Science - USA (2021-11-05)

favored ( 17 ). Thus, biological catalysts (e.g.,
bacteria) are often used to facilitate SRAO
reactions in industrial applications ( 18 ). How-
ever, owing to a lack of direct experimental
evidence, a detailed mechanism has not been
fully elucidated. Taken together, results from
our XPS, FPMD, and DOS analyses indicated
that the two intermediate species (S^2 −and
NO 2 −) appeared to be protonated—i.e., HS−
and HONO. Thus, the relevant reactions ob-
served become

3SO^24 þ4NHþ 4 →3HSþ4HONOþ4H 2 OþHþ

ð 5 Þ

2HONOþ2NHþ 4 →2N 2 þ4H 2 Oþ2Hþð 6 Þ

3HSþ2HONOþ3Hþ→3S^0 þN 2 þ4H 2 O

ð 7 Þ

All relevant surface species (HS−,S^0 , and

HONO) were detected during salt solvation.

Although N 2 was not found, owing to its low

solubility, the observed nitrogen depletion

during the transient period indirectly indi-

cated that N 2 was likely formed and quickly

escaped from the surface. In the atmosphere,

the escape of N 2 to the gas phase is also ex-

pected to occur because of the low solubility of

N 2 and the spontaneity of reactions (4) and

(7). The reactions [(5) to (7)] stopped when

the new steady state (full solvation) was

reached, and only in the transient period

(during deliquescence) did it appear that the

SRAO-type reactions were boosted at the

748 5NOVEMBER2021•VOL 374 ISSUE 6568 science.orgSCIENCE

Fig. 1. Measured and fitted XPS spectra at RHs of 3, 48, and 78%.The
photon energy was set to 435 eV for sulfur XPS and 665 eV for nitrogen XPS.
The probing depth was ~0.8 nm ( 30 ). The binding energies were aligned with
aliphatic C 1s at 284.8 eV. Greek symbols denote the fitted spectra shown
in (C) to (H). (A)MeasuredS2pand(B) N 1s XPS peaks. At RH = 3%, a NH 4 +peak

(B) and a SO 42 −doublet (C) were observed. At RH = 48%, S^0 and HS−doublets

(D) and HONO and NH 3 peaks (G) appeared. At RH = 78%, where full deliquescence

occurred, most sulfate transferred to S^0 (E), and no nitrogen species could be

found (B). When deliquescence was complete, the sulfur region was again

dominated by sulfate (F), and nitrogen species reappeared (H).

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