Retrieval of atmospheric optical parameters from ground-based sun-photometer measurements for Zanjan , Iran

We are reporting the results of ground-based spectroradiometric measurements on aerosols and water vapor in the atmosphere of Zanjan for the period of October 2006 to September 2008 using a CIMEL CE318-2 sun-photometer. Zanjan is a city in Northwest Iran, located at 36.70 ◦ N, 48.51 E, and at an altitude of 1800 m a.m.s.l. (above mean sea level). The spectral aerosol optical depth, Ångstr̈om exponent, and columnar water vapor have been calculated using the data recorded by the sun-photometer through the direct measurements on the sun radiance (sun-mode). The average values of aerosol optical depth at 440 nm, columnar water vapor, and the̊ Angstr̈om exponent,α, during the mentioned period are measured as, 0.28 ± 0.14, 0.57± 0.37 cm and 0.73± 0.30, respectively. The maximum (minimum) value of the aerosol optical depth was recorded in May 2007 (November 2007), and that of columnar water vapor, in July 2007 (January 2008). Using the least-squares method, the Ångstr̈om exponent was calculated in the spectral interval 440–870 nm along withα1 andα2, the coefficients of a second order polynomial fit to the plotted logarithm of aerosol optical depth versus the logarithm of wavelength. The coefficient α2 shows that most of the aerosols in the Zanjan area have dimensions larger than 1 micron. The calculated values forα2 −α1 indicate that 80 % of the aerosols are in the coarse-mode ( >1 μm) and 20 % of them are in the finemode (<1 μm). Comparison of α2 −α1 for the atmosphere over Zanjan with other regions indicates dust particles are the most dominant aerosols in the region. Correspondence to: A. Bayat (a bayat@iasbs.ac.ir)


Introduction
Iran, a country in the Middle East, is located within the Earth's dust belt.The Tigris and Euphrate basins in the west, and the Arabian Peninsula in the south are the main dust sources, as frequently observed in these regions (Prospero et al., 2002).Even though other observations show that other sources within the country, such as the Qom lake (a salt-covered playa) in the center (Mortazavi, 2009), or Hamoun Jaz-Murian in the Introduction

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Full south east have some impact on the atmospheric aerosols in Iran (Leon and Legrand, 2003), outside sources are more dominant.
Zanjan is a city in Northwest Iran (36.70 • N, 48.51 • E, 1800 above m.s.l.), located in a mountainous region.It is subject to frequent dust storms especially in mid and late spring as well as late summer and early fall.A free horizon terrace and excellent weather conditions, with 2850 h of sun per year (Samimi et al., 1997), make this area adequate for aerosol monitoring with remote sensing instruments.The aerosol optical depth (AOD), which is the integral of the optical extinction coefficient of the atmospheric aerosols from the surface to the top of the atmosphere, is an important parameter for visibility degradation (due to atmospheric pollution), solar radiation extinction, climate effects, and tropospheric corrections in remote sensing (Dubovik et al., 2002).Here we are reporting the measurements on the aerosol optical depth of the atmosphere of Zanjan using an automatic sun-tracker sun-photometer CIMEL CE318-2.The measurements were carried out in the period between October 2006 and the end of September 2008.
Recorded data on the 936 nm wavelength channel of the sunphotometer (SPM) in combination with the measurements on the 870 nm and 1020 nm wavelength channels provide a measure of the columnar water vapor (CWV) (Halthore et al., 1997).The Ångstr öm exponent, α, is often used as a qualitative indicator of aerosol particle size ( Ångstr öm, 1929).Using the least-squares method, α was calculated in the spectral interval 440-870 nm.We also used a second order polynomial fit to the log-log plot of AOD versus wavelength (Kaskaoutis et al., 2007) to have a more accurate measure of aerosol size distribution in the atmosphere of Zanjan.
In the next parts of this manuscript, after introducing the measurement instrument in Sect.2, the theoretical basis of the work and calculations will be presented in Sect.3. Section 4 contains the results of the measurements, and finally the work is concluded in Sect. 5. Introduction

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Measurements and instrumentation
The SPM records the sun and sky irradiance on the photometer surface at eight wavelength channels, 440, 670, 870, 936, and 1020 nm where the 870 nm channels consist of one non-polarized and three polarized channels to provide the Stokes parameters.These channels are selected wavelengths that have minimal molecular absorption except for the 936 nm channel that matches one of the water vapor absorption lines (Holben et al., 1998).
The SPM is installed at the Institute for Advanced Studies in Basic Sciences, IASBS, (36.70 • N, 48.51 • E, 1800 m above m.s.l.) and from now on we call it the IASBS site.

Aerosol optical depth
Atmospheric transmittance is a function of the attenuation of extra-terrestrial irradiance by scattering and absorption.When the direct beam is measured over a narrow band-pass (strictly, monochromatic radiation) the Beer-Lambert-Bouguer attenuation law holds and the instantaneous, total optical depth for that wavelength, τ λ , can be derived from: Where, I λ is the intensity of the solar irradiance over the SPM, I 0λ is a measure of solar radiation behind the atmosphere that can be found by drawing a Langly plot for the instrument (Holben et al., 1998).R is the Earth-Sun distance in Astronomical Units at the time of observation and m is the relative optical air mass, which is approximated as the secant of the solar-zenith angle (Kasten et al., 1989).The total optical depth, τ λ , is the result of attenuation by molecules (Rayleigh scattering), aerosols (Mie scattering), and absorption by atmospheric gasses like water vapor and other uniformly mixed Introduction

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Full gases.Each of these components can be separated.The Rayleigh optical depth, τ Rayleigh,λ , as a part of τ λ is readily calculated, depending only on the wavelength and barometric pressure at the surface (Holben et al., 1998).The aerosol optical depth, τ Aerosol,λ , can be obtained through: (2)

Water vapor
Extraction of atmospheric columnar water vapor amount from the SPM measurements generally relies on measurements in 936 nm channel of the SPM that coincides with one of the water vapor absorption bands.The aerosol effect is removed by interpolation between the two adjacent bands of the SPM (870 and 1020 nm wavelength channels) (Halthore et al., 1997) and subtracting the calculated τ Aerosol,λ=936nm from the measured τ λ=936nm .In this case, Eq. ( 1) is not valid anymore since the exponential attenuation applies strictly to monochromatic radiation and is invalid across the broad region of the water vapor absorption band.Atmospheric radiation transmission in the water vapor band T w can be modelled as (Halthore et al., 1997), where w is the water vapor vertical column abundance measured in cm and a and b are constants that depend on the wavelength, transmittance spectrum of the SPM filter, and atmospheric conditions (temperature-pressure lapse and the vertical profile of water vapor band).Halthore (1997) showed that for a narrow band (less than 10 nm) filter, sensitivity to the atmosphere can be removed.A modified-Langley method is used for deriving calibration values for the water vapor absorption band around 936 nm (Bruegge et al., 1992).Introduction

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Ångstr öm exponent
Ångstr öm (1929) proposed an empirical formula to approximate the spectral dependence of atmospheric extinction caused by aerosols: where β is the Ångstr öm's turbidity coefficient which is equal to the AOD at λ=1 µm, and α is the widely known Ångstr öm exponent.Although in this definition α and β are wavelength independent, it is well known that both parameters depend on the wavelength.
A more precise empirical relationship between aerosol extinction coefficient and wavelength is obtained by using a second-order polynomial (King et al., 1976), where the coefficient α 2 accounts for a curvature often observed in sun-photometry measurements when τ Aerosol,λ is plotted against the wavelength on a log-log scale.
The curvature can be an indicator of the aerosol particle size.A negative (positive) curvature indicates aerosol size distribution dominated by fine (coarse) mode particles (Eck et al., 2001a,b;Schuster et al., 2006).
In retrieving the data recorded by a SPM, the second-order polynomial fit (Eq.5) is usually applied to the AOD values at three wavelengths 440, 675, and 870 nm.The AOD's at 1020 nm are not included because of possible water vapor absorption effects at that wavelength, resulting in inaccuracies in the computation of the second order polynomial fit and estimation of the values of α 1 and α 2 parameters (Eck et al., 2001a).Schuster et al. (2006) reported that for α 2 −α 1 <1 aerosol size distributions are dominated by the coarse mode (>1 µm) aerosols.The occurrence of α 2 −α 1 >2 represents an aerosol size distributions dominated by fine mode (<1 µm) particles and when 1<α 2 −α 1 <2 most of the aerosols are in the fine mode.Introduction

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Results
In this work, only the SPM data in the sun-mode have been used.In the sun-mode, the SPM looks directly at the sun at a viewing solid angle of 1.2 • and measures the atmospheric optical depth.

Aerosol optical depth climatology
Figure 1 shows the AOD (440 nm) variations for two years of available cloud-free data.
A main feature of the AOD is its high variability.The variability is higher for lower wavelengths.The occurrence of high turbidity events is higher during spring, as can be seen for example in 2007, where consecutive peaks resulted from the considerable number of desert dust events between April and June (Fig. 1).
Once the main features of the individual measurements were observed, daily and monthly averages were calculated in order to get climatological values for the AOD. Figure 2 shows the AOD monthly averages for 440 nm channel during the two years.The variability is indicated by the standard deviation within the month (error bars).
The pattern of low AOD during winter times is clear in Fig. 2, when the minimum value of τ Aerosol,λ =0.15, occurred in January 2007.These low levels of aerosol loading are due to more frequent rains in winter and few occurrences of desert events.On the other hand, during spring and summer the AOD values are above 0.25 and even get close to 0.40 during April and May 2007 and April 2008, with much higher variability indicated by larger error bars.
The turbidity of the atmosphere increases because of the characteristic spring and summer dry seasons in Zanjan and also by most frequent desert dust outbreaks over this area.These events mostly happen every spring and summer, although their number and intensity shows a high variation from year to year.We can observe two AOD maxima during the year: the first one in spring, and the second at the end of the summer (Fig. 2).It can also be noticed that the AOD remains low from October to January with a decreasing pattern after the summer and an increasing one after the winter while

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Full it has almost the same behavior throughout the two years.
The described behavior is more or less the same for all wavelength channels of the SPM (not shown).The general statistics for the whole data series are summarized in Table 1.For the whole data set the average AOD (440 nm) is 0.27 and the standard deviation is 0.16, representing a 50% variation.At 1020 nm the average AOD decreases to 0.15, showing a moderate wavelength dependence of the AOD (Table 1).

Columnar water vapor climatology
The temporal resolution of the recorded data set allows one to study the temporal evolution of the CWV. Figure 3 shows the CWV (936 nm) variation along the two years for cloud-free available data.The results show that the maximum (minimum) CWV occurred in July and August (December, January, and February).The high levels of CWV loading are due to sunshine in the summer.The average CWV is 0.53 cm for the whole period of the data recording.

Aerosol classification, Ångstr öm exponent
Since the spectral shape of the extinction is related to the particle size, the Ångstr öm exponent is commonly used as an indicator of the predominant aerosol size (type) ( Ångstr öm, 1929).A more precise relation between the aerosol extinction and wavelength is a second order polynomial fit (Eq.5) (King et al., 1976).Variations of α 2 and α 2 −α 1 versus AOD are more precise tools for aerosol classification (Kaskaoutis et al., 2007).While the AOD gives information about the aerosol loading, and α 2 and α 2 −α 1 are related to the aerosol size (type), joint analysis of these parameters makes an interpretation of the data possible (Kaskaoutis et al., 2007).The correlation between α 2 and AOD (440 nm) provides information on the atmospheric conditions under which the spectral variation of α 2 is negligible and so the spectral variation of AOD can be accurately described by the simple Ångstr öm formula.
Figure 4 shows the correlation between the coefficient α 2 computed in the spectral Introduction

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Full interval 440-870 nm and the measured AOD (440 nm) for the IASBS site from October 2006 to September 2008.There is a decreasing trend in α 2 with increasing AOD (440 nm).Ångstr öm exponent and α 2 coefficients show that most aerosols in the atmosphere of Zanjan are in the coarse mode.In Fig. 5, α 2 −α 1 values are plotted against AOD (440 nm) for the IASBS site during the mentioned period.Kaskaoutis et al. (2007) reported the variations of α 2 −α 1 versus the AOD for four different AERONET sites, Alta Floresta (Brezil) a rural site directly influenced by biomass burning smoke during the fire season, Ispera (Italy) an urban/industrial area with significant anthropogenic and industrial activities, Nauru a remote island in the tropical Pacific characterized by very small aerosol loading, and Solar Village (Saudi Arabia) a continental remote site with significant contribution of desert dust particles.Our results in Fig. 5 show a considerable similarity with the superposition of reported data from Solar Village and Ispera.
It should be noted that because of the geographical location of Zanjan, the maritime and biomass burning aerosols will not have a considerable impact on its atmosphere.So, one may conclude that dust and anthropogenic aerosols are the main sources of the aerosol in the atmosphere of Zanjan.The calculated values of α 2 −α 1 show that 70% of the aerosols in Zanjan's atmosphere are in the coarse mode and 30% in the fine mode.
It can be seen from Fig. 5 for clean conditions, i.e. small AOD (440 nm) that there is a wide range of α 2 −α 1 values (from −0.13 to 3.07) for all aerosol types.This is an indication of the existence of bimodal-size distributions at relatively low optical depths.

Conclusions
The IASBS site has the only SPM operating in a region covered by Iran and Iraq.The 2year data series presented here allows for the first time a climatological approach in the study of AOD, CWV, and Ångstr öm exponent.The average AOD (440 nm) and CWV for the mentioned period were 0.27, 0.53 cm, respectively.The maximum (minimum) Introduction

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