Volume 117, Issue D3
Aerosol and Clouds
Free Access

Effects of an extreme desert dust event on the spectral ultraviolet irradiance at El Arenosillo (Spain)

M. Antón

M. Antón

Departamento de Física Aplicada, Universidad de Granada, Granada, Spain

Centro Andaluz de Medio Ambiente (CEAMA), Universidad de Granada, Granada, Spain

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M. Sorribas

M. Sorribas

Departamento de Física Aplicada, Universidad de Granada, Granada, Spain

Centro Andaluz de Medio Ambiente (CEAMA), Universidad de Granada, Granada, Spain

Estación de Sondeos Atmosféricos El Arenosillo, Mazagón, Spain

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Y. Bennouna

Y. Bennouna

Grupo de Óptica Atmosférica, Universidad de Valladolid, Valladolid, Spain

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J. M. Vilaplana

J. M. Vilaplana

Estación de Sondeos Atmosféricos El Arenosillo, Mazagón, Spain

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V. E. Cachorro

V. E. Cachorro

Grupo de Óptica Atmosférica, Universidad de Valladolid, Valladolid, Spain

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J. Gröbner

J. Gröbner

Physikalisch-Meteorologisches Observatorium Davos, World Radiation Centre, Davos, Switzerland

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L. Alados-Arboledas

L. Alados-Arboledas

Departamento de Física Aplicada, Universidad de Granada, Granada, Spain

Centro Andaluz de Medio Ambiente (CEAMA), Universidad de Granada, Granada, Spain

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First published: 14 February 2012
Citations: 15

Abstract

[1] This paper analyzes the effects of an extreme Saharan dust event detected on 6 September 2007 on spectral UV irradiance recorded at El Arenosillo, South Spain. The intensity of the extreme event was detected using the aerosol optical depth (AOD) and Angström exponent series obtained by a Cimel Sun photometer operated at the study site in the framework of the Aerosol Robotic Network (AERONET). This Saharan dust event is characterized by its strong intensity, with a mean daily AOD value at 440 nm of 1.35 ± 0.40 (1.76 ± 0.03 around 13:00 UT). Additionally, a moderate decrease (∼15 Dobson units (1 DU = 2.69 × 1016 molecules cm−2)) in the total ozone column was recorded with a Brewer spectrophotometer during this episode. The spectral UV irradiance was measured from the transportable Quality Assurance of Spectral Ultraviolet Measurements in Europe (QASUME) through the development of a transportable unit reference spectroradiometer. The relative decrease of the UV irradiance at 320 nm on 6 September is about 50% (40%) with respect to days with low (moderate) aerosol loads. This attenuation slightly decreases with increasing wavelength above 315 nm. The relative differences between QASUME measurements and the spectral UV irradiance derived from the Ozone Monitoring Instrument (OMI) were calculated for the desert dust episode. This satellite instrument strongly overestimates the ground-based UV data recorded on 6 September, with differences between 138% at 305 nm and 72% at 380 nm. Finally, the aerosol forcing efficiency (AFE) is evaluated for UV-B (290–315 nm), UV-A (315–400 nm), and erythemal UV (290–400 nm, weighted by the CIE spectrum), showing a notable decrease (in absolute value) with increasing solar zenith angles (SZAs). For instance, the AFE values for the harmful UV-B irradiance change from −0.41 W/m2 per unit of AOD at 440 nm for a SZA of 30° to −0.21 W/m2 per unit of AOD for a SZA of 50°.

Key Points

  • The second dust event of major intensity during the last ten years in Spain
  • Decrease of the UV irradiance at 320 nm about 50%
  • UV attenuation decreases with an increase in wavelength between 315 and 400 nm

1. Introduction

[2] Mineral dust particles produced by wind erosion in arid regions like Sahara and Gobi deserts play an important role on Earth's climate system. Dust particles interact with the solar and thermal radiation, modulating the Earth's radiative budget [Tegen et al., 1996]. It is well known that the Sahara desert is the most important source of mineral dust on a global scale [Papayannis et al., 2005; Liu et al., 2008].

[3] Because of its geographical position near the North African continent, the Iberian Peninsula is frequently affected by African air masses transporting mineral dust particles. Saharan dust aerosol properties and their spatial and temporal variability over the Iberian Peninsula have widely been investigated in the last years using active and passive remote sensing techniques [Díaz et al., 2001; Alados-Arboledas et al., 2003, 2008; Lyamani et al., 2004, 2005, 2010; Escudero et al., 2005; Elias et al., 2006; Pérez et al., 2006; Toledano et al., 2007a; Cachorro et al., 2008; Guerrero-Rascado et al., 2008, 2009; Wagner et al., 2009; Córdoba-Jabonero et al., 2011]. These authors have highlighted the importance of the desert dust aerosols that arrive on the Iberian Peninsula and strongly modulate the aerosol climatology and seasonal patterns.

[4] There is currently a growing interest in analyzing the atmospheric factors that affect the ultraviolet (UV) radiation reaching the Earth's surface. This interest is mainly motivated by the potentially harmful effects of this radiation on human health, ecosystems, and photochemical processes and materials [United Nations Environment Programme (UNEP), 2007]. The most important feature of this narrowband radiation is that it is absorbed by stratospheric ozone, modifying its spectral distribution. [Bais et al., 1993]. In addition, aerosols affect the transmission of solar radiation through the atmosphere, with important consequences for the UV radiation reaching the Earth's surface. This influence strongly depends on the load of atmospheric aerosols and their optical properties. The different types of aerosol (e.g., sulfate, dust, organic carbon, black carbon, and sea salt) have different radiative characteristics, mainly related to the absorption efficiency. Thus, the influence of the different aerosol types on the surface UV irradiance is not identical. This was investigated by Balis et al. [2004], who reported that the UV-B irradiances (290–315 nm) reaching the surface may show differences up to 10% for the same aerosol load, which can be attributed to differences in the aerosol type.

[5] Numerous studies have analyzed the changes of surface UV radiation levels associated with biomass burning aerosols [e.g., Mims, 1996; Eck et al., 1998; Kalashnikova et al., 2007; Arola et al., 2007], anthropogenic aerosols in urban locations [e.g., Lorente et al., 1994; Papayannis et al., 1998; Repapis et al., 1998; Latha and Badarinath, 2005; Chubarova, 2008; McKenzie et al., 2008; Kazadzis et al., 2009a; Panicker et al., 2009], and mixed aerosol conditions [Krzyścin and Puchalski, 1998; García et al., 2006; Díaz et al., 2007; Antón et al., 2011]. However, there are only a few works that analyze exclusively the influence of desert dust aerosols on UV irradiance [e.g., di Sarra et al., 2002; Meloni et al., 2003].

[6] The objective of this work is to analyze the influence of an extreme Saharan dust event on spectral UV irradiance recorded at El Arenosillo, in southern Spain. These measurements were taken as part of the third intercomparison campaign of the Regional Brewer Calibration Center for Europe (RBCC-E), which took place at El Arenosillo during the period 3–15 September 2007. This paper focuses on the characterization of the spectral dependence of the UV irradiance during an extreme aerosol event, the comparison between satellite and ground-based spectral UV irradiance, and the evaluation of the radiative forcing efficiency that is due to desert dust aerosols in several UV intervals: UV-B (290–315 nm), UV-A (315–400 nm), and erythemal UV (290–400 nm, weighted by the CIE spectrum). Therefore, this paper will contribute to improving the understanding of the influence of desert dust aerosol on UV irradiance reaching the Earth's surface.

2. Site, Instruments, and Data

[7] The study site is located at the “El Arenosillo” Atmospheric Sounding Station (ESAt-El Arenosillo) in Huelva, in southwestern Spain (37.1°N, 6.7°W, 20 m above sea level (asl)). This station belongs to the Earth Observation, Remote Sensing and Atmosphere Department, National Institute of Aerospace Technology of Spain (INTA). This center participates in the Global Ozone Observing System (GO3OS) of the Global Atmosphere Watch (GAW) program of the World Meteorological Organization (WMO) as station 213. Data gathering and retrieval and reporting procedures at these stations are standardized by the WMO quality assurance procedures. Regarding the atmospheric conditions, El Arenosillo is characterized by very frequent sunny conditions, around 280 cloud-free days per year. Therefore, this location presents suitable conditions for reliable radiometric observations. In addition, the location of this station allows the detection of dust plumes transported northward over the Atlantic and also many of those transported over the Mediterranean, which directly affect eastern Spain [Toledano et al., 2007a; Cachorro et al., 2008; Córdoba-Jabonero et al., 2011].

[8] Aerosol data are recorded from an automatic CIMEL Sun sky photometer that belongs to the RIMA-PHOTONS networks as part of the NASA Aerosol Robotic Network (AERONET) network (http://aeronet.gsfc.nasa.gov). The CIMEL Sun photometer measures direct Sun and sky radiation at four wavelength channels, 440, 670, 870, and 1020 nm (a full width at half maximum of 10 nm for the visible channels) [Holben et al., 1998]. The aerosol optical depth (AOD) and the Angström exponent from the highest-level data provided by AERONET (level 2, cloud screened and quality assured) [Dubovik and King, 2000] were analyzed to characterize the extreme aerosol dust event detected on 6 September 2007 at the El Arenosillo station. Since this work focuses on the UV region, the AOD at 440 nm was used. Furthermore, the spectral dependency of the AOD has been considered through the Angström exponent evaluated in the range 440–870 nm.

[9] The transportable QASUME instrument consists of a Bentham DM-150 double-monochromator spectroradiometer, which is considered to be the European UV irradiance reference [Gröbner et al., 2005]. The QASUME scale represents the average scale in use at 25 independent European laboratories [Gröbner et al., 2006]. Its wavelength range is between 250 and 500 nm, and the entrance and exit slit widths were chosen to yield a near-triangular slit function with a full width at half maximum resolution of about 0.8 nm [Gröbner et al., 2005]. The uncertainty of the transportable QASUME reference spectroradiometer was estimated at less than 5% for the wavelength range 310–400 nm and for a solar zenith angle (SZA) equal to or smaller than 75°. The contribution of the angular response to this uncertainty is 0.4% at 300 nm and 0.8% at 400 nm [Gröbner et al., 2005].

[10] Total ozone measurements were provided by a Brewer spectrophotometer. This instrument measures the total ozone column based on the ratio of direct sunlight intensities recorded at four wavelengths between 306 and 320 nm with a resolution of 0.6 nm [Kerr, 2002]. The Brewer instrument located at El Arenosillo belongs to the Spanish Brewer Network. All instruments of this network are biannually calibrated by comparison with the traveling references, Brewer 017 belonging to the International Ozone Services and Brewer 185 from the Regional Brewer Calibration Centre–Europe (RBCC-E) [Redondas et al., 2002, 2008].

3. Results and Discussion

3.1. Extreme Aerosol Dust Event

[11] Figure 1 shows the evolution of columnar aerosol optical properties given by the AOD at 440 nm and the Angström exponent (AE) (calculated as a fit of the log-log plot of AOD versus wavelengths using the three standard wavelengths (440,670,870 nm) of the AERONET) for the period 26 August to 19 September 2007 at the El Arenosillo station. This period includes one of the most intense desert dust episodes recorded at this station (pure desert dust particles on 5, 6, and 7 September) and also shows low AOD values (about 0.1 on 29 August) that are representative of summer background low-turbidity conditions given by clear marine air masses. On the days from 30 August to 4 September, moderate AOD values (between 0.2 and 0.3) and medium values of the Angström exponent (between 1.1 and 1.4) were registered. These values correspond to atmospheric conditions of low-moderate summer turbidity conditions (with the exception of 1 September) related to the predominance of clean marine aerosols mixed with continental and local aerosols, typical of a coastal site like El Arenosillo [Toledano et al., 2007b]. We must emphasize that marine coastal aerosols in El Arenosillo are represented not only by low AE values, characteristic of marine aerosol, but also by high values of the AE, which are very frequent in summer because of the mixing with aged air masses favored by atmospheric recirculations. Furthermore, according to Bennouna et al. [2011], we must bear in mind that the mean value of the AOD (440 nm) at this station for the period 2000–2008 is 0.16 ± 0.12, and the monthly mean values during the summer months (June–September) are between 0.18 and 0.20. The corresponding threshold value of the AOD for a day to be considered as predominantly affected by desert dust is 0.22 [Toledano et al., 2007a].

Details are in the caption following the image
Evolution of the aerosol optical depth (AOD) at 440 nm and Angström exponent calculated in the range 440–870 nm for the period 26 August–19 September 2007 at El Arenosillo.

[12] However, in summer, the conditions are often mixed because of the frequent desert dust intrusions over the Iberian Peninsula, and this tends to increase the AOD values, as shown in Figure 1 from 8 to 18 September. For instance, we can also observe another desert dust intrusions on previous days, from 26 to 28 August. On 5 September, a notable increase in the AOD and a significant decrease in the AE occurred, reaching extreme and unusual values on 6 September. This day shows a sharp increase (0.6–1.8) in the AOD and a decrease (0.4–0.1) in the AE with respect to the previous days, achieving the maximum and minimum values, respectively, around 13:00 UT and giving a clear signature of coarse dust particles in the atmospheric column [Lyamani et al., 2005; Cachorro et al., 2008].

[13] The evolution of these two parameters on 7 September shows a strong decay of AOD values and the enhancement of the AE values, but this is not a clear indication of the end of the Saharan dust outbreak because it may be considered that this intrusion extends as late as 18 September, considering the high and variable AOD values (about 0.25 to 0.8). However, the high AE values indicate that desert dust particles may already be well mixed with local aerosols, marine or continental types, depending on the air mass origin. After this intense episode, the AOD and AE parameters do not reach low-moderate values until 19 September. The values of AOD and AE recorded this day are representative of clear air conditions in this area.

[14] Table 1 shows the mean daily values (±1 standard deviation) of the AOD and the AE for the days 1–8 September. Additionally, this table also shows the mean values of these two parameters around 13:00 UT (±15 min) for each day. This Saharan dust event is characterized by its extreme intensity on 6 September 2007, with a mean daily AOD value of 1.35 ± 0.40 (1.76 ± 0.03 around 13:00 UT). The intensity of this episode is clear in the AOD series at El Arenosillo from February 2000 until March 2010 (AERONET level 2 data), being the second event of major intensity for this period of 10 years. This extreme dust episode was also detected in other locations in southern Spain. For instance, Guerrero-Rascado et al. [2009] showed that this event was the most intense episode in the AOD series obtained by Cimel CE 318–4 at Granada (southeastern Spain) from August 2004 until December 2008. These authors also analyzed vertically resolved measurements from a ground-based Raman lidar, showing that the greatest aerosol load was located between 3 and 4 km asl on 6 September 2007, although aerosol particles were also detected up to 5.5 km asl.

Table 1. Daily Mean Values (±1 Standard Deviation) of AOD (at 440 nm) and Angström Exponent (440–870 nm) Measured From a Cimel, With Those of the Total Ozone Column Recorded by a Brewer Spectrophotometer (Upper Rows)a
AOD at 440 nm Angström exponent Total ozone column (DU)
1 September 0.31 ± 0.04 1.18 ± 0.03 320.9 ± 2.9
0.28 ± 0.01 1.13 ± 0.01 319.2 ± 1.2
2 September 0.23 ± 0.02 1.32 ± 0.07 319.5 ± 1.6
0.27 ± 0.01 1.37 ± 0.01 319.8 ± 2.3
3 September 0.23 ± 0.04 1.02 ± 0.08 319.5 ± 3.2
0.27 ± 0.01 1.01 ± 0.01 318.4 ± 2.4
4 September 0.27 ± 0.02 1.02 ± 0.04 316.6 ± 1.5
0.24 ± 0.02 0.98 ± 0.02 317.4 ± 1.2
5 September 0.57 ± 0.05 0.45 ± 0.05 311.5 ± 1.5
0.61 ± 0.01 0.41 ± 0.01 313.2 ± 0.5
6 September 1.35 ± 0.40 0.19 ± 0.10 304.0 ± 3.7
1.76 ± 0.03 0.14 ± 0.01 304.9 ± 0.6
7 September 0.51 ± 0.14 0.29 ± 0.10 306.8 ± 1.4
0.57 ± 0.01 0.26 ± 0.01 306.4 ± 0.6
8 September 0.34 ± 0.07 1.06 ± 0.15 308.1 ± 3.6
—— —— 305.4 ± 0.4
  • a Mean values of these three variables around 13:00 UT (±15 min) are shown in the lower rows.

3.2. Influence on Spectral UV Irradiance

[15] In order to investigate directly the spectral attenuation of UV irradiance from the extreme aerosol dust event, we use the spectral global UV irradiance ratio (measured by the QASUME spectroradiometer at 0.25 nm intervals from 290 to 400 nm) of the day with the highest aerosol load (6 September 2007) to that of a given day, in the period between 3 September and 7 September This period presents cloud-free conditions. Since the solar irradiance reaching the ground is strongly dependent on the SZA, the spectral UV irradiances were recorded at the same observation time (13:00 UT) with very similar SZA values between 30.2° (3 September) and 31.7° (7 September).

[16] Figure 2 shows the values of the UV irradiance ratios from 290 to 400 nm. It can be seen that the irradiance at 320 nm is reduced by about 50% on the hazy day (6 September) as compared with the days with low aerosol loads (3 and 6 September) and by about 40% relative to the days with moderate aerosol loads (5 and 7 September). These high percentages of reduction in UV irradiance highlight the relevance of the extreme aerosol dust episode analyzed in this work, indicating that desert dust aerosols significantly affect the propagation of UV radiation through the atmosphere. The magnitude of these results is comparable to those reported by Di Sarra et al. [2002]. They showed that the Saharan dust aerosols recorded at Lampedusa produced reductions of the surface UV irradiance at 350 nm up to 30% at a SZA of 20° and up to 60% at a SZA of 60°.

Details are in the caption following the image
The spectral irradiance ratio (measured by the QASUME spectroradiometer from 290 to 400 nm, and simulated by the LibRadtran/UVSPEC model from 440 to 550 nm) of the day with the highest aerosol load (6 September 2007) to a given day, in the period between 3 and 7 September.

[17] From Figure 2, the spectral dependence of the UV irradiance ratios is obvious. Indeed, these ratios increase as a function of wavelength longer than 315 nm, indicating that the influence of the Saharan dust aerosols decrease in a relatively strong way with increasing wavelength in the UV range. This spectral dependence is related to the absorbing properties of the desert dust. Thus, in several experimental studies, this type of aerosol has shown enhanced absorption in the UV compared with that of the visible wavelength range [Mattis et al., 2002; Meloni et al., 2004]. This spectral behavior is certainly due to the spectral dependence of the single scattering albedo (SSA) of desert dust particles, which decreases with decreasing wavelength, contrary to other types of aerosols, as has been clearly demonstrated by the AERONET values [Alados-Arboledas et al., 2008; Cachorro et al., 2010]. Table 2 shows the mean daily values (±1 standard deviation) of the SSA for the days 3–7 September. It can be seen that the SSA experiences a notable decrease with decreasing wavelength on 6 September, varying from 0.97 at 675 nm to 0.92 at 440 nm. This behavior of the mineral dust is also observed for the days with moderate aerosol loads (5 and 7 September). In contrast, the days with low aerosol loads show an opposite pattern; for instance, the SSA increases from 0.92 at 675 nm to 0.94 at 440 on 3 September.

Table 2. Daily Mean Values (±1 Standard Deviation) of the Single Scattering Albedo (SSA)
SSA at 440 nm SSA at 675 nm SSA at 870 nm
3 September 0.94 ± 0.01 0.92 ± 0.03 0.92 ± 0.03
4 September 0.93 ± 0.01 0.92 ± 0.01 0.92 ± 0.01
5 September 0.93 ± 0.02 0.96 ± 0.02 0.97 ± 0.01
6 September 0.92 ± 0.01 0.97 ± 0.01 0.98 ± 0.01
7 September 0.95 ± 0.03 0.97 ± 0.01 0.98 ± 0.01

[18] The spectral global irradiance ratios for the wavelengths between 440 and 550 nm have been derived from the libRadtran radiative transfer model [Mayer and Kylling, 2005] using a pseudo-spherical radiative transfer equation solver (sdisort) running in a 16 stream mode and standard atmosphere midlatitude summer (afglms). The impact of the aerosol load in these simulations was expressed by means of the Ångstrom coefficients (α and β) that were obtained from the AOD measured between 440 and 870 nm. Moreover, the spectral SSA values derived from the lineal interpolation between the experimental SSA values measured at 440 and 675 nm have been introduced in the radiative transfer model. Additionally, the total ozone data measured by the Brewer spectrophotometer are used as input in the simulations. Figure 2 shows the variation of the simulated irradiance ratios with the wavelength. It can be seen that these ratios also present a significant spectral dependence, but with a slope smoother than in the UV range. This result suggests that the spectral dependence of the desert dust absorption is stronger for the shortest wavelengths. The global irradiance in the visible region between 440 and 550 nm is reduced by a percentage of about 25%–30% on the hazy day as compared with the days with low aerosol loads and around 20%–25% relative to the days with moderate aerosol loads. These percentages point out the smaller impact of the dust aerosols in the visible region than in the UV range.

[19] Below 315 nm, the effect of ozone absorption is obvious, and a significant increase of ratios is observed with decreasing wavelength because the total ozone column on 6 September is smaller than the total ozone values for all other days. This effect is not seen in the ratio of day 6 to day 7 because the total ozone column for these two days is very similar, with a slight difference of 1.5 DU around 13:00 UT. Table 1 shows the daily mean values of the total ozone column measured with a Brewer spectrophotometer during the analyzed period at El Arenosillo and the mean values recorded around 13:00 UT (±15 min). A moderate decrease (∼15 DU) in the total ozone column can be seen during this desert dust episode. It is well documented that these events are mainly related to the intrusion of air mass from the subtropical latitudes of the Sahara [e.g., Toledano et al., 2007a], which usually present a content of total ozone lower than south of the Iberian Peninsula. Thus, the horizontal advection of the stratospheric air mass associated with desert dust aerosols in the troposphere replaces lowermost stratospheric air over the study area, producing a decrease in the total ozone column. di Sarra et al. [2002] also showed that the occurrence of large aerosol optical depths was associated with low total ozone values during the intrusion of desert dust aerosols at Lampedusa.

[20] The erythema or sunburn of human skin is the adverse effect most usually related to UV radiation exposure, receiving high attention from society. This effect is commonly quantified by weighing the solar UV radiation (280–400 nm) with the erythemal CIE spectral response, resulting in the so-called UV erythemal irradiance. Thus, it is interesting to quantify the impact of the extreme desert dust event on this irradiance derived from the spectral UV irradiances recorded by the QASUME at 13:00 UT. The values of the UV erythemal irradiance change from 173.3 mW/m2 (4 September) to 89.3 mW/m2 (6 September), which means a reduction of 48%. Therefore, the mineral aerosol load may partially filter out UV erythemal irradiance, leading to a notable decrease in their values reaching the Earth's surface.

[21] The largest relative differences between the surface UV irradiance derived from satellite instruments (e.g., the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI)) and from ground-based measurements are observed in urban polluted areas, where UV-absorbing aerosols play an important role [Kazantzidis et al., 2006; Tanskanen et al., 2007; Ialongo et al., 2008; Weihs et al., 2008; Buchard et al., 2008]. Thus, it is expected that extreme episodes of desert dust aerosols (with significant absorption properties in the UV range) may also cause large differences between the satellite-derived and the measured UV irradiances.

[22] The QASUME measurement nearest to the OMI overpass time is selected every day during the campaign in order to gain the best time coincidence. Figure 3 shows the evolution of the spectral UV irradiance at 310, 324, and 380 nm retrieved from the OMI and the irradiance values measured by the QASUME spectroradiometer between 3 and 7 September. It can be seen that the satellite instruments does not detect the extreme desert dust episode analyzed in this work. Thus, the spectral UV irradiance from the OMI strongly overestimates the ground-based UV data recorded on 6 September with relative differences (OMI-QASUME/QASUME) of 138% at 305 nm (not shown in Figure 3), 98% at 310 nm, 114% at 324 nm, and 72% at 380 nm. These percentages show that the OMI overestimation is significantly higher for the shortest wavelengths. This behavior is related to the spectral influence of the desert dust aerosols on global irradiance, which is more important for the shortest wavelengths. The elevated OMI-QUASUME differences shown in this work are mainly due to the fact that the current OMI UV algorithm assumes no absorbing aerosols in the boundary layer [Tanskanen et al., 2007].

Details are in the caption following the image
Evolution of the spectral UV irradiance at 310 (black), 324 (red), and 380 (blue) nm retrieved from Ozone Monitoring Instrument (OMI) and the corresponding spectral irradiance values measured by the QASUME spectroradiometer between 3 and 7 September.

[23] Antón et al. [2010] showed that the OMI irradiance bias under cloud-free conditions at El Arenosillo varies from +8% to +12% for low aerosol load cases (AOD < 0.1) and from +14% to +19% for moderately high aerosol loads (AOD > 0.25). Thus, to our knowledge, the elevated satellite-ground-based differences obtained in the present work are, so far, the largest mentioned in the literature, pointing out the exceptional intensity of the desert dust episode studied in this work. Several studies [e.g., Krotkov et al., 2005; Arola et al., 2005; Kazadzis et al., 2009b; Ialongo et al., 2010] have suggested off-line corrections for absorbing aerosols if the absorbing aerosol optical thickness (AAOT) or aerosol single scattering albedo (SSA) is known or can be estimated at the site. However, there is, as yet, no standard method for measuring AAOT or SSA in the UV region. Cachorro et al. [2010] analyzed off-line corrections at El Arenosillo, showing that, although they reduce the OMI bias, they do not explain completely the differences between satellite and ground-based UV data, which points out that this problem needs further analysis. Otherwise, new satellite aerosol absorption products from the OMI [Arola et al., 2009] could be used for operational corrections in the future version of the OMI UV algorithm.

3.3. Aerosol Radiative Forcing Efficiency in the UV Range

[24] In this subsection, the aerosol radiative forcing (ARF) per unit of AOD at 440 nm, known as the aerosol forcing efficiency (AFE), has been evaluated during the period 3–7 September 2007 in the following UV intervals: UV-B (290–315 nm), UV-A (315–400 nm), and erythemal UV (290–400 nm, weighted by the CIE spectrum).

[25] A well-known method for calculating the AFE is to obtain the slope of the linear regression between the ARF and the AOD [García et al., 2006; Díaz et al., 2007; Kazadzis et al., 2009a; Antón et al., 2011]. Thus, first it is necessary to obtain the ARF values that have been calculated in this work as a function of the downwelling flux and the surface albedo [Bush and Valero, 2003]:
urn:x-wiley:01480227:media:jgrd17578:jgrd17578-math-0001
where F represents the UV irradiance data recorded from QASUME under cloud-free conditions at El Arenosillo, and F0 corresponds to the UV data for the same SZA and atmospheric conditions but for clear aerosol conditions (cloud-free and clear aerosol conditions) that has been simulated by the libRadtran radiative transfer model [Mayer and Kylling, 2005]. Finally, the UV surface albedo (α) at El Arenosillo was derived from the UV albedo climatology over Europe, which was developed within the context of the European Union's action COST-726, “Long term changes and climatology of UV radiation over Europe” [Litynska et al., 2010].

[26] Figure 4 shows the ARF in the UV-B range (290–315 nm) as a function of the AOD at 440 nm for the period 3–7 September 2007. Simultaneous ARF and AOD values are grouped in two data sets according to their SZA: (30 ± 2.5)° in red and (50 ± 2.5)° in blue. It can be seen that ARF is more negative as the AOD increases, and this relationship presents a strong dependence on the SZA. Thus, for a fixed AOD, the ARF is smaller (in absolute value) for 50° than for 30° SZA because of the longer path through the atmosphere with larger zenith angles, attenuating the direct beam component of the solar irradiance but also increasing the diffuse component that is due to multiple scattering. Thus, the diffuse component is dominant at high SZAs, and, consequently, the measured UV irradiance at the surface becomes less sensitive to changes in the aerosol load.

Details are in the caption following the image
Aerosol radiative forcing in the UV-B range as a function of the AOD at 440 for two data sets corresponding to different SZAs: (30 ± 2.5)° in black and (50 ± 2.5)° in red.

[27] The regression lines were added in Figure 4, with their slopes being equal to the AFE values. From this plot, it is clear that these slopes (AFEs) depend on the SZA for the reasons explained above. Table 3 shows the AFEs obtained over ±2.5° intervals of SZA at 30°, 35°, 40°, 45°, and 50° for UV-A, UV-B, and erythemal UV irradiances. It can be seen that the AFEs decrease (in absolute value) for increasing SZAs for the three UV intervals. This AFE dependence on the SZA agrees with recent results found in the literature for the erythemal UV range [Antón et al., 2011] and for the whole shortwave spectrum of the solar radiation [di Sarra et al., 2008; Di Biagio et al., 2009].

Table 3. Aerosol Radiative Forcing Efficiency (AFE) Obtained Over ±2.5° Intervals of SZAs at 30°, 35°, 40°, 45°, and 50° for the Following UV Ranges: UV-B (290–315 nm), UV-A (315–400 nm), and Erythemal UV (290–400 nm, Weighted by the CIE Spectrum)
SZA (°) AFEUV-A (W/m2 per unit AOD at 440 nm) AFEUV-B (W/m2 per unit AOD at 440 nm) AFEErythemal-UV (W/m2 per unit AOD at 440 nm)
30 −13.8 ± 0.6 −0.41 ± 0.01 −0.052 ± 0.001
35 −12.7 ± 0.4 −0.36 ± 0.01 −0.046 ± 0.001
40 −11.9 ± 0.6 −0.31 ± 0.01 −0.038 ± 0.002
45 −11.2 ± 0.9 −0.26 ± 0.02 −0.033 ± 0.003
50 −10.5 ± 0.7 −0.21 ± 0.01 −0.027 ± 0.002

[28] It is interesting to compare the AFE values shown in this study with the values reported in other works. For example, Antón et al. [2011], using three years of data (2006–2008) from Granada, showed AFE values in the erythemal UV range between (−0.054 ± 0.003) and (−0.030 ± 0.005) W m−2 per unit of AOD at 440 nm when the SZA changes from 30° to 50°. These AFE values are in accordance with our results: (−0.051 ± 0.001) W m−2 per unit of AOD at 440 nm for SZA of 30° and (−0.027 ± 0.002) W m−2 per unit of AOD for SZA of 50° (see Table 2). This excellent agreement can be explained by the characteristics of aerosol properties found over Granada (located in southern Spain), which are also strongly affected by the intrusions of desert dust aerosols [Alados-Arboledas et al., 2003, 2008; Lyamani et al., 2004, 2005, 2010; Guerrero-Rascado et al., 2009].

4. Conclusions

[29] An extraordinary strong desert dust intrusion was coincident with the intercomparison campaign of the Regional Brewer Calibration Center for Europe (RBCC-E) at El Arenosillo, southern Spain, during September 2007. Taking advantage of simultaneous measurements of spectral UV irradiances, total ozone column, and aerosol properties, we have carried out a detailed study dealing with the influence of the extreme values of aerosol properties on the surface UV irradiances.

[30] The attenuation of the UV irradiance at 320 nm on 6 September 2007 (day with an extreme aerosol load) is about 50% relative to days with low aerosol loads. This attenuation slightly decreases with a corresponding increase in wavelength between 315 and 400 nm, since below 315 nm the influence of the ozone changes is important. These results show that the desert dust aerosols notably affect the propagation of the UV radiation through the atmosphere.

[31] The spectral UV irradiance from the OMI satellite strongly overestimates the ground-based UV data recorded on 6 September, with relative differences between 138% at 305 nm and 72% at 380 nm. These large differences indicate that in regions like southern Spain, where the intrusions of the desert dust aerosols are frequent, the effect of these events on the UV irradiance derived from the OMI cannot be neglected.

[32] Finally, the detailed evaluation of the aerosol forcing efficiency shows a notable decrease with increasing SZA for UV-A, UV-B, and UV erythemal ranges. For instance, an increase of one unit in AOD at 440 nm reduces harmful UV-B irradiance by 0.41 W/m2 at an SZA of 30° and by 0.21 W/m2 at an SZA of 50°.

[33] We would like to point out the great influence of desert dust aerosols on UV radiation and argue the need for concurrent measurements of UV radiation and atmospheric aerosols in regions affected by desert dust events.

Acknowledgments

[34] Manuel Antón thanks Ministerio de Ciencia e Innovación and Fondo Social Europeo for the award of a postdoctoral grant (Juan de la Cierva). This work was partially supported by the Andalusian Regional Government through projects P08-RNM-3568 and P10-RNM-6299, the Spanish Ministry of Science and Technology through projects CGL2008-05939-C03-03/CLI, CGL2010–18782, and CSD2007–00067, and by the European Union through the ACTRIS project (EU INFRA-2010-1.1.16–262254).