D. Pérez-Ramírez

Extreme Saharan dust event over the southern Iberian Peninsula in september 2007: active and passive remote sensing from surface and satellite

Abstract. This study investigates aerosol optical properties during the extreme Saharan dust event detected from 3 to 7 September 2007 over Granada, southern Iberian Peninsula, with both active and passive remote sensing instrumentation from surface and satellite. The intensity of the event was visualized on the aerosol optical depth series obtained by the sun-photometer Cimel CE 318-4 operated at Granada in the framework of AERONET from August 2004 until December 2008 (level 2 data). A combination of large aerosol optical depth (0.86–1.50) at 500 nm, and reduced Angstrom exponent (0.1–0.25) in the range 440–870 nm, was detected on 6 September during daytime. This Saharan dust event also affected other Iberian Peninsula stations included in AERONET (El Arenosillo and Evora stations), and it was monitored by MODIS instrument on board Aqua satellite. Vertically resolved measurements were performed by a ground-based Raman Lidar and by CALIPSO satellite. During the most intense stage, on 6 September, maximum aerosol backscatter values were a factor of 8 higher than other maxima during this Saharan dust event. Values up to 1.5×10−2 km−1 sr−1 at 355 and 532 nm were detected in the layer with the greatest aerosol load between 3–4 km a.s.l., although aerosol particles were also detected up to 5.5 km a.s.l. In this stage of the event, dust particles at these altitudes showed a backscatter-related Angstrom exponent between –0.44 and 0.53 for the two spectral intervals considered. The results from different measurements (active/passive and ground-based/satellite) reveal the importance of performing multi-instrumental measurements to properly characterize the contribution of different aerosol types from different sources during extreme events. The atmospheric stabilization effect of the aerosol particles has been characterized by computing the solar heating rates using SBDART code.

Analysis of lidar depolarization calibration procedure and application to the atmospheric aerosol characterization

A Raman lidar system is used to monitor the aerosol depolarization features of the urban atmosphere at the Andalusian Centre for Environmental Research (CEAMA), in Granada, southeastern Spain. The lidar system was upgraded in 2010 to enable the application of the ±45° calibration method, which does not require any external optical device. We analyse the method and classify the atmospheric aerosol following the criteria based on depolarization. Backscatter coefficient, backscatter-related Angström exponent (å β), volume linear depolarization ratio (δv), and particle linear depolarization ratio (δp) profiles are studied in Saharan dust and biomass burning smoke events during the summer of 2010. The strength of these events was visualized in the aerosol optical depth (AOD) series obtained by Sun and star photometers operated at CEAMA. During the analysed events, the AOD at 440 nm ranged between 0.2 and 0.3, although the Angström exponent (å AOD) retrieved by the Sun photometer was considerably lower during the Saharan dust event (å AOD = 0.4 ± 0.1) than during the biomass burning event (å AOD = 1.4 ± 0.1). Regarding å β profiles, å β values were similar along the vertical profiles and comparable to å AOD values for each event. In contrast, the particle linear depolarization ratio (δp) at 532 nm showed an opposite behaviour to å β, changing along the vertical profiles. In fact, the aerosol layers located in the free troposphere showed mean values of δp of 0.13 ± 0.08 and 0.03 ± 0.01 in the Saharan dust and biomass burning events, respectively. These results show that the use of depolarization techniques enables an accurate aerosol typing and the understanding of the layers composition in the atmosphere.

Aerosol transport over the western Mediterranean basin: Evidence of the contribution of fine particles to desert dust plumes over Alborán Island

Eight months (June 2011 to January 2012) of aerosol property data were obtained at the remote site of Alboran Island (35.95°N, 3.03°W) in the western Mediterranean basin. The aim of this work is to assess the aerosol properties according to air mass origin and transport over this remote station with a special focus on air mass transport from North Africa. For air masses coming from North Africa, different aerosol properties showed strong contributions from mineral dust lifted from desert areas. Nevertheless, during these desert dust intrusions, some atmospheric aerosol properties are clearly different from pure mineral dust particles. Thus, Angstrom exponent α(440–870) presents larger values than those reported for pure desert dust measured close to dust source regions. These results combine with α(440, 670) − α(670, 870) ≥ 0.1 and low single scattering albedo (ω(λ)) values, especially at the largest wavelengths. Most of the desert dust intrusions over Alboran can be described as a mixture of dust and anthropogenic particles. The analyses support that our results apply to North Africa desert dust air masses transported from different source areas. Therefore, our results indicate a significant contribution of fine absorbing particles during desert dust intrusions over Alboran arriving from different source regions. The aerosol optical depth data retrieved from Sun photometer measurements have been used to check Moderate Resolution Imaging Spectroradiometer retrievals, and they show reasonable agreement, especially for North African air masses.

Application of Sun/star photometry to derive the aerosol optical depth

Atmospheric aerosols play a crucial role in the radiative transfer and chemical processes that control the Earths climate. Aerosol optical depth and other related aerosol characteristics are widely known during daytime through Sun photometers, and so several international networks have been established. However, there are no regular measurements of the spectral optical depth of the atmosphere at night. Such measurements are now possible thanks to the recent CCD camera developments that make feasible the measurements of star radiative fluxes. In this work we present a star photometer based on a CCD, carrying out an exhaustive characterization of the device in order to guarantee the quality of the measurements. We have analysed the applicability of the Langley calibration method in an urban polluted environment. Finally, we present some preliminary results on aerosol optical depth measured by Sun and star photometers under a Saharan dust event.

Retrieval of aerosol microphysical properties by means of sun and star photometry at Granada, Spain

Knowledge of aerosol microphysical properties is essential to understand the role of atmospheric aerosols in radiative forcing. Passive remote-sensing techniques allow the retrieval of columnar aerosol size distribution from spectral measurements of aerosol optical depth by applying numerical methods. These methods are widely applied to sun photometers. For night-time retrievals of aerosol optical depth, the GFAT (Atmospheric Physics Group of the University of Granada), operates a star photometer with channels at 380, 436, 500, 670, 880, and 1020 nm in the city of Granada (southeastern Spain, 36.83° N, 3.58° W, 680 m a.s.l.). The GFAT also operates a Cimel CE-318 sun photometer with filters at the same wavelengths. To our knowledge, the variations between daytime and night-time columnar aerosol microphysical properties have not been studied until now. In this work, the applicability of an inversion code based only on direct irradiance measurements is presented to retrieve aerosol size distributions, for both daytime and night-time. From March 2007 to February 2009, statistical analyses and time evolutions of the effective radius and the properties of the fine mode such as mean radius, width, and particle volume concentration are analysed. Daytime effective radius values are large in summer and small in winter, whereas no clear seasonal pattern emerges at night-time. During daytime, the fine mode radius does not show any remarkable pattern, whereas at night-time, this parameter shows a clear seasonal pattern with large values in summer and small values in winter. On the other hand, fine mode widths do not show any pattern, neither daytime nor night-time, although large values of this parameter are found during daytime in all seasons. The fine-particle volume concentration presents no seasonal pattern during the daytime, whereas at night-time it shows larger values in summer than in other seasons.

Statistical study of day and night hourly patterns of columnar aerosol properties using sun and star photometry

This work focuses on the statistical analysis of day and night hourly pattern of columnar aerosol properties. To that end, we use the large database of star-photometry measurements at the University of Granada station (37.16°N, 3.60°W, 680 m a.s.l; South-East of Spain) for nighttime characterization, and co-located AERONET measurements for the daytime. The aerosol properties studied are the aerosol optical depth (AOD), Angstrom parameter (α(440-870)), aerosol optical depths of fine (AODfine) and coarse mode (AODcoarse) through the Spectral Deconvolution Algorithm (SDA). Microphysical properties are calculated by inverting AOD spectra and include the effective radius (reff) and volume concentration (V) of the total size distribution, and also the effective radius of the fine mode (rfine). The initial analysis for the different air masses that reach the study area reveals that generally day and night values of AOD and α(440-870) are not different statistically. Nighttime values of AODfine, reff and rfine do however, present larger values. The influence of North African air masses is remarkable both during the day and night, with high particle loads and low values of the Angstrom parameters and also with large contribution of coarse particles as AODcoarse and reff values are almost the double than for other air masses. The analyses of day-to-night hourly values reveal an increase in AOD, AODfine and AODcoarse during the day and a decrease during the night. Such a pattern could be explained by the different emission rates, accumulation, aging and deposition of particles. Changes in particle radius are also observed as part of the day-tonight particle evolution process, being rfine variations important mainly at daytime while for reff variations are more important at nighttime. Results of day-to-night evolution were found to be independent of air mass origin, and seem to be mainly associated with local processes.

Characterization of smoke and dust episode over West Africa: comparison of MERRA-2 modeling with multiwavelength Mie–Raman lidar observations

Observations of multiwavelength Mie–Raman lidar taken during the SHADOW field campaign are used to analyze a smoke–dust episode over West Africa on 24–27 December 2015. For the case considered, the dust layer extended from the ground up to approximately 2000 m while the elevated smoke layer occurred in the 2500–4000 m range. The profiles of lidar measured backscattering, extinction coefficients, and depolarization ratios are compared with the vertical distribution of aerosol parameters provided by the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). The MERRA-2 model simulated the correct location of the near-surface dust and elevated smoke layers. The values of modeled and observed aerosol extinction coefficients at both 355 and 532 nm are also rather close. In particular, for the episode reported, the mean value of difference between the measured and modeled extinction coefficients at 355 nm is 0.01 km−1 with SD of 0.042 km−1. The model predicts significant concentration of dust particles inside the elevated smoke layer, which is supported by an increased depolarization ratio of 15 % observed in the center of this layer. The modeled at 355 nm the lidar ratio of 65 sr in the near-surface dust layer is close to the observed value (70 ± 10) sr. At 532 nm, however, the simulated lidar ratio (about 40 sr) is lower than measurements (55 ± 8 sr). The results presented demonstrate that the lidar and model data are complimentary and the synergy of observations and models is a key to improve the aerosols characterization.

Multi-technique analysis of precipitable water vapor estimates in the sub-Sahel West Africa

Precipitable water vapor (PWV) is an important climate parameter indicative of available moisture in the atmosphere; it is also an important greenhouse gas. Observations of precipitable water vapor in sub-Sahel West Africa are almost non-existent. Several Aerosol Robotic Network (AERONET) sites have been established across West Africa, and observations from four of them, namely, Ilorin (4.34° E, 8.32° N), Cinzana (5.93° W, 13.28° N), Banizoumbou (2.67° E, 13.54° N) and Dakar (16.96° W, 14.39° N) are being used in this study. Data spanning the period from 2004 to 2014 have been selected; they include conventional humidity parameters, remotely sensed aerosol and precipitable water information and numerical model outputs. Since in Africa, only conventional information on humidity parameters is available, it is important to utilize the unique observations from the AERONET network to calibrate empirical formulas frequently used to estimate precipitable water vapor from humidity measurements. An empirical formula of the form PWV=aTd+b where Td is the surface dew point temperature, a and b are constants, was fitted to the data and is proposed as applicable to the climatic condition of the sub-Sahel. Moreover, we have also used the AERONET information to evaluate the capabilities of well-established numerical weather prediction (NWP) models such as ERA Interim Reanalysis, NCEP-DOE Reanalysis II and NCEP-CFSR, to estimate precipitable water vapor in the sub-Sahel West Africa; it was found that the models tend to overestimate the amount of precipitable water at the selected sites by about 25 %.