V. Estellés

Columnar aerosol properties in Valencia (Spain) by ground-based Sun photometry

(1) In this paper, we present a climatological study of atmospheric aerosols in coastal eastern Spain, by means of experimental measurements using a Cimel CE318-2 Sun photometer. The aerosol optical depth, Angstrom wavelength exponent, size distribution, complex refractive index, asymmetry parameter, and single scattering albedo have been retrieved from these measurements. The columnar water content, as an important parameter for understanding aerosol growth, has also been retrieved. Statistical results of the annual and seasonal variability analysis, mainly related to the usual summer maximum turbidity found in the Mediterranean and European regions, are also shown. The results are linked to the character of the site, located in an urban environment, near the Mediterranean Sea, and frequently affected by dusty air masses of Saharan origin.

Application of the SKYRAD Improved Langley plot method for the in situ calibration of CIMEL Sun-sky photometers

The in situ procedure for determining the solar calibration constants, originally developed for the PREDE Sun-sky radiometers and based on a modified version of the Langley plot, was applied to a CIMEL instrument located in Valencia, Spain, not integrated into AERONET. Taking into account the different mechanical and electronic characteristics of the two radiometers, the method was adapted to the characteristics of the CIMEL instrument. The iterative procedure for the determination of the solar calibration constants was applied to a 3-year data set. The results were compared with the two sets of experimental calibration constants determined during this period using the standard Langley plot method. The agreement was found to be consistent with the experimental errors, and the method can definitely also be used to determine the solar calibration constant for the CIMEL instrument, improving its calibration. The method can be used provided the radiometer is previously calibrated for diffuse radiance using a standard lamp.

Study of the correlation between columnar aerosol burden, suspended matter at ground and chemical components in a background European environment

Although routinely monitored by ground based air quality networks, the particulate matter distribution could be eventually better described with remote sensing techniques. However, valid relationships between ground level and columnar ground based quantities should be known beforehand. In this study we have performed a comparison between particulate matter measurements at ground level at different cut sizes (10, 2.5 and 1.0 mm), and the aerosol optical depth obtained by means of a ground based sunphotometer during a multiinstrumental field campaign held in El Arenosillo (Huelva, Spain) from 28 June to 4 July 2006. All the PM fractions were very well correlated with AOD with correlation coefficients that ranged from 0.71 to 0.81 for PM10, PM2.5 and PM1. Furthermore, the influence of the mixing layer height in the correlations was explored. The improvement in the correlation when the vertical distribution is taken into account was significant for days with a homogeneous mixing layer. Moreover, the chemical analysis of the individual size fractions allowed us to study the origin of the particulate matter. Secondary components were the most abundant and also well correlated in the three size fractions; but for PM10 fraction, chemical species related to marine origin were best correlated. Finally, we obtained a relationship between MODIS L3 AOD from collection 5.1 and the three PM cut sizes. In spite of being a relatively clean environment, all the techniques were able to capture similar day to day variations during this field campaign.

Altitude‐resolved shortwave and longwave radiative effects of desert dust in the Mediterranean during the GAMARF campaign: Indications of a net daily cooling in the dust layer

Desert dust interacts with shortwave (SW) and longwave (LW) radiation, influencing the Earth radiation budget and the atmospheric vertical structure. Uncertainties on the dust role are large in the LW spectral range, where few measurements are available and the dust optical properties are not well constrained. The first airborne measurements of LW irradiance vertical profiles over the Mediterranean were carried out during the Ground-based and Airborne Measurements of Aerosol Radiative Forcing (GAMARF) campaign, which took place in spring 2008 at the island of Lampedusa. The experiment was aimed at estimating the vertical profiles of the SW and LW aerosol direct radiative forcing (ADRF) and heating rates (AHR), taking advantage of vertically resolved measurements of irradiances, meteorological parameters, and aerosol microphysical and optical properties. Two cases, characterized respectively by the presence of a homogeneous dust layer (3 May, with aerosol optical depth, AOD, at 500 nm of 0.59) and by a low aerosol burden (5 May, with AOD of 0.14), are discussed. A radiative transfer model was initialized with the measured vertical profiles and with different aerosol properties, derived from measurements or from the literature. The simulation of the irradiance vertical profiles, in particular, provides the opportunity to constrain model-derived estimates of the AHR. The measured SW and LW irradiances were reproduced when the model was initialized with the measured aerosol size distributions and refractive indices. For the dust case, the instantaneous (solar zenith angle, SZA, of 55.1°) LW-to-SW ADRF ratio was 23% at the surface and 11% at the top of the atmosphere (TOA), with a more significant LW contribution on a daily basis (52% at the surface and 26% at TOA), indicating a relevant reduction of the SW radiative effects. The AHR profiles followed the aerosol extinction profile, with comparable peaks in the SW (0.72 ± 0.11 K d−1) and in the LW (−0.52 ± 0.12 K d−1) for the considered SZA. On a daily basis, the absolute value of the heating rate was larger in the LW than in the SW, producing a net cooling effect at specific levels. These are quite unexpected results, emphasizing the important role of LW radiation.

Summertime columnar content of atmospheric water vapor from ground-based Sun-sky radiometer measurements through a new in situ procedure

[1] A new in situ technique for the retrieval of atmospheric water vapor content (i.e., precipitable water content) from Sun photometric direct solar irradiance measurements, taken at the 940 nm wavelength during clear‐sky conditions, is presented. The procedure is applied to summer data recorded in 2007, 2008, and 2009 with a Sun‐sky radiometer at the San Pietro Capofiume station in the Po valley, Italy. It is a preliminary development of the retrieval procedure providing the columnar water vapor content from measurements performed with PREDE Sun‐sky radiometers. The technique brings improvement and innovation by retrieving the best values of constants (a and b), characterizing atmospheric water vapor transmittance while reducing simulation errors, and potentially contains information on seasonal changes in vertical profiles of temperature, air pressure, and moisture at measurement sites. Initially, the in situ procedure needs at least 1 week of independent measurements of precipitable water content taken over a range of solar zenith angles simultaneously with radiometric measurements, but it was also tested for cases in which independent measurements are not available. In the latter, the procedure was started using monthly precipitable water content estimates derived from surface observations of relative humidity, pressure, and temperature. Time patterns and absolute values of precipitable water content retrieved using the in situ procedure were in good agreement with Moderate Resolution Imaging Spectroradiometer retrievals and radiosonde measurements, with correlation coefficients of 0.8–0.9 and low percentage median differences of 7%–13%.

Climatology of the Aerosol Extinction-to-Backscatter Ratio from Sun-Photometric Measurements

The elastic lidar equation contains two unknown atmospheric parameters, namely, the particulate optical extinction and backscatter coefficients, which are related through the lidar ratio (i.e., the particulate-extinction-to-backscatter ratio). So far, independent inversion of the lidar signal has been carried out by means of Raman lidars (usually limited to nighttime measurements), high-spectral-resolution lidars, or scanning elastic lidars under the assumption of a homogeneously vertically stratified atmosphere. In this paper, we present a procedure to obtain the lidar ratio at 532 nm by a combined Sun-photometer-aerosol-model inversion, where the viability of the solution is largely reinforced by assimilating categorized air-mass back-trajectory information. Thus, iterative lidar-ratio tuning to reconstruct the Sun-photometric aerosol optical depth (AOD) is additionally constrained by the air-mass back trajectories provided by the hybrid single-particle Lagrangian integrated-trajectory model. The retrieved lidar ratios are validated with inversions of lidar data based on the Klett-Fernald-Sasano algorithm and with the Aerosol Robotic Network (AERONET)-retrieved lidar ratios. The estimated lidar ratios concur with the AERONET-retrieved lidar ratios and with those of the well-known KFS inversion constrained with Sun-photometric AOD values and embedded single-scattering models. The proposed method can be applied to routinely extract climatological values of the lidar ratio using measurements of direct solar irradiance (more numerous than those of sky radiance).

Aerosol size distributions and air mass back trajectories over a Mediterranean coastal site

In this paper we present the results obtained from ground-based measurements using a CIMEL sunphotometer during selected clear days corresponding to the winter and summer periods of 2002 in Valencia, an urban coastal Mediterranean site. From the direct solar extinction data the spectral aerosol optical depth (AOD) has been retrieved. This spectral AOD has been used to obtain the size distribution function from the King inversion algorithm. The results show the great dependence of the optical aerosol characteristics on the dominant winds in this area. This location is subject to winds which have either a land or Mediterranean origin which frequently mask the aerosols coming from the Atlantic or north Africa, respectively.

Ten years of measured UV Index from the Spanish UVB Radiometric Network.

An analysis is made of the UV Index (UVI) obtained from the ultraviolet erythemal solar radiation (UVER) data measured by the Spanish UVB Radiometric Network between the years 2000 and 2009. Previously, the daily UVI has been evaluated using two different criteria: (a) the value corresponding to solar noon; and (b) the daily maximum value. The mean percentage of agreement is 92% if we consider the cases for which the difference is zero or one UVI unit. These results are similar to those obtained in a previous work where only 2 years were analyzed. In all the stations the UVI reaches very high values (8-10) in spring-summer, and the very high and extreme (≥ 11) UVI values are more dependent on the continental effect than on the latitude effect. From the UVI values it is possible to classify the stations into four groups: Coastal stations, Continental stations (more than 200 km from the coast), Southern stations (Coastal stations but with similar values of UVI as the Continental ones due to their low latitude) and Canary Islands stations (1400 km southwest from the Iberian Peninsula thus lower latitude). The monthly mean maximum of UVI is reached in July due to the annual evolution of the total ozone column. This value corresponds, for a skin phototype II, to three times the minimal erythemal dose (MED) in an hour in a Coastal station, 3.5 MEDs in an hour measured in a Continental or Southern station and up five MEDs in an hour in the Izaña station (Canary Islands). The cumulative dose on a horizontal plane over an average year has been calculated for each station. More than 40% of the annual dose is received in summer, about 35% in spring, more than 11% in autumn and less than 10% in winter except for the stations in the Canary Islands where the difference between seasons is less significant.

Measurement and Analysis of Broadband UVB Solar Radiation in Spain

Measurements of broadband UVB irradiance (290–315 nm) at 14 locations in Spain for the period 2000–2009 have been used to generate instantaneous, hourly and daily values of irradiance (W m−2) and radiant exposure (kJ m−2). These measurements, and its statistical indices, have been analyzed. For the UVB irradiance, the values corresponding to July (maximum) and December (minimum) have been analyzed as representative of the year during the whole period for all locations. For the UVB radiant exposure, the temporal evolution of daily values has been evaluated for all locations to estimate an average yearly behavior. The accumulated radiant exposure for an average year has also been studied for each location. Finally, to determine possible trends in the evolution of the UVB levels, the linear regressions for the mean daily values for all locations have been determined.

Diffuse Ultraviolet Erythemal Irradiance on Inclined Planes: A Comparison of Experimental and Modeled Data

Values of measured and modeled diffuse UV erythemal irradiance (UVER) for all sky conditions are compared on planes inclined at 40° and oriented north, south, east and west. The models used for simulating diffuse UVER are of the geometric‐type, mainly the Isotropic, Klucher, Hay, Muneer, Reindl and Schauberger models. To analyze the precision of the models, some statistical estimators were used such as root mean square deviation, mean absolute deviation and mean bias deviation. It was seen that all the analyzed models reproduce adequately the diffuse UVER on the south‐facing plane, with greater discrepancies for the other inclined planes. When the models are applied to cloud‐free conditions, the errors obtained are higher because the anisotropy of the sky dome acquires more importance and the models do not provide the estimation of diffuse UVER accurately.