The dynamic processing of aerosols in the atmosphere is difficult to mimic under laboratory conditions, particularly on a single-particle level with high spatial and chemical resolution. Our new microreactor system for X-ray microscopy facilitates observations under in situ conditions and extends
the accessible parameter ranges of existing setups to very high humidities and low temperatures. With the parameter margins for pressure (180–1000 hPa), temperature (
Aerosol particles play crucial roles in various atmospheric processes and the Earth's climate system
Scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure analysis (STXM-NEXAFS) in the soft X-ray regime (270–2000 eV) has become a widely used and powerful technique to resolve the micromorphology and chemistry of laboratory and ambient aerosol particles on submicron scales
Here, we present the development of a gas flow system coupled with a microreactor as an accessory for STXM instruments for in situ studies of particles in a controlled gas-phase environment; we emphasize its analytical capabilities and show initial results.
The instrument's design and construction was inspired by previous developments of environmental chambers for X-ray microscopes, namely by
The control system is of compact size: the gas mixing and cooling circuits, along with the power converters and electronics are integrated into a 19 in. enclosure with a height of four rack units (total dimensions:
Gas flow and cooling system schematics of the entire system. Left: X-ray microscope chamber with sketched microreactor. Refer to Fig.
The tasks of control and data acquisition are performed by the so-called “VBUS system”, which has been developed at the Max Planck Institute for Chemistry (MPIC). This miniature measurement system consists of microcontroller-based electronic modules and a flexible software environment including scripts and a graphical user interface (GUI). An example GUI screenshot is provided in the Supplement Fig. S1.
The gas humidification system is similar to the one used by
Stable temperature,
Rendered views of the microreactor assembly and the sensor PCB with the most important parts labeled. The four-way microfluidic connector (Dolomite Ltd.) connects to the internal channel structure in the brass metal body of the microreactor via a compression seal at the top face. The signals from the BME280 (
The system's pressure,
The microreactor, displayed in Fig.
The fluidic connections between the front panel of the control box and the microreactor were realized via four Upchurch Scientific (IDEX Health & Science) PEEK tubings (1.6 mm outer diameter
Sectional views in Fig.
With a typical focal length of 1.36 mm at 280 eV, the space between the focal plane and the zone plate (compare Fig.
For a gas-tight seal, the silicon nitride windows must be glued into the sample holder disks. We either used IMI 7031, also known as GE varnish, and let it cure at room temperature, or Apiezon Wax W, which melts at about 373
Behind the front window, the process gas flows in a 300
The rearmost part of the microreactor is a PCB, which is screwed to the brass metal body. It includes read-out electronics (ADS1118 ADC) for the thermocouple, an electrical connector as well as the environmental sensor S2, which reaches into the gas stream through an O-ring-sealed recess in the metal body (Fig.
A wide range
of atmospheric conditions present in the troposphere can be reproduced by the microreactor system. More precisely, the system uses pressures ranging from 180 to 1000 hPa and has cooling capabilities for highly stable
In Fig.
In the particular example shown here, the microreactor was held at 283.2
A
The
Note that the thermocouple in principle is redundant for the temperature measurement inside the microreactor body, as the BME280 sensor (compare Fig.
The BME280 environmental sensors used in this setup were chosen because of their very small dimensions of
Image sequence showing the hydration (a
A hydration–dehydration experiment with
A pressure calibration was not done for the measurements presented here, as the RH is independent from
Hygroscopic growth curve of ammonium sulfate (AS,
As a second proof-of-performance application, presented in Fig.
Tentative observation of ice at
As a rough measurement time estimate, the recording of a full hydration–dehydration cycle with 22 RH steps took 2 h in the case of dataset 1. The scan time itself was about 2 min at each RH step for recording images at two different energies (65 nm pixel size, 1 ms dwell time per pixel on a
Accordingly, some of the results on the standard compounds shown here can be obtained more efficiently and probably more precisely with other techniques, such as hygroscopicity tandem differential mobility analyzer (HTDMA) systems
As a third proof-of-performance application, a water freezing experiment with isoprene SOA particles at high RH was conducted. These results illustrate that the system can be used for controlled freezing experiments as well as for kinetic deceleration of fast processes.
The particles were produced using a potential aerosol mass (PAM) chamber
It should be noted that it is unlikely to find immersion freezing in aqueous isoprene SOA (seeded with AS), as it occurred at an unusually high temperature of
Furthermore, MIMiX was used in a study on the diffusion-limited oxidation of
One of the real analytical strengths of STXM-NEXAFS analysis in combination with MIMiX emerges in the analysis of ambient aerosol particles, since detailed single-particle studies can typically not be conducted on site, particularly at remote locations. Thus, sampling of ambient particles onto suitable substrates for a subsequent investigation by offline techniques is required. Such samples are well-suited for in-depth studies with the MIMiX system. Another analytical strength of the system relates to the unique combination of microstructural, hygroscopic, and chemical information, which can be obtained on the level of individual particles in the submicron particle size range, while being relatively damage-free through the use of soft X-rays in a dose-efficient scanning system.
This study presents the design, construction, and initial testing of a microreactor system for in situ STXM-NEXAFS analyses of aerosol particles under controlled environmental conditions. Its compact size ensures high portability of the setup, without sacrificing functionality. The operating ranges cover a wide spectrum of
The results from initial experiments (i.e., hygroscopic growth of
We intend for the future development of the MIMiX system to include an extension of the cooling capabilities for in situ ice nucleation observations and the introduction of an optical fiber to study photochemically driven multiphase reactions. On the software side, integration with the Experimental Physics and Industrial Control System (EPICS) and the Pixelator software is planned in order to store environmental parameters in parallel with the X-ray microscopic data at per-pixel resolution.
The STXM-NEXAFS data used for Figs.
A video of the microreactor assembly can be found in the same repository as the scientific data and is available in 720p and 1080p resolution under
The supplement related to this article is available online at:
JDF was responsible for the mechanical and electrical design of the microreactor, the conceptual design of the control system, and its assembly. CP supervised the construction work. CG and ML developed the microcontroller-based electronic system and helped JDF with the programming of scripts for the graphical user interface. MA, SSS, MW, and BW were consulted in an early design stage and influenced the final design of the microreactor. HT, CP, and JDF prepared the samples. JDF led the writing of the paper. CP, MOA, and UP supervised the paper writing. The adaptation of the microreactor to the STXM instruments were conducted by JDF, CP, and MOA, with the technical assistance of MW, BW, and JR. The measurements were led by JDF, CP, and MOA and supported by FD and PAA. All authors contributed to the paper finalization.
The authors declare that they have no conflict of interest.
The authors gratefully acknowledge the support provided by the Max Planck Society (MPG). The authors thank Thomas Kennter, Frank Kunz, and the MPIC's mechanical workshop team for their excellent work. We acknowledge the Helmholtz-Zentrum Berlin, Germany, for the allocation of the synchrotron radiation beamtime at BESSY II and the Paul Scherrer Institute, Villigen, Switzerland, for provision of synchrotron radiation beamtime at the PolLux beamline of the SLS. The PolLux endstation was financed by the Federal Ministry of Education and Research (BMBF) through contracts 05KS4WE1/6 and 05KS7WE1. We thank Michael Bechtel and Blagoj Sarafimov for technical assistance during the beamtimes. We further thank David Walter and Nina Löbs for being part of our experiment team and Frank Helleis, Ralf Wittkowski, Mario Birrer, Thomas Berkemeier, Stefan Blanckart, and Berthold Kreuzburg for their support and stimulating discussions.
The article processing charges for this open-access publication were covered by the Max Planck Society.
This paper was edited by Mingjin Tang and reviewed by two anonymous referees.