salt–vapor interface. During efflorescence,
no Srformation was observed with decreas-
ing RH (fig. S5; from RH = 78 to 8%), which
indicates that this mechanism is associated
with surface solvation but not crystallization.
When RH was raised from 3 to 48% (Fig. 1,
D and G), all SRAO-relevant products (S^0 ,
HS−, HONO) were detected, but during tran-
sient conditions at DRH (RH = 78%; Fig. 1, B
and E) only the final products (S^0 and water)
were detected on the surface. This difference
was likely because at high RH there was abun-
dant molecular water and sufficient interac-
tions between intermediate species to complete
the reaction. Furthermore, a layer of pure water
on a (NH 4 ) 2 SO 4 surface was not expected to
be stable, and salt surface dissolution would
proceed instead. Indeed, the gradual saliniza-
tion of the water layer was recorded during a
1-hour period (fig. S1). The slow salinization
was most likely a result of the competition
between the solvation and the SRAO-like re-
action, and when the system reached steady
state the SRAO-like reaction stopped and the
aqueous phase was stabilized as a saturated
solution. The MD simulation trajectories (Fig. 2,
C and D, and figs. S9 and S11) showed that
ammonium and sulfate ions from the crystal
structure started to be solvated in the adsorbed
water quite rapidly (within 40 ps). The den-
sity profiles for ammonium (blue dotted line),
sulfate ions (orange dotted line), and water
(red line) overlap in Fig. 2, B and D, supporting
the concept that a first adsorbed water layer
was sufficient to initiate the dissolution of
(NH 4 ) 2 SO 4 from the crystal into the liquid
layer. Trajectory movie S3 shows that ammo-
nium and sulfate ions initially located in the
crystal structure interacted with the adsorbed
speciesandbegantobesolvatedinthewater
layer. Finally, Fig. 2, C and D, indicates the
propensity of nitrite ions to reside in the in-
ner part of the adsorbed water layer, close to
the crystal structure interface: The FPMD
showed that the hectic proton dynamic in this
environment facilitated HONO formation by
protonation of nitrite ions. When HONO was
formed, it remained stable over all of the MD
trajectory lengths (figs. S11 and S14D).
Dissolved NH 3 was also detected and was
formed from NH 4 +ions after deprotonation.
The NH3(aq)has a higher surface propensity
than NH 4 +ions ( 15 ), as confirmed in this study
by depth profiling and MD simulation results
(Fig. 2D and SM). Notably, the MD results
suggest that the ammonia surface propen-
sity was not limited to the fully deliquesced
steady state but also occurred in the water-
restricted environment of approximately one
water layer adsorbed on the ammonium sul-
fate surface (Fig. 2D and SM). Nevertheless,
NH3(aq)could be stabilized in the condensed
phase by forming clusters with NH 4 +( 20 ).
This was also observed in the MD trajectory
(Fig. 2C, inset) and may be relevant in this
case because NH 4 +was highly concentrated
at the interface in the pre-deliquescence stages.
Ammonia and ammonium ion water-mediated
clustering was also observed in FPMD tra-
jectories (movie S1C). The proposed pathwayand reaction mechanisms are illustrated in
Fig. 3.
Depth profiles of the ratios between elements
and species (i.e., as a function of electron ki-
netic energy) are shown in Fig. 4. The idealized
elemental ratios for pure ammonium sulfate
and ammonium bisulfate are marked by dashed
and dotted lines, respectively.
1) Sr:SO 42 −ratio. First, the ratios of Srand
NH 3 with respect to their primary ions (SO 4 2+
and NH 4 +)areshowninFig.4A(Sr:SO 4 2+) and
Fig. 4B (NH 3 :NH 4 +). Because Srwas formed
during salt solvation, its abundance depended
on the deliquescence process. Thus, Srwas
abundant at RH = 48%, when water likely
partially covered the surface and the SRAO-
like reactions for Srformation were favored.
As the system gradually approached steady
state (as shown by the time-lapse XPS survey
in fig. S1), the aqueous solution at RH = 78%
showed much less Sr,becausetheformedSr
was diluted by the condensing water and
moved away.
2) NH3(aq):NH 4 +ratio. NH3(aq)was relatively
surface-enriched relative to NH 4 +, because
NH3(aq)has a higher surface propensity than
NH 4 +ions ( 15 ) in aqueous solutions. At RH =
48%, the NH3(aq)might be stabilized in the
condensed phase by NH 4 +ions ( 20 ). MD
results supported the NH 3 enrichment at the
vapor/adsorbed liquid layer (even in the pre-
deliquesced state at a relatively low RH; Fig. 2D)
and its stabilization in the condensed phase
by clustering with ammonium ions (Fig. 2C,
inset).SCIENCEscience.org 5 NOVEMBER 2021•VOL 374 ISSUE 6568 749
Fig. 2. FPMD results.(A) Snapshot from a 40-ps
FPMD of ammonium and sulfate ions solvated in an
adsorbed eight–water molecule (WAT) sublayer on
top of the ammonium sulfate crystal [configuration a
(conf-a) in the SM]. Atom color code: N, blue; S,
yellow; O, red; and H, white. (B) Probability
distribution profile, ni/nTOT, normalized to unity as a
function of thez-coordinate perpendicular to the
liquid–crystal interface, obtained by collecting the
center-of-mass position of each species over the MD
trajectory. Dotted blue and orange lines denote
sulfate and ammonium ions that initially belong to
the crystal substrate. (C) Snapshot from a 40-ps
FPMD [configuration c (conf-c) FPMD in the SM] of
ammonia and ammonium, sulfate, sulfide, and nitrite
ions in a 16–water molecule layer adsorbed on top of
the crystal. Orange, ammonium sulfate ion pairing;
pink, hydrogen sulfide formation; magenta, nitrite;
and cyan, ammonia. (Inset) A different time frame
from the same trajectory, which shows proton
shuffling between NH 4 +and NH 3 (green) in the
adsorbed water layer. Shadowed atoms are those
that belonged to the initial ammonium sulfate crystal
structure. (D) Same as (B) but collected over the
40-ps conf-c FPMD trajectory (see SM). For visual
clarity, the density profile is split into two panels.
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