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Effect of relative humidity and sea level pressure on electrical

conductivity of air over Indian Ocean

S. D. Pawar,1 P. Murugavel,1 and D. M. Lal1

Received 17 December 2007; revised 10 October 2008; accepted 30 October 2008; published 24 January 2009.

[1] The electrical conductivity measured over the Indian Ocean (15N, 77E to20S, 58E) during the Indian Ocean Experiment (INDOEX-1999) from 20 January to12 March 1999 has been analyzed. The conductivity values over two oceanic regions, onewith very low aerosol concentration and another with very high aerosol concentration,are studied in relation with meteorological parameters such as relative humidity and sealevel pressure. The average conductivity is as low as 0.295 10

14 Sm1 in the regionof high aerosol concentration and it is 0.783 1014 Sm1 in the region of very lowaerosol concentration. In both the regions, conductivity shows an inverse relationwith relative humidity and this effect is more in the presence of high aerosol concentration.The hydrate growth of aerosol particles in high-humidity condition may be responsible forthe inverse relation between conductivity and relative humidity. Size distributions ofaerosol particles measured in the same cruise during high-humid conditions are alsoanalyzed to show that sizes, rather than numbers, of aerosol particles increase with anincrease in humidity. The relationship between conductivity and sea level pressure in thesetwo regions is also studied and it shows good correlation in the region where the

background aerosol concentration is low and no correlation in the region where aerosolconcentration is high. The inverse relation between sea level pressure and electricalconductivity is attributed to the possible transportation of ultrafine particles from freetroposphere, with subsiding motions associated with high pressure. The positivecorrelation between ultrafine particles and sea level pressure supports this idea.

Citation: Pawar, S. D., P. Murugavel, and D. M. Lal (2009), Effect of relative humidity and sea level pressure on electrical

conductivity of air over Indian Ocean, J. Geophys. Res., 114, D02205, doi:10.1029/2007JD009716.

1. Introduction

[2] The main source of ionization over remote oceans iscosmic rays and the intensity of cosmic rays is almostconstant in the lower latitudes. Therefore the variations inconductivity of air over ocean are always linked with thevariations in background aerosol concentration because theaerosols act as sinks for the small ions and reducethe electrical conductivity. The electrical conductivity has

been used as an indicator for secular changes in backgroundair pollution over ocean [Misaki and Takeuti, 1970; Misakiet al., 1972;Morita et al., 1973; Morita and Ishikawa, 1977;

Kamra and Deshpande, 1995; Kamra et al., 2001]. Thetheoretical calculation of Hogan et al. [1973], however,

show that the conductivity variations over ocean can berelated either to the change in concentration or to the changein size of the aerosol particles. The conductivity shows goodcorrelation with the total volume or surface area occupied

by aerosol particles rather than the total number concentration[Cobb, 1973; Cobb and Pueschel, 1985]. Adlerman andWilliams [1996] studied the relationship between aerosol

particle concentration and electrical conductivity at differentplaces over land and found that such relation is highlynonlinear over land surface. The observations ofKamra etal.[1997] over ocean show that the relation between aerosoland conductivity is also influenced by certain meteorolo-gical parameters like relative humidity. Pawar et al. [2005]measured the ion concentration of three categories, i.e.,small, intermediate and large, and electrical conductivityover Arabian Sea during SouthEast monsoon season andfound that the highly charged large ions generated by

bubble bursting can enhance the conductivity during highwind conditions.

[3] Surface measurements of vertical electric field (E)

made over oceans have been used to study the globalelectric circuit (GEC) since very long time [Parkinsonand Torreson, 1931; Paramonov, 1950]. The variations inair conductivity near the ocean surface, due to variousmeteorological conditions, could directly affect such mea-surements of E and lead to errors in the estimation of GEC

parameters. Information on electrical conductivity and itsrelation with the meteorological parameters in the remoteoceanic environment is useful in various studies such asGEC, secular change in background air pollution and so on.To study the variations of conductivity over ocean and theirrelation with prevailing meteorological conditions, we haveanalyzed the measurements of conductivity made onboard

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D02205, doi:10.1029/2007JD009716, 2009

1I & OT Division, Indian Institute of Tropical Meteorology, Pune,

Maharashtra, India.

Copyright 2009 by the American Geophysical Union.0148-0227/09/2007JD009716

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Oceanographic Research Vessel (ORV) Sagarkanya over theIndian Ocean during 20 January to 12 March 1999 and theresults are presented here.

2. Instrumentation

[4] The measurements of both polarities of conductivityare made with a Gerdien apparatus having two identicalcondensers connected to a common suction fan through aU-tube. The details of the apparatus are given by Dhanorkarand Kamra[1992]. The critical mobility of the apparatus isadjusted at 3.6 10

4m

2V

1s

1. The signals from both

the condensers after amplification with IC AD549 are fed toa data logger through a coaxial cable. The apparatus wasinstalled on Balloon Deck of the ship, which is at a height ofabout 9 m from sea level, vertically with its air inlets facingdownward so as to avoid the effect of the winds and rain

drops falling directly into the condensers. The sensor rod iscleaned periodically with alcohol to avoid any contamina-tion. The data was recorded at 12 samples per minute andthen averaged for 3 hours, 6 hours, 12 hours and 24 hours.We have used total conductivity in this study which iscalculated by adding both the polar conductivities (ltotal =l+ + l). Number and size distribution of aerosol particlesof 3 1000-nm diameter were measured in ten different sizeranges with an Electrical Aerosol Analyzer (EAA) systemof TSI, USA. More details of the instruments are given in

Kamra et al. [2003]. Observations are made using thisinstrument aboard ORV Sagarkanya from Goa (India)to Port Louise (Mauritius), i.e., from 20 January 1999 to

11 February 1999 and aboard ORV Ron Brown from PortLouise to Male, i.e., from 22 February 1999 to 1 march1999. Observations of meteorological parameters such asrelative humidity and sea level pressure were made onboardORV Sagarkanya after every 3 hour by India Meteorolo-gical Department (IMD).

3. Observations

[5] Figure 1 shows the cruise track during INDOEX-1999.The cruise of ORV Sagarkanya started on 20 January 1999from Goa, India, and reached Port Louis, Mauritius on11 February 1999. It departed Port Louis on 16 Februaryand arrived Goa on 12 March 1999, on return. The ORVRon Brown cruised from Port Louis to Male between22 February and 1 March 1999. This period of the cruisefalls in the Asian winter monsoon season during whichnortheasterly wind prevail over northern Indian Ocean andthese winds transport aerosols and trace gases from Asiancontinent to the northern Indian Ocean. The transport ofaerosols from Asian continent and its effect on electricalconductivity of air has been already discussed in detail by

Kamra et al. [2001]. Here we report the effect of meteoro-logical conditions on electrical conductivity of the remotemarine air with different background aerosol concentrations.We have chosen two regions, as shown in Figure 1, withdifferent aerosol concentrations and the prevailing windsshow that the immediate effect of continental air mass tothese regions is minimum. The position of IntertropicalConvergence Zone (ITCZ) during onward and return cruisesis also shown in Figure 1. The position of ITCZ is between27S during this period [Madan et al., 1999] and RegionA and B are falling respectively south and north of theITCZ. The INDOEX measurements revealed that theArabian sea and northern Indian ocean were highly pollutedas compared to southern Indian ocean during that period

[Ramachandran and Jayaraman, 2002; Bates et al., 2002;Hudson and Yum, 2002]. Kamra et al. [2003] reportedabout 4000 to 6000 particles cm

3 along the cruise trackclose to the Indian subcontinent (Figure 1) and only about500 particles cm

3in the south of the ITCZ which we have

chosen as Region A. As reported by Krishnamoorthy andSaha [2000] from their optical depth measurements duringthe same cruise, the aerosol concentration in the Region Bwas comparable or even sometimes higher than the concen-tration observed near the Indian coast. Quinn et al. [2002]also reports the accumulation mode aerosol surface area ofabout 12mm2 cm3, 44mm2 cm3 and 120mm2 cm3 in thesouthern hemisphere Indian ocean, northern hemisphereIndian ocean and Arabia Indian subcontinent regions

respectively. Therefore there was about an order of moreaerosol particles in the Region B than in the Region A duringthe period of observation. Measurements are made overRegion A from 1 to 10 February 1999 and over Region Bfrom 1 to 11 March 1999. Figure 2 shows the six hourlyaveraged relative humidity, air pressure and conductivityduring the period when the ship was in Region A. As seenin Figure 2 the variations in conductivity are opposite to thevariations in pressure almost all the days. However thevariations in conductivity are opposite to the variations inrelative humidity only for first four days (14 February)and again two days at the end (9 and 10 February). The

Figure 1. The cruise track of ORV Sagarkanya (solid line)and ORV Ron Brown (dashed line) during INDOEX-1999along with the regions of interest and the position of theITCZ.

D02205 PAWAR ET AL.: ELECTRICAL CONDUCTIVITY OVER OCEAN

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inverse relation between conductivity and relative humidityin Region A is not observed for few days (i.e., 5 8 February)mainly because the variations in relative humidity for those

days are very small compared to the variations in sea levelpressure. Figure 3 shows the variation of six hourly aver-aged relative humidity, pressure and conductivity during the

period when the ship was in Region B. The inverse relationbetween pressure and conductivity, as observed in theRegion A, is not observed in this region. However theinverse relation between conductivity and relative humidityis more consistent in this region compared to region A.

[6] Figure 4 shows the scattered diagram of six hourlyaveraged conductivity versus relative humidity and pressurein the Region A; the lines of best fit are also plotted. Asshown here the correlation coefficient (r) between conduc-tivity and pressure is higher than the correlation coefficient

between conductivity and relative humidity. The inconsis-

tency of inverse relation between relative humidity andconductivity in region A is also reflected in scatter diagramand therefore the scatter is more in Figure 4b. Figure 5shows the scattered diagram of six hourly averaged con-ductivity versus pressure and relative humidity with best-fitlines in the Region B. In this region, the correlationcoefficient of conductivity and relative humidity is signi-ficantly higher compared to region A. However the corre-lation coefficient between pressure and conductivity is verysmall not only compared to region A but also compared tocorrelation between conductivity and relative humidity inthe same region.As shown, the scatter in Figure 5b is small

because the effect of variations of pressure on conductivity(Figure 5a) is almost negligible in this region.

4. Results

4.1. Effect of Relative Humidity on ElectricalConductivity

[7] As shown byPruppacher and Klett[1978] the radius

of aerosol particles can increase sharply when relativehumidity exceeds 7075% and the attachment coefficientb between small ion and neutral aerosol particle is afunction of radius of aerosol particles [Hoppel, 1985].Therefore the increase in humidity more than 70 75%can increase the size of aerosol particles and remove morenumber of small ions even though the total number ofaerosol particles remains same. The measurements madeover equatorial Indian Ocean and Arabian Sea, as reported

by Kamra et al. [1997], show an inverse relation betweenconductivity and relative humidity and these results wereexplained on the basis of sharp increase in the sizes of ionsand marine aerosols when the relative humidity exceeds75 80%. Experimental study by Moore and Vonnegut

[1988] also shows that bipolar conductivity decreases withincreasing relative humidity. Our observations also showinverse relation between conductivity and relative humidityin both the regions, however, the correlation of conductivitywith relative humidity in the Region B is more than in theRegion A (Figures 4b and 5b). As described in section 3,there is a large difference in aerosol concentration in thesetwo regions. This is also supported by the conductivity

Figure 2. Six hourly averaged relative humidity, sea levelpressure, and conductivity during the period when the shipwas in region A.

Figure 3. Six hourly averaged relative humidity, airpressure, and conductivity during the period when the shipwas in region B.


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values, as the average value of conductivity in Region A isabout 0.783 1014 Sm

1, whereas in Region B it is only

about 0.295 1014 Sm1. The difference in correlationcoefficients in these two regions indicates that the presenceof high aerosol concentration enhances the effect of relativehumidity on conductivity. It also suggests that the maincause of inverse relation between conductivity and relativehumidity may be the increase of sizes of aerosol particleswhen the relative humidity increases more than 7075%.

[8] The aerosol size distributions measured in the samecruise (INDOEX-1999) are analyzed to study the effect ofrelative humidity on growth of aerosols during high-humidityconditions. Two of such cases on 24 and 26 February 1999,during which the humidity increases to about 70% andremains high for few hours, are presented in Figure 6.

Figures 6a and 6b show three hourly averaged size distri-butions of number and surface concentration respectivelyfor a period of about 12 hours starting 1500 hours UTCduring which the humidity increased from 55% to 80% andremained high. It is clearly seen from Figure 6 that thesmaller particles decrease and larger particles increase innumber as the humidity continues to be high. It can also beseen that the surface area of larger particles systematicallyincreases during this period (Figure 6b). Similar featurescan also be seen in Figures 6c and 6d on 26 February 1999during which humidity increased from 65% to 75%. Wehave also analyzed the aerosol size-distribution data pro-vided by NCAR/EOL under sponsorship of the National

Science Foundation, http://data.eol.ucar.edu/ (data set name:Ron Brown Aerosol Number Size Distributions - 55% RH[Bates], URL: http://data.eol.ucar.edu/codiac/dss/id=22.073,data set name: Ron Brown Aerosol Number Size Distribu-tions - dry [Bates], URL: http://data.eol.ucar.edu/codiac/dss/id=22.072), measured during the same cruise. In this dataset, the particles measured with controlled relative humidityof 10% are referred as dry aerosols and with relative

humidity of 55% are referred as wet aerosols. Moredetails of the measurements and description of data setscan be found in the URL. As shown in Figure 7, totalnumber concentration of wet aerosols smaller than 50-nmdiameter is less than dry aerosols of same size range andhowever, the situation is reversed in the size range greaterthan 50 nm. Figure 7 clearly suggests that aerosol particlesgrow to bigger sizes when the humidity increases. Figures 6and 7 clearly indicate that during high-humidity conditions,considerable increase in the total surface area occupied byaerosol particles can occur. This supports our idea thatrelative humidity affects the conductivity mainly by increas-ing the sizes of aerosol particles.

[9] Laboratory and theoretical studies by Tyndall and

Grindley [1926], Harrrison [1992], Sakata and Okada[1994] and Harisson and Aplin [2007] show that themobility of small ions decreases with increase in relativehumidity. Such decrease in mobility of ions can also affectthe conductivity. The correlation coefficient of conductivityand relative humidity, which is 0.37 in the Region A,suggests that the reduction of mobility of ions by hydrategrowth of ions in moist air as hypothesized byHarrison andFigure 4. Six hourly averaged conductivity versus (a) sea

level pressure and (b) relative humidity along with best-fitlines and correlation coefficient (r) in region A.

Figure 5. Six hourly averaged conductivity versus (a) sealevel pressure and (b) relative humidity along with best-fitlines and correlation coefficient (r) in region B.


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Aplin [2007], might be contributing to the reduction in theconductivity.

[10] Our observations along with earlier observations andexperiments by Hogan et al. [1973], Kamra et al. [1997],Tyndall and Grindley [1926],Harrrison[1992],Sakata andOkada [1994] and Harisson and Aplin [2007] clearlyindicate that relative humidity affects the conductivitymainly by increasing the sizes of aerosols and ions. There-fore the variations in relative humidity persisting for longertime scale is expected to affect the conductivity more thanshort time variations of relative humidity. We have plottedscattered diagram of conductivity versus relative humidityin region B with different time averages in Figure 8. Asshown here the correlation coefficient between these two

parameters increases with increase in averaging period(same was observed in region A, however not plotted here).

4.2. Relation Between Conductivity and Air Pressure

[11] The pressure and electrical conductivity of air areinversely related because the mobility of small ion decreaseswith increasing pressure. As shown in Figure 2, the varia-tions in conductivity inversely follow the pressure varia-

tions in Region A, but it does not follow so in Region B.The correlation coefficients between these two parametersare 0.777 and 0.16 in the regions A and B respectively.The inverse relation between air pressure and conductivityin region A can be explained on the basis of ultrafine

particles transported from free troposphere in to the marineboundary layer as reported by Covert et al. [1996] andKamra et al. [2003]. Raes [1995] also have shown fromtheir model calculations that entrainment from free tropo-sphere is a source of the particles in nucleation mode in themarine boundary layer. Such transport of aerosol particleswith subsidence from free troposphere to the marine bound-ary layer is controlled by the sea level pressure. To confirmthis, we have plotted 3-hourly averaged aerosol numberconcentration of 13-nm diameter with sea level pressure(Figure 9). As shown in Figure 9, the aerosol concentrationshow good positive correlation (r= 0.54) with the pressure.Three hourly averaged aerosol data measured onboardSagarkanya from 31 January to 3 February 1999 andonboard Ron Brown from 24 to 28 February are used inFigure 9. Sometimes when the ship was stationary or thewind speed relative to ship was low or wind direction was

Figure 6. Size distribution of aerosol number and surface concentration on 24 and 26 February 1999showing the evolution of sizes of aerosol during high-humidity conditions.


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such that exhaust passed over instruments, the data collectedduring such periods are not considered for analysis. Also to

avoid coastal effects, the data measured close to the coast isnot included in Figure 9. Figure 9 supports the idea that thetransport of ultrafine particles from free troposphere iscontrolled by sea level pressure. Therefore we propose thatthe inverse relation between conductivity and sea level

pressure may be due to the increased aerosol particlestransported by subsidence motions associated with highsea level pressure. In region B, which is highly pollutedcompared to region A, the possibility of new particleformation by gas to particle conversion in presence of highaerosol concentration is less [Covert et al., 1992; Gras,1993;Raes, 1995]. Moreover the effect of ultrafine particlestransported from free troposphere on conductivity will besmall in presence of large background aerosol concentra-

tion. The correlation coefficient between pressure andelectrical conductivity, being very small in Region Bcompared to Region A, supports this idea.

5. Discussion

[12] Adlerman and Williams [1996] have shown that therelationship between air conductivity and background aero-sol concentration is highly nonlinear over land surface. Thereason is that the strength of the sources and sinks of theatmospheric small ions vary from place to place over landsurface. The main source of ionization over land surface isthe radioactive gases released from the earths crust and this

is highly variable over land. However over oceans, there isno radioactive gases from the surface and hence the galacticcosmic rays, which have been found almost constant in thelower latitudes, are the only ionizing agent to influence theconductivity. The aerosol particles act as sink for small ionsin the atmosphere over land as well as over oceans.Therefore the relationship between conductivity and theaerosol concentration can be nonlinear over land surfaces;

however it can be linear over ocean surface. The attachmentcoefficient b is the function of size of aerosol particles[Hoppel, 1985] and as suggested byHogan et al.[1973] and

by Kamra et al. [1997], for the same aerosol numberconcentration the conductivity can vary with variations inthe sizes of aerosol particles.

[13] Our observations support the results ofHogan et al.[1973] andKamra et al.[1997] which show how the changein sizes of aerosol particles with increasing humidity canaffect the conductivity. The observations in both the regionsclearly show inverse relation between the conductivity andrelative humidity over ocean surface (Figures 4b and 5b).The difference in the correlation coefficient between thesetwo parameters in the regions A and B shows how the

increased aerosol concentration can amplify this effect. Wemight not rule out the possibility of hydrate growth of smallions in the high-humidity conditions and subsequent reduc-tion in the conductivity. It is possible that both the process-es, i.e., hydrate growth of small ions and hydrate growth ofaerosol particles, may be combinely contributing for thereduction in conductivity with increasing relative humidity.

[14] As far as the authors knowledge the inverse relationof conductivity with pressure over ocean has been reportedhere for the first time. Subsiding motion is found to beassociated with increased sea level pressure and this cantransport large number of ultrafine aerosol particles fromfree troposphere in marine boundary layer [Covert et al.,1996;Kamra et al., 2003]. In presence of large backgroundaerosol concentration, as found in Region B, the transport ofaerosols associated with subsiding motions may not havesignificant effect on conductivity. The difference in corre-lation coefficient between pressure and conductivity inRegion A and B supports this hypothesis. As shown inFigures 4a and 5a the correlation between sea level pressureand conductivity is almost negligible in Region B whereas itis 0.777 in Region A. Therefore we propose that in the

presence of low background aerosol concentration thedownward transport of aerosols associated with high sealevel pressure amplify the effect of reducing electricalconductivity.

6. Conclusions[15] Our observations show an inverse relationship between

conductivity and relative humidity. Comparison of thecorrelation of these two parameters in two different regionswith different background aerosol concentrations stronglysupports that the hydrate growth of aerosol particles in high-humidity conditions might be the cause of such inverserelationship. We also have reported the inverse relationship

between sea level pressure and conductivity for the firsttime and it has been explained on the basis of transportationof ultrafine particles from free troposphere associated withhigh sea level pressure. Comparison of correlation coeffi-

Figure 7. Total concentration of dry and wet aerosols.(a) Particles larger than 50 nm and less than 1000-nmdiameter. (b) Particles between 20- and 50-nm diameter.


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Figure 8. Scatter diagram of conductivity versus relative humidity in region B with different averagingperiod: (a) 3 hourly, (b) 6 hourly, (c) 12 hourly, and (d) 24 hourly with line of best fit.

Figure 9. Scatter diagram of pressure versus aerosol particles of 13-nm diameter with line of best fit.


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cients shows that such effect is more significant in theregions where the background aerosol concentration is verylow as compared to the regions of high aerosol concentration.

[16] Acknowledgments. Data wereprovided by NCAR/EOL under thesponsorship of the National Science Foundation (http://data.eol.ucar.edu/).

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D. M. Lal, P. Murugavel, and S. D. Pawar, I & OT Division, IndianInstitute of Tropical Meteorology, Dr. Homi Bhabha Road, NCL Post,Pune, Maharashtra 411 008, India. ([email protected])


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