There is also plastic in the atmosphere. It is part of a group of particles known as atmospheric aerosol. These are small solid or liquid particles suspended in the air, with diameters typically ranging from 1 nm to 10 μm (from one nanometre to ten microns). These diameters correspond to nanoscale-sized molecules, such as organic molecules, or to particles like pollen, sand, or salt, which can reach sizes of microns or larger. Atmospheric aerosols are generated in large quantities due to natural phenomena, such as volcanic eruptions or emissions from the surface of the seas, which make up marine aerosol. Furthermore, human activities release a significant amount of aerosols into the atmosphere through the use of both fossil and non-fossil fuels, including biomass combustion, human-caused fires, and industrial, and agricultural activities. In general, the anthropogenic fraction dominates over the natural one, both in rural and urban/industrial areas.
The influence of atmospheric aerosols on climate is well-known. Their impact on human health is also recognized, as particles smaller than 10 µm can be easily inhaled and are potentially harmful to pulmonary and cardiovascular functions. The episodes of smog (an acronym for smoke & fog) that occurred in post-war London due to the combustion of low-quality coal in power plants are outstanding examples. These events culminated in the episode in December 1952 that resulted in several thousand deaths. It’s important to note that particulate matter is not a single pollutant but a complex mixture of different organic and inorganic compounds, in liquid and solid forms. In current air pollution monitoring programs, two ranges of particulate matter are considered: PM10, which consists of particles with a diameter equal to or less than 10 microns, and PM2.5, or fine particulate matter, with a diameter equal to or less than 2.5 microns.
Recently, microplastics have emerged as a component of the anthropogenic aerosols. Microplastics are solid particles of synthetic polymers, with their largest dimension being less than 5 mm. Large microplastics readily sediment, but the smaller ones can remain suspended for prolonged periods. (Their suspension is favoured by the lower density of polymers compared to mineral matter and in the case of non-spherical particles like fibers.) Urban areas are important sources of microplastic emissions into the environment. We release a significant amount of microplastics into freshwater and marine environments through treated wastewater, but they also enter the atmosphere due to the wear and tear of materials such as textiles and other objects made from synthetic polymers. Furthermore, cities facilitate the dispersion of microplastics released into the atmosphere due to the urban heat island effect, which generates upward air currents of relatively warm air. This occurs because cities are built with materials that absorb solar radiation, like asphalt, and they also emit heat due to the operation of heating systems, vehicles, and a wide variety of machinery and electrical devices.
The presence of microplastics in the atmosphere can be quantified using passive or active samplers. Passive samplers (or collectors) are containers that collect atmospheric deposition over a given period. They can be designed to collect dry and wet (during rain episodes) deposition separately or together. Passive samplers allow for the determination of deposition rates in terms of microplastics per square meter per day or equivalent units. Active samplers are filtration systems equipped with a suction pump that passes a certain volume of air through a filter medium. These samplers enable the establishment of the concentration of microplastics per unit volume of air at a specific time and location.
There are few data available on the amount of plastic carried by the air in our cities. The diversity of urban, suburban, rural, and remote environments sampled, along with methodological differences between studies, result in significant dispersion in the published data. It has been estimated that the most polluted areas in Asia emit as much as one thousand tons of plastic into the Pacific and Indian Oceans through the atmosphere each year. Our teams (Universities of Alcalá and Autónoma de Madrid and the Centre for Astrobiology CAB-INTA-CSIC) have measured concentrations of microplastics exceeding ten particles per cubic meter in direct samplings using aircraft flying over urban centres at up to 3500 m above the surface. Trajectory analyses indicate that plastic particles can be deposited hundreds of kilometres away from their point of emission.
The figure below shows microplastic deposition rates from recent studies. The figures range from approximately one thousand microplastics per square meter per day measured in central London to values of less than ten microplastics per square meter per day recorded in a remote area in Iran. The significant dispersion of available data (the ordinate axis is in logarithmic scale) is due to the diversity of sampled environments and the use of different characterization techniques: Raman microscopy allows the analysis of particles about ten times smaller than mid-infrared microscopy, leading to a bias towards higher concentrations as smaller particles are always more abundant. Also, quite a few studies only analyse a small fraction of the total collected particles, which may lead to high sampling errors.
The data for Madrid, Barcelona, Vigo, and Tres Cantos are comparable in terms of methodology, as they come from a larger research project conducted by EnviroPlaNet network groups and recently published in the journal Science of the Total Environment. This study was carried out in ten Spanish cities over four months, one in each of four consecutive seasons. The average deposition rate ranged from 5.6 to 78.6 microplastics per square meter per day, depending on the sampled city. More in Madrid and Barcelona, and less in smaller cities. To put this result into context, the figures obtained correspond to the deposition of over one hundred thousand plastic particles per day on the grass at the Santiago Bernabéu stadium (approximately 50 million microplastics per year). They represent a small mass because they are small particles, ranging from a few tens of microns to less than half a millimetre, with an average size close to that of a human hair’s thickness; but they are there. Most of them are polyester and acrylic fibers, generally of clear textile origin, polyolefins (the most commonly used polymers), and alkyd resins (used in paints).
In summary, the atmosphere carries remnants of our plastic materials, sometimes over long distances from their point of emission, and deposits them at distances that depend on the atmospheric conditions. A significant portion of these plastics consists of textile fibers, and all of them are small-sized particulate matter, small enough to be inhaled, thereby posing risks to human health that are still challenging to assess. For example, these materials are easily colonized by microorganisms, some of which can be pathogens.
Science, Plastic and Environment 