Black carbon particles, produced by combustion of gasoline, diesel fuel, coal, and other organics, have been found to be the second-largest driver of climate warming, after carbon dioxide (CO2), since the Industrial Revolution [Myhre et al., 2013]. Much of black carbon’s role in this warming results from the fact that it contributes to the melting of snow and ice and thus to darkening of Earth’s surface, reducing the amount of sunlight the planet reflects and increasing the amount it absorbs.
These processes have been thoroughly studied, yet measurements made in the past of black carbon particles in snow and determinations of their effects on melting may be inaccurate. To date, most studies have overlooked a major and potentially complicating factor: microplastics. Here we address the possible implications of this issue and offer recommendations to promote more accurate assessments of the effects of both black carbon and microplastics on snow and climate.
Massive Amounts of Uncounted Microplastics
Microplastics (MPs) are tiny plastic particles, fibers, or fragments smaller than 5 millimeters across—and they are everywhere. They have been implicated in a range of environmental effects, including bioaccumulation in fish and in human blood and feces. These particles have reached Earth’s least populated areas, including in the High Alps, the Arctic [Bergmann et al., 2019], and even Antarctica [Kelly et al., 2020]—in fact, everywhere scientists look, they find microplastic particles. MPs and black carbon (BC) have been depositing together in snow since the 1950s, when plastics and petroleum products came into widespread use.
Today the mass of MP particles in the environment is very likely more than that of BC. Dubaish and Liebezeit  found 5 times more MP particles than visible BC particles in a microscope slide count of particles in water samples from Jade Bay along the northwestern coast of Germany. This is the only study to date that has reported simultaneous measurements of MP and BC particles, and it did not address the coexistence of MPs and BC in snow.
Wind can loft MP particles above the ground and carry them onto snow surfaces [Evangeliou et al., 2020], where, calculations have shown, they can remain for hundreds of years before they completely decompose. MPs are now believed to have spread to every human-reached corner of Earth [Brahney et al., 2020; Pabortsava and Lampitt, 2020]. So when field researchers sample snow for BC laboratory measurements, they inevitably bottle or bag MPs together with BC—and both types of particles may be counted together during measurement and analysis.
Very few studies of BC in snow have tried to separate out MPs before making instrumental measurements. This long-term neglect of MPs in snow in past studies may have resulted in overestimations of the BC content in snow and of its effects on glaciers, snow cover, ice sheets, and climate. Unfortunately, climate change assessments, so far, also have not included the role of MPs in snow.
It’s All the Same to the Instruments…
The risk in not separating MPs from BC before conducting lab measurements is that the effects of the two types of particles could be confounded, impairing our understanding of the true effects of each on snow and climate. Why is this? MPs and BC may be indistinguishable in the results of thermal-optical and pyrolysis-based laboratory tests (Figure 1).
The most-used plastics include polyethylene, polypropylene, polystyrene, and polyvinyl chloride, along with smaller amounts of other plastics. Combustion experiments demonstrate that plastics ignite and burn in oxygen at temperatures between 500°C and 1,000°C. Because the main elements composing plastics are carbon (roughly 85%), hydrogen (roughly 14%), and oxygen (less than 1%), the gases released when these plastics are completely combusted are CO2 and H2O [Zevenhoven et al., 1997].
BC particles, which consist mostly of carbon, oxidize in air between 500°C and 700°C and form CO2 if combustion goes to completion [Andreae and Gelencsér, 2006]. The overlap in the oxidation temperature ranges of BC and MPs implies that pyrolysis-related techniques may produce results in which the BC signal is contaminated by MPs in a sample.
Likewise, optical absorption techniques such as aethalometry (light absorption by aerosol particles collected on filters) and laser-induced incandescence (light absorption by nebulized samples) may also combine signals from MPs and BC. Many types of MPs are brightly colored or black and absorb light in the visible and near-infrared wavelengths [Alexander et al., 2008], just as BC particles do.
Until recently, light-absorbing constituents in snow samples measured using the above methods were implicitly presumed to be BC, organic carbon, or dust, while the probable coexistence of MPs was overlooked.
But the Environment Knows the Difference
Almost all “clear” plastics are nearly transparent in the UV–visible wavelengths [Ishaq, 2019]. These plastics, deposited on snow, do not perturb the solar radiation balance. However, most plastic products are dyed or painted, which causes them to absorb light. For example, red plastics absorb green light, blue plastics absorb yellow light, and black plastics absorb a wide range of wavelengths. In addition, as plastics weather and break down, partly because of sunlight absorption, the resulting MPs can turn from transparent to translucent. Tearing, scratching, and aging processes such as these cause MP particles to absorb more light.
Increased light absorption by MPs may contaminate assessments of BC’s impacts on snow albedo (the fraction of sunlight that snow reflects), and thus on its contribution to calculations of radiative balance, in two ways. One is that MPs may be responsible for part of the reduced snow albedo that current lab tests ascribe entirely to BC. The other is that measured amounts of equivalent BC (a value determined using a mass absorption coefficient that may include substances other than BC) can overestimate the amount of actual BC if the equivalent BC amounts include significant contributions from MPs.
Although natural environmental temperatures never get high enough to convert BC and MP particles to CO2 (except perhaps near active wildfires or volcanoes), laboratory analyses using pyrolysis methods are another matter. In such analyses, both BC and MPs are oxidized to CO2, and measurements reflect contributions from both species. Calculations of BC thermally converted to CO2 in instruments may thus be inflated by unrecognized contributions from MPs in current instrument analyses.
A Work-Around to Measure BC and MPs in Snow
Considering the potentially significant complications that MPs pose for our understanding of BC’s effects on snow melting and climate, it is clear that research is urgently needed to determine the extent—or lack thereof—to which scientists can distinguish these particles using different analytical methods. It is also very important to quantify the co-occurrence of MPs and BC in snow samples collected for lab measurements.
So far, no protocols have been applied to separate these particles prior to instrumental analysis of BC in snow or ice. We thus suggest the following simple preprocessing protocol to separate MPs and BC in snow samples before testing:
- Use glass bottles instead of plastic ones when collecting field samples to avoid plastic contamination from sampling bottles.
- Filter melted snow samples through filters with micrometer-sized pores to remove relatively large MP particles.
- Centrifuge samples to separate smaller MP particles. (The density of BC particles is greater than 1.8 grams per cubic centimeter, whereas most plastics have a density of less than 1.4 grams per cubic centimeter.)
In light of potential interference from MPs, we also suggest that previous measurements of BC in snow may need to be reviewed and the radiative effects of BC and MPs in snow co-assessed. Such a reassessment could assist in identifying the true sources of particle pollution and melting in snow and increase the effectiveness of remediation efforts.