The GAW-PFR aerosol optical depth network: The 2008–2013 time series at cape point station, South Africa

Abstract

A ground-based aerosol optical depth (AOD) climatology is presented for Cape Point (CPT) station, South Africa, for the 2008–2013 period. CPT is part of the Global Atmosphere Watch–Precision Filter Radiometer network which conducts long-term AOD measurements at remote background sites. AOD (λ = 500 nm) and Ångström exponent (368 to 862 nm; α368–862) averages for the entire period were 0.059 and 0.68, displaying only a weak seasonality. Based on an established method for air mass classification using the in situ wind direction and222Rn concentration, the following four air mass types were used to further investigate AOD: background marine, marine, mixed, and continental. AOD was similar for all types, but α368–862was distinctly lower (0.43) for background marine and higher (1.07) for continental air masses, illustrating the presence of coarse mode and anthropogenic aerosols, respectively. Trajectory cluster analysis of 5 day back trajectories confirmed/augmented this classification. AOD for background marine and marine air mass types were consistent with ship-based (Maritime Aerosol Network) and island (AErosol RObotic NETwork)measurements, suggesting that CPT is a suitable site to monitor pristine conditions in the South Atlantic and Southern Oceans when222Rn concentrations are <100 mBqm-3.