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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

    mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://aeronet.gsfc.nasa.gov/http://aeronet.gsfc.nasa.gov/http://aeronet.gsfc.nasa.gov/http://aeronet.gsfc.nasa.gov/mailto:[email protected]:[email protected]:[email protected]
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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.

    http://aeronet.gsfc.nasa.gov/http://aeronet.gsfc.nasa.gov/http://aeronet.gsfc.nasa.gov/http://aeronet.gsfc.nasa.gov/
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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

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