Metrological aspects of air pollution dispersion

1. INTRODUCTION

The decrease of an atmospheric variable with height, the variable being temperature unless otherwise specified. The lapse rate is defined as the rate at which atmospheric temperature decreases with an increase in altitude. The terminology arises from the word lapse in the sense of a decrease or decline. While most often applied to Earth's troposphere, the concept can be extended to any gravitationally supported parcel of gas. It can be

• Lapse rates
• Temperature lapse rates
• Environmental lapse rates
• Adiabatic lapse rates

1.1 Lapse rates

The decrease of an atmospheric variable or any meteorological factor with altitude, the variable may be temperature, pressure, density etc. In the case of temperature, a lapse rate is the negative of the rate of temperature change with altitude change.

So y =dT/DZ

Where T=temperature,

Z=altitude,

Y= lapse rate

The lapse rate of rising air—commonly referred to as the normal lapse rate—is highly variable, being affected by radiation, convection, and condensation; it averages about 6.5 °C per kilometer (18.8 °F per mile) in the lower atmosphere (troposphere). The measurable lapse rate is affected by the moisture content of the air (humidity).

1.2 Temperature lapse rates

The rate of change of atmospheric temperature while moving upward from the earth surface is temperature lapse rates.

For a polytrophic model

dT/dz =-(n-1/n)g/R

Where R is universal gas constant

g is accel due to gravity

n is the polytrophic factor

for troposphere n=1.23

for the stratosphere n=1

1.3 Environmental lapse rate

The environmental lapse rate (ELR), is the rate of decrease of temperature with altitude in the stationary atmosphere at a given time and location. The environmental lapse rate found to be 6.5’c/km up to the 12km from the ground level then after nearly constant up to 25km .On a sunny day, it is about 10’c/km up to 200 meters. The temperature of the actual atmosphere does not always fall at a uniform rate with height, For example, there can be an inversion layer in which the temperature increases with altitude. We also assume ideal condition i.e. standard atmosphere contains no moisture. This rate varies from time to time and from place to place. A rawinsonde’s thermometer measures the environmental lapse rate.

1.4 Adiabatic lapse rates

1.4.1 Dry adiabatic lapse rate

The dry adiabatic lapse rate (DALR) is the rate of temperature decrease with altitude for a parcel of dry or unsaturated air rising under adiabatic conditions. Unsaturated air has less than 100% relative humidity; i.e. its actual temperature is higher than its dew point. The term adiabatic means that no heat transfer occurs into or out of the parcel. Air has low thermal conductivity, and the bodies of air involved are very large, so the transfer of heat by conduction is negligibly small.

Under these conditions when the air rises (for instance, by convection) it expands because the pressure is lower at higher altitudes. As the air parcel expands, it pushes on the air around it, doing work (thermodynamics). Since the parcel does work but gains no heat, it loses internal energy so that its temperature decreases. The rate of temperature decrease is 9.8 °C/km (5.38 °F per 1,000 ft) (3.0 °C/1,000 ft). The reverse occurs for a sinking parcel of air.

1.4.2 Saturated adiabatic lapse rate

When the air contains moisture saturated adiabatic lapse rate (SALR) applies which strongly varies with temperature and is about 5 °C/km.It releases heat when it condense, thus decreasing the rate of temperature drop as altitude increases. The saturated adiabatic lapse rate is about one-third of the dry adiabatic lapse rate. If the saturated parcel has a high vapor content the MALR will be much smaller than the DALR. If the saturated parcel has a low vapor content (example, very cold air) the MALR will approach the DALR.

2) Atmospheric stability

The ability of the atmosphere to disperse pollutants emitted into it depends on to a large extent on the degree of its stability Atmospheric stability is a measure of the atmosphere's tendency to encourage or determine vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. Atmospheric instability is a condition where the Earth's atmosphere is generally considered to be unstable and as a result, the weather is subjected to a high degree of variability through distance and time. In unstable conditions, a lifted parcel of air will be warmer than the surrounding air at altitude. Because it is warmer, it is less dense and is prone to further ascent.

There are two primary forms of atmospheric instability

2.1 Convective instability

Under convective instability thermal mixing through convection in the form of warm air rising leads to the development of clouds and possibly precipitation or convective storms.

2.2 Dynamic instability (fluid mechanics)

Dynamic instability is produced through the horizontal movement of air and the physical forces it is subjected to such as the Coriolis force and pressure gradient force. Dynamic lifting and mixing produce cloud, precipitation and storms often on a synoptic scale.

Comparison with environment lapse rate and adiabatic lapse rate give the idea of the stability of the atmosphere. Condition applied,

ELR > DALR i.e. air temp decreases rapidly with height → an unstable atmosphere i.e. super-adiabatic lapse rate (favors vertical mixing)

ELR < DALR i.e. air temp decreases slowly with height or may increase with height (i.e. an inversion) → the atmosphere is stable i.e. sub-adiabatic lapse rates (strongly resists vertical mixing)

ELR = DALR i.e. air temp decreases at the rate of about 9.8oC/km the atmosphere is neutral (no relative tendency for the air parcel to rise or sink)

Same reason can be applied for MALR.

2.3 Air parcels and stability

Stable: if the parcel is displaced vertically, it will return to its original position.

Neutral: if the parcel is displaced vertically, it will remain in its new position.

Unstable: if the parcel is displaced vertically, it will accelerate away from its original position in the direction of the initial displacement.

3) Inversion

It is the extreme case of stable atmosphere occurs at negative lapse rates and practically no mixing of pollutants. Atmospheric inversion influences the dispersion of pollutants by restricting vertical mixing.

Types

3.1 Subsidence inversion

An inversion can develop aloft as a result of air gradually sinking over a wide area and being warmed by adiabatic compression, usually associated with a subtropical high-pressure area. A subsidence inversion develops when a widespread layer of air descends. The layer is compressed and heated by the resulting increase in atmospheric pressure, and as a result, the lapse rate of temperature is reduced. If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes, producing a temperature inversion. Subsidence Inversions take pace in valleys or in places partially surrounded by hills or mountains. When the air blows over the hills, it is heated as it is compressed into the side of the hills. When that warm air comes over the top, it is warmer than the cooler air of the valley. Also, increasing the inversion, as the air comes over the top of the hill, it causes the air in the valley to be compressed, heating the cooler valley air from the top down. When this cool air is trapped and compressed, so is everything that is in the air, such as vehicle emissions, smoke, and smog in general.

3.2 Radiation inversion

This is the result from a normal diurnal cooling cycle, happens in places where it cools off a lot at night.Eg fog in the morning, smog, turbulence, reduced visibility etc. Gentle breezes overnight can cause some mixing the air near the surface leading to the cool layer becoming slightly thicker as the night progresses. It is because of radiation inversions that there is often fog in the morning. The radiation inversion traps the moisture (clouds) under the inversion layer resulting in for or smog, depending where you live.

3.3 Advective inversion

It is formed when the warm air moves over a cold surface or cold air. It can be grounded in the former case and elevated in latter case E.g. hill range forces warm air flows at the high level, a cool sea breeze flow at the low level.

Advection of a thick layer of warm air over a cold surface produces an inversion of temperature in the lower layers of the atmosphere for the warm air is cooled by conduction.

4. Air Pollution Dispersion

The movement of pollutants in the atmosphere is caused by transport, dispersion, and deposition. Transport is movement caused by a time-averaged wind flow. Dispersion results from local turbulence, that is motions that last less than the time used to average the transport. Deposition processes, including precipitation, scavenging, and sedimentation, cause downward movement of pollutants in the atmosphere, which ultimately remove the pollutants to the ground surface. This chapter deals only with transport and dispersion.

During the past decade, the complexities of transport and dispersion of airborne pollutants associated with vehicular emissions have been studied with elaborate field and modeling experiments. Since pollutants can travel distances from meters to hundreds of kilometers, the relative scales of motion involved in distinguishing transport phenomena from dispersion phenomena may vary from problem to problem.

Meteorology is the science of atmosphere and the study of the characteristics of weather elements. Meteorological parameters are having great importance in transportation, dispersion and natural cleansing of the air pollutants in the atmosphere. Thus, meteorological information is very essential in locating the industry and planning the control measures for air pollution. The basic meteorological parameters affecting air pollution dispersion are:

Primary Metrological Parameter

• Wind speed
• Wind Direction
• Atmospheric Stability

Secondary Metrological Parameter

• Sunlight
• Temperature
• Precipitation and Humidity
• Topography
• Energy from the sun and earth’s rotation drives atmospheric circulation

Here we primarily focus on primary metrological parameters and its effects on air pollution dispersion.

a) Wind Speed and direction

When high pollutant concentrations occur at a monitoring station, wind data records can determine the general direction and area of the emissions. Identifying the sources means planning to reduce the impacts on air quality can take place. An instrument called an anemometer measures wind speed. At our monitoring stations, the type of anemometer we use is a sonic anemometer. A sonic anemometer operates on the principle that the speed of wind affects the time it takes for sound to travel from one point to another. Sound travelling with the wind will take less time than sound travelling into the wind. By measuring sound wave speeds in 2 different directions at the same time, sonic anemometers can measure both wind speed and direction.

b) Temperature

Measuring temperature supports air quality assessment, air quality modelling and forecasting activities. Temperature and sunlight (solar radiation) play an important role in the chemical reactions that occur in the atmosphere to form photochemical smog from other pollutants. Favorable conditions can lead to increased concentrations of smog. The most common way of measuring temperature is to use a material with a resistance that changes with temperature, such as the platinum wire. A sensor measures this change and converts it into a temperature reading.

c) Relative Humidity

Like temperature and solar radiation, water vapor plays an important role in many thermal and photochemical reactions in the atmosphere. As water molecules are small and highly polar, they can bind strongly to many substances. If attached to particles suspended in the air they can significantly increase the amount of light scattered by the particles. If the water molecules attach to corrosive gases, such as sulfur dioxide, the gas will dissolve in the water and form an acid solution that can damage health and property. Reporting of the water vapor content of air is as a percentage of the saturation vapor pressure of water at a given temperature. This is the relative humidity. The amount of water vapor in the atmosphere is highly variable—it depends on geographic location, how close water bodies are, wind direction and air temperature. Relative humidity is generally higher during summer when temperature and rainfall are also at their highest. Measuring humidity uses the absorption properties of a polymer film. The film either absorbs or loses water vapor as the relative humidity of the ambient air changes. A sensor measures these changes and converts them into a humidity reading.

d) Rainfall

Rain has a 'scavenging' effect when it washes particulate matter out of the atmosphere and dissolves gaseous pollutants. Removing particles improve visibility. Where there is frequent high rainfall, air quality is generally better. If the rain dissolves gaseous pollutants, such as sulfur dioxide, it can form acid rain resulting in potential damage to materials or vegetation. A common method to measure rainfall is to use a tipping bucket rain gauge.

e) Solar Radiation

It is important to monitor solar radiation for use in modelling photochemical smog events, as the intensity of sunlight has an important influence on the rate of the chemical reactions that produce the smog. The cloudiness of the sky, time of day and geographic location all affect sunlight intensity. An instrument called a pyranometer measures solar radiation from the output of a type of silicon cell sensor.

With all these factors, the environmental lapse rate is set up which in turns defines the stability of atmosphere and shows how the plumes will be dispersed.

The ability of the atmosphere to enhance or to resist atmospheric motions is called atmospheric stability. It is influenced by the vertical movement of air.

If the air parcels tend to sink back to their initial level after the lifting exerted on them stops, the atmosphere is stable. If the air parcels tend to rise vertically on their own, even when the lifting exerted on them stops, the atmosphere is unstable. If the air parcels tend to remain where they are after lifting stops, the atmosphere is neutral.

Plume Rise and Dispersion of Air Pollutants

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.

Three basic type of Environmental lapse rate and two mix lapse rate give rise to plume dispersion techniques are discussed here.

a) Coning

Horizontal dispersion at a right angle to the wind is due to turbulence and diffusion, which occurs at the same rate as the vertical dispersion, which is not being opposed nor encouraged by the stability (or lack of it) in the atmosphere. Plume spreads equally in the vertical and horizontal as it propagates downstream, forming a coning plume. It generally forms in sunny days.

b) Looping

In unstable air, the plume will whip up and down as the atmosphere mixes around (whenever an air parcel goes up, there must be air going down someplace else to maintain continuity, and the plume follows these air currents). This gives the plume the appearance that it is looping around. Vertical dispersion is very high. High probability of high concentrations sporadically at ground level close to stack.

c) Fanning

High wind speed: Night time, High horizontal dispersion, Vertical dispersion is suppressed by a stable atmosphere. In the vertical, dispersion is suppressed by the stability of the atmosphere, so pollution does not spread toward the ground. This results in very low pollution concentrations at the ground

d) Fumigation

Most dangerous plume: contaminants are all coming down to ground level. They are created when atmospheric conditions are inversion 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.

e) Lofting

It forms during evening – night when radiation inversion starts to form. It is 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.

Similarly, trapping is another type of plume dispersion technique not discussed here.

In Context of Nepal

Research done by ICIMOD is briefly discussed here with a view to addressing air dispersion in case of Kathmandu. Kathmandu experiences strong seasonal variation: it has a relatively cool, dry winter and a hot, rainy monsoon in summer. Both precipitation and shifts in wind pattern influence how airborne pollutants are distributed over the city.

Observations by research included the following.

Regardless of the season, pollutant concentrations were high at roadside sampling stations and in the city core areas and low at rural stations.

During the rainy season, the concentration of all measured pollutants was low. Rain and the wind help to clean the pollutants from the lower atmosphere. Higher concentrations of all atmospheric pollutants (especially SO2 and particulates) were observed during the dry season than during the rainy season. Lower wind speeds and substantial emissions from brick kilns, which operate only during the dry season, contributed substantially to pollution in the dry season.

The pollution hot spots were shown clearly by the passive sampling measurements. They are located along the main roads, near industrial areas, hotels, the airport, and other areas with significant emissions. The road network, with daily traffic of more than 15,000–20,000 vehicles, is a major polluter; the major source of SO2, NOX, and particulate emissions is vehicle exhaust. PM10 was higher along the roads and high-traffic areas than elsewhere, while other pollutants such as SO2, CO, and NMVOC were higher at the city centre since these are produced by the fuel used in cooking and industry. NH3 was found in higher concentrations at the periphery of the urban areas as it is a by-product of agricultural activity. Kathmandu being the valley, all these pollutants are heavily trapped by inversion phenomenon.

5. Plume

An enclosed surface where the released gases are supposed to disperse and travel is simply called plume. This is basically the region where we made all our calculations for pollutants dispersions.

Types of Plumes:

• Gaussian Plume
• Non-Gaussian Plume

5.1.1 Gaussian Plume

Those plumes where all the dispersed air are supposed to remain inside certain Gaussian surface are known as Gaussian Plumes.

5.1.2 Non-Gaussian Plume

Those plumes where all the dispersed air aren’t supposed to remain inside certain Gaussian surface are known as Non-Gaussian Plumes.

5.2 Atmospheric Dispersion Models

Atmospheric dispersion models are computer programs that use mathematical algorithms to simulate how pollutants in the ambient atmosphere disperse and, in some cases, how they react in the atmosphere. Atmospheric Dispersion Modeling is the mathematical description of contaminant transport in the atmosphere. This is an excellent example of interdisciplinary mathematics that has direct application. They can be applied to the problems having industrial relevance.

There are many types of Atmospheric Dispersion Models such as:

Box Model

Lagrangian Model

Eulerian Model

Dense Gas Model

Gaussian Model

6. Gaussian Plume Model

Gaussian Plume Model is the simplest of the dispersion modeling techniques which describes three-dimensional concentration field generated by a source. Any source can be corresponded to a continuous point source that emits contaminant into a uni-directional wind in an infinite domain from where the analysis can be made considering the transport of mass within a small control volume.

The Gaussian dispersion modeling equation not only includes the upward reflection of the pollution plume from the ground, it also includes downward reflection from the bottom of any temperature inversion lid present in the atmosphere. The Gaussian Plume Model can be applicable to all the point, line and volume sources to but considering the infinite volume every source can be considered as the point source and the necessary calculations can be made.

6.1 Gaussian Plume Equation

The Gaussian Plume Model is entirely based on the Gaussian Plume equation. With the following sets of assumptions the Gaussian Plume Equation is:

Assumptions:

• Steady state conditions:
• ∂C/∂t=0
• Constant wind speed with height (u does not depend on z)
• Constant eddy diffusivity (K does not depend on y or z)
• No wind shear in horizontal or vertical direction Ø Mass is conserved:

fig: gussian plume

Cdydz=Q [for x > 0]{μg/sec }

X= pollutant concerntration as function of downward wind position

Q= mass emmision rate

H= effective stack height

U= wind speed at efeective release height z= effective release height Ï­z= distn. of mass in vertical dimension Ï­y= distn. of mass in cross flow direction

6.2 Applications of the Gaussian Plume Models

• To estimate the downwind ambient concentration of air pollutants or toxins emitted from sources
• To predict future concentrations under specific scenarios (i.e. changes in emission sources)
• To facilitate various research projects on the behavior of pollution dispersion and control of its flow for the human benefit

6.3 Benefits of the Gaussian Plume Model

• Easiest technique among dispersion model techniques
• Dominant type of model used in air quality policy making
• Useful for pollutants that disperse over large distances & that may react in atmosphere
• Various sources considerations can be made

6.4 Drawbacks

• Complex Technique
• Not quite applicable for the downwind distances below 100m
• Wind velocity is not practically constant
• Preciseness uncertain
• Expensive Research Works
• Difficult for general public to understand

6.5 Conclusion

Atmospheric Dispersion Modeling is very a modern mathematical concept applied to the real world. As there is global concern about the effects of the pollution, the predictability of air pollution dispersion is a very useful research project. Gaussian Plume model is an easiest and most effective Dispersion Modeling Technique, so it has been of great applicability nowadays to many research projects as well as minimization of industrial adversities. So that all atmoshpheric mmeterogical parameters must be considered in Atmospheric Dispersion Modeling.

References:
1. Mackenzie L. Davis & David A. Cornwell, “Introduction to Environmental Engineering”, McGraw Hill.
2. Gilbert M. Masters, Standford University, “Introduction to Environmental Engineering and Science”, Printice Hall.
3. Stephan Konz, Kansas State University, “Work design”, Grid Publishing Inc., Colombus, Ohio
4. C. S. Rao, “Environmental Pollution Control Engineering”, New age International (P) Limited, Publishers, India.