retrieval of columnar aerosol size distributions and radiative-forcing evaluations from...

Atmospheric Environment 34 (2000) 5095}5105

Retrieval of columnar aerosol size distributions andradiative-forcing evaluations from sun-photometricmeasurements taken during the CLEARCOLUMN

(ACE 2) experiment

Vito Vitale*, Claudio Tomasi, Angelo Lupi, Alessandra Cacciari, Stefano Marani

Institute ISAO } C.N.R., via Gobetti 101, I-40129 Bologna, Italy

Received 6 January 2000; accepted 10 May 2000

Abstract

Spectral measurements of direct solar irradiance were taken within 13 narrow-band channels in the 401}3676 nmwavelength range, using the IR}RAD sun-radiometer at the Sagres station in southern Portugal during the CLEAR-COLUMN (ACE 2) experiment, from June 16 to 25 July 1997. The measurements performed on 21 clear-sky days wereexamined in terms of the Bouguer}Lambert}Beer law, following realistic correction procedures for Rayleigh scatteringand absorption by minor atmospheric gases, to determine more than 2100 spectral series of aerosol optical depth. Allthese spectral series were examined with the King inversion method to retrieve the columnar aerosol particle sizedistributions over the 0.07}10 lm radius range, for values of the real part of the particulate refractive index rangingbetween 1.43 and 1.50 and values of the imaginary part between 0.003 and 0.010. Using the well-known 6S computer codefor all the columnar aerosol size-distribution curves determined on ten `golden daysa and both refractive index partsvarying with wavelength, evaluations of the change *FC caused by aerosol particles in the outgoing solar radiation #uxwere made at solar zenith angles h ranging between 15 and 763 and for spectral albedo features of both clear water andgreen vegetation surfaces. The results show that the radiative forcing *FC assumes positive values (associated withcooling e!ects) for the clear water surface on all the measurement days and at all the solar zenith angles, and mostlynegative values (warming e!ects) for the green vegetation surface in the range h(603. The present evaluations also giveclear evidence of the close dependence of *FC not only on the surface albedo, solar zenith angle and aerosol optical depthbut also on the mean single scattering albedo of the columnar aerosols. ( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Aerosol extinction; Aerosol radiative properties; Climatic e!ects; Inversion methods

1. Introduction

Important e!ects on the Earth's radiation budget areproduced by aerosol particles both directly, by scatteringand absorption of solar radiation, and indirectly, actingas cloud condensation nuclei and, hence, modifying thecloud radiative properties (Charlson et al., 1991). In par-ticular, the anthropogenic aerosol particles generated bythe combustion of fossil fuel and biomass burning can

*Corresponding author. Fax: #39-051-639-9658.

cause appreciable radiative forcing e!ects in the regionswhere the aerosol optical depth assumes high values atvisible wavelengths, considerably changing the net #ux ofradiation within the atmosphere. Such e!ects are ex-pected to be particularly intense in the industrialisedregions of Europe, where the aerosols (mainly consistingof sulphates and, in lower concentrations, soot substan-ces) are estimated to cause appreciable variations inthe albedo characteristics of the surface-atmospheresystem. Consequently, signi"cant climatic e!ects canbe caused by aerosols, of comparable intensity to thewarming e!ects induced by the concentration increase of

1352-2310/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 2 6 9 - 7

greenhouse gases (Wigley, 1989). However, large uncer-tainties still exist in the yearly average evaluations of thedirect radiative forcing caused by tropospheric aerosolson the global scale, because of the great spatial variabilityin both aerosol concentration and composition. Forthese reasons, regional studies of the direct e!ects pro-duced by aerosol particles in scattering the solar radi-ation are strongly recommended (Huebert et al., 1996;Bates et al., 1998; Russell et al., 1999a).

In order to reduce the large uncertainties in estimatingthe aerosol radiative e!ects and to obtain, at the sametime, more precise evaluations of the climate forcingproduced by aerosol particles, the Second Aerosol Char-acterization Experiment (ACE 2) was designed. The ulti-mate purpose of the experiment was to de"ne moreaccurate and detailed models describing the radiativeproperties of marine aerosol particles transported bywinds from the North Atlantic Ocean, anthropogenicaerosol particles from the European continent and desertdust from Sahara regions. The Clear-Sky Column Clos-ure Experiment (CLEARCOLUMN) is part of the ACE2 Experiment: it took place in June/July 1997 in south-western Portugal, the Canary Islands, and over the east-ern Atlantic Ocean surrounding and linking those sites(Russell and Heintzenberg, 2000). It provided overdeter-mined sets of volumetric, vertical pro"le and columnaraerosol data from sea level to about the 5 km heightusing samplers and sensors at land sites, on a ship and onfour aircraft. In particular, our group (FISBAT/ISAOInstitute, C.N.R., Bologna, Italy) was entrusted with rou-tine measurements of direct solar irradiance at the Sagres(50 m a.m.s.l.) station in Portugal, taken within severalnarrowband channels centred at selected wavelengthswithin the main windows of the visible and near-infraredsolar spectrum. Precise values of aerosol optical depthwere determined from the measurements at allwavelengths. The spectral series of aerosol optical depthwere subsequently examined through inversion methodsto retrieve the size distributions of the aerosol particlessuspended in the vertical atmospheric column. Thesesize-distribution curves were used together with realisticevaluations of the particulate matter radiative propertiesto obtain the evaluations of the instantaneous directradiative forcing caused by aerosols in the surface}atmo-sphere system for di!erent surface albedo conditions.

2. The sun-radiometric measurements of aerosoloptical depth

The spectral measurements of direct solar irradiancewere taken at the Sagres station on the Atlantic coastusing a new model of multiwavelength sun-radiometer,called IR}RAD, designed and manufactured by ourgroup at the FISBAT/ISAO Institute. The optical, spec-tral and electronic characteristics of the IR}RAD sun-

radiometer were carefully described by Vitale et al.(2000). This instrument basically consists of (i) a sapphirewindow to prevent dust from entering the radiometer, (ii)a collimator tube containing "ve circular diaphragms, 6.5mm diameter, perfectly aligned and placed at regulardistances one from the other to de"ne a circular "eld ofview with an angular diameter of 1316@, (iii) a rotatingdisk divided into solid and open sectors, used as step-by-step chopper to obtain the zero-signals of the sensor, (iv)a rotating circular disk-driven step-by-step by a syn-chronous motor, containing three-"eld stoppers and 13narrow-band interference "lters, with peak-wavelengthschosen in the 401.3}3676.0 nm spectral range, (v) a ther-mopile detector used as sensor, and (vi) a thermostatedbox containing the detector, the chopper disk, the "lterwheel and all the electronic devices. The internal temper-ature of the box is maintained steady at 153C by fourthermoelectric (Peltier) heat pumps regulated by a set ofsingle-supply internal temperature sensors. In this way,errors due to temperature drift e!ects on the "lter trans-mission curves and thermal instability of the thermopileare avoided. The IR}RAD sun-radiometer was mountedon an alt-azimuth automatic tracker capable to followthe Sun with an angular precision of less than 1@ forclear-sky conditions.

The IR}RAD measurements were taken at Sagres dur-ing the period from 16 June to 25 July 1997, on 21 stableclear-sky days, collecting a total number of more than2100 spectral series for cloudless sky conditions. A vari-able number (50}130) of spectral series were recorded oneach clear-sky day, since each spectral scanning wasperformed within less than 5 min. Each spectral seriesconsisted of 13 output voltages J(j) taken within narrow-band channels chosen in the middle of the main transpar-ency windows of the solar spectrum (Vitale et al., 2000).For stable clear-sky conditions of the atmosphere, thevariations of the total atmospheric optical thickness oc-curring within 5 min were estimated to be appreciablysmaller than the overall instrumental and calibrationerrors at all the window wavelengths.

Simultaneous relative measurements of direct solarirradiance were also taken at Sagres by the Leipzig Uni-versity group, using the Aerosol Sun-Photometer, fromwhich evaluations of the aerosol optical depth were ob-tained at several window-wavelengths in the visible andnear-infrared, together with estimates of the atmosphericturbidity parameters a and b (As ngstroK m, 1964). More-over, measurements of sky brightness were routinelytaken in the almucantar and at di!erent angular distan-ces from the solar disk, with the purpose of de"ning theangular dependence characteristics of aureole scatteringintensity, from which realistic estimates of the real part ofparticulate matter refractive index were obtained (vonHoyningen-Huene, private communication).

Considering the narrow "eld of view of the IR}RAD,we assumed that the sky di!use radiation entering the

5096 V. Vitale et al. / Atmospheric Environment 34 (2000) 5095}5105

Fig. 1. Spectral series of aerosol optical depth d1(j) obtained

from IR}RAD measurements taken at di!erent hours of fourclear-sky days (solid symbols), compared with measurements ofd1(j) carried out by the Leipzig University group on the same

days (open symbols), as obtained from the ACE 2 data archive.Vertical bars represent the absolute errors, which were estimatedto vary mostly between 0.005 and 0.015.

instrument was negligible at all the solar elevation anglesand for all the aerosol extinction features observed dur-ing the campaign, as demonstrated by Vitale et al. (2000)using the atmospheric scattering model de"ned by Boxand Deepak (1981). Therefore, each monochromatic sig-nal J(j) given by the IR}RAD is a precise relativemeasurement of the direct solar irradiance reaching theground within the narrow spectral interval de"ned byone of the interference "lters mounted on the IR}RAD.All the output voltages J(j) were examined assuming thatthe direct solar irradiance extinction along the atmo-spheric slant-path is accurately de"ned by theBouguer}Lambert}Beer law at all the window wave-lengths. On this assumption, the monochromatic opticalthickness md(j) measured along the sun-path can becalculated in terms of the following equation:

md(j)"ln[RJ0(j)]!ln J(j), (1)

where (i) m is the relative optical air mass, which gives themeasure of the sun-path length as a function ofthe apparent solar zenith angle h (Kasten, 1966), (ii) d(j)is the total atmospheric optical thickness measured alongthe vertical path, (iii) R is a correction factor taking intoaccount the day-to-day variability of the Earth}Sundistance (Iqbal, 1983), and (iv) J

0(j) is the calibration

constant at wavelength j, which is proportional to themonochromatic extra-terrestrial solar irradiance for themean yearly Earth}Sun distance. The IR}RAD values ofJ0(j) at the 13 window wavelengths were determined

during the RAD-I-CAL 96 intercomparison campaignheld at the Schneefernerhaus Observatory (2665 ma.m.s.l.) on the Zugspitze (Germany) in October 1996(Vitale et al., 2000). Two Langley plots only were per-formed during the campaign because of the inclementweather. Therefore, other calibration measurements werecarried out at Sagres on the two clear-sky days of 24 and25 July, as soon as the CLEARCOLUMN campaign wasclosed. The values of J

0(j) determined at Sagres were

found to agree very well with those determined duringthe RAD-I-CAL 96 campaign, presenting di!erences ofbetween #0.9 and #4.7% at the visible wavelengths,between !6.7 and #6.4% in the 0.78}1.025 lm spec-tral range and between !1.0 and #1.5% at the longerinfrared wavelengths. Bearing in mind that the atmo-spheric transparency conditions observed at theSchneefernerhaus Observatory were greatly more fa-vourable for the Langley plots than those encountered atSagres and taking note of the relatively small discrepan-cies between the calibration constants obtained duringthe two campaigns, we decided to use the RAD-I-CAL 96spectral series of J

0(j) for examining in terms of Eq. (1) all

the output voltages J(j) given by the IR}RAD during theCLEARCOLUMN campaign, from which the corre-sponding monochromatic values of md(j) were deter-mined.

3. The spectral characteristics of aerosol optical depth

The total atmospheric optical thickness md(j) in Eq. (1)is the measure of the atmospheric extinction along theslant-path and, consequently, is given at all the windowwavelengths by the sum of di!erent partial contributionsdue to Rayleigh scattering, aerosol extinction and ab-sorption by di!erent minor gases. Therefore, we cal-culated the aerosol particle optical depth d

1(j) along

the vertical atmospheric path in terms of the followingdi!erence,

d1(j)"d(j)!+

j

(mj/m)d

j(j), (2)

where (i) the relative optical air mass m was calculated asa function of h from the evaluations performed byTomasi et al. (1998), (ii) the optical mass functions m

jfor

Rayleigh scattering ( j"1), water vapour ( j"2), ozone( j"3), nitrogen dioxide ( j"4), and all the other mixedgases ( j"5) were calculated according to Tomasi et al.(1998), assuming that m

1and m

5are both equal to m; and

(iii) the Rayleigh scattering optical depth d1(j), water

vapour optical depth d2(j), ozone optical depth d

3(j),

NO2

optical depth d4(j) and overall optical depth d

5(j)

due to the mixed gases were all calculated following theprocedure described by Vitale et al. (2000).

More than 2100 spectral series of total atmosphericoptical depth d(j) were examined following this proced-ure to determine the corresponding spectral series ofd1(j) in the range of 401}3676 nm wavelength. Fig. 1

shows four spectral series of d1(j) determined at various

hours of four clear-sky days. As can be seen, the series of

V. Vitale et al. / Atmospheric Environment 34 (2000) 5095}5105 5097

d1(j) measured on 19 and 23 June both present values

slowly decreasing with wavelength, showing that impor-tant extinction e!ects were produced by giant particlesand large particles of relatively great size. The two otherspectral series of d

1(j), measured on 18 and 20 July, both

exhibit steeper spectral dependence slopes, indicatingthat remarkable extinction e!ects were also caused by thelarge particles of smaller size. Four spectral series of d

1(j)

carried out by the Leipzig University group on the samedays are also shown in Fig. 1 for comparison. The ac-cordance between the spectral series measured on 23June and 20 July appears to be substantially good. Thediscrepancies observed on the two other days can bereasonably attributed, at least in part, to the di!erentmeasurement hours.

The time-patterns of the atmospheric turbidity para-meters a and b determined from the IR}RAD measure-ments of d

1(j) taken within the 401}1025 nm wavelength

range turned out to be closely related to the origin of theair masses passing over the Sagres station. In fact, thevalues of a were found to range mostly between 0.1 and1.0 together with values of b ranging mostly between 0.02and 0.10 during the "rst measurement period from 16June to 4 July, which was most frequently characterisedby the arrival of air masses from the central and northernregions of the Atlantic Ocean or from the coastal regionsof Portugal. Thus, the columnar particle content pre-dominantly consisted of marine aerosols or mixed (mar-ine and continental) particles. Parameter a was found tovary mainly between 0.2 and 1.3 together with values ofb ranging between 0.04 and 0.26 during the second periodfrom 5 to 25 July, when the air masses were in most casesfrom the western part of the Mediterranean and the cen-tral regions of Europe or from the Northern AtlanticOcean and the northern regions of Europe. On those days,the columnar aerosol content was most frequently givenby particulate matter of prevailing continental origin.

The wavelength dependence features of aerosol opticaldepth are closely related to the shape-parameters of theoverall columnar particle size distribution, throughfeatures carefully described by the Mie theory in termsof multimodal polydispersions of spherical particles(Deirmendjian, 1969). Bearing this in mind and consider-ing that the spectral series of d

1(j) determined during

the Sagres campaign cover the wide wavelength intervalfrom 401 to 3676 nm, we attempted to de"ne thecorresponding columnar particle size distributions usinginversion methods for examining the spectral measure-ments of d

1(j).

4. Retrieval of the columnar aerosol particlesize-distribution

The aerosol optical depth d1(j) is given by the integral

of the monochromatic volume extinction coe$cient of

aerosol particles along the vertical path of the atmo-sphere, and, hence, can be calculated (in the ideal case ofspherical particles) as the integral over the whole-particleradius range of the product of the three following phys-ical quantities: (1) the aerosol size-distribution curve N(r)giving the total number of aerosol particles in the verticalatmospheric column of unit cross section, per unit radiusinterval (consequently, the integral of N(r) calculatedover the whole radius range gives the total columnaraerosol particle number N), (2) the geometrical cross-section of the spherical particle, equal to pr2, and (3) themonochromatic e$ciency factor Q

%95[2pr/j, n(j)!ik(j)]

for aerosol particle extinction, de"ned by the Mie theoryfor spherical particles as a function of ratio 2pr/j and ofboth real part n(j) and imaginary part k(j) of the partic-ulate matter refractive index. It is well known that theextinction e$ciency factor Q

%95[2pr/j, n(j)!ik(j)] in-

creases as a function of the Mie ratio 2pr/j to describea "rst marked maximum, when r/j is near the unity,followed by a sequence of oscillations, whose intensitiesdecrease as Mie ratio increases, tending to an asymptoticvalue through features depending on the refractiveindex characteristics. Thus, a multispectral set of aerosoloptical depth measurements can provide the usefulinformation on the shape of the size distribution N(r).The inverse problem is to infer the shape of function N(r),possibly within the radius range of both large and giantparticles, from a spectral series of d

1(j) by assuming

a "rst guess size distribution and, then, determininga solution which minimises the di!erences between thevalues of d

1(j) calculated according to the Mie theory

and those obtained from the "eld measurements.Applying the linear inversion techniques proposed byPhillips (1962) and Twomey (1963) to "nd numericalsolutions for the Fredholm integral equation of the "rstkind, Yamamoto and Tanaka (1969) were the "rst to usea numerical inversion algorithm in order to infer theparticle size-distribution curve from spectral measure-ments of the aerosol extinction coe$cient. Many otherinversion procedures were subsequently proposed byvarious authors (Grassl, 1971; King et al., 1978; Shaw,1979) in order to obtain realistic solutions for the inverseproblem. The de"nition of the particle radius intervalon which the inversion method can be correctly used andthe assumption of realistic values of both particulaterefractive index parts are the most crucial points inthe rigorous application of the inversion methods tospectral series of the aerosol optical depth. Followingthe suggestions made by GonzaH lez Jorge and Ogren(1996) and according to the criteria de"ned byHeintzenberg et al. (1981), we estimated that theindependent information content on the optical charac-teristics of the columnar aerosols was in the particleradius interval from 0.07 to 10}12 lm, when examiningaerosol optical depth measurements covering the0.4}3.7 lm spectral range (like that of the IR}RAD) and


Fig. 2. Aerosol particle size-distribution curves inferred usingthe King (1982) inversion method from the four spectral series ofd1(j) shown in Fig. 1, for values of n and k constant with

wavelength. The thick curve represents the "ve-mode averagesize-distribution curve of the columnar aerosol particles, asde"ned by the Leipzig University group for the clean maritimeaerosol type (von Hoyningen-Huene, private communication).

considering sets of values of n(j) and k(j) pertinent tomaritime and continental aerosol particles (HaK nel andBullrich, 1978).

On the basis of these considerations, we decided to usethe 0.07}10 lm radius interval in applying the King(1982) inverse method for inferring the particle size-distri-bution curves from the IR}RAD spectral measurementsof the aerosol optical depth. To obtain more informationon the complex refractive index of particulate matter, theKing (1982) inversion method was "rst employed fora preliminary examination of all our clear-sky spectralseries of d

1(j), assuming as a "rst step that (i) the imagi-

nary part k of the refractive index is null at all window-wavelengths, (ii) the real-part n takes only discrete valueswithin the realistic range from 1.35 to 1.60, which wasestablished a priori by us taking into account the evalu-ations of this parameter available in the literature (HaK neland Bullrich, 1978; Shettle and Fenn, 1979), and (iii)parameter n remains constant throughout the spectralinterval from 0.4 to 3.7 lm. Thus, all the spectral series ofd1(j) collected during a single measurement day were

analysed using the King (1982) method for values ofn increasing gradually from 1.35 to 1.60, in steps of 0.025.Among the 11 solutions found for each spectral series ofd1(j), the most realistic value of n was assumed to be that

found for the lowest value of the root mean squaredeviation between calculated and measured values ofd1(j) at the various wavelengths. Following this proced-

ure, the best size-distribution curve was chosen for eachspectral series of d

1(j) together with the best-"t value of

n found within the 1.35}1.60 range. Since a variablenumber of 50}130 spectral series of d

1(j) were recorded

on each clear-sky day, a corresponding number of best-"tvalues of n was found. We then de"ned the daily relativefrequency histograms for the various days and selectedthe corresponding median values of n, "nding daily me-dian values of n varying between 1.43 and 1.47 on themeasurement days characterised by backward trajecto-ries coming from oceanic regions, and values rangingbetween 1.48 and 1.50 on the days characterised by thearrival of air masses from Spain and other Europeanregions.

The daily estimates of the real-part n were found to bein good agreement with those determined at Sagres bythe Leipzig University group on the same days (vonHoyningen-Huene, private communication). Since wewere not able to obtain independent estimates of k, weutilised the evaluations made by the Leipzig Universitygroup for particulate classes closely related to the originsof aerosol particles, assuming: (i) k"0.003 on days char-acterised by the arrival of air masses directly from theAtlantic Ocean, (ii) k"0.005 on days with mixed popula-tions of aerosols, presumably composed of both maritimeand continental particulate matter, and (iii) k"0.01 forcontinental (anthropogenic) aerosols arriving from Euro-pean regions.

For the values of n and k de"ned above, we againapplied the King (1982) method to all the spectral seriesof d

1(j) measured on the 21 clear-sky days and deter-

mined the columnar aerosol size-distributions. Four ofthese size-distribution curves for di!erent air mass trans-port situations are shown in Fig. 2: (i) the curve of 19 June1997 (09 : 20 GMT), in the presence of a predominantcontent of maritime particles arriving from NorthernAtlantic (n"1.43, k"0.003), (ii) that of 23 June 1997(07 : 46 GMT), in the presence of mixed aerosolparticles arriving at various levels from the AtlanticOcean and coastal (industrial) regions of Portugal(n"1.49, k"0.005), (iii) that of 18 July 1997 (17:07GMT), with a prevailing content of anthropogenicparticulate matter from European regions(n"1.48, k"0.01), and (iv) that of 20 July 1997 (16 : 24GMT), in the presence of anthropogenic particlestransported from the European area (n"1.50, k"0.01).


Table 1Values of total columnar number N, total columnar surface S and total columnar volume < of aerosol particles in the 0.08}10.2 lmradius range, found for the four cases shown in Fig. 2, together with the corresponding values of aerosol optical depth d

1(550 nm),

number percentage UN, surface percentage U

Sand volume percentage U

Vof giant particles in the atmospheric column

Day GMT d1(550 nm) N

(cm~2)S(lm2 cm~2)

<(lm3 cm~2)

UN

(%)U

S(%)

UV

(%)

19 June 09 : 20 0.053 2.22]108 4.16]104 2.79] 106 0.08 23.6 45.623 June 07 : 46 0.065 1.16]108 8.26]104 6.86]106 0.42 47.0 75.818 July 17 : 07 0.298 4.67]108 1.50]105 9.96]106 0.06 25.3 35.120 July 16 : 24 0.291 2.77]108 1.63]105 1.09]107 0.14 60.7 70.8

Fig. 2 also shows the "ve-mode average size-distributioncurve of the columnar aerosols determined by the LeipzigUniversity group to provide a general representation ofthe multimodal features characterising clean maritimeaerosol polydispersions. The comparison presents a sub-stantial accordance between our four size-distributioncurves relating to the diverse origins of the particulatematter and the general model proposed by the LeipzigUniversity group for marine aerosols and clean-air con-ditions.

The four columnar particle size-distribution curvesexhibit appreciable discrepancies, in both large and giantparticle size-intervals, presenting multimodal featuresthroughout the radius range from 0.07 to 10.7 lmcovered by the inversion procedure. The correspondingvalues of the total columnar number N, total columnarsurface S and total columnar volume < of aerosol par-ticles, calculated within this radius range, are given inTable 1 together with the corresponding values ofd1(550 nm) and those of the number percentage U

N, sur-

face percentage US

and volume percentage UV

of thegiant particles (with radius r'1 lm) in the atmosphericcolumn. The results given in Table 1 show that therelative number content of giant particles is very low,ranging between less than 0.1% and not more than 0.4%in the four cases. Nevertheless, the percentage U

Swas

correspondingly found to vary between 24% (June 19)and 61% (July 20), clearly indicating that importantscattering e!ects were produced by these particles on allthe measurement days. In these four cases, the volumepercentage U

Vwas found to vary between 35 and 76%.

5. Changes in the outgoing solar radiation 6ux

Fig. 2 gives evidence of the wide shape-variability ofthe columnar aerosol size distributions found on thevarious days of the campaign, suggesting that such varia-bility could be associated with signi"cant changes in theoutgoing #ux FC of solar radiation at the top ofthe atmosphere. In order to evaluate the magnitude ofthe change *FC in the outgoing #ux FC and its variations

with aerosol optical depth, particulate refractive indexand shape-parameters of the columnar aerosol particlesize distribution, we considered four of the `golden daysaindicated by Russell and Heintzenberg (2000), two ofwhich (19 and 23 June) were characterised by prevailingcontents of marine or mixed aerosols and the two other(18 and 20 July) by predominant columnar contents ofanthropogenic aerosols. Optical depth d

1(550 nm) de-

creased on 19 June from values close to 0.15 in the earlymorning (due to the hazy conditions at the ground) tovalues ranging between 0.13 and 0.04 during the rest ofthe day. It varied between 0.04 and 0.13 on 23 June,between 0.20 and 0.42 on 18 July and between 0.20 and0.32 on 20 July 1997.

For all the IR}RAD spectral series of d1(j) measured

at various hours of the four days, we calculated theoutgoing #ux FC of solar radiation at the top level of theatmosphere and over the 2p solid angle, using the 6Scomputer code (Vermote et al., 1997) for (i) the verticalpro"les of temperature and relative humidity determinedfrom the radiosounding measurements performed atSagres (Tomasi et al., 2000), (ii) the vertical contents ofozone, carbon dioxide and oxygen, as de"ned for mid-latitude summer conditions of the atmosphere, for whichthe spectral absorption features were determined usingthe two random exponential band models adopted by the6S code, (iii) the simultaneous values of the solar zenithangle h, (iv) the particle size-distribution curves retrievedabove, (v) the spectral re#ectance curves for both clearwater and green vegetation surfaces, as de"ned in the 6Scomputer code, which present average albedo equal to0.025 and 0.268 at angle h"03, respectively, and increas-ing values with h up to 0.18 and 0.37 at h"763, respec-tively, and (vi) the spectral curves of particulate refractiveindex parts n(j) and k(j), as de"ned at the variouswavelengths from 0.4 to 3.75 lm in terms of linear combi-nations of the values of the two parameters proposed inthe 6S code for three of the four aerosol basic compo-nents (water-soluble, oceanic and soot). More precisely,in the cases of 19 June and 23 June, the spectral values ofn(j) were determined by (1) de"ning for each day thelinear combination of the values of n(j) adopted in the 6S


Table 2Values of aerosol optical depth d

1(550 nm), weighted average real part n and weighted average imaginary part k of the particulate matter

refractive index, weighted average single scattering albedo u, change *FC in the outgoing solar radiation #ux and ratio *FC/d1

(550 nm),the latter two quantities being calculated at solar zenith angle h"153 using the 6S computer code (Vermote et al., 1997) for spectralre#ectance conditions of both clear water (CW) surface (mean spectral albedo equal to 0.025 at h"153) and green vegetation (GV)surface (mean spectral albedo equal to 0.280 at h"153)

Day GMT d1(550 nm) Weighted average

refractive indexWeightedaverage u

*FC (W m~2)(h"153)

*FC/d1

(550 nm)(W m~2) (h"153)

n k CW GV CW GV

19 June 09 : 20 0.053 1.43 0.0037 0.96 3.7 1.2 70 2220 June 07 : 15 0.067 1.49 0.0082 0.81 3.1 !6.2 47 !9323 June 07 : 46 0.065 1.47 0.0067 0.87 5.0 !3.4 77 !5325 June 12 : 32 0.086 1.49 0.0130 0.90 4.6 !4.0 53 !475 July 17 : 17 0.051 1.47 0.0067 0.91 3.4 !3.7 66 !727 July 12 : 08 0.132 1.50 0.0138 0.80 5.0 !6.2 38 !4710 July 11 : 52 0.233 1.49 0.0130 0.85 7.9 !8.2 34 !3511 July 12 : 33 0.217 1.49 0.0082 0.89 12.3 !7.2 57 !3318 July 17 : 07 0.298 1.48 0.0122 0.90 13.7 !1.7 46 ! 620 July 16 : 24 0.291 1.50 0.0138 0.84 11.1 !9.2 38 !32

Fig. 3. (a) Time-patterns of the product m d1(550 nm) giving the

total aerosol optical thickness along the sun-path during fourmeasurement days; (b) time-patterns of the change *FC in theoutgoing solar radiation #ux calculated on the four days, usingthe 6S computer code (Vermote et al., 1997) for the above valuesof m d

1(550 nm), the particle size-distribution curves retrieved

using the King (1982) inversion method and the spectral re#ec-tance characteristics of the clear water surface; (c) as in (b), forthe spectral re#ectance characteristics de"ned by the 6S com-puter code (Vermote et al., 1997) for the green vegetation surface.

code for the water-soluble and oceanic components,which gives the value of n (550 nm) equal to that pro-posed in the previous section, and (2) multiplying thespectral values of n(j) by a correction factor givinga weighted average value of n over the 0.4}3.75 lm spec-tral range equal to that proposed in Table 2 (through theuse of a weight function given by the spectral curve of thedirect solar irradiance reaching the ground along thevertical path of the standard atmosphere). Correspond-ingly, the spectral values of k(j) were determined follow-ing the same weighting procedure and found to yielda weighted average value equal to 0.0037 on 19 June and0.0067 on June 23. In the cases of 18 and 20 July, thespectral values of n(j) were calculated assuming that thecolumnar mass content of aerosol particles consisted ofa constant component of soot particles, "xed equal to1.1% of the total columnar mass loading, and of linearcombinations of the water-soluble and oceanic compo-nents, de"ning the composition of the remaining massfraction equal to 98.9%. The values of n(j) and k(j)determined through the procedure were then also multi-plied by a correction factor to obtain weighted averagevalues of n over the 0.4}3.75 lm wavelength interval,equal to 1.48 on 18 July and 1.50 on 20 July, togetherwith weighted average values of k equal to 0.0122 and0.0138, respectively, as shown in Table 2.

Following this procedure, we calculated the outgoing#ux FC with and without aerosol particles. Each di!er-ence between the two values of the outgoing #ux wasrealistically assumed as the measure of the radiativeforcing *FC caused by the aerosol particles at that time,according to the procedure adopted by HaK nel et al.(1999). The results are shown in Fig. 3, which is dividedinto three parts. Fig. 3(a) shows the time patterns of the


product m d1(550 nm) over 4 days. This quantity was

observed to decrease rapidly during the early hours of themorning, mainly as a consequence of the decrease in therelative optical air mass m but also because of the de-crease in d

1(550 nm) due to the gradual vanishing caused

by solar heating of the haze ground layer formed duringthe night. Subsequently, m d

1(550 nm) usually remained

nearly constant during the middle part of the day andgradually increased throughout the late afternoon, asa consequence of the increasing air mass m.

Fig. 3(b) shows the calculations of *FC obtained on4 days for the clear water surface. All the evaluationsappear to be positive, clearly indicating that columnarcontents of marine aerosol particles or anthropogenicparticles characterised by very weak absorption proper-ties cause an increase in the albedo of the surface-atmo-sphere system, when the surface re#ectance is low likethat of the sea surface. The values of *FC determined onthe two days of June for relatively low values of d

1(j)

were found to range between 1.6 and 7.6 W m~2, show-ing that relatively weak cooling e!ects can be producedby small loadings of marine aerosols over the sea surface.The lower estimates of *FC were obtained during themiddle part of the day, while gradually increasing valueswere found at higher solar zenith angles describing twowide maxima centred at about h"603, that is in the earlymorning and the late afternoon. The values of *FC andtheir diurnal variations are very similar to those found byRussell et al. (1999b) during the TARFOX experiment forsmall aerosol optical depths and refractive index valuestypical of marine aerosols. The values of *FC calculatedon 18 and 20 July, for relatively high values of d

1(j)

mainly due to anthropogenic aerosols, turn out to rangebetween about 7 and 24 W m~2, presenting a wide min-imum in the middle part of the day and two pronouncedmaxima in the early morning and in the late afternoon,respectively, when the solar zenith angle exceeds 553. Theevaluations of *FC and their time variations agree alsovery well with those found by Russell et al. (1999b) forsimilar values of d

1(j) and surface albedo characteristics,

con"rming that the highest values of *FC are producedfor stable particulate extinction conditions at solar zenithangles varying between 55 and 653. Examining theseresults, it is interesting to notice that the maxima becomemore pronounced and their position moves slightly to-wards the lower values of h as d

1(j) increases.

Fig. 3(c) shows the values of *FC obtained on the same4 days for spectral albedo characteristics typical of a sur-face covered by green vegetation; they are partly positiveand partly negative. The positive values are associatedwith cooling e!ects and are appreciably smaller thanthose found in Fig. 3(b) for the clear water surface, sincethey are all smaller than 2.5 W m~2 during the middlehours of 19 June. They clearly indicate that the coolinge!ects caused by low marine aerosol loadings in conti-nental regions are considerably less intense than in

oceanic areas. The negative values of *FC indicate thatthe aerosol particles cause a decrease in the albedo of thesurface-atmosphere system, inducing warming e!ects inthe atmosphere. All were found to be not lower than!16 W m~2 during the middle hours of the 2 days ofJuly. Only in the early morning and in the late afternoon,for values of h higher than 603, were the anthropogenicaerosol particles found to give positive values of *FC,continuously increasing as h increased throughout thelate afternoon. The results shown in Fig. 3(c) are in goodaccordance with those found by HaK nel et al. (1999)during the Lindenberg Aerosol Characterization Experi-ment 98 (LACE 98) "eld campaign carried out atLindenberg/Falkenberg in Germany from 13 July to14 August 1998. The LACE 98 results indicate thatcolumnar contents of aerosol particles with values ofd1(550 nm) mostly ranging between 0.10 and 0.25 and

values of the mean single scattering albedo u between 0.6and 0.9 can cause instantaneous direct radiative forcinge!ects of between !4 and!14 W m~2, producing con-siderable heating within the atmospheric boundary layer.

The results shown in Figs. 3(b) and (c) for relativelyhigh values of d

1(j) due to continental/anthropogenic

particulate matter show that the same columnar aerosolcontent can cause radiative forcing e!ects of oppositesign, when passing from the water surface to a morere#ecting surface. Such variations depend on the magni-tude and the spectral characteristics of aerosol opticaldepth, which are closely related to the shape of theparticle size-distribution, the aerosol scattering and ab-sorption properties, the surface albedo and solar zenithangle. In order to better understand the relationshipsbetween the radiative forcing e!ects and the aerosoloptical parameters, we considered ten cases, each onechosen on one of the `golden daysa of the CLEAR-COLUMN experiment.

6. Dependence of aerosol direct radiative forcing onsurface albedo

The main radiative parameters used in examining the10 cases are presented in Table 2. Optical depthd1(550 nm) was found to range from the very low values

of around 0.05 observed on 19 June and 5 July to therelatively high values close to 0.3 measured on 18 and 20July . The spectral values of n(j) yielded weighted averagevalues of n ranging between 1.43 (marine aerosols only)and 1.50, as in the cases of 7 and 20 July, pertinent tocontinental aerosols. The corresponding weighted aver-age values of k, whose spectral curves were evaluated byfollowing the procedure described in the previoussection, are also given in Table 2: among the 10 cases,only 19 June presents a relatively low value of k, typical ofpoorly absorbing particles like marine aerosols; 4 casesconcerned with mixed aerosols, presenting values of k equal


Fig. 4. Values of the change *FC in the outgoing solar radiation#ux plotted as a function of aerosol optical depth d

1(550 nm),

calculated using the 6S computer code (Vermote et al., 1997)for "ve values of solar zenith angle h and the ten cases listed inTable 2. The left part shows the results obtained for the clearwater surface, the right those found for the green vegetationsurface.

to 0.0067 or 0.0082, and the other 5 cases (25 June and 7,10, 18 and 20 July) continental (anthropogenic) aerosolswith values of k varying between 0.0122 and 0.0138.Correspondingly, we determined the columnar aerosolsize-distribution curves by applying the above inversionprocedure to the spectral series of d

1(j) and calculated

the weighted average values of the single scattering al-bedo u over the 0.4}3.75 lm spectral range, obtainingreliable evaluations of the fraction of incoming solarradiation subject to scattering. These evaluations of uare given in Table 2, varying between 0.80 (as inthe continental particle case of 7 July) and 0.96, as inthe case of 19 June, in the presence of marine aerosolsonly, which in practice do not absorb solar radiation.Table 2 also presents the values of *FC calculatedat h"153 for the clear water surface albedo, whichwere all found to be positive and those obtained ath"153 for the green vegetation surface, which areall negative except for the case of 19 June concerningmaritime particles.

The two last columns in Table 2 show the values of*FC per unit variation of d

1(550 nm), as determined at

h"153: those found for the clear water surface resultedto vary (i) between 34 and 53 W m~2 in the cases withk ranging between 0.0122 and 0.0138 and with weightedaverage values of u varying between 0.80 and 0.90, and(ii) between 47 and 77 W m~2 in the cases with k rangingbetween 0.0067 and 0.0082 and u between 0.81 and 0.91.In the single case with k"0.0037 (19 June), parameters*FC/d

1(550 nm) and u were found to present high values,

equal to 70 W m~2 and 0.96, respectively. The values of*FC/d

1(550 nm) calculated for the green vegetation sur-

face were found to range (i) between !6 and!47 W m~2 in the cases with k higher than 0.01 andu ranging between 0.80 and 0.90, and (ii) between !33and !93 W m~2 in the cases with k varying between0.0067 and 0.0082 and u between 0.81 and 0.91. The onlypositive value was found in the case of 19 June, withk"0.0037 and u"0.96. These "ndings clearly indicatethat both surface albedo and mean single-scattering al-bedo are crucial parameters in calculating the directradiative forcing due to aerosol particles.

The results in Fig. 3 show that *FC is also very sensi-tive to the time variations of h, since it assumes thehighest values for h close to 603, in the cases with the clearwater surface. In the other cases with the green vegeta-tion surface, the term *FC was found to assume mostlynegative values for low values of h and positive values inthe upper range, which increase more markedly forh'603. Showing that the radiative forcing caused byaerosol particles closely depends on the radiative proper-ties of the columnar aerosol particles and, in particular,on the mean single-scattering albedo, the values reportedin Table 2 for h"153 require to be completed with otherestimates of *FC at higher solar zenith angles. Consider-ing that higher values of d

1(j) were generally found on

days when the continental particulate matter fractionwas considerably larger than that of marine aerosols, theaerosol radiative properties observed in the 10 casespresent in reality evolutionary patterns strongly in-#uenced by the gradual increase of aerosol optical depthdue to the growing contents of continental (anthropo-genic) particulate matter. On this basis, we decided tocalculate *FC for the 10 cases in Table 2, not only forh"153 but also for h"253, 403, 603 and 763, and tode"ne the average increasing trend of *FC withd1(550 nm), at the various zenith angles.The values of *FC are plotted as a function of aerosol

optical depth d1(550 nm) in Fig. 4, for the "ve values of h,

separately for the two surface albedo models, on the leftfor the clear water surface and on the right for the greenvegetation surface. The comparison provides clear evid-ence of the substantial di!erences between the resultsobtained for the two surfaces: the values of *FC cal-culated for the clear water surface turned out to becorrelated very well by regression lines presenting posit-ive slope coe$cients equal to 37 W m~2 at h"153,


43 W m~2 at h"253, 48 W m~2 at h"403, 49 W m~2

at h"603 and 34 W m~2 at h"763, together with re-gression coe$cients all higher than 0.87. The values ofthe slope coe$cient are in good accordance with those of*FC/d

1(550 nm) given in Table 2 for h"153 and clearly

con"rm the existence of a wide maximum at aroundh"603. The values of *FC found for the green vegetationsurface appear to be more dispersed at all the "ve zenithangles than in the clear water cases. However, they arebest-"tted by regression lines presenting negative slopecoe$cients equal to !14 W m~2 at h"153,!15 W m~2 at h"253 and !8 W m~2 at h"403.These evaluations of the slope coe$cients agree very wellwith the results shown in Fig. 3(c) for the green vegeta-tion surface and low values of h, suggesting that thehigher the aerosol optical depth, the more intense thewarming e!ects occurring in the atmosphere for relative-ly low solar zenith angles, especially in cases where u islower than 0.9. On the contrary, the slope coe$cientsdetermined by the regression lines drawn in the caseswith h"603 and 763 were found to be positive and equalto about 8 and 15 W m~2, respectively. These resultsfully con"rm our previous interpretation of the resultsshown in Fig. 3(c), suggesting that cooling e!ects can beproduced by aerosol particle layers located above sur-faces presenting relatively high albedo features only forlow solar elevation angles.

7. Conclusions

The present results obtained from the spectral series ofd1(j) through the use of the King (1982) inversion method

show that realistic columnar aerosol size-distributionscan be determined only if the measurements of d

1(j) are

performed at several window wavelengths distributedthroughout a wide spectral range, like the one covered bythe IR}RAD sun-radiometer. The columnar aerosolsize-distributions obtained from the IR}RAD spectralseries of d

1(j) carried out during the CLEARCOLUMN

experiment were used to evaluate the changes *FC in theoutgoing solar radiation #ux caused by aerosol particlescharacterised by di!erent radiative properties. Thesecalculations indicate that the direct radiative forcingproduced instantaneously by aerosol particles stronglydepends on various parameters, such as surface albedo,solar zenith angle, aerosol optical depth, spectraldependence features of particulate extinction and meansingle-scattering albedo, the last quantity being closelyrelated to the particulate refractive index characteristics.In particular, we have found that the same columnaraerosol content can cause radiative e!ects of oppositesign in regions presenting very low and relatively highvalues of surface albedo. In fact, positive changes of theoutgoing solar radiation #ux leading to cooling e!ectswere found at all the solar zenith angles for surface

albedo features similar to those of the sea surface, asso-ciated with average values of *FC/d

1(550 nm) ranging

between 35 and 50 W m~2 throughout the range ofh from 153 to 603, the most intense forcing terms beingobtained for h close to 603. Negative changes of theoutgoing #ux FC were mostly found in the presence ofa green vegetation surface, suggesting that radiative forc-ing terms varying between !8 and !16 W m~2 (caus-ing warming e!ects in the atmosphere) can be producedat relatively low solar zenith angles, while moderatecooling e!ects, varying between 8 and 15 W m~2 areexpected at solar zenith angles higher than 603.

Acknowledgements

This research is a contribution to the InternationalGlobal Atmospheric Chemistry (IGAC) Core Project ofthe International Geosphere}Biosphere Programme(IGBP) and is a part of the IGAC Aerosol Characteriza-tion Experiments (ACE). It has been supported by GrantNo. ENV4CT950108 of the European Community.

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