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Columnar aerosol properties from sun-and-star photometry: Statistical comparisons and day-to-night dynamic

by D. Pérez-Ramírez, H. Lyamani, F. J. Olmo, D. N. Whiteman, L. Alados-Arboledas
Atmospheric Chemistry and Physics ()
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This work presents the first analysis of long-term correlative day-to-night columnar aerosol optical prop-erties. The aim is to better understand columnar aerosol dy-namic from ground-based observations, which are poorly studied until now. To this end we have used a combination of sun-and-star photometry measurements acquired in the city of Granada (37.16 • N, 3.60 • W, 680 m a.s.l.; South-East of Spain) from 2007 to 2010. For the whole study period, mean aerosol optical depth (AOD) around 440 nm (±standard de-viation) is 0.18 ± 0.10 and 0.19 ± 0.11 for daytime and night-time, respectively, while the mean Angström exponent (α) is 1.0 ± 0.4 and 0.9 ± 0.4 for daytime and nighttime. The ANOVA statistical tests reveal that there are no significant differences between AOD and α obtained at daytime and those at nighttime. Additionally, the mean daytime values of AOD and α obtained during this study period are coherent with the values obtained in the surrounding AERONET sta-tions. On the other hand, AOD around 440 nm present evi-dent seasonal patterns characterised by large values in sum-mer (mean value of 0.20 ± 0.10 both at daytime and night-time) and low values in winter (mean value of 0.15 ± 0.09 at daytime and 0.17 ± 0.10 at nighttime). The Angström ex-ponents also present seasonal patterns, but with low values in summer (mean values of 0.8 ± 0.4 and 0.9 ± 0.4 at day-and night-time) and relatively large values in winter (mean values of 1.2 ± 0.4 and 1.0 ± 0.3 at daytime and nighttime). These seasonal patterns are explained by the differences in the meteorological conditions and by the differences in the strength of the aerosol sources. To take more insight about the changes in aerosol particles between day and night, the spectral differences of the Angström exponent as function of the Angström exponent are also studied. These analyses re-veal increases of the fine mode radius and of the fine mode contribution to AOD during nighttime, being more remark-able in the summer seasons. These variations are explained by the changes of the local aerosol sources and by the meteo-rological conditions between daytime and nighttime, as well as aerosol aging processes. Case studies during summer and winter for different aerosol loads and types are also presented to clearly illustrate these findings.

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