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 Earth's Radiation Energy Budget System (CERES) and surface-based data exhibit an increase in SSR between 2003 and 2012, exceeding +10 W m À2 over this period for some areas of the peninsula. The calculations are performed for three surface-based sites: Barcelona and Valladolid (Spain), and Évora (Portugal). Ranges in monthly values of CARE, CRE, and ARE are (À80, À20), (À60, À20), and (À30, 0), respectively (in W m À2 ). The average trends for the analyzed period of CARE, CRE, and ARE are +7, +5, and +2 W m À2 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.

Abstract. Arid regions are a major source of mineral dust aerosol. Transport from these sources can have a great impact on aerosol climatology in distant regions. In order to assess the impact of dust on climate we must understand how dust properties change after long distance transport from sources. This study addresses the changes in columnar aerosol properties when mineral dust outbreaks from western Africa arrive over the eastern Caribbean after transport across the Atlantic Ocean, a transit of 5–7 days. We use data from the NASA Aerosol Robotic Network (AERONET) located at five Caribbean and two western Africa sites to characterize changes in columnar aerosol properties: aerosol optical depth (AOD), size distribution, single scattering albedo, and refractive indexes. We first characterized the local aerosol climatology at each site and then using air mass back trajectories we identified those days when trajectories over Caribbean sites back-tracked to western Africa. Over the period 1996–2014 we identify 3174 days, an average of 167 days per year, when the air mass over the Caribbean sites could be linked to at least one of the two western Africa sites. For 1162 of these days, AOD data are available for the Caribbean sites as well as for the corresponding western Africa sites about 5–7 days earlier, when the air mass passed over these sites. We identified dust outbreaks as those air masses yielding AOD  ≥  0.2 and an Ångström exponent below 0.6. On this basis of the total 1162 days, 484 meet the criteria for mineral dust outbreaks. We observe that the AOD at 440 nm decreases by about 0.16 or 30 % during transport. The volume particle size distribution shows a similar decrease in the volume concentration, mainly in the coarse mode. The single scattering albedo, refractive indexes, and asymmetry factor remain unchanged. The difference in the effective radius over western Africa sites with respect to Caribbean sites ranges between 0 and −0.3 µm. Finally we conclude that in about half of the cases only non-spherical dust particles are present in the atmosphere over the western Africa and Caribbean sites, while in the other cases dust particles were mixed with other types of aerosol particles.

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Abstract. A reliable identification of desert dust (DD) episodes over north-central Spain is carried out based on the AErosol RObotic NETwork (AERONET) columnar aerosol sun photometer (aerosol optical depth, AOD, and Ångström exponent, α) and European Monitoring and Evaluation Programme (EMEP) surface particulate-mass concentration (PMx, x = 10, 2.5, and 2.5–10 µm) as the main core data. The impact of DD on background aerosol conditions is detectable by means of aerosol load thresholds and complementary information provided by HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory Model) air mass back trajectories, MODIS (Moderate Resolution Imaging Spectroradiometer) images, forecast aerosol models, and synoptic maps, which have been carefully reviewed by a human observer for each day included in the DD inventory. This identification method allows the detection of low and moderate DD intrusions and also of mixtures of mineral dust with other aerosol types by means of the analysis of α. During the period studied (2003–2014), a total of 152 DD episodes composed of 418 days are identified. Overall, this means ∼ 13 episodes and ∼ 35 days per year with DD intrusion, representing 9.5 % days year−1. During the identified DD intrusions, 19 daily exceedances over 50 µg m−3 are reported at the surface. The occurrence of DD event days during the year peaks in March and June, with a marked minimum in April and lowest occurrence in winter. A large interannual variability is observed showing a statistically significant temporal decreasing trend of ∼ 3 days year−1. The DD impact on the aerosol climatology is addressed by evaluating the DD contribution in magnitude and percent (in brackets) for AOD, PM10, PM2.5, and PM2.5 − 10, obtaining mean values of 0.015 (11.5 %), 1.3 µg m−3 (11.8 %), 0.55 µg m−3 (8.5 %) and 0.79 µg m−3 (16.1 %), respectively. Annual cycles of the DD contribution for AOD and PM10 present two maxima – one in summer (0.03 and 2.4 µg m−3 for AOD in June and PM10 in August) and another in March (0.02 for AOD and 2.2 µg m−3 for PM10) – both displaying a similar evolution with exceptions in July and September. The seasonal cycle of the DD contribution to AOD does not follow the pattern of the total AOD (close to a bell shape), whereas both PM10 cycles (total and DD contribution) are more similar to each other in shape, with an exception in September. The interannual evolution of the DD contribution to AOD and PM10 has evidenced a progressive decrease. This decline in the levels of mineral dust aerosols can explain up to 30 % of the total aerosol load decrease observed in the study area during the period 2003–2014. The relationship between columnar and surface DD contribution shows a correlation coefficient of 0.81 for the interannual averages. Finally, synoptic conditions during DD events are also analysed, observing that the north African thermal low causes most of the events ( ∼  53 %). The results presented in this study highlight the relevance of the area studied since it can be considered representative of the clean background in the western Mediterranean Basin where DD events have a high impact on aerosol load levels.

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