An optical set-up for the multi-wavelength characterization of carbonaceous particulate matter

Carbonaceous aerosol is a major component of urban particulate matter (PM), in particular of its finer fractions (PM2.5, PM1). The elemental part, often referred to as EC, is strongly light absorbing: when determined by optical methods it is usually called “black carbon” (BC). The two quantities, EC and BC, do not exactly define the same PM component. PM samples are routinely collected in urban areas to monitor PM10 and PM2.5 levels. Low volume sequential samplers (LVS) are often equipped with 47 mm filters and operated on a daily basis. PTFE (Teflon®) membranes are very appropriate for gravimetric and compositional analyses of PM and currently 47 mm filters with 2 um pores are often used to collect PM samples on daily basis (LVS). We have developed a simple, fully automatic and non-destructive new optical system to measure off-line the light absorption and hence the BC content in the PM on the PTFE filters routinely used for PM studies and on other collecting substrates. This gives the opportunity to measure in each sample the concentration of total PM by gravimetric analysis, BC, metals by, for instance, X Ray Fluorescence, and ions by Ion Chromatography. All these pieces of information can be obtained with just one filter being sure to analyze the same PM. The set-up we realized is composed by a collimated laser source and three low-noise photodiodes (PD) placed at 0, 125 and 160 degrees with respect to the laser beam direction. The PDs active surface is about 65 mm2 and a large fraction of the forward scattered radiation is collected by the 0-degree PD positioned just behind the sample. Samples can be analyzed in sequence and in an automatic and controlled way in about 15 minutes thanks to a wheel which can host up to 16 47-mm filters. The wheel is connected to a stepping motor to change the filter under analysis and to two linear translator which allow the scanning of the whole deposition area. All the movements and the acquisition of the photodiode signals are controlled by a PC using a Labview 8.5 home-made software. The set-up is completed by three different interchangeable laser sources: 4 mW - red (l = 635nm), 4 mW - blue (l = 405nm) and 20 mW - IR (l = 850nm). For the calculation of BC we have adopted and extended the method, based on the radiative transfer scheme proposed by Hänel (Hänel, 1987 and 1994), used by the MAAP (Petzold and Schönlinner, 2004). Furthermore, thanks to the computer control, the analysis of non-homogeneous PM samples, as those collected by multi-stage cascade impactors, is also possible. Our new instrument has been validated in different campaigns and, for the first time, has been employed in a long campaign realized in the frame of the MED – APICE program (Common Mediterranean strategy and local practical Actions for the mitigation of Port, Industries and Cities Emissions, http://www.apice-project.eu/). In this case, a 6-month PM2.5 sampling campaign was organized between spring and summer 2011, at three sites in the urban area of Genoa (Italy). A LVS (TRC-Tecora, Italy) was used to collect daily PM2.5 alternatively on quartz fiber and on PTFE filters. All the samples (about 250 per each medium) were first analyzed with the optical set-up and quartz fiber filters only were subsequently analyzed by a SUNSET EC/OC thermal-optical instrument adopting the EUSAAR_2 protocol in order to obtain the EC load on each sample. Comparing the results obtained from the two different techniques it is possible to find the aerosol light absorption cross-section Sabs for the different sites (in Figure 1 the results for one of the three sites). In this way we are able to calculate the BC concentration also for the days in which PTFE filters were used. We will briefly describe the optical method providing details on the set-up and on its components. Results of the analysis on different set of samples will be also presented, with particular attention to the results obtained from measurements at the three different wavelengths with the two different kinds of filters. Hänel, G., 1987. Contrib. Atmos. Res., 60, 241–247. Hänel, G., 1994. Appl. Optics, 33, 7187-7199. Petzold, A., Schölinner, M., 2004. J. Aerosol Sci. 35, 421-441.