Science - USA (2021-11-05)

S^0. For example, such a complicated surface
environment may explain why ammonium
sulfate acts as an efficient ice-nucleating partic-
ulate ( 5 , 25 – 28 ) when compared with other
hygroscopic salts such as NaCl ( 29 ).

Furthermore, because the reactions are spon-

taneous and therefore cost-efficient, poten-

tial industrial applications may be developed

on the basis of this concept. For example,

SRAO has been widely used as an efficient

approach for wastewater treatment, because

it simultaneously converts ammonium and

sulfate into N 2 gas and elemental sulfur with-

out secondary pollution ( 18 ). A major differ-

ence is that the currently used SRAO reaction

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Fig. 4. Depth profiles of the species ratios.(AtoK) The specific species are
indicated in each panel. Background color indicates the elemental ratios (S:N in
green, S:O in purple, N:O in beige). Three RH conditions are included: RH =
3% for dry conditions (crosses), 48% for pre-deliquesced conditions (stars), and
78% for aqueous solution in steady state (circles). The electron inelastic mean
free path (IMFP) in a sulfate salt (Na 2 SO 4 ) is calculated by QUASES-IMFP-TPP2M
version 3.0 ( 30 ), and the probed depth (attenuation length) shown on the upper

xaxis in (C) is calculated as IMFPcos 30°ðÞ, owing to the geometry of the

setup. Note the uncertainty created by the electron attenuation length ( 31 ).

Dotted lines indicate the elemental ratio in pure (NH 4 ) 2 SO 4 , dashed lines show

the ratio of pure NH 4 HSO 4 , and dashed-dotted lines show the identical ratio

for both (NH 4 ) 2 SO 4 and NH 4 HSO 4. Error bars indicate SDs estimated according

to the variations of individual XPS spectra. Depth profiles from the MD

simulations can be found in Fig. 2 and fig. S9.

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