Reduction of NO2 to nitrous acid on illuminated titanium dioxide aerosol surfaces: Implications for photocatalysis and atmospheric chemistry

Article abstract of DOI:10.1039/b609005b

TiO₂, a component of atmospheric mineral aerosol, catalyses the reduction of NO₂ to nitrous acid (HONO) when present as an aerosol and illuminated with near UV light under conditions pertinent to the troposphere. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b609005b

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Reduction of NO2 to nitrous acid on illuminated titanium dioxide
aerosol surfaces: implications for photocatalysis and atmospheric
chemistry
R. Joel Gustafsson, Alexander Orlov, Paul T. Griffiths, R. Anthony Cox and Richard M. Lambert*
Received (in Cambridge, UK) 26th June 2006, Accepted 19th July 2006
First published as an Advance Article on the web 8th August 2006
DOI: 10.1039/b609005b
aerosol relative humidity (RH) was controlled using a silica gel
drying unit in series with a humidifier. The size distribution was
characterised by means of a differential mobility analyser (Hauke
EMS VIE 08 DMA). NO2 (BOC speciality gases, 50 ppm in N2)
was prediluted with nitrogen (BOC, oxygen free) and introduced
into the flow tube via a sliding injector to give an initial mixing
ratio of y100 ppbv. The sliding injector allowed control of the
NO2/aerosol contact time, and NO2 consumption was measured
using a chemiluminescence NOx analyser (API 200E). HONO
concentrations were determined using the method described by
Febo et al..8 The amount of NOy (defined, in this case, as NO +
NO2 + HONO) was measured by passing the output of the flow
tube directly to the analyser as the resultant signal contains a
response from any HONO present. The amount of NOx (NO +
NO2) was also measured by passing the output of the flow tube
through a Na2CO3 denuder that selectively removes HONO and
HNO3. Any contribution from gas phase HNO3 may be excluded
as no change in signal was observed after passing the reactor
output through a NaCl denuder that selectively removed HNO3.
The TiO2 aerosols were stable for the time required to obtain
measurements. A typical particle size distribution is shown in
Fig. 1. This distribution is approximately described by a
superposition of two log–normal distributions, extending up to
1 mm as a result of agglomeration of the 30 nm particles in the
atomiser suspension. The aerosol surface area exposed to reactant
gases was calculated from the measured size distribution by
assuming that the particles were spherical. Irradiation was
TiO2, a component of atmospheric mineral aerosol, catalyses
the reduction of NO2 to nitrous acid (HONO) when present as
an aerosol and illuminated with near UV light under conditions
pertinent to the troposphere.
Nitrous acid (HONO) plays an important role in atmospheric
chemistry, serving as a major source of hydroxyl radicals which
play a primary role in the formation of ozone and other secondary
atmospheric pollutants.1 Laboratory studies suggest that in
the absence of sunlight HONO is formed heterogeneously on the
surfaces of tropospheric aerosol particles according to the
disproportionation reaction (1):2
2 NO2(ads) + H2O(ads) A HONO(g) + HNO3(ads)
(1)
However, daytime HONO concentrations in the troposphere
remain poorly explained as significant differences exist
between the predictions of theoretical models and observa-
tion.3 It has recently been suggested that these discrepancies
must imply the presence of a daytime source of nitrous acid.3
George et al.4 demonstrated enhanced uptake of NO2 by
organic surfaces in the presence of UV light. Very recently,
Stemmler et al.5 have shown that a photo-enhanced reaction of
nitrogen dioxide with humic acids is a potential daytime source
of tropospheric HONO. Here, based on laboratory measure-
ments of the photoreduction of NO2 in an aerosol flow tube
reactor under conditions of partial pressure, humidity and
temperature pertinent to the troposphere, we suggest another
possible contribution to daytime HONO production.
Mineral dust aerosol is an important component of the tropo-
sphere, comprising fine particles of crustal origin advected from
arid regions. However, its possible role in photocatalytic processes
has until now not been investigated. Under UV irradiation,
titanium dioxide is a very efficient photocatalyst for the degrada-
tion of organic molecules to carbon dioxide and water.6,7 Thus,
although it is a relatively minor component of tropospheric mineral
aerosols, it could nevertheless play a significant role in atmospheric
chemistry during the daytime. Here we demonstrate that UV-
activated TiO2 photocatalyses the conversion of nitrogen dioxide
and water to nitrous acid with high efficiency.
An aerosol of P25 TiO2 (3 : 1 anatase to rutile ratio, BET
surface area 2(50 ¡ 5) m2 g21) was generated from an aqueous
(deionised water) suspension using an atomizer (TSI 3076). The
Fig. 1 Typical bimodal aerosol distribution showing an aerosol with a
reactive surface area of 0.43 m2 m23
Department of Chemistry, Lensfield Road, Cambridge University,
Cambridge, UK CB2 1EW
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provided by four fluorescent lamps (Phillips TL D18W/08; lmax
=
365 nm) symmetrically arranged around the flow tube. The light
intensity inside the reactor was 1.6 mW cm22, measured by a
calibrated UV-A light meter.
NO2 consumption rates were measured and HONO production
rates calculated by taking the difference in NOy and NOx
concentrations (see above) and accounting for HONO loss to
the walls of the flow tube. In the absence of light, at all relative
humidities HONO production and NO2 consumption were below
the detection limit. This corresponds to a HONO formation rate
,4 6 1024 s21. When the aerosol was exposed to UV light
substantial HONO production and NO2 depletion occurred.
Control experiments performed with light but without aerosol
showed no measurable NO2 loss or HONO production, confirm-
ing that all the observed photocatalytic activity was due to the
TiO2 aerosol and not the walls of the flow tube. The calculated
HONO formation rate constants are shown in Fig. 2.
Fig. 3 Dependence of NO2 uptake coefficient on relative humidity.
The data shown in Fig. 2 imply that photocatalytic reactions
taking place on the surface of TiO2-containing mineral aerosols are
a potentially significant source of tropospheric HONO. This view
is supported by the NO2 uptake coefficients (c) calculated from the
rate constants and aerosol surface area according to:
known superhydrophilic behaviour of TiO2 surfaces under UV
irradiation.9 It appears that at low relative humidities the uptake
coefficient begins to fall, most likely as the coverage of water on
the surface limits the availability of protons required for the
formation of HONO; due to experimental constraints it was not
possible to examine this effect further at conditions below 10%
R.H.
no: collisions resulting in loss of molecule from gas phase
c~
total no: collisions
TiO2 surfaces irradiated by UV light catalyse both oxidation
and reduction reactions.10 The disproportionation reaction
(eqn (1)), occurs in the dark on a variety of surfaces11 to yield
gaseous HONO as the reduced species and adsorbed HNO3 as
the oxidised species. The stoichiometry of reaction (1) dictates
that the rate of production of HONO must equal half the rate
of consumption of NO2. As the measured rate of photo-
induced HONO production is actually y75% that of total
NO2 consumption, we may conclude that our observations do
not correspond to photo-enhancement of reaction (1): some
other mechanism must be at work. The formation of HONO
necessarily implies formation of some kind of oxidised species,
the identity of which is not clear at present. A plausible candidate
is hydrogen peroxide; further experiments are required to clarify
this point.
~4 k=vAs
where k is the rate of loss (s21), v is the mean molecular speed of
NO2 (ms21) and As is the reactive surface area (m2 m23). Uptake
coefficients ranged from 9.6 6 1024 for 15% RH to 1.2 6 1024
for 80% RH. These values should be compared with the uptake
coefficient of 2 6 1025 recently reported by Stemmler et al.5 for
the reaction of NO2 with humic acid surfaces. Clearly, tropo-
spheric mineral dusts containing significant amounts of titanium
dioxide or other oxides with appropriate bandgaps (e.g. Fe2O3) are
plausible candidates for daytime tropospheric HONO production.
Fig. 3 shows the negative correlation between HONO formation
rate constant and relative humidity. This behaviour may be
understood in terms of increased water adsorption inhibiting
NO2 adsorption and/or electron/hole transfer processes at the
TiO2/gas interface. Such effects could be amplified by the well
In summary, under conditions pertinent to the daytime
troposphere, we have discovered a new source of HONO.
The rate of photo-reaction depends very significantly on
relative humidity. These results are of significance to an
understanding of the NOx and free radical budgets of the
troposphere; they are also relevant to the general field of TiO2
photocatalysis.
This work was supported under Research Grant NE/B503509/1
awarded by the UK Natural Environment Research Council. R. J.
G. and A. O. acknowledge a NERC studentship and a King’s
College, Cambridge Research Fellowship, respectively. P. T. G.
acknowledges the NERC Distributed Institute for Atmospheric
Composition (DIAC) for post-doctoral support.
Notes and references
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Fig. 2 First-order HONO formation rate plots.
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