One of the strengths of the Keutsch group is its development of scientific instrumentation for field and laboratory use. From its beginnings with laser-based instrumentation at the University of Wisconsin - Madison in the development of instruments that measure formaldehyde and glyoxal, the group added mass spectrometric instrumentation when arriving at Harvard in 2015. Together, these laser-based and mass spectrometric instruments allow us to probe the composition of the atmosphere to understand the chemical mechanisms behind ozone and secondary organic aerosol formation.
FILIF: Measuring Gas-Phase Formaldehyde (HCHO)
Operating Principle and History
HCHO is measured via laser-induced fluorescence (LIF); whereby, a custom UV laser at 353 nm excites a rovibrational transition of gas-phase HCHO and the resulting fluorescence is measured at wavelengths greater than 370 nm with a photomultiplier tube (PMT). Data is collected at 10 Hz which allows for the measurement of fluxes via Eddy covariance.
LIF measurements of HCHO were first applied to in situ atmospheric measurements by Hottle, et al. (2009) with a tunable Ti:sapphire laser. Subsequent work by DiGangi, et al. (2011) replaced the Ti:sapphire laser with a custom fiber laser by NovaWave Technologies. Inspired by the work of Cazorla, et al. (2015), Shutter, et al. (2019) briefly describes the replacement of the multi-pass detection cell with a more robust and stable single-pass detection cell. The latest upgrade to the instrument has been the creation of a new fiber laser with commercially-available parts.
FILIF has traveled to many field campaigns including PROPHET forest in Michigan, Manitou forest in Colorado, Blodgett forest in California, onboard a zeppelin in the Po Valley of Italy, the Amazon rainforest, Caltech, and Long Island Sound. FILIF has even traveled along the coasts of Antarctica and across the Southern/Pacific Oceans.
Source Code and Manual
The QNX code that runs FILIF and the associated manual for the instrument are located on GitHub
Madison/Harvard - LIP: Measuring Gas-Phase Glyoxal (CHOCHO)
LIP instrument in protective casing.
Nd:YAG laser pumping Ti:S crystal.
Why Glyoxal (CHOCHO)?
CHOCHO originates largely from VOC oxidation chemistry and has primary sources from biomass burning. CHOCHO can be used alongside HCHO as a tracer for VOC oxidation chemistry. If CHOCHO and HCHO can be predicted using atmospheric chemistry models, then the details of gas phase chemistry are likely well constrained, meaning that predictions of secondary pollutants like organic aerosol and ozone will be more accurate. CHOCHO can also be measured from space via satellites, so surface-based field studies can help validate satellite retrievals. CHOCHO is water soluble and can also contribute to secondary organic aerosol formation, and our group has participated and has ongoing participation in chamber studies investigating this process.
Operating Principle and History
CHOCHO is measured via laser-induced phosphorescence (LIP); whereby, a Nd:YAG laser (532 nm) pumps a Ti:S laser (880 nm) that is frequency doubled to 440 nm. This then excites gas-phase CHOCHO, and the resulting phosphorescence (520 nm) is isolated and measured with optical filters and a photomultiplier tube (PMT).
LIP measurements of CHOCHO were first applied to in situ atmospheric measurements by Huisman, et al. (2008). Recent and planned upgrades include single-pass detection cell for more stable sensitivities and a reference cell to stay on the online wavelength of 440.25 nm following the recent FILIF upgrades at Harvard.
EDB-MS: Observing chemical processes in levitated particles
Past and Ongoing Development
Proof-of-concept: Birdsall, et al., 2018 demonstrated that the evaporation of particles containing polyethylene glycol mixtures agreed with Maxwellian flux predictions. EDB-MS is capable of performing vapor pressure measurements, even in multicomponent particles.
Instrument development: We are currently doing an instrument overhaul. We are implementing a new geometry, specifically a quadrupole (see to the right) as well as a suite of in-situ measurements, including continuous spectroscopic radius, RH, and temperature.
The current EDB-MS can only trap a single droplet at a time, and so has slow experimental throughput and efficiency. The goal is to couple a Linear Quadrupole Electrodynamic Balance (LQ-EDB) that is capable of levitating multiple droplets simultaneously to a mass spectrometer. This multi-particle suspension in a LQ-EDB has been achieved by Hart et al. (2015) for the purpose of optical size measurements during levitation. However, the periodic discharge of each trapped droplet to a mass spectrometer for chemical analysis is a novel idea, one that will accelerate and advance the study of chemical changes in atmospheric aerosol particles on long timescales.
Atmospheric aerosol particles live for multiple days, during which time they can undergo complex chemical changes. Studies of the underlying chemistry in experimental setups that faithfully mimic the atmosphere -- i.e., in authentic particles, on multiday timescales, at high chemical detail -- will improve our understanding of the associated climate and health impacts.
Electrodynamic balances (EDBs) have been used in aerosol research for several decades. EDBs contain a superimposed AC and DC field that fix laterally and vertically a charged object within the center of the fields. As such, charged aqueous droplets entering the EDB can stably levitate for seconds to days. Recently, the Keutsch group and Dr. Uli Krieger (ETH-Zurich) developed a novel instrument that couples an EDB to mass spectrometry (EDB-MS, schematic to the left), enabling molecular-level chemical analysis of the trapped particles (Birdsall, et al., 2018).
DPOPS: counting and measuring the optical size of particles (140 - 3000 nm) for DCOTSS
Operating Principle and History
DPOPS system is based on the POPS (Portable Optical Particle Spectrometer) instrument developed by the NOAA group. The POPS was designed to be a lightweight, high-performance optical particle counter. It uses a 405-nm diode laser to count and measure the optical size of sampled particles using single-particle light scattering between approximately 140 nm and 3 μm (Gao et al., 2016). Though the instrument is commercially available (Handix Scientific, Boulder, CO), it is not well suited for the flight condition in the UT/LS (low P, T, and low particle number density), especially the high-speed air stream. The DPOPS is an optimized POPS system for DCOTSS flight campaign with autonomous operation in flight that requires minimal support between flights. Three major upgrades are: (1) increasing the sampling flow with external pumps to achieve better counting statistics in light of the low particle number density, (2) achieving isokinetic sampling (the velocity of air entering the inlet is equal to the velocity of the approaching gas stream) for the pickoff inlet mounted in the free stream, and (3) optimizing the tubing system to reduce the loss of particles in the sampling tubes.
DPOPS joined the NASA DCOTSS campaign happening in Palmdale, CA and Salina, KS in 2021 and 2022.
Chemical Ionization Mass Spectrometers
Chemical ionization mass spectrometry (CIMS) is an important and widely used analytical tool for measurements of organic molecules in the atmosphere. Two CIMS instruments in our group can be operated in one of two ionization modes: using either proton transfer reactions such as for H3O+ CIMS or PTR-MS (Breitenlechner et al., 2017) or ammonium ion ligand-switching reactions such as for NH4+ CIMS (Zaytsev et al., 2019). Employment of the two ionization modes significantly improves the measurement capability of the instruments and allows for detection of a vast array of compounds covering a wide range of volatilities from VOCs to ELVOCs. In addition, the CIMS instruments can be equipped with an aerosol inlet to quantify particle-phase compounds. The instrument detection limits are below 1 pptv (part per trillion by volume) for a 1 second integration time for select compounds which allows for the flux measurements.
Recent studies and travels
The CIMS instruments have participated in several laboratory and field studies. In 2017, we travelled to Forschungszentrum Jülich in Germany to study photooxidation of several isoprene-derived OVOCs, including ISOPOOH, IEPOX, and MVK (Fuchs et al., 2018) and participated in the PTR-MS intercomparison campaign PICAB in the Netherlands (Holzinger et al., 2019). More recently, we have extensively collaborated with the Kroll group at MIT participating in a series of chamber photooxidation experiments (Zaytsev et al., 2019).
SAPHIR chamber at Forschungszentrum Jülich, Germany
PICAB campaign, the Netherlands
MIT environmental chamber