Ammonia (NH₃) is an essential component of agricultural fertilizers, but can become an environmental pollutant. Roughly half of all nitrogen used in the agricultural industry escapes into the environment; 56 per cent ends up in water bodies and 44 per cent is released into the air, mostly as NH₃. The gas contributes to soil acidification, biodiversity loss, and various respiratory problems. Monitoring NH₃ is particularly difficult due to its high solubility and tendency to stick to surfaces.
A recent study published in Environmental Pollution provides a comprehensive comparison of sorbent materials for Passive Flux Samplers (PFS), a widely used tool in agriculture for estimating emission rates. The research establishes the groundwork for more accurate ammonia monitoring systems.
In an interview with The Tribune, Angela Trivino Arevalo, lead author of this study and doctoral student in McGill’s Bioresource Engineering Department, detailed how the two-year research was conducted using PFS—a tool utilized in agriculture to estimate emission rates.
“The [PFS] is just a tube, and inside you can change the sorbent according to the gas that you want to target and capture. I am working with ammonia, but there are other samplers that can capture [other gasses]. The idea is that you can go to any farm to measure gas emission, […] and the PFS tries to gather data in a way that […] doesn’t ask the farmer too much work. You can then take the data […] to the lab to analyze what was captured.”
The measured mass obtained from the PFS is then converted into an emission rate. The two processes that determine PFS performance are aerodynamics, how freely air flows through the tube, and sorption, the material’s ability to trap molecules.
Although many materials can be used as sorbents, few studies have compared how these materials affect the PFS’s accuracy. Trivino Arevalo’s team used four materials— glass microfibre filters, glass beads, zeolite, and biochar—and attempted to identify which provided the most reliable NH₃ measurements.
“I love this research, because we are comparing materials that have different characteristics. We have zeolite, that is a material that we know has a big porosity, we have glass microfibre, that is something we use to analyze air, and glass beads [….] It’s not only interesting for the side of chemical and material development, but this knowledge is one specific area that develops something to measure air quality,” Trivino Arevalo said.
Trivino Arevalo used a wind tunnel to examine how each sorbent affects airflow through the PFS. They measured inlet airflow, pressure drop, and internal air velocity at wind speeds of one to six m/s. From here, they calculated a K factor, a measure of thermal conductivity ranging from zero to one, that describes how wind speed relates to the air entering the PFS. The higher the K factor, the more efficiently airflow passes through the sampler.
Glass microfibre filters displayed the best aerodynamic behaviour, with the highest K factor of 0.61; zeolite and biochar performed moderately well, with scores of 0.58 and 0.59, respectively. Glass beads restricted airflow the most, with a K factor of 0.47.
The study compared the sorption abilities of these materials. To determine the best overall sorbent, Trivino Arevalo used a scoring system that emphasized four areas: Data variability, aerodynamics, sorbent capacity—the ability for the sorbent inside the PFS to hold ammonia—and handling and practicality. Variability also played a major role in determining the best sorbent. Zeolite and biochar, despite having high capacities, showed inconsistent results due to their complex pore structures. Glass beads and microfibre filters provided lower detection limits for low-concentration sampling.
Sorption results showed zeolite had the longest breakthrough time of 103 minutes and the highest capacity, while glass microfibre filters and glass beads had lower capacities, meaning they saturated quickly. Trivino Arevalo’s research concluded that glass microfibre filters were the most effective sorbent materials for PFS usage. This material offered the most balanced combination of good aerodynamics and moderate capacity.
Improved PFS performances are a step towards more accurate and reliable data related to ammonia emissions, which is an urgent priority as agriculture is increasingly subject to erratic climate changes.
