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. 2020 Jul 6;743:140758. doi: 10.1016/j.scitotenv.2020.140758

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© 2020 The Authors

Since January 2020 Elsevier has created a COVID-19 resource centre with free information in English and Mandarin on the novel coronavirus COVID-19. The COVID-19 resource centre is hosted on Elsevier Connect, the company's public news and information website. Elsevier hereby grants permission to make all its COVID-19-related research that is available on the COVID-19 resource centre - including this research content - immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for free by Elsevier for as long as the COVID-19 resource centre remains active.

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Fig. 2 — Time series of the main chemical compositions of non-refractory PM_2.5. Chl, NO₃, SO₄, and NH₄ represent chloride, nitrate, sulphate, and ammonium, respectively. OOA and HOA represents oxygenated and hydrocarbon organic aerosol, which are derived from the Tof-ACSM dataset based on PMF analysis. Four typical PM_2.5 pollution episodes are presented with corresponding double-headed arrows.