Analysis of aerosol scattering properties measured by a nephelometer at a coastal-rural site in the Atlantic southwest of the Iberian Peninsula

Abstract

Aerosol hemispherical scattering and the backscattering coefficients, σsp, σbsc, have been measured using a 3-wavelength (450, 550 and 700nm) integrating nephelometer over two years (January 2006–May 2008) in the coastal area of the Gulf of Cádiz, in the southwest of the Iberian Peninsula. These coefficients have been carefully analyzed starting with the impact of corrections on the measurements of σsp: i.e., drift calibration constants do not modify the mean value in our data series. However, the selection of dry data (with RH less than 50%) modifies substantially the number of data and the resulting mean value of σsp is now 14% lower, which is compensated when the angular truncation correction is applied. The characterization and features of σsp, σbsc, and the derived parameters αsp (alpha Ångström exponent) and b (the backscatter ratio) has been analyzed, as annual, seasonal and diurnal evolution. A general statistic based on hourly data gives mean values and standard deviation of σsp (500nm)=48±38Mm−1 with a median of 38Mm−1, and σbsc (500nm)=5.6±3.8Mm−1 with a median of 4.6Mm−1. Thus, these values show moderate-low values but with a large range of variation considering the existing measured values over the Iberian Peninsula. The median value of σsp (500nm) is an indicator that events of high aerosol burden are frequent presenting a substantial influence on the daily averages. The alpha Ångström exponent, αsp, derived from the pairs 450/700nm gives a mean value 1.35±0.54 with a median of 1.47 and with the most frequent value of 1.7, thus indicating the prevalence of medium size particles but with a significant influence of fine particles. The b ratio has the same value for mean and median, 0.12±0.02, showing a decrease with increasing values of σsp. Annual and daily cycles have been also analyzed showing the complex behaviour of the optical properties at this coastal site where cold and warn periods show very different characteristics. Besides, the daily cycle is strongly influenced by the breeze conditions. According to the information extracted from the σsp–αsp relationship this rural Atlantic site presents prevalence of mixed marine-continental aerosols but with significant influence of local anthropogenic sources. Desert dust particles were also discriminated which are not frequently reported in the data of “in situ” aerosols.