Comparison of online and offline methods for measuring fine secondary inorganic ions and carbonaceous aerosols in the central mediterranean area

Online hourly concentrations of nitrate (NO3-), sulphate (SO42-) and ammonium (NH4+), and two hourly concentrations of organic carbon (OC) and elemental carbon (EC) were compared with the daily concentrations determined offline from filter samples. The comparison was performed over two months (May and June 2010) at a coastal site characterized by low local pollution, located in the Southern Italy (Central Mediterranean basin). The online measurements of the above inorganic ions and carbonaceous components of the aerosols were carried out using an Ambient Ion Monitor-Ion Chromatograph (AIM IC URG 9000-D) and a Sunset Laboratory Model-4 Semi-Continuous OCEC Field Analyzer, respectively, while the offline analyses were performed with two ion chromatographs instruments (Dionex DX-120 and Dionex 500) and a Sunset Laboratory Dual-Optical Carbonaceous Analyzer (Sunset Laboratory). The collocation of instruments allowed to evaluate the differences between online and offline daily concentrations and to interpret the origin of positive and negative sampling artefacts. As an example, the offline OC concentrations are constantly higher than online concentrations and this is explained by positive artefacts of offline measurements. The analysis was performed separately for days with and without transport of Saharan dust and revealed significant enhancement of concentrations for both inorganic ions and carbonaceous aerosols in the presence of dust. The increase of EC concentrations during dust events may be explained by an increase of emissions transport from sea traffic to the site. Moreover, the results showed that the online instruments may be used at a low polluted site for the acquisition of high temporal resolution data.