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clouds ( 4 ). Although natural sources

can produce ammonium sulfate aero-

sols, most of them are produced by

human activities. In the troposphere,

the lowest layer of the atmosphere,

the sulfur dioxide emitted from fos-

sil fuel combustion dissolves in wa-

ter droplets and is transformed into

sulfuric acid. Precipitation of these

droplets to the ground creates acid

rain. Sulfuric acid in the troposphere

can also react with ammonia emitted

from agricultural activities and pro-

duces ammonium sulfate salt, either

in the form of aqueous solutions or

crystallized particles, depending on

air humidity and temperature. Kong

et al. studied the behavior of am-

monium sulfate crystals exposed to

varying concentrations of water va-

por with an experimental technique

called ambient pressure x-ray photo-

electron spectroscopy. They measured

the binding energy of the inner elec-

trons in sulfur and nitrogen atoms on

the crystal surface, which conveys the

chemical information on what kind of

compounds these atoms are forming.

In dry conditions, only the expected

signatures for sulfate and ammonium

ions are observed. However, when the rela-

tive humidity increases, the ammonium

sulfate salts begin to dilute and react over

the surface, leading to additional signatures

for elemental sulfur, bisulfide ions, nitrous

acid, and ammonia, which was unexpected.

If the relative humidity is further increased

to the deliquescence threshold of 78%, the

salts gradually dissolve completely in wa-

ter. During this transformation, there is a

transient period in which elemental sulfur

dominates the spectra and nitrogen species

signals vanish. As the dilution continues,

however, the sulfate, ammonium, and am-

monia signals reappear, and a new steady

state is reached. The assignment of the sig-

nals and important aspects of the underly-

ing transformation mechanisms have been

guided and supported by quantum chem-

istry and molecular dynamics calculations.

Kong et al. conclude that spontaneous

sulfate-reducing ammonium oxidation re-

action is at play as the salt surface is be-

ing diluted, which does not happen when

the salt is completely dry or completely

diluted. The final products of this reaction

are elemental sulfur and nitrogen gas. The

ease with which the reaction occurs in the

aqueous layer at the vapor-crystal inter-

face is notable because it is an unfavorable

reaction under ordinary conditions. For in-

stance, biocatalysts are typically needed to

stimulate the reaction in wastewater treat-

ment processes.

The spontaneous oxidation reaction im-

plies that, in humid air, highly oxidized

sulfur compounds, such as sulfate, can

be converted to less oxidized compounds,

such as elemental sulfur or bisulfide. This

chemistry was previously not expected to

occur in the atmosphere, which is consid-

ered an oxidizing medium. The counter-

intuitive discovery is likely to change the

current understanding of the sulfur cycle.

In addition, the authors also identified ni-

trous acid as an intermediate product of

this process. Nitrous acid is a key precur-

sor to the extremely reactive hydroxyl radi-

cal sometimes known as the “atmosphere’s

detergent.” The presence of the spontane-

ous reactions on sulfate ammonium could

help explain why the concentration of at-

mospheric hydroxyl radicals is underesti-

mated in current models.

The findings also have broad implica-

tions for the “on water catalysis” phe-

nomenon ( 5 ), where the rate of chemical

reactions is accelerated at water surfaces

and aqueous interfaces. The work by Kong

et al. provides new perspectives on the

current understanding of these acceler-

ated reactions ( 6 ). Though still incom-

pletely understood, “on water catalysis”

has been shown to play a role in several

chemical and photochemical processes on

atmospheric aqueous aerosols (7–9). In

addition, because the interfaces formed

by water droplets coated with an organic

film share similarities with cell mem-

branes, “on water catalysis” may have

played an important part in the origi-

nation of life on Earth ( 10 ). In terms

of applications, the effect of “on wa-

ter catalysis” has been exploited in

organic synthesis that uses dispersed

interface-rich systems such as oil-in-

water microemulsions ( 11 ) or sprayed

aqueous microdroplets ( 12 ). For in-

stance, certain reactions, such as the

oxidation of water to form hydrogen

peroxide, have been identified to pro-

ceed spontaneously “on water” even

if they do not occur “in water” ( 13 ).

These findings open new perspec-

tives for the development of greener

synthetic techniques by reducing the

need for organic solvents.

There is no simple answer to why

some unfavorable chemical reactions

are boosted at aqueous interfaces.

Some fundamental molecular proper-

ties, such as the oxidation potential

( 14 ) or the acidity ( 15 ), change when

the molecules are adsorbed at an

aqueous interface. The local electric

fields associated with the structural

asymmetry of the interface have been

pointed out as a key for explaining

these effects. The alignment and confine-

ment of the reagents, or the large surface-

to-volume ratio exhibited by microscopic

systems, have also been suggested as im-

portant factors to be considered for the

effect. These issues are being intensely

debated in the literature ( 6 ), and future

research should aim at clarifying their re-

spective contributions. In this regard, the

parallel study of selected reactions with

different experimental techniques and

elaborated theoretical molecular simula-

tions would be most valuable. j

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4. J. P. Abbatt et al., Science 313 , 1770 (2006).


5. S. Narayan et al., Angew. Chem. Int. Ed. 44 , 3275 (2005).


6. M. F. Ruiz-Lopez, J. S. Francisco, M. T. C. Martins-Costa, J.
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11. S. Serrano- Luginbühl, K. Ruiz-Mirazo, R. Ostaszewski,
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12. X. Yan et al., Angew. Chem. Int. Ed. 55 , 12960 (2016).


13. J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A. 116 , 19294
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14. M. T. C. Martins-Costa, J. M. Anglada, J. S. Francisco, M. F.
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10.1126/science.abl8914

S N 2 HS– HNO 2

(NH 4 ) 2 SO 4 NH 3

SO 2

H 2 SO 4

Sulfuric acid

Ammonium

sulfate crystals

Oxygen,

water vapor

Sulfur dioxide

Ammonia

How ammonium sulfate crystals break

down upon hydrated surface catalysis

When exposed to water vapor, ammonium sulfate crystals

spontaneously form elemental sulfur, nitrogen gas, bisulfide ions,

and nitrous acid. This unexpected reaction has implications for

the long-term effects of ammonia and sulfur dioxide emissions.

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