Particle optical properties at a Central Mediterranean site: Impact of advection routes and local meteorology

Abstract

A multiwavelength integrating nephelometer and a PM10 sampler have been used to continuously measure optical properties and mass concentrations of particles at the ground level with the main aim of determining airflow and local meteorology effects on particle optical properties and PM10 mass concentrations. Measurements have been performed at Lecce (Italy), a coastal site in the Central Mediterranean and included scattering (σp) and hemispheric backscattering (βp) coefficients, and hemispheric backscattering fraction (βp/σp) at three wavelengths (450, 525, and 635nm), in addition to PM10 concentrations. The scattering Ångström exponent (å (λ1, λ2)) for different wavelength pairs (λ1, λ2), and the scattering Ångström exponent difference (∆å=å (450nm, 525nm)−å (525nm, 635nm)) have been calculated to estimate the airflow impact on the relative weight of fine and coarse mode particles. The yearly mean values±1 standard deviation of σp (450nm), βp (450nm), βp/σp (450nm), å (450, 635nm) and ∆å are equal to 100±50Mm−1, 12±6Mm−1, 0.13±0.02, 1.1±0.4, and 0.2±0.2, respectively. σp is well correlated to PM10 mass concentrations (r=0.96) and the PM10 mass scattering cross section is equal to 3.6±0.1m2g−1. The back trajectory cluster analysis has identified 5, 5, 7 and 7 distinct airflow types reaching the study site at 271, 500, 1500 and 3000m above the sea level, respectively, with only slight differences in airflow type among the four arrival heights. We have found that σp and βp values and their respective dependence on wavelength are strongly dependent on airflows. Therefore, we have shown that the in situ particle properties and the local meteorological parameters vary with advection routes. Given the dependence of å, ∆å, and βp/σp on particle size and shape, their strong association with airflows has indicated that the mean size distribution of sampled particles varies with air mass history and it has been shown that å, ∆å, and βp/σp values allow a satisfactory differentiation of the particle properties associated with different advection routes. More specifically, å and ∆å values have allowed distinguishing the airflows mainly responsible for the advection of fine mode particles from the ones which are mainly responsible for the advection of coarse mode particles. These results have provided a satisfactory understanding of the dependence on air flows of the mass scattering cross section values which vary from 2.9±0.2m2g−1 to 4.3±0.1m2g−1 with air flows. The airflow analysis has also allowed understanding the seasonality of the particle optical properties being linked to the airflow seasonality.