Abstract
Abstract. This work presents the first analysis of long-term correlative day-to-night columnar aerosol optical properties. The aim is to better understand columnar aerosol dynamic 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 deviation) is 0.18 ± 0.10 and 0.19 ± 0.11 for daytime and nighttime, 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 stations. On the other hand, AOD around 440 nm present evident seasonal patterns characterised by large values in summer (mean value of 0.20 ± 0.10 both at daytime and nighttime) and low values in winter (mean value of 0.15 ± 0.09 at daytime and 0.17 ± 0.10 at nighttime). The Angström exponents 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 reveal increases of the fine mode radius and of the fine mode contribution to AOD during nighttime, being more remarkable in the summer seasons. These variations are explained by the changes of the local aerosol sources and by the meteorological 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.
Highlights
Atmospheric aerosol is noted by the Fourth Intergovernmental Panel for Climate Change (IPCC 2007) as a key component on the climate (Forster et al, 2007)
Atmospheric aerosol particles can act as cloud condensation nuclei and, they can modify cloud droplet size and cloud albedo (Forster et al, 2007)
The Angstrom exponent, α(436–880 nm), obtained at nighttime is computed from aerosol optical depth (AOD) at 436, 667, 880 nm and the Angstrom exponent, α(440–870 nm), obtained at daytime is calculated from AOD at 440, 670 and 870 nm
Summary
Atmospheric aerosol is noted by the Fourth Intergovernmental Panel for Climate Change (IPCC 2007) as a key component on the climate (Forster et al, 2007). Atmospheric aerosol particles can act as cloud condensation nuclei and, they can modify cloud droplet size and cloud albedo (Forster et al, 2007). They have effects on air quality and, on the human health (e.g., Pope et al, 2002; Brunekreef and Forsberg, 2005; Miller et al, 2007).
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