ONE YEAR OF SUNPHOTOMETER MEASUREMENTS IN ROMANIA *

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1 ONE YEAR OF SUNPHOTOMETER MEASUREMENTS IN ROMANIA * ANCA NEMUC 1, L. BELEGANTE 1, R. RADULESCU 1 1 National Institute of R&D for Optoelectronics P.O.Box MG-5, RO Bucharest- Magurele, Romania, anca@inoe.inoe.ro, belegantelivio@inoe.inoe.ro, razvan@inoe.inoe.ro Received November 4, 2010 Multi-wavelength sunphotometry provides a quantitative index that relates to total suspended aerosol in the atmospheric air column above the observer. In addition, it has the capability of 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 averaged retrievals of AERONET(AErosol RObotic NETwork) sun photometer measurements from July 2007 to June are used to provide preliminary results on the characterization of aerosol properties and changes over south east Romania, near Bucharest, at Magurele (44.35N, 25.05E). It is shown that aerosol optical and microphysical properties and the dominating aerosol types are influenced by the long range transport of Saharan dust and biomass burning. Aerosol-parameter frequency distributions reveal the presence of individual modes that lead to the assumption that moderately absorbing urban industrial aerosols are usually characterizing the atmosphere above Magurele. The reported data agree well with known aerosol information retrieved from climatology of 10 years of observations of other AERONET sites. Key words: remote sensing, sun photometer, aerosols, AOD, AERONET. 1. INTRODUCTION Aerosols are an integral part of the atmospheric hydrological cycle and the atmosphere s radiation budget, with many possible feedback mechanisms that are not yet fully understood. Different aerosol indirect effects and their sign of the net radiative flux change at the top of the atmosphere have the largest source of uncertainty in the climate change scenarios [1], [2]. When is used in conjunction with other aerosol and meteorological measurements, sun photometry has the capability of delineating characteristic features of different air masses and the aerosol sources that affect them [3], [4], [5], [6]. Aerosol concentrations and size distributions can be derived remotely through solar direct beam measurements at a range of wavelengths and zenith angles. The aerosol single scattering albedo can be also retrieved. The amount of light * Paper presented at the Optoelectronic Techniques for Environmental Monitoring (OTEM- 2009), September 30 October 2, 2009, Bucharest, Romania. Rom. Journ. Phys., Vol. 56, Nos. 3 4, P , Bucharest, 2011

2 2 Sunphotometer measurements in Romania 551 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 light extinction from both scattering and absorption. If the single scattering albedo lies below a critical value, the combined aerosol Earth system reflects less energy back to space than the Earth's surface alone, leading to a net warming of the Earth. But this critical single scattering albedo depends strongly on the Earth's local albedo. [1][2]. The AERONET programme maintains a global network of sunphotometers for this purpose [3]. There are ~450 instruments registered in the network and ours is operating in Romania since July 2007 along with other equipments for measurements of optical properties of aerosol [7]. In this paper we present the results related to air column aerosol characteristics from the first year, July 2007 to June, of continuous sun photometer s measurements in Romania, in a sub- urban area. First part is focus on methodology, followed by results presentation focused on case study analysis for different types of aerosols loads in the atmosphere and discussions. The last part is dedicated to conclusions and 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 sky spectral sun photometer. A sensor head fitted with 25 cm collimators is attached to a 40 cm robot base which systematically points the sensor head at the sun according to a preprogrammed routine. The radiometer makes two basic measurements, either direct sun or sky, both within several programmed sequences. The direct sun measurements are made in eight spectral bands requiring approximately 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 a direct drive stepping motor [3]. Optical depth is calculated from spectral extinction of direct beam radiation at each wavelength based on the Beer-Bouguer Law. In addition to the direct solar irradiance 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 and 1020 nm) along the solar principal plane (i.e., at constant azimuth angle, with varied 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. The approach is to acquire aureole and sky radiances observations through a large range of scattering angles from the sun through a constant aerosol profile to retrieve size distribution, phase function and aerosol optical depth. More than eight almucantar sequences are made daily both morning and afternoon.

3 552 Anca Nemuc, L. Belegante, R. Radulescu 3 All data are processed, cloud-screened and quality assured as part of routine data processing [6]. The V2 AERONET retrieval provides wide number of parameters and characteristics that are important for the comprehensive interpretation of the aerosol retrieval. The output includes both retrieved aerosol parameters (i.e., size distribution, complex refractive index and partition of spherical/non-spherical particles) and calculated on the basis of the retrieved aerosol properties (e.g. phase function, single scattering albedo, Angstrom exponent, spectral and broad-band fluxes, etc.). Accurate retrievals of SSA (with accuracies reaching 0.03) can be obtained for high aerosol loadings and for solar zenith angles less than 50 degrees [3, 6]. The volume particle size distribution dv(r)/dlnr is retrieved in 22 logarithmically equidistant bins in the range of sizes 0.05µm r 15 µm. The real n(λ) (1.33 n(λ) 1.6) and imaginary k(λ) parts of the complex refractive index ( k(λ) 0.5) are retrieved for the wavelengths corresponding to sky radiance measurements. In addition to the detailed size distribution, the retrieval provides the standard parameters for total (t), fine (f) and course (c) aerosol modes. The accuracy of the AERONET aerosol optical depth measurements is ~0.01 for the wavelength 0.44 µm and the uncertainty in measured sky radiances due to calibration error is аbоut 5% [6]. The accuracy assessments quality control criteria and 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 to µm. This minimum is used as a separation point between fine and coarse mode particles. Using that separation, the code simulates optical thickness, phase function and single scattering albedo of fine and coarse mode separately [6]. The Angstrom exponent å, represents the slope of the wavelength dependence of the AOD in logarithmic coordinates 0. In the solar spectrum, å is a good indicator of the size of the atmospheric particles determining the AOD: bigger than 1 are mainly determined by fine mode, submicron aerosols, while å less than 1are largely determined by coarse, supermicron particles (e.g. [10]) 3. RESULTS AND DISCUSSIONS First year of measurements of a sun photometer in Romania, 5km away of Bucharest, at Magurele was used to derive independent aerosol optical properties, following the AERONET procedure. Aerosol Optical Depth (AOD) monthly averages at 500 nm wavelength are given in Table 1. Also the total number of days with quality assured measurements have been specified there. Highest values of AOD are obtained in June 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 range transport (Saharan dust or biomass burning) (table 2) as is going to be explained

4 4 Sunphotometer measurements in Romania 553 further and is consistent with other studies 0. Analyzing the yearly evolution of Angstrom coefficient-å we depicted several days with values below 1(Table 2). The magnitude of the Angstrom exponent is determined by the fraction ratio of fine and coarse modes. If the coarse mode is predominant, the Angstrom exponent is less than 1, and vice-versa [12]. In this study we also observed several instances during which aerosol concentrations were exceptionally low related to monthly averages and other studies (Table 3) [13]. The morning afternoon monthly average time series of the aerosol optical depth at all wavelengths measured during June at Magurele is presented in Fig. 1. Each data point has an upper limit uncertainty of June 26 th, has values well over the monthly average. June 14 th, is a day with an average AOD very close to the monthly average. (These two AOD values at 550nm are marked with arrows in Fig. 1). Three-dimensional back trajectories were calculated with the NOAA HYbrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT Model) [14] to analyze the long range transport influence on local atmosphere. Also we run the DREAM model for dust loading prognosis over Europe [15]. Aerosol types in table 2 have been decided after confirmations from HYSPLIT, DREAM and online fire maps composites of MODIS [16]. Table 1 The monthly average time series of the aerosol optical depth at 500nm wavelength and Angstrom coefficient-å measured by a sunphotometer at Magurele during July 2007 June Jun 2007 Aug 2007 Sept 2007 Oct 2007 Nov 2007 Dec 2007 Jan Feb Mar Apr May Jun no.of days AOD å Table 2 Selected cases daily average of the aerosol optical depth (AOD) at 500nm wavelength, Angstrom coefficient-å and fine mode measured by sunphotometer at Magurele 24.July April May June 08 1Aug Aug June 08 AOD å derived fine mode aerosol type dust dust dust biomass burning Biomass burning and dust Biomass burning and dust urbanindustrial

5 554 Anca Nemuc, L. Belegante, R. Radulescu AEROSOL CHARACTERISTICS AND SOURCES a) Biomass influence June is the month with the largest number of daily measurements. June 1 st and 7 th have been discarded during the cloud screening process but June 26 th remained with the highest AOD value (Fig. 1). Looking at the hourly measurements made during this day a sharp increased in the AOD at all wavelengths have been noted. Also AOD modes for this particular day showed an increase of the fine mode (Fig.2). Almucantar size distribution (Fig.3), small values of Angstrom coefficient (Table 2) and decreasing values of single scattering albedo with 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 the wavelength range nm, SSA can increase by a factor of 2 5 as wavelength increases for biomass burning and urban aerosols, while remaining constant or decreasing in the presence of mineral dust. Biomass burning smoke is known as an absorbing aerosol with high concentration of black carbon produced by combustion [12]. We have analyzed the satellite measurements, MODIS fire alerts composite [16] along with HYSPLIT model of air masses trajectories [14] and the air masses have been proven to come from a region with dense fires (Ukraine). Fig. 1 The morning afternoon average time series of the aerosol optical depth at seven distinct wavelengths measured by the sunphotometer at Magurele during June.

6 6 Sunphotometer measurements in Romania 555 Fig. 3 Size distribution almucantar on June 26, ; 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 by DREAM model and HYSPLIT backward trajectories. Examples are given in Figs.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 event during of July, up to 0.65 in an event on 20 th May and on April 22,. The Angström exponent å reached a minimum of 0.5 in the May 20 th event and was below 1 for the other events (Table 2). Increasing values of single scattering albedo with increasing wavelength were noted on all dust episodes. Examples are given in the upper right panel of Fig.4 for May 20 th, event and Fig. 5 for April 22 nd,. The aerosol size distributions, retrieved from aerosol optical depth using King's method [5], demonstrated how the large size fraction of aerosol associated with 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 0.8µm, and showed an evident increase in the large particles mode with radii in the range µm (Figs. 4 and 5). The small particle concentration during the dust events did not present any marked change, and was similar to those observed on days without Saharan dust (Table 2). The Angström exponent å and aerosol optical depth values during Saharan intrusions agree well with those obtained during the same kind of events over AERONET sites [8, 12, 17].

7 556 Anca Nemuc, L. Belegante, R. Radulescu 7 Fig. 4 Aerosol size distribution derived from sun photometer data on May 20, 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,.

8 8 Sunphotometer measurements in Romania 557 Fig. 6 Air masses back trajectories arriving over Magurele site on April 22, at m show their sources north Sahara. Fig. 7 Air masses back trajectories arriving over Magurele site on August 11 th, 2007, at m 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 fine mode particle concentrations but values of Angstrom coefficient bigger than 1.6: August and August (Table 2). DREAM model predicted intensive Saharan dust intrusions for both periods (Fig. 8 shows August 11 th, 2007) but during summer time there are a lot of fires in Ukraine, Russia and also Greece, as can be observed from the ten days composite MODIS fire map available online at: For August 11 th, 2007 Hysplit backward trajectories showed air masses at 1km altitude 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 and cm respectively, almost double than monthly average), consistent with the air masses trajectories coming from over the sea. Size distribution retrieved during August 11th, 2007 showed two different type of representation one with dominance of small particles (influence from biomass burning influence) and the other one with large particle dominance (influence of dust) (Fig. 11) emphasizing the existence of two different types of aerosol over Magurele.

9 558 Anca Nemuc, L. Belegante, R. Radulescu 9 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, 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. An example is given in figure 2, daily averages of fine and coarse modes for June

10 10 Sunphotometer measurements in Romania 559 proving high influence of local pollution of anthropogenic sulfate as is highlighted in the detailed analysis of Eck et al. [12]. The aerosol absorption for a typical local polluted atmosphere is comparable with the one in suburban Paris [8]. Single scattering albedo at 550 nm, SSA(550)=0.93 showing intermediate absorbing aerosol as is in Table 1 and figure one of the study by Dubrovnik et.al. [8]. Unfortunately there have not yet been reported any other measurements of the single scattering albedo in Bucharest region. Particle size distribution of aerosol over Magurele (fig. 12) is similar to the one reported for the climatology of Creteil-Paris [8] and the total volume of finemode particles are clearly larger than the total volume of coarse mode particle and this results in SSA( λ) decreasing with increasing λ (fig.11 right panel). For June 12, there was no dust intrusion prognosis by DREAM [15]. Fig Aerosol size distribution derived from sun photometer data on June 12, showing a typical size distribution for urban industrial aerosol load; upper right corner wavelength dependence of single scattering albedo. e) Low AOD cases From the data analysis of sunphotometer measurements we have noticed few cases with very low AOD and fine mode particles values related to monthly averages (Table 3). By analyzing air masses trajectories using HYSPLIT we can confirm that during these cases the overhead cold, very clean air was originating from Arctic regions. An example for November 27 th, 2007 is given in Fig. 12. Large and small particle are comparable represented with a minor presence in the middle size range (Fig. 13). AOD values of and single scattering albedo at 500 nm about 0.75 are consistent with characteristic values of clean Arctic air [13], [18].

11 560 Anca Nemuc, L. Belegante, R. Radulescu 11 Table 3 Selected cases with daily average of the aerosol optical depth (AOD) at 500nm wavelength extremely low, Angstrom coefficient-å and fine mode measured by sunphotometer at Magurele AOD å derived fine mode Fig. 12 Air masses back trajectories arriving over Magurele site on November 27 th, 2007, at m show their sources from Arctic regions. Fig. 13 Size distribution almucantar on November 27 th, 2007; upper right corner wavelength dependence of single scattering albedo (SSA). 4. CONCLUSIONS We analyzed data of a multiwavelength sun photometer monitoring particle optical depth from 340nm to 1020nm during daytime. The observations were done during June 2007-July in Magurele, in a sub-urban area of Bucharest, during its first year of operation. Good agreement was found between our observations and previous analysis of sunphotometer data in different locations and atmospheric conditions from AERONET climatological data sets.

12 12 Sunphotometer measurements in Romania 561 Sunphotometer proves to be a very useful tool to distinguish between different types of aerosol loading in the atmosphere. Aerosol-parameter frequency distributions reveal the presence of individual modes that lead to the assumption that moderately absorbing urban industrial aerosols are usually characterizing the atmosphere above the site. The fine fraction dominates the size distribution for the whole year. Aerosol optical properties and the dominating aerosol types are influenced by the long range transport. The size distribution retrievals during the depicted desert dust intrusions episodes are always bimodal with a well-defined mode centered at a radius of 0.8µm, and showed an evident increase in the large particles mode with radii in the range µm in contrast with the ones retrieved when urban aerosol or biomass burning is influencing. The fine mode concentration during the dust events have not shown any change being similar to those observed on days without Saharan dust. Biomass burning smoke proven to come from a region with dense fires (Ukraine) has been influencing the local aerosol. Typical almucantar size distribution, small values of Angstrom coefficient and decreasing values of single scattering albedo with increasing wavelength have been calculated from sunphotometer measurements. Acknowledgements. The authors wish to acknowledge DELICE grant contract FP7 REGPOT- -1 Contract no This work was supported by a grant from Norway through the Norwegian Co-operation Programme for Economic Growth and Sustainable Development in Romania. The AERONET database is maintained and made publicly available by B.N. Holben (NASA s Goddard Space Flight Center, Greenbelt, MA) and Philippe Goloub (Photons, France). REFERENCES 1. Rogner H.-H., D. Zhou, R. Bradley. P. Crabbé, O. Edenhofer, B.Hare (Australia), L. Kuijpers, M. Yamaguchi, Introduction. In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, (2007). 2. Le Treut, H., R. Somerville, U. Cubasch, Y. Ding, C. Mauritzen, A. Mokssit, T. Peterson and M. Prather, 2007, Historical Overview of Climate, Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, (2007). 3. Holben B. N., Eck T. F., Slutsker I., AERONET A federated instrument network and data archive for aerosol characterisation, Remote Sens. Environ., 66, 1 16, (1998). 4. Holben, B. N., Tanre, D., Smirnov, A., Eck, T. F., Slutsker, I., An emerging ground-based aerosol climatology: Aerosol optical depth from AERONET, J. Geophys. Res., 106, , (2001).

13 562 Anca Nemuc, L. Belegante, R. Radulescu Dubovik, O. and M. D. King, A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements, J. Geophys. Res., 105, 20,673 20,696, (2000). 6. Dubovik O., A. Smirnov, Holben, M.D. King, Y.J. Kaufman, T.F. Eck and I. Slutsker, Accuracy assessments of aerosol optical properties retrieved from AERONET sun and sky-radiaometric measurements, J. Geophys. Res. 105, pp , (2000). 7. Doina Nicolae, Jeni Vasilescu, Emil Carstea, Kerstin Stebel, Fred Prata, Romanian atmospheric research 3D observatory: synergy of instruments, in review for publication in, Romanian Reports in Physics, (2009). 8. Dubovik O., B. Holben, T.F. Eck, A. Smirnov, Y.J. Kaufman, M.D. King, D. Tanré and I. Slutsker, Variability of absorption and optical properties of key aerosol types observed in worldwide locations, Journal of the Atmospheric Sciences 59, pp , (2002). 9. Gobbi, G. P., Kaufman, Y. J., Koren, I., and Eck, T. F.: Classification of aerosol properties derived from AERONET direct sun data, Atmos. Chem. Phys., 7, , (2007). 10. Angstrom, A., On the atmospheric transmission of sun radiation and on dust in the air, Geografika Ann., 11, , (1929). 11. Kaufman, Y. J., Aerosol optical thickness and atmospheric path radiance, J. Geophys. Res., 98(D2), , (1993). 12. Eck T.F., B.N. Holben, J.S. Reid, O. Dubovik, A. Smirnov, N.T. O Neill, I. Slutsker and S. Kinne, Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosols, Journal of Geophysical Research 104 (D24), pp , (1999). 13. Saha, Auromeet, Mallet, Marc, Roger, Jean Claude, Dubuisson, Philippe, Piazzola, Jacques, Despiau, Serge, One year measurements of aerosol optical properties over an urban coastal site: Effect on local direct radiative forcing, Atmospheric Research () 14. Draxler, R.R., Rolph, G.D., HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) Model Access via NOAA ARL READY Website: NOAA Air Resources Laboratory, Silver Spring, MD.(2003) Formenti P., H. Winkler, P. Fourie, S. Piketh, B. Makgopa, G. Helas and M.O. Andreae, Aerosol optical depth over a remote semi-arid region of South Africa from spectral measurements of the daytime solar extinction and nighttime stellar extinction, Atmospheric Research 62, pp , (2002) 18. O Neill N. T., O. Pancrati, K. Baibakov, E. Eloranta, R. L. Batchelor, J. Freemantle, L. J. B. McArthur, K. Strong, and R. Lindenmaier, Occurrence of weak, submicron, tropospheric aerosol events at high Arctic latitudes, Geophys. Res. Lett., 35, L14814, doi: /gl ()