S. N. Pereira

Properties of dust aerosol particles transported to Portugal from the Sahara desert

Aerosol properties of mineral particles in the far field of an African desert dust outbreak were investigated that brought Saharan dust over the Mediterranean in different layers to Portugal. The measurements were performed inside the project Desert Aerosols over Portugal (DARPO) which was linked to the Saharan Mineral Dust Experiment (SAMUM). The maximum particle mass concentration was about 150μgm−3 and the corresponding scattering coefficient was 130Mm−1 which results in a mass scattering efficiency of 0.87m2 g−1. The aerosol optical depth reached values up to 0.53 and the lidar ratio was between 45 and 50 in the whole dust loaded column. A comparison between particle size distributions and refractive indices derived from different instruments and models showed a general good agreement but some minor differences could also be observed. Measurements as well as calculations with a particle transport model suggest that there is a relatively higher concentration of very large particles in the upper region of the dust layer than on the surface which is likely connected with meteorological conditions at the observational site ( Évora, Portugal).

PollyNET: a network of multiwavelength polarization Raman lidars

PollyNET is a growing global network of automatized multiwavelength polarization Raman lidars of type Polly (Althausen et al., 2009). The goal of this network is to conduct advanced remote measurements of aerosol profiles and clouds by the same type of instrument. Since 2006 this network assists the controlling and adjustment activities of Polly systems. A central facility receives the data from the Polly measurements. The observational data are displayed in terms of quicklooks at http://polly:tropos.de in near real time. In this way, the network serves as a central information platform for inquisitive scientists. PollyNET comprises permanent stations at Leipzig (Germany), Kuopio (Finland), Evora (Portugal), Baengnyeong Island (South Korea), Stockholm (Sweden), and Warsaw (Poland). Non-permanent stations have been used during several field experiments under both urban and very remote conditions - like the Amazon rainforest. These non-permanent stations were lasting from several weeks up to one year and have been located in Brazil, India, China, South Africa, Chile, and also aboard the German research vessels Polarstern and Meteor across the Atlantic. Within PollyNET the interaction and knowledge exchange is encouraged between the Polly operators. This includes maintenance support in system calibration procedures and distribution of latest hardware and software improvements. This presentation introduces the PollyNET. Main features of the Polly systems will be presented as well as recent instrumental developments. Some measurement highlights achieved within PollyNET are depicted.

Forest Fire Smoke Layers Observed in the Free Troposphere over Portugal with a Multiwavelength Raman Lidar: Optical and Microphysical Properties

Vertically resolved optical and microphysical properties of biomass burning aerosols, measured in 2011 with a multiwavelength Raman lidar, are presented. The transportation time, within 1-2 days (or less), pointed towards the presence of relatively fresh smoke particles over the site. Some strong layers aloft were observed with particle backscatter and extinction coefficients (at 355 nm) greater than 5 Mm−1 sr−1 and close to 300 Mm−1, respectively. The particle intensive optical properties showed features different from the ones reported for aged smoke, but rather consistent with fresh smoke. The Ångström exponents were generally high, mainly above 1.4, indicating a dominating accumulation mode. Weak depolarization values, as shown by the small depolarization ratio of 5% or lower, were measured. Furthermore, the lidar ratio presented no clear wavelength dependency. The inversion of the lidar signals provided a set of microphysical properties including particle effective radius below 0.2 μm, which is less than values previously observed for aged smoke particles. Real and imaginary parts of refractive index of about 1.5-1.6 and 0.02i, respectively, were derived. The single scattering albedo was in the range between 0.85 and 0.93; these last two quantities indicate the nonnegligible absorbing characteristics of the observed particles.

Modulation of urban atmospheric electric field measurements with the wind direction in Lisbon (Portugal)

Atmospheric electric field measurements (potential gradient, PG) were retrieved in the urban environment of the city of Lisbon (Portugal). The measurements were performed with a Benndorf electrograph at the Portela Meteorological station in the suburbs of the city (NE from the centre). The period of 1980 to 1990 is considered here. According to wind direction, different content and types of ions and aerosols arrive at the measurement site causing significant variations to the PG. To the south there are significant pollution sources while to the north such sources are scarcer. The Iberian Peninsula is found east of the station and the Atlantic Ocean covers the western sector, Wind directions are divided in four sectors: i) NW: 270° ≤ θ ≤ 360°; ii) NE: 0 ≤ θ ≤ 90°; iii) SE: 90 ≤ θ ≤ 180°; iv) SW: 180° ≤ θ ≤ 270°. Analysis of weekly cycle, caused by anthropogenic pollution related with urban activity, was undertaken for each wind sector. NW sector has been shown to be less affected by this cycle, which is attributed to the effect of marine air. The daily variation of NE sector for weekends reveals a similar behaviour to the Carnegie curve, which corresponds to a clean air daily variation of PG, following universal time, independent of measurement site.

Multiyear Measurements of the Aerosol Absorption Coefficient Near the Surface in a Small-Sized Urban Area in Portugal

Measurements of the aerosol absorption coefficient, between 2007 and 2013, were made at the ground level in Evora, a Portuguese small town located in the southwestern Iberia Peninsula. Such a relatively long time series of absorbing aerosols is unique in Portugal and uncommon elsewhere. The average aerosol absorption coefficient was close to 9 Mm−1 and clear cycles at both daily and seasonal time scales were found. An average increase by a factor of two (from 6 to 12 Mm−1) was observed in winter if compared to summer season. The daily variations were similarly shaped for all seasons, with two morning and afternoon peaks, but with magnitudes modulated by the seasonal evolution. That was not the case if Sundays were considered. These variations can be explained in terms of the impact of local particle sources, related mainly to traffic and biomass burning and upward mixing of the aerosol due to variable mixing layer heights, either daily or seasonally. Also, a strong negative correlation between the aerosol absorption coefficient and the wind speed was verified, and an exponential decay function was found to fit very well to the data. The wind direction seems to be not correlated with the aerosol absorption coefficient.

Aerosol extinction and absorption in Évora, Portugal, during the European 2003 summer heat wave

Aerosol optical properties are retrieved from measurements acquired during the 2003 summer at the new AERONET station of Evora, Portugal, with a sun/sky photometer, a fluxmeter and a nephelometer. Aerosol optical thickness (aot) derived at several wavelengths shows that an exceptionally long turbid event occurred in July-August. Desert dust particles transported from North Africa increased aot at 873 nm (aot873) to the value of 0.27 with an Ångstrom exponent αC=0.5. Emissions from forest fires in The Iberic peninsula affected Evora since the end of the dust episode, with aot441 reaching 0.81 and aC=1.8. The aerosol scattering coefficient measured at surface level shows that desert dust does not reach the surface level at Evora while the forest fire emissions were uniformly distributed over the atmospheric column. Sky-radiance and flux measurements agree in retrieval of the aerosol single scattering albedo (assa) at several wavelengths. A large absorption rate is found with a high spectral dependence for desert dust particles (assa441=0.86 and ass873=0.93) and with a flat spectral dependence during the forest fires emission episode (assa441=0.88 and assa873=0.87). All measurements as well as back-trajectory calculations indicate mixture of particles during the desert dust.

Vertically resolved optical and microphysical properties of Portuguese forest fire smoke observed in February 2012

For the first time fresh biomass burning aerosol from wild fires in the north of Portugal was studied in detail by analysing profiles of optical and microphysical particle properties obtained from multiwavelength Raman lidar measurements. During the driest February since 1931, in 2012, an unusual high number of forest fires occurred in the north of the country. Despite the strong fires and back-trajectories directly crossing the burning areas, the optical and microphysical properties indicate a mixture of the biomass burning smoke with dust from the dried out soil of the Iberian peninsula or with other aerosol types. The layer means of the particle effective radius, the real part of the complex refractive index (CRI), and the imaginary part of the CRI ranged, respectively, from 0.20 μm to 0.30 μm, from 1.52 to 1.64, and from 0.009 to 0.017. The large single scattering albedo between 0.92 and 0.96, 0.92 and 0.96, and 0.88 and 0.94 at 355, 532 and 1064 nm, respectively, indicate weak absorption.

Analysis of atmospheric vertical profiles in the presence of desert dust aerosols

The present work aims at studying a very recent episode of desert dust transport that affected Iberia in mid May 2015. The dust aerosols were detected over Evora, where a varied set of instrumentation for aerosol measurements is installed, including: a CIMEL sunphotometer integrated in AERONET, a Raman Lidar and a TEOM monitor, as well as ceilometer and a microwave radiometer (profiler). The aerosol occurrence, detected using the columnar, vertically-resolved and in situ measurements, was characterized by a fairly high aerosol optical thickness that reached a value of 1.0 at 440 nm and showed mass concentration peaks at the surface of the order of 100 μg/m3. Subsequently, the tropospheric vertical profiles of humidity and temperature obtained with the passive microwave (MW) radiometer are analysed in order to distinguish possible modifications that can be connected with the transport of desert dust. Modelling results are also examined and the total, SW and LW radiative forcings are investigated, taking into account the different vertical profiles obtained during the desert dust occurrence. It is found that the differences in the atmospheric profiles mostly affect the LW radiative forcing, with an underestimation of about 30% when the actual vertical profile is not considered.

Surface UV radiation in the South of Portugal: Monitoring and assessment of cloud effects

Measurements of both UV B (280 – 315 nm) and UV AB (280 – 400 nm) irradiances taken during about seven years are presented and analyzed here. The UV irradiance data were measured with MACAM radiometers, which are installed in the Atmospheric Physics Observatory of the University of Evora Geophysics Center - CGE (38°34’N, 7°54’W, 300 m above mean sea level) since 2004. Special attention is devoted to calibration issues, since the radiometers were calibrated in Spain, at “El Arenosillo” ESAt/INTA laboratory in 2009 and a methodology based on radiative transfer calculations combined with observations from atmospheric quantities, was developed and applied to retrocalibrate the data from 2009 to 2004. The comparison between the radiative transfer based and the laboratory calibration methodologies yielded a quite promising outcome, with normalized root mean square errors lower than 3% and mean absolute percentage errors lower than 2%. Cloud optical thickness values derived from ground-based spectral irradiance ...