CHAPTER I: INTRODUCTION

The earth's climate is driven by the continuous deluge of solar radiation.

Every activity, biotic and abiotic, in all aspects is tightly linked to the character

of solar radiation. Any change, in the nature of radiation or the magnitude of

radiation received by earth, has significant impact on the overall environment

dynamiCS of the earth. The passage of solar radiation through earth's atmosphere

undergoes a Significant attenuation due to the absorption and scattering. Figure

1.1 shows the solar spectral irradiance distribution received at earth's surface. The

ultra violet part of this solar irradiance (A<400nm) forms only about 9 % of the

total, solar irradiance received by earth (Bolton et. al. 1994).

Although, the solar IN irradiance received by earth constitutes a small part

of the total solar irradiance but it plays (and has played) a central role in origin

of the life and its perpetuation on earth (Chaisson,1988) . Any change in the

quality and the magnitude of the solar IN irradiance is likely to have a far

reaching impact on the functioning of life forms, ranging from the biomolecules

to the biosphere (Biggs and Kossuth,1978; Caldwell et.al.,1975; Dickson and

Caldwell,1978; Teramura,1983; Holm-hamsen et.al., 1993;Varshney and Attri,

1995). Due to the eclectic nature of the interactions between different

1

2000 .

E :::I I' 1500 E ~ )( :::I .... -... o .... o Vl

0 0 0.4 0.8 1.2 1.6

_._.- Solar curv~ extraterrestrial _ - _ 6000 0 K black body curve ____ Solar curve at earth's surface

..... H20 ....... I . \

2.0 2.4 2.8 3.2 Wavelength (}-1m)

Figure I. 1 Spectral distribution of incoming solar radiat ion.

components of natural world, the extent and the direction of such changes are

difficult to anticipate (Varshney and Attri, 1995).

The energy of UV photons elevate the bio-molecules to

excited state (Phillips, 1983). The bio-molecules in excited state are relatively

unstable and tend to loose energy by; i) returning to ground state or, ii) reacting

chemically with surroundings or, iii) undergoing molecular decomposition or iv)

photosensitization of adjacent bio-molecules(Frederick, 1986; Emmett, 1986;

Varshney and Attri,1995). In general, the biological response to UV radiation is

associated with the action spectra of biomolecules in a given wavelength range.

For instance, the DNA absorption spectra lies between 240-290 nm and has

maxima at 260 nm. Similarly, the protein absorption maximum occurs at 280nm.

The action spectra for skin erythema and photo-keratitis occurs at 290 nm

(Diffey,1986). The erythema spectral curve range coincides with the absorption

spectra of most of the nuclear and serum proteins (van der Leun, 1984). Proteins

are the largest UV-B absorbing component inside the living cell having absorption

maxima at 280 nm. The UV-B flux between 280 to 320 nm is found to cause

irreversible changes in lipids, steroids, melanin and uracanic acid (Emmett, 1986).

The schematic representation of UV-B effects on living forms in sequence- from

biomolecules to the cell- are shown in figure 1.2 (Varshney and Attri, 1995).

3

Potential UV-B Effect on Living Cell

l UVB - J ~ I \

Living Cell

I Absorption by Chromophores and Excitation

-10 Response Time 10 to 10 Seconds

J Photochemical Reactions Response Time

10-3 to 10-1 Seconds J

Biological Changes Response Time Seconds to Years

I I / ~ IStructural Changes in DNA:

-...,

Altered Enzyme a] Mutation~ b) Photolesions Response i) Formation of Pyrimidine adduct '-

" iiJ DNA-DNA and DNA Protein cross Links I

C Altered Phenotype Expression ~

Figure 1.2 Interaction of UV-B at the level of biomolecules and cell.

The actinic effects of UV-B on human health are mainly from its

absorption by the skin, which affects DNA and protein synthesis(Hawk and

Parrish, 1982; Quevedo et.al, 1985). Most of the adverse effects attributed to

UV-B are related with the increase in the incidence of cancer, actinic elastosis,

squamous skin cancer and pre-mature aging. Indirect effects of UV-B include

genetic disorders, pellagra, kwashiorkor, suppression of immune response,

metabolic disorders and eye diseases (Frederick, 1986).

The steady increase in the solar UV irradiance has become one of the most

important global issues (Madronich et.al., 1995, Gerstl et.al.,1981). Any

mitigation strategy to counter, or to understand, t~e various facets related to the

increase in solar UV irradiance, requires an exhaustive scientific understanding

about the factors leading to these changes. The single most important factor,

which has caused the increase in UV irradiance at the earth surface is the

depletion of stratospheric ozone ( Roy et. al., 1990; Varshney and Attri, 1995).

To a smaller extent, the atmospheric pollution in troposphere also modulates the

incoming UV radiation reaching the ground (Ilyas, 1987; Frederick et.al., 1990;

Liu et.al., 1991; Michelangeli et.al., 1992; Vogelmann, 1992; IPCC 94; IPCC 95).

In this light, only by understanding the role of various climatic and atmospheric

5

factors controlling the amount of incoming solar UV it is possible to predict and

model the likely increase in the solar UV irradiance at different locations on earth.

[1.1] Stratospheric Chemistry:

Although ozone in stratosphere is present in trace amounts, its role in

stratospheric chemistry is vital and essentially forms an effective shield against

the penetration of short wavelength UV radiation to earth's surface(Madronich

et.al.1995). As UV increase is directly linked with depletion of ozone, the issue

has assumed global importance not only among scientists but also among the

technocrats, diplomats, politicians and social activists. Montreal protocol which

seeks to mitigate the ozone depleting chemicals is the first step towards warding

off the environmental crisis braced with UV-B increase (Lohrer, I 989). Ozone

depleting chemicals, like CFCs (Chloro Fluoro Carbons), undergo photo-

dissociation in stratosphere and subsequently the by products

(e.g. Chlorine) interfere with the overall ozone formation process in the

stratosphere.

In order to address the question 6f increase in incoming UV

irradiance at ground level it is important to understand the photochemistry

operating in stratosphere. The ozone formation and destruction in stratosphere

is the consequence of photochemical reactions. Early work done by Chapman

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(1930), lead to the emergence of the basic steps in photo-reactions for ozone

formation and destruction. The details about the present understanding of the

stratospheric photochemistry are given by Wennberg et.al. (1994). In

stratosphere, ozone is produced photochemically from oxygen in three body

reaction:

O2 + hv =*> ° + ° (A < 243)

° + O2 + M =*> 03 + M

(Where M is the third body and can be N2 or 02)

Ozone thus fonned is destroyed via photochemical pathway as:

03 + hv =*> OeD) + O2 (al~g)

or,

.. (1)

.. (2)

.. (3)

.. ( 4)

( the symbols in parentheses designate the energy levels of ° and 02). If the

formed 02 molecule is excited and in higher vibration mode ( "fi' > > 26) then it

can react with another O2 molecule to produce 03 in following manner.

..(5)

The reactions I to 5, describing the ozone fonnation and the

destruction, essentially act as a effective filter to eliminate most of biologically

hannful UV radiation from reaching the ground. The ozone depleting compounds

7

(ODC) essentially interfere in a way to decrease the overall concentration of

ozone in stratosphere. The ozone depleting reactions as a result of the presence

of CFCs in stratosphere are shown below. The chemistry involved in the

mechanism of ozone destruction by these compounds, in stratosphere, was first

worked out by Molina and Rowland (197 4a, b ). These compounds are inactive so

far they remain in the confines of troposphere . Once transported to the higher

layer of stratosphere they undergo photolysis (Molina and Molina, 1986,

Steinfeld et.al., 1989; Cicerone, 1994) on encountering high energy UV-radiation.

The steps involved are given below:

CCl2F2 + hv(180<A<220 nm) ~> CF2CI + Cl

(Freon 12)

.. (6)

in a different reaction these compounds(ODC) react with electronically excited

oxygen atoms (OeD» to yield chlorine oxides (CIO).

CCl2F2 + (010) ~> CF2CI + CIO .. (7)

These products subsequently participate in a catalytic fashion with ozone forming

reactions as:,

CI + 0 3 ~>CIO + O2

CIO + o~> CI + O2

net: 0 + 0 3 ~> 202

8

.. (8)

.. (9)

The photolyzed product, CI and CIO, in reaction (8) and (9) are capable of

destroying 1,00,000 ozone molecules before they are inactivated (Stolarski,

1988). The major removal process for CI atoms from stratosphere is their reaction

with CH4•

Cl + CH4 =*> H CI + CH3 .. (10)

However, it is observed that CI can again be liberated from Hel or by the

OH radicals, albeit slowly. In addition to CFCs, the presence of (NOx) in

stratosphere also destroys ozone in a catalytic manner, similar to Cl. It is apparent

from the stratospheric chemistry, discussed above, that most of the short

wavelength UV radiation will be removed by ozone formation and destruction.

Whereas, destruction of ozone will lead to the steady percolation of UV towards

ground.

For convenience, the solar ultraviolet irradiance has been divided into three

parts as shown in table 1.1.

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Table 1.1: Nomenclature of incoming UV radiation on the basis of spectral range

Nomenclature Wavelength(}.) range (nm)

UV-A 320-400

UV-B 280-320

uv-c 180-280

This division is frequently used in the field of biology and medicine but

in the area of remote sensing the classification of Huffman (1992) is used, (see

appendix I).

[1.2] The Effects of Increased UV-radiation:

The effects of UV radiation are widespread and pervasive on life forms and

amenity. The anticipated changes due to increased UV flux on air quality, human

health, animals, plants, microorganisms and on materials are described in this

section.

The increase in the magnitude of irradiance, which is showing a steady

shift towards the shorter wavelength of the solar UV spectrum, has already

become significant ( Scotto, 1988; Stamns et.al., 1990; Kerr and McElroy, 1993;

Michaels et.al., 1994). The short range UV radiation (260-320 nm) for long is

known to be a potent mutagenic agent. The UV induced mutations are stochastic

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in nature. In the wake of this, the increased UV irradiance is expected to cause

genetic change by altering DNA, a hereditary material present in plant and animal

cells (Setlow, 1974). In 1960, Beukers and Berends showed that UV radiation

leads to the formation of thymine dimers. More recently, UV has been shown to

induce inter-strand dimer formation, when DNA is in Z -conformation (Love

et.al.,1986; Attri and Minton, 1986). The interaction of UV, in particular

between 260-320 nm wavelength range with important biomolecules is crucial.

The amount of solar UV-B radiation reaching the earth's surface directly

influences the rates of some crucial chemical reactions which occur in troposphere

by enhancing the photo-dissociation rates (Tang and Madronich, 1995; Leighton,

1961). The values for reaction rate coefficient, for some reactions, have been

given by Madronich et.al.(1995) and the comparable sensitivities under different

UV scenarios have been presented by Madronich and Granier (1994) and

Fuglastvedt et.al. (1994). For example, enhanced UV will cause an increase in the

concentration of OH, H02 and H20 2 radicals in troposphere (Fuglastvedt et.al.,

1994 and Neftel et.al.; 1984). The UV-B absorption by formaldehyde(HCHO)

can lead to the formation of odd-hydrogen radicals (Finlayson et.al.1986).

Increased UV-B radiation can produce higher levels of ozone nearer to the (NOJ

emission sources (Gery et.al., 1987 and Whitten 1986). In fact, tropospheric

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ozone concentration is increasingly becoming a common feature of urban

landscape. This increase in UV-B levels may also contribute to the ozone

production in rural areas (Gery, 1993). Enhanced UV radiation also alters the

source and sink relation of greenhouse gases and chemically important trace gases

CO2 , CO, CO(S), Nox ( Zepp, 1994 and Moore, 1993 ).

The relationship between UV-B exposure and h~man health are rather

complex. In small dosage UV-B helps in the formation of Vitamin-D which is very

important for the bone tissue. Other beneficial influence of UV-B exposure is the

suppression of immune reactions in the skin from psoriasis, a hyper-proliperative

skin disorder. But the suppression of immune reactions become a negative factor

under skin tumor formation. Direct adverse effects of UV-B on vision (Taylor

et.al., 1988; Cruickshanks et.al., 1992; Hollowe and Morane, 1981; Pitts et.al.,

1977; Zigman, 1994) and immune system have been studied by a number of

groups (Morison, 1989; Defafo and Noonam, 1993; Jeevan and I<ripke, 1993;

Rivas and Ullrich, 1994). Some studies also include direct effects of UV-B

increase on infection caused by, viruses (Brozet et.al., 1992; Vogel et.al., 1992;

Goethsch et.al., 1994) bacteriae (Jeevan and I<ripke, 1993) and fungi (Denkins

and I<ripke, 1993).

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The most feared and common concern related to UV -B exposure is the

corresponding increase in incidence of skin cancer. Studies done in USA show

that 38,000 new cases of skin cancer will result due to UV-B exposure (Leffell and

Brash, 1996). The cancer resulting from UV-B exposure are of two types,

melanoma and non-melanoma, which have been studied in detail by deGruilj and

van der Leun (I 991, 1993)

Several studies have been carried out on physical and developmental

processes of terrestrial plants on exposure to increased UV-B radiation (Tevini

1993, Borman 1993; Caldwell, 1994). Observations from these experiments show

a decrease in leaf size and shoot length (Tevini, 1989; Varshney and Attri, 1995).

In higher plants, suppression of photosynthesis has also been reported (Caldwell,

1995; Teramura, 1983). In some species, the effects of UV radiation on the

flowering time (Ziska et.al.,1992) and the number of .flowers have been

observed(Hart et.al., 1975). The UV-B also effects the extent of flowering in

plants. Due to enhanced UV-B radiation, reduction in cell division, cell

elongation, seedling elongation, stem elongation, stomatal closure, pollen

germination, pollen tube growth and increase in stem branching, leaf thickness,

pigmentation, wax (cuticular) have been reported ( Hart et.al., 1975; Flint and

Caldwell, 1978; Rau et.al., 1988; Teveni and Teramura, 1989).

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Aquatic systems are very important sources of food for humans. About

20% of the total net primary production on the earth takes place in aquatic

systems. About 30% of animal protein comes from sea (Hader et. aI., 1995). The

anticipated effects of increased UV-B radiation on aquatic ecosystems have been

studied by many workers (Williamson et.al., 1994; Manney et.al., 1994).

Sensitivity of UV-radiation differs from species to species. Piazena and Hader

(1994) have shown that the transparency of water for UV-radiation strongly

depends on the quality of water, and up to a large extent the transparency

becomes the determining factor for the productivity in water bodies. The

phytoplanktons are the primary producers in aquatic ecosystems, thus any

changes due to UV-B increase at this trophic level will lead to corresponding

changes in biomass at next higher trophic levels. The decrease in reproductive

capacity and larval development has been reported (USEPA 1987). But, in one ,

recent study enhanced algal growth(after some lag time) has also been observed

(Bothwell et.al. 1994). This shows that the UV-B effects can be variable and

unpredictable depending on the complexities of the food chain. Cleavage in sea-

urchin eggs of benthic organisms is impaired by UV-radiation (EISayed, 1988).

Coral reefs are also affected by UV-B radiation but, their sensitivity depends on

the depth at which they grow (Jokiel and York, 1982). UV radiation lowers the

14

growth and survival of larval fish (Hunter et.al, 1982).

The susceptibility of a particular material value to UV radiation vvill also

depend upon geographic location and factors like surface characteristics (Hader

et. al., 1995). The effect of UV radiation on materials has been studied by

Tidjani et.al. (1993) and Torikai et.al. (1993a). Exposure of UV-B impairs the

quality of biopolymers and synthetic polymers (Andrady et.al, 1989, 1990; Davis

and Sims, 1983; Torikai, 1993b)

[1.5] Outline and Significance of This Work:

In view of the vvidespread and eclectic interaction of UV-B, with living

and non-living forms, the global concern of scientific community is rightly

justified, for the present and for the posterity. Giving due respect to the complex

interactions, at different levels in biosphere, it is difficult to envisage the long

term changes due to UV-B increase. In addition, as the extent and the irradiance

wavelength span of UV-B varies at different locations, the corresponding observed

changes from one study cannot be extrapolated to evolve general rules. In this

framework the correct knowledge of solar UV-B flux, at a given location, is

essential and pre-requisite to correlate the experimental findings with increased

UV dose. The knowledge, that, up to a large extent, predictable atmospheric and

15

astronomical parameters modulate the incoming solar UV flux (Iqbal, 1983;

Flegale and Businger, 1980) some attempts have been made to model the amount

of UV flux/nin/unit area as a function of the latitude and altitude(Frederick and

Lubin, 1988; Frederick and Snell, 1990; Frederick et. aI., 1993; Green and

Shettie, 1974; Green et.al. 1980; Schippnick and Green 1982). The model

calculations have been compared with actual collected data to further strengthen

the predictability. Hence, the monitoring of ground based UV-flux is fundamental

for validation of any model. The validated model can play an important role in

providing useful information on the variation in UV-B flux over diverse regions.

In addition, the primary data on ground level UV-B flux also assumes significance

in view of the importance of local variability on account of site characteristics

including the atmospheriC pollution which may become an important factor in

urban areas. To date, no exhaustive study on monitoring, modeling and predicting

of UV-flux in urban areas in India has been undertalcen. The pervasive coupling

of UV-B and its significance in relation to biotic and abiotic factors, in general,

has prompted the present research on monitoring and modeling of the incoming

solar UV flux reaching the ground, in Delhi. The measurement procedure for solar

UV flux and the instrument used, are described in chapter II. The present study

entails the measurements of UV-A and UV-B flux reaching ground, in Delhi at

several selected sites. The description of model for predicting UV flux is given in

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chapter III. The diurnal and seasonal variations of measured UV flux and of

model predictions are presented in chapter IV. This chapter also discusses the

experimental and theoretical results in relation to the tropospheric ozone and

aerosols optical thickness. Chapter V outlines the highlights and the main findings

of present work. We have also evaluated the thickness of tropospheric ozone

column over Delhi, at different sites, on different days of the year. Similar

estimate has been made for the thickness of aerosol (scattering and absorption)

column over Delhi. The summary of the work is presented in chapter VI.

17