Nitrous acid (HONO) is one of the
most important photochemical
precursors of the hydroxyl radical in the sunlit urban atmosphere.
The sources of HONO, however, are still poorly characterized, yet
there is a disagreement between the field observations and the model
results. Here, we show that light-induced NO2 heterogeneous
chemistry on authentic urban grime can make an important contribution
to the total HONO levels in the urban atmosphere. The obtained results
indicate that the effective uptake coefficients of NO2 on
urban grime in the presence of ultraviolet light [2.6 × 1015 photons cm–2 s–1 (300
nm < λ < 400 nm)] increased markedly from (1.1 ±
0.2) × 10–6 at 0% relative humidity (RH) to
(5.8 ± 0.7) × 10–6 at 90% RH, exhibiting
the following linear correlation with RH: γ(NO2)
= (7.4 ± 3.3) × 10–7 + (5.5 ± 0.6)
× 10–8 × RH%. The flux densities of HONO
mediated by light-induced heterogeneous conversion of NO2 (46 ppb) on urban grime were enhanced by ∼1 order of magnitude
from (2.3 ± 0.2) × 109 molecules cm–2 s–1 at 0% RH to (1.5 ± 0.01) × 1010 molecules cm–2 s–1 at
90% RH. This study promotes light-induced NO2 chemistry
on urban grime being an important source of HONO and suggests that
further experiments be performed in the future.
The photolysis of nitrous acid (HONO) is the main initiation source of hydroxyl radical (OH) which in turn is the main oxidant controlling the oxidation capacity of the indoor atmosphere.
A vertical
wetted-wall flow-tube technique was used to explore
the ionic strength effects at the air–water interface in mediating
the sea-surface reaction between ozone (O3) and pyruvic
acid (PA). The uptake coefficients of ozone on aqueous PA increase
substantially with the concentrations of bromide (Br–) ions, clearly indicating that the dry deposition of ozone could
be significantly enhanced due to the presence of carbonyl compounds
such as PA at the bromide-rich sea surface. Based on the observed
uptake coefficients, the estimated deposition velocity of ozone (100
ppb) for a nanomolar range of PA concentrations is ∼1 ×
10–3 m s–1, which represents a
significant contribution to the known deposition velocity of ozone
at the sea surface. The analysis of reaction products by ultra-high-resolution
Fourier transform-ion cyclotron resonance mass spectrometry suggests
the formation of oligomers during both the dark and light-induced
heterogeneous reactions between gaseous O3 and PA occurring
at the surface of a dilute aqueous phase (representative of cloud
droplets). The detected high-molecular-weight compounds are much more
complex than the oligomeric species identified during the photolytic
degradation of bulk aqueous PA alone.
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