Heterogeneous photochemistry of imidazole-2-carboxaldehyde: HO<sub>2</sub> radical formation and aerosol growth
The multiphase chemistry of glyoxal is a source of secondary organic aerosol (SOA), including its light-absorbing product imidazole-2-carboxaldehyde (IC). IC is a photosensitizer that can contribute to additional aerosol ageing and growth when its excited triplet state oxidizes hydrocarbons (reac...
Main Authors: | , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-09-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/16/11823/2016/acp-16-11823-2016.pdf |
Summary: | The multiphase chemistry of glyoxal is a source of secondary organic aerosol
(SOA), including its light-absorbing product imidazole-2-carboxaldehyde (IC).
IC is a photosensitizer that can contribute to additional aerosol ageing and
growth when its excited triplet state oxidizes hydrocarbons (reactive uptake)
via H-transfer chemistry. We have conducted a series of photochemical
coated-wall flow tube (CWFT) experiments using films of IC and citric acid
(CA), an organic proxy and H donor
in the condensed phase. The formation rate of gas-phase HO<sub>2</sub> radicals
(<i>P</i><sub>HO<sub>2</sub></sub>) was measured indirectly by converting gas-phase NO into
NO<sub>2</sub>. We report on experiments that relied on measurements of NO<sub>2</sub>
formation, NO loss and HONO formation. <i>P</i><sub>HO<sub>2</sub></sub> was found to be a
linear function of (1) the [IC] × [CA] concentration product and
(2) the photon actinic flux. Additionally, (3) a more complex function of
relative humidity (25 % < RH < 63 %) and of (4) the
O<sub>2</sub> ∕ N<sub>2</sub> ratio
(15 % < O<sub>2</sub> ∕ N<sub>2</sub> < 56 %) was observed, most
likely indicating competing effects of dilution, HO<sub>2</sub> mobility and losses
in the film. The maximum <i>P</i><sub>HO<sub>2</sub></sub> was observed at 25–55 % RH
and at ambient O<sub>2</sub> ∕ N<sub>2</sub>. The HO<sub>2</sub> radicals form in the
condensed phase when excited IC triplet states are reduced by H transfer from
a donor, CA in our system, and subsequently react with O<sub>2</sub> to regenerate
IC, leading to a catalytic cycle. OH does not appear to be formed as a
primary product but is produced from the reaction of NO with HO<sub>2</sub> in the
gas phase. Further, seed aerosols containing IC and ammonium sulfate were
exposed to gas-phase limonene and NO<sub><i>x</i></sub> in aerosol flow tube experiments,
confirming significant <i>P</i><sub>HO<sub>2</sub></sub> from aerosol surfaces. Our results
indicate a potentially relevant contribution of triplet state photochemistry
for gas-phase HO<sub>2</sub> production, aerosol growth and ageing in the
atmosphere. |
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ISSN: | 1680-7316 1680-7324 |