Day and night columnar aerosol properties at Granada (Spain) retrieved from sun-and star-photometry
1Centro Andaluz de Medio Ambiente (CEAMA), Universidad de Granada, Junta de Andalucía, Av. del Mediterráneo s/n, 18006-Granada, Spain
2Departamento de Física Aplicada, Universidad de Granada, Campus de Fuentenueva s/n, 18071-Granada, Spain
3Mesoscale Atmospheric Processes Laboratory, NASA Goddard Space Flight Center, 20771, Greenbelt-Maryland, USA
Abstract. This works present the first analysis of long-term day- and night-time columnar aerosol optical properties. To this end we have used a combination of sun and star photometer measurements at 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) at 440 nm (± standard deviation) is 0.18 ± 0.10 and 0.19 ± 0.11 for day- and night-time, respectively, while the mean Angström exponent (α) is 1.0 ± 0.4 and 0.9 ± 0.4 for day- and night-time. The ANOVA statistical tests reveal that there are no significant differences between the AOD and α obtained at day-time and those obtained at night-time. Additionally, the mean day-time values of AOD and α obtained during this period are within the values obtained in the surrounding AERONET stations. On the other hand, AOD presents an evident seasonal pattern characterized by large values in summer (mean values of 0.20 ± 0.10 both at day- and night-time) and low values in winter (mean values of 0.15 ± 0.09 at day-time and 0.17 ± 0.10 at night-time). The Angström exponent presents clear seasonal pattern 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 day- and night-time). These seasonal patterns are explained by the differences in the meteorological conditions and by the differences in the strength of aerosol sources during day and night. To take more insight about the changes in aerosol particles between day and night, the spectral difference of the Angström exponent (δα) as function of α is also studied. This analysis reveals an increase in the fine mode radius and in the fine mode contribution to AOD during night-time, being more remarkable in summer seasons. These changes are explained by the changes in the local aerosol source emissions and meteorological conditions between day- and night-time, as well as aerosol aging processes.