Ambient measurements of aerosol properties are required to further our understanding of the role of aerosol in climate and the detrimental effects on human health . The dynamic nature of atmospheric processes make the online measurement of aerosol properties very challenging, and the choice of techniques depends on the specific aerosol property of interest and the temporal resolution required. In the field of atmospheric science, the importance of refractory aerosols in atmospheric processes has increased the demand for online measurement of size-resolved composition at low number concentrations.

In single-particle mass spectrometry (SPMS), high-powered laser pulses are used to vaporise and ionise ambient aerosols that have been focussed into a narrow particle beam. The generated ions are usually analysed by time-of-flight mass spectrometry (TOF-MS), providing detailed composition information on a particle-by-particle basis. This approach allows for aerosol particle number concentrations to be evaluated by particle composition as well as the probing of internal mixing state that cannot be done by bulk analysis techniques e.g.. The type of material that can be analysed is determined by the power and wavelength of the pulsed laser system, whilst the temporal resolution is related to the probability that a particle will be coincident with the laser pulse in the ion source, a phenomenon often referred to as hit rate. In many systems, optical particle detection is incorporated into the instrument to provide an external trigger to the pulsed laser – temporally aligning the laser pulse with the presence of a particle in the ionisation region, resulting in an increased hit rate.

The incorporation of two optical detection stages into the instrument allows the determination of particle velocity by measuring the time taken for particles to travel the known distance between two detection stages. When used with an aerodynamic lens inlet, particle velocity is a function of the vacuum aerodynamic diameter, allowing for a particle size measurement to be made in addition to the single-particle composition measurement made by the TOF-MS. In some instruments, the optical detection stage has also been used to optically size particles and to directly measure light-absorbing properties by calculating the scattering cross section from the intensity of detected light. These tandem measurements of single particles allow for the direct linking of composition with physical properties such as size, shape, and density . The design of the optical detection stage has a strong influence on the overall instrument performance and has been the focus of much research . The features of SPMS instruments have previously been described by , ,, and .

Optical particle detection techniques are well established for the size-resolved counting of particles in optical particle counters (OPCs) , which are similar in many respects to the optical detection systems employed in SPMS. Particles are detected by collecting the scattered light generated from the interaction of a particle beam with a continuous-wave (cw) laser. When employed in SPMS, the scattering signals need only exceed a certain threshold to register a particle event, unlike standard OPCs, which optically size the particle based on the magnitude of the scattering signal. Incorporating an optical system into the geometry of a SPMS creates additional design challenges.

Advances in laser technology have influenced the design and development of the optical detection stage in SPMS. Early instruments utilised a helium–neon gas laser with a wavelength of 633 nm with an output in the range of 4–10 mW. The implementation of 532 nm Nd:YAG solid-state lasers with an output in the range of 50–300 mW greatly improved laser fluence and beam quality in later-generation instrument . Other groups have opted for newly developed laser diodes that, while producing less output (40 mW), have shorter wavelengths (405 nm) and are relatively cheap and easy to implement. Shorter wavelengths are desirable when sampling particles whose diameter (D) is smaller than the wavelength of the incident radiation as Csca is proportional to D6 in the Rayleigh regime.

The most efficient particle detection systems use an elliptical mirror to collect scattered light over a wide angle, thus maximising scattering signal at the detector . However, the physical dimensions of a standard elliptical mirror prevent the detection stage being located within the ionisation region. Consequently, it must be located upstream in the vacuum housing, and complex trigger circuits must be made that produce a particle-size-dependent trigger delay . Such systems have excellent particle detection efficiencies but have a hit rate that is limited by the probability of hitting a detected particle with the pulsed laser.

Composition measurements with SPMS are usually considered qualitative due to shot-to-shot variations in instrument function and a strong matrix effect that influences the ionisation process . These phenomena result in ion signals that are not proportional to the mass of the chemical species and a particle number counting bias with respect to particle type. Numerous studies have reported quantitative or semi-quantitative results by using relative sensitivity factors to account for matrix effects , accounting for transmission and hit rate bias , and calculating scaling functions by referencing conventional particle counters . Quantification of particle number concentrations by these methods requires large assumptions to made about particle properties such as shape, density, and refractive index, making the application to ambient aerosol difficult. For instruments that utilise elliptical mirrors for particle detection, the optical particle detection efficiency is usually only considered a limiting factor when assessing the low particle size cut.

Despite these limitations, SPMS has proven very useful in measuring relative trends in particle number concentrations and chemical markers. A number of custom-built instrument are in operation for laboratory, field-based, and aircraft-based studies. In addition, the commercially available aerosol time-of-flight mass spectrometer (ATOFMS) (model 3800, TSI Inc.) is operated by several research groups worldwide. Recent examples of the application of this instrument include source apportionment and mixing-state studies in urban environments , the study of the atmospheric ageing of mineral dust during long-range transport , and the characterisation of particles of low number concentration in the Arctic . The direct comparison of the response of single-particle instruments with different designs highlights the need to consider the factors that affect particle counting statistics in SPMS.

The laser ablation aerosol particle time-of-flight (LAAP-TOF) spectrometer is a type of single-particle mass spectrometer manufactured by AeroMegt (GmbH) and is in an early stage of commercial development. The instrument features an aerodynamic lens inlet (model LPL-2.5, AeroMegt GmbH), a bipolar TOF analyser (TOFWerks AG), a novel particle detection system based on 405 nm laser diode technology, and a compact light collection optics assembly consisting of fibre optic guides that collect scattered light over a narrow scattering angle and is located directly in the ionisation region.

We present an evaluation of the instrument performance with the original instrument manufacturer (OEM) detection stage design that identified the optical detection system as a limiting factor in instrument performance. The instrument performance is compared with a customised detection system in which the detection laser is replaced with a fibre-coupled 532 nm 1 W Nd:YAG solid-state laser system with a collimated laser beam. The influence of detection stage geometry is evaluated using the customised detection system as the laser intensity distribution within a collimated beam is relatively simple to model because, unlike a focused beam, there is no variation along the beam axis due to depth of field.

The instrument design has previously been described by . Here, we provide a brief overview of the instrument layout and describe the modifications made to the optical detection system. A schematic layout of the instrument is shown in Fig. . Aerosol enters the instrument via a 100 µm critical orifice and passes through an aerodynamic lens before beam expansion into the first low-pressure region of the instrument. The particle beam passes through second and third differentially pumped stages separated by skimmers that remove the majority of the gas phase. Detection stage 1 is encountered in the third pumping stage when the particles pass through a cw laser beam arranged orthogonally to the axis of the particle beam. After passing a differentially pumped aperture into the TOF vacuum region, the particles encounter detection stage 2, which is located within the extraction optics of the TOF analyser. Detection stage 2 triggers an excimer laser (ArF λ=193 nm, model EX5, GAM) to fire an intense pulse in a direction that is co-axial but counter-propagate with the particle beam. The cloud of ions generated by the interaction of the material with the high-energy pulse is extracted into two linearly opposing TOF analysers, for positive and negative ions respectively.

Instrument control is performed using IgorDAQ software (AeroMegt Gmbh) based on IGOR (Wavemetrics), which incorporates TofDaq (TOFWerk AG) data acquisition. The instrument can be operated in three modes:

Auto-trigger. The excimer is set to fire at a user-defined frequency (up to 100 Hz), and mass spectra will be generated at a rate that is determined by the probability of a particle being synchronous with sufficient energy from the 8 ns excimer pulse in the ionisation region.

The instrument features two optical particle detection stages which have very similar designs. A high-powered cw laser beam, arranged in a vertical orientation, passes through the instrument to a beam dump on the underside of the vacuum chamber. The instrument is aligned so that the particles in the particle beam interact with the detection laser radiation in a position where scattered light can be collected by a set of 12 optical fibres, arranged in a concentric pattern centred on the axis of the cw laser beam. The diameter of the circle is defined by the naked fibre termini, and the position below the axis of the particle beam is such that light is collected over an angle of ∼ 13–15∘ with respect to the incident laser radiation at detection stage 1 and ∼ 10–12∘ with respect to the incident laser radiation at detection stage 2. The compact design of the collection optics allows the detection of particles within the ionisation region of the mass spectrometer which negates the requirement for size-dependent trigger delays that are a feature of some systems.

The optical fibres for light collection are connected to two photomultiplier tubes (PMTs) so that each PMT receives light from six fibres. A PMT signal pulse height above a user-set threshold results in a transistor–transistor logic (TTL) pulse from the discriminator to the microprocessor control unit (MCU) in the timing electronics. A pulse is required from both PMTs in the detections stage in order for the MCU to recognise the presence of a particle. The thresholds are set experimentally by finding the minimum threshold value at which false triggers (noise) are not created – resulting in the discrimination between high-frequency noise and true particles, improving the signal to noise performance of the detection system in a similar set-up to that described by .

In this study, two different systems of laser delivery are evaluated: detection system A, based on a laser diode emitting light at 405 nm that is focussed by a lens with a long focal length and spatially filtered with a sheath within the vacuum chamber, and detection system B, based on a custom-built fibre-coupled system featuring a Nd:YAG diode-pumped solid-state laser (DPSS), emitting at 532 nm that is collimated by a fiberport and spatially filtered by an orifice outside the vacuum chamber. Schematic diagrams of detection system A and detection system B are shown in Fig. a and b respectively. A summary of the key characteristics of the laser systems is given in Table .

Laser type is a fundamental choice that has an effect on the implementation, usability, and reliability of the system. Laser diodes have the advantage of being relatively cheap and easy to implement. However, diodes have a significantly shorter lifetime than a DPSS system especially if operated at or close to maximum output power. Consequently, the laser diode in detection system A is operated at a fraction of the maximum available output power which is set by adjusting the drive to the diode. The output that is produced by a set drive voltage deteriorates over an diode-specific timescale, and, as the actual output can only be measuring with a laser power meter outside the instrument, there is some uncertainty as to the actual diode power output if the instrument is continually operated over a number of days or weeks.

In contrast, the Nd:YAG DPSS laser is specified as having an mean time to failure (MTTF) of > 400 000 h at full power and automatically maintains output power to the user-requested value. However the system is much larger and more difficult to implement than the diode system, particularly in a transportable instrument. A fibre coupling was chosen to transfer the beam from the laser to the detection stage to eliminate the need for complex optomechanics. The coupling of a 3.3 µm single-mode fibre in this set-up results in a power transmission efficiency of 40 %. The typical operation condition of this system produces an output of 300 mW in a collimated beam after spatial filtering.

Overall, the extra power available from the fibre-coupled system allowed for a larger detection beam width without significantly reducing the peak intensity produced by the tightly focussed diode beam. A more detailed description of the detection system designs can be found in Appendix B.

A number of methodologies were used to evaluate the instrument performance. The creation of aerosols in the laboratory was required for the instrument set-up procedure, instrument sensitivity testing, and the subsequent evaluation of performance of the particle detection system. Ambient sampling was carried out to measure the sensitivity of the instrument in atmospheric conditions and assess the suitability of the system for in situ measurement of size-resolved chemical composition of atmospheric aerosols.

In Sect. 4.1 we compare the results of laboratory measurements of the instrument performance with detection system A and detection system B in second-laser-only acquisition mode (ETriggeredMS), which showed an improvement in instrument performance when the detection system with the collimated beam was implemented. Using these results, along with a measurement of the particle beam width (Sect. 4.2), we calculated the value for the transfer function (K) described in the model by the empirical measurement of the required model parameters with a 600 nm PSL particle. The method used for calculating the transfer function is described in Sect. 4.3.

The input of the transfer function into the numerical model of the optical detection geometry allowed the calculation of the effective detection radius (R) with respect to particle diameter across the transmission range of the instrument (0.2–2.5 µm). The results of modelling the effect of different optical detection system design parameters on R are presented in Sect. 4.4. In Sect. 4.5 we use the modelled data to explain the differences between particle size distributions measured by the LAAP-TOF and those measured by the APS in ambient data.

In the model of the optical detection geometry we describe the optical detection efficiency in terms of the overlap of the detection beam with the particle beam. The Gaussian profile of the detection laser beam results in a particle-dependent variation in the effective beam width, which we term the effective detection radius (R). For a certain instrument design, R is a function of the scattering cross section of the particle, resulting in a particle size dependence. Instrument design parameters such as the detection laser wavelength, the light collection angle, and the signal-to-noise level in the detection stage have a strong influence on R.

Ambient measurements demonstrate the impact that the variation in the effective detection radius may have on the data. Comparison with the size distribution measured with the APS suggests that a size-dependent detection bias is imposed on the measured size distributions. Furthermore, the particle refractive index appears to have a strong influence on this detection bias, which further complicates the interpretation of size distributions. Overall, the LAAP-TOF reports particle concentration in the size range 0.5–2.5 µm, which is approximately 2 orders of magnitude lower than those reported by the APS. Much of this deficit in particle counting may be accounted for by composition-dependent ionisation efficiency on the ion source and a reduction in optical detection efficiency due to particle beam divergence. However, the optical detection efficiency must also be influenced by the size-dependent variation in the effective detection radius. For example, the coincidence of a maximum in R with the mode size of the sea salt class of particles will enhance the number counting statistics of those particle types.

Optical detection of particles in SPMS has been the focus of much research . Early instruments used fibre optics to guide scattered light to PMTs for detection . The introduction of an ion source extraction plate fashioned into an elliptical mirror allowed light to be collected over a wider solid angle within the ion source whilst maintaining relatively high hit rates, but this approach is not amenable to a bipolar TOF analyser due to extraction field distortion. The removal of the detection optics from the ion source region permitted a more complete elliptical mirror to be used but required a more complex trigger system to ensure the UV ablation laser hits particles travelling at different velocities.

The aim of using elliptical mirrors is to collect as much scattered light as practicable over a wide solid angle. The benefit of this approach is demonstrated in the model results that calculate R to be close to the 3σd width across much of the transmission range. Authors have reported detection efficiencies of 0.5–1 for instruments using this type of collection optics over selected size ranges, typically 200–600 nm. Hit rates are generally lower with instruments that do not trigger in the ion source, and overall sampling efficiency is rarely reported in the literature.

An ideal instrument design would have optical detection in the ion source of a bipolar TOF while maintaining the optical detection efficiency of an elliptical mirror. This is very difficult to achieve in practice. The compact design of the light collection optics in the LAAP-TOF allows for the detection stage to be placed in the ion source but collects light over a very narrow solid angle. Our initial laboratory studies with detection system A showed a low detection efficiency in the size range 300–800 nm, which was a limiting factor in the overall instrument performance.

Laser diode light sources have relatively low power and produce a relatively divergent beam that can be difficult to focus with a single lens. The implementation of a 532 nm Nd:YAG laser with high pointing stability allowed for single-mode fibre coupling and efficient collimation to a larger beam waist diameter compared to the focal point of detection system A. The implementation of detection system B resulted in over an order-of-magnitude improvement in ETriggeredMS compared to detection system A, but it came at the expense of a slightly larger particle size cut-off. Larger beam diameters have the potential to increase the detection efficiency for some particle sizes. However, modelling of the effective beam diameter shows that, for a set laser power output, the optimum detection laser beam width is particle size dependent (Fig. a). The particle size distribution of the target application must be considered when choosing the output power and focussing characteristics of the detection laser system if using light collection optics with a narrow collection angle.

Optical particle detection is an established technique in instruments dedicated to measuring particle size distributions of ambient aerosol populations . The smoothing of Mie scattering oscillation is a design requirement when accurate particle size measurement requires a near-monotonic response in pulse magnitude with respect to particle diameter . Instruments utilising monochromatic laser source require a wide collection angle, whereas a monotonic response has been reported for a near-forward scattering instrument using an incandescent (white-light) source .

A detection laser consisting of mixed wavelengths may be beneficial to the near-forward light collection system utilised in the LAAP-TOF. The Mie theory model (Fig. ) indicates that 532 nm may complement 808 nm by covering the deep oscillation in the profile. The mixing of light from two distinct sources is possible with a fibre-coupled system. Figure b shows an example of the modelled R with respect to particle size and detection beam width using equal powered 532 and 808 nm wavelength sources whilst maintaining the signal-to-noise characteristics of detection system B. This model shows less variation in R with respect to particle size and offers the possibility of using larger σd, which would improve the overall sampling efficiency.

A custom detection laser system consisting of a high-powered fibre-coupled Nd:YAG solid-state laser with a collimated beam was implemented in a LAAP-TOF single-particle mass spectrometer without major modifications to instrument geometry. The new laser system resulted in an order-of-magnitude improvement in sensitivity to spherical particles in the size range 500–800 nm compared to a focussed 405 nm laser diode system. A numerical model is presented that allows for a general evaluation of how beam width, wavelength, and light collection geometry affect the particle detection efficiency of the optical detection stage. We used the model to explain number counting bias in an ambient data set.

The laser intensity encountered by a particle in a collimated laser beam is a function of its position within the Gaussian intensity distribution with respect to the laser beam axis. A transfer function was calculated in order to quantify the minimum intensity requirement which defines an effective detection radius (R) that is a function of the scattering cross section (Csca) of the particle. The model predicts that Csca controls the limit of the detection in terms of particle size as expected. However, if light is collected over a narrow collection angle, Mie interference patterns result in an oscillation of R with respect to particle size across the transmission range (0.2–2.5 µm) of the LAAP-TOF, resulting in large particle-size-dependent variation in detection efficiency. We compare the model prediction with an ambient data set acquired during the ICE-D project, a multi-platform field campaign based at the Cabo Verde islands in August 2015. The model is used to partly explain a detection bias towards sea salt particles with an aerodynamic size mode of ≈ 1.5 µm. We also show that the detection bias imposes itself on the measured aerodynamic size distribution as determined by the instrument, an effect that must be considered when interpreting the data.

Modelling of the effective detection radius shows that for a set laser power output the optimum detection laser beam width is also particle size dependent. The particle size distribution of the target application must be considered when choosing the output power and focussing characteristics of the detection laser system if using light collection optics with a narrow collection angle. Variations in the effective detection radius could be minimised by collecting light over a wider angle or by mixing laser wavelengths. The stabilisation of R with respect to particle diameter would result in more accurate aerodynamic size distribution measurements and reduce the variation in particle number concentration measurements of different particle size, shape, and refractive index. A more rigorous evaluation of the effective of particle size and morphology on the overall sampling efficiency of the instrument would require a model of the aerodynamic lens characteristics in order to constrain particle beam divergence. The effect of particle beam divergence on both the variation and absolute sampling efficiency could be improved by reducing the length of the particle flight path by shortening the vacuum housing.

Laboratory-acquired data used to inform the model of the the effective beam radius are available from the author by request.