Fig. 4: Sources of hydrophilic OPEs in PM_2.5.

Relative contributions from production and use of OPEs, traffic and industrial emissions, waste disposal and incineration, and secondary formation sources to the mass concentrations of hydrophilic OPEs in (a) Chengdu, (b) Wuhan, (c) Shanghai and (d) the Pearl River Delta region (PRD) during summer and winter. Markers on the map show the measurement locations. The map is color-coded for the annual mean (January to December 2018) of sulfate (SO₄²⁻) production from aqueous sulfur dioxide (SO₂) oxidation. The data were obtained from Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) observations (https://giovanni.gsfc.nasa.gov/giovanni/). The base map of China and coastline boundaries in the map are originated from Meteolnfo free vector map data (http://meteothink.org/downloads/index.html).