Science - USA (2021-11-05)

3. S:N ratios. The Stot:Ntotratio in Fig. 4C
   lies between the reference dashed and dotted
   lines for (NH 4 ) 2 SO 4 and NH 4 HSO 4 , indicating
   that the sample was stoichiometrically equiv-
   alent to a mixture of the two salts. The mixture
   of (NH 4 ) 2 SO 4 and NH 4 HSO 4 salts in the pre-
   deliquesced state was also supported by the
   MD trajectories, which showed vivid proton dy-
   namics on the adsorbed water layer, with ion
   pairing and proton transfer from NH 4 +to SO 42 −,
   forming HSO 4 −, and NH 4 HSO 4 and (NH 4 ) 2 SO 4
   clustering(Fig.2,AandC;fig.S14B;andmovie
   S2). A closer look at the SO 42 −:NH 4 +ratio in
   Fig. 4D (i.e., the primary ions) showed similar
   trends to those in Fig. 4C, which suggests that
   the primary ions dominated the S and N spe-
   cies. In Fig. 4E (Sr:NH 4 +), the surface enrichment
   of Srwas the main reason for the relatively high
   ratio at the first point at RH = 48%.


4. S:O ratios. In the comparison of sulfur to
   oxygen (Fig. 4F), the dry sample showed the
   expected S:O ratio because SO 42 −was the only
   source of sulfur. At RH = 48%, even though
   the SO 42 −:O ratios decreased (Fig. 4G) because
   of water dilution and SRAO-like reactions,
   the overall Stot:Ototratios increased because
   of the existence of Sr(Fig. 4H). In the aque-
   ous system at RH = 78%, the S:O ratio showed
   the lowest values because of the contributions
   from condensed water.


5. N:O ratios. The N:O ratio in Fig. 4I was
   generally stable over the entire probed depth
   except for the uppermost point of the aque-
   ous solution, which was caused by slight sur-

face enrichment of both NH 4 +(Fig. 4J) and

NH3(aq)(Fig. 4K) with respect to oxygen. The

surface-enriched NH 3 and NH 4 +indicated

that the nitrogen depletion during deliques-

cence totally recovered in the fully solvated

state. At RH = 48%, the NH 4 +:O ratio (Fig. 4J)

showed a gentle surface nitrogen-depletion

trend, which might be due to the slow but

ongoing SRAO-like reactions that generated

N 2 gas. Otherwise, the Ntot:Ototratios located

between the two reference lines were con-

sistent with the Stot:Ntotratios in Fig. 4C.

Redox reactions are essential in the atmo-

sphere because they are responsible for many

processes that govern the formation of gas

molecules and aerosol particles. In this study,

a redox reaction that spontaneously occurs

on typical inorganic aerosol surfaces during

water solvation was discovered and charac-

terized, from both experimental and theo-

retical perspectives. The limited hydration at

the initial solvation stage of the ammonium

sulfate crystal promoted the SRAO-like reac-

tion, favoring transfer and accommodation of

protons from ammonium ions and only par-

tially solvating sulfate ions (see SM for dis-

cussion of FPMD results). Because this surface

chemistry mechanism had not been identified

previously, new implications and applications

arelikelyandwillbenefitfromthismorecom-

plete picture of microscopic surface processes.

In the context of atmospheric science, sul-

fate and ammonium are the primary compo-

nents of secondary inorganic aerosols, and

the thermodynamic properties of the relevant

aerosol particles have been well characterized

( 21 ). It has previously been indicated that a

spontaneous oxidation pathway from SO 2 to

sulfate may exist on salt surfaces ( 22 ), which

implies that the identified surface mechanism

may play a role in atmospheric heterogeneous

chemistry. The sulfur reduction is an essen-

tial component of the full sulfur chemistry

cycle. Also, the revealed mechanism may com-

plement the autocatalyzed sulfur chemistry

pathway of elemental sulfur aerosol formation

in planetary atmospheres ( 23 ). Furthermore,

this mechanism may contribute to the forma-

tion of HONO, which is a key tropospheric

intermediate. Recently, it is has been reported

that the traditionally considered HONO for-

mation pathway—the heterogeneous reaction

of NO 2 on water surfaces—is not sufficient to

explain the observed high HONO concentra-

tions in the troposphere ( 11 ), and an additional

surface catalysis mechanism may have a crit-

ical role ( 24 ). In this study, HONO was formed

as an intermediate product on the ammonium

sulfate surface, indicating an unconventional

pathway of HONO formation directly from

inorganic aerosol particles during surface solv-

ation. Ammonium sulfate particles have also

been recognized as active ice nuclei for cirrus

cloud formation ( 25 ). As with deliquescence,

deposition ice nucleation is initiated by sur-

face water adsorption, which means that it

too may be influenced by the observed pro-

cesses and products, such as HONO, HS−, and

750 5 NOVEMBER 2021•VOL 374 ISSUE 6568 science.orgSCIENCE

Fig. 3. Schematic view of the outlined mechanism, with H 2 O molecules and protons omitted.(A) Dry salt crystal. (B) Pre-deliquescence, when ions are
released from salt crystals. (C) SRAO-like process; reaction (3). (D) SRAO-like process; reaction (4). (E) During full deliquescence, the escape of N 2 drives SRAO
reactions. (F) SRAO end products—i.e., H 2 O, S^0 , and N 2 (escaped)—dominate. (G) Gradually salinized solution. (H) Saturated solution after full salt dissolution.

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