Set-up of a multi wavelength polar photometer for off-line absorption coefficient measurements on 1-h resolved aerosol samples

Abstract

In this paper, a polar photometer (PP_UniMI) was set up to measure the aerosol absorption coefficient (σ ap) at four wavelengths (λ) on 1-h resolved aerosol samples collected using a streaker sampler. Due to the characteristics of such samples (small deposit area, low aerosol load, and limited substrate thickness-10μm), the main technical developments aimed at reaching suitable limits of detection (LODs). To this aim, multiple scattering between the sample and a suitable substrate were exploited to amplify the system sensitivity to absorbing particle load. In the paper, the development and test of this innovative approach is presented. LODs for σ ap in the range 5.0-11.6 Mm-1 were reached, depending on the wavelength. Such values were suitable for the analysis of 1-hour resolved samples collected at an urban background site in Milan (Italy) during a test campaign of 1-week carried out in winter 2015. The methodology was validated comparing σ ap measurements performed by PP_UniMI at λ=635nm on the streaker sample to the data obtained by a Multi-Angle Absorption Photometer (MAAP) operated in parallel. Agreement within 10% was found. To check the results obtained at other wavelengths, Ångström Absorption Exponent (AAE) was calculated from σ ap measurements at 4-λ. The AAE values resulted in the range of expectations for aerosol emitted by fossil fuel combustion (0.8-1.2) and wood burning (0.9-3.5), which are the main sources contributing to absorbing aerosol in urban areas in winter. 2 The analytical methodology can be extended to samples collected with high time resolution using other high-time resolution samplers (e.g. drum rotating impactors). This is-as far as we know-the first time that σ ap measurements are performed on streaker samples collected with 1-h resolution. Our results thus set PP_UniMI as an important tool for the community performing high time resolved sampling to widen the characterisation of such samples and to further develop source apportionment studies.

First Results of the “Carbonaceous Aerosol in Rome and Environs (CARE)” Experiment: Beyond Current Standards for PM10

“…The PP-UniMI polar photometer-a benchtop instrument developed by the Milan research group-was extensively described in [27,28] . Briefly, the instrument is based on the measurement on the scattering plane of the light transmitted and scattered in the forward and back hemispheres by unloaded and loaded samples using a photodiode mounted on a rotating arm.…”

The Multi-Wavelength Absorption Analyzer (MWAA) Model as a Tool for Source and Component Apportionment Based on Aerosol Absorption Properties: Application to Samples Collected in Different Environments

“…The Multi-Wavelength Absorption Analyzer (MWAA) instrument [20,33] and the polar photometer PP_UniMI [34,35] are multi-wavelength devices providing b abs (λ) measurements of aerosol collected on filters. The b abs (λ) measurements are performed at 375, 407, 532, 635, and 850 nm by MWAA [20], and at 405, 532, 635, 780 nm by PP_UniMI [35].…”

“…The b abs (λ) measurements are performed at 375, 407, 532, 635, and 850 nm by MWAA [20], and at 405, 532, 635, 780 nm by PP_UniMI [35].…”

“…Then, the total amounts of the light diffused in the forward and back hemispheres are evaluated by solid angle integration. This allows PP_UniMI to operate also on aerosol collected on other filter matrices (e.g., membrane filters, see [35]) with no need for external information on the phase function shape. Assumptions on the phase function shape are needed only for extrapolating values in the range 173-180 • .…”

“…Nevertheless, the model can be applied to any b abs (λ) data, provided that at least 4-λ measurements are performed. For example, off-line measurements performed by the polar photometer PP_UniMI [34,35] or on-line Aethalometer measurements (properly corrected) [36] can be other possible inputs to the MWAA model. The b abs (λ) measurements carried out by such instruments do not require any sample treatment after particle collection.…”