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.