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Research projects in the Huffman group generally focus on the development, improvement, and application of strategies to detect and quantify biological aerosols, related proteins, and other analytes of environmental or health interest. Projects are almost always borne out of at least one of these broad questions about the natural, human, or built environments:

  • How to bioaerosols affect Earth system properties, i.e. cloud formation and processing?

  • How does heterogeneous chemical reaction of proteins in the atmosphere or human system affect health?

  • What factors influence ambient air quality, and how can improved instrumentation or data access improve community outcomes?

  • How do respiratory aerosols affect human health, and what practical steps can increase agency for individuals to reduce exposure to airborne infectious disease or particulate matter (PM) from pollution or wildfire smoke?

We approach all questions from the perspectives of quantitative, analytical science, but the thought processes and strategies used are deeply multidisciplinary. As a group that sits in the Department of Chemistry and Biochemistry, an emphasis of most projects involves investigation involving the molecular scale. However, we regularly rely on aspects of environmental chemistry, biochemistry, microbiology, microphysics, mechanical engineering, atmospheric science, computer science, building engineering, and many other disciplines in our projects. As a result, we look for team members who are passionate about such interdisciplinary approaches – even if they have little experience in most of the areas.

On-going project groups

1. Fluorescence Instrument Development

   Many commercial technologies use UV-LIF (laser-induced fluorescence) techniques to detect fluorescent particles as a proxy for bioaerosols (i.e. pollen, fungal spores, bacteria, etc.) and other coarse-mode particles. These can provide information highly resolved with respect to particle size and temporal emission patterns, but usually with almost no spectral resolution for composition (i.e., a “spectrum” of two channels) and at a purchase cost of $100k or more. We developed and patented a microscope spectrofluorometer, designed to be >1-2 orders of magnitude less expensive than existing commercial units. It can either provide either bulk fluorescence properties or well-resolved (a few nm) fluorescence emission spectra of individual particles, each at several excitation wavelengths. Various ongoing projects push the development and application of the technology. A collaborative STTR grant in 2024 with CloudSci through the U.S. Department of Energy seeks to advance the work. See publications 42, 49, 57.

Fluorescent streaks from individual particles, imaged by one version of the instrument.

2. CCN and Ultrafine Particle Counting Instrument Development and Application

   Cloud condensation nuclei (CCN) act as the subset of atmospheric particles onto which liquid water vapor condenses to initiate cloud formation. As a result, the concentration and properties of CCN play heavily into the hydrological cycle and changes in global radiative forcing and climate. Over decades, Prof. Emeritus Donald Huffman from the University of Arizona and DH Associates (Prof. Alex Huffman’s father) developed and optimized a small, relatively inexpensive static diffusion chamber (SDC) style CCN counter. Several phases of collaborative work have been undertaken between the University of Arizona and our group at the University of Denver, including two Phase I SBIR grants through the U.S. Department of Energy. Our group is working to improve, characterize, and deploy the instrumentation.

Video above shows activation of water onto CCN to form "rain" within a version of the instrument

3. Nitration of Proteins in Atmospheric Pollen and Human Systems

   Chemical reactions between certain proteins and both aqueous and gas-phase nitrating mechanisms have been linked to a wide range of health problems, from allergies to neurodegenerative diseases. The original motivation for our work was published evidence showing an enhanced allergenic response when mice were exposed to nitrated pollen proteins. Relatively high levels of nitrated proteins have been found in subsequent ambient environmental studies. It has been hypothesized that the surfaces of bioaerosols could undergo oxidation and nitration reactions, i.e. with ozone and nitrogen dioxide secondary pollutants, to produce more allergenic protein products. Using a combination of laboratory experiments and field study, we work with proteins of importance both for the ambient atmosphere and within human systems.  We have been conducting long-term experiments using two field sites in Denver as an ambient air reaction chamber, and a number of individual laboratory projects are also involved. The Max Planck Institute for Chemistry in Mainz, Germany has been a close and important collaborator through many of these projects. Our work has been funded by a combination of fellowships and grants from the Environmental Protection Agency (EPA), the U.S. Army Research Office (ARO), and the Knoebel Center for the Study of Healthy Aging (KIHA) at the University of Denver. See e.g., publications 48, 56, 59.

ELISA trays showing yellow color of nitrated proteins.

Undergraduate students Alyssa Knaus & Olivia Wuttke stand under one set of PM sensors.

4. Regional Air Quality Monitoring

   Ambient air pollution exposure is among the most significant contributors to the global burden of disease and negative public health outcomes. Local and regional sources of air pollution in the Denver metro and Front Range Colorado regions play similarly important roles in the public health of Colorado residents. Leveraging the opportunity afforded by the recent acquisition of the James C. Kennedy Mountain Campus (KMC) by the University of Denver, the Huffman Group has established a small network of particulate matter (PM), gas-phase, and weather sensors to monitor regional air quality (AQ) trends. PurpleAir PM sensors, TSI BlueSky 8145 have been deployed at three sites around the campus, and will eventually provide real-time AQ data to the community via publicly available internet maps. The nascent work is meant to establish a detailed baseline of AQ trends and to showcase the site as a possible location for the development of a more permanent environmental research facility, with ready access to power, internet, and logistical infrastructure, including year-round road access at this low-mountain site. The site is 724 acres and sits at 8,100 feet of elevation within the Roosevelt National Forest, approximately 109 miles from the main DU campus. Several undergraduate thesis projects have already contributed to the establishment of the research site, and a new generation of undergraduate researchers is now taking the lead to move the work into the next phase. The project is being undertaken in collaboration with the DU Kennedy Mountain Campus staff and through student support from the DU Undergraduate Research Center.

5. Field Investigation of Bioaerosols and Ice Nuclei

   The emission sources, atmospheric properties, and environmental effects of various classes of bioaerosols are being more fully understood as techniques to detect them in the ambient atmosphere have progressed (e.g. 17, 43, 54, 55). Our group has frequently participated in and led both short-term (~1 month), intensive or long-term (>12 months) field campaigns to understand bioaerosol properties in a region or inside the built environment. These field studies are most frequently collaborative efforts with many other universities and labs across the world (see summaries and associated publications on a separate page here). Each study is different, but we frequently deploy our UV-LIF instrumentation, and have also collected high-volume filter samples of particulate matter in order to perform off-line analysis of molecular tracers for fungal spores and bacteria (44). One frequent component of the outdoor field studies is the correlation of bioaerosols with ice nucleating particles (INP), because of the importance that certain bacteria, fungal spores, and even pollen fragments can play in the efficient nucleation of ice in the atmosphere. Our group has participated in-person in domestic studies in Colorado, California, and Oklahoma and internationally in rural British Columbia, Cyprus, Barbados, France, and Reunion Island.

Field site in Ucluelet, British Columbia showing instrument trailer with aerosol inlets.

An example portable carbon dioxide monitor.

6. Indoor Carbon Dioxide Monitoring, Indoor Air Quality, & Respiratory Aerosols

   Early in the COVID-19 pandemic, Dr. Huffman partnered with the DU Office of the Provost to deploy carbon dioxide (CO2) sensors in classrooms around the DU campus, with special attention to performance and practice rooms at the Lamont School of Music. The relatively simple Wells-Riley Model of aerosol infectious disease transmission was also utilized to estimate airborne aerosol risk from COVID, with focus on rooms with active CO2 detection. The efforts were important to help keep the University of Denver campus safe during acute phases of the COVID-19 pandemic, in part because they helped elucidate areas that required more targeted ventilation improvements. Data and understanding gathered through this work has also been instrumental in helping further advocacy for broader community improvements in ventilation and awareness of indoor air quality. The CO2 measurement research is on-going, with an overview of preliminary results published in 2022 (61).

  Dr. Huffman has also partnered with the Colorado Department of Public Health and Environment to improve indoor air quality guidance for the state, and is involved in research to understand the aerosol properties of respiratory disease transmission.

7. Aerosol Instrument Characterization

   The Wideband Integrated Bioaerosol Sensor (WIBS) is one of only a few commercially available UV-LIF instruments designed for bioaerosol detection. While groups around the world have applied the instrument to field detection of both indoor and outdoor bioaerosols, there are still many questions about how best to interpret the complex data. Our group has been a leader in characterizing and improving the operational and analysis process to be able to extract the most meaningful results from the UV-LIF instrument for bioaerosol detection (publications 18, 24, 47, 51, 52).

   Similarly, the Resource Effective Bioidentification System (REBS) uses Raman spectroscopy to automatically characterize the composition of individual aerosol particles collected onto a metalized tape within the instrument. Through support from the Army Research Laboratory, we are working to better characterize how the REBS can be applied to ambient bioaerosol detection.

Former students Nicole Savage and Christine Krentz lead aerosol lab studies.

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