Aerosol size properties at Armilla, Granada (Spain)

The size characteristics of atmospheric aerosol over a suburban environment on the outskirts of Granada, a non-industrialized medium sized city in south-eastern Spain, were investigated using five years of continuous spectral measurements in the range 368-778 nm. From these the aerosol optical depths at the corresponding wavelengths were derived. Aerosol columnar size distributions were then retrieved from the spectral optical depths. These, and a set of derived aerosol properties including columnar mass, columnar surface or columnar number of particles, were then examined. For the analysed series two different periods were observed, one with a prevalence towards Junge type distributions and a second where bimodal log-normal distributions were favoured; these coincided with periods of atypical severe drought and typical meteorological conditions, respectively, that affected the measurement area. The bimodal distributions present a clearly defined mode in the range 0.4-0.8 mum radius, with a less well-defined mode for smaller radii. For periods typifying normal conditions the behaviour of the lower and higher modes were analysed. The higher mode is slightly sharper with a consistent position compared to the lower mode. The relationships among the different aerosol size properties and the aerosol optical properties were also analysed. A good correlation was found between the Angstrom exponent, that measures the aerosol optical-depth's spectral dependence, and the ratio of columnar particle density for large and small particle sizes in the accumulation mode. It is also shown that the aerosol optical depth at 500 rim is a good predictor of columnar aerosol properties such as the aerosol mass or surface area.