Production of HONO from NO 2 uptake on illuminated TiO 2 aerosol particles and following the illumination of mixed TiO 2 ∕ammonium nitrate particles
2021
Abstract. The rate of production of HONO from illuminated TiO 2 aerosols in the
presence of NO 2 was measured using an aerosol flow tube system coupled
to a photo-fragmentation laser-induced fluorescence detection apparatus. The
reactive uptake coefficient of NO 2 to form HONO, γ NO 2 → HONO , was determined for NO 2 mixing ratios in the range 34–400 ppb, with γ NO 2 → HONO spanning the range (9.97 ± 3.52) × 10 −6 to (1.26 ± 0.17) × 10 −4 at a
relative humidity of 15 ± 1 % and for a lamp photon flux of (1.63 ± 0.09) ×1016 photons cm −2 s −1 (integrated
between 290 and 400 nm), which is similar to midday ambient actinic flux
values. γ NO 2 → HONO increased as a function of NO 2
mixing ratio at low NO 2 before peaking at (1.26 ± 0.17) × 10 - 4 at ∼ 51 ppb NO 2 and then sharply decreasing
at higher NO 2 mixing ratios rather than levelling off, which would be
indicative of surface saturation. The dependence of HONO production on
relative humidity was also investigated, with a peak in production of HONO
from TiO 2 aerosol surfaces found at ∼ 25 % RH.
Possible mechanisms consistent with the observed trends in both the HONO
production and reactive uptake coefficient were investigated using a
zero-dimensional kinetic box model. The modelling studies supported a
mechanism for HONO production on the aerosol surface involving two molecules
of NO 2 , as well as a surface HONO loss mechanism which is dependent
upon NO 2 . In a separate experiment, significant production of HONO was
observed from illumination of mixed nitrate / TiO 2 aerosols in the
absence of NO 2 . However, no production of HONO was seen from the
illumination of nitrate aerosols alone. The rate of production of HONO
observed from mixed nitrate / TiO 2 aerosols was scaled to ambient
conditions found at the Cape Verde Atmospheric Observatory (CVAO) in the
remote tropical marine boundary layer. The rate of HONO production from
aerosol particulate nitrate photolysis containing a photocatalyst was found
to be similar to the missing HONO production rate necessary to reproduce
observed concentrations of HONO at CVAO. These results provide evidence that
particulate nitrate photolysis may have a significant impact on the
production of HONO and hence NO x in the marine boundary layer where
mixed aerosols containing nitrate and a photocatalytic species such as
TiO 2 , as found in dust, are present.
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