A filter-based light-absorption measurement with polar photometer: effects of sampling artefacts from organic carbon

In this work, light-absorption measurements were carried out on quartz-fibre and PTFE (i.e. Teflon®) filters using a home-made polar photometer in order to assess the role of organic sampling artefacts on optical measurements. Filter-based light-absorption instruments are generally operated with quartz/glass-fibre filter tapes and – as far as we know – currently there are no systematic studies on the sampling artefacts affecting the light absorption coefficient measurements. In this work, particulate matter (PM) samples were collected in Milan (Italy) during different sampling campaigns with the aim of (1) validating the polar photometer; (2) comparing the absorption properties measured on PM collected on different filter media; (3) investigating the causes of the observed differences. The polar photometer was demonstrated to give aerosol light absorption coefficients (σap in Mm−1) fully comparable to those obtained by a Multi-Angle Absorption Photometer (MAAP) when using quartz-fibre filters to collect atmospheric particles. The aerosol light absorption coefficients determined on samples collected in parallel on quartz-fibre and PTFE filters showed significant differences (about 40%). These differences were much lower (i.e. 8%) when the quartz-fibre filter sampling line was equipped with a denuder to remove organic gases from the incoming air stream evidencing a relevant role of organic sampling artefacts in light absorption measurements by filter-based systems.