Flagan Research Group


Suspensions of particles in a gas, called aerosols, play a central role in the atmospheric processes involved in climate change.  Some aerosols are emitted directly into the atmosphere; others form there through oxidation of gaseous precursors.  Both natural processes and anthropogenic sources contribute to the aerosol burden.  Climate change is driven by greenhouse gases that warm the air, and by aerosols, and the cloud droplets that form on them that, depending on their compositions cool the planet by scattering incident sunlight back to space, or warm it when soot and other particles absorb solar radiation.  While the impacts of greenhouse gases are readily quantified, aerosols remain the greatest source of uncertainty in the global radiation budget.  Only by understanding the global distribution of aerosols can their impact be properly assessed.  Unfortunately, data of the atmospheric aerosol remain sparse.  The Flagan group works to change the level of understanding through a combination of laboratory and field measurements, and by advancing the state of the art in aerosol measurements.


Fine particulate matter in the air also has profound impacts on human health.  These particles are usually characterized in terms of the mass concentration of particles that can penetrate to the lower airways when inhaled, so-called PM2.5.  Epidemiological studies reveal a strong association between PM2.5 levels and a number of adverse health effects, but PM2.5 is a blunt instrument to understand fine particle effects.  Near roadway increases in of some of these health effects point to possible contributions of particles in the low nanometer regime, and to heavy-duty diesel traffic as the likely culprit, though, again, the data are too sparse to allow rigorous assessment of the causes. 


We seek to advance our understanding of the atmospheric aerosol, at scales ranging from the very localized effects of near roadway exposures, to that of the urban, regional, and global atmosphere.  By enabling measurements throughout the particle size range from molecular clusters through supermicron particles, we are working to provide the comprehensive data that are needed to probe the fundamental mechanisms of aerosol formation and growth, and to assess their many impacts.  A particular focus of our work is the secondary organic aerosol that dominate the aerosol in many locations, and which account form most of the growth even when species such as sulfuric acid are responsible for new particle formation.  We further study much more localized effects of aerosols, including workplace exposures in the burgeoning nanotechnology arena.  Our research broadly encompasses several strongly overlapping areas:


We further are exploring applications of methods of aerosol science to broader research arenas.  The instruments we have developed have demonstrated potential for high-resolution molecular separations. 


Laboratory chamber studies


Environmental chamber studies at Caltech focus on the organic portion of the atmospheric aerosol, the secondary organic aerosol (SOA).  Caltech has lead the way in the study of the chemistry of SOA formation, providing much of the data on SOA formation from both biogenic and anthropogenic hydrocarbons.   Using state-of-the-art aerosol measurement methods that were developed at Caltech, the Seinfeld and Flagan groups have, together, determined the yield of atmospheric aerosols as a function of the level of nitrogen oxides, background aerosol properties, and other atmospheric parameters.  Data from the chamber studies guide the development of chemical kinetic and aerosol dynamic models of the evolution of the atmospheric aerosol.

The Flagan and Seinfeld Groups standing in the new Linde Robinson Environmental Science Chamber.


The Flagan group also participates in the CLOUD experiment at CERN, in Geneva.  CLOUD had developed the cleanest atmospheric chamber to date.  Experiments at CLOUD probe the fundamental mechanisms of new particle formation with unprecedented resolution.  Mass spectrometric measurements made by collaborators in CLOUD follow the transition of clusters past the so-called critical cluster size molecule-by-molecule.  Our instruments that enable high resolution size distribution measurements of particles as small as 1 nm diameter bridge the gap between the mass spectrometer, and standard in the field, the scanning mobility particle sizer (SMPS), which was also developed by our group.  Our new instruments make it possible to determine size distributions with sufficient resolution to map the new particle formation and growth kinetics that are responsible for nearly half of the cloud condensation nuclei in the global atmosphere.


Field campaigns: Airborne measurements of aerosols and clouds


Laboratory studies reveal the underlying processes involved in secondary aerosol formation and growth, under carefully controlled conditions.  In situ measurements are, nonetheless, essential to understanding their many effects.  Atmospheric aerosols form the seeds on which cloud droplets form when rising air cools and becomes supersaturated with water vapor.  Aerosols scatter sunlight back to space, leading to direct radiative forcing.  Cloud droplets scatter much more sunlight, a process that has been labeled the indirect effect.  In order to understand these complex interactions, and the profound effects that they have on the global radiation budget, the Seinfeld and Flagan groups pursue a wide range of in situ, airborne measurements of the atmospheric aerosol.  By taking a suite of state-of-the-art aerosol and cloud droplet measurements into different environments around the world, we provide in situ truth for remote sensing (mostly satellite) observations of aerosols and clouds, as well as modeling studies. 


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CIRPAS Twin Otter flight crew and research team.

Probing a ship plume using the CIRPAS Twin Otter aircraft.


Our group uses the CIRPAS Twin Otter research aircraft, which is based at the Naval Postgraduate School at Monterey, California,  and operated out of a large, fully-equipped hangar at the Marina Municipal Airport, a few miles north of Monterey.  The Caltech group, which combines the groups of Professors Seinfeld and Flagan, has conducted experiments, typically one per year, using the Twin Otter aircraft at locations around the world in our efforts to understand aerosol-cloud interactions.


Aerosol measurement methods


The Flagan group has repeatedly advanced the state of the art in aerosol measurement, beginning with a low-pressure impactor that enabled the first size resolved sampling of particles as small as 20 nm diameter.  The world standard for measurements of particle size distributions in the submicron size regime is the scanning mobility particle sizer (SMPS), also developed by the Flagan group, that enables measurements of size distributions in as little as a couple of minutes using commercial instruments, down to a few seconds using state-of-the-art instruments developed in our laboratory.  Among numerous other developments, our group has recently developed a nano-radial differential mobility analyzer (nRDMA) that extends the SMPS method down to 1 nm diameter, and simultaneously increases its size resolution.


Our group has further developed a totally new nanoparticle sizing instrument that not only allows high resolution measurements of aerosol particles in the low nanometer regime, but also enables molecular separations sufficient to separate peptide stereoisomers.  This new radial opposed migration ion/aerosol analyzer (ROMIAC) changes the scaling from conventional differential mobility analyzers, lowering the operating voltage at which diffusion begins to degrade the resolving power of the mobility method.  This, in turn, allows new instrument design and fabrication approaches.  On one hand, it allows the development of new classifiers with resolution that is not possible with conventional instrument designs.  On the other hand, it also enables mobility analyzers to be miniaturized without sacrificing resolution.  We are pursuing developments in both directions: advancing biomolecular separations and analysis, and developing simple, low-cost sensors that maintain the performance of present state-of-the-art aerosol measurements while dramatically reducing the cost and complexity of the instruments.


Radial opposed migration ion/aerosol analyzer (ROMIAC). (reprinted from Mui et al., Anal. Chem. 85: 6319-26 (2013).



Nanoparticle inhalation in the environment and in the nanotechnology workplace


Nanotechnology is developing rapidly, but potential risks of inhalation exposures to engineered nanomaterials have received limited attention.  The Flagan group is using the measurement methods developed at Caltech in the study of possible methods by which nanomaterials may become airborne and pose inhalation risks. 


Pollen allergens and measurement


Larger particles are also important to human health.  Beginning with the puzzle of how large (20-100 Ám diameter) pollen particles could trigger the allergic responses deep in the airways such as asthma.  As long ago as the pioneering work of Robert Brown (of Brownian motion fame), had observed that pollen grains in water sometimes undergo osmotic shock, releasing particles of only a few microns in size, or smaller.  We that, when the rupture occurs on the nano-textured, super-hydrophobic surfaces of the flowers of anemophilous (wind pollenated) plants, the fragments can be entrained into the air by a gentle breeze.  Further, we have developed a computer-vision-based, automated pollen identification and counting system (APICS).