20 years of columnar aerosol properties at Valencia area (Eastern Spain) by ground-based sun-photometry

In this paper, a climatological study focused on the analysis of atmospheric aerosols properties over Burjassot and Aras de los Olmos AERONET sites, located at Eastern Spain, is performed. Computed mean values for Aerosol Optical Depth (AOD440), & Aring;ngstrom exponent (alpha(440-870)) and columnar water vapor (w) in Burjassot (Aras de los Olmos) are 0.19 +/- 0.08 (0.10 +/- 0.02), 1.21 +/- 0.16 (1.18 +/- 0.25) and 1.87 +/- 0.68cm (0.94 +/- 0.10cm), respectively. From the monthly statistics of w and AOD, it is concluded that summer months are characterised by higher humidity and turbidity, being the latter favoured by particle stagnation, recirculation, secondary aerosol formation, hygroscopic processes and North African dust intrusions. Regarding the volume size distribution, well developed fine and coarse modes are found in summer. On the other hand, the spectral dependence of the asymmetry parameter (g) describes a decrease in lambda up to 870nm, and then a slight increase due to the influence of dust particles. The computed mean values for the asymmetry parameter (g(440)) are 0.700 +/- 0.013 (0.709 +/- 0.016) for Burjassot (Aras de los Olmos). With respect to the Single Scattering Albedo (SSA) and the complex refractive index, an additional analysis using Level 1.5 data is performed, as relatively few data points pass the filters imposed by the Level 2.0 conditions. In the case of SSA, which is found to be larger in spring and summer months, the obtained mean annual values at 440nm are 0.94 +/- 0.03 (0.95 +/- 0.03) for Burjassot (Aras de los Olmos). Finally, regarding the complex refractive index, the mean values of the real part (alpha 440) are 1.50 +/- 0.03 (1.49 +/- 0.03), and for the imaginary part (divided by kappa(440)divided by) are 0.007 +/- 0.004 (0.004 +/- 0.003) in Burjassot (Aras de los Olmos). The present study is complemented with three analyses based on the distinction between the different aerosol types. To begin with, these are classified according to their nature, being the most abundant continental clean aerosols. Next, Gobbi diagrams are plotted for both sites, depicting a clustered region of coarse aerosols, as well as particle growth traces. Finally, the different air masses affecting the measurement sites are studied, reaching to the conclusion that these originated in the European and the African continents are the most turbid ones. To conclude this study, a seasonal Mann-Kendall test is applied to AOD, alpha, w and SSA measurements in order to study possible trends. Hence, a statistically significant (>80%) decreasing trend in AOD is observed for most months; whereas alpha, w and SSA do not show statistically significant long-term trends, only for a few months of the year.