Emission estimates
So far, no atmospheric emission inventory exists that covers all known MP sources. Traffic emissions are considered one of the largest primary MP environmental sources26,27. We used the bottom-up emissions as described in ref. 24 (Supplementary Text 1), which compiles emissions from tyre wear, brake wear, road markings and road surface wear. Recently, Bucci et al.10 and Evangelou et al.11 estimated the resuspension emissions from the ocean and from bare arid soils, respectively. Both studies reported bottom-up estimates based on process modelling of the respective source. We combined the traffic, ocean and bare arid soil emissions and refer to this emission case as ‘bottom-up’ (BU). Notice that potentially important emissions are missing from this inventory (for example, emissions from clothing, industrial sources and so on). No reliable bottom-up emission estimates exist for these sources but we assume that their spatial distribution is similar to the traffic emissions. Traffic emissions may thus serve as a proxy for all primary MP emissions.
In the study of Brahney et al.20, emissions from the ocean, traffic, mineral dust, agricultural dust and population activities were estimated on the basis of proxies, such as population density, sea spray and mineral dust emissions. An atmospheric transport model was then fed with these emissions and the resulting simulated values were compared with in situ measurements of MP deposition in the western United States. The emissions were then optimized top-down. Following the study of Brahney et al., Evangeliou et al.3 presented a different top-down emission estimate using the same measurements but a different modelling and inversion approach. We refer to these two top-down (TD) emission inventories as TD-B and TD-E for Brahney et al. and Evangeliou et al., respectively.
All three emission datasets were regridded to a 0.5° × 0.5° spatial resolution. The TD-B and TD-E emission estimates are available at yearly and 6-h time resolution, respectively. BU traffic-related MP emissions are given at yearly and the oceanic and resuspension MP emissions at 6-h resolution.
The total mass emissions of each sector were weighted to the four size bins 5–10, 10–25, 25–50 and 50–100 μm used for atmospheric transport modelling (Supplementary Table 1), on the basis of one size distribution per sector (Supplementary Text 1 and Supplementary Fig. 1). Thus, the emission magnitude of the different emission cases can be compared, as the size distribution influence is the same for every estimate.
MP emissions are mostly reported in mass units, whereas most atmospheric MP measurements are reported as number concentrations, which requires a conversion that is sensitive to the underlying assumed size distributions. The emissions were converted from particle mass to particle number units using the geometric mean size of each size bin and assuming a spherical shape, except for the TD-E population sector, for which fibres were assumed.
Compilation of MP measurements
Measurements of atmospheric MP concentration and bulk, wet and dry deposition were collected from reports in the literature. Studies were included only if they reported the sampling location and dates, the identified particle sizes and shapes and the measured air concentration or deposition of MPs, expressed as number (or mass) per volume of air or, for deposition measurements, per m2 per sampling period or day, respectively.
The MP measurement dataset is available with information about the study (author name, year of publication, DOI), the location (longitude, latitude and, if applicable, height above sea level), time of sampling (start and end date), type of measurement (air concentration, bulk, wet or dry deposition) and measured value (in number and mass units). The shapes and sizes of the particles are also included, as well as the code of the region in which we classified the measurement (Supplementary Fig. 2) and the type of sampling region (land, coastal, sea).
Because most studies reported the abundance of MPs in number of particles found, we converted all of the mass data values into number values (Supplementary Text 2). The resulting MP values for concentration and deposition in units of particles m−3 and particles m−2 s−1 are shown in Extended Data Fig. 1.
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