Research on microplastics (plastic particles less than 5000 μm in size) initially focused on the marine environment (1) and has since expanded to include terrestrial ecosystems. (2−6) Accumulating evidence indicates that microplastics in the atmosphere can travel above ocean surfaces and even to distant locales via global atmospheric transport. (7,8) However, key questions remain to be answered regarding the vertical exchange and horizontal transport processes of atmospheric microplastics.

Atmospheric microplastics can be deposited on the ground through dry and wet atmospheric fallout, substantially contributing to the presence of microplastics in water and soil environments. (9) Atmospheric transport is emphasized as a crucial pathway impacting the source–sink dynamics of plastic pollution across different ecosystems. (10) Additionally, owing to their ability to adsorb chemicals, bacteria, and viruses, microplastics are recognized as a novel element within the realm of air pollution. (11−14)

Radon-222 (²²²Rn, half-life = 3.82 days), a noble gas and decay product in the ²³⁸U series, is released from the soil matrix at a constant rate, with >99% of ²²²Rn in the atmosphere emanating from continental sources. (15) As the long-lived daughters of ²²²Rn (Supporting Figure 1), (16,17) ²¹⁰Pb, ²¹⁰Bi, and ²¹⁰Po are rapidly adsorbed onto solid particles upon formation, with their fates aligning with those of aerosol-bound particles. The ²¹⁰Po/²¹⁰Pb ratio enables the estimation of the residence time of aerosols (RTA), based on two assumptions: (1) the measured atmospheric particulate matter (PM) is treated as a single entity, and (2) all particle-reactive radionuclides produced by 222Rn decay are scavenged from the atmosphere before reaching a new area. Beryllium-7 (⁷Be, half-life = 53.3 days), a cosmogenic radionuclide, originates from the spallation of oxygen and nitrogen nuclei induced by cosmic rays in the stratosphere and upper troposphere. (18,19) The majority of sup]7Be production (67%) occurs in the stratosphere, and it typically does not reach the troposphere except during thinning of tropopause folds near the jet stream at midlatitudes in the spring. (15,20) Due to their distinctly different source terms but well-established source distributions, ⁷Be and ²¹⁰Pb are widely employed as powerful atmospheric tracers for studying the origins and transport processes of air masses. (15,21)

As PM contains microplastics, active particulate samplers are considered appropriate tools for collecting atmospheric microplastics. (22) Aerosol samples are typically collected by filtering a large volume of air via specialized aerosol collection equipment, (15) enabling simultaneous analysis of filtered microplastic particles and trapped natural radionuclides on the filters. However, plastic and nonplastic particles are intermixed during the atmospheric filtration process, rendering separation of their adsorbed radionuclides impossible. Chen et al. (23) reported that the transport dynamics of plastic particles are generally similar to those of nonplastic particles. Thus, the fates of aerosols are closely linked to those of plastic particles.

Herein, we utilized aerosol tracers (⁷Be, ²¹⁰Pb, and ²¹⁰Po) to trace the origins and transport processes of atmospheric microplastics for the first time in Tianjin, a coastal city in China. The aims of the study were to (i) document the temporal variations in and features of atmospheric microplastics; (ii) examine the origins, transport processes, and residence times of microplastics; and (iii) explore the correlations between microplastics and other components of the atmosphere...

Figure 1. Study area and sampling device employed in this investigation.

Figure 2. Temporal fluctuations in the abundance of atmospheric microplastics in Tianjin from March 2022 to February 2023.

Figure 3. Variations in shape, color, and size of atmospheric microplastics across four seasons. (a–d) Shapes of microplastics during four seasons; (e–h) colors of microplastics during four seasons; (i–l) sizes of microplastics during four seasons.

Figure 4. Polymer composition and surface morphology of microplastics. (a) Spectra of common polymer fiber types; (b, e) polymer composition and surface morphology of microplastic fibers; (c, f) polymer composition and surface morphology of microplastic fragments; (d, g) polymer composition and surface morphology of microplastic films. PA, polyester amide; PAL, polyacrylate; PBd, polybutadiene; PE, polyethylene; PE–PP, poly(ethylene-co-propylene); PET, poly(ethylene terephthalate); PMA, poly(N-methyl acrylamide); PP, polypropylene; PS, polystyrene; PS-MA, poly(styrene-co-maleic acid); PVA, poly(vinyl alcohol); RA, rayon.

Figure 5. ⁷Be, ²¹⁰Pb, and ²¹⁰Po concentrations and ratios in aerosols throughout the year. (a) Concentrations and ratios of ⁷Be and ²¹⁰Pb throughout the year; (b) concentrations and ratios of ²¹⁰Pb and ²¹⁰Po throughout the year; (c) residence times of aerosol particles throughout the year.

Figure 6. Correlations among atmospheric microplastics, radionuclides, and meteorological parameters (n = 18). MPs, atmospheric microplastics; RT, residence time; PM2.5, fine particulate matter; PM10, inhalable particulate matter; PM, total particulate matter; AQI, air quality index; TEMP, temperature; RH, relative humidity; ATM, atmospheric pressure; AWS, average wind speed; MWS, maximum wind speed; AWD, average wind direction; MWD, maximum wind direction.

Furthermore, Ouyang et al. (41) observed that coal combustion caused an enrichment of ²¹⁰Po in fly ash particles, which could lead to elevated ²¹⁰Po levels in the urban atmosphere when mixed with aerosols in air and long-distance transport. Tianjin begins its urban centralized heating in November. Therefore, we hypothesize that the peak residence time of urban atmospheric particles in November may be correlated with increased ²¹⁰Po levels from coal consumption.

...This study was limited by the range of detectable microplastic particle size, which varied from 20 μm to 5 mm. Additionally, radionuclides were found to leach from all particle sizes. Challenges remain in developing methods to effectively separate microplastic particles from total suspended particulates during active particulate sampling. An important achievement of this study is the development of a method utilizing natural radionuclides ⁷Be, ²¹⁰Pb, and ²¹⁰Po to explore the transport processes and residence times of atmospheric microplastics...