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Measurements and Estimation of the Columnar Optical Depth of Tropospheric Aerosols in the UV Spectral Region

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Measurements and Estimation of the Columnar Optical Depth of Tropospheric Aerosols in the UV Spectral Region CORE Metadata, citation and similar papers at core.ac.uk Provided by Directory of Open Access Journals c Annales Geophysicae (2002) 20: 565–574 European Geophysical Society 2002 Annales Geophysicae Measurements and estimation of the columnar optical depth of tropospheric aerosols in the UV spectral region V. E. Cachorro1, R. Vergaz1, M. J. Martin1, A. M. de Frutos1, J. M. Vilaplana2, and B. de la Morena2 1Grupo de Optica Atmosferica-Universidad´ de Valladolid (GOA-UVA), Depto. de Optica y Fisica Aplicada, Depto de Fisica Aplicada I, Valladolid, Spain 2Estacion´ de Sondeos Atmosfericos´ ESAT “El Arenosillo”, INTA, Huelva, Spain Received: 14 June 2001 – Revised: 15 October 2001 – Accepted: 30 October 2001 Abstract. We report values of the columnar tropospheric of measurements of aerosol optical depth (hereafter AOD) aerosol optical depth at UV wavelengths based on experi- and aerosol properties at UV wavelengths (Marenco et al., mental measurements of the direct spectral irradiances car- 1997; Krzyscin and Sylwester, 1998; Kylling et al., 1998; ried out by a commercial spectroradiometer (Li1800 of Licor Erlick and Frederick, 1998) and much of the information has company) covering the range from 300–1100 nm at two sta- been extrapolated from the behaviour in the visible and near tions with different climate characteristics in Spain. The first infrared regions by means of aerosol models and measure- station is located in a rural site in north central Spain with ments (Mayer et al., 1997). In recent years we have found continental climate. The data extend from March to the end more measurements (Meleti and Cappellani, 2000; Reuder of October of 1995. The other station is a coastal site in the and Schwander, 2000; Groebner et al., 2001; Cachorro et al., Gulf of Cadiz´ (southwest Spain) of maritime climate type. 2001) and many works about this subject, also considering This study is mainly focused on the capability of estimating satellite data (Torres et al., 1998; Veefkind and De Leeuw, aerosol optical depth values in the UV region based on the 1998; Krotkov et al., 2000; Torres et al., 2000). In this paper, extracted information in the visible and near infrared ranges. a major focus on the total columnar atmospheric aerosol op- A first method has been used based on the Angstr˚ om¨ turbid- tical depth (tropospheric aerosols) and their characteristics at ity parameters. However, since this method requires detailed UV wavelengths has been carried out in order to assess their spectral information, a second method has also been used, contribution to UV irradiances. based on the correlation between wavelengths. A correla- Due to the great spatial and temporal variability of atmo- tion has been established between the experimental aerosol spheric aerosol, and their importance in the Earth’s radia- optical depth values at 350 nm and 500 nm wavelengths. Al- tion budget and health effects, their monitoring is an essen- though the type of aerosol seems to be the key factor that tial task for climate and environmental studies (NRC, Na- determines the quality of these estimations, the evaluation cional Research Council, 1996; Charlson and Heintzberger, of the associated error is necessary to know the behavior of 1995; WMO 659 Report, 1993). Measurements of the spec- these estimations in each area of study. tral AOD is one of the main procedures to derive microphys- Key words. Atmospheric composition and structure (aero- ical (effective radius, volume, etc.) and radiative columnar sols and particles; transmission and scattering of radiation; characteristics (simple scattering albedo, asymmetry factor, troposphere – composition and chemistry) etc.) of atmospheric aerosols (Cachorro et al., 2000a, b). The AOD at ground level is currently measured at visi- ble and infrared wavelengths by means of commercial pho- tometers (Holben et al., 1998), radiometers (Harrison et al., 1 Introduction 1994; Fuenzalida, 1998), spectroradiometers (Ahern et al., 1991; Vasilyev et al., 1995; Cachorro et al., 1989, 2000a), or There has been an increasing concern over stratospheric different scientific prototypes (Marenco et al., 1997; Russel ozone depletion and the concomitant increase in solar UV-B et al., 1996). The currently available photometers make the radiation during the past few years (Bais et al., 1993; UNEP, measurements at isolated wavelengths by means of a filter 1998). However, the influence of atmospheric aerosols in with a spectral width of 5 to 20 nm, depending on the instru- the UV irradiances is also an increasing concern and is stud- ment. The number and position of the chosen wavelengths ied more in depth. Until now there have been few reports is also very different, but 340 nm, 350 nm and 380 nm can be Correspondence to: V. E. Cachorro used as representative of this spectral region due to the non- ([email protected]) absorption by ozone. The AOD at 350 nm can be considered 566 V.-E. Cachorro et al.: Measurements and estimation of the columnar optical depth as representative of the UV region in the same manner as 500 about 300 spectra on 78 clear sky days. The southwest sta- or 550 nm is taken as the reference wavelength in the visible. tion “El Arenosillo”, belonging to the INTA (Instituto Na- The determination of the AOD for wavelengths lower than cional de Tecnica´ Aeroespacial) is located in the coastal site 350 nm is linked with the ozone content determination (Bais of the Gulf of Cadiz´ in Huelva (37.1◦ N, 6.7◦ W; sea level), et al, 1996; Slusser et al., 1999) and hence the methodology and catalogued as number 213 in the ozone WMO network. is being developed (Groebner et al., 2001). Therefore, this This area is of special interest due to the influence of mar- type of measurements is infrequent (Vergaz et al. 2001). itime aerosols, African desert dust events and local industrial A major effort has already been devoted to the measure- environment (10 km from an important polluted-industrial ments of UV-B (Gardiner et al., 1993, 1995; Bais et al., 1996) area). The measured period extends from August of 1996 to and its modeling (Wang et al., 1994; Mayer et al., 1997; Ren October of 1997, with more than 407 spectra over 80 clear et al., 1999; Diaz et al., 2000) and now the focus is on the sky days. This latter station also provided ozone content aerosol contribution, which seems to play a significant factor determination by means of a Dobson instrument for more once the ozone content determination methodologies are well than 20 years (Morena de la, et al., 1994). Furthermore, a established. Therefore, the importance of the AOD at the UV Brewer #150 UV-spectroradiometer has also been operating wavelengths is clearly manifested in the field of the UV stud- since 1997. ies and their applications (Madronic, 1987; Ruggaber, et al., Details about the instrument characteristics and calibra- 1994; Cotte, et al., 1997; van Weele, 1996). tion of our Li-1800 spectroradiometer have been described Here, in the paper we report values of the AOD at 350 nm elsewhere (LiCor Manual, 1991; Cachorro et al., 1998) but as representative of the UV region, based on solar direct it is described here briefly. The spectroradiometer is based spectral irradiance measurements performed by a spectro- on a single monochromator with a holographic grating of radiometer (covering the range 300–1100 nm) at two sites 800 grooves/mm and a focal length of 150 mm and the detec- in Spain of different climate conditions and over different tor is a silicon photodiode. The collected radiant power must periods. However, the main aim in this study is how the first pass through a filter wheel (composed of 8 filters) be- measurements and modelling of the AOD at visible and how fore it enters the monochromator. One of the filters serves as the near-infrared ranges can be analysed to extract informa- dark reference. For the direct component of solar radiation, tion at these ranges in order to gain available information at a collimator tube was designed with an IFOV (instantaneous ◦ the UV region. What are the uncertainties associated with field-of-view) of 4.3 and was coupled to the Teflon diffuser these procedures and estimations? Note that the most fre- receptor cosine (IFOV=2π sr) of the spectroradiometer. The quent data about aerosol are usually collected in the visi- receptor cosine is mounted at one end of a fiber-optic light- ble and near-infrared regions. The study is mainly based on guide and the other end is connected to the entrance slit the behaviour of the Angstr˚ om¨ turbidity parameters in dif- of the monochromator. We have determined the cosine er- ◦ ferent spectral regions and their capacity to model the AOD ror that is about 5% for angles < 75 , but greater and az- at other wavelength ranges; in this case, how well can the imuthal dependent for higher angles. The spectroradiometer visible-infrared range values predict the values in the UV. covers the spectral range 300–1100 nm, with a nominal 6- However, this method requires a lot of spectral information, nm spectral resolution given by the LiCor company, but the which is not always available. Therefore, we have also used instrument function has been determined in our optic’s lab- another current method based on the existing correlation be- oratory to have 6.24±0.27 nm FWHM (Full-Width-at-Half- tween wavelengths (or channels). We have determined in Maximum). Each spectrum can be recorded with a variable both areas of study the correlation between the AOD at 350 step but 1 nm was used in our measurements. and 500 nm wavelengths and hence the estimated error given The spectroradiometer was calibrated periodically (about by these two approaches.
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