Abstract
ABSTRACTNotable 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 Evora (Portugal, Southwestern Europe). This is an interesting region affected by different aerosol types and sources. Towards this goal, ARF and ARFE are calculated for Evora during thirteen years (2003–2015), 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.
Highlights
Atmospheric aerosol particles, of natural or anthropogenic origin, interact directly with solar and terrestrial radiation through scattering and absorption as well as through emission processes, modifying the Earth's radiative balance
While no aerosol radiative forcing (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
While no seasonal DARF pattern is observed at the TOA, a clear seasonal cycle is detected at the SURF (Fig. 4)
Summary
Atmospheric aerosol particles, of natural or anthropogenic origin, interact directly with solar and terrestrial radiation through scattering and absorption as well as through emission processes, modifying the Earth's radiative balance. According to the Fifth Assessment Report delivered in 2013 by the Intergovernmental Panel on Climate Change, direct and indirect effects of aerosols in the atmosphere result in an overall radiative forcing of –0.9 (–1.9 to –0.1) W m–2 at the TOA (Boucher et al, 2013). This global cooling effect due to aerosols is well established, significant uncertainties still remain, mainly referred to their spatial and temporal variability at regional and local scales. Within this framework, the Iberian Peninsula is a interesting region affected by occasional intrusions coming from the largest source of dust worldwide: the Sahara Desert. The southwestern interior part of the Iberian Peninsula is exposed to local continental
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