A single-beam photothermal interferometer for in situ measurements of aerosol light absorption

Abstract

Abstract. We have developed a novel single-beam photothermal interferometer and present here its application for the measurement of aerosol light absorption. The use of only a single laser beam allows for a compact optical set-up and significantly easier alignment compared to standard dual-beam photothermal interferometers, making it ideal for field measurements. Due to a unique configuration of the reference interferometer arm, light absorption by aerosols can be determined directly – even in the presence of light-absorbing gases. The instrument can be calibrated directly with light-absorbing gases, such as NO2, and can be used to calibrate other light absorption instruments. The detection limits (1σ) for absorption for 10 and 60 s averaging times were determined to be 14.6 and 7.4 Mm−1, respectively, which for a mass absorption cross section of 10 m2 g−1 leads to equivalent black carbon concentration detection limits of 1460 and 740 ng m−3, respectively. The detection limit could be reduced further by improvements to the isolation of the instrument and the signal detection and processing schemes employed.

Determination of the multiple-scattering correction factor and its cross-sensitivity to scattering and wavelength dependence for different AE33 Aethalometer filter tapes: a multi-instrumental approach

“…Unlike BC, which absorbs radiation in a wide range of wavelengths (from UV to infrared) with a wavelength-independent refractive index, the BrC and mineral dust refractive index increases at shorter wavelengths, close to the UV range (Kirchstetter et al, 2004a;Andreae and Gelencsér, 2006;Bergstrom et al, 2007;Laskin et al, 2015;Cappa et al, 2019). Therefore, having at one's disposal accurate absorption measurement techniques is crucial to determining particles' light absorption which can afterwards be used in climate projections (Mengis and Matthews, 2020;Wang et al, 2020). Moreover, there is also the need for standard aerosol particles to use as a reference for the quality assurance of absorption measurements, such as the recently developed flame-generated soot in Ess and Vasilatou (2019).…”

“…However, in the case of photo-acoustic spectroscopy measurements, the heating of the sample and the evaporation of coating materials on the sample may lead to higher detection limit and artefacts impairing the measurement accuracy (Lack et al, 2006;Linke et al, 2016). The photo-thermal interferometry (PTI) is an absorption measurement technique originally developed for measurements of trace gases that has also been applied to aerosol measurements (Lee and Moosmüller, 2020;Visser et al, 2020). However, the aforementioned techniques have so far proved difficult to deploy in a field setting, thus limiting their broader use in international measurement networks.…”

Response of black carbon and aerosol absorption measuring instruments to laboratory-generated soot coated with controlled amounts of secondary organic matter

“…The photo-thermal interferometer PTAAM-2λ is based on a folded Mach-Zender interferometer design (similar to (Moosmüller and Arnott, 1996;Sedlacek, 2006;Visser et al, 2020)). The He-Ne probe laser beam is split into the sample chamber and reference beams.…”

“…The photo-thermal interferometer MSPTI is an improved version of the instrument presented in (Visser et al, 2020). Briefly, the instrument design is similar to a Mach-Zehnder interferometer (Moosmüller and Arnott, 1996;Sedlacek, 2006), with the optical elements in the interferometer consisting of a combined beam splitter and mirror block and a retroreflector.…”

“…In these experiments the filtered sample (HEPA grade absolute filter) is employed as the "zero" sample in the reference arm of the interferometer. Phase quadrature is maintained via an improved version of the pressure cell from (Visser et al, 2020). The MSPTI is calibrated using NO2 and was operated at a flow rate of 0.25 L min -1 .…”