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Determination of mass, element and black carbon concentrations... Aboh and Ofosu

THE DETERMINATION OF MASS, ELEMENT AND BLACK CARBON

CONCENTRATIONS IN HARMATTAN AEROSOL SAMPLES

COLLECTED AT KWABENYA, GHANA

I.J.K. Aboh and F. G. Ofosu Physics Department, National Nuclear Research Institute,

Ghana Atomic Energy Commission,

P. O. Box LG 80, Legon, Ghana

ABSTRACT

Aerosol particles were sampled between 27th December 2005 to 16th February 2006, using the

Gent sampler and segregated into two size fractions – fine (PM2.5) and Coarse (PM10-2.5) from

Kwabenya, near Accra. The aerosol particles were collected on Nuclepore polycarbonate mem-

brane filters. The mass, Black Carbon (BC) and elemental concentrations in the two size frac-

tions were determined using Gravimetric analysis, black smoke method and EDXRF analysis,

respectively. The aerosol mass concentration was 8.57 µg/m3 for the fine fraction and 110.90 µg/

m3 for the coarse fraction. The average Black carbon concentrations measured were 0.71 µg/m3

and 0.65 µg/m3 for the fine and coarse fractions, respectively. The results were compared with

some literature values and the World Health Organisation Standard values. The high coarse to

fine ratio suggest that most of the aerosol are from natural sources.

INTRODUCTION

The dry season in Ghana, which varies slightly

based on the vegetation zone, is from late Octo-

ber to mid March and is mainly characterised

by the Harmattan (dry and dusty) winds from

the Sahara Desert. The winds blow from the

northeast into West Africa and the Gulf of

Guinea. During the Harmattan period, there is

an increase in particulate matter levels which

are closely linked to the increased atmospheric

stability. This is compounded by it coinciding

with the absence of precipitation, hence mini-

mal washout effect, and effective ventilation

(Cuhadaroglu, 1997; Romero, 1999; Cariňanos,

2000). The absence of leaves and vegetation,

which acts as filters during the rainy season

also enhance the high particulate levels in the

environment. The Sahara Desert is regarded as

the largest dust source in the world (Levin,

1999). The study of the Harmattan dust is cur-

rently amongst one of the focused areas of air

pollution because of the high aerosol load and

its effect on health and climate (Schwanghart,

2008; Wong, 2006; Bou Karam, 2008; Antoine,

2006; Carminade, 2006; Ogunjobi, 2008). Har-

mattan dust has been found to have the poten-

tial to be transported over large distances in the

atmosphere, e.g. Harmattan dust have been

determined by satellite in the Western Hermi-

sphere, South America and even the Arctic.

During the Harmattan, heavy amount of dust in

the air severely affect visibility and block the

sun for several days thereby influencing ambi-

ent temperature. This period is not the best of

Journal of Ghana Science Association, Vol. 12 No. 2 . Dec., 2010 64


weathers for people with respiratory health

problems (asthma, cough and catarrh), allergic

eye diseases, and sickle cell anaemia.

For many centuries, aerosol particles have been

recognised for their potentially negative impact

on human health and ecosystems (Brim-

blecombe, 1999; Ramazzini, 1713; Linné,

1734). However, it was not until the second

half of the 20th century, since the famous smog

incident in 1952 in London that forced scien-

tists, politicians and decision makers worldwide

to take note and put in place the necessary re-

medial actions. Epidemiological studies in

several countries have shown conclusively that

there is a direct link between particulate air

pollution and adverse health effects. Research

has confirmed that low-birth weight, preterm

births and infant mortality are higher in com-

munities with high levels of particulate air pol-

lution (American Academy of Pediatrics, 2004;

2005; Gauderman, 2000; Yang, 2003).

The physical properties of aerosols affect the

transport and deposition of the particles in the

human respiratory system, whilst the chemical

composition and the physical properties deter-

mine their impact on health. For two to three

decades, environmental authorities in devel-

oped countries have been active in sampling

and analyzing particles with (aerodynamic)

diameters smaller than 10 µm, PM10. Studies

during the last ten years indicate, however that

the very small particles are even more hazard-

ous to health, and smaller particles PM2.5, PM1.0

and even PM0.1 (or nanoparticles) have been

more extensively studied (Loomis, 2000; Mark,

1998; Schwartz, 2000; Donaldson, 1998; Harri-

son. 2001; Hauck, 2004). The number of death

from air pollution is estimated to be more than

3 million, with about 1 million in cities and 2

million in rural settings. The largest contributor

to the rural death toll is biomass fuel burning,

as a source of domestic energy (WHO, 2000;

CSIRO, 1999).

Although much progress has been made in air

pollution studies, our knowledge of the chemi-

cal composition of the atmospheric aerosol on a

global scale is still rather limited. Most devel-

oped countries have recognised the adverse

effects posed by aerosol particles, especially on

human health, and this has resulted in increased

routine monitoring of atmospheric particles

(Clarke, 1999). In contrast however, there is

almost no routine monitoring of aerosol data in

developing countries, especially in Africa

(except for South Africa, Egypt and Tunisia).

Hence data on concentrations as well as charac-

teristics of particulate matter are almost non-

existent in most developing countries

(Landsberger, 1995; Kent, 1998). To under-

stand fully how aerosol particles behave under

different climatic conditions there is the need to

characterize aerosol particles in many different

places including developing countries, espe-

cially those in sub-saharan countries like

Ghana. Characterization for aerosol particulates

would not only ensure countries to comply with

international conventions and standards but

would serve as an informed-basis for setting

appropriate standards for the countries them-

selves.

In this work, airborne particulates sampled at

Kwabenya (near Accra, Ghana) from 27th De-

cember 2005 to 16th February 2006) were char-

acterized. Characterization includes size distri-

bution (into fine and coarse particles), mass

concentration, black carbon (BC) and elemental

concentrations.

MATERIALS AND METHODS

Sampling Site

Samples of aerosol particles were collected at

the Ghana Atomic Energy Commission site at

Kwabenya (5º40’35.0 N, 0º11’12 W), near Ac-

cra. Kwabenya is a semi-urban area with agri-

cultural lands which are fast developing into

residential areas. It is less than 20 km and in the

North-Eastern direction from the Accra City

Centre (5º35’00 N, 0º13’08W) and on the ma-

jor city air trajectories. It is not close to any

known manufacturing or industrial setup. These

features are ideal for looking at the emissions

from Accra when the winds are favourable and

not affected by the dominating presence of any

particular source. Two other factors that also

influenced the selection of this sampling site



were availability of electrical power and the

secured environment for the sampler.

Sampling Instrument

The GENT sampler used for the sampling con-

sists of a compact vacuum pump system con-

trolled by a timer (Hopke, 1999; Maenhaut,

1992). It is connected to a stacked filter head

unit, which is about 1.6 m from the ground, into

which the two size fraction (PM10-2.5 or coarse

and PM2.5 or fine filters) are loaded. In this

work, nuclepore track etched polycarbonate

filter of pore size 8 µm and diameter 47 mm

was used for the coarse fraction. The fine parti-

cles were collected on a 47 mm diameter nucle-

pore polycarbonate filter with pore size of 0.4

µm. The sampling in this work was done for

approximately 24 hours.

Analytical Techniques

Gravimetric analysis

Composition of atmospheric aerosols involves

the analysis of deposits collected on filter sub-

strate. It is very important because all other

analytical measurements will necessarily de-

pend on the mass deposited. Gravimetric analy-

ses determines the net mass by weighing the

filter before and after sampling with a Sartorius

MC-5 micro-gramme sensitive balance in a

temperature- and relative humidity-controlled

environment after the filters were conditioned

in desiccators for 5 days. The main problem

that arises is interference from electrostatic

charges, which induces non-gravimetric forces

between the filter and the balance. However the

charges are removed from the filters by expos-

ing them to low-level radioactive source (Po-

210) prior to weighing.

Analysis of black carbon

The equipment used is the Smokestain Reflec-

tometer M43D manufactured by Diffusion Sys-

tem of U.K. The operating principle of the Re-

flectometer is known as the “black smoke

method” (Gagel, 1996). A light source (high-

performance LED with maximum emission at

650 nm) shines on the aerosol filter and the

reflected light is measured by photo-diodes

located in a black housing. The Reflectometer

is calibrated by the manufacturer. Measuring

the reflected light emitted by a white filter (this

is set to 8.0) and a totally black filter (set to

0.4), this enables the calibration parameters of

the manufacturer’s to be used.

The output voltage is converted to a measure of

blackness known as “black smoke number”, RZ

which is determined from the three output volt-

age obtained, i.e. from the aerosol filter to be

evaluated, the totally white filter and the totally

black filter. The equation relating the output

voltage to the black smoke number described in

Moloi (2002) is given as:

maxmax / RZRZORZRZO UUUURZRZ (1)

where

URZ0 is the output voltage with blank

(white) filter (which is set to 8.0V according

to the instructions manual), URZmax the out-

put voltage with totally black filter (set to

0.4 V) and URZ the output voltage with ac-

tual filter to be evaluated.

The RZ is then related to the concentration of

black aerosol using the Lambert-Beer’s law,

given by:

max01 /1 kRZRZRZIn

V

RMCR (2)

where

CR is the the black carbon concentration, V

the sampled air volume, RM1 the black car-

bon mass in a single dust layer on the filter,

RZ0 the black smoke number for a white

(blank) filter, RZ the black smoke number

for the actual filter, RZmax the black smoke

number for a black filter and k the calibra-

tion constant.

The EDXRF spectrometer

The EDXRF spectrometer at the Royal Veteri-

nary and Agricultural University of Denmark

was used in the study (Laursen, 1999). The

spectrometer is a compact, flexible and sensi-

tive unit, using a high power Mo X-ray tube.



The primary beam is monochromatized by a

highly ordered pyrolytic graphite (HOPG) crys-

tal and the detector is a Peltier cooled Si(Li)

detector. The detector has an active area 20

mm2, FWHM (Mn K of 146 eV. The X-ray

tube was operated at a voltage of 40 kV and a

current of 40 mA in the measurements with the

irradiation live time of 2000 s. The samples

were placed in an evacuated irradiation cham-

ber with very minimal air absorption hence

elements from Al to U can be detected, ana-

lysed and quantified.

The fluorescent intensity, I, of an element of

interest, i, is generally given as (Aboh, 1992)

low filter loadings (less than 100 µgcm-2).

Hence the x-ray attenuation by the filter mate-

rial can be neglected, since the particulates are

deposited as a thin layer on the surface of the

filter.

The spectrometer was calibrated using thin film

material from NIST (NBS SRM 1832) and the

X-ray fluorescence spectra analysed using the

fundamental parameters programme (Laursen,

2001). Minimum detection limits for the spec-

trometer are shown in Table 1.

RESULTS AND DISCUSSION

The total mass of particulate matter collected

on the filters varied from day to day as shown

in Fig. 1. Mean elemental concentrations for

the PM2.5 and PM10-2.5 aerosol are shown in

Table 2 and Table 3 respectively, together with

standard deviation (Std Dev.), and range. It

should be noted that the standard deviations

listed in the table, (Std Dev), are not "true" de-

viations which expresses fluctuations in experi-

mental conditions for the analytical methods.

Instead, they are combinations of these and the

variations that occur due to changing weather

conditions and human activities from one day

to another. The mean, median, range and St.

Dev may give better information on the extent

of influence of these extreme conditions. These

are further illustrated in Fig. 2, 3 and 4 showing

how the BC and elemental concentrations var-

ied widely from day to day. The true Standard

deviations for measurements on the same stan-

dard sample on this instrument are in the order

of about 10 % (Lindgren, 2006).

The fine particulates contribution of black car-

bon (BC) has been found to be about 8% of the

mass, while for the Coarse it is about 0.6%.

This shows that the fine fraction is dominated

by BC, which is mostly generated by combus-

tion activities. Generally BC is not measured in

the coarse fraction, but we undertook it know-

ing the peculiar situation in Ghana (particularly

in the Accra Metropolitan Area) where most

people and City Authorities indiscriminately

burn waste. This waste burning, at very low

temperature, in addition to the high incidence

corriii AdKGI 0 (3)

where

Go is the geometric constant which depends

on the intensity of the excitation source and

the source-sample-detector geometry, Ki is a

constant that involves all the fundamental

parameters, and Acorr the absorption correc-

tion factor

The Absorption correction factor is given as:

dadaAcorr /exp1 (4)

where

ā is the total absorption of the primary and

fluorescent x-rays in the sample, ρ the

density of the sample and d the thickness

of the sample.

For a sufficiently thin sample, equation 4 can

be approximated by the expansion of the expo-

nential term, as:

exp (-x) ≈ 1 – x, and becomes

iii dKGI 0

Hence for “thin” samples there is a linear rela-

tionship between the fluorescent x-ray intensity

(Ii) and the concentration (ρd)i.

Aerosol samples collected on membrane filters

can be regarded as thin samples, because of the

(5)



DL Fine

(b)ng/cm3 Coarse

(b)ng/cm3 (a)ng/cm2 (a)ng/cm2

13 Al 208.3 109.08 235.1 123.10

14 Si 87.9 46.02 111.3 58.29

15 P 47.9 25.05 57.5 30.10

16 S 32.3 16.80 37.1 19.30

17 Cl 19.8 10.28 25.2 13.09

18 Ar 31.3 16.29 40.3 20.94

19 K 9.7 5.02 12.1 6.31

20 Ca 5.6 2.89 7.0 3.64

21 Sc 3.8 1.98 4.8 2.47

22 Ti 3.0 1.57 4.1 2.11

23 V 2.3 1.20 2.7 1.42

24 Cr 1.9 0.98 2.4 1.24

25 Mn 1.4 0.73 1.9 1.01

26 Fe 1.5 0.79 2.4 1.23

27 Co 0.9 0.48 1.3 0.67

28 Ni 1.4 0.74 1.7 0.87

29 Cu 1.3 0.68 1.5 0.78

30 Zn 0.8 0.40 0.9 0.45

31 Ga 0.6 0.33 0.7 0.38

32 Ge 0.6 0.32 0.7 0.37

33 As 0.5 0.27 0.6 0.31

34 Se 0.5 0.27 0.6 0.30

35 Br 0.7 0.34 0.9 0.49

36 Kr 0.5 0.27 0.6 0.29

37 Rb 0.6 0.32 0.7 0.36

38 Sr 0.9 0.46 0.9 0.9

82 Pb 0.9 0.47 1.1 0.55

a) DL is calculated as 3 times the square root of background concentration (3σ).

Mo Ka: 17.44 keV, V=40 kV, I=40mA, collection time 2000 s.

b) DL for particle concentration is calculated for a sampling of 24 m3.

Table 1: Minimum detection limit for particulate matter on nuclepore filters with the

EDXRF spectrometer

of bush fires during the Harmattan period give

rise to the coarse fraction BC. The mass of

particulates in the coarse fraction is very high

(about 10 times) compared to the fine fraction.

This shows that most of the air particulates,

during the period, originate from mechanical

(mainly natural) activities rather than combus-

tion (anthropogenic) activities which turn to

create fine particles as a result of gas-to-particle

conversion. The coarse fraction is dominated

by very high Al and Si concentrations showing

that most of the particulate matter in the coarse

mode are of crustal origin.



Element (ng/m3) Mean Median Range Std. Dev

Si 332.4 279.3 119.2 - 877.3

202.6

S 146.4 121.4 49.3 - 567.0

103.0

K 153.1 116.1 44.7 - 462.7

96.7

Ca 34.1 29.5 8.5 – 125.2

22.9

Ti 10.3 9.7 1.67 – 29.3

6.2

Mn 2.74 2.4 1.5 – 7.4

1.4

Fe 85.2 76.1 31.2 – 220.8

49.8

Zn 2.0 1.9 0.7 – 5.0

1.1

Br 1.9 1.6 0.8 – 5.4

1.1

BC (µg/m3) 0.71 0.47 0.04 – 2.48

0.69

Mass (µg/m3) 8.57 7.44 2.03 – 2.96 5.57

Table 2: Average fine particle elemental, BC and mass concentration

Element (ng/m3) Mean Median Range Std. Dev.

Al 1682.1 1518.9 429.4 – 3852.0 621.5

Si 4800.0 4538.9 1078.0-10190.8 1655.3

S 317.4 304.9 107.6 – 684.5 117.9

Cl 603.9 528.2 26.8 – 1861.4 322.3

K 529.6 506.9 175.2 – 1191.4 179.9

Ca 663.7 646.6 141.9 – 1370.6 209.8

Ti 157.0 151.7 35.5 – 329.3 51.8

V 12.3 11.6 3.1 – 24.3 4.4

Cr 6.9 6.8 0.9 – 14.8 2.9

Mn 30.3 28.2 7.3 – 63.2 9.9

Fe 1462.5 1386.0 360.1 – 3149.3 486.0

Cu 2.4 2.3 0.5 – 4.6 0.8

Zn 9.8 7.8 2.4 – 53.9 7.7

Br 2.4 2.2 0.7 – 5.7 1.0

Rb 2.4 2.2 0.6 – 5.4 0.9

Sr 5.7 5.7 1.3 – 12.8 1.9

Pb 3.0 3.0 0.7 – 6.1 1.2

BC (µg/m3) 0.65 0.54 0.00 – 1.69 0.34

Mass (µg/m3) 110.9 103.8 25.43 – 228.17 35.70

Table 3: Average coarse particle elemental, BC and mass concentration



Fig. 1: Daily aerosol mass concentration

Fig. 2: Daily black carbon (BC) mass concentration



Fig. 3: Comparison of mass concentration of some elemental in the fine mode

Fig. 4: Comparison of mass concentration of some elemental in the coarse mode



Table 4 shows the monthly variations in the

particulate mass concentration. There was only

four (4) sampling days in December 2005,

hence the values could not be reliable for any

meaningful comparison. It is however notewor-

thy that the mass concentrations obtained for all

the four days in December were consistently

less than the average for January. The signifi-

cant decrease from January to February 2006

was consistent with normal weather trend

where the Harmattan peaks in January, start to

decrease in February and ends in March.

Comparing the result from this work with some

other works (Table 5) confirms that the coarse

fraction is very high but the fine fraction which

contains most of the respirable particulate mat-

ter is low.

The World Health Organization (WHO, 2006)

gives the following guidelines for particulate

matter (PM):

PM2.5 - 10 µgm-3 (annual mean) and 25

µgm-3 (24-hours mean) and

PM10 – 20 µgm-3 (annual mean) and 50

µgm-3 (24-hour mean) respectively.

It should be noted that in this work since we

have size fractionated the PM10 into PM10-2.5

(Coarse fraction) and PM2.5 (Fine fraction),

both fractions needed to be added to get the

PM10 values. On the average the PM10 value

have been exceeded and in the extreme over

four times the guideline limits has also been

exceeded, while the PM2.5 values are far below

the guideline values. In the said WHO guide-

line it has been stated that no threshold has

been identified below which damage to health

is not observed.

Sampling Fine (PM2.5) Coarse (PM10-2.5) PM10

Month Days Mean (µg/

m3) StDev

Mean

(µg/m3) StDev

Mean

(µg/m3) St.Dev

Dec-05 4 9.193 3.199 84.363 43.347 93.455 46.122

Jan-06 28 8.580 5.464 129.773 38.340 129.773 42.152

Feb-06 15 8.744 6.474 99.216 16.104 107.961 19.859

Table 4: Monthly Comparison of PM Mass Concentration

Place Concentration (µg/m3)

Reference Fine Coarse

Skukuza (South Africa) 9.4 16.2 Maenhaut; 1996

Prestoriaskop (South Africa) 12.3 19.4 ,,

Palmer (South Africa) 18.0 15.2 ,,

Watertown, boston (USA) 17.4 8.6 Chan; 2000

Long Beach, Califonia (USA) 48.6 22.5 ,,

Kashima (Japan) 17.7 17.5 ,,

Tapada du Quterio (Portugal) 18.5 13.1 Alves;1998

Calgary (Canada) 11.1 26.3 Cheng; 2000

Edmonton (Canada) 11.2 19.1 ,,

Hinton (Canada) 8.0 17.0 ,,

Serowe (Botswana) 10.1 18.4 Moloi; 2002

Goteborg (Sweden) 7.0 7.2 ,,

Kwabenya (Ghana) 8.6 110.9 This work

Table 5: Comparison of PM with a selection from Literature



Even though, fine particles can originate from

natural sources in addition to anthropogenic

sources, it is a well known fact that the coarse-

to-fine (C/F) ratio can be used to characterize

crudely both natural and anthropogenic emis-

sions. Usually elements of natural origin have

high C/F ratios and elements of anthropogenic

origin have low C/F ratio (Moloi, 2002; Oblad,

1986). However the results in Tables 2 and 3

show that all the elemental means in the coarse

fraction is higher than those in the fine fraction,

i.e. higher C/F ratios.

CONCLUSION

Most of the aerosols measured at Kwabenya

during the Harmattan period under review are

in the coarse particle mode. The mean value of

119.5 µg/m3 for PM10 is above the WHO guide-

line and Ghana Environmental Protection

Agency (Ghana EPA) guideline (70.0 µg/m3 for

24 hour average and 50 µg/m3 yearly average)

values. The mean value of the fine fraction 8.6

µg/m3 falls very well below the WHO guide-

line.

The BC of the coarse fraction is low but that of

the fine fraction which is about 8% is on the

higher side as this is comparable to values from

developed countries. There is the need to look

at this in future measurements because most of

the bush burning during the Harmattan period

are not at temperatures comparable to burning

in incinerators to produce this high fine fraction

black carbon concentration and should contrib-

ute mainly to the coarse fraction.

The size distribution of all the elements meas-

ured are dominated by the coarse and this high

coarse to fine ratio and the high concentration

of Al and Si in both size fractions gives an indi-

cation that the main factor contributing to par-

ticulate matter are soil/dust from natural origin.

ACKNOWLEDGEMENT

The BC and mass measurement was done at the

University of Boras, Sweden and the EDXRF at

the Royal Veterinary and Agricultural Univer-

sity, Denmark. The authors are grateful to the

International Atomic Energy Agency for the

fellowship awards to I.J. Kwame Aboh to

carryout the measurements. We are also grate-

ful to the following people: Profs. Eva Selin

Lindgren and Dag Henriksson of the University

of Boras and Prof. Jens Laursen of the Royal

Veterinary and Agriculture University in Den-

mark.

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