Fundamentals of air Pollution Engineering METEOROLOGY

The science of meteorology has great bearing on air pollution The science of meteorology has great bearing on air pollution. An air pollution problem involves three parts: the source, the movement of the pollutant and the recipient. All meteorological phenomena are a result of interaction of the elemental properties of the atmosphere, heat, pressure, wind and moisture. In this lecture we will discuss the meteorological conditions, which directly influence the transport and dispersion of pollutants.

METEOROLOGY Pure air is a mixture of gases, containing; 78.0% nitrogen 20.1% oxygen 0.9% argon 0.03% carbon dioxide 0.002% neon 0.0005%helium but pure air is not found in nature and is of interest only as reference.

Earth atmophere can be divided into four major layers: Troposphere – where most of our weather occurs, ranges from 5 km at the poles to about 18 km equator. The temperature decrease with altitude. Over 80% of air is within this well-mixed layer. Stratosphere – a layer of air where the temperature profile is inverted and in which little mixing take place. Pollutant that migrate into this layer can stay for many years. This layer has a high ozone concentration. Mesosphere and Thermosphere – contain only about 0.1 % of the air. Air pollution problem occur in the troposhere. Pollutants in the troposphere wheather produced naturally or emitted from human activities are moved by air currents that we call wind. Wind not only moves the pollutants horizontally, but causes the pollutants to disperse, reducing the concentration of the pollutant with distance away from the source.

Thermosphere A L T I TUD E Mesosphere Stratosphere (km) Troposphere 20 40 60 80 100 A L T I TUD E (km) Troposphere Stratosphere Mesosphere Thermosphere 200 300 400 Temperature (K)

Stability Stability is the tendency of the atmosphere to resist and enhance vertical motion. Lapse rate is the change of air temperature with height. It is used as indicator as the stability condition of the atmosphere. There are 3 stability categories that are: Neutral atmosphere Unstable atmosphere Stable atmosphere

a) Neutral Atmosphere The lapse rate for a neutral atmosphere is defined by the rate of temperature increase or decrease experienced by the parcel of air that expands or contracts adiabatically (without addition or loss of heat) as it raised through the atmosphere. The rate of temperature decrease (dT/dZ) is called dry adiabatic lapse rate and it is designated by Г. dT/dZ is approximately -1.00OC/100 m.

Stability From the figure, since the ambient lapse rate is equal to Г so the atmosphere is said to have neutral stability.

b) Unstable Atmosphere If the temperature rate of atmosphere is greater than Г, the lapse rate is said to be superadiabatic and the atmophere is unstable. If we capture a ballon full of polluted air at elevation A and adiabatically displaced the ballon at 100 m above (B), the temperature inside the ballon will decrease 1 OC (according to Г). The temperature outside the balloon will decrease to 19.90 OC at elevation B (according to ambient lapse rate of –1.25 OC/100 m). Since the outside air is cooler than inside, the balloon will have tendency to rise. If we displace the balloon adiabatically to elevation C the temperature inside the balloon will increase to 22.15 OC, while the ambient temperature would increase to 22.40 OC. The inside air will be cooler than the outside air thus results the balloon to sink.

c) Stable Atmosphere If the temperature of the atmosphere falls at the rate less than Г, it is called subadiabatic and the atmosphere is stable. If we capture a ballon full of polluted air at elevation A and adiabatically displaced the ballon at 100 m above (B), the temperature inside the ballon will decrease 1 OC (according to Г). The temperature outside the balloon will decrease to 20.65 OC at elevation B (according to ambient lapse rate of –0.5 OC/100 m). Since the inside air is cooler than outside, the balloon will have tendency to sink. If we displace the balloon adiabatically to elevation C the temperature inside the balloon will increase to 22.15 OC, while the ambient temperature would increase to 21.65 OC. The balloon will be warmer than the ambient air thus results the balloon to rise.

There are 2 special cases of subadiabatic lapse rate: Isothermal – No changes of temperature with elevation. Inversion – The temperature increases with elevation. It is the most severe form of stable atmosphere but when it occur it often asociated with restricted air volumes that cause air pollution

EXAMPLE Given the following temperature and elevation data, determine the stability of the atmosphere. Z,m Temperature (OC) 2 -3.05 318 -6.21

SOLUTION 1) Begin by determining lapse rate: = -6.21 – (-3.05) = -0.01OC/m 318 – 2 = -0.01OC x 100m = -1.00OC m 2) Compare with Г and find that they are equal. Thus the atmospheric stability is neutral.

Plume Types The plume (smoke trail) from a tall stack located on flat terrain has been found to exhibit a charecteristic shape that is dependent on the stability of the atmosphere. There are 5 plume types that are:- Looping plume – Occur in highly unstable conditions. Unstable conditions are generally favorable for pollutant dispersion, momentarily high ground-level concentrations can occur if the plume loops downward to the surface. 2) Coning plume – is characteristic of neutral conditionsor slightly stable conditions. It is likely to occur on cloudy days and sunny days before the development of unstable daytime condition.

Fanning plume – Occur in stable conditions Fanning plume – Occur in stable conditions. The inversion lapse rate discourages vertical motion without prohibiting horizontal motion and the plume may extend downwind from the source for a long distance. Often occur in the early morning. Lofting – When conditions are unstable above an inversion, the release of a plume above the inversion results in effective dispersion without noticeable effects on ground level concentration around the source. 5) Fumigation and trapping – If the plume is released just under an inversion layer, a serious air pollution situation could develop. When the instability reaches the level of the plume that is still trapped below the inversion layer, the pollutants can be rapidly transported down toward the ground.

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.

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

Land-Sea Valley 11/10/2018

Calculations for Wind Rose % Frequency = Number of observations * 100/Total Number of Observations Direction: N, NNE, ------------------------,NNW, Calm Wind speed: Calm, 1-3, 4-6, 7-10, -----------

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

Determination of Maximum Mixing Height

Power plant Plumes

Power plant Plumes

Try to read questions on pages 35 & 36 at the text book Power plant Plumes Try to read questions on pages 35 & 36 at the text book

Stability Conditions Adiabatic lapse rate Environmental lapse rate

Superadiabatic Lapse Rates (Unstable) Temperature decreases are greater than -10o C/km Occur on sunny days Characterized by intense vertical mixing Excellent dispersion conditions

Isothermal Lapse Rates (Weakly Stable) Neutral Lapse Rates Temperature decreases are similar to the adiabatic lapse rate Results from: Cloudy conditions Elevated wind speeds Day/night transitions Describes good dispersion conditions Isothermal Lapse Rates (Weakly Stable) Characterized by no temperature change with height Atmosphere is somewhat stable Dispersion conditions are moderate

Inverted Lapse Rates (Strongly Stable) Characterized by increasing temperature with height www.co.mendocino.ca.us/aqmd/Inversions.htm Inversion

Plume Rise As you can see from the preceding figure the height a plume rises is very important. No theoretical model has been developed to predict plume rise, but several good empirical models have been developed. One of those is presented below: )h = 2.6 [F/( u S)]1/3 )h = plume rise above stack, m u = average wind speed, m/sec )T/)z = prevailing lapse rate, oC/m Vs = stack gas exit velocity, m/s d = stack diameter, m Ta = temperature of atmosphere, oC Ts = temperature of stack gas, oC F = buoyancy flux, m4sec-3 F = [gVsd2(Ts – Ta)] / [4 (Ta + 273)] S = [g/(Ta + 273)] [ ()T/)z) + 0.01]

Example A stack has an emission exiting at 3 m/sec through a stack with a diameter of 2 m. The average wind speed is 6 m/sec. The air temperature at the stack exit elevation is 28oC and the temperature of the emission is 167oC. The atmosphere is at neutral stability. What is the expected rise of the plume? )h = 2.6 [F/( u S)]1/3 At neutral stability, )T/)z = 0.01 oC/m F = [9.8 x 3 x 22 x (167 – 28)] / [ 4 (28 + 273)] = 13.6 S = [9.8 / (28 + 273)] ( 0.01 + 0.01) = 6.51 x 10-4 )h = 2.6 [13.6 / ( 6 x 6.51 x 10-4)]1/3 = 40 m