Production of HONO from NO<sub>2</sub> uptake on illuminated TiO<sub>2</sub> aerosol particles and following the illumination of mixed TiO<sub>2</sub>∕ammonium nitrate particles
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Published:2021-04-16
Issue:7
Volume:21
Page:5755-5775
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Dyson Joanna E., Boustead Graham A., Fleming Lauren T., Blitz MarkORCID, Stone DanielORCID, Arnold Stephen R., Whalley Lisa K., Heard Dwayne E.ORCID
Abstract
Abstract. The rate of production of HONO from illuminated TiO2 aerosols in the
presence of NO2 was measured using an aerosol flow tube system coupled
to a photo-fragmentation laser-induced fluorescence detection apparatus. The
reactive uptake coefficient of NO2 to form HONO, γNO2→HONO, was determined for NO2 mixing ratios in the range 34–400 ppb, with γNO2→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. γNO2→HONO increased as a function of NO2
mixing ratio at low NO2 before peaking at (1.26 ± 0.17) ×10-4 at ∼ 51 ppb NO2 and then sharply decreasing
at higher NO2 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 TiO2 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 NO2, as well as a surface HONO loss mechanism which is dependent
upon NO2. In a separate experiment, significant production of HONO was
observed from illumination of mixed nitrate/TiO2 aerosols in the
absence of NO2. However, no production of HONO was seen from the
illumination of nitrate aerosols alone. The rate of production of HONO
observed from mixed nitrate/TiO2 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 NOx in the marine boundary layer where
mixed aerosols containing nitrate and a photocatalytic species such as
TiO2, as found in dust, are present.
Funder
Natural Environment Research Council
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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