ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-6355-2016Formation of reactive nitrogen oxides from urban grime photochemistryBaergenAlyson M.DonaldsonD. Jamesjdonalds@chem.utoronto.caDepartment of Chemistry, University of Toronto, Toronto, M56 3H6,
CanadaDepartment of Physical and Environmental Science, University of
Toronto at Scarborough, Toronto, M1C 1A4, CanadaD. James Donaldson (jdonalds@chem.utoronto.ca)24May201616106355636319February20162March20162May20166May2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/6355/2016/acp-16-6355-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/6355/2016/acp-16-6355-2016.pdf
Impervious surfaces are ubiquitous in urban environments and
constitute a substrate onto which atmospheric constituents can deposit and
undergo photochemical and oxidative processing, giving rise to “urban
grime” films. HNO3 and N2O5 are important sinks for NOx
in the lower atmosphere and may be deposited onto these films, forming
nitrate through surface hydrolysis. Although such deposition has been
considered as a net loss of NOx from the atmosphere, there is
increasing evidence that surface-associated nitrate undergoes further
reaction. Here, we examine the gas phase products of the photochemistry of
real, field-collected urban grime using incoherent broadband cavity-enhanced
absorption spectroscopy (IBBCEAS). Gas phase nitrogen oxides are emitted
upon illumination of grime samples and their production increases with
ambient relative humidity (RH) up to 35 % after which the production
becomes independent of RH. These results are discussed in the context of
water uptake onto and evaporation from grime films.
Introduction
Atmospheric NOx(= NO + NO2) is an important reactant in the
formation of urban pollutants such as ground-level O3, while
HONO(g) is an important photochemical source of OH
(Finlayson-Pitts and Pitts Jr., 1999). Therefore, in order to quantify the
local atmospheric oxidative capacity, it is important to understand the
processes mediating the concentrations of these species in the urban
atmosphere. A major sink for nitrogen oxides in the troposphere is the
formation of gas phase HNO3 or N2O5, followed by the
deposition of these species to surfaces and their subsequent hydrolysis to
form nitrate. This anion is considered to be a sink for the gas phase
NOx because its aqueous phase photochemistry is very slow. However,
there is an increasing body of literature which suggests that surface bound
nitrate and HNO3 are not terminal sinks, but rather can undergo further
recycling back to the gas phase. For example, HNO3 has been shown to
react on surfaces with gas phase NO and HONO to form NO2 (Mochida and
Finlayson-Pitts, 2000; Rivera-Figueroa et al., 2003; Saliba et al., 2001), and
photochemical mechanisms for the conversion of HNO3 and nitrate anion to
gaseous nitrogen oxide species have been proposed on a variety of surfaces
including glass (Zhou et al., 2003), snow (Grannas et al., 2007; Mochida and
Finlayson-Pitts, 2000; Rivera-Figueroa et al., 2003; Saliba et al., 2001),
organic films (Handley et al., 2007), leaves (Zhou et al., 2011), plants (Ye
et al., 2016), building materials (Ye et al., 2016) and mineral oxide
surfaces such as aluminum oxide and zeolite (Gankanda and Grassian, 2013;
Nanayakkara et al., 2014; Rubasinghege and Grassian, 2009; Schuttlefield et
al., 2008). There is particular interest surrounding whether such processes
could explain an as of yet unconfirmed source of daytime HONO in urban
centers. Field studies have indicated that this missing source is
photochemical in nature and acts at or near ground level (Lee et al., 2016;
Wong et al., 2013, 2012; Young et al., 2012). Other processes, such as
reactions of NOy (total reactive nitrogen) on aerosols (Ma et al., 2013)
and soil-mediated processes (Oswald et al., 2013; Scharko et al., 2015), have
also been proposed but have not been confirmed at this time.
When studying atmospheric surface reactions, an often-overlooked surface is
that of human-made structures (e.g., buildings, roadways). These surfaces,
when exposed to the atmosphere, become coated in a complex surface film over
time due to the deposition and subsequent processing of atmospheric
constituents
(Chabas
et al., 2008, 2012; Diamond et al., 2000; Duigu et al., 2009; Favez et al.,
2006; Ionescu et al., 2006; Lam et al., 2005; Law and Diamond, 1998; Liu et
al., 2003; Lombardo et al., 2010, 2005; Simpson et al., 2006; Wu et al.,
2008a, b). Referred to as “urban grime”, these films have generally been
thought of as merely a surface for deposition as a terminal sink for species.
However, there is increasing understanding that these films could also play a
role in mediating environmental cycling. Most attention has been brought to
the idea that the films can sequester gas phase compounds and enhance
pollutant concentrations in rainfall runoff (Diamond et al., 2010, 2001;
Priemer and Diamond, 2002), but there is evidence suggesting that they can
also impact the reactivity of species contained within the film, such as PAHs
(Ammar et al., 2010; Kwamena et al., 2007), and nitrate / HNO3
(Baergen and Donaldson, 2013; Baergen et al., 2015).
Additionally, it has been predicted that there is enough water present on
all environmental surfaces, even those hydrophobic in nature, to influence
heterogeneous reactions (Sumner et al., 2004). Rubasinghege and
Grassian (2013) have discussed the role of water on environmental surfaces outlining a wide
range of mechanisms through which water can impact reactivity. These include altering reaction
pathways, promoting hydrolysis reactions, ionic dissociation and solvation
of ions, inhibiting reactivity through blocking reactive sites, enhancing
ion mobility on the surface and altering the stability of surface species.
For example, both the extent of reaction and the distribution of products
change as a function of relative humidity (RH) for nitrate photolysis on
aluminum oxide and Pyrex substrates, but the response is substrate
dependant (Rubasinghege and Grassian, 2009; Zhou et al., 2003). Such
studies have generally investigated the impact of water on atmospheric
surface chemistry by varying the ambient RH. However, because different
surfaces have different water affinities, they may be expected to display
different responses to changes in relative humidity. For example, a study by
Nguyen et al. (2015) shows that estimating water content in the aerosol, rather than just
using RH data, is important for predicting the formation of biogenic
secondary organic aerosol. Related to grime
surfaces, Sumner et al. (2004) have shown how different surfaces, representing building
surfaces, vary in their water uptake behavior.
There are only minimal studies performed looking at water interactions with
grime, but they show that grime films impact water uptake on surfaces
(Baergen and Donaldson, 2013; Chabas et al., 2014). Thus it
is important to characterize the change in surface water content as a
function of RH as well as grime photochemistry. In the following we present
results of experiments which monitor the photochemical release of gas phase
nitrogen oxides from urban grime as a function of RH, in conjunction with
water uptake measurements on grime.
ExperimentalSample collection
Grime samples were collected by placing the substrate, either 3 mm diameter
glass beads (Fisher Scientific) or quartz crystal microbalance (QCM)
crystals, outside in downtown Toronto, Canada, for up to 1 year. The beads
were placed on metal mesh shelves underneath a building overhang, sheltering
the sampler from precipitation. Sunlight was blocked with a black cloth
covering the front of the sample and a building blocking the sunlight from
the other direction. The QCM crystals were placed in holders where the face
of the crystal was facing the ground while the back was within the holder,
preventing the collection of grime on this backside, which would impact the
QCM response. In this way both types of samples were shielded from light and
precipitation while still being open to the atmosphere.
Gas phase product formation was determined using IBBCEAS. The system is
described in full elsewhere (Reeser et al., 2013). Briefly, a 10 W LED with
a maximum intensity at 372 nm was focused into a 100 cm cell. The cell was
sealed with two highly reflective mirrors (> 99.95 % between 367 and
380 nm). The light escaping through the back mirror was collected by a lens
and focused onto a fiber optic bundle, which was directed into a spectrograph
with a CCD detector. The mirrors were continually purged using a flow rate of
25 mL min-1 of N2 directed onto the mirror surfaces. Transmission
spectra were collected for 30 s (averages of 30 scans with an integration
time of 1 s each) over a wavelength range of 362 to 385 nm.
The concentrations of HONO and NO2 were calculated using the method
described by Fiedler and Gherman (Fiedler et al., 2003; Gherman
et al., 2008) and previously used by us (Reeser et al., 2013). This uses
measured mirror reflectivity (Washenfelder et al., 2008), Rayleigh cross
sections of the carrier gas (Bodhaine et al., 2010) and the
absorption cross sections of NO2 (Vandaele et al., 1998) and
HONO (Stutz et al., 2000) to fit the experimental spectra with DOASIS
software (Lehmann, 2009). DOASIS uses a linear least-squares method to
fit the absorption bands to reference spectra and a polynomial to fit broad
features such as those from Rayleigh scattering, Mie scattering and
temperature drifts. The fit is optimized by including terms that allow for
small shifts in absorption wavelengths and spread of peaks. A sample fit is
displayed as the solid line in Fig. 1. Calculated detection limits
(signal/noise = 3) are 1.50×1011 molecules cm-3
(∼ 6 ppb) for NO2 and 6.5×1010 molecules cm-3
(∼ 3 ppb) for HONO.
Experimental absorption spectrum fit with a reference HONO spectrum
(Stutz et al., 2000) using DOASIS (Lehmann, 2009). This spectrum was measured
at RH = 37 % and represents a concentration of 2.17×1011 molecules cm-3.
Photochemistry
Samples of 10.0 g of exposed glass beads were weighed into a glass petri
dish for illumination. These were placed within a stainless steel chamber
(3.2′′× 2.2′′× 1.5′′), and nitrogen was flowed
through the chamber into the IBBCEAS cell at a rate of 0.3 L min-1. RH
and temperature in the chamber were monitored using a
Traceable® Memory Hygrometer/Thermometer. The
reported accuracy is ±2 % at mid-range and ±4 % elsewhere in
the range of 10 to 95 % RH. The calibration was checked by measuring the RH
above a series of saturated salt solutions in comparison to the known
deliquescent RH and was the same as the reported values within the stated
uncertainties.
Nitrogen was flowed through the system for 1 h prior to illumination to
establish a stable background in the spectrum, and equilibrate the water
vapor in the chamber for the RH used in the experiment. The samples,
initially at a RH of 35 %, were illuminated through a quartz window at the
top of the sample chamber with a xenon arc lamp (λ>295 nm)
for 60 min. The light was then blocked, the signal allowed to return to
baseline, and the RH adjusted for the next illumination period. After 60 min
the sample was again illuminated for 60 min before blocking the light and
repeating the cycle for a third time. Average concentrations were calculated
for the second 30 min of illumination, where the signal appeared to reach
steady state, and then normalized to the initial steady-state value at the
RH of 35 % to adjust for experiment variability such as variations in
sample, light intensity and temperature. These experiments were carried out
without temperature control, with the chamber operating at a temperature
between 28 and 34∘ C during illumination.
Control experiments were carried out in which 10.0 g of clean beads was
illuminated for 1 h at a relative humidity of 35 %. In addition, 10.0 g of the grime coated beads was subjected to heating up to 36 ∘C
to study the impact of increased temperature on product formation. Both of
these tests were completed in triplicate with a different sample being used
each time. Neither experiment showed detectable levels of HONO or NO2.
The setup was further tested by flowing a known concentration of NO2
through the empty chamber and IBBCEAS cell. A flow containing 6.0 ppm
NO2 in N2 was diluted in a stream of N2 down to (4.76 ± 2.4) × 1012 molecules cm-3
at varying RHs. NO2 and HONO
steady-state concentrations measured at each RH were used to characterize
the IBBCEAS response to changes in humidity and the efficiency of NO2
hydrolysis to HONO on the walls of the reaction chamber and IBBCEAS cell.
Ion analysis
Two different ion extraction techniques were used. For ion analysis of the
illuminated beads, 4.0 g of grime-coated beads was shaken for 5 min with
3.00 mL of deionized water with a resistivity of greater than
18 MΩcm (Baergen et al., 2015). For ion analysis to accompany water
uptake measurements, samples were collected on 5 cm × 7.6 cm
pieces of window glass over the same time period as the quartz crystals and
extracted with 45 mL deionized water. Glass samples were first placed in
25 mL of water and sonicated for 1 min. Each side of the glass was then
washed twice with 5 mL of water. Solutions were filtered through a
0.2 µm IC Millex®-LG syringe
filter before being analyzed by ion chromatography on a Dionex ICS-2000;
1.33 mL samples were injected onto a concentrator/analytical column system:
Ionpac® TAC-ULP1/AS19 with KOH eluent for
anion detection and Ionpac® TCC-ULP1/CS17
with methanesulfonic acid eluent for cation detection. A second extraction
resulted in concentrations of less than 10 % of the first extraction for
all ions. The inorganic ion content of the grime used for photochemistry
experiments is given in Table S1 in the Supplement.
Water uptake
The mass of water taken up onto an urban grime film as a function of RH was
measured using a quartz crystal microbalance (QCM), as described in Demou et
al. (2003). Grime was collected directly onto a quartz
crystal and then placed in the QCM. The QCM was housed in a Plexiglas
chamber whose humidity was increased by flowing air through a water bubbler
at room temperature at a variable flow rate to maintain a rate of change of
RH of 1 % min-1 and decreased by flowing dry air through the chamber at
variable flow rates to maintain a rate of change of -1 % min-1. The
frequency change of the microbalance from the change in water content of the
film was converted to a mass using the Sauerbrey equation (Δm=CΔf), where Δm is the mass change, C is a
proportionality constant and Δf is the frequency change from the
deposited mass. In a previous study, the Sauerbrey relationship was
confirmed to hold for this apparatus only up to 1 % of the fundamental
frequency of the crystal (Demou et al., 2003). Due to this mass
restriction, samples for QCM analysis were collected for only 4 weeks
instead of the 1 year for the photochemistry samples. The value of the
constant C is reported to be 8.147×107 Hz cm2 g-1 for the
0.550-inch, 6 MHz crystals used in this study (Sauerbrey, 1959). The RH
was measured using a Traceable® Memory Hygrometer/Thermometer.
ResultsPhotochemical production of nitrogen oxides
Figure 1 shows a typical absorption spectrum collected upon illumination of a
grime sample. One can see two features, typical of HONO absorption: a
stronger signal at 368nm, and a second peak appearing at 384 nm, at the
longer edge of our wavelength range. The IBBCEAS is also sensitive to
NO2, which absorbs in this wavelength region. However, this molecule was
not detected in any of the photochemical experiments performed here. We argue
in the Supplement that NO2 hydrolysis on the walls of the
chamber and/or IBBCEAS cell (Finlayson-Pitts et al., 2002) would
prevent NO2 from being detected even if it was originally formed in the
chamber. Because of this hydrolysis there is an uncertainty as to whether the
HONO we observe in the illumination experiments was originally NO2,
which was hydrolyzed prior to detection, or if it is HONO being produced
directly from the sample. Therefore, we cannot attribute the observed HONO
product exclusively to direct photochemistry of the grime sample; rather we
use the HONO signal to indicate the combined total emission of NO2 and
HONO. We further note that the total product detected decreases when NO2
is flowed through the apparatus in the light as compared to in the dark by
approximately 60 %, indicating gas phase photolysis of products (see
Fig. S1 in the Supplement). This highlights the importance of such
considerations to be made whenever HONO and NO2 are being measured. Each
system needs to be classified individually over a range of RH conditions.
Time trace of an experiment where the sections highlighted in yellow
indicate when the sample is exposed to light. The relative humidity in the
chamber during each illumination period is indicated. The HONO detection
limit is indicated by the dashed line.
Figure 2 depicts the results of a typical experiment where a grime sample is
placed within the chamber and exposed to three separate 60 min illumination
periods at different relative humidities. The yellow highlighted regions
indicate illumination. It is clear that nitrogen oxides are released to the
gas phase during illumination and that the amount of products formed is
dependent on the relative humidity. A repeat illumination of a sample at an
RH of 35 % showed an average ratio of 0.88 ± 0.06 compared to the
original illumination. This provides evidence of some precursor depletion
due to illumination; however, the smaller signal level at 20 % RH, apparent in Fig. 2,
indicates that there is an RH dependence to product formation, in addition to any sample depletion.
The nitrate to sulfate ratio of the grime before and after illumination was used to examine
nitrate behavior. Sulfate is not expected to have any photoreactivity on
the film and thus was used to account for sample variability as was done in
our previous work (Baergen et al., 2015). No nitrate loss was
detected between water extracts of beads before and after illumination at
35 % for three 1 h periods. The average change in the nitrate to
sulfate ratio from before to after illumination was 3.6 ± 6.6 %.
There was also no nitrate loss detected for the samples that were heated for
3 h; these show an average change in the nitrate to sulfate ratio
from before to after heating of 1.0 ± 3.6 %. The amount of nitrate
loss expected during illumination, based on the integrated amount of gas
phase nitrogen oxides produced, is in agreement with the above results.
In order to further investigate the RH dependence on product formation, the
initial illumination period at 35 % RH was used to normalize the
concentrations detected for the next illumination periods. These data are
shown for a range of RH values in Fig. 3. Up to an RH of approximately
35 % the amount of products formed increases, after which product
formation becomes independent of RH. At a RH of 0 %, no products were
detected. However, from the NO2 control experiments, there was evidence
that gas phase NO2/ HONO is lost to the chamber walls for these dry
conditions, and thus this value was not plotted in Fig. 3.
HONO production as a function of relative humidity. Values are
normalized to the steady-state concentration of HONO formed during an initial
illumination period at a relative humidity of 35 %. The average of at least
three measurements on different samples is shown; error bars represent 1 standard
deviation.
Water uptake by grime samples
This interesting RH dependence of the amount of nitrogen oxides emitted
photochemically from urban grime motivates the study of water uptake onto
grime. Grime–water interaction has been reported before using ATR-FTIR with
1-week-old grime, showing equilibrium with ambient water vapor
(Baergen and Donaldson, 2013). Chabas et al. (2014) also reported that mass
measurements on 100-month-old films showed enhanced water uptake on
grime-coated substrates compared to clean ones.
Here we use 4-week-old samples collected throughout the year-long collection
onto the glass beads, and look at both water uptake and evaporation, to
better probe water–grime interactions. The uptake and loss curves displayed
in Fig. 4 are an average of 16 curves collected at different time points
through the year normalized to the mass of major ions in the film (Cl-,
NO3-, SO42-, Na+, K+, Mg2+ and
Ca2+), extracted from a glass slide exposed to the atmosphere for the
same length of time as the quartz crystal and scaled to the same surface
area as the crystal. The shaded region indicates the 95 % confidence
interval. The water uptake onto a clean crystal was subtracted from each
sample's uptake curve before averaging, so the figure displays the mass of water
taken up mediated by the grime itself. The degree of uncertainty captures
some of the seasonality of grime water uptake, which will be discussed in an
upcoming paper along with the seasonality of grime ion content. The water
uptake onto and evaporation from grime are both smooth curves, with no
indication of phase changes over the RH values spanned here. The lack of
hysteresis also gives confidence that the illumination experiments reflect
the true state of the “real” urban grime, as the film remains equilibrated
with the ambient RH as this changes.
Average ratio of water mass to total ion mass within grime as a
function of relative humidity. Water uptake onto clean crystals was
subtracted from the grime uptake curves, and thus only grime-mediated uptake
is shown here. The shaded region indicates a 95 % confidence interval base
on 16 measurements of different samples.
Discussion
The illumination of urban grime results in the release of gas phase nitrogen
oxides in the form of NO2 and/or HONO. While previously predicted
(Baergen and Donaldson, 2013; Baergen et al., 2015), this is
one of the first observations of such gas phase products. Field-collected
grime samples were illuminated, without any alteration, clearly showing that
urban grime is a source of nitrogen oxides back into the atmosphere. Our
previous work, as well as that of others, has shown that nitrate is present
within urban grime films (Favez et al., 2006; Lam et al.,
2005) and that this nitrate is photolabile (Baergen
and Donaldson, 2013; Baergen et al., 2015; Ye et al., 2016). Nitrate
photolysis is known to form NO2 within other media. Recent work by Ye
et al. (2016) also looked at surfaces exposed to the atmosphere for much shorter exposure
times detecting HONO and NO2 at varying ratios depending on the
surface (Ye et al., 2016). If HONO is a product of this chemistry one
possible source is via the protonation of nitrite, another known product of
aqueous nitrate photochemistry. However, our previous study was consistent
with an alkaline film due to the loss of ammonium; therefore, we do not
expect the film to be acidic enough for this mechanism to be important
(Baergen et al., 2015).The organic fraction of the film could also
play a role in the conversion of NO2 to HONO, such as has been seen on
organic surfaces such as humic acid (Stemmler et al., 2006) and PAH
films (Ammar et al., 2010). NO2 to HONO conversion could also
occur through NO2 hydrolysis within the film. Although it seems likely
that nitrate is responsible for the observed chemistry due to its high
concentration and known photoactivity on other surfaces, it is also possible
that photochemically active organo-nitrogen compounds may be present, though
they have yet to be detected within grime films. If present, these compounds
may react as indicated by Han et al. (2013), who
have reported the formation of R-NO, R-NO2 and R-ONO2 species on
NO2 exposed soot, which can photolyze to form NO and HONO.
In contrast to our previous studies showing the photolability of nitrate in
grime (Baergen and Donaldson, 2013; Baergen et al., 2015), the current study
does not show nitrate depletion upon illumination. This apparent discrepancy
can be explained by the difference in experimental methods between studies.
In both previous studies, the films were “younger”, with between 1 and
6 weeks of collection time, in comparison to the year-long collection here.
In addition, for the Leipzig samples described in Baergen et al. (2015) the
“light” sample was continually exposed to ambient sunlight, whereas in the
present experiment, like the previous Toronto study (Baergen and Donaldson,
2013), the samples were shielded from the light for the entire collection and
then illuminated in a controlled laboratory setting. Both of the previous
studies suggested that only a portion of the film is photolabile and the
current result indicates that this non-photoactive proportion of the film
forms a greater proportion of the film over time. Continued growth of the
film may block photoactive sites or bury photoactive components of the film,
making a smaller portion available for reaction. Ye et al. (2016) found a
logarithmic relationship between surface density of nitrate / HNO3
and reaction rate which could also indicate only nitrate / HNO3 on
the surface remains reactive in comparison to the nitrate / HNO3
within the film. Whether a film grown under continual exposure to ambient
light would show the same trend is an open question. Exposure to
precipitation could also impact the photoactive fraction, both in potential
compositional changes as different fractions are removed from the film during
precipitation and in preventing such long-term film growth. This large
non-photoactive fraction may also explain the disparity between the depletion
of gas phase products over time and the lack of a corresponding nitrate drop;
the photolabile fraction is small enough that the approximately 12 % loss
of reactive precursor implied by the gas phase result is too small of a
proportion of the total nitrate to be detected within the extracts of the
whole film.
The photochemical release of gas phase NO2 and/or HONO is clearly
dependant on relative humidity and therefore, as seen through the water
uptake experiments, on the water content of the film. In particular, the
product formation increases as the amount of water on the film increases, up
to a relative humidity of 35 % after which case, the chemistry is not
impacted by further addition of water up to 60 %. This behavior is
different from what has been seen from nitrate photolysis experiments on
other surfaces. In a study performed on HNO3 deposited on Pyrex glass,
the combined NOx and HONO formation rate was highest at 0 % and
decreased for 20 and 50 % while the reported HONO production rate was
lowest at 0 % and then increased up to 80 % (Zhou et al., 2003).
However, the authors assumed a constant NO2 to HONO wall conversion
independent of relative humidity taken from a measurement in a different
system, and thus the determined ratios may not reflect the real distribution
of products emitted as a result of the photochemistry (Zhou et al.,
2003). Humidity dependence has also been seen for nitrate photochemistry on
mineral dust surfaces. In this case, a minimum was seen for nitrate loss and
NO2 production at 0 % and a maximum at 20 % which subsequently
decreased between 20 and 80 %, while NO production continually decreased
from 0 to 80 % (Rubasinghege and Grassian, 2009). HONO production was
not reported in this study.
The difference in nitrate photolysis behavior between grime and other
surfaces as a function of RH is indicative of the grime providing a unique
environment for the photochemistry. Many different mechanisms for the role
of water in surface reactions have been discussed, such as enhancing the
mobility of reagents, allowing them to move to more photolabile positions
within in the film or enhanced hydrolysis and dissociation of species such
as HNO3, NH4NO3 or N2O5 producing more of the
photolabile precursors (Rubasinghege and Grassian, 2013). The increased
reactivity could also be the result of a viscosity change within the film.
It is known that the viscosity of particles changes based on relative
humidity (Renbaum-Wolff et al., 2013), and therefore it is expected
that the same would be true for grime, with particles being a source to
the film. The film's water uptake/evaporation curve is consistent with
continuous viscosity change rather than phase transitions over the RH region
studied. In a highly viscous film, the photochemical products are more
likely to be trapped and thus recombine. However, a less viscous film would
allow for faster diffusion and thus the release of products could become
competitive and then dominate in comparison to recombination. Such an impact
has recently been suggested to explain a smaller mass loss from illuminated
secondary organic aerosol under low RH conditions in comparison to high (Wong et al., 2014),
and faster PAH ozonation within a secondary organic aerosol coated particles at high RH as
compared to lower RH (Zhou et al., 2013). This sort of behavior would
not be anticipated for a clean glass or metal oxide surface. The leveling
off of product formation at relative humidities greater than 35 % could
indicate that a critical amount of water has been reached. The case of a
viscosity effect would suggest that the process is no longer diffusion
limited. Another process that could be playing a role is the re-adsorption
of the products to the film, as discussed by Rubasinghege and Grassian (2009), which would compete with further
growth causing a net leveling off of product formation.
While specific atmospheric implications require a better speciation of
products, the production of such species can be discussed in the context of
multiple recent field studies. In SHARP 2009, field measurements indicate that there
was a photolytic source of HONO within 20 m of the ground (Wong et al.,
2012, 2013). Studies done in other urban centers such as London (Lee et al.,
2016) and Los Angeles (Young et al., 2012) also suggest there is an unknown
photochemical HONO source. Many suggest that this source is correlated to
NO2; however, in a study carried out in Bakersfield and Pasadena, the
HONO source does not correlate with NO2 (Pusede et al., 2015). As
discussed by those authors, the formation of HNO3 and its subsequent
incorporation into aerosol as ammonium nitrate can extend the lifetime of
airborne nitrate, causing the nitrate which is deposited to not correlate
temporally with NO2(g). Grime would likely have a similar delayed
response; in addition, the RH dependence of the grime photochemistry could
serve as a further mechanism for an offset in NO2 values and HONO
production, due to the cycling of RH conditions in the atmosphere and,
therefore, the cycling of this source strength. However, more quantification
and speciation is required to evaluate the importance of such a grime source.
Conclusions
Urban grime was collected onto glass substrates without modification and
illuminated. Grime photochemistry produced nitrogen oxides in the form of
NO2 and/or HONO. Such chemistry is not currently included in urban air
models, but could impact NOx and/or HONO levels in these centers. The
production of these species is dependant on RH, again highlighting the need
to consider water content when studying environmental surfaces.
The Supplement related to this article is available online at doi:10.5194/acp-16-6355-2016-supplement.
Acknowledgements
This work was funded by NSERC. A. M. Baergen thanks NSERC for a CGS-D award
and the government of Ontario for an Ontario Graduate
Scholarship.Edited by: A. Laskin
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