Thirteen Years of Aerosol Radiative Forcing in Southwestern Iberian Peninsula

Abstract

Notable uncertainties in the aerosol impact on the Earth's radiative budget still remain, mainly at local scales. This study aims to accurately quantify the shortwave aerosol radiative forcing (ARF) and its efficiency (ARFE) at Évora (Portugal, Southwestern Europe). This is an interesting region affected by different aerosol types and sources. Towards this goal, ARF and ARFE are calculated for Évora during thirteen years (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015), which is the longest period ever analyzed for this region. For this calculation, the most updated AERONET data have been used to simulate irradiance with the libRadtran code. Thus, 1676 daily values were analyzed, obtaining a mean ARF of -7.4 W m -2 at the Earth's surface (SURF) and -1.1 W m -2 at the top of the atmosphere (TOA), indicating a cooling of the Earth-atmosphere system. The difference in ARF between TOA and SURF indicates a mean net gain of 6.3 W m -2 for the atmosphere. While no ARF seasonal pattern is observed at the TOA, a clear seasonal cycle is detected at the SURF with the most negative values occurring in summer. On the other hand, ARFE shows mean values of -62.7 W m -2 τ -1 at the SURF and -14.3 W m -2 τ -1 at the TOA. Results show that ARFE highly depends on the aerosol single scattering albedo, showing opposite behavior at the SURF and at the TOA. The large temporal variability detected in the aerosol properties in the region of study confirms the general need for long time series of measurements to achieve an accurate estimation of the ARF and ARFE.

Spatial and Temporal Variation of Aerosol and Water Vapour Effects on Solar Radiation in the Mediterranean Basin during the Last Two Decades

“…This may be due to the fact that their study period is 2000-2009, and the economic crisis had not greatly affected AOT values yet. On the contrary, trends obtained by Floutsi et al [17] and Alfaro-Contreras et al [62] show values of −0.030 and −0.023 per decade, respectively, being higher, in absolute terms, than that the one obtained in our study. This difference may be due to the fact that their study periods are 2002-2014 or 2002-2015, and therefore, the increase in AOT starting in 2014 has not been completely included.…”

“…In the case of AOT, the trend is equal to −0.021 per decade during nearly 19 years, resulting in an absolute AOT decrease of 0.040. Statistically significant negative trends of AOT have also been obtained by other authors for the Mediterranean region (e.g., [17,61,62]). However, the values of these trends vary, depending on the study period considered.…”

“…Due to great interest on the Mediterranean basin, several studies have analysed aerosols, water vapour and their effects on solar radiation in this area [9][10][11][12][13][14][15][16][17][18]. However, most of these studies are local (measurements taken at specific locations) or use a short period of study (specific campaigns), preventing a complete analysis of the spatial and temporal variability.…”

“…The main limitation of the existing studies comes from the fact that they examine the effect of aerosols (e.g., [9][10][11][12][13][14]17]) or water vapour (e.g., [15,18,22]) individually. Thus, to our knowledge, only the study by Obregón et al [23] has analysed the joint effect of both atmospheric constituents and focuses on nine specific stations representative of different regions of the world.…”

“…Satellites provide long-term data records with a large spatial coverage, being the ideal source of information for this type of studies. They have already been used to monitor aerosols [17,24] and water vapour [25] in the Mediterranean basin but, as mentioned above, none of the previous studies have analysed the combined effect of both components.…”