Spectral characteristics of the annual rainfall series for Northeast Brazil

The rainfall series for Fortaleza (Ceará) is compared with similar series for several other locations in Northeast Brazil. It is shown that the correlations are high for distances up to about 600 km from Fortaleza. The Fortaleza series shows prominent periodicities at T = 2.1, 10.1, 12.9, 25.1, and 61.0 years, all significant at a 3 a priori level. Amongst these, T = 12.9 and 25.1 years are significant at a 4 a priori level. A master Curve (for 1912–1978 only), obtained by averaging rainfall data for 93 stations having good correlations with Fortaleza, shows very prominent periodicities at T = 5.6, 12.3 and 47.3 years, significant at a 3 a priori level. T = 12.3 is significant at a 4 a priori level. Predictions from both the Fortaleza series (1849–1976) and the Master Curve (1912–1978) indicated droughts during 1979–1983. This prediction seems to have come true. In the future, droughts during 1992–1994 and 2002–2006 are indicated.     

Solubilities of pyruvic acid and the lower (C1-C6) carboxylic acids. Experimental determination of equilibrium vapour pressures above pure aqueous and salt solutions

Henry's law constantsK _H (mol kg^–1 atm^–1) have been determined at 298.15 K for the following organic acids: formic acid (5.53±0.27×10³); acetic acid (5.50±0.29×10³); propionic acid (5.71±0.34×10³);n-butyric acid (4.73±0.18×10³); isobutyric acid (1.13±0.12×10³); isovaleric acid (1.20±0.11×10³) and neovaleric acid (0.353±0.04×10³). They have also been determined fromT=278.15 K toT=308.15 K forn-valeric acid (ln(K _H)=–14.3371+6582.96/T);n-caproic acid (ln(K _H)=–13.9424+6303.73/T) and pyruvic acid (ln(K _H)=–4.41706+5087.92/T). The influence of 9 salts on the solubility of pyruvic acid at 298.15 K has been measured. Pyruvic acid is soluble enough to partition strongly into aqueous atmospheric aerosols. Other acids require around 1 g of liquid water m^–3 (typical of clouds) to partition significantly into the aqueous phase. The degree of partitioning is sensitive to temperature. Considering solubility and dissociation (to formate) alone, the ratio of formic acid to acetic acid in liquid water in the atmosphere (at equilibrium with the gas phase acids) is expected to increase with rising pH, but show little variation with temperature.     

Henry's law constant and hydrolysis of peroxyacetyl nitrate (PAN)

The effect of temperature on the solubility of PAN and on its hydrolysis rate in near-neutral and slightly acidic water were studied in a bubble column apparatus. The results obtained are a Henry's law coefficient H=10^–9.04±0.6 exp[(6513±376)/T] M atm^–1, and a first-order hydrolysis rate constant k=10^6.60±1.0 exp[(–6612±662)/T] s^-1, which was independent of pH in the range 3.2pH6.7. The products formed are nitrite and nitrate in approximately equal proportions under near-neutral conditions. At a pH<4, nitrite is oxidized in a secondary reaction, and nitrate becomes the only product at low pH. Previously measured deposition velocities of PAN on stagnant water surfaces are shown to be hydrolysis rate limited.     

Power spectrum analysis of the annual rainfall series for Massachusetts (NE,U.S.A.)

Maximum Entropy Spectral Analysis of the annual rainfall series for 1887–1976 (90 years) for Massachusetts (northeastern USA.) shows T = 17.8 (very near the 18.6 year luni-solar signal) as the most prominent periodicity. However, it explains only 12% variance. Also, the next prominent periodicity is T = 2.72 years, i.e. in the QBO (Quasi-Biennial Oscillation, T = 2–3 years) region. Also, regular periodicities account for only 50% variance, leaving 50% as a random component. Hence, predictions are unreliable. Roughly, excess rainfall during 1990–1994 and droughts during 1992–2002 are indicated; but occasional years of opposite behavior cannot be ruled out.     

Annual variation of atmospheric carbonyl sulfide in the marine atmosphere in the Southern Indian Ocean

Measurements of the concentrations of carbonyl sulfide (COS) in the marine atmosphere were made over a period of two years in the southern Indian Ocean (Amsterdam Island, 37°50 S–77°31 E; March 1987–February 1988 and April 1989–February 1990). The mean atmospheric COS concentration for the whole period was 475±48 pptv (n=544). Atmospheric COS concentrations show no significant seasonal variation with a summer to winter ratio of 1.05. Taking into account the observed variability of the atmospheric COS concentration (10%), a value of 1.4 yr is estimated as a lower limit for the atmospheric COS lifetime. A comparison of the COS data at Amsterdam Island with those obtained in the Southern Hemisphere in the past 12 yr does not reveal any significant trend in the tropospheric background COS mixing ratio.     

Power spectrum analysis of the time-series of annual mean surface air temperatures

Maximum Entropy Spectral Analysis of the annual mean surface temperature series for land masses and sea in the northern and southern hemispheres indicated long-term linear warming trends of (0.12 to 0.56) °C/century with superposed significant periods in the ranges T = 5–6 yr, 10–11 yr, 15 yr, 20 yr, 28–32 yr, and 55–80 yr. Extrapolation in future indicated for 2000–2030 a departure of (+0.4 °C) above the 1950–70 level. However, for the 1980s, the observed values are above the expected level, probably indicating large greenhouse effects due to human intervention. In that case, our predictions would be underestimates.     

A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH

The design and performance of a smog chamber for the study of photochemical reactions under simulated environmental conditions is described. The chamber is thermostated for aerosol experiments, and it comprises a gas chromatographic sample enrichment system suitable for monitoring hydrocarbons at the ppbv level. By irradiating NO _x /alkane-mixtures rate constants for the reaction of OH radicals with n-alkanes are determined from n-pentane to n-hexadecane to be (k±2)/10^–12 cm³ s^–1=4.29±0.16, 6.2±0.6, 7.52 (reference value), 8.8±0.3, 10.2±0.3, 11.7±0.4, 13.7±0.3, 15.1±0.5, 17.5±0.6, 19.3±0.7, 22.3±1.0, and 25.0±1.3, respectively at 312 K. Rate constants, (k±2)/10^–17 cm³ s^–1, for the reaction of ozone with trans-2-butene (21.2±1.0), cis-3-methylpentene-(2) (47.2±1.7), cyclopentene (62.4±3.5), cyclohexene (7.8±0.5), cycloheptene (28.3±1.5), -pinene (8.6±1.3), and -pinene (1.4±0.2) are determined in the dark at 297 K using cis-2-butene (13.0) as reference standard.     

Rate constants for the reactions of the NO3 radical with HCOOH/HCOO− and CH3COOH/CH3COO− in aqueous solution between 278 and 328 K

The kinetics of the aqueous phase reactions of NO₃ radicals with HCOOH/HCOO^– and CH₃COOH/CH₃COO^– have been investigated using a laser photolysis/long-path laser absorption technique. NO₃ was produced via excimer laser photolysis of peroxodisulfate anions (S₂O ₈ ^2– ) at 351 nm followed by the reactions of sulfate radicals (SO ₄ ^– ) with excess nitrate. The time-resolved detection of NO₃ was achieved by long-path laser absorption at 632.8 nm. For the reactions of NO₃ with formic acid (1) and formate (2) rate coefficients ofk ₁=(3.3±1.0)×10⁵ l mol^–1 s^–1 andk ₂=(5.0±0.4)×10⁷ l mol^–1 s^–1 were found atT=298 K andI=0.19 mol/l. The following Arrhenius expressions were derived:k ₁(T)=(3.4±0.3)×10¹⁰ exp[–(3400±600)/T] l mol^–1 s^–1 andk ₂(T)=(8.2±0.8)×10¹⁰ exp[–(2200±700)/T] l mol^–1 s^–1. The rate coefficients for the reactions of NO₃ with acetic acid (3) and acetate (4) atT=298 K andI=0.19 mol/l were determined as:k ₃=(1.3±0.3)×10⁴ l mol^–1 s^–1 andk ₄=(2.3±0.4)×10⁶ l mol^–1 s^–1. The temperature dependences for these reactions are described by:k ₃(T)=(4.9±0.5)×10⁹ exp[–(3800±700)/T] l mol^–1 s^–1 andk ₄(T)=(1.0±0.2)×10¹² exp[–(3800±1200)/T] l mol^–1 s^–1. The differences in reactivity of the anions HCOO^– and CH₃COO^– compared to their corresponding acids HCOOH and CH₃COOH are explained by the higher reactivity of NO₃ in charge transfer processes compared to H atom abstraction. From a comparison of NO₃ reactions with various droplets constituents it is concluded that the reaction of NO₃ with HCOO^– may present a dominant loss reaction of NO₃ in atmospheric droplets.     

Henry's law coefficients for aqueous solutions of acetone,acetaldehyde and acetonitrile,and equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite

Vapor phase concentrations of acetone, acetaldehyde and acetonitrile over their aqueous solutions were measured to determine Henry's law partition coefficients for these compounds in the temperature range 5–40 °C. The results are for acetone: ln(H ₁/atm)=–(5286±100)T+(18.4±0.3); acetaldehyde: ln(H ₁/atm)=–(5671±22)/T+(20.4±0.1); and acetonitrile: ln(H ₁/atm)=–(4106±101)/T+(13.8±0.3). Artificial seawater of 3.5% salinity in place of deiionized water raisesH ₁ by about 15%. A similar technique has been used to measure the equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite in aqueous solution. The results are ln(K ₁/M ^–1)=(4972±318)/T–(11.2±1.1) and ln(K ₁/M ^–1)=(6240±427)/T–(8.1±1.3), respectively. The results are compared and partly combined with other data in the literature to provide an average representation.     

Kinetics of the reactions of hydroxyl radicals with aliphatic ethers studied under simulated atmospheric conditions: Temperature dependences of the rate coefficients

Rate coefficients have been measured for the reactions of hydroxyl radicals with five aliphatic ethers over the temperature range 242–328 K. Competitive studies were carried out in an atmospheric flow reactor in which the hydroxyl radicals were generated by the photolysis of methyl nitrite in the presence of air containing nitric oxide. The reaction of OH with 2,3-dimethyl-butane was used as the reference reaction and the following Arrhenius parameters have been obtained for the reactions: OH+RORproducts:

Autoxidation of N(III), S(IV), and other Species in Frozen Solution – A Possible Pathway for Enhanced Chemical Transformation in Freezing Systems

The chemistry of glycolaldehyde (hydroxyacetaldehyde) relevant to the troposphere has been investigated using UV absorption spectrometry and FTIR absorption spectrometry in an environmental chamber. Quantitative UV absorption spectra have been obtained for the first time. The UV spectrum peaks at 277 nm with a maximum cross section of (5.5± 0.7)×10^–20 cm² molecule^–1. Studies of the ultraviolet photolysis of glycolaldehyde ( = 285 ± 25 nm) indicated that the overall quantum yield is > 0.5 in one bar of air, with the major products being CH₂OH and HCO radicals. Rate coefficients for the reactions of Cl atoms and OH radicals with glycolaldehyde have been determined to be (7.6± 1.5)×10^–11 and (1.1± 0.3)×10^–11 cm³ molecule^–1 s^–1, respectively, in good agreement with the only previous study. The lifetime of glycolaldehyde in the atmosphere is about 1.0 day for reaction with OH, and > 2.5 days for photolysis, although both wet and dry deposition should also be considered in future modeling studies.     

Carbon tetrachloride lifetimes and emissions determined from daily global measurements during 1978–1985

Seven years of daily gas chromatographic measurements of CCl₄ at the five globally distributed ALE/GAGE surface sites are reported. It is determined that CCl₄ has been accumulating in the atmosphere at a rate of 1.3±0.1%/yr over the period 1978–1985 and that the releases of CCl₄ into the atmosphere have remained fairly constant, with the smallest releases in 1981–1982. Using an inversion scheme based on a nine box model of the atmosphere, we infer a CCl₄ lifetime of approximately 40 yr, an inventory on 1 July 1978 of (2.08±0.07)×10⁹ kg and an average rate of release over the period 1978–1985 of (0.9±0.9)×10⁷ kg/yr. These results produce excellent agreement with a release scenario derived from global production estimates for CCl₄ and the major CCl₄ byproduct, the chlorofluorocarbons. However, to obtain this consistency, it is necessary that our current ALE/GAGE absolute calibration standard be reduced approximately 25% thus bringing it into agreement with measurements by Yokohata et al. (1985) and Hanst et al. (1975).     

The extent of the unstable Monin-Obukhov layer for temperature and humidity above complex hilly grassland

Potential temperature, specific humidity and wind profiles measured by radiosondes under unstable but windy conditions during FIFE in northeastern Kansas were analyzed within the framework of Monin-Obukhov similarity. Around 86% of these profiles were found to have a height range over which the similarity, formulated in terms of the Businger-Dyer functions, is valid and for which the resulting surface fluxes are in good agreement with independent measurements at ground stations. When scaled with the surface roughness z ₀ = 1.05 m and the displacement height d ₀ = 26.9 m, for the potential temperature this height range was 45 (±31) (z – d ₀ )/z ₀ 104 (±54) and the comparison of the profile-derived surface fluxes with the independent measurements gave a correlation coefficient of r = 0.96. For the specific humidity these values are 42 (±29) (z – d ₀ )/z ₀ 96 (±38) and r = 0.94. In terms of the height of the bottom of the inversion H _i , in the morning hours the upper limit of (z – d ₀ ) in the Monin-Obukhov layer is approximately 0.3H _i , whereas for a fully developed ABL it is closer to 0.1H _i . Probably, as a result of the short sampling times and perhaps also of the small gradients under the windy conditions, the exact height range of validity was difficult to establish from a mere inspection of these profiles.     

The CO2 climate response problem a statistical approach

Summary The anthropogenic increase of the atmospheric carbon dioxide (CO₂) concentration leads to a global warming of the atmospheric surface layer, whereas the stratosphere is cooled. This greenhouse effect postulated by a number of climate models (on a physical basis) can be conditionally verified by statistical multiple regression techniques. In this study the following climatic time series are used (all data yearly averages): northern hemisphere mean temperatures near surface 1781–1980 (alternatively since 1851 or 1881) and corresponding stratospheric data 1958–1983, sea surface temperatures 1856–1980, northern hemisphere or global average, alternatively, and the global mean sea level fluctuations 1881–1980. In order to account for an appropriate part of explained variance, volcanic and solar forcing parameters are implied and the data are low-pass filtered suppressing variations of the period rangeT < 10 years. Based on the recently assessed preindustrial CO₂ concentration of c. 280 ppm and the Mauna Loa value of c. 344 ppm in 1984 this industrial CO₂ increase reveals a northern hemisphere temperature increase near surface of c. (.7±.1) K (average and standard deviation of all statistical regression runs), statistically significant at the 95% level. A CO₂ doubling (300 to 600 ppm) leads to a statistically derived signal of (3.1±.6) K, satisfactorily congruent with the results of most of the (deterministic) climate models: c. (3±1.5)K. A stratospheric cooling trend in recent time may be existent but is highly non-significant. Similarly, the SST data do not allow to evaluate a significant CO₂ signal to noise ratio. In contrast to that the observed long-term global mean sea level increase (9.3 cm) can be predominantly attributed to the CO₂ effect (99.9% level).

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Zusammenfassung Der anthropogen bedingte Anstieg der atmosphärischen Kohlendioxid-(CO₂-)Konzentration führt zu einer Erwärmung der bodennahen Luftschicht, während die Stratosphäre abgekühlt wird. Dieser Glashauseffekt, von zahlreichen Klimamodellierungen (auf physikalischer Basis) postuliert, kann auf statistischem Weg durch multiple Regressionsrechnungen bedingt verifiziert werden. Die vorliegende Studie basiert auf den folgenden Klima-Zeitreihen (alle Daten in Form von Jahresmittelwerten): nordhemisphärische Mitteltemperatur in Bodennähe 1781–1980 (alternativ seit 1851 bzw. 1881) und in der Stratosphäre 1958–1983, Meeresoberflächentemperatur 1956–1980, nordhemisphärisch bzw. global gemittelt, und mittlere globale Meeresspiegelschwankungen 1881–1980. Um einen angemessenen Teil erklärter Varianz zu erfassen, wurden vulkanische und solare Parameter mit einbezogen und eine Tiefpaßfilterung mit Unterdrückung des PeriodenbereichsT < 10 Jahre zugrunde gelegt. Auf der Basis des kürzlich abgeschätzten vorindustriellen CO₂-Konzentrationswertes von ca. 280 ppm und dem Mauna Loa Wert des Jahres 1984 von ca. 344 ppm entspricht dieser industrielle CO₂-Anstieg einer Erhöhung der bodennahen nordhemisphärischen Mitteltemperatur von ca. (0.7±0.1) K (Mittelwert und Standardabweichung aller Regressionsrechnungen), was einen auf dem 95%-Niveau signifikanten Temperaturanstieg darstellt. Eine CO₂-Verdoppelung (300 auf 600 ppm) führt, ebenfalls auf statistischem Weg, zu einem Temperatursignal von (3.1±0.6) K, in befriedigender Übereinstimmung mit den meisten der (deterministischen) Klimamodelle: ca. (3±1.5) K. In der Stratosphäre könnte in letzter Zeit ein Abkühlungstrend aufgetreten sein, der aber höchst insignifikant ist. Auch die SST-Daten erlauben keine signifikanten Schätzungen des Signal-Rausch-Verhältnisses. Im Gegensatz dazu kann der langfristige Trend des globalen Meeresspiegelanstiegs (9.3 cm) weitgehend dem CO₂-Effekt zugeschrieben werden (99.9%-Signifikanzniveau).

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With 7 Figures     

Determination of rate constants for reactions of some hydrohaloalkanes with OH radicals and their atmospheric lifetimes

Rate constants have been measured for the gas-phase reactions of hydroxyl radical with partly halogenated alkanes using the discharge-flow-EPR technique over the temperature range 298–460 K. The following Arrhenius expressions have been derived (units 10^–13 cm³ molecule^–1 s^–1): (8.1 _–1.2 ^+1.5 ) exp{–(1516±53)/T} for CHF₂Cl (HCFC-22); (10.3 _–1.5 ^+1.8 ) exp{–(1588±52)/T} for CH₂FCF₃ (HFC-134a); (11.3 _–1.6 ^+2.1 ) exp{–(918±52)/T} for CHCl₂CF₂Cl (HCFC-122); (9.2 _–2.0 ^+2.5 ) exp{–(1281±85)/T} for CHFClCF₂Cl (HCFC-123a).The atmospheric lifetimes for the substances have been estimated to be 12.6, 12.9, 1.05, and 4.8 years, respectively, and the accuracy of the estimates is discussed.     

Solubility of HOCl in water and aqueous H2SO4 to stratospheric temperatures

Henry's law constants K_H (mol kg^–1 atm^–1) for the reaction HOCl_(g)=HOCl_(aq) near room temperature, literature data for the associated enthalpy change, and solubilities of HOCl in aqueous H₂SO₄ (46 to 60 wt%) at temperatures relevant to the stratosphere (200 KT230 K) are shown to be thermodynamically consistent. Effective Henry's law constants [H*=mHOCl/pHOCl, in mol kg^–1 atm^–1] of HOCl in aqueous H₂SO₄ are given by: ln(H*)=6.4946–mH₂SO₄(–0.04107+54.56/T)–5862 (1/T_o–1/T) where T(K) is temperature and T_o=298.15K. The activity coefficient of HOCl in aqueous H₂SO₄ has a simple Setchenow-type dependence upon H₂SO₄ molality.     

An Examination of the Atmospheric Chemistry of Mercury Using 210Pb and 7Be

Measurements of Hg (total gas-phase, precipitation-phase andparticulate-phase), aerosol mass, particulate ²¹⁰Pb and⁷Be and precipitation ²¹⁰Pb were made at an atmosphericcollection station located in a near remote area of northcentral Wisconsin,U.S.A. (46°10N, 89°50W) during the summers of 1993, 1994and 1995. Total Hg and ²¹⁰Pb were observed to correlate strongly(slope = 0.06 ± 0.03 ng mBq^-1; r ² =0.72) in rainwater. Mercury to ²¹⁰Pb ratios in particulate matter(0.03 ± 0.02 ng mBq^-1; r ² = 0.06) wereconsistent with the ratio in rain. Enrichment of the Hg/mass ratio (approx.5–50×) relative to soil and primary pollutant aerosols indicatedthat gas-to-particle conversion had taken place during transport. Comparisonof these results with models for the incorporation of Hg into precipitationindicates that atmospheric particles deliver more Hg to precipitation than canbe explained by the presence of soot. A lack of correlation between totalgas-phase Hg (TGM) and a ⁷Be/²¹⁰Pb function suggests novertical concentration gradient within the troposphere, and allows an estimateof TGM residence time of 1.5 ± 0.6 yr be made based on surface airsamples.     

Characterization of Organic Acids in Dew Collected on Surrogate Surfaces

Levels of formate and acetate in dew were measured at Dayalbagh, India, usingsurrogate surfaces. The dew formed per night ranged between 0.06 lm^–2 and 1.38 l m^–2, with an average of 0.59l m^–2. pH ranged between 6.7 and 7.4. Mean concentrations offormate and acetate in dew were 10.2 ± 10.2 eql^–1 and 7.5 ± 4.5 eq l^–1,respectively. The correlation coefficient between the two ions was 0.80 (p =0.001), which suggested that concentrations of these species in dew are linkedtogether. They have either common or different sources with fairly constantstrengths or products of same reaction. Good correlation of formate andacetate with Ca (r = 0.82 and r = 0.70, respectively) and Mg (r = 0.74 and r= 0.71, respectively) suggested that these ions may be associated with Ca andMg after the neutralization process. Deposition rates for formate and acetatein dew per night were 10.2 ± 7.22 mol m^–2 pernight and 4.6 ± 2.2 mol m^–2 per night,respectively. The theoretical Henry's law constant (K^* _H)and the field-observed Henry's law coefficient (K^* _H) ascalculated from concurrent measurements of gas phase and dew for both acidsshowed large discrepancies of three orders of magnitude.     

Chemical Assessment of Oceanic and Terrestrial Sulfur in the Marine Boundary Layer over the Northern North Pacific during Summer

Dimethylsulfide (DMS) in surface seawater and the air, methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO₄ ²–) in aerosol, and radon-222 (Rn-222) were measured in the northern North Pacific, including the Bering Sea, during summer (13 July – 6 September 1997). The mean atmospheric DMS concentrations in the eastern region (21.0 ± 5.8 nmole/m³ (mean ± S.D.), n=30) and Bering Sea (19.9 ± 9.8 nmole/m³, n=10) were higher than that in the western region (11.1 ± 6.4 nmole/m³, n=31) (p<0.05), although these regions did not significantly differ in the mean DMS concentration in surface seawater. Mean sea-to-air DMS flux in the eastern region (21.0 ± 10.4 mole/m²/day, n=19) was larger than those in the western region (11.3 ± 16.9 mole /m²/day, n=22) and Bering Sea (11.2 ± 7.8 mole/m²/day, n=7) (p<0.05). This suggests that the longitudinal difference in atmospheric DMS was produced by that in DMS flux owing to wind speed, while the possible causes of the higher DMS concentrations in the Bering Sea include (1) later DMS oxidation rates, (2) lower heights of the marine boundary layer, and (3) more inactive convection. The mean MSA concentrations in the eastern region (1.18 ± 0.84 nmole/m³, n=35) and Bering Sea (1.17 ± 0.87 nmole/m³, n=13) were higher than that in the western region (0.49 ± 0.25 nmole/m³, n=28) (p < 0.05). Thus the distribution of MSA was similar to that of DMS, while the nss-SO₄ ²– concentrations were higher near the continent. This suggests that nss-SO₄ ²– concentrations were regionally influenced by anthropogenic sulfur input, because the distribution of nss-SO₄ ²– was similar to that of Rn-222 used as a tracer of continental air masses.     

On the attenuation of direct solar irradiance in Athens due to aerosols

Summary In this study the trend of the time sequence of the integral aerosol optical depth (k _a), as proposed by Unsworth and Monteith, was determined for clear days in summer for the period 1962–1988 in Athens. The trend was found by fitting a third degree polynomial curve and it was concluded that (k _a) showed a considerable increase (i.e. from the value of 0.18 to 0.31) in the period 1962–1976 and remained approximately constant until 1979, after which it started decreasing again slowly until 1988. The increase of (k _a) in the period 1962–1976 is likely attributable to the rapid development of the city in this period, while the decrease of (k _a) after 1979 probably reflects the efficiency of some restrictions which were imposed on the pollutant emissions during this period. In addition, an analysis of the percentage frequency distribution found that while 95% of the values of (k _a) ranged from 0.100 to 0.400 in the beginning of the period (1964–1967), in recent years (1984–1987) the same percentage of the values of (k _a) ranged from 0.100 to 0.500.With 3 Figures