JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D20, PAGES 26,183-26,192, OCTOBER 27, 1998

Spatial Gradients in Aerosol Characteristics over the Arabian Sea and Indian Ocean

S. K. Satheesh, K. Krishna Moorthy, and B. V. Krishna Murthy Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India

Abstract. Aerosol spectral optical depths (ZpX) estimated at 10 wavelengths in the visible and near infrared range over oceanic regions covering near and far coastal regions of the Arabian Sea and remote areas of the equatorial Indian Ocean adjoining the Indian peninsula have been used to study the spatial variation of aerosol optical depths over the ocean. Size parameters and concentrations of aerosols along the downwind direction have been derived with a view to studying the effect of the continent on aerosol properties over the ocean. The studies showed a

gradual decrease in ZpX with fetch distance D with an e -• scaling distance in the range of 1700 to 2400 km under calm to moderate wind conditions. The scaling distance increases with increase in wind speed. A significant increase in the relative concentration of small (accumulation size range) particles is seen as the continent is approached from deeper oceanic regions downwind, indicating increased influence '-• •,,•h ....... ;,' •-ti,,itie • The coarse- particle concentration appears to be of oceanic origin and remains nearly steady over the entire region when the winds are low to moderate.

1. Introduction

Extensive studies of aerosol characteristics in the marine

boundary layer have been made over various oceanic

environments using ship-borne and aircraft measurements [e.g.,

Blanchard, 1963; Prospero, 1979; Blanchard and Woodcock,

1980; Blanchard et al., 1984; Monahah et al., 1983; 1986; Hoppel et al., 1990; Fitzgerald, 1991; O'Dowd and Smith, 1993; Smirnov

et al., 1995; Moorthy et al., 1997] over the Atlantic, Pacific, and

Indian Ocean regions. Transport of continental aerosols (mineral

and anthropogenic) over oceanic regions has been a major focus of studies relating to aerosol characteristics and radiative

interactions in marine atmosphere [e.g., Prospero, 1979; Hoppel

et al., 1990]. Large-scale wind systems (like trade winds) carry

continental aerosols to remote and pristine marine atmosphere

environments and produce effects at locations very far from the

sources. Prospero et al. [1981 ] have reported transport of Saharan

dust to Caribbean islands across the Atlantic Ocean. Transport of aerosols from South Africa to regions over the Atlantic and Indian

oceans is also observed [Tyson et al., 1996]. Most of the earlier studies have revealed a decrease in aerosol concentration near the

surface [e.g., Hoppel et al., 1990] and spectral optical depths [e.g., Moorthy et al., 1997; Jayaraman et al., 1997, Krishnamufti et al.,

1997], with increase in distance from the main land. However,

Copyright 1998 by the American Geophysical Union.

Paper number 98JD00803. 0148-0227/98/98JD-00803 $09.00

quantitative estimates of the gradients over oceans are scarce,

particularly over the Arabian Sea and Indian Ocean. Estimates of

such gradients are essential in assessing the possible impact of

aerosols from the Indian subcontinent and adjoining regions in

influencing aerosol characteristics over the remote Indian Ocean.

The impact of transport of aerosols from the Indian subcontinent

has not been really studied, and this is an important aspect being

investigated under the Indian Ocean Experiment (INDOEX) program [Rama,•athan et al., 1006]. In this paper, we present

observational results on the spatial gradients in aerosol properties over the oceanic ,'egions from a series of measurements of aerosol

spectral optical depths and derived size characteristics over the near coastal and far and remote areas of the Arabian Sea and the

Indian Ocean adjoining the Indian peninsula.

2. Experimental Details, Data, and Analysis

The aerosol spectral optical depth data have been obtained

during the period January to April 1906 over the coastal areas of

the Arabian Sea al•d remote regions of equatorial Indian Ocean

between approximately 18øN and 5øS latitudes and 60 ø to 80øE

longitudes, usin,: a multiwavelength solar radiometer (MWR). The MWR has been token onboard the oceanographic research vessel

Sagar Kanya cn its cruises 100, 110 and 110A. Using this

instrument, thc "etails of which are given elsewhere [Moorthy et

al., 1007], spectral extinction measurements of directly transmitted ground-reaching (ship deck) solar flux have been carried out at 10

narrow spectral ',:ands (full width at half maximum in the range

26,183

26,184 SATHEESH ET AL.: ARABIAN SEA AND INDIAN OCEAN AEROSOL PROPERTIES

6 to 10 nm) ceatered at 38(}, 40(}, 450, 500, 6(}0, 65(}, 750, 85(},

035, and 1(}25 nm. The cruise tracks are shown in Figure la, for cruise 100 [from ¾[oorthy et al. 1007] and in Figure lb combined cruises for l i(; (:;olid line) and 110A (dashed line). The solid points along th(: cruise tracks in Figure la show the locations

where the MWR observations were made, and the dates of

observations are given by the side of the points. Occasions when

two sets of MWR observations were made on the same day at two

distinct locations :ire identified by 1 and 2 in the parentheses

beside the dates •n Figure la, while these occasions are identified

30

"o 25

ß -• 8o

• 15

Q• 10

o.05 o

r• o

-5

-lO 5o

30

INDIA BOMBAY

25

'i,,,I,,,,I .... I,,,,I,,,,I ....

55 60 65 70 75 80 85 90

Geographic Longitude

(b)

co 20

•0 15 ¸

1o

5 55 60 65 70 75 80 85

Geographic Longitude Figure 1. The ,:ruise tracks for (a) cruise 100, reproduced from Moorthy et al. [1907] and (b) combined for cruises 110 (solid line) and 110A (dashed line).

SATHEESH ET AL.' ARABIAN SEA AND INDIAN OCEAN AEROSOL PROPERTIES 26,185

by two close-by points about the date in Figure lb. More details

of these are ava!!able elsewhere [Moorthy et al., 1007; Satheesh

and Moorthy, 1907] and are not repeated here.

The data oi:tained from the MWR were analyzed tollowing

the Langley tecimique [e.g., Shaw et al., 1073; Moorthy et al., 1089] to estimate columnar total optical depths, from which the

aerosol optical depths -rpx are deduced after removing the molecular contrilmtions (scattering and absorption due to ozone and water vail, r) tollowing the details given lay Satheesh and

Moorthy [10•7! In all, 37 sets of 're> ' values have been thus estimated over v. lious oceanic regions during the period of study.

Other supplemectary data such as (true) wind speed and direction, deck-level temperature and relative humidity, and sea surface

temperature were also obtained at regular intervals from

instruments onboard the ship, and these have been used to assess

the general meteorological conditions prevailing during the cruise as well as to delineate air mass movements.

The aeros,•t optical depths, obtained as a function of

wavelength ovc: the range 38(} nm to 1(}25 nm, have been used to derive the columnar size distributions (CSD) of aerosols lay

numerical inw:rsion of the integral equation following the

constrained inversion technique [King, 1082; Moorthy et al.,

•00•].

'• --. f •r r 2 D,•r (r, 2, m) n c (r) dr (].)

In (1), Q•,,,, is the Mie extinction efficiency parameter which is a function of the particle radius r, wavelength ),, the aerosol

refractive index m, and n•(r) is the CSD function giving the number density of aerosols in a vertical column of the atmosphere over a small radius range dr centered at r, r• and r•,, the lower and

upper radii limits respectively, that contribute significantly to the integrand of (1) _}nd depend on the wavelength range used in the MWR. The lower and upper radii limits are (}.(}5 and 3.0 Bin,

respectively, in the present study. From these derived size distributions, the eft'ective radius (reff) which is the ratio of the total volume to, the total area of the aerosols and the columnar

mass loading (mr) have been estimated [Moorthy et al., 1006]. The retrie. vtd size distributions are parameterized using a

combination o: •'o lognormal distributions of the form

2

• Noi n c (_r) = -_ 2Vl-•-ffo:t r • exp (lnr_ lnrt,• ) 2

_ (2)

where r,, and o, are the mode radii and standard deviations,

respectively, w,th i=1 representing the primary (small particle) mode and i=2 representing the secondary (large particle) mode. The parameter ¾oi depends on the concentration of aerosols. The parameters r•, and o, are estimated from the derived distributions by solving (2) using tour points around each mode depicted lay n•(r) and the two values of No• are adjusted to evolve a best fit with minimum rms error [Moorthy et al., 1006]. All these

parameters, -rpx. he(r), rao rn•, r,,, and o i are obtained for all 37 sets of data, and they formed the database.

3. Synopti,, Meteorological Conditions

During the ?eriod of this study, the general synoptic scale

patterns reveal surface-level northeasterly/easterly winds (the trade

winds) in regions lying north of the equator, so that the oceanic

regions covered lay the cruises are on the downwind side of the

continent and thus are subjected to the continental influence (lay

transport) as far as the aerosol characteristics are concerned. The mean synoptic wind patterns at 85(} hPa level during the cruise

period are sho,•n for two representative months in Figure 2:

January (Figure. 2a) and for April 1006 (Figure 2b). These are obtained from the National ('entre for Medium-Range Weather

Forecasting, New Delhi. Figures 1 and 2 together suggest that the oceanic regiona covered during the cruises are subjected to a general continental air mass influence. The meteorological data obtained from the ship were also in general conformity with this

[Moorthy et al, lq07; Satheesh and Moorthy, 1007]. However, the daily average wind speed showed large variations during cruise 100 ranging from ~3.5 m s 4 to as high as 12 m s 4, whereas during the subsequent cruises the average wind speed has been, in general, much lower with peak values less than 6 m s 4. The weather charts available at the meteorological center, Trivandrum

(8.55øN, 77øE), did not show any large-scale weather phenomena during the observation periods over the oceanic regions covered.

4. Resttits and Discussion

4.1. Aerosol qpcctral Optical Depths

In order ta study the spatial gradients in aerosol optical

depths and the effect of proximity of the continent on marine aerosol characteristics, the variation of the aerosol parameters is

examined as a function of distance D (kilometers) from the

continent along the mean (down) wind direction to the various oceanic 1ocati(,•ts where the MWR observations have been made.

The variation of -rpx (in log scale), at the various wavelengths, with D is shoxvn in Figure 3, where the individual points

correspond to the -rpx values and the line is least squares fitted. In evolving Figme 3, it is assumed implicitly that the spatial distribution of at'rosol characteristics, in general, remains the same

temporally over the area considered and during the period of the study, as the stt,C.' period is meteorologically calm and devoid of any strong weather phenomena such as reversal of air mass movements, large-scale precipitation, and cyclonic activity. Thus the individual .,I•servation points are treated as samples obtained

from a statist,,. •11y stable population for the location. Earlier studies over ma:'•e environments have shown that the wind speed

histories prevai;•:}g over ocean areas are important in influencing aerosol characteristics and optical depths. Strong winds lead to

increased concentration (due to production) of marine aerosols

leading to increase in optical depths [Lovett, 1078; Exton et al.,

26,186 SATHEESH ET AL.' ARABIAN SEA AND INDIAN OCEAN AEROSOL PROPERTIES

MEAN ANALYSIS 850 hPa WIND(ra/s) JAN 1996

,," ,' : :':: ß - :l

o

LONGTUDE(øE)

Figure 2. Monthly mean wind patterns at 850 hPa level over the study area, obtained from the National Centre for Medium-Ra[•ge Weather Forecasting, New Delhi, (a) for January 1906 and (b) for April 1096, indicating the mean flow patterns. The length of the arrow on the right-hand top corner of Figure 2 represents a speed of 10 m

1985; Hoppel e' al., 1000; Moorthy et al., 1997]. In view of these findings and as ,•he aim of the current study is to determine the influence of continental aerosols over oceanic environments, we

have excluded in Figure 3 those 'c•, values obtained on days when the average w;nd speed ([0 exceeded 7 m s 4. This is to avoid high-wind conditions which might lead to direct sea-spray

production, and this aspect is discussed in detail later in this

section. These o.,nstituted eight sets of observation points made

during cruise 100. Furthermore, during these days the wind had

a dominant westerly/southerly component from the open ocean

[Moorthy et al., 1007] and thus constituted a marine air mass. In

view of the ai: ),,.'•, the remaining 20 sets of 'c•, x values obtained during days ot !,•w to moderate winds with the winds directed

from the contimmt only are incorporated into Figure 3. In Figure

3, the measurement points lie around the least squares fitted line,

revealing an expouential decrease in 'c•, (at all wavelengths) with increase in the "fetch" distance D of the form

x r = xrc exp (-D/D o) (3)

where Do is considered as a "scaling distance" at which (i.e., for

D -- Do) x•, decreases to e 4 (scaling distance) of its value, x•, at the coast. Thus D o can be considered as a measure of the distance

over the ocean up to which the continental effects are felt

significantly. The high values of 'cp• can be mainly attributed to the high aerosol loading close to the coast, arising mainly from

SATHEESH El' AL.: ARABIAN SEA AND INDIAN OCEAN AEROSOL PROPERTIES 26,187

o 1ooo •ooo o 1ooo •ooo

Distance (km) Figure 3. The variation for 'r•,x with distance D along the mean downwind direction. The points are individual observations, and the line is least squares fitted. Note that the ordinate is in logarithmic scale.

the anthropogenic activities over the coastal regions. It is well known that the ,•'estern coastal regions of central India (Mumbai

and Gujarat) are highly industrialized and urbanized and the aerosols resultlag from these activities would dominate the near-

coastal regions [Satheesh and Moorthy, 1007]. As we move out

more and m(:re into the open ocean regions, these aerosols

undergo both •,: •,,.'. transtbrmation (by microphysical processes

such as coagula:ion, cloud cycling, and condensation) and loss by

sedimentation and (occasional) precipitation over ocean areas. With the fresh inputs over oceans being small (as generally low-

wind days only are considered), the aerosol concentration and thus

'c•, decreases. The best fit line yields a value for D o in the range ~2000 to 3(}(}(} km (for the data at different wavelengths) with a mean value of 2446 km and standard deviation 575 km. The

scatter of the points in Figure 3 and varying values of Do at

different wavelengths may be due to the various factors perturbing

the aerosol characteristics such as varying degree of marine

aerosol generation even within the wind speeds considered

(U<7 m s 't) me:,c, scale changes in meteorological conditions, and the variations in aerosol size distributions. (Nevertheless, the fit

is significant, and the values of correlation coefficients (p)

between !n('rr,; and D are in the range 0.43 to 0.64 and are significant at/' = 0.02 (08%) level [Fisher, 1{)70]).

It is furthe,' ,,hserved that D O changes significantly and so too

the correlation• (p) when measurements of 'rp at higher wind speeds are also considered. In Figure 4, the variations of D o (at

various wavelengths) are shown when 'rp data sets below three different threshold wind speeds (Ut) are considered. The mean

values of p and D O for these cases along with the relevant

statistical parameters are given Table 1. The rationale for

considering the three wind speed thresholds has been to assess the

effect of changes in the surface wind speed on the gradients

estimated. The selection of the threshold has the following

physical bearing also. It is well known that winds are potential

producers of marine aerosols by breaking of bubbles and whitecap

films over the :.cean surfaces as well as by direct sea-spray

production [e.g., Lovett, 1078; Monahan et al., 1083; Hoppel et

al., 1000; Andreas et al., 1005]. It is well known that whitecaps start occurring •),,'er the sea surface at wind speeds > 3 to 4 m s 't [Blanchard anti, Woodcock, 1080], and at speeds below this the ocean surface can be considered fairly tree of whitecaps (at least

over remote oceanic regions). Similarly when the speed exceeds

7 to 10 m s 4, direct particle production occurs associated with spume productiq [Andreas et al., 1005] which is more coarse in nature. The abundance and size spectrum of the aerosols depend

on the wind speed characteristics. With these considerations, we

26,188 SATHEESH ET AL.: ARABIAN SEA AND INDIAN OCEAN AEROSOL PROPER'lIES

:O 0 All Points '•..&U < ? m •-I + .... +U <4m• -1

I I I I I

4OO 6OO 8OO 1000

Wavelength (nm) Figure 4. Effect of threshold wind speeds on the e 4 scaling distance.

have set three wind speed thresholds: the entire wind speed

regime during the study, wind speeds below spume production level, and only calm wind conditions. Besides, the speed 7 m s 'l works out as the median value of wind speeds observed over

remote oceanic environment during our study.

The effect of wind speed changes on D O are readily seen from

Figure 4 and Table 1. For case 1, the entire data sets are

considered without any threshold for the highest wind speed, so

that measurements made at wind speeds up to 12 m s 4 are also included. It can •e seen from Figure 4 that D O values are quite

high and are spread over a wide range with a mean value of

~5943 km and .• high standard deviation (Table 1). Moreover, the

values of Do show an increasing trend toward longer wavelengths,

suggesting tha• the gradients in x•, become less and less at longer wavelengths. (Exen if the extremely large value of D o at 935 nm is not considered, D o has a mean value of ~3676 km and a

standard deviation of 14(10 kin). The high value of D o and the

increase in D o at longer wavelengths are attributed mainly to

increased (local) production of sea-spray aerosols over the ocean

at high wind ,½peeds, leading to increase in x•,. It has been observed that there is a significant increase in the aerosol

concentrations, total volume scattering coefficient (risc), and aerosol optical depths associated with increase in wind speed over

remote marine environments [e.g., Exton et al., 1085; Hoppel et

al., 1990; Moorthy et al., 1997]. The effects of increase in wind

speed on aerosol scattering coef_ficient (fl,½) and xt, are observed to be stronger at !o,'ger (near IR) wavelengths [1toppel et al., 1990;

Moorthy, et al., 1097], and this is attributed to the increased

production of !a. rger aerosols. This increase in x•, with wind speed would partially compensate for the gradual decrease in x•, with D from the sho,'c (owing to particle removal) and leads to an

increase in Do, n'•re so at longer wavelengths. In case 2, U t is

limited to under '7 m s 4 by eliminating data obtained during days of strong winds; the mean wind speed and its standard deviation

are significantly reduced as seen in Table 1. There is a large

decrease in the mean value of D O to ~2446 kin; its standard

deviation decre.•ses to 575 km. In case 3, which represents only

calm conditions with I[• <4 m s 4 and mean speeds of 2.6 m s 't, D O drops down further to 167(} _ 41(} km. The above change in D O

(with wind speed) might be associated with particle residence

times also. Ignoring local production (under low and moderate wind conditions) for a given residence time of aerosols, the

particles will cover longer distances when average wind speeds are higher, and this could explain (at least partly) the increase in

D O from case 3 to case 2. Under calm conditions it can reasonably be taken that an e '• decrease occurs over a distance of ~17(10 km

offshore, which when translated to temporal scale corresponds to

about 7 days for an average wind speed of 2.6 m s 4. However, during the sam•; period the aerosols can travel as far as ~2700 km

if the mean wind speed is 4.5 m s 4 (case 2) or even to ~33(10 km for wind speeds of 5.5 m s 4 (case 1). Thus it is also possible that at higher wind speeds the continental aerosols travel longer

distances over oceans. As the coastal regions of continental India

are industrialized [Satheesh and Moorthy, 1997], it is possible that

Table 1. Values of p for Different Wind Speed Thresholds

Case U,, Mean /•L Data p for P=0.02 Mean Value Mean Value of

m s' m s 4 Sets Significance Level of p D o

1 nil 5.5 +_ 2.3 37 0.40 0.35__+0.17 5943_+5375

0.42_+0.10' 3676-+1400'

2 7 4.5 -+ 1.6 29 0.41 0.51_+0.11 2446 _+ 575

3 4 2.6 _+ 0.8 10 0.72 0.74_+0.10 1670 _.+ ,•10

The values given after plus/minus symbol in the mean value column are the standard deviations. * The value obtained at Z. = 935 nm is excluded.

SATHEESH ET AL.: ARABIAN SEA AND INDIAN OCEAN AEROSOL PROPERTIES 26,189

these anthropogenic aerosols (which are generally of

submicrometer size and thus have a longer residence time) are transported to greater distances over the ocean. Such possibilities of transport of aerosols and pollutants from India as well as mineral dust from arid regions in western India, Saudi Arabia, and

North Africa have been reported t¾om detailed air trajectory analysis by Kris'hnamurti et al. [1007]. A chemical analysis of aerosol samples collected over different regions of the ocean under varying wind conditions would be needed to delineate the

different sourc•:• and their relative significances. From the available information t¾om our study, it can only be inferred that

the combined e :t•cts of increased sea-spray production and longer transport (during a given period) would both be causing the

reduction of ti•,e spatial gradients of x•,.

4.2. Size Distributions and Related Parameters

With a view to examining the spatial variation of aerosol size

characteristics, the columnar size distributions (CSD) are retrieved from the spectral optical depths by inverting (1). The CSDs are, in general, ftmnd to be bimodal in nature. The parameters

describing the CSDs and physical characteristics (such as r,•t, r,•2, or, 02, mt• and r•t-f) are derived as described in section 2. In view

of the observed influence of wind speed changes, only those cases associated with '[I<7 m s 4 (case 2) are considered. The variation of mt• and rea; r,• t and rm2 and o t and 02 with D are shown in

Figure 5 in the top, middle, and bottom graphs respectively. The

columnar mass !,,ading m r (top left in Figure 5) decreases with distance, (accol'ding to (3)), with an e 't scaling distance of ~1600

km and has a o,)rrelation coefficient of ~0.6. This might be caused

by the loss o.'; (larger) particles over ocean by sedimentaii,•n

and/or washot,:. 'I'he effective radius, however, does not show any trend with D, and the correlation coefficient is 0.17, which is not

significant. The- continuous size transformation by the

microphysical processes can be partly responsible for this feature.

The mode radii, shown in the middle graphs of Figure 5, do not

show any significa•t trend with distance. The individual estimates

lie around 0.06 ,urn tbr r,,t and ~0.75/•m for rm2. However, the

standard deviations o• and o 2 show interesting behaviors as seen

in the bottom graphs of Figure 5, where, o• increases (with a

correlation coefficient 0.5) from 0.38 close to the coast to ~0.5 at

~1000 km oft•shore, indicating a broadening of the primary mode

with distance away from the coast. However, o2 shows no trend and remains at ~0.2 with a lower value for the correlation

coefficient (0.38). The above observations clearly indicate that

under calm to moderately conditions of wind the following occur:

(1) There is a gradual reduction in the overall aerosol loading as one moves away from the coast due to particle removal processes.

(2) The basic nature of the CSDs is rather unchanged, being bimodal with a r)road accumulation mode at ~0.06 •tm and a

narrower secondary mode at ~0.75 •tm. (3) The width of the accumulation mode increases with distance while that of the

secondary mode. is not significantly altered. The aerosol size distribution over marine environments is

found to be generally bimodal in nature [e.g., Hoppel et al., 1900;

Moorthy et al., 1007]. The primary (small) particle mode can be

attributed to (1) the formation of new small particles over the

0 1000 2000 d) 1000 2000 0.50- 2, -

. ß • 1,0, ß • • :t. o. t0. ß . .

• 0.05,, •.. ß '1 ' 0,5 0.3 0-01 , • , • ß

o ooo ooo ' ' 20 0

ß ß

o.ot . . ß [ .. ". 0,1

0.2• ß ' ß ' • 0.0/ , • , 0 1000 2000 0 1000 2000

Distance (km) Figure 5. The variation with D of various derived parameters of aerosols. (top) From left to right are those of and re•, (middle) t',•t and r,,> and (bottom) o• and 0 2. The lines are least squares fittc,l.

26,190 SATHEESH ET AL.: ARABIAN SEA AND INDIAN OCEAN AEROSOL PROPERTIES

ocean by the gas to particle conversion processes involving

different gaseous species of continental and marine (e.g., Di-

Methyl Sulphide) origin, (2) transport of submicrometer continental aero•,,ols, and (3) the size transformation of small

particles by the microphysical processes such as coagulation,

condensation, and cloud cycling.

In addition, breaking of whitecaps over sea surface is known

to produce substantial amounts of submicrometer aerosols over the

marine environment [Cipriano et al., 1087; O'Dowd and Smith,

1003]. The co::centration of these submicrometer aerosols

increases expollentia!ly with increase in wind speed, and at high

wind speeds ihe majority of the condensation nuclei in clean

marine enviro:iu•ents are associated with the sea-spray aerosols

[0 'Dowd and S.•ith, 1093]. Further, sul•tantial amounts of coarse particles are prodaced by breaking of waves and bubble jet action

[Blanchard ami Woodcock, 108(}; Fitzgerald, 1001], and these

mechanisms are primarily responsible for the secondary large-

particle mode. The concentration of the coarse particles increases,

more rapidly at larger sizes, with increase in wind speeds [Lovett,

1078; Exton et ai., 1085; Hoppel et al., 1901]. The observed

(near) steady values of the mode radius r,, 2 and standard deviation o 2 (in Figure 5) indicate that the large-particle mode does not

undergo significant changes over the oceanic regions. The larger

scatter of o 2 values compared to o• (and the lower correlation of

0.38 for 02 as compared to 0.5 for %) would I• at least partly due to the rather wide range of wind speeds considered. Other factors

such as the 1ocal',zed generation of the sea-spray aerosols over the

ocean and their shorter residence times [Monahah et al., 1083] also would be i•nportant. It is unlikely that these particles are of

continental origia, in which case the mode radius r,, 2 and/or 02 should show •;-,l:•fl changes with distance owing to the loss of

particles by sedimentation (and the loss rate increases rapidly with

particle size), and this is not noticed in our studies.

This aspect is further examined by considering the two modes in the CSDs tn be associated chiefly with the two different

processes discu'•:,ed above. Considering all the CSDs for U < 7

m s 4, it is found that r,, 2 has a mean value of •0.74_0.04/xm and that o 2 has a mean value of •0.21. Thus more than 70% of the coarse aerosols lie above r,•0.5/xm. Similarly, r,,• has a mean

value of 0.065_.0.01 •m with o• increasing t¾om 0.38 to 0.5 •m with distance. Here it should be borne in mind that toward sizes

lower than r,,•, the particles are increasingly short-lived and lost

rapidly by coagulation as is also evidenced by broadening of the

primary model with distance in this study. Also, by inverting x•, x data from the MWR, no useful information can be obtained on

still lower sizes as the lowest wavelength used in the MWR is

380 nm, and wry small particles do not produce significant

extinction at th,'•se wavelengths. Thus o• corresponds to values of

r > r,,,•. Thus, on average, more than 70% of small particles lie

below r•0.5 ?ra. Dividing the CSDs into two parts with r--0.5/an

to demarcate ,•l•e two size regimes, the total concentration of

accumulation model particles N,• and coarse particles N½ are

estimated as tht. •',rea under the portion of the CSD curves below

and above r=0.5/an, respectively. In general, N½ constituted only

about 0.1 to 1% of the total particle concentration N, (equal to

N•+Nc). The variat;ons of N,•, N• and the ratio N•/N• with D are shown in Figure 6 t¾om top to bottom, respectively. The

corresponding values of the correlation coeffident are given at the

top of each graph. It is seen that there is a substantial decrease in

the concentration of accumulation model particles as they move

away from the coast (i.e., away from the source region) with the concentrations decreasing on average by a factor of 4 by the time

they reach 1000 km. If we consider an average wind speed of 2.6 m s 4, it take/, the air mass about 4.5 days to cover this distance. It appears that the microphysical processes taking place

during this period transforming smaller particles to larger sizes lead to subst•ntial decrease in N,• and broadening of the

accumulation :m•de as indicated by the increases of o• in Figure

5. It is well knc, wn that coagulation is an efficient process that

rapidly transtbrrr. s particles of small sizes to larger sizes [e.g., Junge, 1072; Hoppel et al., 1000], and the simple conservation of

volume itself requires that at least eight particles of a given size

have to combine to yield one particle of double the size. The

coarse-particle concentration, however, does not show any

significant variation with distance (Figure 6, middle) thereby suggesting that they are more likely to be produced locally (over

14

1o

1

11

1o

I I I

1000 :2000

p =-0.11

t I • I

1øo lOOO 2000 o

1 o f p: +0,45 ß. z 10

i I I !

o lO0O 2000

Distance (km) Figure 6. Variation of (top) N,, (middle) N• and (bottom) N•N• with distant. The •rrelation coefficients are given in the top right •rner, the lin• are least squares fitted, and the ordinate s•!es are logarithmic.

SATHEESH ET AL.' ARABIAN SEA AND INDIAN OCEAN AEROSOL PROPERTIES 26,191

the ocean) and as such have a uniform spatial distribution. This

is further supported by the fact that compared to the accumulation

mode particles the coarse particles have much shorter residence

times [Monahan et al., 1083] and thus are unlikely to travel great distances. So 1oc.,i generation is essential to support a near steady concentration t,f these particles over the vast ocean areas. The

variation of NJN, in the bottom graph of Figure 6 shows that as the air mass m•ves offshore there is a substantial decrease in the

relative conce::,• )tion of small particles in the size distribution, or ,

in other words there is a large increase in the accumulation model

particle concentration as one approaches peninsular India. This

may be attributed to the anthropogenic activities over the continent. The gas to particle conversion of effluvia t¾om

anthropogenic activities appears to be the major cause for this.

5. Conclusions

Our studies have shown that during conditions of calm to moderate winds the following occur:

(1) The aerosol optical depth gradually decreases with increase in distance from the continent with an e 4 scaling distance of ~1700 to 2. t00 km. When high wind speed data are also

considered. the decrease in •:•, is very slow and the scaling distance increases substantially, particularly at longer wavelengths, due to the influence of coarse sea-spray aerosols produced locally over the oceans.

(2) The aerosol parameters estimated from the columnar size

distributions derived from the optical depths show that even

though the mode radii do not show any significant change, the columnar mass loading decreases with increasing distance from the coast. There is a considerable broadening of the accumulation mode with increase in distance from the coast

possibly due to the microphysical processes. (3) There is a significant enhancement in the concentration of

small particles as the coastal regions are approached from

deep ocearic regions; concentration of large particles remains nearly steady. The increase in small particle concentration is

attributed tq the anthropogenic activities over the continent,

and the co,,me particles in the CSD appear to be mainly associated w•th marine aerosols.

Acknowledglnent. The study was carried out as part of the INDOEX precampaign. The amhors acknowledge the Director, NIO, Goa, and chief scientists of the respective cruises for providing the shipboard facilities. We are extremel) thankful to B.K. Basu of NCMRWF. New Delhi, for

providing us with the synoptic :,scale •vind data for the period. We thank the reviewers for the useful comments to improve the paper.

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(Received June 10, 1997; revised Februm'y 4, 1998: accepted March 6, 1998)