Unraveling the Marine Microplastic Cycle: The First Simultaneous Data Set for Air, Sea Surface Microlayer, and Underlying Water

Microplastics (MP) including tire wear particles (TWP) are ubiquitous. However, their mass loads, transport, and vertical behavior in water bodies and overlying air are never studied simultaneously before. Particularly, the sea surface microlayer (SML), a ubiquitous, predominantly organic, and gelatinous film (<1 mm), is interesting since it may favor MP enrichment. In this study, a remote-controlled research catamaran simultaneously sampled air, SML, and underlying water (ULW) in Swedish fjords of variable anthropogenic impacts (urban, industrial, and rural) to fill these knowledge gaps in the marine-atmospheric MP cycle. Polymer clusters and TWP were identified and quantified with pyrolysis-gas chromatography–mass spectrometry. Air samples contained clusters of polyethylene terephthalate, polycarbonate, and polystyrene (max 50 ng MP m–3). In water samples (max. 10.8 μg MP L–1), mainly TWP and clusters of poly(methyl methacrylate) and polyethylene terephthalate occurred. Here, TWP prevailed in the SML, while the poly(methyl methacrylate) cluster dominated the ULW. However, no general MP enrichment was observed in the SML. Elevated anthropogenic influences in urban and industrial compared to the rural fjord areas were reflected by enhanced MP levels in these areas. Vertical MP movement behavior and distribution were not only linked to polymer characteristics but also to polymer sources and environmental conditions.


Text Section S1. S³ blanks
Blanks of the S³ were taken by pumping pre-filtrated water (0.3 µm glass fiber filter, Whatman TM ; pretreated in a muffle furnace at 500 °C for 4 h) through the flow-through system of the S³. for ULW, representing potential contamination for ULW samples (SI, Table S10).Determination of blanks of the SML sampling system could not be performed as blanks were taken on land and glass discs were exposed to elevated levels of dust, resulting in a significant overestimation of the SML blanks.However, SML samples passed through an identical tube and pumping system as ULW with the only difference being the rotating glass discs with PC wipers.As four of the 18 SML samples did not contain any C-PC, a general C-PC contamination originating from the wipers seemed to be unlikely.However, further discussions concerning C-PC require appropriate caution.

S-13
Table S8.Limit of detection (LOD) and limit of quantification (LOQ) for respective polymers analysed in this study.S/N calculation based on the peak heights of the respective indicator ions for each basis polymer given in Table S7.The data are representative for a system in optimum condition.

Text Section S2. Polymer identification, quantification, and calibration with Py-GC/MS
Particulate standards were weighed in with a Cubis ultramicro balance MSE2.7S-000-DM(Sartorius, Germany; readability 0.0001 mg, repeatability 0.00025 mg).The calibration range was between 1 and 50 µg.For PS, PC, PET, and PVC dissolved standards in concentrations down to 0.05 µg were applied additionally.CTT formed an exception with a calibration range from 20 to 200 µg due to its relatively high limit of detection (LOD, 20 µg, since applied as entire tire tread, cf.(Goßmann et al., 2021)) for the characteristic decomposition product combined with high-expected mass loads.To all measured standards, 2 µg dPS (absolute) as internal standard, and 20 µL TMAH 12.5% in methanol for thermochemolysis were added.Internal standardized calibration curves were created for each polymer individually; here, for each concentration corresponding peak ratios were calculated based on the respective peak area of the corresponding polymer indicator signal divided by the peak area of the dPS trimer indicator signal (m/z 98).For further details, refer to
n.d.= not detectable, n.q.= not quantifiable S-17  S-20 Lab experiments were conducted to generate knowledge about the residence time of small PET particles below 100 µm in the SML.For the experimental set-up, four 10 L tanks were covered with Teflon foil to avoid the adherence of particles on the glass of the tanks.Then they were thoroughly rinsed with prefiltered water and ethanol (96%).Each tank was filled with pre-filtered (artificial) seawater (pore size 1 µm) and covered with aluminum foil.In a separate container, the thickness of the SML in the respective seawater was measured with microelectrodes (pH-microelectrode, Unisense A/S, Denmark).The SML is detectable based on its pH value which differs slightly from the underlying water (Zhang et al., 2003).According to the pH profile (Fig. S9 a)), the SML had a thickness below 50 µm.
Together with the size of the tank, the total volume of the SML was calculable.

Fig. S2 .
Fig. S2.GPS Data of the S³ in the three sampling areas for each sampling day (#1 Uddevalla Byfjord, #2 Askeröfjorden, #3 Gullmar fjord).The blue trajectories represent the sampling route for the respective days.
z] = mass to charge ratio; [M] = molecular ion; bold = indicator ions used for calibration and quantification; a Mean of n-C16-C26-alkadiens used for quantification of PE. b isotactic.c heterotactic.d syndiotactic.e only after TMAH treatment.
standards; b derived from lowest calibration point for orientation only; c lower calibration impossible due to particulate standard.With respect to CTT and TTT that are both used as entire tire treads see alsoGoßmann et al. 2021.

Fig. S6 .
Fig. S6.Mean polymer concentration in µg L -1 of the "triplets" of SML and ULW samples with error bars.

Fig
Fig. S9.a) pH profile of seawater measured with Unisense microelectrode.b) Experimental set-up of the tanks

Fig. S10 .
Fig. S10.Residence time of small PET particles (< 100 µm) in artificial and real seawater in the SML based on lab experiments.

Table S1 .
Summary of marine atmospheric MP data, acquired by active sampling published according to

Table S2 .
Literature overview of MP documentation in the SML.

Table S3 .
Overview of air samples including field blanks.
*not measured, fell down in the laboratory

Table S4 .
Overview of SML and ULW samples.

Table S5 .
Plastic standards used for quantification.

Table S7 .
Goßmann et al., 2022.ic Py-GC/MS decomposition products used for identification and quantification of detected polymer clusters indicated by "C-" as pure polymers (cf.Fig.S3) and potentially included polymer-related derivatives according summarized byGoßmann et al., 2022.

Table S9 .
Overview of calibration measurements.

Table S10 .
Quantitative results of S³ blank in µg L -1 ..d. = not detectable, n.q.= not quantifiable; Quantification was based on calibration curves from TableS9(25.05.22).The measurement sequence of the S³ blanks did not allow a reliable quantification. n