Atmosphere Aerosol Properties Measured with AERONET/PHOTONS Sun-Photometer over Kyiv During 2008-2009

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285F. Kogan et al. (eds.), Use of Satellite and In-Situ Data to Improve Sustainability, NATO Science for Peace and Security Series C: Environmental Security,DOI 10.1007/978-90-481-9618-0_32, © Springer Science+Business Media B.V. 2011

Abstract The PHOTONS network, as a part of the AERONET ground network for aerosol remote sensing of Earth’s atmosphere, covers more than 40 sites in Europe, Africa and Asia, providing sun-photometer measurements, calibration and data processing. Within the framework of scientific cooperation between the Lille 1 and the National Taras Shevchenko Kyiv Universities, the CIMEL CE 318-2 sun-photom-eter has been operated at Kyiv from the end of March, 2008. This article describes the AERONET/PHOTONS measuring equipment, procedure, data processing and the preliminary analysis of columnar aerosol properties retrieved during April 2008–March 2009. Spectral aerosol optical thickness (AOT), Angström parameter and precipitable water vapor thickness were measured and analysed.

Keywords Aerosol remote sensing • AERONET/PHOTONS network • Aerosol optical thickness • Angström parameter • Precipitable water vapor

Introduction

In recent years, scientific community, governments and non-government organiza-tions are giving much attention to research of the atmospheric aerosols content, dynamic and physical properties since it is one of the air pollutants that can be potentially hazard for biosphere and also contributor to global climate change (Penner et al. 2001; Forster et al. 2007). Present increase in the amount of aerosols

V. Danylevsky (*), V. Ivchenko, and G. Milinevsky National Taras Shevchenko University of Kyiv, Kyiv, Ukraine e-mail: [email protected]

M. Sosonkin Main Astronomical Observatory of National Academy of Science of Ukraine, Kyiv, Ukraine

P. Goloub, Z. Li and O. Dubovik Université de Lille, France

Atmosphere Aerosol Properties Measured with AERONET/PHOTONS Sun-Photometer over Kyiv During 2008–2009

Vassyl Danylevsky, Vassyl Ivchenko, Gennadi Milinevsky, Michail Sosonkin, Philippe Goloub, Zhengqiang Li, and Oleg Dubovik

Author's personal copy286 V. Danylevsky et al.

in the atmosphere creates negative radiative forcing counteracting to global warming (Forster et al. 2007). Key parameters for determining both direct and indirect radia-tive forcing are: (1) the aerosol optical properties, which vary as a function of a wavelength and relative humidity, (2) the atmospheric loading and geographical distribution of the aerosols, which vary as a function of time, and (3) the aerosol particles sizes, shapes and chemical compositions.

Lack of aerosol temporal and spatial data and insufficient accuracy of aerosol properties determination create some problem for accurate estimation of aerosol radiative forcing (Penner et al. 2001; Forster et al. 2007; Kokhanovsky 2008; Dubovik et al. 2002). It is important to separate radiative forcing created by anthropogenic aerosol contribution from radiative forcing created by the natural aerosol. The atmosphere aerosol particle properties are usually estimated by an inverse problem solution (King et al. 1999; Dubovik et al. 2002; Kokhanovsky 2008). The Earth atmosphere-surface system is characterized by great number of parameters which have to be retrieved simultaneously. The best results are obtained by joint analysis data of space-borne and ground-based remote sensing.

In order to monitor aerosol properties and dynamics at regional and global scales, a network of ground-based sites, equipped with standardized measuring devises was set up. Ground-based network for passive aerosol measurements is the AERONET (AERosol Optical NETwork, http://aeronet.gsfc.nasa.gov/)–established in early 1990 by NASA and Laboratoire d’Optique Atmosphérique (LOA) University Lille 1, the Centre National d’Etudes Spatiales (CNES) and Centre National de la Recherche Scientifique (CNRS) of France (Holben et al. 1998). The AERONET consists of hundreds of automatic sun-photometers. The PHOTONS (PHOtométrie pour le Traitement Opérationnel de Normalisation Satellitaire, http://loaphotons.univ-lille1.fr) is French subdivision of the AERONET, operates about 45 observational sites: about 30 in Europe (France), 10 in Africa and 5 in Asia. They provide sun-photometer measurements, calibration and data processing. But AERONET/PHOTONS sites distributed unevenly, especially in East Europe (in Ukraine particularly). At the end of 2007, following scientific cooperation between LOA, Lille 1 (France) and National Taras Shevchenko (Kyiv, Ukraine) universities, the AERONET/PHOTONS site was set up. This article describes equipment, data reduction procedures and the preliminary analysis of columnar aerosol properties retrieved from Kyiv site.

Instrument and Data

Currently, automatic sun photometers (spectral radiometers) CIMEL CE-318 (http://www.cimel.fr/photo/sunph_us.htm) are used by the AERONET/PHOTONS as the main instrument (Holben et al. 1998). The CE-318 sun tracking photom-eters have been designed and realized to be a very accurate motorized, portable, autonomous (powered by solar battery) and automatic instruments. The most

Author's personal copy287Atmosphere Aerosol Properties Measured with AERONET/PHOTONS

currently widespread models of CIMEL sun-photometers over AERONET/PHOTONS sites are standard СЕ 318-1 and polarized model СЕ 318-2. In order to derive total column aerosols properties, water vapor and ozone, these photom-eters measure solar irradiance, sky radiance (aureole brightness), polarization (if polarized model is used), along the almucantar and vertical of the Sun with cer-tain angle intervals. The photometers have two optical channels with two colli-mators and independent detectors of different sensitivities to measure bright direct sun irradiance and dark sky radiance in the standard wavebands. The pho-tometers equipped with sets of optical filters, which wavelengths were selected to avoid strong influence of gaseous constituents of atmospheric extinction and to take into consideration water vapor absorption. The CE 318-1 model is equipped with filters for wavelengths of 340, 380, 440, 675, 870, 940, 1020 nm, and the CE 318-2 model for wavelengths 440, 675, 870 (2 polarization analyzers added), 940, 1020 nm. Spectral bandwidth of each channel is equal to 10 nm at half maximum. The 940 nm channel is used to determine the water vapor amount in atmosphere column because water vapor has a maximum of absorp-tion at this wavelength. The data are transferred from sun-photometer to the AERONET/PHOTONS data base in two ways: via the special data collection systems on a geostationary satellite, or via Internet. Description of the sun-photometers characteristics is provided by http://www.cimel.fr, http://aeronet.gsfc.nasa.gov/new_web. The polarized CE 318-2 sun-photometer model has been installed at Kyiv site.

The pre-programmed microprocessor control measurement procedure pro-vides several scenarios depending on Sun position on celestial sphere, season and time of the day (Holben et al. 1998). The data obtained from observations are used both for aerosol optical thickness (AOT), water vapor content measurements and for the instrument self-calibration. Following Holben et al. (1998) and Li et al. (2008), calibration techniques is used to convert the instrument outputs to AOT and radiance (W/m2 sr mm). Two types of calibration procedures are used: direct-Sun irradiance and diffuse-sky radiance. Also different techniques are used to calibrate reference (master) and field instruments. Sun-channels of reference sun- photometers are usually calibrated at special high-altitude sites with clear stable atmosphere conditions by the Langley plot method, which uses the Sun as a reference light source. Field instruments are generally calibrated by comparison with the master instrument at low-altitude calibration sites (e.g., Goddard Space Flight Center, USA, and Carpentras, France). Sky-radiance channels are cali-brated in the laboratory by using an integrating sphere or a “vicarious” calibration method (Li et al. 2008). The errors is less than 2% for solar channels and less than 5% for sky-radiance channels. These values correspond to the total uncertainty in AOT from a newly calibrated field instruments under cloud-free conditions typically not more than 0.01 for l ³ 440 nm. The aerosol optical depth is com-puted for three data quality levels: level 1.0 – unscreened data, level 1.5 – screened for cloud contamination, and level 2.0 – cloud-screened and quality-assured data. Level 2.0 data are also corrected after photometer’s yearly recalibration.


Method

For the most reliable Level 2.0 data the following parameters are determined: spectral AOT, Angström exponent and water vapor content (thickness of precipitation water layer) in atmosphere column over observational site (Holben et al. 1998, 2001). AOT is proportional to the number of aerosol particles in sun-photometr’s field of view and is a proper measure of aerosol content in atmosphere over the observational site. Spectral AOT approximation is based on the Beer–Lambert–Bouguer law:

( ) ( ) ( )( )λ λ τ λ= −0 · expE E

where E(l) is spectral solar irradiance measured with the calibrated sun-photometer at the time of observations, E

0(l) is solar irradiance at the top of the atmosphere,

computed using the sun-photometer calibration coefficients, and t(l) is atmosphere optical thickness in the direction of the Sun, it is computed from the equation above. To obtain AOT the optical thickness due to water vapor, Rayleigh scattering and trace gases (O

3, CO

2, NO

2 etc.) must be subtracted from t(l):

( ) ( ) ( ) ( ) ( )λ τ λ τ λ τ λ τ λ −= − − −2water Rayleigh CO

AOT

Water vapor content is a very important factor for deriving AOT. The total column water vapor is derived from three spectral channels: 675, 870 and 940 nm. Firstly atmosphere optical thickness is computed for 675 and 870 nm using Rayleigh optical thickness and AOT only. Then the atmosphere optical thickness for 940 nm is computed extrapolating the data obtained above. Hence, the water vapor optical thickness t

W for 940 nm is found using measured and extrapolated data:

( ) ( ) ( )τ τ τ= −940 940ln ln ln .W measured extrapolated

The total thickness TW

of the precipitable water layer in atmosphere column is determined using equation:

( )1

ln bW

WW

aT

m

− =

τ

where a and b are filter-dependent constants, and mW is water vapor optical air mass.

Angström parameter a is power exponent in equation that is used for calculation of AOT dependence on light wavelength:

( ) αλ λ −= ·AOT B

where B is AOT at l = 1 mm. The parameter a is calculated from data measured at two or more wavelengths, using a least squares fit, as

( )( )( )

λ

λ

=

ln.

ln

d AOT

da


AOT obtained for 440 and 870 nm is used for a calculations, as a rule. Angström parameter determined in this way is the simplest qualitative indicator of aerosol particle size averaged on atmosphere column over observational site because aerosol particles optical properties and, as a consequence, spectral extinction coefficient of aerosol depend on the ratio 2p · a/l, where a is the characteristic size of the particle. The coefficient a increases when the particles’ size decreases. Studies of optic atmosphere properties show the Angström parameter change range from −0.1 (coarse particles with a ~ 1–10 mm) to 2.5 (fine aerosol fraction with a ~ 0.01–0.1 mm). Representative value of a for inland aerosol of various sources is about 1.3, but its peak value for practically molecular atmosphere (with a << l) can reach 4 (Dubovik et al. 2002; Kokhanovsky 2008). But detailed research shows that Angström parameter determined from two spectral channels mentioned above is more sensitive to the ratio V

fine/V

total, where V

fine is the aero-

sol fine particles volume and Vtotal

is the total particles volume, than to effective radius of particles observed in atmosphere column. The dependence of the ratio V

fine/V

total on a for the sun-photometer spectral range can be more precisely

determined from AOT obtained at more than two spectral channels using a sec-ond-order polinomial fit of the logarithm of equation AOT(l) = B · l−a (Schuster et al. 2006).

Special inversion algorithm and software of Version 2 have been developed by AERONET team for aerosol optical and physical properties retrieval (Dubovik and King 2000; Dubovik et al. 2002, and Version 2 Inversion Products/Inversion Product Description at http://aeronet.gsfc.nasa.gov/new_web/publications.html). The software inverts sky radiances simultaneously at all available wavelengths for the complete solar almucantar scenario or principal plane scenario together with AOT measured at the same wavelength. The retrieval accounts for different levels of accuracy in the measurements: the standard deviation for error in AOT is assumed ±0.01, the standard deviation of error in the sky radiance measurements is assumed ±5%, and the standard deviation for error in scattered angles are ±0.1°. The inverse solution is based on a set of assumptions, principal of them are: (1) atmosphere is plane-parallel; (2) aerosol particles are partitioned into two compo-nents: spherical, which is modeled by an ensemble of polydisperse homogeneous spheres, and non-spherical, which is a mixture of polidisperse randomly-oriented homogeneous spheroids; (3) vertical distribution of aerosol is homogeneous in the almucantar inversion and bi-layered in the principal plane inversion; (4) errors of measurements are uncorrelated and log-normally distributed. The output includes both retrieved aerosol parameters (particles size distribution, volume concentration, volume median and effective radii, complex refractive index, partition of spherical/non-spherical particles etc.) and calculated on the basis of retrieved aerosol proper-ties (single scattering albedo, phase function and its asymmetry). The output also provides estimates for both random and possible systematic errors for most of the retrieved characteristics. According to those estimates, 68% confidence intervals are presented for most of retrieved characteristics. The detailed retrieved aerosol properties are used for calculating downward and upward radiant fluxes in broad spectral range and aerosol radiative forcing.


Study Area

The Ukraine’s AERONET/PHOTONS site is located at the Golosiiv forest, 10 km from the center of the Kyiv. The major sources of the anthropogenic atmosphere pollution over Kyiv are fumes from fuel-burning boilers, combustion plants, cars and airplanes. The surrounding landscape enables sun-photometer to scan entire celestial hemisphere. The site has been equipped with the polarized sun-photometer model CIMEL CE-318-2. Level 2.0 data were produced during April 2008–March 2009 and Level 1.5 thereafter. There are other facilities to study aerosol remotely: Sevastopol AERONET site, which is at very southern coast of Ukraine, approxi-mately 700 km from Kyiv; Chişinău (Moldova), about 450 km and Minsk (Belarus), about 550 km.

Results and Discussion

The first year sun-photometer data were used to estimate climatology and micro-physics of aerosol particles over Kyiv. Figure 1 shows variations of the aerosol properties obtained from Level 2.0 data during April 2008–March 2009. As seen, AOT, the Angström parameter and precipitable water layer thickness are changing considerably in the course of the year. The largest AOT and water vapor (WV) were in August–September. The range of Angström parameter indicates that the fine mode aerosol dominates in the Kyive’s atmosphere (Fig. 1b). This is mainly anthro-pogenic aerosol, which is confirmed by other studies (Penner et al. 2001; Forster et al. 2007). But coarse-mode aerosol particles dominate in some days, especially during spring and summer of 2008 (see Fig. 1b).

Table 1 shows the number of days in each month with the measurements of Levels 1.5 and 2.0. As seen, the number of observations in spring and summer is significantly larger than during November–February.

Comparison of the climatology of aerosol optical properties obtained at Kyiv with other AERONET sites is shown in Table 2 (Holben et al. 2001; Dubovik et al. 2002). The AOT over Kyiv during April 2008–March 2009 was rather low compared to other continental sites except for Dalanzadgad presented by desert dust aerosol. The lower limit of the Angström parameter over Kyiv is close to lower limit of a at maritime and desert continental sites. The upper limit of the Angström parameter over Kyiv is closer to sites with urban-industrial and biomass burning aerosols. Table 2 also shows that AOT over Kyiv was lower than over boreal forests, tropical forests and savannas where biomass burning aerosol predominates. Besides, AOT was lower than at urban-industrial regions. The Angström parameter range at Kyiv indicates larger variation probably due to the sizes and, may be, microphysics of aerosol particles.

The AOT and Angström parameter over Kyiv for two days one in summer and one in spring are shown in Fig. 2.


Fig. 1 Dynamics of aerosol columnar climatology parameters (sun-photometer СЕ-318): (а) monthly average АОТ, standard deviations »±0.01; (b) the Angström parameter a (daily average) in the range 440–870 нм; (c) precipitable water layer thickness (daily average)

Table 1 Numbers of days with the measurements, Kyiv

Month, year Level 1.5 Level 2.0

April 2008 9 9May 2008 25 25June 2008 19 19July 2008 22 22August 2008 29 29September 2008 14 14October 2008 18 18November 2008 6 6December 2008 2 2January 2009 3 3February 2009 5 4March 2009 10 9


On May 20 (Fig. 2a and b) coarse aerosol particles predominated relative volume more than 50% over a day (a £ 1). AOT increase was accompanied by a decrease in a, because relative content of coarse aerosol particles was increasing.

The Angström parameter for August 19 values indicate that aerosol fine mode predominated (a » 2), which relative volume was more than 50% over a day, and AOT diminution in the morning with the corresponding a increase (Fig. 2c and d). It indicates that coarse particles number is decreasing.

Table 2 Aerosol climatology properties at Kyiv in comparison with other AERONET sites. The AOT daily average for Kyiv site (column 2), whereas the AOT ranges for other sites include all data obtained at 440 nm (Dubovik et al. 2002) and monthly average at 550 nm (Holben et al. 2001)

Site and time of observations

Range of АОТ for l = 440 nm or l = 500 nm

Mean value of АОТ for l = 440 nm or l = 500 nm

Range of the Angström parameter

Aerosol typell = 440–870 нмKyiv

2008–20090.76 ³ t

440 ³ 0.06 t

440 = 0.23 2.1 ³ a ³ 0.4 –

GSFC, Greenbelt (MD, США) 1993–2000

1.0 ³ t440

³ 0.1 t440

= 0.24 2.5 ³ a ³ 1.2 Urban-industrial and mixed

Crete-Paris, France 1999

0.9 ³ t440

³ 0.1 t440


Mexico City 1999–2000

1.8 ³ t440

³ 0.1 t440


Amazonian forest, Brazil 1993– 1994; Bolivia 1998–1999

3.0 ³ t440

³ 0.1 t440

= 0.74 2.1 ³ a ³ 1.2 Biomass burning

African savanna, Zambia 1995–2000

1.5 ³ t440

³ 0.1 t440


Boreal forest, USA and Canada 1994–1998

2.0 ³ t440

³ 0.1 t440


Dalanzadgad, Mongolia

1997–2000

0.25 ³ t500

³ 0.05 t500

= 0.13 1.94 ³ a ³ 0.61 Desert dust

Lanai, Hawaii 1995–1999

0.12 ³ t500

³ 0.06 t500

= 0.08 0.96 ³ a ³ 0.56 Oceanic

San Nicolas Island, California 1998–2000

0.13 ³ t500

³ 0.04 t500

= 0.08 1.10 ³ a ³ 0.78 Oceanic


Conclusions

For the first time measurements of aerosol properties are performed in the atmosphere column over Kyiv with the sun-photometer CIMEL CE-318-2 (polarized model) starting from the end of March, 2008. The PHOTONS network, as a division of AERONET, is in charge of the Kyiv site providing the sunphotometers calibration and data processing. Version 2 of the AERONET inversion retrievals techniques have been applied to derive atmosphere column of aerosol optical and microphysical prop-erties and aerosol climatology over Kyiv using data of Level 2.0 quality obtained from the Sun direct irradiation and the Sky radiation measurements for a period from April 2008 to March 2009.

Daily means AOT at 440 nm were changed in the range from 0.06 to 0.76 over Kyiv for the period mentioned, and daily means AOT at 675, 870 and 1020 nm have lower values. Maximum of AOT was observed in August–September, and minimum in November. Yearly average AOT at 440 nm over Kyiv is equal to 0.23 and is rather low as compared with some other continental sites except the site of desert dust aero-sol. The precipitable water vapor thickness on atmosphere column over Kyiv had maximum during summer months and was not more than 3 cm, but in winter it could be less than 0.2 cm. Range of the Angström parameter values obtained during of a year for spectral range of 440–870 nm (2.1 ³ a ³ 0.4) is rather wide as compared with some other AERONET sites, but the parameter a vary between approximately 1.5

Fig. 2 Spectral AOT and the Angström parameter variations at Kyiv site during one day in the spring (a, b) and one in the summer (c, d)


and 2.0 during the most part of a year showing that aerosol fine mode predominated in atmosphere over Kyiv. Spectral AOT and the Angström parameter can vary appre-ciably during a day pointing on variations of relative content of the fine or coarse aerosol particles in atmosphere column over Kyiv.

This work was supported by the Ministry of Education and Science of Ukraine, and EGIDE, France.

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Atmosphere Aerosol Properties Measured with AERONET/PHOTONS Sun-Photometer over Kyiv During 2008-2009