Sizing Up the Nanoscale: Measuring Nanocluster Aerosol in Indoor Air

Brandon E. Boor, the Dr. Margery E. Hoffman Associate Professor in the Lyles School of Civil and Construction Engineering at Purdue University, recently delivered a talk at the American Chemical Society (ACS) Fall 2026 Meeting in Chicago, Illinois, titled “Indoor Atmospheric Nanoparticle Formation from Scented Cleaning Products.”^1,2 Boor’s talk presented recent work by his team on the measurement of indoor air quality.^3,4

In this multi-part Q&A, Boor expands on the material presented in his ACS talk. In the first installment, he describes the instrumentation and methodology behind measuring nanoparticle size distributions at the nanocluster scale (1–3 nm), and outlines the technical challenges of acquiring reliable, real-time data at these dimensions.

You conducted both field and laboratory experiments in residential and office settings. How did the two environments differ in what they let you observe, and how did results from one inform or validate the other?

At Purdue, much of our controlled laboratory characterization of indoor air pollutant dynamics is conducted in the zero Energy Design Guidance for Engineers (zEDGE) test house, a full-scale residential test house on campus. It provides a controlled environment in which we can carry out common household activities such as cooking, cleaning, and the application of personal care products. The house allows us to track how these activities alter the composition of indoor air under realistic conditions. It was developed by my colleague, Professor Nusrat Jung, and includes a full kitchen, a bathroom, and a loft, together with the indoor materials and furnishings found in a typical home.

At the same time, we can control the ventilation conditions, including the source of the outdoor air and the manner in which it enters the space. We design tightly controlled experimental sequences in which our graduate students carry out a prescribed routine on a fixed schedule, which gives us a precise record of what occurred and when.

This design supports much of the work we publish, including in ACS journals, where our aim is to produce generalizable results. Because the test house is a controlled environment, we can construct a mass balance model for the space. We couple that model with measured concentrations of the pollutants present, and from it we report parameters such as emission rates that others can use to better understand how these activities shape indoor air quality.

The controlled environment therefore allows us to run experiments repeatably and to develop mechanistic insight into how a given household activity affects the composition of indoor air, in this case with respect to nanoscale aerosol particles and gases such as volatile organic compounds (VOCs) and ozone. Because the facility is on campus, we can also deploy our full suite of instrumentation there. We have a variety of high-resolution analytical equipment to track the particles that form in the air or that are emitted into the air, as well as the gases present.

We deploy aerosol spectrometers that provide size-resolved concentration data and online mass spectrometers that quantify VOCs and their oxidation products in real time. Some of this instrumentation can be deployed in the field, but for other instruments it is more practical to keep them on campus and conduct the experiments at Purdue.

We also conduct field work. We have carried out measurements in the Herrick Living Laboratories at Purdue, the Purdue Retrofit Net-zero: Energy, Water, and Waste (ReNEWW) House near campus, and in the France A. Córdova Recreational Sports Center (CoREC), where the large indoor aquatic center allows us to study indoor chemistry in a very different setting.

Field measurements are valuable, but they are inherently observational, because we cannot control what occurs in those environments. They lend realism to the data, yet we cannot dictate what happens and when, as we can in the full-scale test house.

Overall, the laboratory and field work are complementary: each addresses a different class of research question.

You measured nanoparticle size distributions down into the 1–3 nm nanocluster regime. What instrumentation made that possible, and what were the biggest technical challenges in getting reliable, real-time data at that scale?

In our recent work we have used an instrument called a particle size magnifier–scanning mobility particle sizer (PSMPS). That instrument allows us to both detect and size-classify airborne nanoparticles down to approximately 1.2 nm in electrical mobility diameter.

Particles below about 10 nm have historically been very difficult to measure. The instrumentation that made it possible was developed largely to study outdoor atmospheric new particle formation, for example over forested regions.

That instrumentation has since found its way indoors, and we now use it to measure these very small particles. The measurement proceeds in several steps.

Nanocluster aerosol in the 1–3 nm range can be produced by secondary processes such as ozone reacting with certain VOCs, or emitted directly by primary sources such as a gas stove or a candle. Regardless of their origin, we need to determine how large they are and how many are present in each size fraction. To do that, we first apply a known charge distribution to the particles. In the air, they may or may not be charged, and they may have positive or negative charges, but we need to pass them through a bipolar charger (neutralizer). This can be done with a radioactive source such as krypton-85, or with a soft X-ray source. In either case, ions are generated at a controlled rate. They attach to the particles, bringing the population to a known and predictable charge distribution. That is the first step in detecting these nanoparticles. The second is to classify them by size.

Particle size governs how these particles are transformed in the air around us, so it must be determined. To do this, we pass them through an instrument called a differential mobility analyzer, which consists of two concentric electrodes. One is held at high voltage, up to roughly 10 kilovolts, and the other at ground, establishing a controlled electrical field between them. The particles charged in the bipolar charger enter that field. Depending on their size, they are classified by their electrical mobility, which governs their trajectory through the electrical field and whether they reach the outlet of the differential mobility analyzer. By varying the applied voltage, we vary the electrical field, and particles of different sizes are transmitted in turn.

At each voltage, a narrow size fraction is transmitted and passed to a detector for counting. For example, we may transmit and count 2 nm particles, record the concentration in that size fraction, then step the voltage to transmit 3 nm particles, and so on. Stepping through the full voltage range yields the particle number size distribution.

Counting is equally important, because conventional techniques fail at these sizes. An optical particle counter of the kind found in a low-cost air quality sensor cannot detect these particles at all; most optical instruments cannot resolve particles below roughly 300 nm, because they scatter too little light. These particles therefore have to be handled differently, and in our instrument we first grow them in a particle size magnifier.

A nanoparticle enters a growth tube supersaturated with a working fluid, in this case diethylene glycol. The vapor condenses onto the particle, which acts as a condensation nucleus, and the particle grows into a droplet that then exits the device. It then enters a second instrument, a condensation particle counter, which contains a further growth tube supersaturated with a second working fluid, in this case butanol. The butanol condenses further onto the droplet, enlarging it. The droplet then passes through a laser beam and is counted optically. In this way, particles far too small to detect directly are grown to sizes that can be counted.

Once these measurements are in hand, we can begin to understand how the particles form in the indoor environment and how they are subsequently transformed: how they grow by condensation and coagulation, how they deposit onto surfaces, and how they are removed by HVAC filters or portable air cleaners.

References

1. Purdue University, Brandon E. Boor Profile. Purdue University, 2026. https://engineering.purdue.edu/CCE/People/ptProfile?resource_id=124718 (accessed September 8, 2026).
2. Boor, B. E. Indoor Atmospheric Nanoparticle Formation from Scented Cleaning Products. Presented at the American Chemical Society Fall 2026 Meeting, Chicago, Illinois. Available at: https://acs.digitellinc.com/live/37/session/591663
3. Patra, S. S.; Liu, J.; Jiang, J.; Ding, X.; Huang, C.; Keech, C.; Steiner, G.; Stevens, P. S.; Jung, N.; Boor, B. E. Rapid Nucleation and Growth of Indoor Atmospheric Nanocluster Aerosol during the Use of Scented Volatile Chemical Products in Residential Buildings. ACS ES&T Air 2024, 1 (10), 1276–1293. DOI: 10.1021/acsestair.4c00118
4. Jiang, J.; Ding, X.; Tasoglou, A.; Huber, H.; Shah, A. D.; Jung, N.; Boor, B. E. Real-Time Measurements of Botanical Disinfectant Emissions, Transformations, and Multiphase Inhalation Exposures in Buildings. Environ. Sci. Technol. Lett. 2021, 8 (7), 558–566. DOI: 10.1021/acs.estlett.1c00390