Air Pollution Meteorology

1. Air Pollution Meteorology • Atmospheric thermodynamics • Atmospheric stability • Boundary layer development • Effect of meteorology on plume dispersion

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

3. Atmospheric Thermodynamics • A parcel of air is defined using the state variables • Three important state variables are density, pressure and temperature • The units and dimensions for the state variables are • Humidity is the fourth important variable that gives the amount of water vapor present in a sample of moist air

4. 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 = 0.287 Joules / gm /oK • R for water vapor = 0.461 Joules / gm /oK • R for wet air is not constant and depend on mixing ratio

5. 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"

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

7. Processes in the Atmosphere • 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 ) • The adiabatic law is P. αγ = constant • E =

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

9. Statics of the Atmosphere • Integration of the above equation gives Using the initial condition Z=0, P = P0 • 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. • 12.24 mb per 100m.

10. Lapse Rate: • Lapse rate is the rate of change of temperature with height • Lapse rate is defined as Γ = -δT δz • 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 = To = T 1000 R/Cp P

11. 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).

12. Atmospheric Stability • Atmospheric stability can be determined using adiabatic lapse rate. • Γ is environmental lapse rate • Γd is adiabatic lapse rate (10c/100m) and dT/dZ = -10c /100 m

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

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

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

16. 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

17. Wind Velocity • A power law profile is used to describe the variation of wind speed with height in the surface boundary layer • U = U1 (Z/Z1)p • Where U1 is the velocity at Z1 (usually 10 m) • U is the velocity at height Z. • The values of p are given in the following table.

18. Beaufort Scale • This scale is helpful in getting an idea on the magnitude of wind speed from real life observations

19. 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: • Analysis for wind direction • Determination of frequency of wind in a given wind direction • Analysis for mean wind speed • Preparation of polar diagram

20. 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 themaximum mixing height for the day