Introduction
The catalytic cycling of nitrogen oxides (NOx= NO + NO2)
plays a key role in the formation of tropospheric ozone (O3) from the
photooxidation of volatile organic compounds (VOCs).
Reactive nitrogen species, such as alkyl and
multifunctional nitrates (ANs, RONO2), peroxy nitrates (PNs,
RO2NO2), and nitric acid (HNO3) serve as reservoirs and sinks
of NOx. The formation of these species results in a chain termination
that determines the efficiency of the O3 production cycle and can also
transport NOx far from the original emission source. For this reason,
total reactive nitrogen (NOy= NO + NO2+ RONO2+ RO2NO2+ HNO3+ HONO + NO3+2× N2O5+ aerosol nitrates) is an important tracer in monitoring
tropospheric O3 production. Its accurate detection is critical in field
measurements of ambient air quality, as O3 is a known health risk, and
a number of regions across the US are currently in non-attainment or near
non-attainment with national ambient air quality O3 standards
(EPA, 2016). However, the sources and fates of NOy species are
complex and remain poorly characterized in some regions. Measured total
reactive nitrogen has in some cases deviated significantly from the sum of
the measured individual components, ΣNOy,i (see Fahey et
al., 1986; Bradshaw et al., 1998; Neuman et al., 2012; and others referenced
within). This unmeasured NOy, sometimes referred to as “missing
NOy”, indicates the need for a more complete understanding of total
and speciated reactive nitrogen and for accurate analytical instrumentation
for NOy measurement (Crosley, 1996; Williams et al., 1998; Day et al.,
2003).
Techniques that detect the major individual components of NOy include
detection of NO and NO2 by chemiluminescence (Ridley and Howlett, 1974; Kley and McFarland, 1980),
cavity ring-down spectroscopy (CRDS; Fuchs et al., 2009), or
laser-induced fluorescence (LIF; Thornton et al., 2000), as well as
detection of HNO3 by chemical ionization mass spectrometry (CIMS; Fehsenfeld et al., 1998; Huey et al., 1998; Neuman et al., 2002; Huey,
2007) or mist chamber sampling (Talbot et al., 1990).
Additionally, speciated peroxyacyl nitrates (PANs) have been detected by
gas-chromatography electron capture detection (Darley et al., 1963; Flocke
et al., 2005) and CIMS (Slusher et al., 2004), while
N2O5 and ClNO2 have been detected by CRDS (Dubé et
al., 2006; Thaler et al., 2011) and CIMS (Kercher et
al., 2009). HONO has been detected by long path differential optical
absorption spectroscopy (Perner and Platt, 1979), and NO3 has
been detected by CRDS (King et al., 2000). However, fewer
methods have been developed for detection of the broad suite of individual
alkyl and multifunctional nitrates, which have been suggested to comprise
upwards of 20 % of NOy in the mid-latitude continental boundary layer
and may be higher in remote locations (O'Brien et al., 1995; Day et al.,
2003; Worton et al., 2008; Beaver et al., 2012; Xiong et al., 2015; Lee et al.,
2016). An alternative to detecting individual components of NOy is the
use of a molybdenum oxide or gold catalyst in the presence of CO to reduce
all NOy species to NO, followed by NO detection by chemiluminescence
(Winer et al., 1974; Fahey et al., 1986), though catalyst-based techniques
are known to require frequent cleaning and are potentially sensitive to
contamination and to interferences at ambient levels of ammonia, HCN,
acetonitrile and R-NO2 compounds (Crosley, 1996; Kliner et al.,
1997; Bradshaw et al., 1998; Williams et al., 1998; Day et al., 2002). An
alternative method developed by Day and co-workers (Day et
al., 2002) uses a quartz thermal dissociation (TD) inlet to rapidly
thermally convert nearly all NOy species to NO2, which is then
detected by laser-induced fluorescence. The NOy species in the TD
inlet undergo the following reaction:
XNO2+heat→X+NO2,
where X is HO, RO, or RO2. Heated inlets had previously been used to
dissociate PNs (Nikitas et al., 1997), but the TD inlet developed
by Day et al. (2002) takes advantage of the different O-N
bond energies of ANs, PNs, and nitric acid to separately and selectively
detect these three classes of NOy. A plot of measured NO2 signal
as a function of inlet temperature (hereafter referred to as a
“thermogram”) yields a stepwise dissociation curve with increases in
signal near 100, 300, and 500 ∘C, corresponding to the
dissociation of PNs, ANs, and HNO3 respectively. By setting the TD oven
temperature to one of the three plateaus, they were able to measure each
class of NOy, by comparison of the NO2 signal in a given channel
to the signal measured at the adjacent lower temperature plateau.
In recent years, a suite of other instruments have incorporated this
NOy TD inlet method into existing techniques that measure NO2 or
the radical co-fragment X in Reaction (R1), such as chemical ionization mass
spectrometry (TD-CIMS; Slusher et al., 2004; Zheng et al., 2011; Phillips
et al., 2013), cavity ring-down spectroscopy (TD-CRDS; Paul et al.,
2009; Thieser et al., 2016), and cavity attenuated phase shift spectroscopy
(TD-CAPS; Sadanaga et al., 2016). Each instrument has its own
advantages and disadvantages. For example, TD-LIF detects NO2 at low
pressure following thermal dissociation. Secondary recombination reactions
of the dissociated radicals would thus be suppressed in the detection
region, although the thermal dissociation inlet may be operated at either
high or low pressures in these instruments. However, it is subject to
interferences from urban levels of NO and NO2 (Paul et al.,
2009; Wooldridge et al., 2010). TD-CIMS can differentiate between the
different types of PNs but requires regular calibration of each species,
not all of which have native standards readily available. TD-CAPS is subject
to interferences from glyoxal and methylglyoxal (Sadanaga et
al., 2016). TD-CRDS is an absolute measurement but can be subject to other
interferences, as discussed in Sect. 3.
Recent TD inlet studies (Day et al., 2002; Paul et al., 2009; Thieser et
al., 2016) have measured the conversion efficiency for several AN and PN
species with known concentrations in a laboratory setting. These studies all
note the possibility of secondary reactions that either increase or decrease
the NO2 signal. For example, recombination reactions to reform the AN
or PN species prior to reaching the detector will result in a negative bias
in NO2 (too little NO2 measured). Likewise, ambient levels of
O3 in the sampled air may react in the oven with NO to form NO2,
resulting in a positive bias (Pérez et al., 2007),
though this reaction rate depends on the TD inlet pressure and flow rate
(Wooldridge et al., 2010).
Day et al. (2002) found that recombination reactions were
significant for PNs but caused minimal problems for nitric acid, since the
OH radical is far more likely to be lost to the walls of the oven than to
recombine with NO2. More significant is the reaction of dissociated
RO2 and HO2 radicals with ambient levels of NO and NO2.
Thieser et al. (2016) parameterized the bias in
peroxyacyl nitrate and 2-propyl nitrate detection in their inlet as a
function of ambient NO and NO2 concentrations but noted that these
parameterizations may vary for other PNs or ANs. In cases where the
concentration of one category of NOy species far exceeds the others,
such as the high HNO3 : ANs ratios in Pusede et al. (2016),
speciated measurements can be significantly affected by biases in
measurements of the other NOy compounds.
A four-channel CRDS instrument (hereby referred to as the NOAA TD-CRDS
instrument) for detection of nitrogen oxides was recently developed (Wild
et al., 2014). In this instrument, one channel is equipped with a TD inlet
set at 650 ∘C and is used to measure all NOy species
(including NO2, as well as NO by chemical conversion with an O3 addition
to NO2). Two other channels simultaneously monitor NO2 and NO, and
so a measurement of NOz (= NOy - NOx) can be derived.
Because NO is intentionally detected as NO2 in the NOy channel,
this instrument avoids the majority of the NO ↔ NO2
interconversion interferences that affect many other thermal dissociation
instruments. Analogous to the studies which measured the conversion
efficiencies of ANs and PNs (Day et al., 2002; Paul et al., 2009; Sadanaga
et al., 2016; Thieser et al., 2016), we present here an analysis of the
conversion efficiencies of several other NOy species and the
interferences that affect the operation of this high temperature inlet.
These interferences include the temperature dependence of HNO3
conversion, which is important to understanding both its quantitative
conversion at 650 ∘C as well as its potential to interfere with
measurements of ANs at lower temperatures. We also compare these results to
those from the TD-LIF instrument of Day et al. (2002),
hereby referred to as the Berkeley TD-LIF instrument. Additionally, we
report the temperature dependence of N2O5 conversion, which is
shown to occur in two steps; first, the conversion efficiency of ammonium nitrate
aerosol and finally the interference of NH3 through its partial
conversion to NO.
Methods
Thermal dissociation cavity ring-down spectroscopy (TD-CRDS)
Cavity ring-down spectroscopy is a direct absorption technique for measuring
the concentration of trace gases (O'Keefe and Deacon, 1988; Fuchs et al.,
2009). The four-channel 405 nm NOAA TD-CRDS instrument, which has been used
by our group in both lab-based studies and atmospheric sampling (Wild et
al., 2014, 2016), simultaneously measures ambient NO2 in
one channel, while chemically converting NO and O3 to NO2 in the
second and third channels, and thermally converting NOy to NO2 in
a TD oven in the fourth channel. In this study, we have used only the
NOy channel to study the conversion efficiency of several reactive
nitrogen species to NO2. Figure 1 shows a schematic of the relevant
instrument plumbing and optical cavity. The details of the optical cavity
can be found in Wild et al. (2014); only a brief description of the
optical system and the details of the TD inlet that deviate from that study
will be described here.
Sampled air is pulled into a 50 cm long high-finesse optical cavity capped
by highly reflective end mirrors, with purge flows of 25 sccm (standard cubic
centimeters per minute at 273.15 K and 1 atm) added in front of each mirror
to maintain mirror cleanliness. The output of a 0.5 nm bandwidth, continuous
wave diode laser centered at approximately 405 nm and modulated at 2 kHz is
passively coupled into one end of the optical cavity. The laser light builds
up in the cavity, and when it is modulated off the decaying output light
intensity is monitored by a photomultiplier tube on the far side of the
cavity. The measured light decay profiles are summed and fit at a 1 Hz
repetition rate to yield the ring-down time τ. The ring-down time is
inversely related to the concentration of the absorbing gas, NO2 in
this case, which can be derived as
NO2=RLcσ1τ-1τ0,
where RL is the ratio of d, the mirror separation length, and l, which is the
distance over which the sample is present. The speed of light is represented
by c, σ is the absorption cross section of NO2, and τ0 is the ring-down time of a reference cavity without any absorbing
gases, which is obtained by flushing the cavity with an excess flow of zero
air for 30 s every 10 to 20 min. If purge volumes were not used,
the RL term in Eq. (1) would simply be 1, but, since purge volumes are
used here, σ/RL is characterized regularly by filling the
cavity with several different known NO2 concentrations (obtained by
reacting the output of an O3 standard source with excess NO) and
calculating the slope of the measured optical extinction vs. [NO2] as
described in Washenfelder et al. (2011). This value was
measured approximately once per month during laboratory tests with this
instrument but was constant to within ±1 %, with an average value
of 6.25×10-19 cm2. More regular calibrations of the
σ/RL value during recent field studies show similar stability.
The NO2 signal can be measured with a lower detection of 18 pptv
(1σ) in 1 s (Wild et al., 2014).
Instrument schematic of the TD-CRDS instrument used in this study.
An NOy source (HNO3 permeation tube, N2O5 cold trap,
NH4NO3 particle atomizer + DMA size selector, or NH3
permeation tube) is diluted by a zero air flow (with an option for adding
O3, VOCs, RH, or CO through the secondary addition port) and passed
through the TD oven. A portion of the flow is sampled prior to entering the
oven with one of several types of auxiliary measurements (CIMS for
N2O5; an ultra-high sensitivity aerosol spectrometer, UHSAS, for NH4NO3 particles; or commercial CRDS for
NH3). After flowing through a cooling region, the sample passes through
a particle filter and then is mixed with a ∼ 30 ppmv addition of
O3 in a mixing volume before entering through the optical cavity, where
NO2 is measured by CRDS.
The NOy TD oven inlet consists of a quartz tube (0.39 cm ID and 63 cm in
length, of which 38 cm is heated) wrapped in nichrome wire and insulated
with fiberglass. The flow rate through the inlet and optical cavity is
controlled by a mass flow controller on the downstream side of the optical
cavity. Because the standard flow rate is held constant during each
experiment, the volumetric flow rate, and therefore the TD residence time,
varies with oven temperature. For example, the 4.5 cm3 inner volume of
the oven results in an oven residence time of 30–100 ms at a flow rate of
1.9 slpm (standard liters per minute; at 273.15 K and 1 atm) for temperatures
from 25 to 650 ∘C. A flow rate of 1.9 slpm represents the normal operating
conditions of this instrument, but flow rates between 0.25 and 3 slpm were
tested, which provides oven residence times between 20 and 400 ms. The
temperature of the TD oven is monitored by a thermocouple mounted to the
outer side of the quartz tube and therefore is slightly lower than the
temperature of the gas. However, inserting a temperature probe into the
inner part of the TD inlet yields a temperature profile, shown in Fig. S1 in the Supplement,
which approaches the temperature set point by the end of the inlet. All oven
temperatures described hereafter refer to the measured thermocouple
temperature. After passing through the TD oven, the gas cools to room
temperature in the non-heated portion of the quartz tube, passes through a
particle filter (47 mm diameter, 1 µm pore size PTFE membrane) to
remove non-volatilized particles, and then enters a 15 cm3 mixing
volume prior to entering the CRDS cavity. There, O3 (∼ 30 ppmv after dilution) is added to the sampled air to convert any NO that
formed in the thermal dissociation to NO2. As the rate constant for the
NO + O3→ NO2+ O2 reaction is more than 3
orders of magnitude faster than the NO2+ O3→ NO3+ O2 reaction, conversion of NO2 to NO3 (and subsequently to
N2O5) is at most 1–2 % in this mixing volume and is corrected
for using a previously described method (Fuchs et al., 2009).
To measure the thermograms shown in this paper, the oven temperature was set
to a sequence of temperatures spanning 300 to 650 ∘C and spaced
by 25 ∘C in a random order. The measured NO2 concentrations
are averaged at each temperature set point for approximately 10–15 min.
NOy samples and additions
Samples of reactive nitrogen species (labeled as “NOy source” in Fig. 1) were introduced into the TD oven in several ways. HNO3 and NH3
were obtained by passing a 50 sccm flow of zero air through a calibrated 45 ∘C permeation tube containing HNO3 (VICI Metronics) or
NH3 (KinTek), providing
gaseous outputs of 64 and 23 ng min-1,
respectively (Neuman et al., 2003). Subsequent dilution in 0.5–4 slpm zero (synthetic) air resulted in HNO3 and NH3
concentrations of 5 to 40 ppbv. Because both these species readily adsorb to
instrument surfaces (Neuman et al., 1999), only fluorinated ethylene propylene (FEP) Teflon
tubing was used between the permeation tube and the TD oven, and all tubing was
kept as short as possible (typically less than 30 cm) and was wrapped in
100 ∘C heating tape to reduce losses to the walls. However, these
precautions were found to be unnecessary in this laboratory study, since the
constant flow from the permeation tube resulted in an equilibrium in which
the adsorption losses to the walls were equal to the rate of off-gassing.
NO was obtained by dilution of the output of a calibrated standard
(Scott-Marrin, 0.2 % in N2). N2O5 was synthesized via a
procedure adapted from Davidson et al. (1978) and Bertram et al. (2009),
which has been used as a calibration for the N2O5 channel of a CRDS
NO3 instrument (Dubé et al., 2006; Wagner et al., 2011). Pure
samples of NO and O2 were mixed to yield NO2, and this mixture was
reacted in a flow tube with excess O3, yielding NO3 which then
reacted with NO2 to form N2O5. The resulting mixture flowed
through a glass trap at -78 ∘C, where N2O5 solidified as
a white crystal. A gaseous sample of N2O5 was obtained by flowing
20–50 sccm of zero air over the solid -78 ∘C sample and then
diluting further in zero air. Gas-phase N2O5 prepared in this way
is known to contain variable but significant amounts of HNO3 (Bertram et
al., 2009), and thus efforts were made to minimize this interference by
baking all glassware for several hours before use and by distilling the
solid N2O5 sample regularly by bringing it to room temperature
under an O3 flow for 10 min. Nevertheless, some HNO3 was always
present in the sample, and therefore the output of the trap was passed
through a nylon wool scrubber prior to entering the TD oven, which removed
HNO3 without significantly perturbing the N2O5 concentration.
Finally, ammonium nitrate particles were generated by running a
0.1 g L-1 solution of
aqueous NH4NO3 through an atomizer and size-selecting particles of
a certain diameter with a custom-built differential mobility analyzer (DMA).
Conductive tubing, rather than Teflon, was used to minimize electrostatic
build-up and loss of particles to the walls before entering the TD oven.
In order to test whether common atmospheric gases would interfere with the
conversion efficiency, some additional species were added to the sample
prior to entering the oven. Water was added by passing the dilution zero air
through a water bubbler prior to mixing with the HNO3 sample. Various
amounts of O3 were added by running the dilution zero air through an
O3 calibrator (Thermo Fisher Scientific 49i) that is also capable of
generating up to 200 ppm O3 in 1–3 slpm of zero air. We also
investigated the effect of various VOCs, including a high concentration of
propane (∼ 5 ppmv) and a standard mixture of VOCs (Air
Liquide) consisting of n-hexane (1.234 ppm), propanal (0.397 ppm),
2-butanone (1.237), benzene (1.151 ppm), methylcyclohexane (0.938 ppm),
ethylbenzene (1.213 ppm), 2,2,4-trimethylpentane (1.186 ppm), isopropyl
benzene (1.148 ppm), and ethanol (0.994 ppm). This mixture is commonly used
to calibrate gas chromatography–mass spectrometry (GC–MS)
instruments but here provides common atmospheric species
with a range of masses, bond strengths, and degrees of oxidation. It was
diluted to 50 ppbv total VOCs by addition of zero air prior to entering the
oven. We also added CO in varying quantities to the HNO3 and NH3
samples.
Ancillary measurements
Several instruments were used as ancillary confirmation for some of the
NOy sample concentrations. In each case, a Teflon tee split the sample
input and a portion of the flow was pulled into the secondary instrument
prior to entering the TD oven, as shown in Fig. 1. In the case of NH3,
a Picarro G2103 NH3 analyzer with a manufacturer's specified 1 ppbv
detection limit at 5 s integration time was used. A custom-built iodide
adduct chemical ionization mass spectrometer (Lee et al.,
2014), described in further detail in Veres et al. (2015), was used to monitor
the N2O5 and HNO3 concentrations from the N2O5
solid sample prior to dissociation in the oven. In this instrument,
N2O5 and HNO3 mixed with I- ions produced by passing
CH3I through a 210Po source, and the resulting
HNO3 ⋅ I- and N2O5 ⋅ I- ions were
detected by quadrupole mass spectrometry at m/z = 190 and 235. This
measurement has a detection limit of 4 pptv and 70 pptv and error bars of
25 and 25 % (3σ) for N2O5 and HNO3,
respectively. Lastly, an ultra-high sensitivity aerosol spectrometer
(Droplet Measurement Technologies) was used to monitor the size distribution
of the size-selected ammonium nitrate particles (Cai et al.,
2008).
HNO3 and NH3 conversion efficiencies were also tested using
ambient air for dilution (rather than synthetic air), as sampled during
daytime in August 2016 in Boulder, CO. Ambient air was drawn into the two of
the four channels of the NOAA TD-CRDS instrument, through two side-by-side
identical quartz ovens heated to 650 ∘C at a flow rate of 1.4 slpm, and the output of either the NH3 or HNO3 permeation tube was
inserted directly into the exposed inlet of one of the ovens, for a duration
of approximately 6 min. The NO2 signal was measured by one of the
remaining channels in the NOAA TD-CRDS instrument, and the conversion
efficiency of each species was calculated by comparing the difference in
NO2 signal between the two ovens relative to the calibrated output of
the permeation tube to correct for small differences in NO2 signal
between the two ovens.
We also present results measured in the Berkeley TD-LIF instrument. It is
described in greater detail elsewhere (Day et al., 2002),
but briefly HNO3 and n-propyl nitrate samples were provided by
permeation tubes similar to those described in Sect. 2.2, diluted in dry
zero air, and passed through 20 cm heated length quartz ovens, held at
ambient pressure, at a flow rate of 2 slpm. This resulted in residence times
of approximately 50 ms. The NO2 released in the thermal conversion was
supersonically expanded into the detection region and measured by
laser-induced fluorescence from an individual rovibronic NO2 line. The
NOy conversion ratio was calculated as the measured NO2
concentration relative to the maximum NO2 signal at high temperatures,
as the oven temperature was changed at a rate of -10 ∘C per minute.
Box modeling
A simple kinetic box model was used to support the experimental findings.
Reaction rates for ∼ 60 reactions possibly involved in the
dissociation and secondary chemical reactions of each NOy species
(listed in the Supplement) were obtained from the Jet Propulsion Laboratory (JPL) Kinetics
Database (Sander et al., 2011) and the
NIST Chemical Kinetics Database (Manion et al., 2015) at
temperatures spanning the 25–650 ∘C range of the experimental
thermograms. For every HNO3, N2O5, and NH3 thermogram, a
simulation was run at each temperature, assuming a starting concentration of
the NOy species equal to that observed in the experiment and lasting
the duration of the residence time in the oven. The simulation was then
allowed to keep running at room temperature for an additional
∼ 1 s to mimic the conditions between the oven and the
instrument. During this additional low temperature time, 30 ppmv of O3
was added to the simulation to convert NO to NO2 as in the TD-CRDS
instrument. The final concentration of NO2 at the end of the simulation
was recorded for each temperature, which resulted in a simulated thermogram.
Several simplifying assumptions were made here. We assume instantaneous
heating and cooling of the sample and a uniform temperature profile along
the 38 cm length of the TD oven. We also only consider gas-phase reactions
and neglect any surface-mediated reactions. When possible, JPL-recommended
values for the rate constants were used, but many of those listed did not
span the full temperature range of the thermograms. When JPL values were not
available, reaction rates from the NIST database were used (see Table S1).
We also derive temperature-dependent wall loss constants for O and OH using
the procedure outlined by Thieser et al. (2016), but
we find that better agreement in some simulations can be achieved with the
experimental data by using an empirical value or no wall loss at all. As
can be seen in Sect. 3, these simulations successfully replicated a major
portion, but not all, of the experimental results, likely due to these
simplifications.
Results
HNO3 thermograms
Figure 2 shows the conversion efficiency of HNO3 to NO2 as a
function of temperature for several flow rates through the NOAA TD-CRDS.
Conversion efficiency was calculated as the measured NO2 mixing ratio
divided by the input HNO3 mixing ratio. The box model simulations for
each flow rate are shown as solid lines of corresponding color. The
HNO3 permeation tube has a calibrated output of 64 ng min-1, which
corresponds to an expected HNO3 concentration of between 5 and 40 ppbv,
depending on the zero air dilution required for each flow rate. The output
of the permeation tube was found to contain approximately 2.5 % NO2,
and all HNO3 thermograms have had this 2.5 % baseline signal
subtracted. At a flow rate of 1.9 slpm (where the oven residence time is 30–100 ms depending on temperature), we observe 100 % conversion of
HNO3 at oven temperatures above 600 ∘C, whereas the
thermograms obtained at 1 and 3 slpm reach a maximum conversion of
100 % at 550 and 650 ∘C, respectively. The 0.5 slpm thermogram
has a slightly lower maximum conversion efficiency (95 %), possibly due to
the recombination reaction of OH and NO2 during the extended time in
the “cooling region” prior to detection.
The box model simulations in Fig. 2 mimic the shape of the experimental data,
but some are slightly shifted to higher or lower temperatures, likely because
the simulation is extremely sensitive to the flow rate and may be affected by
the simplifying assumptions detailed in Sect. 2.4. The shape of the simulated
thermogram is entirely controlled by the reaction rate of the initial
dissociation reaction of HNO3 to NO2+ OH. This reaction has a
third-order rate constant of k0(T)=1.82×10-4×(T/298)-1.98×e(-24004/T) and a high-pressure limit of
k∞(T)=2×1015×e(-24054/T) (Glänzer and
Troe, 1974), and thus at a midrange temperature, such as 500 ∘C, the
HNO3 lifetime is approximately 250 ms. The inner volume of the oven is
4.5 cm3, and so at a flow rate of 1.9 slpm, the gas has a plug flow
residence time of 38 ms in the 500 ∘C oven, compared to a residence
time of 77 ms at 1.0 slpm and 153 ms at 0.5 slpm. The simulated
conversion efficiency in these midrange temperatures is therefore extremely
sensitive to the flow rate, in agreement with our experimental results.
However, the experimental 100 % conversion efficiency at high
temperatures indicates that there is virtually no recombination of OH and
NO2 once formed, because the recombination rate for OH + NO2 is
quite low and because OH radicals are far more likely to be lost to the
walls of the oven at a diffusion-limited rate determined by Day et al. (2002)
of ∼ 46 s-1 for 1/4 in (0.63 cm) OD tubing, which is far higher than the
pseudo-first-order recombination rate coefficient of 0.075 s-1 at
[NO2] = 10 ppbv. No attempt was made to dilute the output of the
HNO3 permeation tube any further, as recombination effects would likely
only be less important at lower starting HNO3 concentrations. Similarly,
increasing the starting NO2 concentration, to mimic conditions in highly
polluted environments, was not attempted in this set of experiments, but
increasing the starting NO2 concentration in the kinetic model up to
50 ppbv shows that there is no recombination expected even with elevated
NO2 in the oven. This is in contrast to ANs and PNs, for which the
reaction of the dissociated peroxy and alkyl radicals with NO2 is a
significant interference (Thieser et al., 2016), but in good agreement with
the HNO3 results of Day et al. (2002) and Sobanski et al. (2016).
HNO3 thermograms measured at several flow rates in the NOAA
TD-CRDS. Conversion efficiency is calculated as measured NO2 signal
relative to the expected concentration of HNO3. Parentheses in the
legend indicate the range of residence times experienced by the sample in the
heated inlet. The grey dashed lines indicate 0 and 100 % conversion.
Solid lines show simulations using a simple kinetic box model, as described
in the text.
At a flow rate of 1.9 slpm, we observe a ∼ 6 % conversion of
HNO3 to NO2 at an oven temperature of 400 ∘C. Although
this efficiency is specific to the conditions of the oven used here, it is a
key finding since 400 ∘C is in the vicinity of the temperature
set point chosen for selective detection of total alkyl and multifunctional
nitrates by TD-LIF (Day et al., 2002) and other TD
instruments. This result is in good agreement with Thieser et al. (2016), who found a ∼ 10 % HNO3 conversion at 450 ∘C. Sadanaga et
al. (2016) report a ∼ 15 % HNO3 conversion at 360 ∘C at a TD residence time of 3.4 s, which exceeds the range of
our study but follows the trend in Fig. 3. In a previous study (Wild et
al., 2014), we presented thermograms designed to demonstrate quantitative
conversion efficiency at high temperatures. The temperature dependence of
thermal conversion was not well constrained at lower temperatures and
showed, for example, 30 % conversion at 400 ∘C. As discussed by
Sobanski et al. (2016), the large conversion
efficiency presented by Wild et al. (2014) at this temperature is likely
incorrect. The extent of HNO3 conversion is dependent on the residence
time in the oven, but because residence time for a given flow rate changes
with oven temperature, it is easier to observe this effect by plotting
conversion efficiency versus residence time, as in Fig. 3, for five
different temperatures (350, 400, 450, 500, and 600 ∘C). This
plot represents transects through Fig. 2 at these five temperatures. Figure S2 shows a log scale plot to highlight the low conversion efficiency region.
Most instruments utilizing the TD oven technique use a set point between 350
and 450∘ and a residence time between 30 and 100 ms to selectively
detect ANs and not HNO3 (Day et al., 2003; Paul et al., 2009; Thieser
et al., 2016), but Fig. 3 demonstrates that there is significant variability
in the HNO3 conversion efficiency that depends nonlinearly on oven
residence time.
Conversion efficiencies of HNO3 to NO2 plotted as a
function of plug flow residence times in the oven (see text) for five different
temperatures. Values were obtained by scaling the measured conversion
efficiency in Fig. 1 to the overall maximum and minimum of the thermogram, to
account for slight differences between thermograms. Solid circles indicated
measurements at ambient pressure, whereas open circles indicate measurements
at low pressure. Different line traces indicated different temperatures. A
temperature set point between 350 (short dashed line) and 450∘ (long
dashed line) and a residence time less than 200 ms are the conditions
normally selected for selective detection of alkyl nitrates with no detection
of HNO3. However, under these conditions HNO3 conversion may be
anywhere between 1 and 30 %.
We further measure the effect of pressure on the conversion by placing a
heated stainless steel needle valve in front of the oven, thus lowering the
pressure inside the oven to 250 mbar. The low pressure transects for each of
the five temperatures can be seen in open circles in Fig. 3, and the full
thermograms are displayed in Fig. S3. The low pressure transects are
slightly lower than those at ambient pressure for the 450 and 500 ∘C set points but match reasonably well at low and high
temperatures, indicating that the onset and final conversion of HNO3
are not strongly sensitive to pressure. To ensure that HNO3 was not
lost on the walls of the stainless steel valve, the conversion efficiency
was measured with the valve fully open and was found to match that taken
with no valve. These experiments demonstrate the importance of verifying
that a given temperature set point and that flow rate is suitable for measurement
of alkyl nitrates without interference from HNO3 conversion.
To demonstrate the variability within individual TD ovens, an example of the
HNO3 conversion efficiency near the alkyl nitrate temperature set point,
as measured by the Berkeley TD-LIF instrument, is shown in Fig. 4. This
inlet's alkyl nitrate set point temperature was chosen to be just past the
plateau in the n-propyl nitrate signal at 410 ∘C. The HNO3
conversion to NO2 was found to be 2.5 %, which for most TD-LIF
experiments would be negligible compared to other uncertainties in measured
ANs (±15 %) and no correction would be applied. One example where a
correction was significant was for the NASA DISCOVER-AQ California
deployment, which took place in California's central valley during a period
of high NH4NO3 aerosol loading. Ratios of (HNO3+ NH4NO3) to ANs were high enough that a correction was necessary
and applied to both observations (Pusede et al., 2016).
As HNO3 is derived by subtraction of the ANs, any HNO3 conversion
at the AN temperature results in a high bias for ANs and an equal low bias
for HNO3. The sum of the two remains correct, independent of the onset
of the HNO3 conversion. The Berkeley group has found the HNO3
conversion to be oven dependent even for identical pressure and flow
conditions, indicating that some but not all ovens have impurities at the walls
that effectively catalyze HNO3 decomposition. Ovens with high HNO3
conversion efficiencies at low temperatures were discarded. These results
highlight the importance of careful evaluation and calibration of each TD
oven, even when the inner volumes and flow rates are similar.
HNO3 thermograms with additions
Tests for other interferences to HNO3 and AN measurements included
adding several different chemical species to the HNO3 sample prior to
entering the oven. These were designed to test the hypothesis that certain
trace gases found in ambient air would interact with radicals in the oven or
would themselves dissociate to form radicals which could react with NO,
NO2, OH, or HNO3. The results are shown in Fig. 5. In Fig. 5a, a
portion of the dilution air was passed through a distilled water bubbler
prior to diluting the HNO3, bringing the relative humidity up to
66 %. The change in RH does not alter the shape, onset, or total
conversion efficiency of the thermogram. This is to be expected, as the oven
temperature is not high enough to dissociate H2O to OH + H, and
reactions between H2O and the relevant species formed in the oven from
HNO3 dissociation are far too slow to be important here. However, it
should be noted that both H2O and HNO3 are sampled in this
experiment at a steady concentration, and it is possible that, during ambient
sampling, rapid changes in the RH or HNO3 concentration could change the
overall efficiency. Additionally, we did not test the conversion efficiency
at very high RH levels, and it is possible there could be a nonlinear effect
of water. Figure 5b shows the measured thermogram with the addition of
∼ 50 ppbv VOCs (described in Sect. 2.2) with and without the addition
of 90 ppbv O3 as well as the addition of 5 ppmv of propane, to mimic
conditions found in highly polluted wintertime atmospheres. If organic
radicals were produced thermally in the TD oven, they could potentially react
with NO2, thus altering that signal. However, the bond dissociation
energy of the C-H or C-C bonds most likely to thermally dissociate in each of
the VOCs are all significantly higher (typically
> 100 kcal mol-1) than that of the O-N bond in HNO3
(∼ 50 kcal mol-1), making it unlikely that organic radicals are
formed inside the oven from dissociation of VOCs. Reactions of unsaturated
hydrocarbons with O atoms or OH radicals tend to be rapid and would produce
organic radicals, but these tend to be unstable, and any stable radicals
would likely only react with NO2 to form ANs or PNs. The oven is set at
sufficiently high temperatures to dissociate ANs and PNs back to NO2 +
the organic radical. Addition of these VOCs does not affect the measured
conversion efficiency, even in the presence of ambient levels of O3.
Ozonolysis of the unsaturated hydrocarbons is slow enough (typically on the
order of 1×10-17 cm3 molecule-1 s-1) to not
have any effect here (we would expect < 0.0001 % reaction for
the duration of the oven residence time). An extremely high concentration of
propane also has no effect on the overall conversion efficiency, within the
error bars of the measurement, for the same reasons as detailed above.
HNO3 and n-propyl nitrate thermograms taken with the Berkeley
TD-LIF instrument used in the NASA DISCOVER-AQ California mission.
(b) Only HNO3 with the y axis expanded to illustrate the
dissociation onset. The oven is from the instrument's alkyl nitrates channel.
The flow rate was approximately 2 slpm, and the measurement set point was
410 ∘C. The dataset was corrected for the 2.5 % dissociation of
HNO3 in the alkyl nitrates channel. A different oven was used for
HNO3 at a set point of 620 ∘C.
HNO3 thermograms (1.9 slpm, ambient pressure) taken with the
NOAA TD-CRDS, with various additions added prior to the TD oven. In each
frame, the black solid circles indicate the case without additions.
(a, b) No effect is observed when the thermogram is taken at high
relative humidity or when VOCs are added. (c) Varying amounts of
O3 were added, ranging from ambient levels (75 ppbv) to extremely
polluted levels (1200 ppbv), which decreases the overall conversion at high
temperatures. (d) The addition of 400 ppmv CO alters the shape of
the thermogram.
Figure 5c shows the addition of both small and large quantities of O3 to
the HNO3 sample. Small quantities do not change the onset or overall
conversion efficiency, but larger amounts of O3 reduce the conversion
efficiency at high temperatures. The kinetic box model does not predict this
reduction, as it predicts 100 % conversion efficiency to NO2 at all
O3 levels. The dominant bimolecular reaction of O3 in the model is
the reaction with NO2 to make NO3, but, since these reactions are
occurring at high temperature, any NO3 formed will immediately
dissociate to NO2 (see Sect. 3.3). O3 also thermally dissociates
to O + O2 at temperatures above 200 ∘C (see Fig. S4), but
the dominant fate of the O radicals should be loss to the walls. Of the O
atoms that are not lost to the walls, their primary reaction is also with
NO2 to form either NO + O2 or NO3, but NO should be
converted back to NO2 after the oven. Nevertheless, there is an apparent
reduction in the conversion of HNO3 to NO2 with increasing O3.
While the O3 concentration range in Fig. 5c exceeds that found in
ambient air, highly polluted areas may have large enough O3
concentrations to make this reduction in conversion efficiency significant.
Finally, the addition of 400 ppmv CO in Fig. 5d has a marked effect on the
onset, shape, and final conversion of the HNO3. This addition was tested
because gold catalytic NOy converters require a 1 % CO addition to
drive the dissociation forward. We find that ∼ 0.5 % CO is
sufficient to promote HNO3 dissociation even in the absence of a gold
catalyst. However, our kinetic model does not replicate the results of the CO
addition. Since the rate-limiting step in these thermograms is the initial
dissociation of HNO3, it is unlikely that the reaction between CO and OH
or NO2 plays a role here. It must therefore be caused by a reaction
which changes the rate kinetics of the initial dissociation step. However, to
our knowledge there have been no laboratory kinetics studies on the
CO + HNO3 reaction. It is likely that there is some surface reaction
that affects the HNO3 conversion in the presence of CO.
We also note that previous work on TD ovens (Day et al., 2002; Thieser et
al., 2016) has cautioned that the elevated temperature of the oven may
accelerate the reaction between ambient levels of NO and O3 to generate
NO2, thereby creating an NO2 signal that is in fact due to ambient
levels of NO. This issue does not affect the TD-CRDS NOy detection
scheme, as excess O3 is intentionally added to the mixing volume after
the oven to convert NO to NO2 to measure total NOy. Nevertheless,
we have investigated how NO responds in the oven, and the results are shown
in Fig. S5. A 15 ppbv NO sample was passed through the oven. When no excess
O3 is added to the mixing volume, no NO2 signal is seen, and, when
mixing volume O3 is added, full conversion of NO to NO2 is
observed, as expected. However, when 100 ppbv of O3 is added to the
oven (with no mixing volume O3 addition), an approximately 2.2 ppbv
NO2 signal was observed, or a 15 % conversion. This is consistent
with the kinetic rate expressions for NO + O3 and NO + O, but we do
not differentiate between these two mechanisms in these experiments, as
O3 will always form O at the elevated oven temperatures.
N2O5 thermograms
Figure 6 shows the measured thermogram of N2O5 in the NOAA TD-CRDS
at ambient pressure and flow rates of 1.9 and 1.0 slpm, with the kinetic
model simulations for each flow rate shown in solid and dashed lines. Two
distinct dissociation steps are observed and confirmed by the kinetic model:
one between 30 and 110 ∘C corresponding to the dissociation of
N2O5 to NO2+ NO3 and one above 300 ∘C
corresponding to the dissociation of NO3. The N2O5 synthesis
method also produces HNO3 (Bertram et al., 2009), and, because the bond
enthalpies of NO3 and HNO3 dissociation are similar (both
∼ 50 kcal mol-1), the thermograms of these two species are
expected to overlap at high temperatures. Thus, a nylon wool scrubber was
used to remove HNO3, and the scrubbed sample was simultaneously
monitored with an iodide chemical ionization mass spectrometer, described in
Sect. 2.3, to ensure the HNO3 (and not the N2O5) was
completely removed. The flow rate was lowered to 1.0 slpm in the high
temperature scans to accommodate both instruments with a better signal-to-noise ratio. The CIMS measured
approximately 120 pptv HNO3, possibly due to hydrolysis of
N2O5 after the scrubber, and thus more than 99.5 % of the
NO2 signal we observe is attributed to N2O5.
Thermogram of N2O5 measured in the NOAA TD-CRDS at two
flow rates. The red squares and red dashed line show the 1.9 slpm thermogram
and simulation, while the blue circles and blue solid line show the analogous
result at 1.0 slpm. The first dissociation corresponds to N2O5→ NO2+ NO3 and the second to NO3→ NO2+ O. The
second curve reaches a maximum of 200 %, while the first reaches
90–95 %, depending on the flow rate, due to recombination of NO2
and NO3 in the cooling region prior to the detector region. The black
dashed line is the experimental HNO3 thermogram from Fig. 2, offset by
100 %. The green triangles indicate measurements of the conversion
efficiency without the O3 addition, confirming that the second
dissociation must occur via NO3→ NO2+ O rather than NO3→ NO + O2.
At high temperatures, each N2O5 is expected to produce two
NO2 molecules. Conversion efficiency is calculated from the measured
NO2 concentration relative to the N2O5 concentration measured
by the CIMS instrument, which samples prior to the TD oven. However, the
CIMS instrument requires an empirical calibration factor for any species it
measures, and, while the HNO3 signal may be calibrated using the
permeation tube described in Sect. 3.1, there was no independent calibration
available for N2O5 – only the signal measured using the TD-CRDS
instrument. Therefore, the CIMS N2O5 signal was assumed to
correspond to a 200 % conversion efficiency in the TD-CRDS at 650 ∘C, and the relative conversion was measured at lower
temperatures. The first dissociation step of N2O5 to NO2 and
NO3 is expected at oven temperatures above 110 ∘C, but,
because the sample must then travel through a cooling region prior to
entering the CRDS optical cavity (see Fig. 1), approximately 10 % of the
NO2 and NO3 is expected to recombine back to N2O5, based
on the rate constant and the residence time in the mixing volume. This
behavior has been well characterized previously (Fuchs et al.,
2009) and is accounted for in the data analysis, and as expected we observe
a 91 % conversion efficiency of N2O5 to NO2 between 110 and
300 ∘C. At higher temperatures, NO3 dissociates in the oven
before recombining with NO2, and thus a 200 % conversion efficiency
is observed. While this is not an absolute measure of conversion efficiency,
the relative conversion efficiency is consistent with N2O5
dissociation and recombination reaction rates to generate two NO2
molecules in a distinct stepwise manner. At 150 and 400 ∘C, the temperature set points often used for detection of PANs
and ANs, we find 90 and 105 % conversion of N2O5 to
NO2, respectively. The exact values are highly dependent on the
residence time in both the oven and in the cooling region but serve to
highlight the importance of characterizing the N2O5 response in
every thermal dissociation oven.
We also measured the conversion of N2O5 without the mixing volume
O3 addition at two relevant temperatures in order to determine the
mechanism for NO3 dissociation. These data are shown in green triangles
in Fig. 6 and show no difference in onset or maximum conversion efficiency,
whether or not mixing volume O3 is added. As the mixing volume O3
converts ambient or thermally produced NO to NO2, the similarity of the
two spectra indicates that the NO3 dissociation mechanism must be
NO3→ NO2+ O. However, there are no published rate expressions
for this reaction, and the few studies on NO3 thermal dissociation have
disagreed about whether the reaction proceeds to NO + O2 (Johnston et
al., 1986) or NO2+ O (Schott and Davidson, 1958). The former argued
for the NO3 → NO + O2 mechanism based on thermodynamics,
as this reaction is exothermic. However, this implies that NO3 would be
thermally unstable at room temperature, which is not the case. It is likely
that there is a significant energy barrier to this reaction. The bond
enthalpy of the NO3→ NO2+ O reaction, on the other hand, is
50.4 kcal mol-1, nearly identical to that of HNO3→ NO2+ OH, and the two thermograms are very similar in shape and are centered at
the same temperature (500 ∘C). The simulation shown in Fig. 6 is a
fit rate expression of k(T)=1×10-2×(T/298)9×exp(-1500/T) obtained by taking the rate expression of HNO3
dissociation and iteratively adjusting it until it matched the data.
Essentially identical results were observed in the TD-LIF instrument
(R. C. Cohen, personal communication, 2016).
NH4NO3 thermograms
NH4NO3 particles were generated in situ from an aqueous solution,
dried, and size selected by a differential mobility analyzer set at
250 nm prior to entering the TD oven. The conversion efficiency was
calculated by comparing the measured NO2 concentration in the TD-CRDS
instrument to the expected number of NH4NO3 molecules in the
aerosol particles, derived from the number and size of the aerosol particles
as measured with an ultra-high sensitivity aerosol spectrometer (UHSAS). The
measured UHSAS histogram was used, along with the literature value for the
density of NH4NO3, to convert particle diameter to particle
volume and then to the total number of NH4NO3 molecules. We
demonstrate here that the dissociation pathway is NH4NO3→ NH3+ HNO3, and we assume that NH3 is not converted in any
significant fraction. A temperature-dependent baseline NO2 signal is
observed when the DMA voltage is set to zero (i.e. when no particles are
transmitted), which is attributed to gas-phase HNO3 molecules which have
evaporated from the particles and adsorbed to the tubing walls, and which
are subtracted from the total signal. Figure 7 shows the measured thermogram
of NH4NO3 with the thermogram of gas-phase HNO3 from Fig. 2
overlaid. The close agreement between the two thermograms demonstrates that
the dissociation pathway is NH4NO3→ NH3+ HNO3
and that this reaction is rapid at the temperatures reached in the TD inlet.
For particles that pass through the DMA at a given size set point, the UHSAS
measures a size histogram that peaks at a diameter approximately 8 %
lower, likely because the NH4NO3 particles are slightly
non-spherical, and therefore the electrical mobility diameter is slightly
larger than the geometric diameter. This phenomenon has been discussed at
length elsewhere (DeCarlo et al., 2004), and we make no
attempt to further characterize NH4NO3 particle behavior in the
DMA – we have simply taken the UHSAS histogram data to calculate the
particle volume, even though this is also subject to slight differences
based on the refractive index of NH4NO3. However, if the TD oven
were to fail to volatilize and convert all NH4NO3 particles to HNO3
and then to NO2, the measured thermogram would deviate from the
HNO3 spectrum at lower temperatures, where perhaps the heat is not
sufficient to drive the NH4NO3 out of the condensed phase. The
close match between the two is a good indication that the conversion goes to
completion. Additionally, Figs. S6 and S7 show a sample NO2 measurement
measured by TD-CRDS at 650 ∘C as the particle diameter set point
is changed. There is no correlation between particle size and conversion
efficiency, indicating that the oven is completely converting all particles
without a size dependence.
Measured thermogram of NH4NO3 particles as solid circles
from the NOAA TD-CRDS. The black solid line indicates the measured thermogram
of gas-phase HNO3 at a 1.9 slpm flow rate (from the gold squares trace
in Fig. 2). The close match of these two thermograms indicates that the
NH4NO3 particles go through HNO3 as an intermediate and is a
good indication that complete conversion is achieved.
NH3 thermograms
A previous study (Wild et al., 2014) investigated whether ambient levels of
ammonia would represent an interference to NOy conversion and found
that it made at most a 1 % difference to the NO2 signal in dry air,
but that this effect was suppressed when RH > 10 %. We find
in the present study that there is a significant interference when ambient
levels of both NH3 and O3 are present in the oven, but that this
effect is potentially suppressed by other species found in ambient sampling.
Figure 8 shows a thermogram of NH3 with and without 100 ppbv O3
present in the oven. The conversion of NH3 to NO2 at
650 ∘C, calculated as the observed NO2 signal relative to the
added NH3 concentration, is small without O3. This is consistent
with the previous study of Wild et al. (2014). However, when 100 ppbv of
O3 is added, the thermogram reaches a maximum molar conversion
efficiency of 8 %, with an onset near 400 ∘C (red circles). In
contrast to the HNO3 thermograms, however, this signal does not appear
to plateau at 650 ∘C but rather continues to grow at higher
temperatures. This result is similar to the interference reported by Dillon
et al. (2002), which was attributed to a reaction between NH3 and
O3. The interference is only present when O3 is added to the mixing
volume, indicating that the conversion of NH3 must be producing NO,
rather than NO2, and is subsequently unimportant to instruments that
measure NO2 only, such as TD-LIF instruments. A kinetic model simulation
of both experiments is shown in solid lines in Fig. 8. This simulation was
carried out with 35 relevant reactions between NH3, O3, and the
radicals that are formed from these two species in the oven, with the most
important reactions listed below. The reaction between NH3 and
O3 is far too slow to be relevant here, and the oven temperature is not
high enough to dissociate NH3 to NH2+ H (ΔH = 108 kcal mol-1). However, O3 dissociates readily at oven
temperatures above 200 ∘C, and once formed the O atoms may react
with NH3 to form NH2.
O3→O2+ONH3+O→NH2+OHNH3+OH→NH2+H2O
The reactions of NH3 are the slowest steps, but once formed NH2
reacts readily with O atoms.
NH2+O→HNO+H→OH+NH→NO+H2
HNO then reacts with O, OH, and H to form NO or can also directly
dissociate to form H + NO.
HNO+O→OH+NOHNO+OH→H2O+NOHNO+H→H2+NOHNO→H+NO
The OH and H atoms formed in reactions (R3) and (R4) then drive Reaction (R2) further.
This mechanism takes place entirely in the gas phase and does not take into
account any surface-mediated reactions. Many of these reactions have only
limited published studies, so the simulation used rate constants that have
not been extensively tested. Additionally, to achieve a significant
conversion of NH3 to NO, it was necessary to decrease the O and OH wall
loss constants in the model. This rudimentary simulation predicts the
initial signal increase starting at 300 ∘C, though it has a
maximum conversion efficiency of just under 2 %, which is below that
observed in the experiment. In Fig. 9, we adjusted the amount of added
O3, while monitoring the conversion efficiency of NH3 to NO2 at an inlet temperature of 650 ∘C. We find that increasing the
O3 increases the conversion, which is consistent with NH3+ O
being the limiting reaction to make NH2. Figure 9 also demonstrates
that the conversion of NH3 is partially quenched by the addition of
ambient levels (∼ 100 ppbv) of CO, likely because the CO + O → CO2 reaction competes with those in Reaction (R2). Figure 10 shows
that the average conversion efficiency of NH3 when measured in ambient
air in Boulder, CO, in August 2016 (which contains 40–60 ppbv O3,
> 80 ppbv CO, ∼ 15 % RH, and other species) is
0.5 ± 2.4 %, or zero to within the 1σ error from repeated
measurements. This is in contrast to the conversion efficiency of HNO3
in ambient air, shown in the upper right frame of Fig. 10, which is largely
unchanged from that measured in zero air. Thus, constituents present in
ambient air, such as methane, CO, and water, are possibly suppressing the
conversion of NH3 to NO, likely through the reaction with O atoms.
Thermogram of NH3 taken with the NOAA TD-CRDS, with 100 ppbv
of O3 added before the oven shown as red circles. The blue circle
represents an analogous measurement at 650 ∘C with no O3 added.
Kinetic box model simulations shown in solid lines of corresponding color.
Conversion efficiency of NH3 to NO2 as a function of
O3 added to the TD inlet. Red circles show 10 ppbv NH3 with
O3 ranging from 0 to 300 ppbv, and the green and blue traces show similar
data, but with 100 and 2000 ppbv CO added. The partial depletion of the
signal (∼ 25 %) with the addition of CO indicates that the oxygen
atoms formed from O3 pyrolysis are reacting with CO instead of
NH3.
Measurement of HNO3 and NH3 conversion in ambient air at
an inlet set temperature of 650 ∘C. (a) Measured conversion
efficiencies for standard additions of HNO3 and NH3 to the NOAA
TD-CRDS inlet sampling ambient air in Boulder, CO, on 9 August 2016.
(b, c) Time series of measured NOy during standard additions.
The data are the difference between two NOy measurement channels, one
with and one without the standard addition, to cancel the variation in
ambient NOy during the tests.
Discussion
Using a thermal dissociation cavity ring-down spectrometer, we have
quantitatively added reactive nitrogen species to the TD inlet in order to
test the efficiency of the thermal conversion of each species to NO2
and the effect of any interferences from other trace gases which may be
present in the ambient troposphere. We have determined that the TD-CRDS
converts HNO3, N2O5, and NH4NO3 particles to
NO2 with 100 % efficiency at temperatures above 600 ∘C,
but that the onsets of the dissociation are highly dependent on oven
residence time. Despite their similar residence times, the NOAA TD-CRDS and
Berkeley TD-LIF instruments measure HNO3 conversion efficiencies
ranging from 2.5 to ∼ 8 % at 410 ∘C. It is
therefore important that the oven residence time is well characterized in
instruments designed to selectively detect ANs without interference from
HNO3. Even two TD ovens with identical inner volumes may exhibit
different response functions if they have different ratios of surface area
to volume.
We find that high levels of ambient O3 (> 500 ppbv) and CO
(> 400 ppmv) significantly changed the final conversion
efficiency and the onset of the conversion, respectively, of the HNO3
thermogram but that ambient levels of a group of representative VOCs and
high RH did not affect the measured thermogram. Modest levels of O3
converted a portion of NH3 to NO2. The conversion mechanism
likely arises from a gas-phase reaction between oxygen atoms and NH3
which produces NO. To our knowledge, the NH3+ O3 reaction in TD
ovens has not been studied in detail, but previous studies of NH3
conversion in catalytic converters have noted similar results to those
presented here – water and CO suppress the NH3 conversion to NO, while
O3 enhances it (Fahey et al., 1985; Kliner et al., 1997). If not
quenched by other species present in ambient air, this effect could
represent a potentially significant interference in field sampling for
instruments that are sensitive to NO directly or via conversion to
NO2. For example, at 50 ppbv O3, the 6 % conversion of NH3
would present an interference of more than 10 % if NH3 / NOy > 1.7, which is not an uncommon condition in agricultural
regions. This signal was suppressed in ambient air, indicating that NH3
may not interfere with NOy under most conditions. However, ambient air
in Boulder is not representative of all sampling conditions, and, since the
species responsible for quenching the reaction remains unclear, more work
must be done to better understand the mechanism of the NH3 / O3
thermal reaction. This result, along with the others detailed above, serves
to emphasize that great care must be taken to characterize the potential
interferences in TD NOy-conversion ovens.
The measured N2O5 thermogram exhibits a double dissociation curve,
corresponding to the initial dissociation of N2O5 to NO2 and
NO3 and the subsequent dissociation of NO3. Our results indicate
that the mechanism of the second step is NO3→ NO2+ O, in
contrast to earlier literature that reported NO3→ NO + O2
as the dominant mechanism. To our knowledge, this is the first published
thermogram of NO3. TD-NOy instruments often operate in the daytime
when N2O5 is not a significant fraction of NOy, though some
groups have operated at night and have typically assumed complete conversion
to NO2+ NO3 at the TD inlet set point for PNs (Di Carlo et al., 2013) and complete
conversion to 2NO2+ O at the set point for HNO3 (Wild et al.,
2014). These results confirm that there is approximately quantitative
conversion at these set points, though there are slight deviations from
100 % conversion near the PN set point. Therefore, care must be taken to
select a set point carefully and ensure complete conversion at that
temperature. However, this interference would only be significant during
nighttime or during very cold weather sampling.
The thermogram of particulate ammonium nitrate matches the thermogram of
HNO3, within the margin of error of the UHSAS measurement. TD ovens
have not typically been used explicitly for particle detection, with a few
exceptions (Voisin et al., 2003; Smith et al., 2004; Rollins et al., 2010),
though very fine particles may be sampled by the inlet, unless they are
excluded aerodynamically or physically. These results demonstrate that the
volatile portion of the particulate ammonium nitrates will be driven into
the gas phase at low oven temperatures, consistent with
Rollins et al. (2010), who used a denuder to remove gas-phase
nitrates and to detect aerosol organic nitrates in a 325 ∘C oven.
Their results indicate it is likely that particulate organic nitrates would
be converted to NO2 with 100 % efficiency in the NOAA TD-CRDS, but
this result has not been explicitly tested here. Other NO3 salts might
also be detected via thermal dissociation, although it is expected that they
would be non-volatile at the temperatures of these TD-inlets.
Bertram and Cohen (2003) examined NaNO3 and determined that
those particles would not be detected in TD inlets. However, these studies
measured pure aerosols, and results may vary with heterogeneously mixed
particles with multiple components. The initial dissociation of
NH4NO3 will produce an NH3 molecule in addition to an
HNO3 molecule, which means that particles may be subject to the same
NH3 / O3 interference when sampling in ambient air, which was not
considered in this study. Additionally, the particles sampled in this paper
were generated and injected directly into the inlet. The efficiency of
particle sampling in ambient air will depend on particle size and inlet
design, particularly during aircraft measurements. In future studies, a TD
inlet that either effectively samples aerosol or effectively excludes
aerosol (such as a cyclone), or a combination of the two, could be used to
specifically measure aerosol nitrates, which may make up a substantial
fraction of NOy, particularly in polluted wintertime urban atmospheres.
Based on the results of this paper, we make the following three
recommendations. (1) TD ovens should be characterized with the appropriate
reactive nitrogen compounds regularly at the oven set points using the oven
residence time and gas pressure that will be used in ambient sampling. This
is especially important given the findings of the Berkeley group regarding
impurities found in otherwise identical ovens, as discussed in Sect. 3.1.
(2) In addition to the AN and PN calibrations recommended by Day et al. (2002), Thieser et al. (2016), and others, these calibrations should include
HNO3. HNO3 calibration will be especially important if sampling in
regions where HNO3 is in large excess over other NOy species.
(3) Potential non-NOy species such as NH3 should also be regularly
introduced into the inlet under conditions where O3 is present in
ambient air to check for potential conversion. These recommendations are
similar to those detailed in Bradshaw et al. (1998). The
results of Fig. 9 indicate that calibration results may also vary
significantly when sampling in ambient air, due to the large number of
possible gas-phase reactions available to the wide variety of trace
atmospheric species. The last step is particularly important in instruments
that detect NO as well as NO2. Comprehensive calibration of these
interferences is key to these
instruments' NOy measurement accuracy, which in turn will provide
valuable information about tropospheric NOx chemistry.