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ONE YEAR OF SUNPHOTOMETER MEASUREMENTS INROMANIA

ANCA NEMUC1, L. BELEGANTE1 , R. RADULESCU1

1National Institute of R&D for Optoelectronics P.O.Box MG-5, RO-077125 Bucharest-Magurele,

Romania, E-mail:[email protected],[email protected],[email protected]

Abstract. Multi-wavelength sunphotometry provides a quantitative index that relates to totalsuspended aerosol in the atmospheric air column above the observer. In addition, it has the capabilityof delineating characteristic features of different air masses and the aerosol sources that affect them,when used in conjunction with other aerosol and meteorological measurements. Daily averagedretrievals of AERONET(AErosol RObotic NETwork) sun photometer measurements from July 2007to June 2008 are used to provide preliminary results on the characterization of aerosol properties andchanges over southeast Romania, near Bucharest, at Magurele (44.35N, 25.05E). It is shown thataerosol optical and microphysical properties and the dominating aerosol types are influenced by thelong range transport of Saharan dust and biomass burning. Aerosol-parameter frequency distributionsreveal the presence of individual modes that lead to the assumption that moderately absorbing urbanindustrial aerosols are usually characterizing the atmosphere above Magurele. The reported data agreewell with known aerosol information retrieved from climatology of 10 years of observations of otherAERONET sites.

Key words: remote sensing, sun photometer, aerosols, AOD, AERONET.

1. INTRODUCTION

Aerosols are an integral part of the atmospheric hydrological cycle and theatmospheres radiation budget, with many possible feedback mechanisms that arenot yet fully understood. Different aerosol indirect effects and their sign of the netradiative flux change at the top of the atmosphere has the largest source ofuncertainty in the climate change scenarios [1][2].

When used in conjunction with other aerosol and meteorologicalmeasurements, sun photometry has the capability of delineating characteristicfeatures of different air masses and the aerosol sources that affect them

[3][4][5][6]. Aerosol concentrations and size distributions can be derived remotelythrough solar direct beam measurements at a range of wavelengths and zenith

angles. The aerosol single scattering albedo can be also retrieved. The amount oflight absorbed by each particle is measured by its single scattering albedo (SSA)the ratio between the light extinction due to scattering alone and the total lightextinction from both scattering and absorption. If the single scattering albedo liesbelow a critical value, the combined aerosolEarth system reflects less energy backto space than the Earth's surface alone, leading to a net warming of the Earth. Butthis critical single scattering albedo depends strongly on the Earth's local albedo.[1][2].

The AERONET programme maintains a global network of sunphotometersfor this purpose (http://aeronet.gsfc.nasa.gov). There are ~450 instruments

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registered in the network and one is operating in Romania since July 2007 alongwith other equipments for measurements of optical properties of aerosol [7].

In this paper we present the results related to air column aerosolcharacteristics from the first year of continuous sun photometers measurements inRomania, in a pre-urban area. First part is focus on methodology, followed byresults presentation focused on case study analysis for different types of aerosolsloads in the atmosphere and discussions. The last part is dedicated to conclusionsand further work.

2. METHODOLOGY

The instrument used for the measurements is a CIMEL Electronique 318A

spectral radiometer, solar-powered, weather-hardy, robotically-pointed sun and skyspectral sun photometer. A sensor head fitted with 25 cm collimators is attached toa 40 cm robot base which systematically points the sensor head at the sunaccording to a preprogrammed routine. The radiometer makes two basicmeasurements, either direct sun or sky, both within several programmed sequences.The direct sun measurements are made in eight spectral bands requiringapproximately 10 seconds. Seven interference filters at wavelengths of 340, 380,440, 500, 670, 870, and 1020 nm are located in a filter wheel which is rotated by adirect drive stepping motor (http://aeronet.gsfc.nasa.gov).

Optical depth is calculated from spectral extinction of direct beam radiationat each wavelength based on the Beer-Bouguer Law. In addition to the direct solarirradiance measurements that are made with a field of view of 1.2 degrees, these

instruments measure the sky radiance in four spectral bands (440, 670, 870 and1020 nm) along the solar principal plane (i.e., at constant azimuth angle, withvaried scattering angles) up to nine times a day and along the solar almucantar (i.e.,at constant elevation angle, with varied azimuth angles) up to six times a day. Theapproach is to acquire aureole and sky radiances observations through a large rangeof scattering angles from the sun through a constant aerosol profile to retrieve sizedistribution, phase function and aerosol optical depth. More than eight almucantarsequences are made daily both morning and afternoon.

All data are processed, cloud-screened and quality assured as part of routinedata processing [6]. The V2 AERONET retrieval provides wide number ofparameters and characteristics that are important for the comprehensiveinterpretation of the aerosol retrieval. The output includes both retrieved aerosol

parameters (i.e., size distribution, complex refractive index and partition ofspherical/non-spherical particles) and calculated on the basis of the retrievedaerosol properties (e.g. phase function, single scattering albedo, Angstromexponent, spectral and broad-band fluxes, etc.). Accurate retrievals of SSA (withaccuracies reaching 0.03) can be obtained for high aerosol loadings and for solarzenith angles >50 degrees [3][6].

The volume particle size distribution dV(r)/dlnr (m3/m2) is retrieved in 22logarithmically equidistant bins in the range of sizes 0.05m r 15 m. The realn() (1.33 n() 1.6) and imaginary k() parts of the complex refractive index(0.0005 k() 0.5) are retrieved for the wavelengths corresponding to skyradiance measurements. In addition to the detailed size distribution, the retrievalprovides the standard parameters for total (t), fine (f) and course (c) aerosol modes.

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The accuracy of the AERONET aerosol optical depth measurements is 0.01for the wavelength0.44 m and the uncertainty in measured sky radiances due

to calibration error is

5%[4]. The accuracy assessments quality control criteriaand data limitations have been described in details by Dubrovnik at al. [5][6][8].Fine and coarse mode separation can be obtained by using the inversion code

which finds the minimum within the size interval from 0.194 to 0.576 m. Thisminimum is used as a separation point between fine and coarse mode particles.Using that separation, the code simulates optical thickness, phase function andsingle scattering albedo of fine and coarse mode separately [6].

The Angstrom exponent , represents the slope of the wavelength dependenceof the AOD in logarithmic coordinates [9]. In the solar spectrum, is a goodindicator of the size of the atmospheric particles determining the AOD: bigger than1 are mainly determined by fine mode, submicron aerosols, while less than 1arelargely determined by coarse, supermicron particles (e.g. [11])

3. RESULTS AND DISCUSSIONS

First year of measurements of a sun photometer in Romania, 5km away ofBucharest, at Magurele was used to derive independent aerosol optical properties,following the AERONET procedure.

Aerosol Optical Depth (AOD) monthly averages at 500 nm wavelength aregiven in Table 1. Also the total number of days with quality assured measurementshave been specified there.

Highest values of AOD are obtained in June 2008 and August 2007; AOD

averages remain below 0.2 during months with a lot of rain (November, January).The highest aerosol concentrations coincide with influence from long rangetransport (Saharan dust or biomass burning) (table 2) as is going to be explainedfurther and is consistent with other studies [9]. Analyzing the yearly evolution of440-870 Angstrom coefficient- we depicted several days with values below1(Table 2). The magnitude of the Angstrom exponent is determined by the fractionratio of fine and coarse modes. If the coarse mode is predominant, the Angstromexponent is less than 1, and vice-versa [12].

In this study we also observed several instances during which aerosolconcentrations were exceptionally low related to monthly averages and otherstudies (Table 3). [13]

The morningafternoon average time series of the aerosol optical depth at all

wavelengths measured during June 2008 at Magurele is presented in Fig. 1. Eachdata point has an upper limit uncertainty of 0.025 [6]. June 26th, 2008 has valueswell over the monthly average. June 14 th, 2008 is a day with an average AOD veryclose to the monthly average. (These two AOD values at 550nm are marked witharrows in Fig. 1).

Three-dimensional back trajectories were calculated with the NOAA HYbridSingle-Particle Lagrangian Integrated Trajectory Model (HYSPLIT Model) [14] toanalyze the long range transport influence on local atmosphere. Also we run theDREAM model for dust loading prognosis over Europe(http://www.bsc.es/projects/earthscience/DREAM/). Aerosol types in table 2 havebeen decided after confirmations from HYSPLIT, DREAM and online fire maps

composites of MODIS (http://rapidfire.sci.gsfc.nasa.gov/).

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Table 1The monthly average time series of the aerosol optical depth at 500nm wavelength

and 440-870 Angstrom coefficient- measured by a sunphotometer at Magureleduring July 2007-June 2008

Jun

2007

Aug

2007

Sept

2007

Oct

2007

Nov

2007

Dec

2007

Jan

2008

Feb

2008

Mar

2008

Apr

2008

May

2008

Jun

2008

no.of

days24 24 13 15 14 5 9 4 13 16 25 26

AOD 0.272 0.356 0.289 0.297 0.115 0.307 0.144 0.333 0.145 0.220 0.312 0.304

1.359 1.501 1.427 1.362 1.659 1.527 1.527 1.630 1.458 1.163 1.307 1.545

Table 2Selected cases daily average of the aerosol optical depth (AOD) at 500nm

wavelength, 440-870 Angstrom coefficient- and fine mode measured bysunphotometer at Magurele

24.July07.

22.April08

20.May08

26.June08

1Aug 07 21.Aug 08 12-14.June08

AOD 0.3 0.465 0.65 0.551 0.514 0.493 0.3

0.632 0.541 0.5 0.886 1.652 1.636 1.4derivedfine mode

0.3 0.3 0.16 0.435 0.444 0.426 0.27

aerosoltype

dust dust dust biomassburning

Biomassburningand dust

Biomassburningand dust

urban-industrial

3.1 Aerosol characteristics and sources

a) Biomass influence

June 2008 is the month with the largest number of daily measurements. June1st and 7th have been discarded during the cloud screening process but June 26 thremained with the highest AOD value (Fig. 1). Looking at the hourlymeasurements made during this day a sharp increased in the AOD at allwavelengths have been noted. Also AOD modes for this particular day showed anincrease of the fine mode (Fig.2). Almucantar size distribution (Fig.3), small valuesof Angstrom coefficient (Table 2) and decreasing values of single scattering albedowith increasing wavelength (upper right side of Fig.3) show typical evolution for

biomass burning influence[9][12] Eck et al. [12] have shown how in thewavelength range 380870 nm, SSA can increase by a factor of 25 as wavelengthincreases for biomass burning and urban aerosols, while remaining constant ordecreasing in the presence of mineral dust.Biomass burning smoke is known as an absorbing aerosol with high concentrationof black carbon produced by combustion [12]. We have analyzed the satellitemeasurements, MODIS fire alerts composite (http://rapidfire.sci.gsfc.nasa.gov/)along with HYSPLIT model of air masses trajectories [14] and the air masses havebeen proven to come from a region with dense fires (Ukraine).

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Fig. 1 The morningafternoon average time series of the aerosol optical depth at sevendistinct wavelengths measured by the sunphotometer at Magurele during June 2008

Fig. 2 Derived AOD fine (triangles), coarse modes (crosses) and total (squares)at 500nm

during June, 2008; pronounced increased values of fine mode can be noted on June 26

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Fig. 3 Size distribution almucantar on June 26, 2008; aerosols are characterized by

lognormal distributions, small particles dominating-typical for biomass burning influence; upper

right corner wavelength dependence of single scattering albedo (SSA)

b) Dust intrusion influence

The dust intrusions episodes examined by our team have been confirmed byDREAM model and HYSPLIT backward trajectories. Examples are given inFigs.6-9. All depicted events were associated with marked increases in aerosol

optical depth at all wavelengths. Thus AOD (500 nm) increased from a value of~0.3 corresponding to non-polluted conditions over the site, up to 0.8 in an eventduring 22-24 of July, up to 0.65 in an event on 20 th May 2008 and 0.465 on April22, 2008. The Angstrm exponent reached a minimum of 0.5 in the May 20 th2008 event and was below 1 for the other events (Table 2). Increasing values ofsingle scattering albedo with increasing wavelength were noted on all dustepisodes. Examples are given in the upper right panel of Fig .4 for May 20 th, 2008event and Fig.5 for April 22nd, 2008.

The aerosol size distributions, retrieved from aerosol optical depth usingKing's method [5], demonstrated how the large size fraction of aerosol associatedwith Saharan dust dominated during these events. When Saharan dust was present,the retrieved aerosol size distributions were bimodal with a well-defined mode

centered at a radius of 0.8m, and showed an evident increase in the large particlesmode with radii in the range 0.910m (figs.4 and 5). The small particleconcentration during the dust events did not present any marked change, and wassimilar to those observed on days without Saharan dust (Table 2).

The Angstrm exponent and aerosol optical depth values during Saharanintrusions agree well with those obtained during the same kind of events overAERONET sites[6] [12] [15].

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Fig. 4 Aerosol size distribution derived from sun photometer data on May 20, 2008

showing a typical desert dust size distribution; upper right corner wavelength dependence of

single scattering albedo (SSA)

Fig.5 Similar as in Fig.4 but for April 2, 2008

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Fig. 6 Air masses back trajectories

arriving over Magurele site on April 22,2008 at

1500-3000m show their sources north Sahara

Fig.7 Air masses back trajectories

arriving over Magurele site on August 11th,

2007, at 1000-3000m show their sources from

north Sahara and Ukraine

c) Mixed influence from long range transport

During August 2007 there are 2 periods with high values of AOD and finemode particle concentrations but values of Angstrom coefficient > 1.6: August 10-

12 and August 21-22 (Table 2). DREAM model predicted intensive Saharan dustintrusions for both periods (Fig.8 shows August 11th, 2007) but during summertime there are a lot of fires in Ukraine, Russia and also Greece, as can be observedfrom the ten days composite MODIS fire map available online at:https://reader009.{domain}/reader009/html5/0527/5b0a401d704d8/5b0a4032bd466.jpg.For August 11th, 2007 Hysplit backward trajectories showed air masses at 1kmaltitude arriving from Ukraine. Upper air (3 Km altitude) travelled from Sahara,over fires in Greece, then over Black Sea and finally reached Magurele (Fig.7).High water vapor values are characterizing both periods in August (3.106 cm and3.101 cm respectively, almost double than monthly average), consistent with the airmasses trajectories coming from over the sea. Size distribution retrieved duringAugust 11th, 2007 showed two different type of representation one with dominanceof small particles (influence from biomass burning influence) and the other onewith large particle dominance (influence of dust) (Fig. 11) emphasizing theexistence of two different types of aerosol over Magurele.

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Fig.8 DREAM forecast, dust loading predicted for

August 11, 2007, showing a dust intrusion over

Romania from Sahara;

Fig.9 DREAM forecast, dust loading

predicted for April 22, 2008 showing a

strong intrusion over Romania of dust from

Sahara; the arrows indicate the wind at

3km altitude

Fig. 10 Size distribution almucantar on August 11th, 2007; upper left corner wavelength

dependence of single scattering albedo (SSA)

d) Local pollution

The fine fraction dominates the size distribution for the whole year. Anexample is given in figure 2, daily averages of fine and coarse modes for June 2008proving high influence of local pollution of anthropogenic sulfate as is highlightedin the detailed analysis of Eck et al.[12].

The aerosol absorption for a typical local polluted atmosphere is comparablewith the one in suburb Paris [6]. Single scattering albedo at 550nm,SSA(550)=0.93 showing intermediate absorbing aerosol as is in Table 1 and figure

one of the study by Dubrovnik et.al.[6]. Unfortunately there have not yet been

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reported any direct measurements of the single scattering albedo in Bucharestregion. Particle size distribution of aerosol over Magurele (fig.12) is similar to the

one reported for the climatology of Creteil-Paris [6] and the total volume of fine-mode particles are clearly larger than the total volume of coarse mode particle andthis results in SSA( ) decreasing with increasing (fig.11 right panel). For June12, 2008 there was no dust intrusion prognosis by DREAM.

Fig. 11 Aerosol size distribution derived from sun photometer data on June 12, 2008 showing a

typical size distribution for urban industrial aerosol load; upper right corner wavelength dependenceof single scattering albedo

e) Low AOD cases

From the data analysis of sunphotometer measurements we have noticed few caseswith very low AOD and fine mode particles values related to monthly averages(Table 3). By analyzing air masses trajectories using HYSPLIT we can confirmthat during these cases the overhead cold, very clean air was originating fromArctic regions. An example for November 27th, 2007 is given in Fig.12. Large andsmall particle are comparable represented with a minor presence in the middle sizerange (Fig.13). AOD values of 0.05-0.07 and single scattering albedo at 500nmabout 0.75 are consistent with characteristic values of clean Arctic air [13] [16].

Table 3Selected cases with daily average of the aerosol optical depth (AOD) at 500nm

wavelength extremely low, 440-870 Angstrom coefficient- and fine modemeasured by sunphotometer at Magurele

16.10.2007 24.10.2007 27.11.2007 28.01.2008 02.03.

2008

AOD 0.050 0.096 0.044 0.068 0.063

1.74 1.884 1.818 1.809 1.374derivedfine mode

0.048 0.091 0.043 0.056 0.053

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Fig. 12 Air masses back trajectories arriving

over Magurele site on November 27th, 2007,

at 1000-3000m show their sources from

Arctic regions

Fig. 13 Size distribution almucantar on November

27th, 2007; upper right corner wavelength

dependence of single scattering albedo (SSA)

4. CONCLUSIONS

We analyzed data of a multiwavelength sun photometer monitoring particleoptical depth from 340nm to 1020nm during daytime. The observations were doneduring June 2007-July 2008 in Magurele, in a suburban area of Bucharest, duringits first year of operation. Good agreement was found between our observationsand previous analysis of sunphotometer data in different locations and atmosphericconditions from AERONET climatological data sets.

Aerosol-parameter frequency distributions reveal the presence of individualmodes that lead to the assumption that moderately absorbing urbanindustrial

aerosols are usually characterizing the atmosphere above the site. Aerosol opticalproperties and the dominating aerosol types are influenced by the long rangetransport. The size distribution retrievals during the depicted desert dust intrusionsepisodes are always bimodal and confirm the domination of large particles incontrast with the ones retrieved when urban aerosol or biomass burning isinfluencing.

Sunphotometer proves to be a very useful tool to distinguish betweendifferent types of aerosol loading in the atmosphere.

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AcknowledgementsThe authors wish to acknowledge DELICE grant contract FP7 REGPOT-

2008-1 Contract no. 229907.This work was supported by a grant from Norway through the NorwegianCo-operation Programme for Economic Growth and Sustainable Development inRomania.

The AERONET database is maintained and made publicly available by B.N.Holben (NASAs Goddard Space Flight Center, Greenbelt, MA) and PhilippeGoloub (Photons, France).

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