Particles suspended in the air all around us are linked to millions of premature deaths yearly worldwide. Some of these particles are formed naturally, but the reason for our current challenges are the particle emissions which result from human activities, especially traffic and combustion of fuels for heating or energy. Understanding where the particles come from and how they relate to health is crucial for improving air quality. And necessary for understanding is comprehensive measurements.
The most common way to measure particles is to measure their mass. But this leaves a lot of information out. Particle sizes span several magnitudes, from just a few nanometers to tens of micrometers, and just a few large particles can completely overshadow the presence of huge numbers of ultrafine particles (particles smaller than 100 nm).
“Current evidence shows that ultrafine particles are very toxic despite having very little mass, which is why mass measurements are not sufficient”, Laura Salo says.
In her doctoral dissertation, Salo explores how electrical measurement methods can improve our understanding of particle emissions and particle concentrations which are relevant to health. The thesis includes measurements in cities and an underground mine: these environments suffer from particle pollution from vehicle engines, which produce mostly ultrafine particles.
Electrical measurement methods are very good at targeting these smaller particles, as small, charged particles move easily in electrical fields. The current detected from the charged particles can be transformed into lung-deposited surface area—a particle metric which specifically targets health outcomes.
A new sensor for detecting ultrafine particles
In addition to established electrical methods, the dissertation introduces a new particle sensor based on measuring particles charged naturally in the atmosphere. Previous similar sensors use an electrical charger.
A major finding in Salo’s research is that in low-pollution environments, the majority of particles entering lungs are very small and from local sources, whereas in polluted environments they tend to be larger particles that have travelled in the air for some time. This could explain inconsistencies that have previously been found in the health effects associated with particle mass.
Mass concentration is a good metric for the larger particles, but to measure the smaller, locally emitted particles, other methods are needed, and for that purpose Laura’s thesis presents the new sensor. A prototype of the sensor was tested near a busy road just south of Delhi. Based on the test results, the sensor response correlated very well with particle surface area, which has previously been shown to correlate with health outcomes.
“We used the new sensor in a very polluted environment next to a busy road in Delhi, where we were pleasantly surprised at how well it performed despite the difficult conditions. The sensor needs to be evaluated for accuracy of course, but practical aspects such as robustness, price and maintenance needs are also important and have been taken into account in the design,” she says.
Public defence on Friday 16 May
The doctoral dissertation of MSc (Tech) Laura Salo in the field of aerosol physics titled Characterization and Development of Electrical Methods for Aerosol Measurement will be publicly examined at the Faculty of Engineering and Natural Sciences at Tampere University at 12 o’clock on Friday the 16th of June 2023, at Hervanta campus, Tietotalo auditorium TB104 (Korkeakoulunkatu 1, Tampere). The Opponent will be Professor Naomi Zimmerman from the University of British Columbia. The Custos will be Professor Topi Rönkkö from Tampere University.
The doctoral dissertation is available online.
The public defense can be followed via remote connection.
Photo: Laura Salo