Atmospheric turbidity and the diffuse irradiance in Lagos, Nigeria

PII: S0038-092X(97)00020-0

Solar Energy Vol. 61, No. 4, pp. 241–249, 1997 © 1997 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain 0038-092X/97 $17.00+0.00

ATMOSPHERIC TURBIDITY AND THE DIFFUSE IRRADIANCE IN LAGOS, NIGERIA A. A. L. MADUEKWE* and M. A. C. CHENDO†** * Department of Physics, Usmanu Danfodiyo University, Sokoto, Nigeria ** International Centre for Theoretical Physics, Trieste, Italy Received 8 December 1995; revised version accepted 3 February 1997 Communicated by RICHARD PEREZ Abstract—Values of the Angstrom turbidity coefficient, b, at 0.50 mm wavelength have been determined in Lagos, a tropical coastal city in Nigeria, for 17 months between 1990 and 1991 using a Volz sun photometer. The results obtained indicate high variability of aerosol loading, though the city, on average, experiences high turbidity for most parts of the year. An annual average of 0.299 with a standard deviation of 0.187 was found. On average, the month of January experienced the highest turbidity, with a mean value of 0.497, while November experienced the lowest aerosol loading on average with a value of 0.225. The aerosols influencing the city are both of maritime and Saharan origin, coupled with locally produced particulates from industries and automobile combustion. Possible effects of the Mount Pinatubo haze may be the cause of the increase of turbidity values in the second half of 1991 when compared with values for the same period in 1990. Addition of b in the Liu and Jordan type diffuse fraction correlation indicates little improvement in standard error. © 1997 Published by Elsevier Science Ltd.

1. INTRODUCTION

There are numerous articles in the literature on the effects of atmospheric aerosols on solar radiation and the possible effects on climatic change (Liou and Sasamori, 1975; Wang and Domoto, 1974; Lacis and Hansen, 1974; Braslau and Dave, 1973a,b). The influence of aerosol on radiation passing through the atmosphere cannot be neglected, especially in urban or industrialized areas. Some aerosols such as soot or soil dust also absorb light which is thus lost from the radiation flux, but the aerosol layer itself heats up and re-radiates the energy at other (thermal ) wavelengths. The diffuse fraction of the total hemispherical solar radiation plays an active role in thermal, chemical and biological processes on the earth’s surface. There have been several investigations in the Nigerian environment on the relationship between the daily and monthly average of the diffuse fraction and the ratio of the global horizontal to extraterrestrial solar radiation (Sambo, 1988; Doyle and Sambo, 1988; Ideria and Bamiro, 1982). Doyle and Sambo (1988) have also investigated the effect of changing air masses on the diffuse fraction received in Kano, Nigeria. However, not much work has been carried out at any Nigerian location in relation to the influence of atmospheric aerosols on the †ISES member. 241

diffuse solar radiation received on the earth’s surface at these locations. In this work an attempt is made to obtain relationships between turbidity, using Angstrom turbidity coefficients, and the diffuse component of solar radiation through simple linear equations. The sun photometric method has been used to obtain the atmospheric turbidity, b, at 0.50 mm wavelength using a Volz sun photometer, #573, which has four wavelength channels at 0.38, 0.50, 0.88 and 0.94 mm 1.1. Lagos climate Lagos, like most towns of West Africa, experiences two major weather regimes in a year, namely the dry and wet seasons. During the dry season (January, February, March, October, November, December) the prevailing winds are dry and dust laden north-east trades ( locally called harmattan) blowing from the Sahara desert. In the wet season (April, May, June, July, August, September) moist south-west monsoon winds blow inland from the Atlantic Ocean. These seaborne winds influence the available solar radiation in the location in several ways, including: (1) They are heavily moisture laden most of the time, thus giving rise to precipitation during the wet season. This invariably limits the number of days of clear sky. (2) The moisture laden atmosphere is highly

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humid leading to cloud formation and subsequent extinction of solar radiation. (3) The sea sprays and wind carry fine droplets containing sodium chloride salt, dissolved organic compounds and other organic matter, resulting in high turbidity within the locality. 2. DATA ACQUISITION

2.1. Hourly global and diffuse solar irradiation The hourly global and diffuse solar radiation data were collected with Middleton pyranometers mounted at the Science Faculty building at the University of Lagos. The diffuse radiation measurements were thereafter corrected using the approach of LeBaron et al. (1990). The measured solar radiation data were checked for data that violated the conservation principle or that violated the physical limits as given by Reindl et al. (1990): Limit 1: I /I<0.90 and I/I <0.20 d o Limit 2: I /I>0.80 and I/I >0.60 d o where I is the hourly measured global horizontal solar radiation, I is the hourly measured diffuse d solar radiation on a horizontal surface and I o is the calculated hourly extraterrestrial solar radiation on a horizontal surface. Limit 1 places a restriction on the diffuse fraction under cloudy overcast sky conditions. If an hour had a measured diffuse fraction that was less than 0.90 for a clearness index less than 0.20, it was eliminated from the data set. Similarly, Limit 2 places a limit on the diffuse fraction under clear sky conditions. These quality tests help to reduce spurious data and minimize any impact that suspect data would have on a derived correlation. Altogether 1740 h of data were used for which 851 h of turbidity measurements were actually realised. Table 1 shows the number of realized measurements of direct spectral irradiance at wavelength 0.50 mm made with the sun photometer for each month. 2.2. Angstrom turbidity coefficients Direct sun photometry has been used to determine aerosol loading and the amount of water vapor in the atmosphere. It is a simple radiometric technique based on the qualitative changes in the diminution of direct beam radiation in a narrow selected spectral interval. The sun photometer, like the pyrheliometer, detects not only the direct beam but also a circumsolar component. A second photometer #222 with

Table 1. Number of measurements realized with the sun photometer for each month of the two years (1990–1991)

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

Number of measurements realized with sun photometer 67 96 48 71 100 84 12 51 65 202 33 22

three optical filters (0.50, 0.88 and 0.94 mm) was also used whenever photometer #573 was taken for calibration. The photometers have a 2° field of view, which in the worst case underestimates the 0.50 mm optical depths by 4% as indicated by Volz (1974). The expression used to describe the aerosol optical thickness t at wavelength l is obtained a from the instrument measurements by: t (k)=M−1[ln I −ln(I F )−(t +t )KM ] a e ok r o p (1) where I is the extraterrestrial solar radiation at e mean solar distance; I the observed direct ol beam solar radiation at ground level at wavelength l; F the correction factor for the mean sun–earth distance applied to the extraterrestrial radiation I ; t the Rayleigh optical thickness at e r sea level; p =1013 mb, t the ozone optical o o thickness; t the aerosol optical thickness above a station at M =1.0; M the calculated relative p ‘‘air mass’’ calculated using the equation of Kasten (1966); M =M·p/p the absolute optical p o air mass at station pressure p; and K a factor taking into account the different optical path lengths at M>4 of air, ozone, and low level haze ( Volz, 1974). I is determined from measol urements taken with the photometer under cloudless atmospheric conditions. The sun photometer #573 was recalibrated in October 1990 and March 1991 and the quantity I was determined at each wavelength by applyol ing the Langley method to sets of I . This was l,Z measured at various zenith angles, Z, of the sun throughout the morning hours that had clear sky and clean air conditions. In fact, the photometer was taken to Sokoto, a semi-arid town (13°N ), a location in Nigeria that has clearer sky conditions than Lagos (especially between

Atmospheric turbidity and the diffuse irradiance in Lagos, Nigeria

March and May and September to early November) and which has a visibility exceeding 25 km on many days. The second photometer was then recalibrated with photometer #573. 3. RESULTS AND DISCUSSION

3.1. Monthly and seasonal means The monthly means of the clearness index,

I/I , the diffuse index, I /I , and the beam o d o index, I /I , are shown in Fig. 1 with error b o bars showing the standard errors of the means (SEM ). The relationship between the indices is

I/I = I /I + I /I cos Z, where Z is the o d o b o zenith angle. The figure shows that the annual trend for the means of the indices, I/I , is o highest in the month of April with a value of 0.64, while it is lowest in July with a value of 0.43. Similarly, I /I  has highest and lowest b o values in the months of April and July, respectively. The SEMs for I/I  and I /I  are o b o highest in July due to the fewer number of data points obtained for this month for both years. July was very cloudy with rain falling most of the time. Table 2 shows simple descriptive statistics of the four variables of interest. Fig. 2 shows the means of the Angstrom turbidity coefficients at 0.5 mm for the period under consideration as measured at the site. The turbidity has been calculated based on the power law expression given by Angstrom (1961): t (l)=bl−a a

(2)

Table 2. Monthly means, standard deviations and standard error of means of four variables

Variable

Mean

Standard deviation

Standard error of mean

I/I 0 I /I d I /I b 0 b

0.582 0.585 0.370 0.299

0.209 0.264 0.250 0.189

0.007 0.009 0.008 0.007

where b and a are the Angstrom turbidity parameter and wavelength exponent, respectively. Angstrom (1964) had earlier determined an average value of 1.3 for a when the wavelength is given in microns. In the present work, a=1.3 has been used in Eq. (2) to determine the coefficient b. From Fig. 2, January is the most turbid month while November has the lowest mean value of b. The value of b in July was high because, as explained earlier, the number of data points for July was very few. The SEM shows that the dispersion of the coefficients for this month is higher than for the other months. Table 3 shows the variation of the means of some parameters of interest in the two seasons. The results are contrary to the assertion of Gueymard (1993) that ‘‘The equatorial region is generally characterized by stable humidity and turbidity conditions throughout the year’’. The humidity in Lagos is indeed stable with a mean of over 70% most of the year, but the turbidity ranges from the very low value of 0.005 in October to 1.222 in January on an

0.8 Components of of Total Total Soloar Solar Radiation Components Radiation

243

I/Io Id/Io Ib/Io

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

5

6

7

8

Monthly Month Fig. 1. Monthly means of clearness index, diffuse index and beam index.

9

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A. A. L. Maduekwe and M. A. C. Chendo

0.8

TURBIDITY COEFFICIENT b

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

6 5 Month

7

8

9

10

Fig. 2. Monthly means of the Angstrom turbidity coefficient, b, at 0.50 mm wavelength. Table 3. Seasonal means and standard deviations of some variables of interest Wet season

Dry season

Variable

Mean

Std

Mean

Std

I/I 0 I /I d b a Water vapor (cm) Relative humidity (%) Ambient temperature (°C )

0.60 0.56 0.270 1.457 4.199 0.80 30.2

0.205 0.266 0.182 1.437 1.579 0.047 2.244

0.54 0.64 0.361 2.253 3.704 0.77 32.3

0.213 0.254 0.187 1.192 0.979 0.058 2.112

hourly basis, though on a monthly average basis it ranges from 0.225 in November to 0.497 in January. Unlike the findings of Flowers et al. (1969) for the United States, where minimum and maximum values occur in December–January and June–July, respectively, the minimum values in Lagos occur in October– December, while the maximum values occur in December–March. Incidentally, this is the period when the Lagos regime is experiencing the harmattan haze associated with the dust laden north-east trade winds from the Saharan region. In general, the values obtained in Lagos are high. The turbidity of the atmosphere measured at any site depends partly on local weather conditions, which determine the input of aerosols from domestic and industrial sources, and partly on the synoptic history of the prevailing air masses which transport aerosol particles from more distant sources to the locality (Hansen, 1974).

According to Hansen (1974), the seasonal variations observed may be caused by changes in air mass type and not by any local source of aerosol. In the wet season in Lagos the tropical maritime air mass moves inland from the Atlantic Ocean carrying with it a lot of moisture. Aerosols can thus be formed by the aggregation of hygroscopic nuclei into large nuclei. In the dry season, the increased heating of the atmosphere may lead to the rupturing of molecular aggregates which then gives rise to a larger numbers aerosol particles with smaller radii (Hansen, 1974). 3.2. Turbidity values and wind directions Fig. 3 shows the distribution of the Angstrom turbidity coefficient b for all wind directions, as well as for three other directions, namely the western, the southern and the northern directions, in the order of their importance. The distribution shows a log-normal feature with the majority of measured values between 0.2 and 0.5. The histograms of the turbidity coefficients are plotted at intervals of 0.2. The means of the wind speed for each interval and for each direction are shown in the figure. The wind data were obtained from the meteorological station at Oshodi, which is about 8 km from the University of Lagos. It is assumed that, on average, the wind speed and direction should be similar for the two locations. The percentage of data available for each group of directions are:

Atmospheric turbidity and the diffuse irradiance in Lagos, Nigeria

ALL W W S

FREQUENCY

320

S N N

240

10 8

6

160

4

80

2

0

MEAN WIND SPEED (m/s)

400

245

0 0.1

0.3

0.5 0.7 0.9 INTERVALS OF b

1.1

1.3

Fig. 3. Histograms of the Angstrom turbidity coefficient, g, for all wind directions and for the western, southern and northern directions. The curves are the average wind speeds for the intervals and directions W WS—mean wind speed in the western direction.

From the percentage of data available it is certain that most of the aerosol bearing winds that affect the Lagos environment blow in from the ocean from the westerly direction. Lagos is virtually an ‘‘island’’ surrounded by the Atlantic Ocean. Consequently, the ocean represents a source of natural aerosols which can contain sodium chloride, salts, dissolved organic compounds, etc. In the western directions, comprising SWW, NWW, and W directions, the majority of the turbidity values are below 0.5, as shown in Fig. 3. The mean of b and the standard deviation in this direction are 0.283 and 0.176, respectively. The mean wind speeds are below 10 m s−1 in all cases. For the southern directions, which include SSE, SSW, and S, the mean wind speeds are generally below 10 m s−1, and most of the turbidity coefficients are below 0.6. The mean of b for this direction and the standard deviation are 0.266 and 0.197, respectively. In the northern directions, comprising NNE, NNW, and N directions shown in Fig. 3, the mean wind speeds are in all cases lower than 8 m s−1. The turbidity values are generally below 0.6. The mean and the standard deviation for the northern direction are 0.369 and 0.265,

respectively. In the eastern direction the mean of b and the standard deviation are 0.318 and 0.177, respectively. Thus, in general, the largest values of b are obtained from the northern and eastern directions. These directions correspond to the path through which the dust laden harmattan winds arrive in the months of December, January, February and to a lesser extent March. 3.3. Variation of the wavelength exponent a Figure 4 shows a plot of the hourly variation of the Angstrom wavelength exponent, a, for 0.50 mm using the 0.38 mm wavelength as a

a-coefficient

southerly directions, 21.60%, northerly directions, 10.86%, westerly directions, 64.92%, and easterly directions, 2.62%.

6 4 2 0 _2 _4 _6 0

100

200

300

400

500

600

0

100

200

300

400

500

600

1.2

b

(1) (2) (3) (4)

0.9 0.6 0.3 0.0

observations

Fig. 4. Variations of the Angstrom turbidity coefficients a and b for 605 observations made with Volz sun photometer #573.

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reference. The relationship used, which was adapted from Cachorro et al. (1987) and also used by Hansen (1974), is given as: al (l )=ln[t (l )/t (l )]/ln(l /l ) (3) R i a i a R R i where l is the reference wavelength and l is a R i second wavelength. In the present work the reference wavelength was taken as 0.50 mm, while the second wavelength was 0.38 mm. Only 605 data points were realised, as shown in the figure, since the photometer #573 used was taken for calibration in October 1990 and March 1991, while photometer #222, which did not have the 0.38 mm filter, was used temporarily for the measurement of b which was of primary interest in this work. The figure shows that, for the pair of filters, a varies between −4 and 4. It should be borne in mind that the exponent a of Angstrom’s equation decreases as the weight of large and giant particles of optical thickness t (l) increases. The different a values of a found can be interpreted as the variable contribution of the different modes to the size distribution of the particles. The values of a close to zero are associated with high loads of large and giant particles, whereas values between 1.5 and 2 indicate that the particle population is mostly composed of Aitken nuclei (Hansen, 1974; Prodi et al., 1984). The average value of a for the period of measurement was 1.705 and b=0.315 (for the 605 observations). This is an intermediate value of a which implies a contribution of particles in all size ranges and is typical of a sea-level station in an industrialized plain (Prodi et al., 1984). The negative values of a obtained are not strange as several authors (Hansen, 1974; Quentzl, 1970; Porch et al., 1971; Ramanathan and Karandikar, 1949; Røssler, 1971) have obtained such values. The reasons for the negative results may not be different from that given for the same phenomenon occurring in north and central India by Hansen (1974): ‘‘during the hot and dry months the atmosphere over India gets much polluted, when a large number of giant dust particles are introduced into it due to thermal instability and dust storms’’. Although Lagos is nearer the equator than India, the situation described above may well describe what happens in the mostly hot and humid climate of the city of Lagos. The interested reader is referred to the discussion of Hansen (1974) on the findings of Røssler (1971) at Hammaguir in Algeria and how he related ˚ s in Norway. this to his findings for A

Table 4 shows the monthly averages of a for Lagos as found from the measurements made within the period. The table shows that the months of January, February, November and December have high a values, indicating that the aerosol particles present are mostly composed of Aitken nuclei. Again, these months are easily identified with the presence of northeasterly trade winds, locally called ‘‘harmattan winds’’, which bear dust particles from the Sahara. The remaining months show somewhat average values of a which can partly be attributable to the presence of large sea salt particles and other industrial effluent. Lagos is a coastal city and consists of an island and a mainland. The month of August shows exceptional deviation from the discussions reported so far. In general, this month experiences less rainfall and is fairly dry. Locally, this period is called the ‘‘August break’’ due to the fact that the rains cease temporarily. The high value of a in August suggests that the influence of the south westerly winds blowing in from the Atlantic is minimal and particles of maritime origin are not much present. Seasonality is clearly suggested by the means of a as seen in Table 3. 3.4. Turbidity values and wind speed Figure 5 shows the analyses of the influence of wind speed on the turbidity values. Intervals of 1 m s−1 were chosen for the wind speed and the means of the turbidity coefficients were calculated and then plotted for these wind speed intervals. Fig. 5 shows high turbidity values for low and very high wind speeds similar to those found for a semi-rural area ( Katz et al., 1982a,b). The high turbidity levels for low wind speeds arise because conditions are favourable for high pollution levels, while for high wind Table 4. Monthly mean values of the wavelength exponent a, in Lagos, showing also the standard deviation and the minimum and maximum values observed for each month Month Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

a

Std

Minimum

Maximum

2.402 2.232 1.185 1.296 1.340 1.467 1.847 2.175 0.974 0.951 3.163 3.193

0.749 1.056 0.950 1.129 1.316 1.220 0.974 1.200 1.225 1.555 0.842 0.900

+0.76 −1.48 −2.10 −1.52 −3.79 −1.27 +0.39 −2.12 −3.22 −1.49 +1.42 +1.43

3.63 4.13 3.60 3.90 3.91 3.74 3.48 3.89 3.97 2.95 3.84 3.97

Atmospheric turbidity and the diffuse irradiance in Lagos, Nigeria

measurements impossible. It can be seen, however, that the turbidity values for August, September and October show marked increases for the year 1991. We do not have data for the remaining part of 1991. The large increases noticed may indeed point to the effect of the Mount Pinatubo haze which started in the second half of 1991. The similarity in the records for June and the subsequent increase may lend credence to this view.

1.0 Means of Angstrom turbidity coefficient b

247

0.8

0.6

3.6. Correlating b with the diffuse fraction

0.4

The relationship between the atmospheric turbidities measured at 0.5 mm wavelength and the diffuse fraction of the total hemispherical solar radiation has been investigated. 0.2 The relative influence of the turbidity on the diffuse fraction received can also be determined using the measured turbidities in a linear regres0.0 sion with the diffuse fraction. Iqbal (1980) and 1 3 5 7 9 11 13 15 17 Skartveit and Olseth (1987) have suggested that apart from the clearness index (K ) the next Wind speed intervals m/s t most important variable affecting the diffuse Fig. 5. Mean Angstrom turbidity coefficients for different radiation is the solar elevation. In the relationwind speed intervals. ships developed below the atmospheric turbidispeeds the turbidity levels are high because of ties have been included together with the the increase in the amount of suspended par- clearness index and the sine of the solar elevaticles. The turbidity levels for the intermediate tion to obtain models for predicting the diffuse irradiance at Lagos. levels of wind speed are generally low. Piecewise regression analysis was performed for the intervals 0≤K ≤0.30, 0.30
Jun. Jul. Aug. Sep. Oct.

1990

1991

Mean of b

Std

Mean of b

Std

0.264 0.359 0.285 0.240 0.194

0.263 0.263 0.219 0.211 0.110

0.261 No data 0.336 0.415 0.363

0.104 No data 0.169 0.180 0.162

Interval 0≤K ≤0.30, constraint I /I≤1.0: t d I /I=1.021−0.151K (4) d t interval 0.30
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A. A. L. Maduekwe and M. A. C. Chendo

Table 6. CRSS and RMSE as measures of the effect of the atmospheric turbidity on the diffuse solar radiation reaching Lagos Model I II III

CRSS

% difference

RMSE

% difference

2.40 2.22 2.19

7.5 8.8

0.0523 0.0503 0.0500

3.8 4.5

other words the atmospheric turbidity does affect the diffuse radiation reaching the surface of the earth at the location. Using the CRSS as the basis for comparison we see that the addition of the turbidity has increased the performance of Model II by 1.3% and Model I by 8.8%. 4. CONCLUSIONS

Model II: The expressions found when the sine of the solar elevation, sin h, is included in the Liu and Jordan type equation are: interval 0≤K ≤0.30, constraint I /I≤1.0: t d I /I=1.019−0.159K +0.0058 sin h (7) d t interval 0.30
The trends in the atmospheric turbidity in Lagos, Nigeria, have been studied with a Volz sun photometer using the Angstrom turbidity coefficient b at 0.50 mm wavelength. The variations of b for the city indicate that the atmosphere is mostly turbid throughout the year. The high turbidity is mostly of maritime and Saharan origin, with the highest values occurring during the harmattan haze in the months of December, January, February and March. When the diffuse fraction was regressed with the clearness index, K , the sine of the solar t elevation, and the turbidity coefficient, b, the standard error of the usual Liu and Jordan (1960) type correlation was reduced by 8.8%. Acknowledgements—The authors are grateful to the Nigerian Meteorological Services, Oshodi-Lagos, for providing them with some of their data. This paper was completed during the second author’s (MACC ) Associate Scheme Visit to the International Centre for Theoretical Physics (ICTP), Trieste, Italy. He expresses his gratitude to Professor Abdus Salam, President of ICTP, and the Swedish Agency for Research Cooperation with Developing Countries (SAREC ) for their financial support.

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