Evaluating the influence of laser wavelength and detection stage geometry on optical detection efficiency in a single-particle mass spectrometer

Abstract

Abstract. Single-particle mass spectrometry (SPMS) is a useful tool for the online study of aerosols with the ability to measure size-resolved chemical composition with a temporal resolution relevant to atmospheric processes. In SPMS, optical particle detection is used for the effective temporal alignment of an ablation laser pulse with the presence of a particle in the ion source, and it gives the option of aerodynamic sizing by measuring the offset of particle arrival times between two detection stages. The efficiency of the optical detection stage has a strong influence on the overall instrument performance.A custom detection laser system consisting of a highpowered fibre-coupled Nd:YAG solid-state laser with a collimated beam was implemented in the detection stage of a laser ablation aerosol particle time-of-flight (LAAP-TOF) singleparticle mass spectrometer without major modifications to instrument geometry. The use of a collimated laser beam permitted the construction of a numerical model that predicts the effects of detection laser wavelength, output power, beam focussing characteristics, light collection angle, particle size, and refractive index on the effective detection radius (R) of the detection laser beam. We compare the model predictions with an ambient data set acquired during the Ice in Clouds Experiment -Dust (ICE-D) project.The new laser system resulted in an order-of-magnitude improvement in instrument sensitivity to spherical particles in the size range 500-800 nm compared to a focussed 405 nm laser diode system. The model demonstrates that the limit of detection in terms of particle size is determined by the scattering cross section (C sca ) as predicted by Mie theory. In addition, if light is collected over a narrow collection angle, oscillations in the magnitude of C sca with respect to particle diameter result in a variation in R, resulting in large particlesize-dependent variation in detection efficiency across the particle transmission range. This detection bias is imposed on the aerodynamic size distributions measured by the instrument and accounts for some of the detection bias towards sea salt particles in the ambient data set.

Aircraft and ground measurements of dust aerosols over the west African coast in summer 2015 during ICE-D and AER-D

“…On the ground, the SP2-measured hematite is highly correlated with the number concentrations of particles classified as silicate from clustering of single particle mass spectra from LAAP-TOF measurements (Fig. 10e, as detailed in Marsden et al, 2016), suggesting the measured hematite is a valid tracer for dust. Also, as Fig.…”

“…For comparison, a second Grimm OPC measured in situ on the roof of the container to avoid any of the inlet losses associated with sampling down the 10 m inlet line. A laser ablation aerosol particle time-of-flight (LAAP-TOF) single particle mass spectrometer (Marsden et al, 2016) was deployed to measure the composition of refractory aerosol in the size range 0.5-2.5 µm. Other instruments included a WIBS-4 bioaerosol spectrometer (Gabey et al, 2010) for biological aerosol detection, and aerosol filter samples were also collected for offline compositional particle analysis (these and related measurements will be reported elsewhere).…”

“…The dust observed in Period II is dominated by constant transport from the dust-laden coast along Morocco and western Sahara with a timescale of 2 days. Both pathways advected dust but from different source origins, and this is reflected in the different size and compositions of mineral dust in the two air masses (Marsden et al, 2016).…”

Online differentiation of mineral phase in aerosol particles by ion formation mechanism using a LAAP-TOF single-particle mass spectrometer

“…Consequently, particle-to-particle variations in the peak power and effective pulse duration can reduce the reproducibility of the measurement. A previous study with this instrument configuration has shown a sampling efficiency for ambient mineral dust of approximately 0.01 with a particle-size-dependent optical detection bias that favours the detection of particles with a mode of around 1.5 µm in diameter (Marsden et al, 2016). Sampling efficiency is important in SPMS as it influences the particle-counting statistics of an aerosol population and can introduce a bias of compositional measurement towards a particular particle type (Zelenyuk et al, 2009).…”

“…The LAAP-TOF used in this study is a modified version of the commercially available single-particle mass spectrometer manufactured by AeroMegt (GmbH) that features a modified optical detection system as described in detail by Marsden et al (2016). Briefly, particle-laden air enters the instrument via an aerodynamic lens inlet (Liu et al, 1995) which produces a narrow but divergent particle beam along the instrument axis.…”