Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces

Article abstract of DOI:10.1039/c6cp00436a

During the last few decades, nitrous acid (HONO) has attracted significant attention as a major source of the OH radical, the detergent of the atmosphere. However, the different daytime sources identified in the laboratory are still the subject of controversial discussion. In the present study, one of these postulated HONO sources, the heterogeneous photolysis of nitric acid (HNO₃), was studied on quartz glass surfaces in a photo flow-reactor under atmospherically relevant conditions. In contrast to other investigations, a very low HNO₃ photolysis frequency for HONO formation of J(HNO₃ → HONO) = 2.4 × 10^-7 s^-1 (0° SZA, 50% r.h.) was determined. If these results can be translated to atmospheric surfaces, HNO₃ photolysis cannot explain the significant HONO levels in the daytime atmosphere. In addition, it is demonstrated that even the small measured yields of HONO did not result from the direct photolysis of HNO₃ but rather from the consecutive heterogeneous conversion of the primary photolysis product NO₂ on the humid surfaces. The secondary NO₂ conversion was not photo-enhanced on pure quartz glass surfaces in good agreement with former studies. A photolysis frequency for the primary reaction product NO₂ of J(HNO₃ → NO₂) = 1.1 × 10^-6 s^-1 has been calculated (0° SZA, 50% r.h.), which indicates that renoxification by photolysis of adsorbed HNO₃ on non-reactive surfaces is also a minor process in the atmosphere.

Full text of DOI:10.1039/c6cp00436a

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Investigations on HONO formation from
photolysis of adsorbed HNO3 on quartz
glass surfaces
Cite this: Phys.Chem.Chem.Phys.,
2016, 18, 9616
Sebastian Laufs and Jorg Kleffmann*
¨
During the last few decades, nitrous acid (HONO) has attracted significant attention as a major source of
the OH radical, the detergent of the atmosphere. However, the different daytime sources identified in
the laboratory are still the subject of controversial discussion. In the present study, one of these
postulated HONO sources, the heterogeneous photolysis of nitric acid (HNO3), was studied on quartz
glass surfaces in a photo flow-reactor under atmospherically relevant conditions. In contrast to other
investigations, a very low HNO3 photolysis frequency for HONO formation of J(HNO3 - HONO) =
2.4 ꢀ 10ꢁ7 sꢁ1 (01 SZA, 50% r.h.) was determined. If these results can be translated to atmospheric
surfaces, HNO3 photolysis cannot explain the significant HONO levels in the daytime atmosphere.
In addition, it is demonstrated that even the small measured yields of HONO did not result from the
direct photolysis of HNO3 but rather from the consecutive heterogeneous conversion of the primary
photolysis product NO2 on the humid surfaces. The secondary NO2 conversion was not photo-
enhanced on pure quartz glass surfaces in good agreement with former studies. A photolysis frequency
for the primary reaction product NO2 of J(HNO3 - NO2) = 1.1 ꢀ 10ꢁ6 sꢁ1 has been calculated (01 SZA,
50% r.h.), which indicates that renoxification by photolysis of adsorbed HNO3 on non-reactive surfaces
is also a minor process in the atmosphere.
Received 20th January 2016,
Accepted 9th March 2016
DOI: 10.1039/c6cp00436a
www.rsc.org/pccp
In summary, mainly ground surface sources have been
identified to date in laboratory studies to explain atmospheric
1. Introduction
During the last two decades, unexpectedly high nitrous acid HONO formation during daytime,41 which has also been con-
(HONO) mixing ratios have been observed during daytime in firmed by recent flux measurements in the atmosphere.25,42,43
remote,1–5 semi urban6–9 and urban regions.10–17 These results
Among the different proposed sources, the photolysis of
stimulated laboratory investigations on potential daytime adsorbed nitric acid (HNO3) has been suggested as an impor-
HONO sources, by (i) the reduction of nitrogen dioxide (NO2) tant HONO formation pathway, especially in rural areas.
in the presence of organic photosensitisers,18–21 (ii) the photo-
JðHNO3!HONOÞ
lysis of adsorbed nitric acid22–26 and (iii) bacterial production of
HNO3ðads:Þ þ hn
HONO þ O;
(R1)
ꢀꢀꢀꢀꢀꢀꢀꢀꢀ!
nitrite in soil27–30 and/or desorption of adsorbed HONO from
soil surfaces during daytime.31–33 Another intensely discussed Zhou and coworkers24 have published a photolysis frequency
source, the gas-phase reaction of excited NO2 with water,34 was of 1.2 ꢀ 10ꢁ5 sꢁ1 for HNO3 adsorbed on Pyrex glass, at 50%
found to be of minor importance as demonstrated by laboratory relative humidity under tropical noontime conditions, which is
studies.35,36 Also the photolysis of nitro-phenols or similar at least two orders of magnitude larger compared to those
compounds is too slow to explain significant HONO levels in of liquid- and gas-phase HNO3. In addition, the same group
the daytime atmosphere.37 Finally, a gas-phase HONO source recently presented a correlation of HONO fluxes above a forest
recently postulated by Li et al.,38 i.e. the reaction of HO2ꢂH2O canopy with HNO3 adsorbed on leaves from the surrounding
complexes with NO2, could not be confirmed by the same group trees.25 These observations are in contrast to a smog chamber
in simulation chamber experiments39 and is also in conflict study, in which photolysis of adsorbed HNO3 was excluded as a
with recent aircraft measurements.40
source of photochemical HONO formation on Teflon surfaces.44
In these experiments significant photochemical HONO forma-
tion was still observed at wavelengths 4370 nm, for which no
significant light absorption of HNO3 is expected. However, for
¨
¨
Physikalische und Theoretische Chemie/Fakultat fur Mathematik und
¨
Naturwissenschaften, Bergische Universitat Wuppertal, 42097 Wuppertal, Germany.
E-mail: kleffman@uni-wuppertal.de; Fax: +49 202 439 2505; Tel: +49 202 439 3534 HNO3 adsorbed on fused silica and aluminium, a red-shift of
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Table 1 Calculated and measured (by chemical actinometry, in brackets)
photolysis frequencies of NO2, HONO and HCHO inside the flow-reactor
for the two sets of lamps used (lw: long wavelength; sw: short wavelength).
In the last column, upper limit atmospheric values (01 SZA)54 are also listed
the n - p* HNO3 absorption band and more than two orders of
magnitude larger UV cross-sections compared to the gas phase
have been recently observed by Brewster angle cavity ring-down
spectroscopy.45–48 These observations have rekindled interest
in HNO3 as a potential HONO and NO2 source. However, the
mechanism of heterogeneous HNO3 photolysis is still under
discussion, and besides reaction (R1) secondary HONO forma-
tion pathways have also been proposed.22,24,25,48
lw: Philips,
TL05, 40 W
sw: Wolff System,
Helarium, 40 W
Atmosphere
(01 SZA)
J(NO2) (sꢁ1
J(HONO) (sꢁ1
J(CH2O) (sꢁ1
)
0.027 (0.ꢁ0239) 0.036
0.010
)
5.1 ꢀ 10
8.7 ꢀ 10ꢁ3
2.2 ꢀ 10ꢁ3
)
8.4 ꢀ 10ꢁ5
4.8 ꢀ 10ꢁ4 (4.2 ꢀ 10ꢁ4
)
8.9 ꢀ 10ꢁ5
To elucidate the importance of reaction (R1) as a HONO
source in the atmosphere, photolysis of adsorbed HNO3 was
studied in a photo flow-reactor under atmospherically relevant measured by chemical actinometry of NO2 and HCHO in the
concentrations, humidity and actinic flux conditions.
flow-reactor and showed good agreement (see Table 1).
Corrections of the observed HONO formation for its gas-
phase photolysis were typically only ca. 5 and 8% for the two
types of lamps used, respectively. Photolysis frequencies of
HONO ( J(HONO), see Table 1) were calculated using published
cross-sections.53
2. Experimental
2.1. Experimental setup
Experiments were performed in a cylindrical flow-reactor
(5.0 cm i.d.) made of smooth quartz glass with an illuminated
length of 56.5 cm (see Fig. 1). For a few experiments a similar
flow reactor made of Pyrex glass was used. The reactor was
irradiated with 6 UV-Vis lamps (Philips TL/05 40 Watt,
300–500 nm, lmax E 370 nm; or Wolff System Helarium 40 W,
290–450 nm, lmax E 350 nm), which were placed in an air-cooled
(fan), UV-reflecting aluminium box. The actinic flux inside
the photoreactor (see Fig. 2) was measured using a spectro-
radiometer (Dl = 1 nm; Metcon Inc., USA). Calculated photolysis
frequencies J(NO2) and J(HCHO) using published cross-sections
and quantum yields49–52 were compared with values directly
2.2. Instruments
HONO was measured using the sensitive LOPAP technique10,55
with a detection limit and an accuracy of 2 pptV and 10%,
respectively. NO2 and NO were detected by a recently developed
NO2-LOPAP56 which was modified for the additional detection
of NO, details of which will be published elsewhere. In short,
NO passing the NO2 collection stripping coil was oxidized to
NO2 by an acidic KMnO4 solution in a consecutive stripping coil
and was detected downstream using the Griess–Saltzman reac-
tion, similar to the NO2 channel of the instrument. Detection
limits and accuracies for NO2/NO under the experimental
conditions applied were 10/20 pptV and 7/10%, respectively.
For the experimental determination of the photolysis frequency
of HCHO an Aero Laser AL4001 Hantzsch instrument was used.57
Finally, adsorbed HNO3 was quantified by ion-chromatography
(IC)58 after elution from the flow-reactor surface with ultra-pure
water (MilliPore). IC detection was also used to measure gas-
phase levels of HNO3 after bubbling through ultrapure water.
The accuracy of the nitrate concentration determined by IC was
7% and those of HNO3 in the gas phase and on the surface were
10 and 12%, respectively.
Fig. 1 Experimental setup (MFC: mass flow controller, MFM: mass flow
meter, NV: needle valve).
2.3. Experimental procedure
Prior to the experiments the reactor was cleaned with HF (5%)
followed by ultra-pure water to minimize the contamination
on the irradiated reactor surfaces. This was confirmed in blank
experiments in which neither in the dark nor under irradiation
any significant formation of HONO or NOx was observed.
Different reactant mixtures were generated by mixing humidi-
fied synthetic air with a pure HNO3 gas-phase source58 and
were flushed through the flow-reactor. For some experiments
low levels of NO2 (Messer, 2 ppmV) were also added using a
calibrated 10 mL minꢁ1 flow meter and a needle valve. During
one experiment a mixture of 29 VOCs (methane, ethane, propane,
n-butane, iso-butane, n-pentane, n-hexane, n-heptane, n-octane,
ethene, propene, iso-, trans-, and cis-butene, isoprene, b-pinene,
limonene, ethine, benzene, toluene, o-, m-, and p-xylene, methanol,
ethanol, n-propanol, n-butanol, acetone, and methacrolein) with
Fig. 2 Actinic flux (F) spectra of the short-(sw) and long-(lw) wavelength
lamps and from the atmosphere (01 SZA).50 The absorption spectrum s of
HNO3 adsorbed on fused silica is also shown.45,47
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individual mixing ratios in the range 48–69 ppbV was added using
a 100 mL minꢁ1 flow controller. In some experiments additional
quartz glass plates were added to the flow-reactor to increase the
irradiated surface area. The total gas flow rate was typically
2.7 L minꢁ1, but was varied between 1.1 and 3.4 L minꢁ1 in a
few experiments leading to reaction times in the range 26–80 s.
When constant reactant concentrations were reached, the
lamps were switched on and the system was analysed for photo-
chemical HONO and NOx formation. Immediately after each
experiment and after disconnection from the HNO3 source, the
reactor was eluted with ultra-pure water and the amount of
adsorbed HNO3 was quantified. After saturation of the reactor
surfaces with the continuously running HNO3 source (typically
overnight), adsorbed HNO3 levels were assumed to be constant
during each experiment. This assumption is considered plausible
as the HNO3 source produced fairly stable gas-phase levels, as
demonstrated also in a former laboratory study.58 In addition,
constant gas phase HNO3 levels at the end of the flow tube
confirmed equilibrium adsorption during the experiments. This
observation also indicates that the surface concentration of HNO3
was homogeneously distributed on all the irradiated surfaces. For
the calculation of surface concentrations and flux densities the
geometric surface area of the flow tube was used.
Fig. 4 HONO formation in the photolysis of adsorbed HNO3 as a function
of the reactor surface area ([HNO3]surf = 1.6 ꢀ 1014 cmꢁ2, [HNO3]g
=
86 ppbV, 59% r.h.). The straight line represents the linear regression of the
data (r2: 0.93) with its 95% confidence limits (dashed lines).
the surface area of the reactor was varied. For these experiments,
additional quartz glass plates of different sizes were added to the
flow-reactor. The HONO production during irradiation increased
linearly with the surface area (Fig. 4), which demonstrates the
heterogeneous nature of the process involved.
This result is in agreement with theoretical calculations
performed using known gas-phase absorption cross-sections of
HNO3,51 measured actinic flux, HNO3 gas-phase concentration,
residence time in the flow-reactor and assuming an upper limit
quantum yield for HONO formation of one (for calculations, see
eqn (3)). The calculated maximum HONO formation by HNO3
photolysis in the gas phase of only 6 pptV is much smaller
compared to the experimental results shown in Fig. 4 and thus,
a heterogeneous reaction is proposed here.
3. Results and discussion
In Fig. 3, a typical experiment is shown in which adsorbed
HNO3 was irradiated without (first and last step in Fig. 3) and
with different levels of added NO2. In the absence of additional
NO2, clear positive steps in the HONO, NO and NO2 concentra-
tions were observed, pointing to their photochemical production.
3.1. Surface dependency
To verify whether HONO formation was caused by the photolysis of
adsorbed HNO3, reaction (R1), and not by its gas-phase photolysis,
3.2. HNO3 dependency
By varying the gas-phase concentration of HNO3 in the range
2–120 ppbV, different quantities of surface adsorbed HNO3 in
the range 0.15–3.3 ꢀ 1014 molecules cmꢁ2 were investigated.
Recently, a surface monolayer concentration of HNO3 of 1.1 ꢀ
1014 molecules cmꢁ2 was estimated based on its van der Waals
radius,45 which is in fair agreement with the measured value for
SiO2 particles (7 ꢀ 1013 molecules cmꢁ2).59 Thus, the investi-
gated surface levels varied from sub-monolayer to multilayer
coverage, which were obtained at atmospherically relevant low
ppbV mixing ratios of the sticky HNO3.
Photochemical HONO formation was linearly correlated
with the concentration of adsorbed HNO3, both, for the long
and short wavelength lamps used (see Fig. 5). This observation is
in line with a first-order surface photolysis process, reaction (R1),
described by a photolysis frequency of adsorbed HNO3 for HONO
formation, J(HNO3 - HONO), which is defined by:
Fig. 3 HONO and NOx formation from photolysis of adsorbed HNO3
(7.8 ꢀ 1013 cmꢁ2) in the flow-reactor continuously flushed with humid
(54% r.h.) gas-phase HNO3. After first irradiation, NO2 was added stepwise
to the HNO3 mixture (marked with vertical black lines). For the last step,
the NO2 addition was set to zero again. The vertical bars above the plot
represent dark (black) and irradiated (white) periods.
d½HONOꢃsurface
dt
1
JðHNO3 ! HONOÞ ¼
ꢀ
:
(1)
½HNO3ꢃsurface
Here, d[HONO]surface/dt represents the HONO formation rate
described as a surface flux density (molecules cmꢁ2 ꢁ1).
s
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To calculate atmospherically relevant HNO3 photolysis frequencies
for HONO formation according to:
JðHNO3 ! HONOÞ
ln
(3)
X
¼
Fl ꢀ sðHNO3Þl ꢀ fðHNO3 ! HONOÞl ꢀ Dl;
l1
spectroscopic information on the absorption cross-section
(s(HNO3)l), the quantum yield (f(HNO3 - HONO)l) and the
actinic flux spectra (Fl) of the laboratory system and the atmo-
sphere are necessary. Since the absorption cross-sections and
quantum yields are still uncertain, extrapolation to atmospheric
conditions, as done in another study,24 is highly questionable. To
better estimate the atmospheric implications of HNO3 photolysis,
a limited wavelength dependence of the HONO formation was
investigated in the present study using two different sets of lamps
(see Fig. 5), for which the spectra are shifted by B20 nm (see
Fig. 2). The ratio of the experimentally determined photolysis
frequency, J(HNO3 - HONO)sw/J(HNO3 - HONO)lw, of 3.6 (see
Section 3.2) is much lower compared to theoretical ratios of 10.9
and 14.1 for the liquid- and gas-phase photolysis, respectively. The
latter was calculated using eqn (3) and known liquid-60 and gas-
phase51 absorption cross-sections, measured actinic fluxes and
assuming a wavelength independent quantum yield.49 This result
is another indication of surface photolysis of HNO3 and thus
confirms recent publications on a red-shift of the n - p* absorp-
tion of adsorbed HNO3.45,47 Using these published cross-sections,
measured HONO formation (see Fig. 5) and actinic fluxes (see
Fig. 2), formal quantum yields for HONO formation, f(HNO3 -
HONO), for the heterogeneous photolysis reaction (R1) of 6.9 ꢀ
10ꢁ5 and 8.4 ꢀ 10ꢁ5 were calculated using eqn (3) for the short-
and long-wavelength setups, respectively. Taking into account the
uncertainties of the measured HONO formation, actinic flux
spectra and published absorption cross-sections, the similarity of
the two values indicates no strong wavelength dependence of the
quantum yield, and thus an average value of f(HNO3 - HONO) =
(7.6 ꢄ 1.5) ꢀ 10ꢁ5 is preferred and applied in further calculations.
Fig. 5 HONO flux density as a function of the measured surface concen-
tration of HNO3 using two different sets of UV lamps (B42–57% r.h.).
The straight lines represent linear regressions of the data with their 95%
confidence limits (dashed lines).
Surface concentrations of adsorbed nitric acid, [HNO3]surface
(molecules cmꢁ2), measured after the experiments, were assumed
to be constant during each experiment, since the HNO3 source
was running continuously. Thus, J(HNO3 - HONO) was calcu-
lated from the slope of a plot of the measured average surface flux
density (D[HONO]surface/Dt) against [HNO3]surface
:
D½HONOꢃsurface
Dt
¼ JðHNO3 ! HONOÞ ꢀ ½HNO3ꢃsurface
: (2)
From weighted orthogonal regression plots according to eqn (2)
and after correction for gas-phase photolysis of HONO, photo-
lysis frequencies J(HNO3 - HONO) of (1.24 ꢄ 0.19) ꢀ 10ꢁ6 sꢁ1
and (3.4 ꢄ 1.8) ꢀ 10ꢁ7 sꢁ1 were determined at B50% r.h. for the
short- (sw) and the long-wavelength (lw) setups, respectively,
(see Fig. 5). In between the experimental errors photolysis
frequencies were found to be independent of the gas flow rate.
The measured values are much lower than those obtained in
another study on HONO formation from the photolysis of
adsorbed HNO3 on glass surfaces,24 for which photolysis fre-
quencies of B10ꢁ5 sꢁ1 were calculated under atmospheric con-
ditions. The difference between the two studies is actually even
higher, since the small values determined in the present study
are still upper limits as the lamps used in the experiments have
higher actinic fluxes compared to the atmosphere (see Fig. 2,
Table 1 and Section 4). Potential underestimation of the surface
area of the used quartz tube, e.g. by any surface roughness, will
not affect the measured J(HNO3 - HONO) since the surface area
cancels out in eqn (2) in the two terms D[HONO]surface and
[HNO3]surface. In addition, the potential inhomogeneous surface
concentration throughout the flow tube would also not affect the
measured J(HNO3), since first order kinetics was observed.
3.4. Influence of relative humidity
Similar to the study by Zhou et al.,24 a strong influence of the relative
humidity on the HONO formation inside the flow-reactor was
observed. HONO immediately decreased to very low levels after
stopping the humidifier, indicating a direct influence of water on
the HONO production. There are different potential explanations for
this observation: (i) photochemical HONO formation is active only
for H2O/HNO3 clusters26 and/or (ii) is caused by secondary chemistry
via the additional reaction product NO2, for which strong humidity
dependence of the heterogeneous HONO formation is known.61
3.5. NO2 dependency
3.3. Wavelength dependency
To distinguish between these two explanations, the NO2 depen-
dence of the HONO formation was studied. Besides HONO,
NO2 is a main product of the HNO3 photolysis (see Fig. 3):
To estimate the atmospheric implication of the HNO3 photolysis,
the results obtained in laboratory experiments cannot be used
directly, since the actinic flux spectra of laboratory systems are
typically very different from those of the atmosphere (cf. Fig. 2).
JðHNO3!NO2Þ
HNO3ðads:Þ þ hn
NO þ OH:
(R2)
ꢀꢀꢀꢀꢀꢀꢀꢀ!
2
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Thus, in addition to possible direct HONO formation via HNO3 extrapolation to 0 ppbV NOx indicates no additional HONO
photolysis (R1), the reaction product NO2 may also form HONO formation by HNO3 photolysis alone (R1) under the experimental
via secondary heterogeneous conversion:61
conditions applied. Thus, it is proposed that the observed HONO
formation is caused by the heterogeneous conversion of NO2 on
the reactor surfaces, reactions (R3) or (R4).
surface
ꢀꢀꢀ!
2NO þ H O
HONO þ HNO ;
(R3)
2
2
3
(b) HONO formation by heterogeneous NO2 conversion in the
dark and under irradiation is similar, confirming a proposed
dark formation of HONO, e.g. by reaction (R3) on clean quartz
glass surfaces. This result is in excellent agreement with other
studies,26,44 in which no photoenhancement of reaction (R3) was
observed. Thus, the experimental results of the present study do
not confirm photochemical HONO formation via excited NO2,
reaction (R5), as proposed in another study by Abida et al.48
when using excimer laser photolysis. Very recently, the same
group64 also excluded the direct formation of excited NO2 at least
for the gas-phase photolysis of HNO3 and proposed secondary
HONO formation by the heterogeneous reaction (R3). This
conclusion is in agreement with studies of reaction (R5) in the
gas phase,35,36 in which HONO formation via excited NO2 was
excluded at atmospherically relevant actinic fluxes. Finally, the
missing light-enhancement also excludes photolysis of com-
plexes of HNO3 with nitrogen oxides as sources of HONO.63
Based on these two conclusions a two-step formation mecha-
nism of HONO is proposed here. First, NO2 is formed in the
photolysis of adsorbed HNO3, reaction (R2), at longer wave-
lengths compared to the gas and liquid phases. Second, NO2 is
consecutively converted into HONO via the heterogeneous dark
reactions (R3) or (R4) on the pure quartz glass surfaces. This
consecutive formation of HONO via the intermediate NO2 was
already proposed for the photolysis of nitrate in snow22,65 and
in the aqueous phase.66 However, for snow the significant red-
shift of the nitrate absorption compared to the liquid nitrate
spectra was not observed leading to negligible NOx formation at
wavelengths 4345 nm.67 Finally, the observed NO formation
(see Fig. 3) is explained by the gas-phase photolysis of NO2,
reaction (R6).
or by the reaction of NO2 with chemisorbed water on the reactor
surfaces:62
surface
2NO2 þ Hchem þ OHchem
HONO þ HNO :
(R4)
ꢀꢀꢀ!
3
Furthermore, formation of electronically excited NO2 was recently
proposed in the photolysis of adsorbed HNO3 on aluminium
surfaces, which may react with adsorbed water:46,48
surface
NO2ꢅ þ H O
HONO þ OH:
(R5)
ꢀꢀꢀ!
2
Finally, based on theoretical calculations, photolysis of complexes
of HNO3 with nitrogen oxides has been proposed to explain HONO
formation by the photolysis of adsorbed HNO3.63
To elucidate the contribution of NO2 to the observed HONO
formation, NO2 was added to the reaction mixtures (see Fig. 3).
From this and other similar experiments total HONO production
rates were calculated from the concentration differences between
the flow-reactor bypass and the dark or light steps, respectively, and
plotted against the final NOx level at the flow-reactor exit (see
Fig. 6). NOx was used here instead of the proposed precursor NO2 to
take into account the significant NO2 photolysis in the flow-reactor
(see Table 1). Thus, NO measured at the exit of the reactor mainly
originated from the fast gas-phase photolysis of NO2:
NO2 + hn - NO + O(3P),
(R6)
and was available as a reactant for heterogeneous HONO forma-
tion, reactions (R3)–(R5).
Two important conclusions can be drawn from the experi-
mental data:
(a) HONO formation depends strongly on the NO2 concentration
and within the error of the intercept (ꢄ2 ꢀ 108 molecules cmꢁ2 sꢁ1
)
To determine HNO3 photolysis frequencies for the primary
reaction product NO2:
ln
X
JðHNO3 !NO2Þ¼
Fl ꢀ sðHNO3ÞlꢀfðHNO3 !NO2Þl ꢀDl;
l1
(4)
the sum of photochemically formed NO2, HONO and NO was
considered in calculations similar to eqn (2). Using weighted
orthogonal linear regression plots, photolysis frequencies
J(HNO3 - NO2) of (6.6 ꢄ 1.3) ꢀ 10ꢁ6 sꢁ1 and (1.3 ꢄ 1.0) ꢀ
10ꢁ6 sꢁ1 were determined at B50% r.h. for the short and the
long wavelength setups, respectively.
Using the absorption cross-sections of adsorbed HNO3 from
the studies of Zhu et al.45 and Du and Zhu,47 measured actinic
fluxes of both setups and measured values of J(HNO3 - NO2)
from the present study, similar quantum yields for NO2 for-
mation, f(HNO3 - NO2), of (3.6 ꢄ 0.8) ꢀ 10ꢁ4 and (3.2 ꢄ 2.5) ꢀ
10ꢁ4 were determined for the short and the long wavelength
Fig. 6 HONO flux as a function of the NOx concentration calculated from
the differences (dark-bypass) and (illuminated-bypass), ([HNO3]surf r 0.8 ꢀ
1014 cmꢁ2, 51–54% r.h.). Within the experimental errors (95% confidence
intervals, dashed lines), logarithmic fits (solid lines) show no difference
between HONO formation in the dark (black) and under illumination (grey). setups, respectively. The averaged quantum yield for both
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setups of (3.4 ꢄ 2.6) ꢀ 10ꢁ4 is orders of magnitude lower the VOCs. A similar result was obtained when the flow tube was
compared to the studies of Zhu et al.46 and Abida et al.,48 in not cleaned after flushing with laboratory air for one day before
which quantum yields near unity were observed in excimer laser the photolysis experiment. Thus, at least for the smaller VOCs
photolysis experiments at 308 and 351 nm. Reasons for the strong and at atmospherically relevant concentrations, HONO formation
discrepancy are still unclear and may be explained by:
by photosensitized conversion of HNO3 could not be confirmed.
(a) Overestimation of the HNO3 absorption cross-sections in However, further experiments using more complex VOCs, known
the studies of Zhu et al.45 and Du and Zhu.47 In these studies, as photosensitizers (e.g. aromatic carbonyls, hydroxy-PAHs, humic
the concentrations of adsorbed HNO3 were only indirectly acid etc.), are recommended in the future.
determined by adsorption isotherms based on measured light
absorption and by using an estimated van der Waals radius for
HNO3 resulting in a monolayer coverage only at very high gas-
phase levels (15–20 ppmV) of the highly sticky HNO3. In
4. Atmospheric implications
contrast, in the present study three orders of magnitude lower Based on the experimental results HONO formation from the
gas-phase concentrations (B10 ppbV) were necessary to obtain photolysis of adsorbed nitric acid was estimated under typical
similar levels of adsorbed HNO3, which were directly measured atmospheric conditions. The present study indicates no direct
by ion chromatography. If surface levels of HNO3 were much photochemical HONO formation via reaction (R1), but secondary
higher than proposed in the studies of Zhu et al.45 and Du and formation by the heterogeneous dark conversion of NO2 on humid
Zhu,47 absorption cross-sections may be overestimated, which surfaces, reaction (R3) or (R4). At high NOx levels, reaction (R3) can
68
´
was also recently proposed by Tadic;
be a significant night-time ground surface source of HONO in the
(b) Overestimation of the quantum yields f(HNO3 - NO2) atmosphere,12,61 but is too slow to explain daytime HONO levels.41
in the studies of Zhu et al.46 and Abida et al.48 caused by higher Here, other NO2 reactions, e.g. the photosensitized conversion on
photon flux densities of the excimer laser used for the photo- humic acid surfaces have been proposed.19,20
lysis experiments compared to atmospheric conditions. High
Although HONO is not the primary HNO3 photolysis product,
photon flux densities may have caused stronger NO2 formation, reaction (R1), the formal quantum yield for HONO formation of
e.g. by a potential multi-photon excitation, which was also f(HNO3 - HONO) = (7.6 ꢄ 1.5) ꢀ 10ꢁ5, as determined in Section
recently observed for the gas-phase reaction of NO2* + H2O.36
3.3., was considered here. Using this wavelength independent
Independent of which explanation is correct, photolysis of quantum yield, published absorption cross-sections of adsorbed
45,47
pure HNO3 will be of much lower importance for renoxification HNO3
and the upper limit actinic flux in the atmosphere at
than previously assumed, since the presented NO2 flux densities 01 SZA50 a formal photolysis frequency J(HNO3 - HONO) of only
were determined under realistic atmospheric conditions (HNO3 2.4 ꢀ 10ꢁ7 sꢁ1 was derived using eqn (3). This value is almost two
level, humidity, actinic flux). A strong indication for an over- orders of magnitude lower compared to another laboratory study
estimation of either the absorption cross-sections or the quantum by Zhou and coworkers,24 in which atmospheric HONO forma-
yield of adsorbed HNO3 in the former studies45–47 is the resulting tion in a rural environment could be well explained by reaction
unrealistic short photolytic lifetime of adsorbed HNO3 of only (R1). In contrast, using the results of the present study and
B5 min (see Section 4). If published absorption cross-sections assuming a high surface coverage of HNO3 on atmospheric
are overestimated, our calculated quantum yields, which are surfaces of 1014 cmꢁ2 (Bone monolayer45), an upper limit HONO
based on these values, will be strongly underestimated.
flux density of only 2.4 ꢀ 107 cmꢁ2 sꢁ1 can be calculated using
eqn (1) at 01 SZA. This upper limit is 2–3 orders of magnitude
lower compared to HONO flux densities typically observed over
3.6. Influence of VOCs
To explain enhanced HONO formation, photosensitized con- irradiated surfaces in the atmosphere.25,42,43 Thus, HONO forma-
version of HNO3/nitrate by adsorbed organic compounds was tion by photolysis of pure HNO3 is too slow to explain field
recently proposed.69–72 In addition, increased HONO formation observations, even in rural environments. Reasons for the strong
by scavenging of OH radicals formed via reaction (R2) by discrepancy between the present study and the one by Zhou
organic compounds was proposed for the liquid phase photo- et al.24 are yet unclear but may possibly be explained by:
lysis of nitrate.66 To test the influence of organic compounds
(a) the indirect extrapolation of the laboratory results to the
on the reaction system, in one experiment a GC calibration atmosphere via the chemical actinometry used by Zhou et al.,24
mixture of 29 VOCs (rC10, see Section 2.3. for details; each by which OH radicals formed in the liquid phase photolysis
with a final mixing ratio of 3–4 ppbV) was continuously added of nitrate were used to quantify J(HNO3) for the red-shifted
to the humidified HNO3 mixture, both, during surface satura- (compared to the liquid phase) photolysis of adsorbed HNO3.45,47
tion overnight and during the photolysis. While some of the In contrast, in the present study a well calibrated spectro-
used VOCs clearly do not adsorb onto surfaces (e.g. C1–C4), radiometer was used to quantify actinic fluxes and to calculate
others are expected to be surface active (e.g. b-pinene, limonene, J(HNO3) using eqn (3). Here, the wavelength dependent absorp-
xylenes, n-butanol, methacrolein) and substituted aromatic com- tion cross-sections of adsorbed HNO345,47 were considered;
pounds are even proposed to participate in redox reactions of
(b) different surface properties of the reactors and the differ-
NOy.18 However, within the experimental errors photochemical ent NO2 levels used. While in the present experiments, the flow-
HONO formation was not enhanced compared to the absence of reactor was chemically cleaned by HF prior to each experiment,
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Paper
impurities on irradiated surfaces and higher NO2 levels in the Acknowledgements
experiments by Zhou et al.24 may have caused increased HONO
The financial support for this work by the Deutsche Forschung-
sgemeinschaft (DFG) under contract number (KL 1392/3-1) is
gratefully acknowledged.
yields via reaction (R3) and (R4), or by photosensitized conver-
sion of NO2 on adsorbed organic impurities.18–21 Alternatively,
direct photosensitized conversion of HNO3/nitrate by organic
impurities69–72 may also be another explanation for the higher
HONO yields observed by Zhou et al.24
However, in the present study the latter mechanism could
not be confirmed at least for a mixture of 29 smaller VOCs
(rC10) or when the reactor was not cleaned after flushing with
laboratory air for one day before the experiment. Thus, in the
future, HONO formation from the photolysis of adsorbed HNO3
should be studied on more realistic organic surfaces, for
example, on leaves, urban grime or humic acid. However, for
the photolysis of pure HNO3 adsorbed on non-reactive surfaces,
HONO formation is of minor importance.
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