Distribution Trends of UV-Absorbing Aerosols in India

Under Prof. Sagnik Dey – ASL720

Utkarsh

2010CS50299 Nikhil Gupta

2010CS50289 Harshal Bidasaria 2010CS50283

Akshay Singhal 2010CS10207

Dushyant Behl 2010CS50282

Introduction

India follows a cyclic process of an increase in

concentrations of UV-absorbing aerosol during the

summer months. These aerosol concentrations which

consistently remain constant for the rest of the year

show a sudden tendency to rise in the month of March,

particularly in the regions of North India. In this paper we

have attempted to analyze the variations that we see

with regards to Absorbing Aerosol Index (AAI) values for

a period of 30 years over the region of Indian Peninsula.

We have gone through a process of analyzing monthly

trends for the year of 1988, and tried to show how the

AAI values start increasing from the period of March,

reaches its peak value in May, and gets nearly receded

till July. We have also taken up two separate regions, that

of Kanpur and Thar Desert that show considerably high

variations throughout the year. The comparison of

Kanpur and Thar Desert is done for the periods separated

by nearly a decade, the years being 1980, 1990 and 2000.

We have tried to see what is the difference between the

trends that follow over the desert regions of Thar, and

how is it different from the trends that follow in one of

the highly populated cities in the Indo-Gangetic basin.

Following these observations we have tried to find out

how Monsoons may possibly be playing a role in

reduction of aerosol concentrations at the end of

summers.

We have also tried to establish some long term trends.

The long term trends are done in a month wise fashion.

We have taken up two months – May (1999 –2004) and

July (1984 – 1987). The month of May gives the highest

AAI values for most of the years. So, the analysis of May

helps us in understanding the trends in aerosol

concentrations when the values are at their peaks. We

may find out if there is an overall increase in the aerosol

concentrations as the years have progressed through the

recent rapid increase in industrialization across the areas

of Uttar Pradesh. The month of July shows us the trends

of aerosol concentrations when the aerosol levels start

decreasing. We find out using the analysis of July, if the

time during which aerosol concentrations remain high

for a year remains the same across a consecutive period

of five years or not.

We have also covered up the monthly

distribution patterns over a period of five consecutive

years in Kanpur, and monthly distribution patterns

separated by a decade. We are trying to see if the aerosol

variation trends in Kanpur are also cyclic in nature or not,

and that if atmospheric transport is one of the major

reasons in high AAI values over the Indo-Gangetic plains

or not.

Data

The primary source of our data for visualization and

analysis was TOMS daily level 3 global 1.0°x1.25°. The

data was retrieved from three different satellites for

different time ranges

Nimbus – 7 : 1978 – 93

Meteor – 3 : 1992 – 94

Earth Probe : 1996 – 05

We also obtained certain UV Aerosol Index plots of the entire World were also obtained to find any dependencies or relationships that the trends in India may have with the trends that may be observed for the World in general, particularly the area covering Saudi Peninsula and Africa.

Monthly Trends for 1988

In March, the aerosol content is low and initially exists

because of biomass burning, atmospheric transport of

aerosols from nearby deserts like Thar and other less

significant factors like industrial activity, vehicular

emissions etc. However, it is relatively higher in specific

regions such as The Thar Desert of Rajasthan and The

Kutch Desert of Gujarat since the desert dust is a major

source of UV absorbing aerosols.

As the months progress, atmospheric transport carries

aerosols from source locations to other remote areas.

The higher AAI (absorbing aerosol index) is observed

over India (especially the Indo-Gangetic plains). This

observed trend can be attributed to the following

reasons. First is the inflow of aerosols from Sahara desert

through atmospheric transport. The Mediterranean

winds are the major source of atmospheric transport of

aerosols from Sahara desert to remote areas like India.

Some examples of Mediterranean winds that may be

involved are Sirocco, Khamsin etc. Second is the

increased biomass burning and agriculture crop residue

burning in the Indo-Gangetic plains (IGP) during the dry

seasons.

As can be seen from the plots, the AAI is highest in the

month of May. This supports the atmospheric transport

theory by saying that the tropospheric winds reduce in

intensity after the month of May. This is in conjunction

with the durations of many Mediterranean winds. Also

since biomass burning is concentrated in activities like

cultivation, deforestation, forest fires etc., higher values

of AAI is observed in peak dry seasons.

As the inflow of aerosols reduces and the dry season

starts coming to an end (with the advent of monsoon),

the average AAI starts to decrease and ultimately

reduces down to the initial low values.

Annual trends – Kanpur

Kanpur-1980

Kanpur-1990

Kanpur-2000

Kanpur is an industrial city with a large population. In the

three graphs shown above it can be observed that the

Aerosol Absorbing Index is higher in the summer months.

The AAI picks up starting from March and it increases

gradually through the summers, and then drops

dramatically with the onset of monsoon.

It can also be noted that the average AAI values as well

as the maximum AAI values have increased over the

years. This trend can be attributed to high industrial

growth in the region, causing greater emissions of

particulates.

Annual trends – Thar Region

Thar-1980

Thar-1990

Thar-2000

The trends for Thar is visibly different from that of

Kanpur. The Thar region receives no rain in the Monsoon

period and so it doesn’t cause any reduction in the AAI

levels. Instead we see a more gradual rise and decline of

the AAI levels over the months with the peak values at

around the June-July period.

Here again we see an increase in the average and the

peak AAI values over the years. This could be attributed

to increasing temperatures in the region and increasingly

arid climate, leading to higher amounts of fine

suspended dust particles.

Monthly Trends over Kanpur for Consecutive Years

(1984-86)

Monthly Trend for 1984

Monthly Trend for 1985

Monthly Trend for 1986

The above graphs present information about AAI over

Kanpur region for 3 consecutive years (1984-86). The AAI

value for each month is the temporal and spatial

average. As can be clearly seen the shape of the plot for

each year in the given range is the same.

This presents a very interesting observation i.e. the cyclic

nature of aerosol variation. Every year, AAI over Kanpur

undergoes the same variation with time (more or less).

Some small variations can be noticed like value at the

peak.

The same shape suggests a cyclic nature in the variation

and distribution of aerosol content over Kanpur region

which strongly supports the assertion that India

experiences a lot of inflow of aerosols from Sahara region

(atmospheric transport because the flow of tropospheric

winds is cyclic in nature i.e. the duration and timing of

winds is approximately the same every year).

A question arises here is that why is Saharan desert being

speculated as the source. Well, the answer lies in the fact

that desert dust is one of the largest sources of UV

absorbing aerosols and Saharan desert is the largest

desert in the world where aerosols are constantly

transferred to the Troposphere especially in the dry

seasons. Also its proximity to the Mediterranean sea

results in easy transport of the aerosols through

Mediterranean winds to remote areas (in a specific belt

determined by the carrier wind).

Monthly Trends over Kanpur Over decades

Presented below are plots which represent the variation

of average AAI values over Kanpur region over 3 decades.

As can be clearly seen, the cyclic nature observed in the

earlier plots is preserved over decades leading us to the

conclusion that a periodic yearly cause – possibly

atmospheric transport of aerosols (originally originating

from desert dust in Sahara) is one of the major reasons

in high AAI values over Kanpur region (and by extension

over the Indo-Gangetic plains) apart from biomass

burning (which is also cyclic).

1980

1991

2000

Yearly Trends in May over Indian Peninsula (1999 – 2004) – Analysis on Next Page

The changing graphs show an influx of UV-absorbing aerosols from the North-West into the Indian

Peninsula. This is the peak time of the year when AAI gives the highest values. The Absorbing Aerosol

Index shows a general trend in increasing values over this year range. The year of 2001 has an

exceptionally low aerosol cover as compared to other years.

Yearly Trends For May (1999 - 2004)

The Absorbing Aerosol Index consistently shows the

highest values in the month of May over the Indian

Peninsula. The visualized data on the previous page

shows that the highest values lies over the range of 3+.

The regions with highest AAI values are found around

Central India, prominently Delhi, and eastern Uttar

Pradesh.

If we compare the data of May for the year range

1999 to 2004, we see a trend that there is a relatively

higher presence of UV-absorbing aerosols in the region

that extends from beyond North-West of Rajasthan to

somewhere in the middle of Indo-Gangetic planes. The

Deccan Plateau has relatively lower levels of AAI values,

but it still shows a trend of increasing AAI over the period

of these five years. The data from May 2002 and May

2003 clearly shows that the increase in UV-absorbing

aerosol concentration in Deccan Plateau is an extension

of the regions of Central India that already showed high

AAI values in the previous years. The region that is south

of this and the Arabian Sea also show a relative increase

in AAI values but this increase is relatively low as

compared to other parts of India that we have covered.

The regions around Uttar Pradesh and Rajasthan

show relatively higher AAI values as compared to other

parts of the country. There’s also a gap in between these

regions, that is more prominent in the years 2000, 2001,

2002 and 2004. The reasons for these separated high

aerosol concentration can be different. Uttar Pradesh

has the highest population in country. It is possible that

the local emissions due to human activity in Uttar

Pradesh is resulting in relatively higher AAI values in that

specific region. Industrialization, and high population

together may be the two chief contributing factors for

higher aerosol concentrations around Uttar Pradesh.

Uttar Pradesh also forms the central part of the

agricultural belt of India. So, one of the other possible

reasons for higher concentrations around this region

may also be agriculture crop residue burning.

Rajasthan on the other hand has a large portion

of desert area. The population density is comparatively a

lot less as compared to Uttar Pradesh. Since, desert dust

is one of the primary sources of UV-absorbing aerosols

we can assume that Thar Desert is the reason that

Rajasthan has relatively higher aerosol concentrations as

compared to other surrounding regions in India. But

desert dust itself may not be enough to explain the cause

of increasing AAI values when yearly trends are

observed. It is possible that one of the possible reasons

for this increasing trend is an external factor. When we

check the world AAI values for May in 2002, we noice

that the higher AAI values in Rajasthan are an extension

of the regions of Uttar Pradesh that has the highest

relative AAI values in the surrounding regions. The whole

of Indo-Gangetic Planes and Thar Desert form a

contiguous area. It is also possible that Thar and Uttar

Pradesh have localized high concentration of UV-

absorbing aerosols, but due to the action of winds the

aerosol concentration evenly spreads out across the

whole of the Indo-Gangetic Plains and the Thar Desert.

Yearly Trends for July (1984-1987)

July is the period when Aerosol Index for UV-absorbing

aerosols starts decreasing over the Indian Peninsula. If

yearly data is observed for this month it can be seen that

the averaged out Aerosol Index value over the Indian

Peninsula increases over the period of 1984-1987. The

western half of the Indian Region has comparatively

higher AAI values as compared to the eastern half,

especially in the year of 1985. The receding UV-absorbing

Aerosol cover shows comparatively higher levels of AAI

towards the North-West Region of Indian Desert. This

pattern is consistent for the range of 1984 – 1987.

In the region of Central India, around Delhi and

Western border of Uttar Pradesh, we notice a higher

concentration of UV-absorbing aerosols, especially year

1985 onwards. An AAI value in the range of 2 - 2.5 can be

observed for this area of Central India and Eastern

Pakistan in the year 1985. For the same month in 1987,

both of these areas seem to merge into a single

contiguous area of AAI 2 - 2.5. It may also be noticed that

other than the area around Delhi and UP, no other region

in India seems to have high localized AAI values.

The AAI values over this period of time for

Arabian Sea also shows an increase from 1 - 1.5 in 1984,

to 1.5 - 2 in 1985. It then stays consistent for the years

following 1985. There seems to be no primary source of

absorbing aerosols over this region, hence it’s only

possible that the increase in aerosol concentration is due

to transport of UV-absorbing aerosols from some other

region. India on the east of this region, does not seem to

be a source of any UV-absorbing aerosol. On South,

Arabian Sea is surrounded by Indian Ocean, which also

cannot be a source of UV-absorbing aerosols. The only

possible regions which may be responsible for this

increase in AAI values can be Arabian Peninsula and

Africa in the West, or Indian Desert and Pakistan in the

North. The global AAI pattern in July for the year 1987

shows that the reason of increased AAI values is possibly

inflow of UV-absorbing aerosols from both African

Desert and Arabian Peninsula in West, and Indian Desert

and Pakistan in North.

Conclusion

The analysis of collected data shows that the major

sources of UV absorbing aerosols are desert dust and

biomass burning. The Aerosol Absorbing Index is

observed to be maximum in the summers in the Indian

Subcontinent, with the values peaking around the May-

June period.

The monthly variations in AAI were observed to follow

similar trends over a period of three decades showing

increase around the months of April-May-June and then

decreasing afterwards.

The aerosol content was observed to be increasing over

the years due to increasing industrial activities,

consumption of coal and vehicular emissions.

We also noticed the role of the global weather patterns

in the distribution of aerosols. Mediterranean winds

carry over aerosols from the Saharan regions into the

Indian subcontinent via the Arabian Peninsula.

AAI levels were seem to be cyclic in nature and showed

this nature over a fixed belt which strongly indicated

towards the presence of Tropical winds mainly flowing

eastwards.

Some highly industrialized regions like Kanpur had higher

AAI values as compared to the surrounding regions. The

reasons for this included higher biomass burning and

industrial activity.

We also noticed that Monsoon played a role in

controlling the AAI levels. The regions which received

rainfall such as Kanpur during the monsoon season saw

a dramatic drop in AAI values, while other regions such

as Thar which do not receive any rainfall showed no such

drop in AAI levels.

Work Distribution

Nikhil and Utkarsh: Analyzing yearly trends over Indian

peninsula

Harshal And Dushyant: Analyzing monthly trends over

Indian peninsula

Akshay : Correlating observations and conclusions with

global AAI data

References

http://disc.sci.gsfc.nasa.gov/

Global distribution of UV-absorbing aerosols

from Nimbus 7/TOMS data by J.R Herman and P.

K. Bhartia

http://earthobservatory.nasa.gov/

http://www.nasa.gov/centers/langley/news/fac

tsheets/Aerosols

Absorbing Aerosol Index and Aerosol

Characterization from UV-Visible satellite

instruments by Pepijn Veefkind et al

Long-range transport of aerosols from

agriculture crop residue burning in Indo-

Gangetic Plains—A study using LIDAR, ground

measurements and satellite data by K.V.S.

Badarinath et al.