Maria João Costa

Simulations of time‐coincident, co‐located measurements from ENVISAT‐1 instruments for the characterization of tropospheric aerosols: a sensitivity study including cloud contamination effects

Abstract A sensitivity test of the aerosol properties retrieval is conducted using nadir viewing instruments on board the ENVISAT-1 platform trying to reproduce the natural variability of the aerosol field. The analysis highlights problems arising in the retrieval when aerosol loading, relative humidity, and cloud coverage are simultaneously varied.

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).

Quantifying the respective roles of aerosols and clouds in the strong brightening since the early 2000s over the Iberian Peninsula

The contribution of clouds and aerosols to the decadal variations of downward surface shortwave radiation (SSR) is a current controversial topic. This study proposes a method, which is based on surface-based SSR measurements, aerosol observations, and radiative transfer simulations (in cloud-free and cloud- and aerosol-free scenarios), to evaluate cloud-aerosol (CARE), cloud (CRE), and aerosol (ARE) radiative effects. This method is applied to quantify the role played by, separately, clouds and aerosols on the intense brightening of the SSR observed in the Iberian Peninsula. Clouds and Earths Radiation Energy Budget System (CERES) and surface-based data exhibit an increase in SSR between 2003 and 2012, exceeding +10 W m−2 over this period for some areas of the peninsula. The calculations are performed for three surface-based sites: Barcelona and Valladolid (Spain), and Evora (Portugal). Ranges in monthly values of CARE, CRE, and ARE are (−80, −20), (−60, −20), and (−30, 0), respectively (in W m−2). The average trends for the analyzed period of CARE, CRE, and ARE are +7, +5, and +2 W m−2 per decade, respectively. Overall, three fourths of the SSR trend is explained by clouds, while the other one fourth is related to aerosol changes. The SSR trends explained by the clouds and aerosol radiative effects are in line with the observed reductions in total cloud cover and aerosol load (both at the surface and in the whole atmospheric column). Furthermore, the CRE values are compared against CERES data showing good agreement between both data series, although some discrepancies are observed in their trends.

Infrared lidar overlap function: an experimental determination.

The most recent works demonstrate that the lidar overlap function, which describes the overlap between the laser beam and the receiver field of view, can be determined experimentally for the 355 and 532 nm channels using Raman signals. Nevertheless, the Raman channels cannot be used to determine the lidar overlap for the infrared channel (1064 nm) because of their low intensity. In addition, many Raman lidar systems only provide inelastic signals with reasonable signal-to-noise ratio at nighttime. In view of this fact, this work presents a modification of that method, based on the comparison of attenuated backscatter profiles derived from lidar and ceilometer, to retrieve the overlap function for the lidar infrared channel. Similarly to the Raman overlap method, the approach presented here allows to derive the overlap correction without an explicit knowledge of all system parameters. The application of the proposed methodology will improve the potential of Raman lidars to investigate the aerosol microphysical properties in the planetary boundary layer, extending the information of 1064 nm backscatter profiles to the ground and allowing the retrieval of microphysical properties practically close to the surface.

Remote sensing of water quality parameters over Alqueva Reservoir in the south of Portugal

In this study, the potential of MEdium Resolution Imaging Spectrometer (MERIS) to describe variations of optically active substances over Alqueva artificial lake is investigated. Limnological laboratory analyses of the water samples collected monthly, from 2003 to 2006, are used in combination with MERIS. The water surface spectral reflectance is derived from Level1b MERIS data, using radiative transfer calculations to account for the atmospheric effects. The lake water spectral surface reflectance is combined with laboratory analyses of cyanobacteria total densities as well as chlorophyll a concentrations and empirical algorithms for both quantities are derived. The results obtained are compared with independent laboratory analyses from 2007, with good correlation coefficients obtained both for cyanobacteria (R = 0.93) and chlorophyll a(R = 0.80). The methodology proposed here has been developed to inexpensively monitor Alqueva Reservoir water quality in terms of cyanobacteria and chlorophyll a on a regular basis, and to provide useful information to the authorities.

Aerosol optical thickness and classification: use of METEOSAT, GOME, and modeled data

Satellite measurements at high spectral resolution and span that avoid gas absorption bands and determine aerosol spectral optical properties are necessary for obtaining aerosol optical thickness values at the reference wavelength of 550 nm (hereinafter AOT). GOME (Global Ozone Monitoring Experiment on board the ERS-2 spacecraft) measurements fit such requirements, with a suitable spectral resolution over the region between UV and near IR while presenting a low spatial (320 X 40 Km2) and temporal resolution. The present new method overcomes these limitations by combining aerosol optical characteristics retrieved from GOME with METEOSAT visible data; the latter allow for monitoring aerosol events with adequate temporal resolution over wide cloud-free oceanic areas. The AOT results from fitting the measured broad-band visible METEOSAT radiance with the simulated radiance from radiative transfer calculations; aerosol properties estimated from GOME data are the essential input parameters. Several parameterizations of aerosol microphysical quantities have been tested to improve the AOT retrievals.

Monitoring of atmospheric ozone and nitrogen dioxide over the south of Portugal by ground-based and satellite observations.

The SPATRAM (Spectrometer for Atmospheric TRAcers Monitoring) instrument has been developed as a result of the collaboration between CGE-UE, ISAC-CNR and Italian National Agency for New Technologies, Energy and the Environment (ENEA). SPATRAM is a multi-purpose UV-Vis-scanning spectrometer (250 - 950 nm) and it is installed at the Observatory of the CGE, in Evora, since April 2004. A brief description of the instrument is given, highlighting the technological innovations with respect to the previous version of similar equipment. The need for such measurements automatically taken on a routine basis in south-western European regions, specifically in Portugal, has encouraged the development and installation of the equipment and constitutes a major driving force for the present work. The main features and some improvements introduced in the DOAS (Differential Optical Absorption Spectroscopy) algorithms are discussed. The results obtained applying DOAS methodology to the SPATRAM spectrometer measurements of diffused spectral sky radiation are presented in terms of diurnal and seasonal variations of nitrogen dioxide (NO(2)) and ozone (O(3)). NO(2) confirms the typical seasonal cycle reaching the maximum of (6.5 +/- 0.3) x 10(+15) molecules cm(-2) for the sunset values (PM), during the summer season, and the minimum of (1.55 +/- 0.07) x 10(+15) molecules cm(-2) for the sunrise values (AM) in winter. O(3) presents the maximum total column of (433 +/- 5) Dobson Unit (DU) in the spring season and the minimum of (284 +/- 3) DU during the fall period. The huge daily variations of the O(3) total column during the spring season are analyzed and discussed. The ground-based results obtained for NO(2) and O(3) column contents are compared with data from satellite-borne equipment (GOME - Global Ozone Monitoring Experiment; SCIAMACHY - Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY; TOMS - Total Ozone Monitoring Spectrometer) and it is shown that the two data sets are in good agreement. The correlation coefficient for the comparison of the ground-based/satellite data for O(3) is of 0.97.

Measurements of stratospheric ozone and nitrogen dioxide at Èvora, Portugal

The Spectrometer for Atmospheric TRAcers Monitoring (SPATRAM) has been developed as a result of collaboration between the Geophysics Centre of Évora University (CGE-UE), the Institute for Atmospheric Sciences and Climate of the National Research Council (ISAC-CNR) in Italy and the Italian National Agency for New Technologies, Energy and the Environment (ENEA). SPATRAM is a multipurpose ultraviolet (UV)–visible scanning spectrometer (250–950 nm). It has been installed at the Observatory of the CGE, in Évora, since April 2004 and is currently used to carry out measurements of the zenith scattered radiation, the so-called ‘Passive mode’, to retrieve the vertical content and distribution of some atmospheric tracers such as ozone (O3) and nitrogen dioxide (NO2) using Differential Optical Absorption Spectroscopy (DOAS) methodology. The lack of such measurements taken automatically on a routine basis in southwestern European regions, specifically in Portugal, motivated the effort for its installation and constitutes a major driving force for the present work. For continuous NO2 monitoring the 425–455 nm spectral range is investigated. For O3 retrieval the spectral interval 320–340 nm is chosen. The measurements are in good agreement with the photochemical theory of NO2 (O3), showing maximum values during the summer (spring) and minimum values during the winter (autumn) seasons. Moreover, the application of sophisticated inversion schemes to the output of the DOAS program, using the Air Mass Factor (AMF) matrix as the kernel of the inversion algorithm, allowed for the determination of the vertical distribution of NO2 and O3 atmospheric compounds. In addition, the influence of desert dust aerosol absorption on ozone retrieval is assessed, revealing values of about 3.5% for an aerosol optical depth (AOD) of 1.0, in the case simulated. A correction factor is derived and applied whenever desert dust is detected. The ground-based results obtained for the ozone column content are compared with data from the satellite-borne Ozone Monitoring Instrument (OMI), and the two data sets are found to be in good agreement, with a correlation coefficient of 0.96.

Aerosol characterization and direct radiative forcing assessment over the ocean. Part I: Methodology and sensitivity analysis

A method based on the synergistic use of low earth orbit (LEO) and geostationary earth orbit (GEO) satellite data for aerosol-type characterization, as well as aerosol optical thickness (AOT) retrieval and monitoring over the ocean, is presented. These properties are used for the estimation of the direct shortwave aerosol radiative forcing at the top of the atmosphere. The synergy serves the purpose of monitoring aerosol events at the GEO time and space scales while maintaining the accuracy level achieved with LEO instruments. Aerosol optical properties representative of the atmospheric conditions are obtained from the inversion of high-spectral-resolution measurements from the Global Ozone Monitoring Experiment (GOME). The aerosol optical properties are input for radiative transfer calculations for the retrieval of the AOT from GEO visible broadband measurements, avoiding the use of fixed aerosol models available in the literature. The retrieved effective aerosol optical properties represent an essential component for the aerosol radiative forcing assessment. A sensitivity analysis is also presented to quantify the effects that changes on the aerosol model may have on modeled results of spectral reflectance, AOT, and direct shortwave aerosol radiative forcing at the top of the atmosphere. The impact on modeled values of the physical assumptions on surface reflectance and vertical profiles of ozone and water vapor are analyzed. Results show that the aerosol model is the main factor influencing the investigated radiative variables. Results of the application of the method to several significant aerosol events, as well as their validation, are presented in a companion paper.

Variability of the Daily-Mean Shortwave Cloud Radiative Forcing at the Surface at a Midlatitude Site in Southwestern Europe

The shortwave cloud radiative forcing is calculated from surface measurements taken in Evora from 2003 to 2010 with a multifilter rotating shadowband radiometer (MFRSR) and with an Eppley black and white pyranometer. A new approach to estimate the clear-sky irradiance based on radiative transfer calculations is also proposed. The daily-meanvalues ofthe cloud radiativeforcing (absolute andnormalized)aswell astheir monthlyand seasonal variabilities are analyzed. The study shows greater variability of radiative forcing during springtime with respect to the other seasons. The mean daily cloudy periods have seasonal variation proportional to the seasonal variation of the cloud radiative forcing, with maximum values also occurring during springtime. The minimum values found for the daily-mean cloud radiative forcing are 2139.5 and 2198.4Wm 22 for MFRSR and Eppley data, respectively; the normalized values present about 40% of sample amplitude, both for MFRSRandEppley.Inaddition,aquantitativerelationshipbetweentheMFRSRandEppleycloudradiative forcings applicable to other locations is proposed.