Development, characterization, and first field deployments of a novel aerosol mass spectrometer combining laser ablation and flash vaporization techniques for aircraft application at high altitudes

Aerosol particles are affecting human health and the environment. To estimate the effects, a detailed characterization of these particles is required. Particles can be characterized according to their size, shape, and chemical composition. The latter property can be determined by mass spectrometry. Two main types of mass spectrometers are currently used for in-situ chemical analysis of aerosol particles from the ambient atmosphere. One uses a pulsed laser to vaporize and ionize submicron to micrometer sized single particles before injecting ions into the mass spectrometer. The other uses thermal vaporization and electron impact ionization to analyze small particle ensembles. These ions are also detected by a mass spectrometer. In the here presented novel aerosol mass spectrometer ERICA (acronym for: ERc Instrument for Chemical composition of Aerosols) the two techniques were combined to obtain complementary chemical information about the aerosol. Such an instrumental design is unique. The first part is called ERICA-LAMS (ERICA Laser Ablation Mass Spectrometer) and the second part ERICA-AMS (ERICA Aerosol Mass Spectrometer). After being focused by an aerodynamic lens, the sampled ambient aerosol particles enter a high vacuum chamber, where they are detected by two laser light scattering units, namely the detection units, before they reach the laser ablation region. Here, the particles are vaporized and ionized by a pulsed, frequency quadrupled Nd:YAG laser. The ions are then measured by a bipolar Time-of-Flight mass spectrometer. Particles which are too small to be detected by the detection unit or are missed by the ablation laser continue their travel to the ERICA-AMS section of the instrument, within which they are flash-vaporized on a heated tungsten surface and ionized by electron impact ionization. The resulting positive ions are guided into a Compact Time-of-Flight mass spectrometer. The goal of this work was to set up, characterize, and deploy the ERICA instrument. To characterize the focused detection laser and ablation laser beams, dedicated experiments as well as the particle beam comprehensive laboratory measurements were conducted. For the laser beam characterization, the so-called knife-edge technique was adopted. The detection efficiency of the particle detection units was measured for various aerodynamic lens positions with PSL and ammonium nitrate particles in various sizes (91 nm – 5,145 nm, vacuum aerodynamic diameter). An adopted curve fitting procedure provided the particle beam parameters, such as the particle beam width, the effective detection radius, the particle beam divergence, and the transmission efficiency of the aerodynamic lens. Furthermore, two different particle size calibration functions were compared and validated. In addition, pure chemical substance particles (sodium chloride, ammonium nitrate, gold-spheres, and benz[a]anthracene) were sampled to validate the ERICA-LAMS mass spectra evaluation. By that it was found that the erosion of gold fragments from the gold-plated aircraft inlet is sufficiently low and the inlet reliably deployable. The dimensions of the instrument are 600 mm x 740 mm x 1,400 mm (height x width x length) with a total weight of around 200 kg. The first field deployment of ERICA took place during a test campaign in Kalamata, Greece, from August to September 2016 aboard the Russian high-altitude research aircraft M-55 Geophysica. This was the first aircraft phase of the StratoClim (acronym for: Stratospheric and upper tropospheric processes for better Climate predictions) project. Here, positive and negative ion mass spectra of single particles were concurrently measured in the lower stratosphere for the first time. The results of the data evaluation show that the ERICA-LAMS is capable of measuring elemental carbon and meteoric dust particles in altitudes where these particle types were expected besides organic and inorganic particle types. However, some improvements needed to be implemented to be enable measuring the mass concentration of particulate sulfate, nitrate, ammonium, and organic content quantitatively with the ERICA-AMS. The second aircraft phase was performed in Kathmandu, Nepal, from July to August 2017, where single particle and particle ensemble mass spectra were recorded in altitudes of up to 20 km. Here, nitrate-containing single particles and an enhanced nitrate mass concentration were observed in an aerosol layer that was previously identified as particulate nitrate-consisting layer by a remote sensing instrument. For the first time an AMS type of instrument was operated at such altitudes. By means of ERICA, answers concerning the long- standing scientific question about the chemical composition of ATAL (Asian Tropopause Aerosol Layer) aerosol could be provided and were published 2019 in Nature Geoscience. Furthermore, the data set was investigated for metal-containing particles. Overall, this work documents the high quality of the ERICA instrument and its reliable performance. The field campaigns have shown that the instrument is able to operate continuously under demanding conditions like heat, vibration, and turbulences aboard an aircraft covering an altitude range from mean sea level to 20,477 m above mean sea level, an ambient temperature range from -87 °C to 32 °C, and an ambient pressure range from 55 hPa to 1015 hPa. Thus, design, implementation, and fully autonomous flight operation, especially on a high-altitude aircraft, of ERICA can be considered as veritable experimental success