Routine aerosol measurements have been carried out at INTA-El Arenosillo (Huelva, Spain) since February 2000 with a Cimel sun photometer integrated in the global aerosol monitoring network AERONET. A 5-year aerosol database allows characterization and classification of aerosol properties, which define the local aerosol climatology. In this work two basic parameters for aerosol characterization, aerosol optical depth (AOD) andÅngström exponent, α, are used for aerosol analysis. The mean AOD is 0.18 (±0.14) and the mean α is 1.05 (±0.43). The AOD presents two peaks during the year, at the end of the winter and during the summer, which are related to the seasonal patterns of the desert dust aerosols which arrive at the south-western Iberian Peninsula from North Africa. TheÅngström exponent presents two frequency modes, related to the two main aerosol types present at El Arenosillo: coastal marine aerosols and desert dust. A main aerosol scenario (66% of data) is defined as coastal marine aerosols, with the influence of local sources of continental and polluted aerosols. Continental aerosols are present in 11% of cases, while the desert dust has a very relevant occurrence in around 20% of the data.

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[1] Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and for different time periods. They were examined to estimate ensemble average, long-term trends of the summer background aerosol optical depth AOD(500 nm) in the polar regions (omitting the data influenced by Arctic haze and volcanic eruptions). The trend for the Arctic was estimated to be between À1.6% and À2.0% per year over 30 years, depending on location. No significant trend was observed for Antarctica. The time patterns of AOD(500 nm) and Å ngström's parameters a and b measured with Sun photometers during the last 20 years at various Arctic and Antarctic sites are also presented. They give a measure of the large variations of these parameters due to El Chichon, Pinatubo, and Cerro Hudson volcanic particles, Arctic haze episodes most frequent in winter and spring, and the transport of Asian dust and boreal smokes to the Arctic region. Evidence is also shown of marked differences between the aerosol optical parameters measured at coastal and high-altitude sites in Antarctica. In situ optical and chemical composition parameters of aerosol particles measured at Arctic and Antarctic sites are also examined to achieve more complete information on the multimodal size distribution shape parameters and their radiative properties. A characterization of aerosol radiative parameters is also defined by plotting the daily mean values of a as a function of AOD(500 nm), separately for the two polar regions, allowing the identification of different clusters related to fifteen aerosol classes, for which the spectral values of complex refractive index and single scattering albedo were evaluated. Citation: Tomasi, C., et al. (2007), Aerosols in polar regions: A historical overview based on optical depth and in situ observations,

Routine aerosol measurements are carried out at El Arenosillo station (37.1°N, 6.7°W), in Huelva (Spain), since February 2000 with a Cimel Sun photometer included in the AERONET network. This station, located in the coast of the southwestern Iberian Peninsula, is an appropriate site for the detection and characterization of desert dust aerosols arriving from North Africa. The aerosol database allows us to make an inventory of desert dust events over 6 years corresponding to the period 2000–2005. The inventory is based on threshold values for the aerosol optical depth (AOD) and Ångström exponent, the analysis of the size distributions, a manual inspection, and the use of air mass back trajectories as complementary information. This inventory shows the presence of desert aerosols 65 d yr−1 (18% of days), although there are significant differences among years. Desert dust events have a mean duration of 4 days, with mean AOD (440 nm) of 0.31 and mean Ångström exponent of 0.66. The desert dust episodes are more frequent in February, March, and the summer months. During autumn, winter, and April the episodes are less frequent. This seasonal pattern of the desert events influences the AOD at El Arenosillo, which presents an analogous seasonal pattern. Because of the desert events, AOD levels increase 28% at 440 nm and 48% at 870 nm. Desert dust intrusions have been also catalogued according to four basic meteorological scenarios. The analysis of the desert inventory has made it possible to establish an automatic method for the detection of the desert dust intrusions over the Iberian Peninsula with Sun photometer data. Finally, a classification of air masses (based on back trajectories) has been compared with the occurrence of desert dust intrusions.

The Aerosol Robotic Network (AERONET) site “El Arenosillo,” equipped with a Cimel sun photometer, has been in operation since 2000. The data collected there are analyzed to establish an aerosol synoptic climatological description that is representative of the region. Different air masses and aerosol types are present over the site depending on the synoptic conditions. The frequent intrusion of dust from the Sahara Desert at El Arenosillo suggested the use of back trajectories to determine the airmass origins of other types of aerosol observed there. The focus of this study is to classify the air masses arriving at El Arenosillo by means of back-trajectory analyses and to characterize the aerosol within each type by means of the aerosol optical depth (AOD) and its spectral signature, given as the Ångström exponent (AE). The goal is to determine how aerosols observed over the station (receptor site) differ depending on source region and transport pathways. Two classification methods are used, one based on sectors and a second based on cluster analysis. The period analyzed is from 2000 to 2004. Both methods show that maritime air masses are predominant, occurring 70% of the time and having relatively low AOD (≈0.1 at 440 nm) and a wide range of AE (from about 0 to 2.0). Air masses with continental characteristics are moderately turbid and have values of AE that average ≈1.4. Air masses arriving from the south and southwest show the distinct features of the desert dust, having moderate to high values of AOD (0.30–0.35 at 440 nm) and low values of AE.