Fig. 1: Particulate Matter (PM) sample fractionation by solid-phase extraction.

Mass-to-charge ratio (m/z) vs. retention time space showing single organic compounds extracted from a representative Handan ambient PM_2.5 filter (Handan 1: 18.10.2018, daytime) with negative electrospray ionization. Each circle represents a detected compound with the circle size illustrating the signal intensity. The six compound classes are classified by color (CHO, dark cyan; CHNO, dark blue; CHOS, orange; CHNOS, yellow; CHOP, pink; other, light blue). The compound class names refer to the elemental composition of the classes (i.e., CHNOS must contain the elements carbon, hydrogen, nitrogen, oxygen and sulfur); “other” refers to the compounds excluded from the major compound classes mentioned above. Graph a shows the native extraction: ambient PM_2.5 filter extracted with a water/methanol mixture (98/2 (v/v)), b the first solid phase extraction (SPE) elution: fraction containing (nitro-)aromatics and organic acids, c the second SPE elution: isolated fraction containing organosulfates (OSs) and nitrooxy-OSs, and d the flowthrough: native extraction after passing through the SPE cartridge, containing mostly neutral and polar compounds.