6 Questions with Amara Holder, PhD

Amara Holder, PhD
Research Mechanical Engineer
Office of Research and Development
United States Environmental Protection Agency (U.S. EPA)


1. How did you get involved in the aerosol science community, or how did you first become interested in aerosol science?

I started out my master’s program in a combustion research group at UT Austin led by Professor Ofodike Ezekoye. He was interested in particle synthesis and was working on optical measurements in the post-flame region. I spent the summer in the library reading about fractal aggregate light scattering, which was my introduction to aerosols. It was fun.

2. Which people or programs in our field have been the most influential to you and your path, or who have most influenced your ideas about aerosol research?

I grew up, so to speak, at Lawrence Berkeley National Laboratory where there are many fantastic aerosol scientists with a great history of contributions across the different topics in aerosol science. Whenever a question came up the typical answer was, “let’s measure it”. I think that principle guides my research today and is perhaps the reason why a lot of my research is focused on improving the measurement as well as understanding the phenomena I am trying to measure.

3. What factors inspired your interest in studying aerosol emissions from biomass burning?

I had just started at EPA when a colleague asked me if I wanted to piggyback on their field effort to measure smoke from prescribed fires. I was interested in measuring black carbon from different combustion sources and it was the perfect opportunity to get at one of the largest sources of black carbon to the Earth’s atmosphere. At about the same time EPA was planning a major change in how they estimated emissions from wildland fires by separating the emissions from flaming and residual smoldering combustion phases. So, my research expanded to fill this sudden need for information about the optical, physical, and chemical properties of what gets emitted from fires under these varying combustion conditions.

4. What are, in your opinion, the most interesting or important research contributions you’ve made so far?

During my PhD I was trying to identify the components of diesel exhaust that caused cellular toxicity. I was working on sorbents and denuders to remove just one compound or class of compounds from the exhaust stream. But we were having a hard time killing our cells, at least according to the assays we were using. With some detective work we found that the diesel particles were interfering with the assays. We found that the large surface area of combustion generated particles could absorb the dyes and other molecules commonly used to identify cell viability or death. We also found that by modifying the particle surface characteristics we could modify the interaction with the assay molecules. It was a small study, but the potential impact was large. This is still one of my most cited works.

At EPA I am more involved in applied research and finding solutions to our environmental challenges. Evaluating PM sensors for wildfire smoke is my most impactful research for public health. There had already been many PM sensor evaluations under typical ambient conditions, and they showed a lot of promise for providing accurate data with corrections. However, the public is often most interested in sensor data when air quality is poor. So, I wanted to focus in on times when the concentrations are elevated, like during a wildfire. Now sensor data is frequently used by communities impacted by fires to know when to go outside to walk the dog and when to come inside and turn on an air cleaner. Sensors have made the often invisible pollution visible and I hope that will have a positive impact on public health as people become more aware of pollution and take actions to protect their health.

5. Are there new aerosol research directions that you see as particularly important or interesting?

It is exciting to see so much public interest in topics that intersect with aerosol science. I am curious to see what happens in some of these crowdsourced endeavors. One effort that comes to mind is the development of improved masks/respirators in response to the pandemic. Respirators are the last line of protection for many environmental hazards but are nearly unbearable to wear for an extended duration. Better, more comfortable, respirators could potentially change the lives of the thousands of outdoor workers that have to work in smoky conditions during wildfire season every year.

I’m also interested in seeing what comes of the rapid expansion of PM observations from crowdsourced low-cost sensors. Thanks to these sensors and new satellite aerosol products we have so much more data on PM concentrations in the atmosphere at global, regional, and even local scales. I am hopeful that we can harness this information to advance smoke models and air quality models to better understand what controls atmospheric PM. We also have this very interesting experiment with the pandemic shutdowns to give us an idea of what sources impact air quality most. This holds promise to inform what regulations might be most effective.

6. You have been one of the leaders in understanding the performance of low-cost aerosol sensors. To expand on the previous question, what do you see as new opportunities in this area? 

These low-cost sensing networks combined with data mining offer extensive opportunities for improving science in numerous areas including aerosol health effects through epidemiological studies and smoke plume modeling. Multi-pollutant, low-cost sensors hold promise for new source apportionment approaches in the ambient and indoor environments. Low cost sensors still need continued evaluation, it would be interesting to look at the impact of aerosol composition (available at CSN network sites) on aerosol sensor response.