmospheric Stability
• Affects dispersion of pollutants
• Temperature/elevation relationship principal
determinant of atmospheric stability
• Stable
– Little vertical mixing
– Pollutants emitted near surface tend to stay there
– Environmental lapse rate is same as the dry
adiabatic lapse rate
Stability Classes
• Developed for use in dispersion models
• Stability classified into 6 classes (A – F)
• A: strongly unstable
• B: moderately unstable
• C: slightly unstable
• D: neutral
• E: slightly stable
• F: moderately stable
Vertical Temperature Profiles
Environmental lapse rate (ELR)
Dry adiabatic lapse rate (DALR)
If:
ELR > DALR =sub adiabatic condition, atmosphere is stable.
ELR >> DALR= Inversion conditions. Very stable atmosphere.
ELR= DALR= atmosphere is neutral.
ELR< DALR = super adiabatic condition, atmosphere is
unstable.
Shapes of plumes depends upon atmospheric stability conditions.
Stability and Thermodynamic Model
(Air movement behavior)
thermodynamic property – measurements or soundings
day
γa > γ₀ unstable
γb = γ₀ neutrally stable
γc < γ₀ stable
On day a a parcel will cool more slowly than surroundings – air will be
warmer and rise.
On day b a parcel will always have same temperature as surroundings –
no force of buoyancy.
On day c a parcel will cool more quickly than surroundings – air will be
cooler and return to original altitude.
Copyright R. R. Dickerson 2011
19
Central Concepts
• The dry adiabatic lapse rate is one degree Celsius of cooling for every
100 meters (-1°C/100m, -10°C/kilometer). This is the parcel of Dry air in
the atmosphere!!!
• As the parcel of air rises and it cools, it will eventually cool to the dew
point when condensation can begin and clouds will form
• However- Air that is saturated with water has reached the dew point
temperature and is carrying as much moisture as that parcel of air is
capable of holding at that temperature.
• This saturated parcel of air has a saturated adiabatic lapse rate (also known
as wet adiabatic lapse rate) of 0.5°C/100 m (5°C/kilometer).
Creating Adiabatic Graph
• First must plot Environmental Lapse Rate Data.
• Create graph of the adiabatic temperature change
for the air parcels.
• Label where the air is cooling at the DAR and
SAR, identify the level of condensation where
clouds start to form, and where the air is stable
and unstable.
Graphing ELR and DALR
• Lapse rate = -DT/DZ = (T2-T1)/(Z2-Z1)
– DALR = 1°C/100m and WALR = 0.60C/100m
• Ground level Temp = 150C
• Dew Point Temp = ?
• Need to graph the adiabatic temperature change.
To do this I need three points:
– Point 1 – Ground level air temperature
– Point 2 – The condensation level
– Point 3 – The end point (highest elevation for the
problem)
Condensation Level
•
•
Ground Level: 150C
Mixing Ratio: 8 g/kg ~ Mixing ratio (w) is the amount of water vapor that is
in the air i.e. w is the grams of vapor per kg of dry air. w is an absolute
measure of the amount of water vapor in the air.
•
Mass of Water Vapour to Mass of Dry Air:
•
Cond… Level: 100c
•
Need an ELR Graph
10
Height in (M)
ELR Graph
Temp C0
DAR and Condensation
Level
(a)15o C – 10o = 50C.
(a)But need to know the
actual level in meters:
(a)50C * 100m = 500m
500m
Step 3: Plot Ending Temperature
• Calculate change (∆) in elevation:
= starting elevation (h) – ending elevation (h)
= 500m – 3000m
= -2500
Calculate change (∆) in Temperature:
= ∆ elevation x Lapse Rate (dt/dz)
= -2500 x 0.60C/100m
= -15
Calculate ending Temperature:
= starting temperature + ∆ temperature
=100C +-150C
= -50C
Plot Change in Temperature from CL
Stable and Unstable
Mixing Height of atmosphere
The height of the base of the inversion layer from ground
surface.
General Characteristics of Stack
Plumes
• Dispersion of pollutants
• Wind – carries pollution downstream from source
• Atmospheric turbulence -- causes pollutants to
fluctuate from mainstream in vertical and crosswind
directions
• Mechanical & atmospheric heating both present at
same time but in varying ratios
• Affected plume dispersion is differently- location &
time
Plume Types
• Plume types are important because they help
us understand under what conditions there
will be higher concentrations of contaminants
at ground level.
Looping Plume
• High degree of convective
turbulence
• Superadiabatic lapse rate -- strong
instabilities
• Associated with clear daytime
conditions accompanied by strong
solar heating & light winds
• High probability of high
concentrations sporadically at
ground level close to stack.
• Occurs in unstable atmospheric
conditions.
Coning Plume
• Stable with small-scale
turbulence
• Associated with overcast
moderate to strong winds
• Roughly 10° cone
• Pollutants travel fairly long
distances before reaching
ground level in significant
amounts
• Occurs in neutral
atmospheric conditions
Fanning Plume
• Occurs under large negative
lapse rate
• Strong inversion at a
considerable distance above
the stack
• Extremely stable atmosphere
• Little turbulence
• If plume density is similar to
air, travels downwind at
approximately same elevation
Lofting Plume
• Favorable in the sense
that fewer impacts at
ground level.
• Pollutants go up into
environment.
• They are created when
atmospheric conditions
are unstable above the
plume and stable
below.
Fumigation
• Most dangerous plume:
contaminants are all coming
down to ground level.
• They are created when
atmospheric conditions are
stable above the plume and
unstable below.
• This happens most often
after the daylight sun has
warmed the atmosphere,
which turns a night time
fanning plume into
fumigation for about a half
an hour.
Vertical Temperature
Profiles
Environmental lapse rate (ELR)
Dry adiabatic lapse rate (DALR)
If,
ELR > DALR =sub adiabatic
condition, atmosphere is stable.
ELR >> DALR= Inversion
conditions. Very stable atmosphere.
ELR= DALR= atmosphere is
neutral.
ELR< DALR = super adiabatic
condition, atmosphere is unstable.
Shapes of plumes depends upon
atmospheric stability conditions.
THANK YOU