Air Pollution Meteorology Ñ Atmospheric thermodynamics Ñ Atmospheric stability Ñ Boundary layer development Ñ Effect of meteorology on plume dispersion

Atmosphere Ï Pollution cloud is interpreted by the chemical composition and physical characteristics of the atmosphere Ï Concentration of gases in the atmosphere varies from trace levels to very high levels Ï Four major layers of earth’s atmosphere are: ° Troposphere ° Stratosphere ° Mesosphere ° Thermosphere

Atmospheric Thermodynamics Ø A parcel of air is defined using the state variables Ø Three important state variables are density, pressure and temperature Ø The units and dimensions for the state variables are Ø Humidity is the fourth important variable that gives the amount of water vapor present in a sample of moist air Density (mass/volume) gm/cm 3 ML -3 Pressure (Force/Area) N/m 2 ( P a ) ML -1 T -2 Temperature o F, o R, o C, o K T

Equation of State « Relationship between the three state variables may be written as: « f ( P, ρ,T) = 0 « For a perfect gas: « P = ρ.R.T « R is Specific gas constant R for dry air = Joules / gm / o K R for water vapor = Joules / gm / o K R for wet air is not constant and depend on mixing ratio

Laws of Thermodynamics First Law of Thermodynamics: ­ This law is based on law of conservation of total energy. ­ Heat added per unit mass = (Change in internal energy per unit mass) + (Work done by a unit mass) Second Law of Thermodynamics: ­ This law can be stated as "no cyclic process exists having the transference of heat from a colder to hotter body as its sole effect"

Specific Heat Ø Defined as the amount of heat needed to change the temperature of unit mass by 1 o K. Ø Specific heat at constant volume C v = lim δQ δT 0 δT α = const Ø Specific heat at constant pressure C p = lim δQ δT 0 δT p = const Ø Relationship between C v and C p is given by Carnot’s law: ¬ For perfect gas, C p – C v = R ¬ For dry air C p = (7/2). R C v = (5/2). R ¬ Ratio of C p and C v for dry air is 1.4

Processes in the Atmosphere Ô An air parcel follows several different paths when it moves from one point to another point in the atmosphere. These are:  Isobaric change – constant pressure  Isosteric change – constant volume  Isothermal change – constant temperature  Isentropic change – constant entropy (E)  Adiabatic Process – δQ = 0 (no heat is added or removed ) Ô The adiabatic law is P. α γ = constant Ò E =

Statics of the Atmosphere  Vertical variation of the parameters = ? Hydrostatic Equation: ³ Pressure variation in a "motionless" atmosphere ³ Pressure variation in an atmosphere: ³ Relationship between pressure and elevation using gas law:

Statics of the Atmosphere ± Integration of the above equation gives Using the initial condition Z=0, P = P 0 ± The above equation indicates that the variation of pressure depends on vertical profile of temperature. ± For iso-thermal atmosphere ± Therefore pressure decreases exponentially with height. ± mb per 100m.

Lapse Rate: ² Lapse rate is the rate of change of temperature with height ² Lapse rate is defined as Γ = -δT δz ² Value of Γ varies throughout the atmosphere Potential Temperature: ¯ Concept of potential temperature is useful in comparing two air parcels at same temperatures and different pressures ¯ θ = T o = T 1000 R/C p P

Atmospheric Stability ¶ The ability of the atmosphere to enhance or to resist atmospheric motions ¶ The stability depends on the ratio of suppression to generation of turbulence ¶ The stability at any given time will depend upon static stability ( related to change in temperature with height ), thermal turbulence ( caused by solar heating ), and mechanical turbulence (a function of wind speed and surface roughness).

Atmospheric Stability Γ > Γ d Unstable Γ = Γ d Neutral Γ < Γ d Stable ­ Atmospheric stability can be determined using adiabatic lapse rate. ³ Γ is environmental lapse rate ³ Γ d is adiabatic lapse rate (1 0 c/100m) and dT/dZ = -1 0 c /100 m

Atmospheric Stability Classification Schemes to define the atmospheric stability are: Ò P- G Method Ò P-G / NWS Method Ò The STAR Method Ò BNL Scheme Ò Sigma Phi Method Ò Sigma Omega Method Ò Modified Sigma Theta Method Ò NRC Temperature Difference Method Ò Wind Speed ratio (U R ) Method

Turbulence Fluctuations in wind flow which have a frequency of more than 2 cycles/ hr Types of Turbulence Ø Mechanical Turbulence Ø Convective Turbulence

Boundary Layer Development ­ Thermal boundary Layer (TBL) development depends on two factors: ³ Convectively produced turbulence ³ Mechanically produced turbulence ­ Development of TBL can be predicted by two distinct approaches: ³ Theoretical approach ³ Experimental studies ­ Theoretical approach may be classified into three groups: ³ Empirical formulae ³ Analytical solutions ³ Numerical models ³ One layer models ³ Higher order closure models

Effects of Meteorology on Plume Dispersion ¯ Dispersion of emission into atmosphere depends on various meteorological factors. ¯ Height of thermal boundary layer is one of the important factors responsible for high ground level concentrations ¯ At 9 AM pollutants are pulled to the ground by convective eddies ¯ Spread of plume is restricted in vertical due to thermal boundary height at this time

Wind Velocity ¯ A power law profile is used to describe the variation of wind speed with height in the surface boundary layer U = U 1 (Z/Z 1 ) p Where U 1 is the velocity at Z 1 (usually 10 m) ² U is the velocity at height Z. ² The values of p are given in the following table. Stability ClassRural pUrban p Very Unstable Neutral Very Stable

Beaufort Scale ­ This scale is helpful in getting an idea on the magnitude of wind speed from real life observations Atmospheric condition Wind speedComments Calm < 1mphSmoke rises vertically Light breeze 5 mphWind felt on face Gentle breeze 10 mphLeaves in constant motion Strong 25 mphLarge branches in motion Violent storm 60 mphWide spread damage

Wind Rose Diagram (WRD) ¯ WRD provides the graphical summary of the frequency distribution of wind direction and wind speed over a period of time ¯ Steps to develop a wind rose diagram from hourly observations are: Ø Analysis for wind direction Ø Determination of frequency of wind in a given wind direction Ø Analysis for mean wind speed Ø Preparation of polar diagram

Determination of Maximum Mixing Height Steps to determine the maximum mixing height for a day are: ¯ Plot the temperature profile, if needed ¯ Plot the maximum surface temperature for the day on the graph for morning temperature profile ¯ Draw dry adiabatic line from a point of maximum surface temperature to a point where it intersects the morning temperature profile ¯ Read the corresponding height above ground at the point of intersection obtained. This is the maximum mixing height for the day