Introduction
The nitrate radical (NO3) and dinitrogen pentoxide
(N2O5) are the most important reactive nitrogen species in
night-time chemistry (Wayne et al., 1991). The NO3 is dominantly formed
by the reaction of NO2 with O3 (Reaction R1), which contributes to
the night-time oxidation of volatile organic compounds (VOCs; Reaction R2)
and the production of the organic nitrate (Fry et al., 2009; Riemer et al.,
2003). NO3 can react with NO rapidly (Reaction R3), and NO3 absorbs
red light effectively by its strong B2E′-X2A2′ electronic
transition centred around 662 nm (Yokelson et al., 1994). Such strong
absorption in the visible light range promotes the development of optical
instruments applied in ambient NO3 detection. After formation, NO3
holds a thermal exchange with N2O5 (reactions R4a and b), which is
defined by the NO2 concentration and the ambient temperature (Brown et
al., 2003a). The heterogeneous reaction of N2O5 on ambient aerosols
plays an important role on the NOx removal from regional to global
scales (Brown et al., 2006; Brown and Stutz, 2012; H. C. Wang et al., 2015)
and shows a potentially significant impact on the ambient ROx chemistry
and photochemical ozone productions through halogen activation (Reaction R5;
Osthoff et al., 2008; Thornton et al., 2010; Phillips et al., 2012; Tham et
al., 2016).
NO2+O3→NO3,NO3+VOCs→ products,NO3+NO→2NO2,
NO2+NO3+M→N2O5+M,N2O5+M→NO2+NO3+M,
N2O5+(H2O or Cl-)→(2-ϕ)NO3-+ϕClNO2.
NO3 is a radical with a short lifetime, high reactivity, and an extremely
low mixing ratio in ambient air (Wayne et al., 1991), which means that field-deployable NO3 measurement techniques should feature high sensitivity,
high selectivity, and high temporal and spatial resolutions. The requests
are less rigorous for detection of N2O5, since, in general, it has
higher concentrations and less reactivity than NO3.
There are several existing methods based on optical spectroscopy and mass
spectrometry for the in situ detection of NO3 and N2O5. With
respect to the optical approaches, NO3 detection is based on its strong
absorption around 662 nm (Yokelson et al., 1994), and N2O5 can be
measured following thermal dissociation to NO3. Since the 2000s, cavity
ring-down spectroscopy (CRDS) has been used in the field measurement of both
NO3 and N2O5 (Simpson, 2003; Brown et al., 2002; Dubé et
al., 2006; Nakayama et al., 2008; Schuster et al., 2009). CRDS has high
temporal and spatial resolution with high sensitivity and accuracy.
Cavity-enhanced absorption spectroscopy (CEAS) was proposed later by Fiedler
et al. (2003) and has been successfully deployed to measure a number of
atmospheric trace gas compounds like HONO, H2O, IO, O3, O4,
I2, IO, OIO, SO2, NO3, N2O5, glyoxal (CHOCHO), and
methylgloxal (CH3COCHO; Washenfelder et al., 2008, 2013, 2016; Thalman
and Volkamer, 2010; Gherman et al., 2008; Axson et al., 2011; Kahan et al.,
2012; Min et al., 2016). The measurement of NO3 was shown to be
successful in simulation chamber conditions with an open-path incoherent
broadband CEAS setup (Venables et al., 2006; Varma et al., 2009), and shortly
afterwards, the closed cavity type of IBBCEAS was successfully deployed on
the ground (Langridge et al., 2008; Benton et al., 2010) and airborne
(Kennedy et al., 2011) for measurements of both NO3 and N2O5.
According to comparison experiments at the SAPHIR chamber (Fuchs et al.,
2012; Dorn et al., 2013), the CEAS technique shows a similar detection
capability for N2O5 and NO3 to that of CRDS (e.g. Brown et
al., 2003b, 2006; Benton et al., 2010; Kennedy et al., 2011; Crowley et al.,
2010; Sobanski et al., 2016). For the laser-induced fluorescence (LIF), its
detection sensitivity is, in general, smaller than that of the
cavity-assisted absorption techniques due to the low fluorescence quantum
yield of NO3 (Matsumoto et al., 2005). In addition to optical
approaches, different chemical ionization mass spectrometry (CIMS) methods
have been used for the detection of ambient N2O5 (Slusher et al.,
2004; Fortner et al., 2004; Kercher et al., 2009; Chang et al., 2011).
Slusher et al. (2004) utilized ion reaction (I-+ N2O5→ NO3-) to detect N2O5 at 62 amu (NO3-).
Nevertheless, this approach showed cross sensitivity towards NO3 (I-+ NO3→ NO3-) and additional interference from species
like ClONO2 and BrONO2. A strong unknown interference at 62 amu
was found for the detection of N2O5 under a high-NOx regime in
Hong Kong (Wang et al., 2014). Kercher et al. (2009) introduced an
ion-molecule region (IMR) module wherein the ion reaction, I-+ N2O5→ I(N2O5)-, is enhanced so that
N2O5 can be detected specifically at 235 amu. With this method, a
direct measurement of N2O5 is achieved, showing a good comparison
with the well-established CRDS system in Hong Kong (Wang et al., 2016).
Until now, field measurements of NO3 and N2O5 have been
extensively conducted in both the United States of America (USA) and Europe
but have been sparse in China (i.e. Brown et al., 2006; Crowley et al., 2010;
Benton et al., 2010), with only a few conducted in Hong Kong, Shanghai, and
the North China Plain (Wang et al., 2016, 2013; Brown et al., 2016; Tham et
al., 2016). From satellite observations, it was found that the USA, Europe,
and China are the three major high-NOx regions worldwide (e.g. Richter
et al., 2005). Moreover, in the North China Plain areas, the high-NOx
air masses often overlap with high aerosol loadings from both secondary
aerosol particle formation as well as nearby natural sources (e.g. dust from
the Gobi Desert in the spring) and serve as ideal locations for the study of
NO3 and N2O5 chemistry. To probe such potentially interesting
chemistry in China, we developed a new light-emitting diode (LED)-based
IBBCEAS (incoherent broadband cavity-enhanced absorption spectrometer)
instrument for the detection of NO3 and N2O5. In this study,
the detailed setup of our instrument, lab characterizations, and its first
field applications in Beijing are presented.
A schematic of the newly developed IBBCEAS instrument for the
detection of NO3 and N2O5. (a) Overview of the
optical layout (LEDs, collimating optics, high-finesse cavity, and
spectrometer) and the flow system (aerosol filter, inlet, NO titration
module, preheating tube, and detection cell). (b) The schematic
layout of the mirror mounts, which enables a mechanical alignment of the
high-reflectivity (HR) mirrors. (c) A photograph of the mirror
mounts. (d) The schematic layout of the NO titration module; the red
arrow denotes the N2 gas flow, and the blue arrow denotes the NO gas
flow.
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The instrument
Our IBBCEAS instrument is designed to measure the ambient NO3 and
N2O5 and features a small size, easy portability, and low power
consumption. The total weight is less than 25 kg, approximate dimensions are
95×40×25 cm, and the power consumption is less than 300 W,
which potentially meets the requirements for future applications on mobile
platforms. The system is distinct from previous IBBCEAS systems
(Langridge et al., 2008; Schuster et al., 2009; Kennedy et al., 2011) because
of its rigid cavity design, fast setup, and stable operation (e.g. the
thermal alignment drift is minimized) in field campaigns. A dynamic NO
titration setup was used to obtain the reference spectrum, which removed the
influence of ambient water vapour so that the fitting precision was
significantly enhanced.
Optical layout
The schematic layout of the instrument is shown in Fig. 1a. The optical
layout consists of a temperature stabilized light source, collimating optics,
and a commercial spectrograph with a charge-coupled device (CCD) detector. The light
source and collimating optics were concentrically integrated on an aluminium
profile (75×8×5 cm).
A single-colour LED (LZ1-10R200, LED Engin, Marblehead, MA, USA) is used as
the light source and is mounted on a three-dimensional (3-D) adjustable
bracket. The manufacturer-specified full luminosity output is about 800 mW,
centred at the deep red light region (660 nm), and the full width at half
maximum (FWHM) is 25 nm. To minimize the wavelength shift and intensity
drift caused by the LED temperature drift, the LED plate is mounted on an
aluminium block and uses a thermoelectric cooler
control module to stabilize the temperature of the aluminium block at
17.5 ± 0.1 ∘C. The aluminium block is thermally insulated to
reduce heat exchange with the ambient surroundings.
Before light is emitted into the high-finesse cavity, a plano-convex lens (f=30 mm) is concentrically installed into the lens tube (shown in Fig. 1a
as L1) to collimate the red light into the high-finesse cavity. The
high-finesse cavity is formed by a pair of high-reflectivity (HR) mirrors
(102116, Layertec GmbH, Mellingen, Germany) with a diameter of 25.0 mm
(+0.00/-0.10 mm). The peak reflectivity of the HR mirrors at 660 nm is
reported to be larger than 99.99 % with a radius curvature of
100 ± 5 cm. The two HR mirrors are mounted on two customized lens
tubes, and the lens tubes are mechanically mounted on two matching lens tube
holders, which are installed on an aluminium profile to maintain the distance
between the two mirrors at 50.0 cm. Due to the high-precision machining and
assembly, the two lens tubes (shown as the scheme in Fig. 1b and the
corresponding photograph in Fig. 1c) and the HR mirrors are mechanically
aligned to have concentricity (< 0.01∘). Each HR mirror is
continuously purged by 100 mL min-1 high-purity nitrogen flow to
prevent particle contamination by the sample gas flow.
The optical cavity is enclosed by a sample gas detection cell with a sample
inlet, outlet, and two welded corrugated pipes connected at two ends. The
light exiting the cavity is further imaged by a plano-convex lens (f=50 mm) installed on the lens tube (shown in Fig. 1a as L2) that couples the
output light onto the lead of a 100 µm diameter, 0.22 numerical
aperture optical fibre (QP100-2-UV-VIS, Ocean Optics, Dunedin, FL, USA). The
lead of the fibre is mounted on a 3-D adjustable bracket and integrated on
the aluminium profile. The other lead of the fibre directs the cavity output
light into the spectrometer (QE65PRO, Ocean Optics, Dunedin, FL, USA). The CCD in the
QE65000 spectrograph is thermally regulated at -20.0 ∘C to
minimize the dark current. The line density of the diffraction grating of the
spectrometer is 1200 mm-1, the entrance slit width is 100 µm,
and the spectral resolution FWHM is 0.85 nm with the wavelength coverage of
580–710 nm. The instrument works under a signal-to-noise
ratio estimated to be larger than 500 : 1.
Flow system
The instrument sample flow system includes the aerosol filter, the inlet
tube, the preheating tube and heated detection cell, and sensors for
temperature, pressure, and relative humidity. During the field measurements,
we operate with a sample flow rate of 2.0 L min-1. A Teflon
polytetrafluoroethylene (PTFE) filter
(25 µm thickness, 4.6 cm diameter, 2.5 µm pore size,
Typris, China) is used in the front of the sample module to remove ambient
aerosols. After the filtration of the aerosols, the sample gas flow is
delivered into the preheating tube through a 1.5 m perfluoroalkoxy
alkanes (PFA) inlet tube (Entegris,
I.D. = 4.35 mm). A 35 cm long PFA tube (Entegris, I.D. = 4.35 mm)
is installed in the front of the inlet interface as a preheating tube to
dissociate N2O5 to NO3. This preheating tube is heated and
stabilized at 120 ∘C. With this setup of temperature and residence
time, N2O5 is completely decomposed to NO3 in the preheating
tube.
In Fig. 1b, the central part of the detection cell is constructed using a
35.6 cm long PFA tube (marked in light green; Entegris, I.D. = 10.0 mm),
enclosed by a stainless steel tube (marked in grey). Each end of the stainless
steel tube is connected with a PFA interface (marked in green) which set up the
inlet and outlet and further connected with the corrugated pipes. With this
combination, the loss of NO3 during detection is minimized. The total
PFA cell length is 44.0 cm, but the length from the inlet to the outlet is
only 39.2 cm. The sample gas flow cell is heated and stabilized at 80 ∘C to prohibit the reverse reaction of NO3 and NO2
producing N2O5.
Dynamic reference spectrum
The effect of non-Beer–Lambert behaviour of
water vapour absorption lines near 660 nm has to be well accounted for to
achieve accurate and precise NO3 detection (Langridge et al., 2008; Dorn
et al., 2013). A few groups reported that the water vapour absorption can be
determined with an “effective” water vapour absorption cross section from a
look-up table approach or a real-time iterative calculation approach under
atmospheric conditions (Varma et al., 2009; Langridge et al., 2008; Kennedy
et al., 2011). In this work, we solve this problem by using a dynamic
reference spectrum through frequent addition of NO into the inlet. This
method has previously been used to acquire the chemical zero in the CRDS
method (Brown et al., 2002; Crowley et al., 2010). Through the frequent
addition of NO, the reference spectrum contain optical extinction from other
absorbers in this spectral region, for example water vapour, NO2,
O3, and any aerosols not removed by the filter. For the CRDS method, the
NO addition is carefully designed so that the resulting extinction of
increased NO2 in the reference measurement compared to that of NO3
is negligible. For the CEAS method, the NO addition is a little less precise
since this method allows for observation of NO3 separately from NO2
due to their different spectral shapes. In both cases, the effect of water
vapour is removed, and the fitting precision increases significantly in our
applications.
The NO titration module is connected to the inlet tube by a PFA tee-piece.
Using a computer-controlled solenoid valve, the instrument measures
reference and sample spectrum sequentially by switching the NO injection on
and off (NO = 98.0 ppmv, flow rate = 10.0 mL min-1). A high-purity N2
line (I.D. = 1.50 mm) is added at the exit of the solenoid valve by a PFA
tee-piece to flush the residual NO after the NO injection is switched off
(Fig. 1d). The resulting NO mixing ratio is about 480 ppbv in the sample
flow when NO injection is performed. Since 8.0 ppbv N2O5 was once
observed and reported in Hong Kong (Wang et al., 2016) as an extreme case,
the ambient NO3, N2O5, and O3 were set at about 1,
10, and 100 ppbv, respectively, for the simulation, proving that the
ambient NO3 and N2O5 can be removed within a time scale of
0.05 s when NO is injected (Fig. 2).
The NO2 impurity in the used NO standard is analysed by a commercial
NOx instrument (TE-42i). The NO2 impurity is found to be around 0.8 %, which means 4 ppbv of NO2 is present in the reference spectrum
measurement with the presence of 480 ppbv NO. The NO3 and O3 in
the preheating tube and detection cell react with the high concentration of
NO and generate NO2. In the case shown as Fig. 2, the additional
NO2 produced during the measurement of the reference spectrum can reach
up to 55 ppbv (with the initial additional NO2 set at 4 ppbv).
Therefore, to use this dynamic reference spectrum, we normally fit both
NO3 and NO2 to cover the limiting cases when the generated
NO2 is high. Nevertheless, the fitted NO2 concentration will be
negative since the NO2 concentrations are higher in the reference
spectrum.
Simulation of the change of the mixing ratios of NO3,
N2O5, and NO2 during the NO titration mode in the preheating
tube and detection cell for an extremely high NO3 and N2O5
case. The initial ambient NO3, N2O5, and O3 were set at 1,
10, and 100 ppbv, respectively. The initial NO2 was set at 4 ppbv from
the impurity of the used NO standard.
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Characterizations
The principle of the IBBCEAS system was systematically introduced by Fiedler
et al. (2003) and will only be introduced briefly here. The extinction
coefficient (α(λ)) in the cavity intrinsically consists of
the absorption, the Rayleigh, and the Mie scattering, caused by the gas
samples (Eq. 1). The α(λ) can be determined through
measurements of the intensity of the sample spectrum, the reference spectrum,
the mirror reflectivity, and the effective cavity length
αλ=I0λIλ-11-Rλdeff=∑ini×σiλ+αMieλ+αRaylλ.
In Eq. (1), λ is the wavelength of light; ni and σi(λ) are the number density and absorption cross section of the
ith gas compound, respectively, which causes absorption of the incident
light; deff is the effective cavity length; R(λ) is the mirror
reflectivity; αRayl(λ) is the extinction due to
Rayleigh scattering; αMie(λ) is the extinction due to
Mie scattering; I0(λ) is the reference spectrum; and
I(λ) is the sample spectrum. According to Eq. (1), the parameters
include the cross section of the strong light absorbing gases in the
target wavelength range, the effective cavity length, and the mirror
reflectivity that has to be quantified.
The absorption cross section (<inline-formula><mml:math id="M217" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="italic">σ</mml:mi><mml:mi>i</mml:mi></mml:msub><mml:mo>(</mml:mo><mml:mi mathvariant="italic">λ</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math></inline-formula>)
The effective absorption cross section of the abundant ambient absorbers,
NO3 and NO2, in the wavelength window of 640–680 nm needs to be
determined to retrieve the molecule number density of NO3. Since we used
a dynamic reference spectrum, which contains the same amount of water vapour
as that of the measured sample spectrum (Sect. 2.3), the calculation of the
strong nonlinear absorption lines of H2O in this wavelength window is
avoided. The NO3 absorption cross section is known to be temperature
dependent (Wangberg et al., 1997; Sander, 1986; Ravishankara and Mauldin,
1986; Yokelson et al., 1994; Orphal et al., 2003; Osthoff et al., 2007).
Under the heated cavity conditions (353 K), the effective absorption cross
section of NO3 is calculated by two steps: (1) the reported cross
section of NO3 (Yokelson et al., 1994) is scaled to the ratio of the
band's peak intensity at 662 nm between 298 and 353 K according to Osthoff
et al. (2007) and (2) the scaled absorption cross section is further
convoluted with an instrument function determined with the neon emission line
at 659.48 nm. Consequently, the calculated effective cross section at 353K
is about 1.77×10-17 cm2 molecule-1 at 662 nm, and the
uncertainty of the temperature correction and convolution is estimated to be
13 %. Under cold cavity conditions (298 K), the NO3 cross section
is convoluted directly to our spectral resolution with a peak value of
2.02×10-17 cm2 molecule-1 at 662 nm, and the
uncertainty of the convolution is estimated to be 10 % (Kennedy et al.,
2011). The NO2 cross section is reported to be not sensitive to the
temperature change (Voigt et al., 2002), so that only convolution is
performed to derive its effective absorption cross section for our instrument
setup. Figure 3 shows the temperature-scaled and instrumental resolution
convolved NO3 absorption cross section at 353 K and the convolved
NO2 absorption cross section, respectively. The cross section of
NO3 near 662 nm is three orders of magnitude larger than that of
NO2.
Absorption cross section of NO3 and NO2 from 640 to
680 nm. The green thin line is the original cross section of NO2 at
298 K determined by Voigt et al. (2002), the green thick line is the
convolved result, the orange thin line is the original cross section of
NO3 at 298 K determined by Yokelson et al. (1994), and the red thick
line is the temperature-scaled and convolved cross section at 353 K.
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The mirror reflectivity (<inline-formula><mml:math id="M241" display="inline"><mml:mi>R</mml:mi></mml:math></inline-formula>(<inline-formula><mml:math id="M242" display="inline"><mml:mi mathvariant="italic">λ</mml:mi></mml:math></inline-formula>))
The mirror reflectivity (R(λ)) is an important parameter to be
determined for the CEAS type of instrument. In previous work, R(λ) had
been determined through four different methods, including the detection of a
stable trace gas compound with known concentrations (Venables et al., 2006),
the differentiation of pure gases with distinct Rayleigh scattering cross
sections (Chen and Venables, 2011; Washenfelder et al., 2016; Min et al.,
2016), the usage of low-loss optics (Varma et al., 2009), and the
determination of the phase shift or ring down time (Langridge et al., 2008;
Schuster et al., 2009; Kennedy et al., 2011). In this study, R(λ) is
determined through the differentiation of pure gases (N2 and He) in the
cavity (Eq. 2) during the field campaigns. The Rayleigh scattering cross
sections for N2 (σRayl,N2(λ)) and He
(σRayl,He(λ)) are found in Sneep and
Ubachs (2005) and Shardanand and Rao (1977), respectively.
Rλ=1-d×IN2λ×nN2×σRayl,N2λ-IHeλ×nHe×σRayl,HeλIHeλ-IN2λ.
In Eq. (2), d is the distance between the two high-reflectivity mirrors
(50.0 cm); IN2(λ) and IHe(λ)
represent the light out spectrum determined when the cavity is filled by
N2 or He through the purge flow injection lines, respectively; and
nN2 and nHe are the calculated number density of
N2 and He, respectively, at the measured temperature and pressure in the
cavity. Figure 4 shows the mirror reflectivity calibration results during the
field measurements performed at the campus of the University of Chinese
Academy of Science (UCAS) in Beijing during winter 2016. The bold black line
is the average reflectivity of the five measurements of R(λ). It is
noted that the peak of R(λ) is 0.999936 ± 0.000002 at
662 nm. Under the protection of the purge flow and due to the mechanically
aligned setup of the cavity system, the determined R(λ) is
remarkably stable during this field campaign. The bold red line is the
average cavity loss, which is equal to (1-R(λ))/d, with the
maximized point near 662 nm of (1.28 ± 0.01) × 10-6
(1σ). The total uncertainty of the reflectivity is about 5 %,
which is dominated by the scattering cross sections of N2, according to
Sneep and Ubachs (2005). The uncertainty for He makes a negligible
contribution (Washenfelder et al., 2008).
Mirror reflectivity and cavity losses calibrated with high-purity He
and N2 in the current experimental setup during the field measurements.
The original calibration results were depicted by varying coloured lines: the
smoothed black bold line is the average R(λ), and the smoothed bold
red line is the average cavity loss ((1-R(λ))/d) from five
measurements. The mean (±1σ) value at 662 nm of reflectivity and
the cavity loss are 0.999936 ± 0.000002 and
(1.28 ± 0.01) × 10-6, respectively. The effective path
length at 662 nm reached 6.13 km.
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The effective cavity length (<inline-formula><mml:math id="M277" display="inline"><mml:mrow><mml:msub><mml:mi>d</mml:mi><mml:mi mathvariant="normal">eff</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula>)
The effective cavity length (deff) represents the cavity length
occupied by the absorbing gas when the sample flow is stable. Since the
continuous purge flow occupies the two ends of the cavity to protect the
mirrors, the deff is usually shorter than the distance between
the two high-reflectivity mirrors (defined as d, which is 50.0 cm in our
setup) and longer than the distance between the sample inlet and outlet
(defined as dsample, which is 39.2 cm in our setup). We
determine the deff by supplying an NO2 gas standard
(200 ppbv) with a constant flow into the cavity with purge flow and by
retrieving the deff based on Eq. (1). The 200 ppbv NO2
sample is prepared by a bottle standard of NO2 (80.8 ppm) diluted with
high-purity synthetic air (O2: 20.5 %, N2: bal) through a gas
mixer (TE-146i). The uncertainty of the prepared NO2 standard is
estimated to be 2 %, while the uncertainty of the NO2 absorption
cross section is estimated to be 4.7 % according to Voigt et al. (2002).
In our measurement, the deff is determined to be 45.0 cm, which
occupies 90.0 % of the total length of the optical cavity. Moreover,
dsample is 78.4 % of the length of the total optical cavity.
The dsample is shorter than deff, indicating
that there is turbulent mixing of sample gas into the purge
volumes. Since the possibility of this
turbulent mixing is slow relative to the rate of the NO3 wall losses,
the determination of the deff for NO3 is associated with an
additional uncertainty of 12 %, and the total uncertainty of the
determined deff with this approach is about 13 %.
Results and discussion
Spectral fitting
A least-squares spectral fitting software package was developed for
retrieving the molecule number densities of NO3 and NO2. The
optimized spectral fitting window was found to be from 640 to 680 nm, and a
third-order polynomial was applied to fit the background drift and
unaccounted scattering effect. Figure 5 shows an example of the spectral
fitting of a measurement spectrum of NO3 at 5 s integration time with
ambient measurement. By using the dynamic reference spectrum, the spectral
fitting is targeted at NO3 and NO2 as explained above. The
retrieved mixing ratio of NO3 (that actually represents the
NO3+ N2O5 concentration in the gas samples) is 64 pptv and
that of NO2 is -33 ppbv, which was mainly caused by the conversion of
ambient O3 (80 ppbv) with the added NO in the measurement of the
reference spectrum. The corresponding fitting residual is in the range of
±4.0×10-9 cm-1, and the H2O absorption is found to
be cancelled out in the residual spectrum. Moreover, an example time series
of NO3+ N2O5 measurement results during ambient measurements
is shown in Fig. 6. During ambient measurement, the NO titration is performed
periodically to acquire the dynamic reference spectrum. In Fig. 6, the red
points mark the effective ambient measurement result, which covered 4 min
20 s of every 5 min, and the blue points include 20 s for the zero points
and a 20 s switching phase between the two modes, which is discarded from
the data analysis.
An example of the spectral fit for an extinction spectrum measured
(5 s average) during field measurements. The fitted results of NO3 and
NO2 are shown as well as the third-order polynomial, the total fit
result, and the residual.
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An example time series of NO3+ N2O5 detection
performed at a rural site in Beijing with 5 s spectrum integral time. The
red points denote the ambient measurement mode without NO addition, and the
blue points denote the 20 s zero points (determination of the dynamic
reference spectrum) with NO addition and 20 s switching time between the
titration mode and the sample mode.
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Transmission efficiency of NO<inline-formula><mml:math id="M322" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:math></inline-formula> and N<inline-formula><mml:math id="M323" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>O<inline-formula><mml:math id="M324" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">5</mml:mn></mml:msub></mml:math></inline-formula>
For the accurate measurement of NO3 and N2O5, the wall loss
reactivity of the sample manifold and the detection cell need to be
determined. This includes (1) the wall loss on the filter, (2) the wall
loss on the inner surface of the inlet tube, and (3) the wall loss in the
preheating tube and the detection cell. To determine the wall loss
reactivity, an NO3/ N2O5 source module with stable mixing ratio
(±2 %) is set up in the lab. In this module, high-purity synthetic
air and NO2 is supplied to a gas mixer (TE-146i), and O3 is
generated in this gas mixer through the irradiation of a mercury lamp. The
supplied NO2 and the produced O3 is further delivered into a 160 L
smog chamber to generate stable concentrations of NO3 and
N2O5. Commercial NOx (TE-42i) and O3 monitors (TE-49i) are
operated to quantify the mixing ratio of NO2 and O3 in the
chamber. According to the detected concentration of NO2 and O3, we
can modulate the delivered concentration levels of NO3 and
N2O5 with the help of a box model.
Filter loss
The filter transmission efficiency of NO3 and N2O5 is
determined through the differentiation of an inlet without a filter, with a
clean filter (25 µm thickness, 4.6 cm diameter, 2.5 µm pore
size, Typris, China), and with used filters saved during typical pollution
episodes during field measurements. According to previous field measurements
of NO3 and N2O5 (e.g. Brown et al., 2001; Schuster et al.,
2009), frequent filter change is suggested, and the frequency is proposed to
be 0.5–3 h, depending on the aerosol loadings to reduce the impact of the
filter aging caused by aerosol accumulation. For this reason, we changed the
filter with a regular time interval (once every hour) during pollution
episodes. For clean conditions, the filter exchange frequency was reduced to
be once every 2 h.
For the determination of the NO3 filter transmission efficiency, an
additional preheating tube is inserted in front of the detection system to
convert all the generated N2O5 delivered by the calibration source
to NO3. The determined clean filter transmission efficiency is 75 %
for NO3 and is slightly lower than the previous results of NO3
transmission efficiency on Teflon filters (Aldener et al., 2006; Schuster et
al., 2009). The filter transmission efficiency of NO3 on used filters is
determined to be 5 % less than on the clean filter. For the field
calculation of the NO3 concentrations, the filter transmission
efficiency is then estimated to be 72 ± 3 %. For the determination
of the N2O5 filter transmission efficiency, the mixing ratio of
NO2 and O3 is modulated to achieve a high ratio of
N2O5/ NO3 (> 100) before being fed into the
instrument. The transmission efficiency of the N2O5 on the clean
filter is determined to be 96 %, which is consistent with the previous
studies on the filter loss of N2O5 (Fuchs et al., 2008; Aldener et
al., 2006; Schuster et al., 2009). The filter transmission efficiency of
N2O5 on a used filter is determined to be 6 % smaller than on
the clean filter. Therefore, the filter transmission factor for
N2O5 is estimated to be 93 ± 3 %.
Wall loss of the inlet tube, the preheating tube, and the
detection cell
To determine the wall loss reactivity of NO3, the heated detection cell
is used as a flow tube. Gas samples with a stable amount of N2O5
are delivered by the NO3/ N2O5 source described above. By
stopping the sample gas flow, the observed NO3 versus the elapsed time
determines the first-order loss rate of NO3 in the heated detection
cell. In this experiment, the fitted first-order uptake coefficient of
NO3 reflects the contribution from three processes: (1) the wall loss of
NO3 in the detection cell, (2) the change of the effective cavity length
due to the adding of the purge flows, and (3) the production of NO3 from
the reaction of NO2 and O3. The NO2 concentration determined
in the running sample gas flow is used to determine the change of
deff corresponding to the elapsed time after stopping the sample
flow (in the way it is used to quantify the deff in Sect. 3.3). A
time series of deff is determined with high time resolution data
acquisition (0.5 s) that is then used to quantify the mixing ratio of
NO3 in the corresponding time intervals. Figure 7 shows the decay of the
observed NO3 concentrations on a logarithmic scale versus the elapsed
time. The fitted first-order decay rate is 0.13 ± 0.02 s-1, with
a good correlation coefficient (R2=0.991). Finally, the fitted
first-order decay rate is corrected by the chemistry of reactions R1 and R4
with a box model constrained to observed NO2 and O3. The NO3
wall reactivity of the heated detection cell surface is determined to
0.16 ± 0.02 s-1, which is similar to previous results of
0.1–0.3 s-1 (Brown et al., 2002; Crowley et al., 2010; Kennedy et al.,
2011; H. C. Wang et al., 2015).
The determined concentration decay of the NO3 radical in the
heated detection cell caused by the wall loss. The red crosses denote the
observation results, and the black line depicts the corresponding exponential
fit. The net wall loss reactivity of NO3 is corrected to be
0.16 ± 0.02 s-1 with a box model simulation of the chemical
reactions occurring in the detection cell.
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The surface materials are the same as that of the inlet tube, the preheating
tube, and the detection cell. Therefore, the wall loss reactivity of
NO3 in the detection cell will be applicable for the inlet and the
preheating tubes. As shown in Fig. 1a, our instrument has only one mixing
point at the setup of the NO titration module. In addition, there is no
blockage of the main sample gas flow of the PFA tee-piece. Therefore, we
think the influence of the mixing point can be neglected. As reported by
Kennedy et al. (2011), the NO3 wall loss reactivity in the cold PFA
piping (inlet) is the same as in the heated ones with a value of 0.27 s-1. Nevertheless, we noticed that Crowley et al. (2010) reported that
the NO3 wall loss reactivity of the cold PFA tube could be a factor of
2 larger than that of the heated tube. We assume our NO3 wall loss
reactivity for the cold PFA tube to be between 0.16 and 0.32 s-1, and the average NO3 wall loss reactivity for the cold PFA
tube is estimated to be 0.24 s-1 with an uncertainty of 0.08 s-1.
The transmission efficiency of NO3 and N2O5 for the
sample module setup for the developed instrument.
Gases
Filter
Inlet tube (0.7 s)
Preheating tube (0.14 s)
Cavity (0.46 s)
Total
NO3
72 ± 3 %a
84 ± 4 % (k=0.24 s-1)b
98 % (k=0.16 s-1)
93 % (k=0.16 s-1)
55 ± 6 %
N2O5
93 ± 3 %a
99 % (k=0.019 s-1)
99 ± 1 %c
93 % (k=0.16 s-1)
85 ± 3 %
a Filter aging contributed an uncertainty of 3 %;
b the uncertainty of the NO3 wall loss reactivity in the
cold inlet tube caused an uncertainty of 4 %; c the location
of the N2O5 dissociation in the preheating tube had an uncertainty
of 1 %.
To determine the wall loss reactivity of the N2O5 in the PFA inlet
tube, PFA tubes (Entegris, I.D. = 4.35 mm) with different lengths (0.5, 3.5, 5.5, 7.5, and 10.5 m) are inserted between the outlet of the
NO3/ N2O5 source and the inlet of the preheating tube. The
apparent first-order loss rate of N2O5 (0.015 s-1) is deduced
by an exponential fit of the observed N2O5 concentrations to the
varied residence times with different tube lengths (Fig. 8). The actual
situation is more complicated in these PFA tubes due to the reaction of R1,
R4a and b, and the wall losses of both NO3 and N2O5.
The wall loss reactivity of N2O5 is retrieved from the observed
apparent decay rate with a box model. In this model, initial NO2 and
O3 are the observed values of the NO3/ N2O5 source. The
retrieved N2O5 wall loss reactivity is 0.019 s-1 ± 0.002 s-1. Moreover, the variation of ambient mixing ratio of NO2
will change the N2O5-dissociated location in the preheating tube,
which would also influence the transmission efficiency of N2O5. By
assuming the N2O5 is totally dissociated in the middle of the
preheating tube, we determine the transmission efficiency of N2O5
in the preheating tube to be 99 ± 1 %.
The determined concentrations of N2O5 versus the residence
time of the sample gas flows in the inlet tube. The change of the residence
time is achieved by changing the inlet tubes having different lengths. The red
crosses denote the observation results, and the black line depicts the
corresponding exponential fit. The net wall loss reactivity of N2O5
is corrected to be 0.019 ± 0.002 s-1 with a box model simulation
of the chemical reactions occurring in the inlet tubes.
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The total transmission efficiencies as well as the detailed contributions due
to the corresponding filter and wall loss for NO3 and N2O5 are summarized in Table 1
for the experimental setup during field applications. The total estimated
transmission efficiency of NO3 (TNO3) and N2O5
(TN2O5) is determined to be 55 ± 6 and
85 ± 3 %, respectively. TNO3 is dominated by the
loss on the filter and the inlet tube, and the difference of
TNO3 between cold cavity and heated cavity is negligible,
while the TN2O5 is dominated by the loss on the filter and
the detection cell.
Uncertainty and the limit of detection
As outlined above, the uncertainty of the NO3 absorption is estimated
to be 10 % (298 K) and 13 % (353 K), respectively; the uncertainty of
the effective cavity length calculation is about 13 %, mainly due to the
fast NO3 wall loss; the uncertainty of the mirror reflectivity
determination is about 5 %, controlled by the error of the scattering
cross section of N2; and the uncertainty of the TNO3 is about
6 %, according to the Gaussian error propagation, and the associated
uncertainty is estimated to be 19 % for the ambient NO3 measurement.
The uncertainty of the transmission efficiency in the heated cavity is
estimated at about 4 and 11 % when N2O5 or NO3 dominate
the concentrations of NO3+ N2O5, respectively, according to
the Gaussian error propagation, and the associated uncertainty for the ambient
NO3+ N2O5 measurement is estimated to be 19–22 %.
The uncertainties of the observed mixing ratios of NO3 and
NO3+ N2O5 are summarized in Table 2.
The instrument performance with different integration times.
(a) Allan deviation plots for measurements of NO3 with 1 s
integration time. Panels (b) and (c) show the histogram
analyses of the measurements of NO3 with 1 and 30 s integration time,
respectively.
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For the ambient N2O5 measurement, two parallel cavities are required
with one cold cavity measures NO3 and another heated cavity measured
NO3+ N2O5 like previous studies (Brown et al., 2003b;
Langridge et al., 2008; Crowley et al., 2010). Here we estimated the
uncertainty of N2O5 by following the expression proposed by
Dubé et al. (2006).
δN2O5=[δ(SUM)SUM]2+[δTNO3TNO3NO3]2[SUM-TNO3NO3]2+δTN2O52.
In Eq. (3), the δ(N2O5) represents the
uncertainty of N2O5 measurements, SUM is the measured
NO3+ N2O5 in the heated cavity, NO3 is the ambient
mixing ratio of NO3 derived by the cold cavity, δ(TNO3) and δ(TN2O5) denote the
uncertainty of TNO3 and TN2O5, and δ(SUM) denotes the uncertainty of NO3+ N2O5
measurement in the heated cavity. As reported by Osthoff et al. (2007) and
Kennedy et al. (2011), the uncertainty of N2O5
increases with the
decreasing of the ratio of N2O5/ NO3; when
N2O5/ NO3 is larger than 1 in the field measurement, the
uncertainty of N2O5 is in the range of 22–36 %.
The best integration time is determined through an Allan variance method
(Allan, 1966; Werle et al., 1993). Figure 9a depicts the Allan variance
analysis of the 12 000 zero measurement spectrums in the laboratory with
1 s integration time. According to the Allan deviation plot, increasing the
integration time could improve the sensitivity of our instrument when the
averaging time is smaller than 30 s. When the average time interval ranges
from 30 to 100 s, the best detection capability is achieved; and when the
average time interval is larger than 100 s, increasing the average time does
not improve the sensitivity further and actually decreases it, which is most
likely due to the drift of the light source. The limit of detection can be
estimated by the standard deviation calculation from zero air measurements
with the best integration time estimated above. Figure 9b and c show the
histogram analysis of 12 000 zero measurement results for a 1 and 30 s
average, respectively. The limit of detection is 2.4 pptv (1σ) for
the 1 s data and improved to be 1.6 pptv (1σ) for the 30 s data.
Due to the smaller cross section of NO3 that was applied in the
measurement of N2O5 at 353K, the LOD is estimated 2.7 pptv
(1σ) with 1 s integral time. Referring to the observed mixing
ratios of NO3 and N2O5 in the typical regions (H. C. Wang et
al., 2015), the developed instrument has the ability to measure NO3 and
N2O5 in the field. The LOD and uncertainty of our instrument is
further compared with the existing field measurement techniques for NO3
and N2O5 (Table 3). For the NO3 measurement, CRDS, CEAS, and
LIF are available with LOD values of 0.2-10 pptv and uncertainties lower
than 25 %. For the N2O5 measurement, the three methods
mentioned above and CIMS are available with LOD values of 0.5–12 pptv and
uncertainties lower than 40 %. Our instrument compares well with the
available field instruments for the detection of NO3 and N2O5.
Nevertheless, we have so far only probed the field sites with the presence of
high concentrations of NO3+N2O5, and, therefore, the NO3
measurement mode is not used in the field studies.
Details of the uncertainties of the measurement of ambient NO3
and NO3+ N2O5.
Parameters
Uncertainty
Uncertainty
(NO3)
(NO3+ N2O5)
Cross section of NO3
13 %
10 %
Mirror reflectivity
5 %
5 %
Transmission efficiency
6 %
4–11 %
Effective cavity length
13 %
13 %
Total
19 %
19–22 %
Limits of detection (LODs) and uncertainty of the existing field-deployable instruments of NO3 and N2O5.
Reference
Method
NO3
N2O5
LOD
Uncertainty
LOD
Uncertainty
This work
CEAS
2.4 pptv (1 s)
19 %
2.7 pptv (1 s)
22–36 %
Kennedy et al. (2011)
CEAS
1.1 pptv (1 s)
11 %
2.4 pptv (1 s)
14 %
Bitter et al. (2005)
CEAS
1 pptv (100 s)
Schuster et al. (2009)
CRDS/CEAS
2 pptv (5 s)
14 %
2 pptv (5 s)
13 %
Nakayama et al. (2008)
CRDS
1.5 pptv (100 s)
Dubé et al. (2006)
CRDS
0.2 pptv (1 s)
25 %
0.5pptv (1 s)
20–40 %
Ayers et al. (2005)
CRDS
2 pptv (25 s)
D. Wang et al. (2015)
CRDS
3.2 pptv (10 s)
8 %
Matsumoto et al. (2005)
LIF
10 pptv (600 s)
17 %
12 pptv (600 s)
17 %
Slusher et al. (2004)
CIMS
12 pptv (1 s)
Kercher et al. (2009)
CIMS
2.7 pptv (60 s)
20 %
Wang et al. (2016)
CIMS
4 pptv (60 s)
20 %
Two example time series of the observed mixing ratios of
NO3+ N2O5 measured during the UCAS winter campaign 2016 and
the PKU(CP) summer campaign. The grey box indicates the time span for
night-time. Panel (a) depicts the map of the two sites, indicating
the UCAS site and the PKU(CP) site, that are about 60 and 40 km away from the
centre of Beijing, respectively. Panel (b) shows a typical
development of the observed mixing ratio of NO3+ N2O5 from
clean to polluted air masses at UCAS. Panel (c) shows the observed
mixing ratio of NO3+ N2O5 during a typical pollution episode
at PKU(CP).
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Performance in field campaigns
The instrument has been deployed in two comprehensive field campaigns in
Beijing in 2016. The first campaign took place at the campus of the
University of Chinese Academy of Sciences, and the data shown in Fig. 10b are
from 27 February to 4 March, while the second campaign took place at the
Peking University Changping, PKU(CP), campus and the data shown in Fig. 10c
are from 23 to 29 May. As shown in Fig. 10a, both sites are located in the
northern rural areas in Beijing, about 60 and 40 km from the centre of
Beijing, respectively. According to our current understanding of the
NO3–N2O5 chemistry, rural areas lack fresh NO emissions, and
the air masses transported from urban areas are well aged and featured with
high NO2 and O3 and low NO, so that the influence of the
NO3–N2O5 chemistry can be maximized. We therefore expected these two
sites to be ideal locations to probe the NO3–N2O5 chemistry
in Beijing.
During the UCAS campaign, our instrument was deployed at a roof lab, and the
sample inlet was about 15 m above the ground. The measurement site was close
to the mountainous area in Beijing and also was influenced by nearby traffic
emissions. When the northerly wind appeared, we sampled clean air masses
entrained with local traffic and residential emissions; when the southerly
wind appeared, we could then capture the outflow from Beijing. In this
campaign, the average night-time temperature and NO2 mixing ratio is
-4.3 ∘C and 15.5 ppbv, respectively. The calculated ratio of
N2O5/ NO3 based on the thermodynamic equilibrium was found to
be larger than 300; therefore, the mixing ratio of NO3 was ignorable
compared with N2O5. Therefore, the amount of the detected
NO3+ N2O5 represented that of N2O5 for this
campaign. Figure 10b shows the mixing ratio of NO3+ N2O5
during a typical time when such air mass changes from clean to polluted
conditions. High mixing ratios of NO3+ N2O5 were observed
near the ground surface at the UCAS site. During the pollution episodes, the
maximum NO3+ N2O5 reached more than 1 ppbv on the night of
2–3 March 2016. A rapid variation of NO3+ N2O5 was also
observed, which may have been due to local traffic emissions during stagnant
conditions. In all these days, the observed NO3+ N2O5
continuously accumulated during a few hours after sunset, reached its maximum
before midnight, and then gradually decreased to zero before sunrise. The
decrease of NO3+ N2O5 at night in this location may be
related to the typical running style of the heavy-duty vehicles (HDVs). It is
known that HDVs would emit large amounts of fresh NO. The emitted NO is
titrated with the O3 and NO3 and then reduces the accumulation of
N2O5 or enhances the loss of N2O5 during the time scale
of thermal dissociation (0.1–20 min from summer to winter time). Typically,
more heavy-duty cars appear on the nearby street after 22:00 CST (China Standard Time, UTC + 8 h),
since the ban of HDVs entering downtown Beijing is lifted after 22:00 CST.
The NO3+ N2O5 measurement results of the PKU(CP) summer
campaign are presented in Fig. 10c. During the summer campaign, the
instrument was set up on the fifth floor of the main building at the PKU(CP)
campus. The inlet was also about 15 m above ground. The average night-time
temperature and NO2 mixing ratio were 10.0 ∘C and 17.5 ppbv,
respectively. High-O3 events frequently occurred in this season compared
to that of winter. Together with the atmospheric processes with high-NO2
conditions, the calculated ratio of N2O5/ NO3 based on the
thermodynamic equilibrium was estimated to be larger than 20, and the amount
of NO3+N2O5 also represented that of N2O5 mostly at
the PKU(CP) site.