RESEARCH

Our research focuses 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 generally borne out of at least one of three broad questions:

How to bioaerosols affect Earth system properties, i.e. cloud formation?
How does nitration of proteins in the atmosphere or human system affect health?
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 chemistry, but the thought processes and strategies used are deeply multidisciplinary. 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 transdisciplinary approaches – even if they have little experience in most of the areas.

1. Fluorescence Instrument Development

Many commercial technologies use UV-LIF (laser-induced fluorescence) techniques to detect fluorescent particles as a proxy for bioaerosols. 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 a microscope spectrofluorometer, envisioned to be >1-2 orders of magnitude less expensive than existing commercial units. It provides well-resolved (a few nm) fluorescence emission spectra of individual particles collected onto a substrate, each at several excitation wavelengths. Various ongoing projects push the development and application of the patented technology. See publications 42, 49, 57.

Image shows fluorescent streaks from individual particles, as viewed by the instrument.

2. 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.

Nicole Savage and Christine Krentz lead aerosol lab studies.

3. 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 on Pacific Coast showing instrument trailer with aerosol inlet masts sticking up.

4. CCN 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 a small, relatively inexpensive static diffusion chamber (SDC) style CCN counter. As a collaborative effort involving Prof. Emeritus Huffman and our group at DU, Handix Scientific licensed the technology and further developed the CCN Counter (now referred to as the CloudPuck) into a more robust, commercial instrument through two Department of Energy SBIR Phase I grants. Our group is helping to characterize and deploy the CloudPuck in various capacities.

Video above shows activation of water onto CCN to form "rain" within the CloudPuck.

5. Nitration of Proteins in Atmospheric Pollen and Human Systems

The original motivation for this portion of our projects was literature evidence that showed an enhanced allergenic response when rats were exposed to nitrated pollen proteins instead of the native proteins. Following those studies, nitrated proteins have been found at relatively high levels in limited atmospheric studies. It has been hypothesized that the surfaces of bioaerosols could undergo oxidation and nitration reactions, i.e. with NO2 and O3 secondary pollutants, to produce more allergenic protein products. Using a combination of laboratory experiments to improve detection protocols and field study, we have been working on proteins of atmospheric importance and within the human system (lactoferrin). A portion of these studies have been through collaboration with partners at the Max Planck Institute for Chemistry. 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.

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 Office of the Provost at DU 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.

​Huffman Research Group