Transport and dispersion of air pollution Topic 4 Ms Sherina Kamal Transport and dispersion of air pollution 25/4/2017

Outline pollutant transport in the atmosphere Vertical Horizontal Adiabatic & environmental lapse rate Atmospheric stability Temperature inversion Horizontal Wind speed Wind direction Plume rise Dispersion model Gaussian plume equation derivatives 25/4/2017

Vertical pollutant transport in the atmosphere Adiabatic lapse rate the rate at which atmospheric temperature decreases with increasing altitude in conditions of …………………………….. is when two objects that are at different temperatures balance each other out the condition under which two substances in physical contact with each other exchange no heat energy Environmental lapse rate The actual decrease in temperature with an increase in altitude through the troposphere 25/4/2017

Vertical pollutant transport in the atmosphere Atmospheric stability Resistance of the atmosphere to vertical motion Temperature inversion Warm air on top of cooler air 25/4/2017

Temperature inversion Play a significant role in air pollution meteorology 4 ways to produce inversion: Cool a layer of air from below Heat a layer of air from above Flow a layer of warm air Flow a layer of cold air under a layer of warm air 25/4/2017

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Vertical pollutant transport in the atmosphere Wind speed Increases with elevation, most of the time, in most of the troposphere Reasoning: ground friction slows the wind 25/4/2017

Vertical pollutant transport in the atmosphere Wind direction the direction from which the wind is blowing it is expressed in degrees measured clockwise from geographical north Wind vanes do not respond to changes in wind direction when the wind speed is less than one metre per second or two knots 25/4/2017

Wind direction In estimating wind direction at any time and any location; can use the following rule of thumb: Major, rapidly moving storms and fronts overwhelm all local influences; local ground-level winds blow the way the major storms dictate In deep valleys the daily alternation-wind up the valley in the daytime, down at night Onshore and offshore breezes dominate when there is no major storm Absent of all preceding/any other effects of local topography

Plume rise Gases that are emitted from stacks are often pushed out by fans As the turbulent exhaust gases exit the stack they mix with ambient air This mixing of ambient air into the plume - entrainment As the plume entrains air into it, the plume diameter grows as it travels downwind 25/4/2017

Plume rise These gases have momentum as they enter the atmosphere Often these gases are heated and are warmer than the outdoor air In these cases the emitted gases are less dense than the outside air and are therefore buoyant Buoyant: able/tend to keep afloat or rise to the top of a gas or liquid 25/4/2017

Plume rise A combination of the gases' momentum and buoyancy causes the gases to rise. This is referred to as plume rise and allows air pollutants emitted in this gas stream to be lofted higher in the atmosphere Since the plume is higher in the atmosphere and at a further distance from the ground, the plume will disperse more before it reaches ground level 25/4/2017

Plume rise 25/4/2017

Effect of temperature profile on plume rise 25/4/2017

Dispersion model Gaussian plume 25/4/2017

Gaussian plume 25/4/2017