Abstract
Online hourly concentrations of nitrate (NO_3 ^-), sulphate (SO_4 ^(2-)) and ammonium (NH_4^+), 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.
Highlights
In the last two decades an increasing interest for understanding the relationship between atmospheric aerosol and population health had led to the development of online techniques and instruments able to quantify the concentration of specific chemical with a high time resolution (Chow, 1995)
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)
Online measurements of inorganic water-soluble ions and carbonaceous compounds obtained with an Ambient Ion Monitor-Ion Chromatograph (AIM-IC) system and an OCEC Semi-Continuous analyzer were compared with offline filter measurements on daily basis when the monitor data completeness was fulfilled
Summary
In the last two decades an increasing interest for understanding the relationship between atmospheric aerosol and population health had led to the development of online techniques and instruments able to quantify the concentration of specific chemical with a high time resolution (one hour or less) (Chow, 1995). Some online measurements require daily operator intervention and maintenance, especially when continuous long data series are intended to be obtained in AQ monitoring networks. Further tests of the online methods are still needed for assessing their performances in environments with different/ various aerosol mixtures and concentrations in order to gain a deeper understanding of the limitations and strengths of instruments in different atmospheric conditions (Bauer et al, 2009; Nie et al, 2010; Markovic et al, 2012). Online measurements of inorganic water-soluble ions and carbonaceous compounds obtained with an Ambient Ion Monitor-Ion Chromatograph (AIM-IC) system and an OCEC Semi-Continuous analyzer were compared with offline filter measurements on daily basis when the monitor data completeness was fulfilled. The comparability and correlations between the monitor and filter-based data is shown
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