The Grand Challenges Workshop for Aerosol Science and Technology was organized for the International Aerosol Conference (IAC), in St Louis, September 2–7, 2018. The purpose of the workshop was to identify “Grand Challenges” for aerosol science and technology in the next decade and thereby indicate a viable research road map for the aerosol community

Baltensperger, Urs

Flagan, Richard C.

Pui, David Y. H.

Sorensen, Christopher M.

Caltech Authors - Main

WORKSHOP REPORT
Grand challenges for aerosol science and technology
Christopher M. Sorensena, Richard C. Flaganb, Urs Baltenspergerc, and David Y. H. Puid
aDepartment of Physics, Kansas State University, Manhattan, KS, USA; bDepartment of Chemical Engineering, California Institute of
Technology, Pasadena, CA, USA; cLaboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland; dDepartment of
Mechanical Engineering, University of Minnesota, Minneapolis, MN, USA
The Grand Challenges Workshop for Aerosol Science
and Technology was organized for the International
Aerosol Conference (IAC), in St Louis, September
2–7, 2018. The purpose of the workshop was to iden-
tify “Grand Challenges” for aerosol science and tech-
nology in the next decade and thereby indicate a
viable research road map for the aerosol community.
The organization consisted of a committee of seven
eminent aerosol scientists from around the world.1 With
their advice, ca. 70 distinguished aerosol scientists from
around the world with interests spanning the sub-fields
of aerosol science were invited to attend the workshop.
All were asked to submit a single paragraph statement
of a single most important grand challenge. In addition,
a call was sent out to all members of all the aerosol soci-
eties across the world telling them about the workshop
and inviting them to attend if they so wish.
Prior to the workshop, the submitted paragraphs
were reviewed and organized. Then at the IAC meet-
ing on Sunday, September 2, the workshop com-
menced with 150 attendees. A review of the
paragraphs led to a dynamic and fruitful three-hour
discussion. After the workshop and during the meet-
ing, many impromptu discussions occurred among
the conference attendees that refined the ideas that
resulted from the workshop. A brief PowerPoint pres-
entation describing the grand challenges that resulted
from the workshop was presented Thursday,
September 6, in a 1 h, plenary, open forum on the
topic. The open floor was dynamic and led to very
useful advice and comments that were recorded.2 All
these have been sources of the document pre-
sented here.3
Aerosols
The public knows that aerosols come out of spray cans
and smoke stacks. However, sitting in a darkened room
with sunbeams gliding through the windows, or observ-
ing rays of sun (crepuscular rays) projected through the
clouds on an otherwise sunny day, alerts one to the fact
that aerosols are more than spray cans and smoke
stacks, and hints to the fact that aerosols are everywhere.
Aerosols are dispersions of particles, either liquid or
solid, in a gas; the particles in an aerosol span a wide
range of sizes, compositions, and morphologies.
Aerosols are, indeed, ubiquitous. The aerosols from
smoke stacks, vehicle exhaust, indoor cooking, road-way
dispersion and dust storms pollute our environment and
do us harm. Premature death due to aerosols is a
world-wide problem. On the other hand, aerosols are
the synthetic pathway for many useful materials such as
carbon black that makes tires remarkably durable, or
titania that is the base material for essentially all paints
manufactured today. Aerosols are also important in
medicine to deliver drugs or detect illnesses; life makes
aerosols. Both natural and human-made aerosols fill our
skies and thereby affect the global environment as much
as greenhouse gases but with much greater uncertainties.
So, there can be no doubt that aerosols are a major
part of the human environment with significant effects
for humanity. Therefore, it is essential that we scien-
tifically study aerosols in their manifold situations and
apply this science to deal with the technology of their
applications and to protect us from harm.
The purpose of this summary report is to outline
significant grand challenges for aerosol science and
 2019 The Author(s). Published with license by Taylor & Francis Group, LLC.
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-
nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed,
or built upon in any way.
CONTACT Christopher M. Sorensen sor@phys.ksu.edu Department of Physics, Kansas State University, Manhattan, KS 66506, USA.
1Urs Baltensperger, Paul Scherrer Institute, 5232 Villigen, Switzerland; Junji Cao, Chinese Academy of Sciences, Xi’an, 710075, China; Richard Flagan,
Caltech, Pasadena, CA 91125, USA; Y.S. Mayya, Indian Institute of Technology, Mumbai 400 076, India; Yoshio Otani, Kanazawa University, Kanazawa,
Ishikawa 920-1192, Japan; David Pui, Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA; Christopher Sorensen, Kansas State
University, Physics, Manhattan, KS 66506, USA.
2We thank our very capable note takers Yuli Wang Heinson, William Heinson, and Pai Liu of Washington University, St. Louis.
3We thank C Y Wu and Pratim Biswas for conceiving this workshop and nurturing its progress. We acknowledge the funding from AAAR.
AEROSOL SCIENCE AND TECHNOLOGY
2019, VOL. 53, NO. 7, 731–734
https://doi.org/10.1080/02786826.2019.1611333
technology and thereby create a roadmap for future
research. The grand challenges presented here are
indeed “grand” and represent the most pressing issues
for both the science and the technology of aerosols.
We address both the areas where aerosols are typically
referred to as “bad aerosols” and as “good aerosols”,
followed by a summary of the physical, chemical, and
biological properties of aerosol that need to be
addressed to cope with these challenges.
Environment
The aerosol community needs to develop a quantita-
tive description of aerosols in the present, past, and
future, over length scales from indoor and local
through the global. This description must deal with
the inherent complexity of aerosols in the environ-
ment and their multiple and many-dimensional
impacts, including climate, visibility, ecosystems, as
well as the numerous involved negative and positive
feedbacks. This requires a better source apportion-
ment of aerosols, such as discrimination between nat-
ural and anthropogenic aerosols, and quantification of
the corresponding radiative forcings via aerosol–radia-
tion interaction and aerosol–cloud interaction. The
latter relates to the number of cloud condensation
nuclei and ice nucleating particles (CCN and INP,
respectively), i.e., particles that are able to form a
cloud droplet/ice crystal at the given supersaturation
in a cloud. As about 50% of these particles are not
directly emitted but rather formed in the atmosphere
from a variety of precursor gases, it is essential to
understand the underlying processes. Research should
give a special focus to the most vulnerable areas, such
as the Arctic.
Health
The aerosol community needs to provide mechanistic
links between aerosol chemical and physical properties
with toxicity and biological end points. This will require
the community to define the most important biological
end points. A comprehensive ensemble of measurements
will be required in order to link aerosol properties to
physiological mechanisms that lead to those end points.
Personal exposure must be differentiated from total inte-
grated ambient concentrations, and dose from exposure.
Measurements—both indoor and outdoor—need to pro-
vide sufficient spatial and temporal resolution to quantify
exposure. These should include long-term measurements
that can be used in connection with epidemiology stud-
ies, and that provide the data needed to establish links
between aerosol properties and the physiological mecha-
nisms that impact human health, extending beyond the
commonly used approach of just PM10 or PM2.5 (par-
ticulate matter with an aerodynamic diameter smaller
than 10 or 2.5lm, respectively). The community needs
to develop a methodology that is sufficiently simple and
cost-effective, including accurate, cost-effective sensors,
and then realize the potential scientific and societal bene-
fits of networking these sensors and methods.
Measurements are also needed that can evaluate long-
term versus acute health effects in terms of toxicity. All
of these studies need to be applied to understand the role
of aerosols on the health of the individual in order to
correlate/associate the links to end points. Finally, the
community must effectively communicate the results of
these studies to society.
The aerosol community needs to develop sound
methods to detect and predict the spread of aerosol vec-
tors of disease. This will enable appropriate agencies to
take proactive measures to warn or protect the populous.
To do this, massively distributed, yet affordable, meas-
urement networks based on recent advances in molecu-
lar assay techniques are needed. Sampling of the ambient
aerosols must occur across a wide range of scales and
environments such as the home, schools, work-place,
hospitals, and the surrounding communities.
The aerosol community needs to advance the state-of-
the-art in sampling, detection, and analysis of exhaled
breath biomarkers for disease, to monitor health.
Bioaerosols
A grand challenge for the aerosol community is to
understand the influence of personal exposure to bioaer-
osols on long-term health. Important aspects of this chal-
lenge are how to determine personal exposure to
bioaerosols within integrated exposure to indoor and
outdoor agents. We don’t know the dose–response curve
for various microorganisms in different environments.
We also don’t know how modifications in human envi-
ronments, such as urban pollution, feedback to alter the
microbes via, for example, evolutionary pressure.
Another important question is what role does cumulative
exposure to aero-allergens play in chronic inflammation
leading to other negative health outcomes?
The aerosol community needs to understand the
roles of bioaerosols in the environment: the hydro-
logic cycle, cloud condensation and ice nucleation,
and impacts on ecosystems such as crops, grasslands,
and forests.
The aerosol community must seize the future and
prepare for changes in biodiversity due to climate
732 C. M. SORENSEN ET AL.
change and evolution of microorganisms. It must
anticipate its primary role for understanding and con-
trolling bioaerosols in space vehicles, and lunar or
planetary installations.
Control technology
People spend most of their time indoors, and indoor
air pollution is shortening lives world-wide. The aero-
sol community must provide the means to protect the
public and ensure that people breathe healthy air.
This challenge has multiple dimensions which should
be pursued in parallel.
Filtration is the dominant method of removing par-
ticles from indoor air. It is a mature technology, but
there are opportunities for improved filters through the
use of new materials, and integration into filtration sys-
tems of additional functions such as gaseous and bio-
logical pollutant removal. The aerosol community must
explore these opportunities to produce breakthroughs
using nontraditional technologies.
The design of indoor environmental space typically
follows slowly-developed codes and practices that do
not incorporate the latest human health studies, or the
latest knowledge on the patterns of indoor and outdoor
pollutant sources. Narrowing the gap between engineer-
ing practice, cutting edge technology, and aerosol sci-
ence to make smart building engineering and controls a
standard practice is a grand challenge in itself.
Building occupants have a large degree of control
over their indoor environment, but because the effects
of air pollution are typically subtle and even imper-
ceptible to most occupants, indoor air quality has a
lower priority than it deserves. The aerosol commu-
nity must accept the challenge to create innovative
programs in education to enhance public awareness.
Materials
The aerosol community should aspire to be able to
make any material at any scale. A simply stated but
daunting goal is to create materials of arbitrary com-
plexity with tailored properties of composition, size
and shape. The ability to tailor, to control, to predict
the results will need the combined efforts of labora-
tory experimentation and theoretical modeling. The
community should explore and take advantage of
developing artificial intelligence methods for mining
the vast parameter space for product synthesis and
process development. We should aspire to make stuff
that our predecessors never dreamed of.
Making a desirable material at the lab bench is just
the beginning; the challenge is then the scale-up to
yield commercially significant products. Here the
community must address a key tool that is presently
lacking—the ability to measure aerosol properties,
such as size distribution and temperature, space and
time resolved, within the production reactor. This
daunting challenge must often contend with high tem-
perature, chemically reactive, and often corrosive
flows, and mass loadings as high as 10%. Such data
are essential for process control, addressing scale-up
challenges, and for computational-fluid-dynamics-
reaction-aerosol coupled modeling for process
optimization.
Finally, we must not forget that the aerosol com-
munity must be responsible for their science. Hence
life-cycle analysis, toxicity, and environmental impact
analysis of new materials must be performed.
Physics and chemistry
The aerosol community must bring together physical
and chemical experiment and modeling to better
understand a broad variety of fundamental issues. It is
important to stress that communication between mod-
elers and experimentalists is essential for rapid and
relevant progress.
There is a significant need to develop a viable
description of light scattering and absorption by par-
ticles of any size, shape, and composition. This need
arises because many ambient aerosols, as well as
aerosols found in materials manufacturing processes
are non-spherical and can have complex, heteroge-
neous morphologies, and can even contain multiple
phases. Perhaps the most significant use of such
knowledge is to quantify the impacts of aerosols on
the radiation budget of the Earth and, thereby, deter-
mine aerosol impacts on global warming. This need
applies to other applications including optical remote
sensing of ambient aerosols, especially via satellites,
and calculation of atmospheric visibility and visibility
impairment. In the manufacturing realm another
important application is for real time, in situ meas-
urements of aerosols in process flow streams during
materials production. In this regard, as highlighted in
the Materials section of this report, the problem of
optical diagnostics of aerosols with high mass load-
ing, on the order of 10%, where multiple scattering
leads to confusion, is an important impediment that
needs to be overcome.
Another grand challenge is the need to develop a
viable understanding of particle nucleation and
AEROSOL SCIENCE AND TECHNOLOGY 733
growth, spanning the transition from molecules, to
clusters, to stable condensed-phase materials. This
growth transition is made very complex by the fact
that the physical and chemical properties of the evolv-
ing entities are changing with size. A fundamental
understanding of the growth transition would be the
backbone on which to support the advancement of
many related applications, namely air pollution chem-
istry, cloud formation, species transport and diffusion,
particle formation, human health related toxicology
and materials synthesis.
As alluded to in the paragraph above, there is a sig-
nificant need at the fundamental level to develop a
viable understanding of the evolution of physical and
chemical properties of particles in the transition from
the atomic/molecular to “bulk” particle scales. New
sciences lies in this intermediate realm, and as most
of us are aware, we live well today based on the new
science of yesterday.
Overarching grand challenge
While simple metrics of an aerosol, whether number
concentration, mass concentration, PM2.5, or some
other measure yet to be defined, may be useful for spe-
cific applications, multidimensional characterization of
aerosols is needed to understand their complex effects
throughout all the areas reviewed above. Regulatory
agencies often seek a single measure, such as PM2.5, but
no single parameter can describe all of the effects of
concern which often result via a synergy of properties.
Thus, an overarching need for aerosol science in gen-
eral is to advance the art of aerosol measurement and
apply this art for multidimensional characterization.
Many of the above activities will result in a vast
amount of data that must be openly accessible and
rigorously preserved in perpetuity. Perhaps the best
way to ensure this preservation is to engage the only
institutions that have, as their primary mission, pres-
ervation of knowledge for the long term—Libraries.
734 C. M. SORENSEN ET AL.
