“…Using bulk chemistry and physical properties in analyzing sulfate formation in different size ranges might lead to significant bias and wrong conclusions; (2) while PM 2.5 is generally more acidic, showing pH values within the mostly reported range of 3–5, coarse particles typically reveal a pH higher than 5 (Figure S13a), , which (alone from the pH perspective) is in favor of the photosensitization, O 3 , and NO 2 oxidation pathways, among which the NO 2 route has been often proposed to be the dominating sulfate production route in the North China Plain. ,, However, compared to PM 2.5 , ambient coarse particles lack in liquid water contents (Figure S13b), which is not in favor of the proposed aqueous phase sulfate formation routes (including the aqueous phase SO 2 oxidation by H 2 O 2 , NO 2 , O 3 , or TMI catalysis). Interfacial sulfate formation routes (such as interfacial H 2 O 2 oxidation) should also be more favored by fine particles in terms of the ambient aerosol surface area . Thus, it remains less understood how dust particles promoted sulfate formation.…”