1. Introduction
The aerosols are atmospheric components with a very important role in establishing the earth’s radiative balance. They act both in a direct way, through the solar radiation scattering and absorption phenomena, and indirectly, through influencing the microphysical and radiative properties of clouds (Lyamani et al. 2010). At the global scale, the main aerosol types originate from natural processes such as dust storms, agricultural activities, biomass burning, and volcano eruptions (Solomon et al. 2007). The anthropogenic aerosol, which is mainly derived from various combustion processes (urban traffic and industrial activity), dominates in densely populated areas, very industrialized zones, and areas where intense biomass burning takes place (Houghton et al. 2001). Examples of aerosol types that strongly scatter and absorb the solar radiation are organic particles, water-soluble inorganic particles (sulfates and nitrates) from biomass/fuel burnings, ammonium from fertilizers, sea salt, dust, etc. (Dubovik et al. 2002; Houghton et al. 2001).
To date, significant uncertainties persist in our understanding of the aerosol effects on climate (Houghton et al. 2001; Solomon et al. 2007). This is a consequence of the complexity of the interaction processes between aerosols and water vapors (Vardavas and Taylor 2007). The high space–time variability and the heterogeneity associated to the short lifetime both contribute to the persistence of those uncertainties (Vardavas and Taylor 2007). It is therefore very important to understand the aerosols effects in the radiative transfer phenomena and to obtain their optical properties with maximum accuracy, both in real time and over the largest possible area of the earth. The optical properties that offer a thorough picture of the aerosol size distribution and mass are the aerosol optical depth (AOD), the Ångström exponent, and the fraction of fine-mode aerosol.
In this paper, the results of the statistical analysis of certain representative optical properties of the atmospheric aerosol from Măgurele (Ilfov, Romania) are presented. The analysis covers the period between 1 June 2008 and 31 December 2009. The nature of the various aerosol sources in Măgurele, which is a suburban area of Bucharest, the capital of Romania, underlines the importance of studying the aerosol properties in this area. The various sources of atmospheric aerosol in the area originate in agricultural parcels, in the primary sources of dust from biomass burning, in concrete mixing units, and in forest vegetation. The placement of the measurement sites is discussed in section 2.
A TSI 3653 integrated nephelometer has been used to measure the aerosol scattering coefficients for three wavelengths (Charlson 2005). The obtained data have been compared to the results provided by a sun photometer placed in the same area. The following optical parameters have been obtained from these measurements: the total scattering and backscattering coefficients, the Ångström parameter, and the AOD. Details of the instruments and the methods used for the measurements are presented in section 3. The temporal, seasonal, and diurnal evolution of the aerosol optical parameters during the considered period is described in section 4. The related conclusions are summarized in the final section of the paper.
2. Obtaining and processing numerical data
a. The aerosol sources in the studied area
The measurements have been performed in a suburban area of Bucharest, in the town of Măgurele, positioned at 44°21′N latitude and 26°2′E longitude. The characterization of aerosol properties in Măgurele is important because this region lies adjacent to both the agriculture region, which is a large dust source, and the southern part of Bucharest, which is the location of power plants that are sources of aerosols and trace gas emissions. In addition, a substantial amount of smoke and pollution are generated locally from the rapid growth of economic activity, with associated increases in fossil fuel combustion.
It was found (Solomon et al. 2007) that up to 50% of the total amount of atmospheric aerosols comes from land processing. The aerosols produced by these sources have diameters within 2–4 μm (Solomon et al. 2007). The forest surrounding the research facilities from the central city of Măgurele is a source of biogenic aerosol. This type of aerosol, which consists of vegetation residue and microbial particles (e.g., pollen, spores, bacteria, fungi, etc.), absorbs the solar radiation mainly in the ultraviolet B (UVB) range of the solar spectrum (Havers et al. 1998). The sizes of biogenic aerosols are usually between 3 and 150 μm (Solomon et al. 2007). In the industrial area on the northeastern part of the town there are other aerosol sources: the waste pit (with particles produced from the biomass and chemical burning) and the concrete mixing unit, which is placed 1 km away from the measurement spot (with particles produced from cement processing activity). Moreover, one should consider the mechanical disintegration and the gas–particle conversion processes (which dominate in the industrial northeastern part of the town) that produce aerosols with diameters larger than 0.1 μm. The gaseous pollution is generated locally due to the rapid development of economic activity, which is associated with the increase in the combustion of fossil fuel (in urban transportation as well as in traffic on the nearby belt road of Bucharest city).
The climate in Măgurele is temperate–continental characterized by the clear differentiation between the four seasons, especially between summer and winter.
b. Description of the instruments used in the measurements
The main equipment used in acquiring data between June 2008 and December 2009 was a TSI 3653 integrated nephelometer. The instrument is placed in the Laboratory of Atmospheric Physics of the Faculty of Physics of the University of Bucharest. The nephelometer inlet is located about 15 m above the ground. The air sample is absorbed through a smooth Teflon tube of 7-m length and approximately 10-cm diameter. We used Teflon as the material for the inlet tube in order to minimize the aerosols losses; Teflon is a material that prohibits an electrostatic charge. The tube’s vent is protected from rain and insect intrusion. The instrument’s extracting-type exhausting turbine absorbs an aerosol sample through the high-pitched duct into the measuring room. The sample is then illuminated with a halogen lamp under incident angles of between 7° and 170°. The dichroic filters placed in the nephelometer cavity select three wavelengths of visible light (450, 550, and 700 nm, each with a bandwidth of 50 nm) from the entire radiation scattered by aerosols. Three total scattering coefficients are thus obtained, corresponding to these exploring wavelengths. The backscattering disk of the instrument serves to integrate the radiation scattered backward in both the 90°–170° and 7°–170° range. In this way, three hemispherical backscattering coefficients are obtained. An automatic internal valve is acting to diversify the aerosol sample with a high-efficiency filter [high-efficiency particulate air (HEPA)]. Thus, the six optical parameters corresponding to the three different wavelengths (viz., the total scattering coefficients and the backscattering coefficients) are continuously mediated, acquired, and saved in the computer.
For scattering coefficients measurement we set up the averaging time to be 1 min and the zero background measure to be 1 h. The pressure and the temperature inside the nephelometer were recorded and monitored continuously. In Fig. 1 we represented the dependence of the backscattering coefficients with relative humidity. As can be noted in Fig. 1, the humidity inside the nephelometer was lower than 50% and there were no major variations of backscattering coefficients with humidity. Targino et al. (2005) wrote that if the relative humidity inside the instrument is lower than 50%, then the air sampling can be considered dry. Anderson and Ogren (1998) also wrote that the scattering properties of aerosols are affected by humidity only if it is larger than 50%.
The backscattering coefficient dependency on relative humidity for Jun 2008–Dec 2009. Diurnal values of backscattering coefficient and relative humidity were used.
Citation: Journal of Atmospheric and Oceanic Technology 28, 10; 10.1175/2011JTECHA1532.1
In conclusion, the measurements were performed with no aerosol heating and no aerosol size cutoff. Also, in order to track the performance of the nephelometer, we performed periodical span gas checks and calibrations using CO2 as the high span gas and filtered air as the low span gas (Anderson and Ogren 1998). The nephelometer calibration results showed the very good behavior of the equipment, with typical values for K2 for all three wavelengths in the range from 2 × 10−3 to 8 × 10−3 (Sheridan and Ogren 2006). The K2 constant is responsive for the quantity of light detected during the nephelometer calibration for each chopper cycle, by the three PMTs. Depending on the chopper shutter optical defects, the values of K2 can vary more or less. The K4 constant, which is related to the fraction of the scattering volume illuminated during the backscatter measurement, had typical values around 0.5.
The columnar AOD was obtained from two CIMEL CE-318 sun photometers placed in Măgurele National Institute of Research and Development for Optoelectronics (INOE) (used from June to December 2008) and Baneasa (used from March to December 2009). This type of instrument has a full-view angle of 1.2° and is equipped with eight interferential filters and a temperature sensor for the temperature correction of the signal for temperature-dependent channels. Both sun photometers are part of the Aerosol Robotic Network (AERONET; Holben et al. 2001) and measure direct sun and diffuse sky radiances in eight spectral channels at 340, 380, 440, 523, 675, 870, 940, and 1020 nm. All of the channels are used to obtain the AOD, minus the spectral channel corresponding to 940 nm, which is used to compute total precipitable water (Lyamani et al. 2006).
The particulate matter (PM)10 mass concentrations were measured using a gravimetric system; the samples were collected for a 24-h period on a glass fiber filter (daily measurements). For filters weighing an analytical balance (with 0.000 01 g precision) was used.
To see the aerosol source regions responsible for the most important aerosol episodes in the study area for the period of June 2008–December 2009, a backward trajectory was calculated using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Hess 1998).
We used the Goddard Earth Sciences (GES) Data and Information Services Center (DISC) Interactive Online Visualization and Analysis Infrastructure (GIOVANNI) application to visualize parameters, such as the aerosol optical depth at 550 nm and the Ångström parameter from satellite measurements for our area of interest. GIOVANNI is a Web-based application developed by the GES DISC (Acker and Leptoukh 2007).
3. The obtained aerosol optical parameters
Aerosol optical thickness (AOT; also called the AOD) is another important parameter that characterizes the atmospheric aerosol on a vertical column. AOT is defined as the measure of radiation extinction resulting from the interaction of radiation with aerosols in the atmosphere, primarily resulting from the processes of scattering and absorption.
4. Results and discussions
a. Seasonal variations of the daily averages of the optical scattering parameters of the aerosols
In a first stage, the seasonal evolution of the daily averages of the aerosol parameters in 2008 and 2009, in the Măgurele area, has been analyzed. The averaging time set for the nephelometer was of 5 min and the dark signal has been measured once every hour. The statistics of the daily averages of the aerosol optical properties during the 2-yr period is presented in Fig. 2. The values of the total scattering coefficients (Fig. 2a) σs for the three wavelengths of the instrument have varied overall with two orders of magnitude. For σs (at 550 nm), during 2009, the range was (5.45; 173.29) × 10−6 m−1, with an average value (
(a) Total scattering and (b) the Ångström parameter statistics for the years 2008 and 2009. The statistical data are represented through the box–whiskers method; the upper and the lower sides of the box represent the maximal and minimal values, respectively. The horizontal line of each box is the median. The extended lines from each end of the box represent the confidence percentages of 5% and 95%, respectively.
Citation: Journal of Atmospheric and Oceanic Technology 28, 10; 10.1175/2011JTECHA1532.1
The monthly averages of the optical parameters obtained for 2008–09 in the Măgurele area: (a) the total scattering coefficient, (b) the backscattering rate 
Citation: Journal of Atmospheric and Oceanic Technology 28, 10; 10.1175/2011JTECHA1532.1
(a) Ångström parameter variation during Jun 2008–Dec 2009 obtained from two methods: using (bottom) sun photometer data and (top) nephelometer data. (b) AOT variation during Jun 2008–Dec 2009 obtained from two types of equipment using sun photometer data.
Citation: Journal of Atmospheric and Oceanic Technology 28, 10; 10.1175/2011JTECHA1532.1
In the spectral range of 450–550 nm, the Ångström parameter mean values for each season were 1.69 ± 0.18 in spring, 1.74 ± 0.12 in summer, 1.69 ± 0.4 in autumn, and 1.39 ± 0.37 in winter (Fig. 2b). During the June–December 2008 interval, when the Ångström parameter was under 1.5, the coarse-mode aerosol is dominant while between March and December 2009 the fine-mode aerosol prevails (Seinfeld and Pandis 1998). The size-increase tendency, over 1 μm, appears during spring and summer, coinciding with the Saharan dust intrusion and biomass burning episodes (Fig. 7). The rather high aerosol concentrations in the winter of 2008 cannot be found again in the winter of 2009 (Fig. 2a). The dominance of the coarse-mode particles during the autumn and winter of 2008 underline the very different characteristics of the two seasons in the consecutive years (Fig. 2b). The highest values for the total backscattering were observed in spring for the wavelength of 450 nm (122.17 ± 51.27) × 10−6 m−1, followed by that for the wavelength of 550 nm (88.45 ± 39.00) × 10−6 m−1 (not shown). The smallest value of the total scattering coefficient was obtained during winter at the same wavelength of 450 nm (45.65 ± 28.97) × 10−6 m−1 (Fig. 2a).
The monthly evolution of the aerosol optical parameters during 2008–09 can be observed in Fig. 3. However, one can get more insight from the analysis of monthly (Fig. 3) averages, which allow conclusions on the aerosol load in the atmosphere. A decreasing tendency of the total scattering coefficient (Fig. 3a) from spring to the next winter was observed and, as expected, the trend for the Ångström exponent increases from spring to the next winter (Fig. 3c). One can also notice a certain periodicity in the monthly evolution of both the total scattering coefficient (Fig. 3a) and the Ångström parameter (Fig. 3c). The period in the variation of the ratio between the backscattering and the total scattering coefficients—the backscattering ratio—is about 1 month, except for the last 3 months of the year (Fig. 3b). Because this ratio gives the fraction of the backscattered energy, it is very useful in radiative transfer computations (Heintzenberg and Charlson 1996), when an account is taken of the angular distribution of the scattered radiation. The backscattering ratio showed very low seasonal variability. The obtained values for spring were 0.14 ± 0.013, for summer were 0.15 ± 0.02, for autumn were 0.15 ± 0.02, and for winter were 0.13 ± 0.02. It can be seen that the dominant size of the aerosol during all of the months was in the range of the fine mode. This can be also seen from Fig. 4b because particles larger than the wavelength are mainly forward scattered.
Figure 4b represents the daily AOD averages variations from June 2008 to December 2009. For 2008, if we compared the ground level measurements (total scattering coefficients) with the columnar sun photometer measurements (AOD), then we can see the opposite value variations. These differences can be related to the vertical variations, meteorological factors, and different aerosol sources on the vertical column.
The monthly averages of AOD are represented in Fig. 3d for 440, 500, and 675 nm. We represented data from June to December 2008 and from March to December 2009. The smallest values were in June 2009 (0.17 ± 0.1) and the highest values for AOD were in December 2009 (0.49 ± 0.27).
b. Diurnal variations of the optical parameters
On the other hand, the diurnal variations of the optical parameters can help us to determine the local and regional aerosol sources. The analysis for the diurnal variation of the total scattering of α parameter shows (Fig. 5) that, for the visible range of the spectrum (550 nm), the total scattering coefficient has a different diurnal variation in each season.
The diurnal variations of (a) the total scattering coefficient of the 550-nm-wavelength radiation (corresponding to the used nephelometer), and (b) the Ångström parameter in 2009 for the four seasons.
Citation: Journal of Atmospheric and Oceanic Technology 28, 10; 10.1175/2011JTECHA1532.1
One can observe the high values during spring and summer after sunrise, and the shift of high values toward noon during autumn and mostly during winter. During spring and summer, the coefficients’ values decrease at noon and then increase again toward their averages at about 1800 local time (LT). The same behavior is observed for the total scattering coefficient during autumn too, but for smaller values than in either spring or summer (Fig. 5a). These maximum values of total scattering coefficients (Fig. 5a) can be associated with intense emissions resulting from the morning and afternoon traffic, when people go to and return from work. The decrease in coefficients values can be also related to the gradual increase in solar radiation and air convection, which decrease the atmospheric loading at the surface (Lyamani et al. 2010). The ratio of the spring–summer maximal values is 1.3, for spring–autumn it is 2.3, and it increases for the spring–winter case to 3.1. This is normal behavior if one takes into account the different meteorological conditions in the four seasons. The rapid sequence of spring circulations and their dominance from the southern part of the continent allow advections of air masses loaded with aerosols. Moreover, human activity grows stronger during spring and injects aerosols in the atmosphere. The vegetation has the same effect, which also begins its vital cycles of growing and blossoming during spring. During summer, the high values of the total scattering coefficient are explained by the advection of tropical air loaded with aerosols. Also in spring and summer, human agricultural activity and thermal convection during the afternoons (resulting from intense sunlight) both lift aerosols from the ground up to quite high levels. In wintertime, the masses of cold air brought from the northern part of the continent are cleaner and the local pollution is much reduced. Moreover, winter precipitation effectively scrubs the atmosphere of its pollutants. These assertions are sustained by observing the diurnal variation of the seasons (Fig. 5).
Regarding the diurnal variation of the Ångström exponent, it may be observed that for the spectral range of 700–550 nm in autumn and winter 2009 the aerosols are from the fine, submicrometer mode, and in spring and summer the coarse-mode (over 1 μm but not extending beyond 10–15 μm) particles dominate (Fig. 5b). During winter 2009, domestic heating using fossil fuel contributes to the increase of the fine-mode aerosols. Independent measurements of the size distribution of sub-10-μm aerosols, performed by the Bucharest Environmental Agency at its Măgurele station, have confirmed the increase of the concentration of fine-mode particles (see Figs. 6a,b for the situation in 2008). The PM10 mass concentrations are lower in autumn and winter than in summer (Fig. 6a). The total scattering coefficient has similar behavior (Fig. 5a). The good correlation (R2 = 0.803) between the total scattering coefficient determined using the nephelometer and the PM10 mass concentrations can be observed in Fig. 6b.
Comparison of the data used in characterizing the size distribution of aerosols obtained with gravimetric systems and with the nephelometer for the Măgurele area during 2008. (a) PM10 frequency ratio during 2008 at 550 nm, and (b) the correlation factor between the total scattering coefficients and PM10 corresponding to 550 nm.
Citation: Journal of Atmospheric and Oceanic Technology 28, 10; 10.1175/2011JTECHA1532.1
5. Summary and conclusions
In this article a temporal analysis of the daily averages of aerosol optical parameters in the Măgurele area during 2008 and 2009 is reported. Significant differences between the results obtained during these 2 yr were observed for the autumn–winter period. For 2009, the daily averages of the optical parameters were computed and variations of two orders of magnitude over the whole year were observed. The maximal values of the total scattering coefficient suggest the abundance of coarse-mode particles larger than 1 μm during the spring–summer period of 2009. Air circulations normally carry the Saharan dust and biomass burning aerosols over the studied area (see Fig. 7). Human activities and the vegetation cycles, which are intensified during those two seasons, contribute to the episodes of atmosphere’s loading with aerosols. The minimal values of the scattering coefficients were obtained during the cold seasons (autumn–winter 2009) and were associated with northern airmass advections. The average values of the Ångström exponent showed an increasing tendency from spring to winter.
Aerosol optical depth at 550 nm from (left) Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data and (right) back trajectories at 100, 1000, and 3500 m on (a) 31 Oct 2008 and (b) 9 Apr 2009 at the Măgurele site.
Citation: Journal of Atmospheric and Oceanic Technology 28, 10; 10.1175/2011JTECHA1532.1
From the seasonal analysis, a certain monthly periodicity of the optical parameters was noticed. The wavelength dependence of the scattering coefficients suggested the presence of dry aerosols and the dominance of fine-mode aerosols extended over the whole year.
The results of the diurnal analysis for Măgurele area showed visible differences between the maximal and the minimal values for the spring–summer seasons. The maximum values occurred between 0600 and 0900 and between 1800 and 2300 LT, while the minima showed up between 1300 and 1600 LT. For colder seasons, these variations were much smaller, with maxima appearing after 0800 LT in winter, between 0800 and 1100 and between 1700 and 2400 LT.
The author Laura Mihai gratefully acknowledges the support of the POSDRU Program of University of Bucharest. The work of Sabina Stefan was supported by the RADO Project, Contract STVES 2008/115266 from Norway’s INNOVATION program.
Airmass backtrajectories were produced with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT-4.6) model (NOAA). Analyses and visualizations used in this paper were produced with the GIOVANNI online data system, which is developed and maintained by the NASA GES DISC. We also acknowledge the MODIS mission scientists and associated NASA personnel for the production of the data used in this research effort.
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