Clouds form when water vapor – an invisible gas in the atmosphere – sticks to tiny floating particles, such as dust, and turns into liquid water droplets or ice crystals. In a recently published study, we show that microplastic particles can have the same effects, producing ice crystals at temperatures 5 to 10 degrees Celsius (9 to 18 degrees Fahrenheit) higher than droplets without microplastics.
This suggests that microplastics in the air may affect weather and climate by producing clouds in conditions where they would not otherwise form.
We are atmospheric chemists who study how different types of particles form ice when they come into contact with liquid water. This process, which occurs constantly in the atmosphere, is called nucleation.
Clouds in the atmosphere can be made up of liquid water droplets, ice particles, or a mixture of both. In the clouds of the middle to upper atmosphere, where temperatures are between 32 and minus 36 F (0 and minus 38 C), ice crystals normally form around mineral dust particles from dry soils or particles biological, such as pollen or bacteria.
Microplastics are less than 5 millimeters wide, or the size of a pencil eraser. Some are microscopic. Scientists have found them in the deep seas of Antarctica, on the summit of Mount Everest, and in fresh Antarctic snow. Because these fragments are so small, they can be easily carried through the air.
The importance of ice
Ice in clouds has significant effects on weather and climate, as most precipitation usually begins with ice particles.
In nontropical areas around the world, many cloud tops extend high enough into the atmosphere that cold air causes some of their moisture to freeze. Once formed, ice draws water vapor from the liquid droplets surrounding it, and the crystals become heavy enough to fall. If ice does not form, clouds tend to evaporate instead of causing rain or snow.
Children learn in primary school that water freezes at 0°C, but this is not always true. In the absence of something to attach to, such as dust particles, water can be supercooled to temperatures as low as minus 36 F (minus 38 C) before freezing.
For freezing to occur at warmer temperatures, a material that does not dissolve in water must be present in the droplet. This particle provides a surface where the first ice crystal can form. The presence of microplastics can lead to the formation of ice crystals, which could increase rain or snowfall.
Clouds also influence weather and climate in several ways. They reflect incoming sunlight away from the Earth’s surface, which has a cooling effect, and absorb some radiation emitted by the Earth’s surface, which has a warming effect.
The amount of reflected sunlight depends on the amount of liquid water or ice in the cloud. If microplastics increase the presence of ice particles in clouds relative to liquid water droplets, this change in ratio could alter the effect of clouds on Earth’s energy balance.
How we worked
To determine whether microplastic fragments could serve as nuclei for water droplets, we used four of the most common types of plastic in the atmosphere: low-density polyethylene, polypropylene, polyvinyl chloride, and polyethylene terephthalate. Each has been tested in its pure state and after exposure to ultraviolet rays, ozone and acids. All of these elements are present in the atmosphere and could affect the composition of microplastics.
We suspended the microplastics in small water droplets that we slowly cooled to observe them freezing. We also analyzed the surfaces of the plastic fragments to determine their molecular structure, because ice nucleation could depend on the surface chemistry of the microplastics.
For most plastics studied, 50% of the droplets were frozen when cooled to minus 8 F (minus 22 C). These results match those of another recent study by Canadian scientists, who also found that certain types of microplastics produce ice at warmer temperatures than droplets without microplastics.
Exposure to ultraviolet light, ozone, and acids tended to decrease ice nucleation activity on particles. This suggests that ice nucleation is sensitive to small chemical changes on the surface of microplastic particles. However, these plastics still nucleated ice, and therefore could still affect the amount of ice in the clouds.
What we don’t know yet
To understand how microplastics affect weather and climate, we need to know their concentrations at altitudes where clouds form. We also need to know the concentration of microplastics relative to other particles that can nucleate ice, such as mineral dust and biological particles, to determine if microplastics are present at comparable levels. These measurements would allow us to model the impact of microplastics on cloud formation.
The plastic fragments come in various sizes and compositions. In our future research, we plan to work with plastics containing additives, such as plasticizers and colorants, as well as smaller plastic particles.
Miriam Freedman, Professor of Chemistry, Penn State and Heidi Busse, PhD Student in Chemistry, Penn State
This article is republished from The Conversation under a Creative Commons license. Read the original article.
