Aerosol climatology: on the discrimination of aerosol types over four AERONET sites

Abstract

Abstract. Aerosols have a significant regional and global effect on climate, which is about equal in magnitude but opposite in sign to that of greenhouse gases. Nevertheless, the aerosol climatic effect changes strongly with space and time because of the large variability of aerosol physical and optical properties, which is due to the variety of their sources, which are natural, and anthropogenic, and their dependence on the prevailing meteorological and atmospheric conditions. Characterization of aerosol properties is of major importance for the assessment of their role for climate. In the present study, 3-year AErosol RObotic NETwork (AERONET) data from ground-based sunphotometer measurements are used to establish climatologies of aerosol optical depth (AOD) and Ångström exponent α in several key locations of the world, characteristic of different atmospheric environments. Using daily mean values of AOD at 500 nm (AOD500) and Ångström exponent at the pair of wavelengths 440 and 870 nm (α 440–870), a discrimination of the different aerosol types occurring in each location is achieved. For this discrimination, appropriate thresholds for AOD500 and α 440–870 are applied. The discrimination of aerosol types in each location is made on an annual and seasonal basis. It is shown that a single aerosol type in a given location can exist only under specific conditions (e.g. intense forest fires or dust outbreaks), while the presence of well-mixed aerosols is the accustomed situation. Background clean aerosol conditions (AOD500<0.06) are mostly found over remote oceanic surfaces occurring on average in ~56.7% of total cases, while this situation is quite rare over land (occurrence of 3.8–13.7%). Our analysis indicates that these percentages change significantly from season to season. The spectral dependence of AOD exhibits large differences between the examined locations, while it exhibits a strong annual cycle.

Inferring aerosol types over the Indo-Gangetic Basin from ground based sunphotometer measurements

“…Uncertainty in quantifying the climatic impacts of aerosols continues to be greater than that of greenhouse gases (IPCC, 2007) due to variety of their sources, varying trends in aerosol loading and extreme heterogeneity in the spatial and temporal variability of their optical and microphysical properties (Morgan et al, 2006;Kaskaoutis et al, 2007). Accurate assessment of the aerosol impacts on radiative forcing is a complex task, since various aerosol types cause different effects on the solar radiation (Kaskaoutis and Kambezidis, 2008) and also have different effects on the sign and magnitude of the aerosol radiative forcing (Heintzenberg et al, 1997;Satheesh and Moorthy, 2005).…”

Aerosol Optical Depth measurements at 340 nm with a Brewer spectrophotometer and comparison with Cimel sunphotometer observations at Uccle, Belgium

“…This results in an accumulation of high aerosol concentrations above midlatitude regions. Kaskaoutis et al (2007) explain the higher summer AODs at Ispra as a result of the absence of wet removal processes. According to Kazadzis et al (2007), the enhanced evaporation and the higher temperatures during summer in Thessaloniki cause a rise in the turbidity of the boundary layer.…”

Ship-Borne Measurements of Columnar and Surface Aerosol Loading over the Bay of Bengal during W-ICARB Campaign: Role of Airmass Transport, Latitudinal and Longitudinal Gradients

“…The curvature of the lnAOD vs. lnλ was also used to have some insight on the aerosol distribution and fine-tocoarse mode dominance Schuster et al, 2006;Kaskaoutis et al, 2007). The curvature is characterized by the coefficient α 2 (Eq.…”