2 research outputs found
Direct Formation of Electronic Excited NO<sub>2</sub> Contributes to the High Yield of HONO during Photosensitized Renoxification
Photosensitized renoxification of HNO3 is
found to produce
HONO in an unexpectedly high yield, which has been considered an important
source for atmospheric HONO. Conventionally, the production of HONO
is ascribed to the secondary photolysis of the primarily formed NO2. In this study, by using humic acid (HA) as a model environmental
photosensitizer, we provide evidence of the direct formation of NO2 in its electronic excited state (NO2*) as a key
intermediate during the photosensitizing renoxification of HNO3. Moreover, the high HONO yield originates from the heterogeneous
reaction of the primarily formed NO2* with the co-adsorbed
water molecules on HA. Such a mechanism is supported by the increase
of the product selectivity of HONO with relative humidity. Further
luminescence measurements demonstrate clearly the occurrence of an
electronic excited state (NO2*) from photolysis of adsorbed
HNO3 on HA. This work deepens our understanding of the
formation of atmospheric HONO and gives insight into the transformation
of RNS
A Novel Mechanism for NO<sub>2</sub>‑to-HONO Conversion on Soot: Synergistic Effect of Elemental Carbon and Organic Carbon
Soot, mainly composed of elemental carbon (EC) and organic
carbon
(OC), plays an important role in the formation of atmospheric nitrous
acid (HONO) through the heterogeneous reaction with nitrogen dioxide
(NO2). In this study, we found that fresh soot exhibits
a much higher HONO yield than its EC or OC components alone. This
does not support the previously proposed hydrogen-abstraction mechanism
for HONO formation at a single reductive site on the soot surface.
Based on our observations of infrared (IR) spectroscopy and flow tube
experiments, we propose a new mechanism that involves the synergistic
participation of two sites: one located at OC and the other at EC.
They provide a proton and an electron necessary for NO2 reduction to HONO, respectively, via a proton-coupled electron transfer
pathway. This mechanism highlights the important roles of OC and EC
in HONO release from soot and underscores the significant implications
of soot compositions in atmospheric oxidative capacity