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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. 3 — Average mass spectra and corresponding chemical proportions of NR-PM_2.5 during the three phases (a–c). The mean concentration with one standard deviation of (d) PM_2.5, SO₂, NO_x, CO, O_x, (e) chloride (Chl), nitrate (NO₃), sulphate (SO₄), ammonium (NH₄), oxygenated and hydrocarbon-like organic aerosols (OOA and HOA), and (f) nitrogen and sulphur oxidation ratios (NOR and SOR) during the three phases. Note that some species are presented with different units and are multiplied by 10 for clearance, which are marked on their labels.