Aerosol Monitoring and Radiative Forcing Assessment in Southwestern Iberia

Aerosols are short-lived atmospheric constituents, acting as climate forcers and accountable for major uncertainties in the total radiative forcing estimates, as reported by the Intergovernmental Panel on Climate Change (IPCC). Several aerosol microphysical, optical, and geometrical properties are measured at Évora ground-based station since 2002, using in situ instrumentation, as well as remote sensing techniques. Data obtained with sun photometers included in AErosol RObotic NETwork (AERONET) are analyzed here. The intra-annual variability of aerosol parameters is examined on a monthly basis, and the inter-annual trends are evaluated. Results show that the region is frequently affected by long-range transport of atmospheric aerosols from different origins as desert dust or forest fire, with higher aerosol optical depth (AOD) and lower Ångström exponent (AE) values in Spring and Summer with respect to Autumn and Winter. Trend analysis point to decreasing trends of AOD and AE during the 20-year period. However, when only extreme events are considered (AOD > 0.4), the AOD trend inverts and shows an increasing tendency at a rate of 1% per year, even more pronounced in Springtime at a rate of 2.9% per year, both trends statistically significant at the 95% confidence level. Aerosol radiative forcing (ARF) presents the lowest (more negative) values at the surface, and moderately low (negative) values at the top of the atmosphere, indicating a cooling effect at both levels, whereas a warming effect is obtained for the atmosphere. ARF values are consistent with aerosol properties measured and present significant trends indicating more pronounced effects of extreme events at all levels, especially during Winter and Spring.