the atmosphere is a thin layer of air that surrounds our planet

8/8/2019 The Atmosphere is a Thin Layer of Air That Surrounds Our Planet

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Objective of the studyThe present work is done to find out the relation between the meteorological parameters

that affect the tropospheric ozone concentration. Since tropospheric ozone is toxic in nature, the present study is relevant so that remedies can be taken for reducing the ozone concentration.

Ozone in the lower atmosphere is mainly produced by anthropogenic pollutions such as fromautomobiles and fossil fuel burning. So its concentration will be large in cities like New Delhi,

Calcutta, Chennai, Mumbai etc.Here the objective of the study is to get an idea about the vertical profile of ozone

concentration over Mumbai and its concentration. To find out whether there is any relation between the meteorological parameters such as temperature and relative humidity vertical

profiles of these parameters can be plotted and hence the relation can be concluded.

IntroductionThe atmosphere is a thin layer of air that surrounds our planet, the Earth. On a small

model of the earth its thickness would be hardly noticeable, not thicker than the skin of an apple.But its weight at the ground level gives us the air pressure under which we and all land livingcreatures evolved. The higher up we go, the thinner the atmosphere becomes. At a height of

several hundred kilometers, it fades away into the vacuum space.The atmosphere is a mixture of gases, mainly composed of nitrogen, oxygen, and argon,

which together constitute the major gases of the atmosphere. The remaining gases are oftenreferred to as trace gases, among which are the greenhouse gases such as water vapor, carbon

dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many otherchemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air

sample, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine

compounds, elemental mercury, and sulfur compounds such as sulfur dioxide [SO2].

Composition of dry atmosphere, by volume

Gas Volume

Nitrogen (N2) 780,840 ppmv (78.084%)

Oxygen (O2) 209,460 ppmv (20.946%)

Argon (Ar) 9,340 ppmv (0.9340%)

Carbon dioxide (CO2) 390 ppmv (0.039%)

Neon (Ne) 18.18 ppmv (0.001818%)Helium (He) 5.24 ppmv (0.000524%)

Methane (CH4) 1.79 ppmv (0.000179%)

Hydrogen (H2) 0.55 ppmv (0.000055%)

Nitrous oxide (N2O) 0.3 ppmv (0.00003%)

Carbon monoxide (CO) 0.1 ppmv (0.00001%)

Ozone (O3) 0.0 to 0.07 ppmv (0 to 7 106%)


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Not included in above dry atmosphere:

Water vapor (H2O) ~0.040% over full atmosphere

Components of atmosphere (source: Wikipedia)

Mixing ratio of any gas is defined as the ratio of the number density of that species to the

number density of the air.The atmosphere can be divided into a number of layers in accordance with the

temperature variation as illustrated in the figure 1.2. The lowermost layer is called the troposphere.Its height from the ground level varies from 10-16 Km from the poles to the equator. All the

physical activities that affect the weather takes place in this region.

The vertical temperature profile from earth surface to 120 km, (Source:

www.engr.colostate.edu).

. The top of the troposphere is a theoretical boundary called the tropopause, where thetemperature is merely constant with height. Above tropopause, temperature is increasing with

height. This layer that extends about up to 50 km is called the stratosphere. Most military and longdistant aircraft operate in this region. The ozone layer, with in this region absorbs much of the

insolation, so that the temperature is quite high in this region.


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Above stratopause- the upper limit of the stratosphere- lies the mesosphere, whichextends up to about 80 kms. Mesosphere is characterized by a decrease in temperature with

altitude, reaching about 180 K at 80 Km. This drop in temperature is due to a decrease in ozoneconcentration and an increase in distance from the warm surface of the Earth. But it is still thick

enough for meteorites to burn up as they pass through.

Beyond its upper boundary, mesopause, and the mesosphere give rise to thermosphere.Atmospheric gases are so thin in this layer that the protons of the solar wind can easily passthrough. The atmospheric atoms and molecules hit by these protons move faster, causing the

temperature to rise.In addition, the solar wind removes electrons from atoms to form ions. This ionization

takes place in the lower parts of the atmosphere. Aurora are created in this region of higherlatitudes. The upper portion of the thermosphere called exosphere; here the atmospheric gases are

so thin that a gas molecule travels an average distance of 650 km before hitting another molecule.

OzoneOzone is a tri molecule of oxygen which is naturally formed by the photolysis of normal

oxygen by UV radiation, mainly in stratosphere. By the formation process the UV radiation will be blocked from entering into the lower atmosphere (below tropopause). Since UV radiation isharmful to life in the earth, ozone present in the stratosphere acts as an umbrella that prevents the

UV to enter into the surface.Ozone production in the stratosphere was first explained by Sidney Chapman as follows

O2 + photon (< 240 nm) 2 OO + O2 + M O3 + M

O3 + O 2 O2The energy gained by the ozone molecule by the absorption of UV radiation will be

immediately transferred to other molecules which are not directly involved in the ozone reaction.So the kinetic energy of the third particle increases and there by the temperature over that region

also increases. Since the UV is almost shielded in the stratosphere itself, ozone will not be

produced in the troposphere by photochemical reactions, which is infact toxic to living organisms(bad ozone). The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx),

carbon monoxide (CO) and volatile organic compounds (VOCs) react in the atmosphere in thepresence of sunlight.

There is a slow meridional circulation seen in the stratosphere that leads to the transportof newly produced ozone from the equatorial mid stratosphere to the lower polar stratosphere. The

velocity of this circulation is only about 9m/day. This circulation is called as Brewer Dobsoncirculation.

So due to this circulation, the highest ozone concentration is seen in the polar region, but

not in the equatorial region. Also the maximum concentration is observed during spring, but not insummer and the lowest is observed during autumn not in winter.Low level ozone (or tropospheric ozone) is an atmospheric pollutant

[15]. It is not emitted

directly by car engines or by industrial operations, but formed by the reaction of sunlight on aircontaining hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the

pollution or many kilometers down wind.The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx),

carbon monoxide (CO) and volatile organic compounds (VOCs), such as xylene, react in the


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atmosphere in the presence of sunlight. NOx, CO, and VOCs are called ozone precursors. Motorvehicle exhaust, industrial emissions, and chemical solvents are the major anthropogenic sources

of these chemicals. Another source is windshield washer fluid. Although these precursors oftenoriginate in urban areas, winds can carry NOx hundreds of kilometers, causing ozone formation to

occur in less populated regions as well. Methane, a VOC whose atmospheric concentration has

increased tremendously during the last century, contributes to ozone formation but on a globalscale rather than in local or regional photochemical smog episodes. In situations where thisexclusion of methane from the VOC group of substances is not obvious, the term Non-Methane

VOC (NMVOC) is often used.The chemical reactions involved in tropospheric ozone formation are a series of complex

cycles in which carbon monoxide and VOCs are oxidised to water vapour and carbon dioxide. Thereactions involved in this process are illustrated here with CO but similar reactions occur for VOC

as well. Oxidation begins with the reaction of CO with the hydroxyl radical. The hydrogen atomformed by this reacts rapidly with oxygen to give a peroxy radical HO2

OH + CO H + CO2H + O2 HO2

Peroxy radicals then go on to react with NO to give NO2 which is photolysed to give

atomic oxygen and through reaction with oxygen a molecule of ozone:

HO2 + NO OH + NO2NO2 + h NO + O

O + O2 O3

The net effect of these reactions is:

CO + 2O2 CO2 + O3

This cycle involving HOx and NOx is terminated by the reaction of OH with NO2 to form

nitric acid or by the reaction of peroxy radicals with each other to form peroxides. The chemistryinvolving VOCs is much more complex but the same reaction of peroxy radicals oxidizing NO to

NO2 is the critical step leading to ozone formation.

Commonly used unit for ozone measurement is Dobson unit. It is defined as the number ofmolecule required to create a pure layer of ozone of 0.01 mm thick at STP (0 C and 1 atmospheric

pressure) per unit area.

1 DU = 2.691016 molecules /square centimeter

The average ozone concentration over the earth atmosphere is about 300 DU. I.e. if all theseozone is compressed to the earth surface at STP, its thickness will be 3mm only.

Ozone measurement techniques

Scientists have been studying the concentration of ozone in the atmosphere since

the 1920s. Since then, instruments have evolved from ground based spectrometers to balloons,


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aircraft, rockets, and satellites. Developments in ozone instrumentation have enabledmeasurements to expand from the atmosphere above an isolated ground station to daily global

coverage and profiles of ozone in the atmosphere.

Ground stations have been measuring ozone levels for most of this century. They provide

long term data of both total column ozone and ozone distribution with altitude, but only over asmall area. Instruments that are commonly used to measure overhead ozone from the ground arethe Dobson spectrophotometer and Light Detection and Ranging (LIDAR).

Airborne measurements of ozone provide a direct (or in situ) method of determining

ozone concentrations in the atmosphere. Balloons, rockets, and aircraft carry instruments into theatmosphere, resulting in the most accurate and detailed methods of measuring ozone. However, the

measurements are made only over localized regions and cannot provide a global picture of ozonedistribution.

The Total Ozone Mapping Spectrometer (TOMS) aboard Nimbus-7 and Meteor-3

provided global measurements of total column ozone on a daily basis and together provide acomplete data set of daily ozone from November 1978 - December 1994. After an eighteen monthperiod when the program had no on-orbit capability, ADEOS TOMS was launched on August 17,

1996 and provided data until June 29, 1997.

Dobson spectrometerThe Dobson spectrophotometer is a ground-based instrument that measures the

amount of ozone present in the atmosphere. The Dobson spectrophotometer was designed by

Gordon Dobson in the 1930's. The Dobson spectrophotometer measures ultraviolet light from theSun at 2 to 6 different wavelengths from 305 to 345 nm. By measuring UV light at two different

wavelengths, the amount of ozone can be calculated. One of the wavelengths used to measure

ozone is absorbed strongly by ozone (305 nm), whereas the other wavelength is not absorbed byozone (325 nm). Therefore the ratio between the two light intensities is a measure of the amount ofozone in the light path from the sun to the observing spectrophotometer.

The ratio between the two intensities are determined by an R-dial located on thetop of the Dobson spectrophotometer. The R-dial controls a filter wedge that gradually blocks out

the 325 nm light. As the R-dial rotates from 0 to 300 degrees, the filter wedge increasingly blocksout more light. At 0 degrees the wedge does not block out any light. At 300 degrees, the wedge

nearly blocks all of the light. The filter wedge gradually blocks more and more light until theintensity of the 325 nm and 305 nm light are equal. The R-dial is calibrated with the filter wedge,

so that the original intensity of the 325nm light can be determined from the R-dial reading. Bytaking the R-dial reading when the intensities of the two wavelengths are equal, their ratio is

determined. The simple animation below demonstrates how the filter wedge works.

Ozonesonde

Ozonesonde contains two small chambers containing KI solutions of different

concentration. The sample air is pumped into one of the chamber so that the following reactiontakes place.

Titration of ozone in a potassium iodide (KI) solution according the redox reaction:


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2 KI + O3 + H2Op I2 + O2 + 2 KOH

Ozonesonde(source: www.iup.uni-bremen.de/)

Measurement of "free" iodine (I2) in electrochemical reaction cell(s). The iodine makes contact

with a platinum cathode and is reduced back to iodide ions by the uptake of 2 electrons permolecule of iodine:

I2 + 2 e-on Ptp 2 I

-[cathode reaction]

The two chambers are connected by an metallic bridge so that electrons can move through it.This flow of electrons through the cells external circuit can be measured and is directly

proportional to the partial pressure of the ozone content in the sample air.

PV = i t K T / 2e

Where P= Partial pressure of ozone e= Charge of electronK= Boltzmann constant t=Time period of pumping

T= Temperature in Kelvini= Current measured

the life span of an Ozonesonde is only 3-4 hrs. So for a long time observation it cannot be used.


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UV photometerA sample air is pumped into the chamber. A mercury lamp flashes UV radiation of 253.7 nm

passes through the chamber and the intensity is detected when comes out.

A simplified layout of the ozone analyzer showing the working principle of the system.(Source: Research thesis, Lokesh Sahu)

By Beer Lamberts law

where


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Then the experiment is repeated with air but without any ozone, then

Then

From this we can calculate the number density of ozone in the sample air.Compared to the Ozonesonde, the life time of the instrument is very high so that in MOZAIC

program UV photometer is used.MOZAIC program (Measurements ofozone and water vapor by airbus in service

aircraft)

In this program the ozone and water vapor sensors are installed in airlines which take themeasurements at the scheduled flight times. The ozone analyzer is a dual beam UV absorption

instrument which has a detection limit of 2ppbv. The response time of the instrument is 4s,which translates into a vertical resolution of 30 m. The relative humidity and temperature are

measured with a capacitive sensor and a platinum resistance sensor. Measurements are takenduring both take off and landing of the aircraft to get a vertical profile data over the airports.

MOZAIC data combined with meteorological data is used to 1)establish quasi global climatologyof the large scale distribution of ozone and water vapor in the troposphere, 2)establish the

temporal and vertical distribution of O3 and H2O and 3) to validate the chemistry and transport

models 4) investigate the seasonal and geographical variations in the relations to their natural andanthropogenic sources.

Data and methodologyIn the present study the data used is ozone concentration in ppbv, relative humidity in

percentage, temperature in0C in different heights, provided by MOZAIC program. This data is

available from a height of 75 m from the ground to about 10 km at a vertical interval about 50 m.

The data used for this study are 27 august 1997, 20 may 1998, 28 may 1998, 08 July 1998, 22July 1998, 11 September 1998.

The vertical profile of temperature, ozone concentration and the relative humidity areplotted by using Sigma plot. Then the layers of temperature inversions in graphs of each day are

recognized. The strength of each temperature inversion is estimated and corresponding to that thechange in the ozone concentration and the relative humidity are calculated. Then these values are

compared with the other days and concluded for a general result.

Case studies of Ozone profiles over Mumbai

The wind pattern over India is changing for each winter and summer. Typically north easterlywind prevails during winter and in summer it reverses completely to become south westerly. This south


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westerly wind brings moisture from the Indian Ocean leading to the summer monsoon rain almost allover India. Most of this summer monsoon rain will be getting all over the western coastal area of India.

While the north east monsoon rain is very low compared to the SW monsoon rain and is prominentlyobtained over the eastern coast of India.

During winter time, the average wind pattern over India (1996-2001) shows that at 925 hPa

level the wind is coming from the Pakistan region in a curved path covering Rajasthan and Gujarat. At

700 hPa level an anticyclone circulation is seen over Andhrapradesh and so that the wind reaching overMumbai at this level is from that region. At 400 hPa level the wind over Mumbai is purely westerlythat comes from North Atlantic Ocean through the northern Africa. At 250 hPa level also the windpattern is same as that at 400 hPa.

During the summer time, the average wind pattern shows that the wind reaching overMumbai at 925 hPa level is from the Southern Indian Ocean via Arabian Sea, as south westerly. At 700hPa level the wind is coming from Pakistan via Arabian Sea, so that the air will be more humid. At the400 hPa level, the wind reaching over Mumbai is from the central India so that the wind will be dry. At

the 250 hPa level the wind reaching over Mumbai is purely easterly and which actually originated overeastern India and blow through the Bay of Bengal.

(Source: L K Sahu et la)

Mumbai is a metro city in India which is located at 18 53' North, Longitude 72 50' east, theeastern coast of India. The average annual temperature is 27.2 C, and the average annual precipitationis 216.7 cm. In the City, the average maximum temperature is 31.2 C), while the average minimumtemperature is 23.7 C.

The vehicle population in Mumbai during 1997-1998 is about 8,59734 (Western India

Automobile Association - WIAA ). The organic and inorganic pollutants released from the vehicleshave an important role in the production of the ozone.

Since surface ozone is mainly formed due to photochemical and dynamical processes, the

vertical profile of ozone shows large diurnal and seasonal variability. The magnitude of variability can

change significantly with altitude, sometimes exhibiting marked stratification, with a single or multiplepeaks.

In this study, the observed features of ozone are studied along with relative humidity and

temperature in the troposphere.

27 August 1997

The vertical profile of ozone, RH and temperature over Bombay is shown in the figure .


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The minimum ozone value of 11 ppbv is observed at the ground and its concentration isalmost constant 17 ppbv up to 2 km. Then a steep increase in seen showing two peaks of 73 ppbv at

6km and 81 ppbv at 6.6 km altitude. Then the concentration decreased with height up to 8 km. Therelative humidity is seen maximum 83 % up to about 2 km and then shows a steep decrease. Twominimum value peaks are observed at 4.5 km and 6.5 km.

All these values show an opposite correspondence with the ozone concentration. i.e. a

maximum peak of ozone will be corresponds to a minimum relative humidity value and vice versa.

Since Mumbai is a coastal city, wind blowing from the sea will reduce the ozoneconcentration and increases the relative humidity. August includes in the summer monsoon period sothat the relative humidity is seen high.

A temperature inversion is seen at 2.175 km. Here the relative humidity shows a steepdecrease of magnitude 53.65 and the ozone concentration shows a steep increase of 18 ppbv. Thestrength of this inversion is only 2.93 K. The average lapse rate of this temperature profile is about6.59K/km.

20 May 1998

The surface ozone concentration on this day is about 3 ppbv. It is seen almost constant up to1.75 km. from there a steep increase seen upto3.8 km (78 ppbv). Then the concentration reduces to 61ppbv at 5.75 km. The ozone concentration then increases to 92 ppbv at 7.2 km.


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In an inverse manner relative humidity is maximum from the surface to 1 km, about 82 5.Then it reduces to 16% at 3.25 km. Then a maximal peak is seen at 5.75 km, 61%. Then the value

again reduces and is nearly a constant with height up to 9 km and the relative humidity is about 11.5%.

Temperature inversion is seen in two heights, 1.7 km and 6.07 km. The strength of these

inversions is 1.75 K and 3.45 K respectively. At 1.75 km ozone shows an increase of 55 ppbv and therelative humidity shows abrupt decrease of 43%. At 6.07 km ozone shows an increase of 13 ppbv andthe relative humidity shows abrupt decrease of 70%. The average lapse rate of this profile is about 7.4

K/km.

28 May 1998

The ozone concentration is almost constant up to 1.2 km and then shows a peak at 1.75 km(35 ppbv). The next peak is seen at 5 km, 63 ppbv. Then after a steady concentration with height again

it shows a peak at 6.3 km, 79 %. And then also the ozone concentration is almost constant with heightup to 9.5 km, 78 ppbv.

The surface relative humidity is 76% which increases with height and shows a maximum at0.975 km, 95%. Then it reduces and shows a minimal value at 2.3 km, 20%. It again increases and a

43% is seen at 4 km. Then the value reduces and shows a steady value of about 10% with height.


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In this vertical profile also two temperature inversions are seen, at 1.275 km and at 4.125 km.

The strength of these inversions is 2.75 K and 0.5 K respectively. At 1.27 km the ozone concentrationincreases by 26 ppbv and the relative humidity shows an abrupt decrease of 60 % . The average lapse

rate is about 7.66 K/km.

08 July 1998

The surface ozone concentration is about 12 ppbv in this day and which is almost a constant

up to 4 km. 2 maximum peak values and minimum peak values are seen in the figure. A maximum of78 ppbv at 5.1 km and 70 ppbv at 7.5 km and 2 minimal peaks of 66 ppbv at 5.7 km and 29 ppbv at 7.5km.

The atmosphere is seen supersaturated up to 2.7 km. At 2.9 km, the relative humidity reducesto 53% and remains almost constant up to 4.8 km. at 6.2 km the atmosphere is again supersaturated and

the RH is 129%. The at 7.2 km a minimum value of 70% and at 8.2 km a maximum of 101 % RH isrecorded.


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Two significant temperature inversions are seen in this graph. At 2.775 km, the strength ofthe inversion is about 2.54 K and at 6.4 km the strength is also 1.42. Very abrupt changes of relativehumidity and the ozone concentrations can be seen in the inversion layers. At 2.775 km, the ozone

concentration increases by 10 ppbv and the relative humidity decreases by 82%. At 6.375 km the ozoneconcentration reduces by 17 ppbv where as the relative humidity reduces by 43 %. The average lapserate is about 7.19 K/km.

22 July 1998

The vertical profile of ozone shows a very low variability on this day. Its surface

concentration is only about 1 ppbv. It shows a gradual steady increase up to 5.5 km. Even after thatlevel also the ozone shows low variation with height, two maximal peaks, one at 7 km (54 ppbv) andthe other at 7.6 km (52 ppbv). A minimum value of 44 ppbv is seen at layer of 6.7 km.


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The relative humidity is seen super saturated up to 1 km, as it is a typical summer monsoonrainy day. At 8.6 km a peak of 92 % is seen and a minimum peak of 29% is seen in 6.5 km.

A very slight temperature inversion is seen at 1.875 km. Since its strength is too small (0.25K), it seem to be isothermal with height and so it does not show much impacts on ozone concentration(an increase of 4 ppbv) and on the relative humidity (14%). The average lapse rate is 6.7 K/km

11 September 1998

The ozone concentration and the relative humidity show high variability with height in thisgraph. While temperature inversion is not seen at that time of observation. The surface ozoneconcentration is about 10 ppbv only and is seen linearly increasing with height. Maximal values ofpeaks are seen at 3.5 km (53 ppbv) and at7.1 km (75 ppbv) during this observation.


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The atmosphere was super saturated about up to 1.9 km. At 6.1 km and 7.9 km relativehumidity peaks as 113% and 87% respectively. At 5 km and 7.5 km minimal value of humidity layersare observed as 65 % and 45 %.

The lapse rate of this day is 5.77 K/km.


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Conclusion of the case studies

Tableshowing the variation of ozone concentration and the relative humidity corresponding totemperatureinversion.

From the present study it can be clearly concluded that ozone concentration undergoes amarked increase when temperature inversion occurs. This variation seems to be dependent on the

strength of the inversion as well as the concentration of ozone below the inversion layer. Also when thehumidity of the atmosphere increases it causes in the ozone concentration variation also. Since Bombayis a coastal station, humid air frequently blows to the land area due to sea breeze. This leads to a largetemporal and special variation of ozone in the troposphere.


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References

L K Sahu, S Lal : Climatology of the tropospheric ozone and water vapour over Chennai: a studybased on MOZAIC measurement over India, 2010.

Ozone measurement techniques www.albany.edu, (State University of Albany)

the atmosphere is a thin layer of air that surrounds our planet

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