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Tonnes Of Airborne Microplastics Drop Onto Cities Every Year

Tonnes Of Airborne Microplastics Drop Onto Cities Every Year

Whether you’re talking about the environment or news headlines, it’s becoming clear that microplastics are everywhere. These three stories: “Synthetic fibers discovered in Antarctic sea ice”, “Blue whales may ingest 10 mil. pieces of microplastic daily”, and “Microplastics settling into soil”, are from the past month alone. Together, they highlight the ubiquitous nature of these tiny fragments of different polymers, all of which were created and discarded via human activity, and circulated around the planet via natural forces.

A couple of days ago, a paper caught my eye. Published in the journal Environmental Science & Technology ($), and written by researchers at the University of Auckland / Waipapa Taumata Rau, it reported on the microplastics found in the air we breathe. Led by aerosol chemist Dr Joel Rindelaub*, these scientists estimate that an astonishing 74 metric tonnes of microplastics – about the same weight as three million plastic bottles – fall out of the atmosphere and onto the city every year.

The research, which was initiated during a COVID lockdown in 2020, involved putting sampling units at two sites in Tāmaki Makaurau / Auckland. One was placed on the rooftop of a six-story building at the central city university campus. The second was placed on a garden wall at a house in Remeura, a residential neighborhood.

Over a Zoom call, Rindelaub described the sampling process as “extremely low-technology” – at each site, a glass funnel sat untouched, exposed to the air for a week at a time. Anything that fell out of the atmosphere during that period, including airborne microplastics, would be collected by the funnel and drop into a glass bottle beneath. In order to prevent degradation from UV light and evaporation of the samples, the funnel neck and bottle were encased in a wooden box.

An almost-identical unit – save for a piece of aluminum foil that sealed the funnel mouth – was set up alongside it to act as a field blank or control. Each week for nine weeks, the glass bottles were collected and taken to the lab for analysis, and new bottles put in their place.

Great care was taken not to contaminate the sample at all stages – as the authors write in the paper, “When collecting samples, the collector always wore cotton clothing and stood downwind of the sampling unit…. All water used in this study was tested for microplastics prior to use.” In addition, once they reached the lab, the contents from the glass sampling bottles were carefully filtered, before being treated with heated hydrogen peroxide to help remove organic materials (like cotton) that can lead to false positives in later tests. And while the collection process was fairly simple, the lab analysis that followed it was decidedly high-tech.

One of the major challenges with studying the prevalence and chemical makeup of microplastics in the air is that there isn’t just one single standard measurement technique.

Different groups use different approaches, which unsurprisingly, leads to different results. And perhaps even more importantly, there’s no international agreement on the minimum particle size that should be included in studies – some researchers might exclude particles with a diameter of less than 200 µm (microns), while others might include everything down to 50 µm. “Whatever you use,” said Rindelaub, “your lower limit is going to dictate what you can actually see.” As a result, “inter-comparison between studies is basically impossible unless you’re using the same method.”

The Auckland team wanted to provide analysis that both complemented previous studies, and pushed it further, in order to paint a bigger picture of what’s going on with airborne microplastics.

The first step was dye-assisted fluorescence microscopy. Widely-used in lots of research fields, this technique involves staining samples using a specific dye, illuminating them with a light source that makes the dye glow (or fluoresce), and imaging them with a microscope. In microplastics research, it’s become the go-to tool for counting the number of particles in a sample, and to look at the morphology (size and shape) of those particles. So, it’s very useful, but it has its limits. For one thing, it can’t detect particles smaller than 10 μm.

This is important because, as the authors write, “The size of the microplastics observed in this study increased exponentially as the size of the bins decreased”. In other words, the more they zoomed in on their samples, the more particles they saw.

In total, 9464 microplastics were detected across all samples. Secondly, fluorescence microscopy really only shows you your sample – it is solely a visual technique, which isn’t helpful if you’re an analytical chemist, like Rindelaub. “From a toxicological standpoint, you need to know what is there [the chemistry] and how much of it you have [the mass]. The morphology matters, too. But those first two are the big ones.”

To delve into those bigger questions, Rindelaub and his team used another technique – pyrolysis-gas chromatography-mass spectrometry (Pyr-GC/MS). While this has been used in some recent marine and sediment microplastic studies, this is the first time it has been used to study atmospheric microplastics. The samples were placed on quartz filters and heated to the point of destruction inside a controlled, inert environment. Though brutal, this process produces smaller, stable molecules that act as a distinct chemical fingerprint for the material. It is a quantitative way to identify all of the polymers present in any given sample.

Together, these techniques provided a vast amount of information on the microplastics found in Auckland’s air. Here’s a quick overview:

  • Of the microplastics detected in this study, 95% were classified as particles (fragments), while 5% were classified as fibers. This differs from London, where a 2020 study found that fibers were the dominant morphology. The reason for this difference is not clear. It may be a sampling issue, or it may be reflective of the types of pollution present in each city.
  • Eight polymer types were identified in the collected samples. Polyethylene (PE) was the most abundant by mass (39%), followed by polycarbonate (PC, 26%), and polyethylene terephthalate (PET, 22%). PE and PET are widely used in packaging materials. PC is used in everything from electrical and electronic devices to automotive applications. The authors point out that all three are also used in the building and construction industry.
  • The average atmospheric microplastic deposition rate by mass for Auckland was 334 ± 81 μg m−2 day−1. If you scale to the total land area of Auckland (~607 km2), it would correspond to the deposition of 56−92 metric tons of common consumer microplastics every year.
  • The average atmospheric microplastic deposition rate by number was 4885 ± 1858 MPs m-2 day-1. This is much, much higher than has been found in other cities. For example, the 2020 London study measured a deposition rate of 771 ± 167 MPs m-2 day-1. Another from Hamburg, Germany measured 275 MPs m-2 day-1. This does not mean that Auckland is the most polluted city in the world. Rather, it’s a result of the aforementioned lack of consistency across studies. A situation that Rindelaub hopes this work will help change is, “One thing we wanted to say was hey, we need more standardization here, because all the data we have so far is almost meaningless.”

In the paper, the researchers also discuss the origin of these microplastics, and a key aspect is the coastal nature of Auckland. “Wave-breaking is a well-known mechanism for the generation of aerosols in general,” Rindelaub explained. “So, our thinking was that, if there are plastics in the ocean, which we know there are, they’re probably also going to be in those aerosols that are created.” To test their hypothesis, the team looked at meteorological data from the same test period.

They found that at wind speeds of 15–20 meters per second (54–72 kph or 33–45 mph), the microplastic deposition rate increased. In the sheltered Hauraki Gulf on which Auckland sits, these wind speeds are also known to generate increased sea spray. In addition, the most abundant polymer types found in the samples were also the dominant polymers found in the global marine environment and were reflective of a separate study of Auckland beach sediment. So, although they can’t say definitively that these specific microplastics came from the ocean, it looks likely.

What does all this mean for human health? At this stage, we don’t know. “It’s way too early to tell at this point because we don’t yet have definitive toxicological effects or impacts like we do with so many other airborne pollutants,” said Rindelaub. Something we do know is that, regardless of the material, when fragments of it get small enough, they can make their way into our systems and cause havoc (see: asbestos).

And so, nano plastics – the very smallest plastic fragments and fibers, ranging in size between 1 nm and 100 μm – are the plastics that are of the highest concern for people like Rindelaub, “They can get deepest in the lungs, transfer into the bloodstream, translocate throughout the body, and possibly even cross the blood-brain barrier”.

While the current study didn’t look at nano plastics (the smallest particle they could resolve was 10 μm), the exponential increase in the number of particles Rindelaub and his colleagues found at these smaller scales suggests that they are probably present. Analyzing these nano plastics is the next target for the team. “We have funding to expand on this research,” he said. “And I have some ideas on how to do it.

It’ll involve doing some size selection on the front end while you’re collecting. It’s already possible to collect for things like PM10, PM2.5, and all the way down to things in the nanometer scale. If we can pre-sort, we’ll have size information ahead of time along with our pyrolysis analysis for chemical characterization.”

“We’re still very much in the early stages of this emerging field.”

* As well as being a brilliant scientist and science communicator, Dr. Joel Rindelaub is one of my friends. He is also the owner of a truly superb mullet-and-mustache combo that has made him something of a style icon here in NZ.

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