Environmental Sciences, Vol. 3, 2015, no. 1, 17 - 29

HIKARI Ltd, www.m-hikari.com

http://dx.doi.org/10.12988/es.2015.4118

Aerosol Radiative Forcing of Desert Regions

Abdelouahid Tahiri and Mohammed Diouri *

Department of physics, University Mohamed First

Faculty of science, 60020 Oujda, Morocco * Corresponding author

Copyright © 2015 Abdelouahid Tahiri and Mohammed Diouri. This is an open access article

distributed under the Creative Commons Attribution License, which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Mineral dust plays an important role in Earth’s climate system. In this paper, we

analyse the results of the aerosol optical thickness (AOT), aerosol particle size

distribution (PSD) and aerosol radiative forcing (ARF) of desert aerosol

measurements for six sites covering the main desert areas: Tamanrasset (Algeria),

Birdsville (Australia), Jaipur (India), Solar_village (Saudi_Arabia), Sede_Boker

(Israel) and Frenchman_Flat (USA). The annual cycle of the aerosol optical

thickness monthly averages shows variable values due to the changeable weather

and the Sahara source. The AOT at 0.5μm was found between 0.02 and 0.75. The

maximum values coincide with a peak of activity production of dust from each

region and were recorded in spring and summer. The monthly averages volume

particle size distributions are characterized by fine particles at 0.15 μm and large

particles near 2.5μm, with the maximum amplitude observed in May 2011 at

Solar_Village and Saharian source. The ARF values at surface is ranged from

-3.58 W/m2 to -86.72 W/m2, it reveals that the desert dust aerosol reduced

significantly the solar radiation reaching the ground level producing a large

surface cooling. The ARF at top of the atmosphere (TOA) varies from -24.5 W/m2

to 2.7 W/m2. The negative values at TOA indicates that desert dust aerosol caused

an increase of light scattered back to space inducing thus a significant

Earth-atmosphere cooling. While, the positive values indicate a greater amount of

radiative energy reflected by passenger’s cirrus.

Keywords: AERONET, aerosol optical thickness, particle size distribution,

aerosol radiative forcing, desert aerosol

18 Abdelouahid Tahiri and Mohammed Diouri

1 Introduction

Mineral dust particles are one of the main constituents of the atmospheric

aerosol which influence the radiation budget of the atmosphere. The evaluation of

the dust-radiation interaction is essential for climate forcing assessment at both

local and regional scales. However, large uncertainties still remain in assessing

the dust climate impacts. To understand the radiative effects of dust it is crucial to

characterize its optical properties. One of the major sources of the large

uncertainties in dust radiative forcing is associated with dust optical and physical

properties due to the complexity in dust size distribution and mineral composition

[14]. The Sahara desert is the most important source of mineral dust in the

Northern Hemisphere. North African dust is injected into the atmosphere through

resuspension processes at the source areas, being transported particularly with

dust storms at different altitudes (up to 7km) to different areas in the world [15].

Remote sensing results of dust optical properties indicate that dust is nearly

non-absorbing [12], while earlier laboratory measurements suggested dust to be

partly absorbing at visible wavelengths [14]. Many measurements campaign in

desert zone were conducted in the past, the recent and successful one with

SAMUM consortium undertaken on May-June 2006 at Ouarzazate and Zagora

(Morocco) close to Sahara gives very important description including chemical

and physical properties of desert aerosol near ground and at different altitude

levels during its transport [10]. The omnipresence of dust causes a direct radiative

forcing, but the magnitude (its sign and its global significance) is actually

discussed [1]. Saharan desert dust contributes significantly to the global dust

burden [13]. Physical properties of the mineral dust as well as its chemical and

mineralogical composition and state of mixing change its influence on climate and

atmospheric chemistry [9]. Aerosol optical properties are difficult to characterize

globally due to their large spatial and temporal variability. In fact, the short

residence times in the atmosphere (days to weeks) and the large variability in

composition and particle size distribution (wide range of sources) contribute of

both natural (primarily sea salt and desert dust) and anthropogenic (primarily

combustion of biomass and fossil fuels) aerosol disseminations. This study is

developed on AERONET (AErosol RObotik NETwork, http://aeronet.gsfc.nasa.gov/)

network data concerning the optical parameters characterizing the aerosol in six

representative desert areas (Table 1) for the years 2011, 2012 and 2013.

2 Sites and meteorology

The principal sites analyzed in this paper (Figure 1, Table 1) had years 2011,

2012 and 2013 measurement databases for every month.

Tamanrasset instrument site is located on the roof of the Regional

Meteorological Center (Algeria). This area, free of industrial activities, is in the

high lands of the Algerian Sahara (home of the Kel Ahaggar Tuareg).

Aerosol radiative forcing of desert regions 19

Tamanrasset has a hot desert climate, with very hot summers and mild winters.

There is very little rain throughout the year, although occasional rain does fall in

late summer from the northern extension of the Intertropical Convergence Zone.

Birdsville is located near the maximum of dust storm frequency on the

continent. It lies just east of the Simpson desert, and to the north of the Strzelecki

desert so receives dust from the prevailing southerly and westerly winds. The

climate at Birdsville is very hot and dry in summer and cold in the Southeast area

in winter.

Jaipur site is at the Birla Institute of Technology. It sits on a platform at the

top of the building collocated with a broadband radiometer. Major sources of

pollution are local traffic emissions, dust transport from the Thar Desert. The

climate is desert in the west, it is very hot in April-May-June. The annual’s

temperatures prevent exceed 33°C with maxima in May (41°C). The rainy season

(monsoon) extends from June to September with 205 mm in August.

Solar_Village instrument is located on a rooftop at Riyadh in Saudi Arabia,

approximately 50 km northwest of Riyadh. Solar_Village is the world's largest

solar powered electricity generating system. Saudi-Arabia has a desert climate.

Riyadh, which located in the middle of the lands, is hotter in summer and is colder

in winter. Temperatures can climb to 50°C in summer. The sand-laden winds are

frequent, blowing from the northern deserts for days and weakening brightness.

Sede-Boker is located in the south of Israel in the Har HaNegev (Negev

Heights) region of the Negev Desert. Annual rainfall ranges between 85-100 mm.

The climate is mild with a summer maximum of 32 to 34°C with humidity around

30 percent during the day with nighttime temperatures dropping to 16-19°C and

humidity raising to 90 percent. The summer temperatures are kept comfortable

during the day by a prevailing wind from the Northwest. In the winter nighttime

temperatures can drop below freezing, and while snow is not common it is also

not unheard of. It is the climate with its drastic differences between daytime and

nighttime temperatures along with the sunlight both direct and reflected from the

light Loess soil.

Frenchman_Flat site is located at Nevada in the South west part of the United

States of America. These are the lands of the desert in the shadows at the foot of

California Mountains of the Sierra Nevada. The landscape of Nevada is wrinkled

by hundreds of intervals of mountainous aligned parallel of North at South.

Between these intervals are nestled of thousands of valleys of argil soil or of sand

or of flat land of salt. The peaks and ridges the highest mountain receives pretty

moisture. The climate of the desert tends to become very hot in summer and cozy

in autumn and spring and cooler in winter.

20 Abdelouahid Tahiri and Mohammed Diouri

Table 1. Sites informations (AERONET)

Geographic region Site Coordinates Elevation Years Level

Northern Africa Tamanrasset 22.79°N; 5.53°E 1377 m 2013 1.5

Australia and Pacific Birdsville 25.89°S; 139.34°E 46.5 m 2012 2.0

Asia Jaipur 26.90°N ; 75.80°E 450 m 2013 1.5

Middle-East Solar-Village 24.90°N; 46.39°E 764 m 2011-2012 2.0

Middle-East Sede-Boker 30.85°N ; 34.78°E 480 m 2013 2.0

United States West Frenchman-Flat 36.80°N ;115.93°W 940 m 2013 2.0

Figure 1. AERONET Stations selected for desert aerosol.

Frenchman_Flat

Sede_Boker

Tamanrasset Solar_Village

Birdsville

Jaipur

Aerosol radiative forcing of desert regions 21

Zenith

Ai θv

Figure 2. Almucantar measurements

3 Aerosol Optical Thickness (AOT)

The optical thickness of the aerosol represents the total optical attenuation

induced by the particles for a given wavelength. The decrease of solar flux is

expressed by the Bouguer law:

𝐼(𝜆) = 𝐼0(𝜆)𝑒−𝑚𝑎𝑖𝑟 𝜏(𝜆) (1)

with τ (λ) total optical thickness which take account the contribution of aerosols

aér , Rayleigh scattering τ Ray (λ) and atmospheric gasesgaz (λ) absorption (O3,

NO2 et H2O) [6] :

aér Ray gaz (2)

I (λ): Solar flux measured by the sun photometer (W⁄m2)

I0 (λ) : Extraterrestrial solar flux (W⁄m2)

mair: Air mass [11]

Ai horizontal plane

θv vertical plane

22 Abdelouahid Tahiri and Mohammed Diouri

The AOT values at 0.5 µm varies from 0.01 to 0.75 for all sites and shows high

monthly variation. The maxima values are recorded in spring and summer.

4 Inversion methods

The CIMEL sky radiance measurements in the almucantar geometry (fixed

elevation angle equal to solar elevation and a full 360° azimuthal sweep) at 0.44,

0.675, 0.87 and 1.02 μm in conjunction with the direct sun measured τ (λ) at these

same wavelengths were used to retrieve optical equivalent, column-integrated

aerosol size distributions and refractive indices. Using this microphysical

information the spectral dependence of single scattering albedo is calculated. The

algorithm of [4] with enhancements detailed in the work of [5] was utilized in

these retrievals, known as version 2 AERONET retrievals. Almucantar sky

radiance measurements were made at optical air masses of 4, 3, 2 and 1.7 in the

morning and afternoon and once per hour in between.

For the mineral dust aerosol contained in an air column, the total cross section

of the aerosol contained in this column is the sum of the cross sections of all the

particles contained in the column. The extinction coefficient is then given by:

𝐾𝑒(𝜆) = ∫ 𝜋𝑟2𝑄𝑒(𝜆, 𝑟, 𝑚)

∞

0𝑛(𝑟)𝑑𝑟 (3)

Where 𝜋𝑟2𝑄𝑒(𝜆, 𝑟, 𝑚) is the cross section of a particle with radius r (µm),

complex index of refraction m corresponding to a wavelength λ (µm) and n(r) is

the particle size distribution per unit volume. The aerosol optical thickness is

defined by:

𝜏𝑎𝑒𝑟(𝜆) = ∫ 𝐾𝑒(𝜆)∞

0𝑑𝑥 (4)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Aero

sol o

ptica

l thick

ness

(0.5m

)

Tamanrasset 2013

Birdsville 2012

Jaipur 2013

Solar_village 2011-2012

Sede_Boker 2013

Frenchman_Flat 2013

Figure 3. Monthly mean AOT (0.5 μm) at Tamanrasset, Birdsville, Jaipur,

Solar_Village, Sede_Boker and Frenchman_Flat.

Aerosol radiative forcing of desert regions 23

The measurement of atmospheric aerosol optical thickness (AOT) derived

from the light intensities which are measured by the sun photometer and then used

as input data in the resolution of equation (3) which becomes an equation system

for discrete radius ri of 22 size and discrete λj of 8 values (AERONET algorithm),

give valuable information about the mean columnar atmospheric aerosol

distribution. Many methods of resolution of the inversion problem were used as

data reduction techniques for the determination of aerosol size distribution from

different instrument measurements [16], [2] and the adapted method for SP2H

Sun-photometer measurements [3]. Application obtained for the measurements

performed in Oujda [7] and [8].

4 Volume particle size distribution (VPSD)

The nature of laws governing the aerosol properties changes with particle size

distribution. All properties of aerosols depend on their particle size, which is one

of the most important parameter. The volume particle size distribution can be

obtained from AOT through the resolution of the inversion problem described

before. The AERONET algorithm for particle size distribution retrieval provides

the volume distribution data of 22 size bins from 0.05 μm to 15μm. The best fit

for the size distribution data is a two-mode log-normal size distribution described

by equation:

2 2

2 2

ln ln ln lnexp exp

ln 2 ln 2 ln 2 ln2 ln

f cm mf c

f c cf

r r r rC CdV

d r

(5)

where the subscripts f and c denote fine and coarse modes respectively. σ is the

geometric standard deviation (fine, coarse) and rrf (rrm) is the geometric fine

(coarse) mean radius.

Volume median radius (mean logarithm of radius):

𝑙𝑛𝑟𝑣 =∫ 𝑙𝑛𝑟

𝑑𝑉(𝑟)

𝑑𝑙𝑛𝑟

𝑟𝑚𝑎𝑥𝑟𝑚𝑖𝑛

𝑑𝑙𝑛𝑟

∫𝑑𝑉(𝑟)

𝑑𝑙𝑛𝑟

𝑟𝑚𝑎𝑥𝑟𝑚𝑖𝑛

𝑑𝑙𝑛𝑟 (6)

Standard deviation from volume median radius (mean logarithm of radius):

𝜎𝑣 = √∫ (𝑙𝑛𝑟−𝑙𝑛𝑟𝑣)2𝑑𝑉(𝑟)

𝑑𝑙𝑛𝑟

𝑟𝑚𝑎𝑥𝑟𝑚𝑖𝑛

𝑑𝑙𝑛𝑟

∫𝑑𝑉(𝑟)

𝑑𝑙𝑛𝑟

𝑟𝑚𝑎𝑥𝑟𝑚𝑖𝑛

𝑑𝑙𝑛𝑟 (7)

Volume concentration (µm3/µm2):

𝐶𝑣 = ∫𝑑𝑉(𝑟)

𝑑𝑙𝑛𝑟

𝑟𝑚𝑎𝑥

𝑟𝑚𝑖𝑛𝑑𝑙𝑛𝑟 (8)

The monthly averages particle size distribution are characterized by two

modes, the first, for fine particles is at 0.15μm and the second, for coarse particles

24 Abdelouahid Tahiri and Mohammed Diouri

is near 2.5 μm for Tamanrasset, Jaipur and Solar_Village (Figure 4a) with very

high volume concentration reaching 0.216 μm3/μm2. For Birdsville,

Frenchman-Flat and Sede-Boker (Figure 4b) we observe similar modes for fine

and coarse particles with large variation around 2.8 μm for the coarse mode for

Frenchman-Flat. We remark that volume concentrations for this second group are

five times lower. The annual mean VPSD characteristics for a given sites are

described on Table 2.

Table 2. Annual mean VPSD characteristics Fine mode Coarse mode

Site cv (μm3/μm

2) rv σ cv (μm

3/μm2) rv σ

Tamanraaset 0.011 0.170 0.569 0.134 2.320 0.620

Solar-Village 0.030 0.124 0.496 0.216 2.140 0.564

Jaipur 0.047 0.166 0.473 0.169 2.681 0.620

Birdsville 0.060 0.169 0.474 0.013 2.739 0.734

Frenchman-Flat 0.008 0.169 0.474 0.019 2.836 0.714

Sede-Boker 0.016 0.153 0.475 0.073 2.499 0.641

Aerosol radiative forcing of desert regions 25

0,01 0,1 1 10 100

0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,10

Birdsville 2012

January

February

March

April

May

June

July

August

September

October

November

December

dV(r

)/dl

n(r)

(m

3 /m

2 )

Rayon (m)

0,01 0,1 1 10 100

0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,10

Frenchman_Flat 2013

January

February

March

April

May

June

July

August

September

October

November

December

dV(r

)/dl

n(r)

(m

3 /m

2 )

Rayon (m)

0,01 0,1 1 10 100

0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,10

Sede_Boker 2013 January

February

March

April

May

June

July

August

September

October

November

December

dV(r

)/dl

n(r)

(m

3 /m

2 )

Rayon (m)

0,01 0,1 1 10 100

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

dV(r)

/dln

(r) (m

3 /m

2 )

Rayon (m)

Solar_Village 2011-2012

January

February

March

April

May

June

July

August

September

October

November

December

0,01 0,1 1 10 100

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

Tamanrasset 2013 January

February

March

April

May

June

July

August

September

October

November

December

dV

(r)/

dln

(r)

(m

3/

m2)

Rayon (m)

0,01 0,1 1 10 100

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

Jaipur 2013 January

February

March

April

May

June

July

August

Spetember

October

November

December

dV(r

)/dl

n(r)

(m

3 /m

2 )

Rayon (m)

Figure 4.a. Monthly means of Particle Size

Distribution at Tamanrasset, Solar-Village and

Jaipur.

Figure 4.b. Monthly means of Particle Size

Distribution at Sede_Boker, Frenchman_Flat

and Birdsville.

Aerosol radiative forcing of desert regions 25

5 Aerosol Radiative Forcing (ARF)

Aerosol radiative forcing at surface is defined as the instantaneous increase or

decrease of the net radiation flux at the surface due to an instantaneous change of

aerosol atmospheric content. The atmosphere free of aerosols is the reference case.

Thus, the ARF values can be derived from the following expression:

0 0ARF F F F F (9)

where F and F0 denote the global irradiances with aerosol and without aerosol

respectively. The arrows indicate the direction of the global irradiances, ↓

indicating downward irradiance and ↑ indicating upward irradiance.

Figure 5 shows the monthly means ARF values at surface for six sites,

Tamanrasset, Solar-Village, Jaipur, Birsdville, Sede-Boker and Frenchman-Flat.

The monthly means ARF values is ranged from -3.58 W/m2 to -86.72 W/m2. The

negative value at surface reveals that the desert dust aerosol reduced significantly

the solar radiation at the ground level producing a large surface cooling.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

Ae

roso

l ra

dia

tive

fo

rcin

g a

t su

rfa

ce (

W/m

2) Tamanrasset 2013

Birdsville 2012

Jaipur 2013

Solar-Village 2011-2012

Sede-Boker 2013

Frenchman-Flat 2013

Figure 5. Monthly mean of ARF at surface at Tamanrasset, Birdville, Jaipur, Solar

_Village, Sede_Boker and Frenchman_Flat.

26 Abdelouahid Tahiri and Mohammed Diouri

The aerosol radiative forcing at top of atmosphere for six sites, Tamanrasset,

Birdville, Jaipur, Solar _Village, Sede_Boker and Frenchman_Flat are given in

figure 6. The monthly means ARF values is ranged from -24.5 W/m2 to 2.7 W/m2,

and aerosol radiative forcing at the top of the atmosphere is near 0 W/m2 on the

whole domain of our study except for the three sites in Tamanrasset,

Solar_Village and Jaipur. The passages from negative values to positive values

can be explained by the greater amount of the radiative energy available that has

been reflected by the passenger’s cirrus clouds.

6 Conclusion

The values of monthly means of the aerosol optical thickness obtained at

given sites, Tamanrasset, Birdsville, Jaipur, Solar_Village, Sede_Boker and

Frenchman_Flat, confirm the importance of the influence of the desert aerosol

with relatively high values at the source in spring and summer. A volume particle

size distribution for the coarse mode is around 2.5 μm and for the fine mode, the

mean radius is in the vicinity of 0.15 μm. In sources and in Sahara, the coarse

mode is very important (Solar village, Jaipur and Tamanrasset), this mode is less

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

Aer

osol

rad

iativ

e fo

rcin

g at

top

of a

tmos

pher

e (W

/m2 )

Tamanrasset 2013

Birdsville 2012

Jaipur 2013

Solar-Village 2011-2012

Sede-Boker 2013

Frenchman-Flat 2013

Figure 6. Monthly mean of ARF at top of atmosphere at Tamanrasset,

Birdsville, Jaipur, Solar Village, Sede_Boker and Frenchman_Flat.

Aerosol radiative forcing of desert regions 27

important (Sede_Boker), and becomes much less far from the Sahara (Birdsville,

and Frenchman_Flat). They indicate the importance of diffusion phenomena and

absorption of radiation that appear in summer in and close to the Sahara source.

The average monthly values of the radiative forcing of the aerosol observed at top

of the atmosphere ranges from -24.5 W/m2 to 2.7 W/m2, the negative values

indicates an increase of light scattered back to space inducing thus a significant

Earth-atmosphere cooling, while the positive values indicate a greater amount of

radiative energy reflected by passenger’s cirrus.

The values of the aerosol radiative forcing observed at surface varies between

-3.58 W/m2 to -86.72 W/m2, are fairly representative of the importance of forward

diffusion of coarse modes particles characteristics which result in desert aerosol

decrease in net flux at surface.

From this first sample of desert sites, the measurement analysis shows the

existence of two groups of desert aerosol. The first one compound by Tamanrasset,

Solar-Village and Jaipur for which we observe very high values of AOT and high

volume concentration and radiative forcing. The second important group

Birdsville, Sede-Boker and Frenchman-Flat register moderate values of AOT, and

very low volume concentration and radiative forcing, near five time little than

those of the first group.

Acknowledgements. Authors want to thank all P.I of AERONET sites: Emilio

Cuevas-Agullo, Ross Mitchell, Swagata Payra, Brent N. Holben, Arnon Karnieli

and Carol J. Bruegge.

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Received: December 5, 2014; Published: March 9, 2015