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
Senior Lecturer (Environmental Health and Safety )
1oC/100m = 10oC/km = 5.4oF/1000 ft
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.
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 downstream.
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.
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.
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.
PBL below stack top: little or no concentration of pollutants at the surface
PBL well above stack top: decreased concentrations of pollutants at the surface
PBL just above stack top: increased concentrations of pollutants at the surface
2. ATMOSPHERIC STABILITY
Unstable Conditions: leads to greater dispersion of pollutants
Stable conditions: lead to less dispersion of pollutants
Unstable Conditions: Lead to lower concentration of
pollutants at surface
Stable Conditions: Leads to greater concentration of
pollutants at surface
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)
Reverses at Night as Water Remains Warmer than Land to Make Land Breeze
1. Mountain/Valley Winds
2. Urban/Heat Island (Night)
u2 / u1 = (z2 / z1)p
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)
A = very unstable
B = moderately unstable
C = slightly unstable
D = neutral
E = slightly stable
F = stable