Nitric oxide (NO) and nitrogen dioxide (NO2) are relevant to air
quality due to their roles in tropospheric ozone (O3) production. In
China, NOx emissions are very high and NOx emissions exhausted from on-road
vehicles make up 20 % of total NOx emissions. In order to detect the NO
and NO2 emissions on road, a dual-channel cavity ring-down spectroscopy
(CRDS) system for NO2 and NO detection has been developed. In the
system, NO is converted to NO2 by its reaction with excess O3
in the
NOx channel, such that NO can be determined through the difference
between two channels. The detection limits of NO2 and NOx for the
system are estimated to be about 0.030 (1σ, 1 s) and 0.040 ppb (1σ, 1 s), respectively. Considering the error sources of NO2
absorption cross section and RL determination, the total uncertainty of
NO2 measurements is about 5%. The performance of the system was
validated against a chemiluminescence (CL) analyser (42i, Thermo Scientific,
Inc.) by measuring the NO2 standard mixtures. The measurement results
of NO2 showed a linear correction factor (R2) of 0.99 in a slope
of 1.031±0.006, with an offset of (-0.940±0.323) ppb. An
intercomparison between the system and a cavity-enhanced absorption
spectroscopy (CEAS) instrument was also conducted separately for NO2
measurement in an ambient environment. Least-squares analysis showed that the
slope and intercept of the regression line are 1.042±0.002 and
(-0.393±0.040) ppb, respectively, with a linear correlation factor
of R2=0.99. Another intercomparison conducted between the system and
the CL analyser for NO detection also showed a good agreement within their
uncertainties, with an absolute shift of (0.352±0.013) ppb, a slope
of 0.957±0.007 and a correlation coefficient of R2=0.99. The
system was deployed on the measurements of on-road vehicle emission plumes
in Hefei, and the different emission characteristics were observed in the
different areas of the city. The successful deployment of the system has
demonstrated that the instrument can provide a new method for retrieving
fast variations in NO and NO2 plumes.
Introduction
In recent years, with the improvement of people's living standards, more and
more attention is given to the improvement of the living environment. Thus the management of environmental pollution has gradually become one of
the focus issues, and it is believed that the detection of pollutants is
prerequisite for environmental governance. As one of the main air
pollutants, NOx (NOx=NO+NO2) is well known as one
by-product of organic decay, the emission from natural forest fires, and the main anthropogenic emission from both stationary sources (electric
power generation using fossil fuels) (Jaramillo and
Muller, 2016) and mobile sources (motor vehicles and catalytic converters of
most cars) (Carslaw, 2005). NOx can determine the
tropospheric O3 levels and lead to the formation of photochemical
“smog”. Furthermore, NOx is also known as the precursor of nitric
acid (Brown et al., 2004). In
addition, NOx can do harm to the human body and animals by damaging the
respiratory system and leading to pulmonary edema (Yang and Omaye,
2009). Moreover, it is believed that accurate NO2 measurement plays a
key role in accurate measurement of other species, such as organic nitrate
(Thieser et al., 2016; Paul et al., 2009; Day et al., 2002) and RO2
radicals (Chen et al., 2016).
During the last few years, many direct and indirect techniques for
monitoring NO2 have been established, such as chemiluminescence (CL)
detection (Yuba et al., 2010; Sadanaga et al., 2008; Fahey et al., 1985),
differential optical absorption spectroscopy (DOAS) (Platt et al.,
1984; McLaren et al., 2010), tunable diode laser absorption spectroscopy (TDLAS)
(Li et al., 2004), cavity ring-down spectroscopy (CRDS) (Castellanos
et al., 2009; Fuchs et al., 2009, 2010; Osthoff et al., 2006;
Brent et al., 2013; Hu et al., 2016), cavity-enhanced absorption
spectroscopy (CEAS) (Wu et al., 2009; Gherman et al., 2008; Kasyutich et
al., 2006; Wada and Orr-Ewing, 2005), cavity-attenuated phase shift
spectroscopy (CAPS) (Kebabian et al., 2008),
laser-induced fluorescence (LIF) (Taketani et al., 2007; Matsumi et al.,
2010; Sadanaga et al., 2014; Matsumoto et al., 2001), long-path absorption
photometry (LOPAP) (Villena et al., 2011) and gas-based sensing
(Novikov et al., 2016), with CL being the most widely used for in situ
ambient sampling. CL can achieve direct measurement of NO and indirect
measurement of NO2. The method is based on the reaction between NO and
O3, which can form an electronically excited molecule of NO2*.
When NO2* reaches the ground state, it emits fluorescence, which is
proportional to the initial NO concentration. NO2 is measured
indirectly by conversion to NO firstly through heated (300 to
350 ∘C) molybdenum (Mo) surfaces (Ridley and
Howlett, 1974) or photolytic NO2 converters like Xenon lamps or UV-emitting diodes at specific wavelengths (320–400 nm). The CL instruments
have typical NO2 detection limits of 50 ppt min-1
(1σ)
(Wang et al., 2001). By contrast, CRDS, CEAS, CAPS and TDLAS,
generally relying on scanning a light source through a range of frequencies
of interest, are all direct absorption techniques. It has been demonstrated
that these optical methods can achieve a high detection sensitivity and the
detection limit is several parts per trillion with a time resolution of
several seconds
(Li et al., 2004; Wild et al., 2014; Gherman et al., 2008; Kebabian et
al., 2008). Among these techniques, CRDS has become a promising technique
for ambient NO2 detection due to its advantages of high time
resolution, low detection limit and portability, in which pulsed
(Fuchs et al., 2009) or continuous-wave (cw) lasers (Wada and
Orr-Ewing, 2005) were utilised. Wada and Orr-Ewing (2005) demonstrated a cw diode CRDS system operating at 410 nm for the
retrieval of NO2 mixing ratios in ambient air with a detection limit of
0.1 ppb in 50 s at atmospheric pressure. Osthoff et al. (2006) constructed a pulsed cavity ring-down spectrometer where a pulsed
(20–100 Hz, up to 25 mJ) frequency-doubled Nd:YAG laser was used for the
simultaneous measurements of NO2, nitrate radical (NO3), and
dinitrogen pentoxide (N2O5) in the atmosphere, and the detection
limit of 40 ppt (1σ) for 1 s data was achieved for NO2 with
an uncertainty within ±4 % under laboratory conditions. Fuchs et al. (2009) used a simple, lightweight, low-power, and
commercially available Fabry–Pérot (FP) diode laser with a centre wavelength
of 403.96 nm as a light source to detect NO and NO2 in two separate
channels. The limit of detection is 22 ppt (2σ precision) for
NO2 at 1 s time resolution. Karpf et al. (2016) used a high-power, multimode Fabry–Pérot (FP) diode laser with a broad
wavelength range (Δλlaser∼0.6 nm) to excite
a large number of cavity modes, thereby reducing the susceptibility of the
detector to vibration and making it well suited for field deployment. A
sensitivity of 38 ppt was achieved using an integration time of 128 ms for
single-shot detection in their work. To evaluate the uncertainty and the
accuracy of the aforementioned individual instruments, a number of
intercomparison studies have been carried out (Xu et al., 2013; Dunlea et
al., 2007; Villena et al., 2012). The comparison results show that the
method based on Mo converters can be affected by significant interferences
such as N2O5, HONO, HNO3, PAN, etc., whereas the method based
on optical absorption is relatively immune to interferences. Therefore,
direct techniques are considered to be more reliable methods than the CL
method for the measurement of NO2 and have been adopted gradually in field
experiments (Ayres et al., 2015; Wagner et al., 2013; Sobanski et al.,
2016).
In addition to the direct measurement of NO with the CL method, NO
concentrations can also be measured directly based on their absorption
feature at 1585.282 cm-1. For example, a tunable infrared laser
differential absorption spectroscopy (TILDAS) instrument based on an infrared distributed feedback
(DFB)
laser emitting sequentially at 1600 and 1900 cm-1 has
been used by Jagerska et al. for the measurement of NO and NO2
(Jagerska et al., 2015), where an astigmatic multi-pass Herriott cell
(Herndon et al., 2004) and a dual-wavelength spectrometer are utilised.
The 1 s precision for NO measurement of the TILDAS instrument was 550 ppt,
whereas that for the field experiments was 1.5 ppb. Compared with the CL
method, it seems that this technique may suffer from low detection
sensitivity. Given the rapid changes of nighttime oxidation, i.e. NO3
radical, the rapid changes of its precursors NO and NO2 are thus a
prerequisite for developing a nighttime atmospheric chemistry model. To
achieve high sensitivity and high resolution, conversion to NO2 by
adding excess O3 may be one of the best methods for NO
detection (Fuchs et al., 2009; Wild et al., 2014).
The developments of different technologies provide the potential for NOx
measurements on different platforms such as ground sites, vehicles and aircraft (Yamamoto et al., 2011; Wagner et al., 2011; Castellanos et
al., 2009). Due to the rapid economic growth in 2000–2010, China has become
the second largest economy in the world. With the rapid growth of energy
consumption, NOx emissions are increasing. Motor vehicles are one of the
major sources for NOx, especially in urban areas (Westerdahl, 2008).
Exhaust from on-road vehicles makes up 20 % of total NOx emissions in
China (Shi et al., 2014). So various methods have been used to measure
the vehicle emissions to assess exposure to air pollutant and specifical
impacts due to traffic-related emissions (Vogt et al., 2003; Carslaw and
Beevers, 2004; Herndon et al., 2005; Lal et al., 2005; Burgard et al.,
2006a, b; Hueglin et al., 2006; Wild et al., 2017).
However, the measurements were usually carried out at several fixed
locations in large cities, which is not adequate to show the large scale
patterns of the cities. Hence, a direct on-road mobile instrument is needed
to obtain the spatial and temporal variations in NOx pollutants.
In this work, a dual-channel CRDS system has been developed based on the
chemical conversion of NO to NO2, which realises the simultaneous
measurement of NO2 and NOx. In one channel, only ambient NO2
is measured. In another channel, the sum of ambient NO2 and the
converted NO2 from ambient NO is determined to provide a direct
measurement of NOx. Then the difference of the two channels provides a
direct measurement of NO. To assess the accuracy of the dual-channel CRDS
instrument, contrast measurements and comparison were conducted between the
dual-channel CRDS instrument and other instruments. In addition,
the instrument was deployed in the measurement of on-road vehicle emission
plumes in the city of Hefei, China, on 17 December 2018 and 25 February 2019. The main advantages of this instrument compared with CL instruments
are its low detection limit and high sensitivity as well as its potential
ability for trace measurements without calibration and interferences.
Setup of the instrument
Due to its high detection sensitivity, cavity ring-down spectroscopy has
been applied to measurement NO3 radical and N2O5 in our group
(Wang et al., 2015; Li et al., 2018a, b). In this work, the
technique was applied to measure NO2 and NO simultaneously. A schematic
diagram of the dual-channel CRDS system developed in the present work is
shown in Fig. 1. The instrument mainly consists of two identical cavities
for NO2 and NOx detection, a gas handling system, a NO convertor and
an activated carbon device for NOx removal.
Schematic of the dual-channel cavity ring-down spectroscopy system.
CRDS systems
A blue diode laser is used as the light source and the wavelength of the
laser is monitored in real time by a spectrometer. The output of the diode
laser, with a centre wavelength at 403.64 nm and a line width of 0.5 nm, is
directly modulated by a square wave signal (on/off) at a repetition of 2 kHz
with a duty cycle of 50 %. And generally the output power is about 60 mW.
The laser beam first passes through an isolator to prevent the reflected
light back into the laser cavity, is then divided into two approximately
equal beams after a 50/50 beam splitter and enters into two identical
cavities separately. Each optical cavity is made of an aluminum tube with an
inner diameter of 9.4 cm, which is fixed rigidly by a separate frame. Two
high-reflectivity mirrors are held in stable, adjustable mounts. The
distance between the two highly reflective mirrors (LGR, 1 in diameter, 1 m
radius of curvature) in each channel is 75 cm, which corresponds to the
ring-down time constants of 22.90 and 24.12 µs for NOx
and NO2 cavities in zero air. The light emitted through the back mirror
of the cavity passes through a narrowband filter to filter stray light, and
then is directed into a photomultiplier tube (PMT). The signal is amplified firstly, then acquired
with a digital acquisition card (NI USB-6361, 16-bit, 2.0 Ms s-1). The sampling
frequency for each digital acquisition card is 1 MHz. During a continuous
1.0 s data acquisition period, 2000 decay traces are transferred to the PC
using a single transfer command and averaged to get a fitted decay trace at
a laser modulation rate of 2 KHz. NO2 concentrations can be calculated
according to Eq. (1) from the ring-down times, τ and τ0, the ring-down times when the NO2 is in the presence and
absence of the cavity, respectively; the NO2 absorption cross section,
RL, is the ratio of the total cavity length to the length over which the
absorber is present in the cavity, and c is the speed of the light.
[NO2]=RLcσNO21τ-1τ0
NO convertor
NO is measured by its conversion to NO2 in the presence of excess
O3. The principle is based on the following chemical equation
(Sander et al., 2006).
NO+O3→NO2+O2k1,
where k1=3.0×10-12exp(-1310/T) cm3 molec.-1 s-1. Ozone is produced from O2 photolysis
at 185 nm by flowing 100 sccm (standard cubic centimetres per minute) of sampling air over a low-pressure discharge mercury
lamp. The mercury lamp is inset into a quartz glass tube with a length of 50 mm and an inner diameter of 10 mm. The flow rate passing through the mercury
lamp was controlled by a mass flowmeter controller (MFC)
and the resulting mixing ratio of O3 was
detected by an O3 analyser (49i, Thermo Scientific). The O3
concentration is approximately 11.2 ppm after mixing with the sampled air. A
Teflon tubing (length 1 m, i.d. 3.8 mm) serves as a reactor for the NO
conversion.
Activated carbon device
Accurate measurement of the ring-down time, τ0, when there is not
any absorber in the cavity, is pivotal for accurately retrieving the
absorber concentration in a practical measurement as well as for checking
the cleanliness of the cavity mirrors. Usually zero air or chemical scrubber
is used to acquire zeros (Wada and Orr-Ewing, 2005). In our
system, zeros are obtained by passing sampled air through an activated
carbon-filled tube with a length of 26.0 cm and an outer diameter of 6.0 cm.
The τ0 is measured for 60 s every 10–16 min. It is found that
this frequency of zero measurements is sufficient to track drifts in zero
ring-down time constant measurements, i.e. the fluctuation of τ0 is less than 0.1 % for 15 min intervals.
Cross section of the NO3 radical, NO2, O3, water
vapour and diode laser spectrum.
Gas handling system
The gas handling system of the instrument consists of a sampling module and
a purge flow. The sampled air initially flows through a filter device, where
a filtering membrane (1 µm pore size) is loaded to prevent
light-scattering aerosols from entering the cavity. And subsequently it will
pass through the activated carbon device to provide the background
measurement or enter the PFA tube directly just depending on whether the
three-way solenoid valve is open or closed. After the PFA tube, the air flow
is divided into three lines. The first flow (100 sccm) is introduced into a
quartz flow tube equipped with a mercury pen-ray lamp (Oriel 6035) to
generate O3 by air photolysis as mentioned, then it will be merged
with the second sampling flow (900 sccm) and pulled into the NOx
cavity. The third one with a flow rate of 1 slm (standard litres per minute) is directed into the
NO2 cavity. The whole system is pumped by a rotary pump (K86KNE) and the
flow rates of the different lines are controlled by mass flow controllers
separately. To prevent the degradation of the mirror reflectivity, each
mirror is continuously flushed with high-purity nitrogen at a rate of 25 mL min-1 to avoid the potential pollution from the sample flow.
Results and discussionDetermination of absorption cross sections
To retrieve the gas concentration, it is prerequisite to determine the
effective absorption cross section at peak absorption of the laser. The
output waveforms of the laser, with a centre wavelength of 403.64 nm and
full width at half maximum of 0.5 nm, was monitored by a spectrometer
(QEPB0828) (red line shown in Fig. 2). The centre wavelength selected can
cover the strong absorption of NO2 and avoid the interference from
other species, such as H2O (pink line in Fig. 2). The effective
absorption cross section was determined to be 5.63×10-19 cm2 molecule-1
by convoluting the NO2 absorption cross
section with the laser spectrum (blue line in Fig. 2) using Voigt profile
(Voigt et al., 2002). It is well known that a shift in the
laser centre wavelength would result in a change in the effective NO2 cross section, so the laser output was monitored as the index of the
day-to-day variability of the laser centre wavelength. It was shown that the
variability is less than 1 % during the few experiment days. The largest
uncertainty of the absorption cross section is about 3 % according to
Voigt (Voigt et al., 2002).
Different cavity ring-down signals and fitting results in the
absence and presence of NO2. The small figure in the upper right corner
is the fitting residual.
The retrieval of RL
Due to the purge gas to the mirrors, the RL value cannot be simply
determined by the ratio of the distance between two mirrors to that between
the inlet and outlet. The RL value of the system was determined from the
absorption measurement of various concentrations of NO2 ranging from 20 to 70 ppb with or without purge flow. The ratio of the two extinction
measurements yielded a RL value, which is independent of the cross
section and the concentration of NO2. The RL value is determined
to be 1.10±0.03 for both the NOx and NO2 channels.
(a, b) Continuous time series measurement when the instrument
sampled only zero air, averaged to 1 s for NO2 and NOx channels
(black dots); the red dots show the data averaged to 30 s. (c, d) Allan
deviation plots for NO2 concentration in two channels. The minimum
value equals the optimum integration time.
The retrieval of τ0
In order to accurately determine the concentrations of trace gases by CRDS,
it is very important to confirm background cavity loss measurements of τ0 when the target gases are not inside the cavity. Several alternative
background measurement methods have been reported, where various kinds of
air were used as background gas, such as zero air, a mixture of oxygen and
nitrogen, chemically scrubbed laboratory air (using hydroxyapatite), and
laboratory air sampled through the stainless-steel tubing coil (Wada
and Orr-Ewing, 2005). In our instrument, an activated carbon device was used
for background measurement. The ring-down times when the sampled air pass
through the activated carbon device were determined to be 24.12±0.01 and 22.90±0.01µs in two cavities, respectively. These
values are close to those of measurements of zero air at the same sample
rate for a 15 min period.
Two representative ring-down signals corresponding to with and without
NO2 in the cavity for the CRDS system are shown in Fig. 3. And the
fitted ring-down times were 24.12 and 20.30 µs, so
that the NO2 concentration is determined as 20.28 ppb using the
constants determined above.
Detection limit and measurement accuracy of two cavities
The measurement precision of the dual-channel CRDS instrument for NO2
and NOx detection was investigated with time series measurement of zero
air (Fig. 4). The acquisition time for the spectral data was 1.0 s with an
average of 2000 spectra. In order to analyse the stability of the
instrument, the Allan variance had been calculated for the intensity
measurements. For the two channels, the 1σ detection limits were 30
and 40 ppt (1 s) for the NO2 and NOx channels, respectively.
The minima in the Allan plots indicated that the optimum average times for
optimum detection performance (Fig. 4c, d) is about 30 s. With 30 s integration time, the 1σ detection limits were 16 and 14 ppt
for the NO2 and NOx channels, respectively.
The minimum detection can be written as follows:
[A]min=2RLcσΔτ0τ02.
For continuous zero NO2 measurements, the Δτ0 was 0.008 µs in both NOx and NO2 channels and τ0
was 22.90 and 24.12 µs in NOx and NO2 channels,
respectively, for 1 s averaging. Given the RL value to be 1.10 and
σ to be 5.63×10-19 molec. cm-2, the
1σ minimum detection limits for the NOx and NO2 channels
were determined as 39 and 35 ppt, respectively, at an integration time of
1 s, which are close to the Allan variance analysis described above.
The total uncertainty of NO2 measurement by CRDS is expected to be from
the uncertainties on the measurement of the RL and NO2 absorption cross
section. The uncertainty in RL was determined to be less than
3 %, and the uncertainty in the NO2 absorption cross section was
about 4 %, so the total uncertainty of NO2 measurement was estimated
to be 5 % for the system in this work.
Compared with the existing field measurement techniques for NO2
measurements, it seems that the minimum detection and uncertainty of this
instrument are superior to the other methods (see Table 1).
O3 mixing ratio when the bypass flow passing through the Hg lamp
changes.
NO conversion efficiency
The main influence factor on the NO conversion efficiency is the flow rate
passing through the mercury pen-ray lamp as it can determine the generated
O3 concentration. The mixing ratio of O3 in the NOx channel
line was investigated, which changes with the flow rate that passes through
the mercury pen-ray lamp. The result is shown in Fig. 5. As a result, the
bypass flow passing through the Hg lamp was determined to be 100 sccm. Under
this flow rate, when the residence time of O3 in the cavity is 1 s and
the ambient NO2 concentration is 50 ppb, NO conversion efficiency with
different NO concentrations (10–1000 ppb) was simulated and NO conversion
efficiency in the range of experimental concentrations is larger than 98 %.
Comparison of NO2 detection limits based on optical methods.
Principle of measurementLaser powerWavelength range/nmDetection limitReferenceCw-CRDS5 mW (1 MHz)41080 ppt/50 s (1σ)Wada and Orr-Ewing (2005)Nd:YAG laser CRDS1 mJ53240 ppt/1 s (1σ)Osthoff et al. (2006)pDL-CRDS40 mW (2 KHz – 10 %)40411 ppt/1 s (1σ)Fuchs et al. (2009)Fabry–Pérot (FP) pDL-CRDS1.1 W (4 KHz – 10 %)40038 ppt/12 ms (1σ)Karpf et al. (2016)Commercial DL-CRDS1.2 KHz407.3820 ppt/60 s (1σ)Castellanos et al. (2009)LED-based commercial CRD355 mW397–412 (405)27 ppt/60 s (1σ)Brent et al. (2013)LED-CEAS340 mW4552.2 ppb/100 s (1σ)Wu et al. (2009)Xe lamp DOAS295–492 nm1 ppb/480 s (1σ)McLaren et al. (2010)CAPS44020 ppt/10 s (1σ)Kebabian et al. (2008)Diode-pumped Nd:YAG laser-LIF15 mW (14 KHz)47370/60 s (1σ)Taketani et al. (2007)Blue LED-IF17.7 mW (10 KHz)4354.9 ppb/60 s (1σ)Matsumi et al. (2010)Pulsed blue light LED-LIF22 mW4303.5 ppb/60 s (1σ)Sadanaga et al. (2014)pDL-CRDS60 mW403.6430 ppt/1 s (1σ)This work
(a) Time series of NO2 concentration standard
mixtures sampled by the CRDS instrument in two channels with mercury pen-ray lamp
switched on. (b) A correlation plot between the data from two channels.
(a) Time series of NO2 concentration standard
mixtures sampled by the CRDS instrument and CL analyser. The time resolution for the CRDS
instrument and CL analyser are 1s and 1min, respectively. (b) A correlation
plot between the data from the CRDS instrument and the CL analyser (data
averaged to 1 min). The fitting result gave a gradient of 1.031 and an
intercept of -0.940 ppb, with a linear correlation factor of 0.99.
Because the cross section of O3 is about 4 orders of magnitude
smaller than that of NO2 at the centre wavelength of the laser, the
absorption of O3 generated by mercury photolysis is negligible.
According to Fuchs et al. (2009), under conditions when NO is
rich, further oxidation of NO2 to NO3 and N2O5 has only
a slight effect on NOx measurement, which means that the correction of
the NOx measurement can be neglected. However, under conditions when NO
is absent, the loss of NO2 due to oxidation by high concentration of
ozone is indeed one of the main factors that is attributed to the errors in the
NOx channel. The reaction equation is expressed as follows:
R2NO2+O3→NO3+O2k2,R3NO3+NO2↔N2O5keq,
where k2=1.2×10-13exp(-2450/T) cm3 molecule-1 s-1
(T=298 K, k2=3.2×10-17 cm3 molecule-1 s-1) (Sander et al.,
2006) and
keq=(5.1±0.8)×10-27exp(10871/T) cm3 molecule-1
(T=298 K, keq=3.5×10-11 cm3 molecule-1) (Osthoff et al., 2007). The loss rate
will increase with the increase in the NO2+O3 reaction rate
constant when temperature in the cavity increases. Moreover, the loss rate
is also sensitive to the NO2 mixing ratio. Diluted NO2 standard
mixture was introduced into the two channels to characterise the effect of
high ozone on NO2 measurement. The NO2 concentrations in the two
channels and their correlation are shown in Fig. 6. The interference of
O3 in the NOx channel when NO is absent can be neglected. The
discrepancy between two different channels may be the result of the systematic
errors in two different channels and can be corrected with the coefficient
obtained from Fig. 6b.
(a)NO2 mixing ratios by CEAS (1 min average) and CRDS (1 s
average) instruments. (b) Scatter plots for the NO2 dataset from CRDS
and CEAS instrument. The red lines illustrate the linear regression (data
averaged to 1 min base).
(a) Time series of NO by dual-CRDS instrument and CL analyser. (b) A
correlation between two instruments is shown and data for correlation
analysis are averaged in 1 min.
The diagram of the movable van loaded with the CRDS instrument.
Field applicationsContrast measurement of standard mixtures of NO and NO2
The comparisons of NO2 measurements between CRDS and the NOx analyser
have been carried out on NO2 standard mixtures. Different mixing ratios
of NO2 were obtained by gas-phase titration of NO with excess O3
generated by an ozone generator (OC500). The 10.3 ppm NO standard mixture
was initially diluted by N2 and subsequently oxidised by O3. The
amount of NO2 generated from excess ozone can be calculated from the
known initial concentration of NO. The generated pure NO2 standards in
clean air were in the concentration range of 20–70 ppb. The CL analyser used
for comparison in this laboratory experiment was separately calibrated and
the linearity of this instrument was checked using a mixture containing NO.
Figure 7a shows the concentrations of the standard NO2 in the
laboratory measured by CRDS and a commercial CL analyser (42i, Thermo
Scientific, Inc., 0.4 ppb (1σ) detection limit) simultaneously. A
correlation analysis between the data from the two instruments was carried
out. The fitting results shown in Fig. 7b indicate that
NO2(CRDS)=NO2(CLanalyser)×1.031-0.940, with a linear
correlation factor (R2) of 0.99. The results in Fig. 7a also
indicate that the CRDS instrument can capture the NO2 variations more
rapidly than CL analyser.
Results of the NOx(a), NO2(b) and NO (c) concentrations
around Hefei, China (data are averaged to 5 s).
Ground-based measurements of NO2 and NO
To verify its performance and applicability, the dual-channel CRDS
instrument was further compared with a CEAS instrument (Duan et al.,
2018) on ground-based measurement of NO2 during the period from
3 to 5 November 2017 in the western suburb area of Hefei, Anhui, China. The
reason why the CEAS instrument not the CL analyser was selected for NO2
intercomparison is that for CL analyser, NO2 must be converted to NO
first and then can be detected, which exposes the analyser to chemical
interferences, while for the CEAS instrument, NO2 can be detected
directly. Measurement precision (1σ) for NO2 is about 170 ppt
in 30 s. The time resolutions of the CRDS and CEAS instruments are 1 s and
1 min,
respectively. The CRDS and CEAS instruments were set up on the sixth
floor of the building in the Anhui Institute of Optic and Fine Mechanics. The area
lies to the northeast of the Dongpu reservoir and is about 1.5 km from
the reservoir. The northwest of the area is surrounded by a forest. The
significant NO2 pollution during the measurement was found to be the
emission of the cars along the road (100 m radius). The air originating from
the sector between the south and east (5 km) may bring anthropogenic
emissions to the site. Ambient air was introduced into the instruments
through a 6 mm outer diameter Teflon tube. The data for comparison were
averaged to 1 min. Figure 8a shows the temporal variations in NO2
concentrations measured by the CEAS and the CRDS instruments.
It was found that the nighttime NO2 was in the range of 3 to 35 ppb. Generally, the NO2 concentrations and variations measured by the
CRDS instrument show good consistency with those measured by the CEAS
instrument, and the slope and intercept of the regression line from the
least-squares analysis are 1.042±0.002 and (-0.393±0.040) ppb, respectively, as shown in Fig. 8b. However, the results revealed a
discrepancy when rapid NO2 variations appeared. We attribute this
discrepancy to the slight difference between the two inlets of the
instruments when large NO2 was rapidly emitted into the atmosphere. In
general, the CRDS instrument has substantive advantages for retrieving rapid
variations in NO2 plums due to its high time resolution and high
sensitivity.
The 4 h drive around Hefei, China, coloured by the measured NO (a) and
NO2(b) concentrations, respectively.
Results of the NO2, NO and O3 concentrations around Hefei,
China.
Time series of NO from the CRDS instrument and CL analyser (42i). (a) Data averaged to 5 s for the CRDS instrument and (b) the data averaged to
1 min.
The comparison of NO concentrations measured by the dual-channel CRDS
instrument and CL analyser was conducted under a variety of sampling
conditions for a total of 7 d at the site described previously. Both
instruments were attached to the same air sample inlet. The datasets from
the CRDS instrument and CL analyser were highly correlated over wide
concentration ranges of NO (see Fig. 9a). Figure 9b shows the
relationship between NO concentrations observed by the CRDS and CL methods.
The slope and intercept of the regression line were 0.959±0.007 and
0.352±0.013 ppb. The correlation coefficient is R2=0.99. As the CL
method is believed to be reliable for NO measurement, the reliability
of the dual-channel CRDS instrument was validated for the measurement of
NO.
On-road measurements of vehicle NO2/NOx emission
In order to retrieve the vehicle emissions on road, field measurements were
performed in Hefei from 15:00 to 16:00 CST on 17 December 2018. The CRDS
instrument was powered by a lithium battery, and ambient air was pumped into
the system though an inlet fixed on the roof of the car. The vehicle speed
is about 50 km h-1. In order to obtain the discrepancy of vehicle emissions in
urban and suburban areas, the car travels around the whole area. Figure 10
shows a picture of the movable van loaded with the CRDS instrument, and the
position of the sampling inlet is about 1.5 m above the ground. Figure 11
illustrates the route in Hefei and the drive track is coloured
logarithmically with respect to measured NOx, NO2 and NO. The
NO2 concentration ranged from 1.5 to 133.3 ppb and NO ranged from
the detection limit to 554.7 ppb. The mean concentrations of NO and NO2
were 140 and 54.9 ppb, respectively. On the whole, the NO and NO2
concentrations in the urban area were higher than those in the suburban area. Large
plumes of NO were found at the crossroads with heavy traffic or the sites
converged with heavy-duty diesel vehicles. The [NO2]/[NOx] ratio was about
19 %, which is larger than the results observed in the USA
(Wild et al., 2017).
In 25 February 2019 another measurement of vehicle emissions was performed
on the road to further verify the instrument performance. A NO analyser (42i),
an O3 analyser (49i) and the CRDS instruments were all placed in the same
car. Ambient air was pumped through an inlet fixed on the roof of the car
and then divided into three lines to the three instruments.
Figure 12 illustrates the 4 h drive track involving highway, urban and
suburban area around Hefei, which is coloured with respect to the measured NO
and NO2. Vehicle speeds varied greatly on three different road types
and were around 100 km h-1 on the highway. Influenced by the vehicle emissions,
the NOx plumes on urban roads are higher than those on suburban roads and
highway.
Figure 13 shows the time series of NO2, O3 and NO. NO2
concentrations ranged from the detection limit to 110.2 ppb and O3
concentrations ranged from 6.9 to 85.3 ppb. Several NO plumes were
observed and the maximum value was up to 767.1 ppb. O3 and NO showed a
significant negative correlation, which is attributed to the quick titration
of O3 by NO. Figure 14 shows the NO data measured by CRDS and the CL
analyser (42i, 1 min), Fig. 14a shows the data averaged to 5s for the CRDS instrument and
Fig. 14b shows the data averaged to 1min for the CRDS instrument. The good agreement
between the measurement results from the two instruments proves that the
CRDS instrument can be applied for fast vehicle NOx emissions.
Because the NO2-to-NOx emissions ratio affects ozone production and
spatial distribution, more efforts should be made to provide a constraint on
emissions inventories used in air quality modelling. The mobile CRDS
instrument provides a good method to retrieve the direct vehicle NOx
emissions and NO2-to-NOx ratio of plumes due to its easy deployment and
high temporal resolution.
Conclusions
A compact, sensitive and accurate instrument based on diode-laser cavity
ring-down spectroscopy with the centre wavelength of 403.64 nm has been
demonstrated for detection of trace amounts of NO2 and NOx in
ambient air. Minimum detection limits of NO2 and NOx were
estimated to be 0.030 and 0.040 ppb at an integration time of 1 s when
zero air is sampled with a measurement accuracy of ±5 %. Contrast
measurements between the dual-channel CRDS instrument and a CL analyser on
NO2 standard mixtures were performed, which showed a good correlation
between the two different techniques. In order to confirm the reliability of
the dual-channel CRDS instrument in the field atmosphere, continuous
measurement was conducted and the stability of the instrument was
investigated. During the intercomparison measurements of NO2 and NO in
the field, the dual-channel CRDS instrument showed a good correlation with
the CEAS instrument for NO2 measurement, and with the CL
analyser for NO measurement.
The CRDS instrument was further deployed in a movable car to monitor NO and
NO2 emissions on the road. The advantage of the high time resolution of the
instrument has been demonstrated, which means the instrument provides a new
direct method for on-road vehicle plume measurement. Meanwhile, the high
detection sensitivity of the instrument was also shown in this work, which
indicates it can act as a new detection technique for chemistry model
verification. It is expected that the instruments developed will lead to the
wider application for ambient air quality monitoring and will be useful to
investigate photochemistry in the atmosphere more precisely.
Data availability
The data used in this study are available from the corresponding author upon request (rzhu@aiofm.ac.cn).
Author contributions
RH and ZL contributed equally to this work.
RH, PX, JL and WL contributed to the conception of the study. ZL and HC built the CRDS instrument. ZL, HC, XL, SL, FW, YW and CL performed the experiments. DW developed the data processing software. ZL performed the data analyses and wrote the paper. RH edited and developed the paper.
Competing interests
The authors declare that they have no conflict of
interest.
Special issue statement
This article is part of the special issue
“Advances in cavity-based techniques for measurements of atmospheric aerosol and trace gases”. It is not associated with a conference.
Financial support
This research has been supported by the
National Natural Science Foundation of China (grant nos. 91644107, 61575206,
41571130023 and 61805257) and the National Key Research and Development
Program of China (grant nos. 2017YFC0209401, 2017YFC0209403).
Review statement
This paper was edited by Weidong Chen and reviewed by Hongbing Chen and one anonymous referee.
ReferencesAyres, B. R., Allen, H. M., Draper, D. C., Brown, S. S., Wild, R. J.,
Jimenez, J. L., Day, D. A., Campuzano-Jost, P., Hu, W., de Gouw, J., Koss,
A., Cohen, R. C., Duffey, K. C., Romer, P., Baumann, K., Edgerton, E.,
Takahama, S., Thornton, J. A., Lee, B. H., Lopez-Hilfiker, F. D., Mohr, C.,
Wennberg, P. O., Nguyen, T. B., Teng, A., Goldstein, A. H., Olson, K., and
Fry, J. L.: Organic nitrate aerosol formation via
NO3+ biogenic volatile organic compounds in the southeastern
United States, Atmos. Chem. Phys., 15, 13377–13392,
10.5194/acp-15-13377-2015, 2015.Brent, L. C., Thorn, W. J., Gupta, M., Leen, B., Stehr, J. W., He, H.,
Arkinson, H. L., Weinheimer, A., Garland, C., Pusede, S. E., Wooldridge, P.
J., Cohen, R. C., and Dickerson, R. R.: Evaluation of the use of a
commercially available cavity ringdown absorption spectrometer for measuring
NO2 in flight, and observations over the Mid-Atlantic States,
during DISCOVER-AQ, J. Atmos. Chem., 72, 503–521,
10.1007/s10874-013-9265-6, 2013.Brown, S. S., Dibb, J. E., Stark, H., Aldener, M., Vozella, M., Whitlow, S.,
Williams, E. J., Lerner, B. M., Jakoubek, R., Middlebrook, A. M., DeGouw, J.
A., Warneke, C., Goldan, P. D., Kuster, W. C., Angevine, W. M., Sueper, D.
T., Quinn, P. K., Bates, T. S., Meagher, J. F., Fehsenfeld, F. C., and
Ravishankara, A. R.: Nighttime removal of NOx in the summer
marine boundary layer, Geophys. Res. Lett., 31, 1–5,
10.1029/2004gl019412, 2004.Burgard, D. A., Bishop, G. A., Stadtmuller, R. S., Dalton, T. R., and
Stedman, D. H.: Spectroscopy applied to on-road mobile source emissions,
Appl. Spectrosc., 60, 135a–148a, 10.1366/000370206777412185, 2006a.Burgard, D. A., Bishop, G. A., Stedman, D. H., Gessner, V. H., and
Daeschlein, C.: Remote sensing of in-use heavy-duty diesel trucks, Environ.
Sci. Technol., 40, 6938–6942, 10.1021/es060989a, 2006b.Carslaw, D. C. and Beevers, S. D.: Investigating the potential importance of
primary NO2 emissions in a street canyon, Atmos. Environ., 38,
3585–3594, 10.1016/j.atmosenv.2004.03.041, 2004.Carslaw, D. C.: Evidence of an increasing NO2/NOx
emissions ratio from road traffic emissions, Atmos. Environ., 39, 4793–4802,
10.1016/j.atmosenv.2005.06.023, 2005.Castellanos, P., Luke, W. T., Kelley, P., Stehr, J. W., Ehrman, S. H., and
Dickerson, R. R.: Modification of a commercial cavity ring-down spectroscopy
NO2 detector for enhanced sensitivity, Rev. Sci. Instrum.,
80, 113107,
10.1063/1.3244090, 2009.Chen, Y., Yang, C. Q., Zhao, W. X., Fang, B., Xu, X. Z., Gai, Y. B., Lin, X.
X., Chen, W. D., and Zhang, W. J.: Ultra-sensitive measurement of peroxy
radicals by chemical amplification broadband cavity-enhanced spectroscopy,
Analyst, 141, 5870–5878, 10.1039/c6an01038e, 2016.Day, D. A., Wooldridge, P. J., Dillon, M. B., Thornton, J. A., and Cohen, R.
C.: A thermal dissociation laser-induced fluorescence instrument for in situ
detection of NO2, peroxy nitrates, alkyl nitrates, and
HNO3, J. Geophys. Res.-Atmos., 107, 4046, 10.1029/2001jd000779,
2002.Duan, J., Qin, M., Ouyang, B., Fang, W., Li, X., Lu, K., Tang, K., Liang, S.,
Meng, F., Hu, Z., Xie, P., Liu, W., and Häsler, R.: Development of an
incoherent broadband cavity-enhanced absorption spectrometer for in situ
measurements of HONO and NO2, Atmos. Meas. Tech., 11, 4531–4543,
10.5194/amt-11-4531-2018, 2018.Dunlea, E. J., Herndon, S. C., Nelson, D. D., Volkamer, R. M., San Martini,
F., Sheehy, P. M., Zahniser, M. S., Shorter, J. H., Wormhoudt, J. C., Lamb,
B. K., Allwine, E. J., Gaffney, J. S., Marley, N. A., Grutter, M., Marquez,
C., Blanco, S., Cardenas, B., Retama, A., Ramos Villegas, C. R., Kolb, C. E.,
Molina, L. T., and Molina, M. J.: Evaluation of nitrogen dioxide
chemiluminescence monitors in a polluted urban environment, Atmos. Chem.
Phys., 7, 2691–2704, 10.5194/acp-7-2691-2007, 2007.Fahey, D. W., Eubank, C. S., Hubler, G., and Fehsenfeld, F. C.: Evaluation of
a Catalytic Reduction Technique for the Measurement of Total Reactive
Odd-Nitrogen NOy in the Atmosphere, J. Atmos. Chem., 3,
435–468, 10.1007/bf00053871, 1985.Fuchs, H., Dube, W. P., Lerner, B. M., Wagner, N. L., Williams, E. J., and
Brown, S. S.: A Sensitive and Versatile Detector for Atmospheric
NO2 and NOx Based on Blue Diode Laser Cavity
Ring-Down Spectroscopy, Environ. Sci. Technol., 43, 7831–7836,
10.1021/es902067h, 2009.Fuchs, H., Ball, S. M., Bohn, B., Brauers, T., Cohen, R. C., Dorn, H.-P.,
Dubé, W. P., Fry, J. L., Häseler, R., Heitmann, U., Jones, R. L.,
Kleffmann, J., Mentel, T. F., Müsgen, P., Rohrer, F., Rollins, A. W.,
Ruth, A. A., Kiendler-Scharr, A., Schlosser, E., Shillings, A. J. L.,
Tillmann, R., Varma, R. M., Venables, D. S., Villena Tapia, G., Wahner, A.,
Wegener, R., Wooldridge, P. J., and Brown, S. S.: Intercomparison of
measurements of NO2 concentrations in the atmosphere simulation
chamber SAPHIR during the NO3Comp campaign, Atmos. Meas. Tech., 3, 21–37,
10.5194/amt-3-21-2010, 2010.Gherman, T., Venables, D. S., Vaughan, S., Orphal, J., and Ruth, A. A.:
Incoherent broadband cavity-enhanced absorption spectroscopy in the
near-ultraviolet: Application to HONO and NO2, Environ. Sci.
Technol., 42, 890–895, 10.1021/es0716913, 2008.Herndon, S. C., Shorter, J. H., Zahniser, M. S., Nelson, D. D., Jayne, J.,
Brown, R. C., Miake-Lye, R. C., Waitz, I., Silva, P., Lanni, T., Demerjian,
K., and Kolb, C. E.: NO and NO2 emission ratios measured from
in-use commercial aircraft during taxi and takeoff, Environ. Sci. Technol.,
38, 6078–6084, 10.1021/es049701c, 2004.Herndon, S. C., Shorter, J. H., Zahniser, M. S., Wormhoudt, J., Nelson, D.
D., Demerjian, K. L., and Kolb, C. E.: Real-time measurements of
SO2, H2CO, and CH4 emissions from in-use
curbside passenger buses in New York City using a chase vehicle, Environ.
Sci. Technol., 39, 7984–7990, 10.1021/es0482942, 2005.Hueglin, C., Buchmann, B., and Weber, R. O.: Long-term observation of
real-world road traffic emission factors on a motorway in Switzerland, Atmos.
Environ., 40, 3696–3709, 10.1016/j.atmosenv.2006.03.020, 2006.Hu, R. Z., Wang, D., Xie, P. H., Chen, H., and Ling, L.: Diode Laser Cavity
Ring-Down Spectroscopy for Atmospheric NO2 Measurement, Acta.
Optica. Sinica, 36, 0230006, 10.3788/AOS201636.0230006, 2016.Jagerska, J., Jouy, P., Tuzson, B., Looser, H., Mangold, M., Soltic, P.,
Hugi, A., Bronnimann, R., Faist, J., and Emmenegger, L.: Simultaneous
measurement of NO and NO2 by dual-wavelength quantum cascade laser
spectroscopy, Opt. Express, 23, 1512–1522, 10.1364/oe.23.001512, 2015.Jaramillo, P. and Muller, N. Z.: Air pollution emissions and damages from
energy production in the US: 2002–2011, Energy Policy, 90, 202–211,
10.1016/j.enpol.2015.12.035, 2016.Karpf, A., Qiao, Y. H., and Rao, G. N.: Ultrasensitive, real-time trace gas
detection using a high-power, multimode diode laser and cavity ringdown
spectroscopy, Appl. Optics, 55, 4497–4504, 10.1364/ao.55.004497, 2016.Kasyutich, V. L., Martin, P. A., and Holdsworth, R. J.: Phase-shift off-axis
cavity-enhanced absorption detector of nitrogen dioxide, Meas. Sci. Technol.,
17, 923–931, 10.1088/0957-0233/17/4/044, 2006.Kebabian, P. L., Wood, E. C., Herndon, S. C., and Freedman, A.: A practical
alternative to chemiluminescence-based detection of nitrogen dioxide: Cavity
attenuated phase shift spectroscopy, Environ. Sci. Technol., 42, 6040–6045,
10.1021/es703204j, 2008.Lal, D. R., Clark, I., Shalkow, J., Downey, R. J., Shorter, N. A., Klimstra,
D. S., and La Quaglia, M. P.: Primary epithelial lung malignancies in the
pediatric population, Pediatr. Blood Cancer, 45, 683–686,
10.1002/pbc.20279, 2005.Li, Y. Q., Demerjian, K. L., Zahniser, M. S., Nelson, D. D., McManus, J. B.,
and Herndon, S. C.: Measurement of formaldehyde, nitrogen dioxide, and sulfur
dioxide at Whiteface Mountain using a dual tunable diode laser system, J.
Geophys. Res.-Atmos., 109, D16S08, 10.1029/2003jd004091, 2004.Li, Z. Y., Hu, R. Z., Xie, P. H., Wang, H. C., Lu, K. D., and Wang, D.:
Intercomparison of in situ CRDS and CEAS for measurements of atmospheric
N2O5 in Beijing, China, Sci. Total Environ., 613–614, 131–139,
10.1016/j.scitotenv.2017.08.302, 2018a.Li, Z. Y., Hu, R. Z., Xie, P. H., Chen, H., Wu S. Y., Wang, F. Y., Wang, Y.
H., Ling, L. Y., Liu, J. G., and Liu, W. Q.: Development of a portable cavity
ring down spectroscopy instrument for simultaneous, in situ measurement of
NO3 and N2O5, Opt. Express, 26, A433–A449,
10.1364/OE.26.00A433, 2018b.Matsumi, Y., Taketani, F., Takahashi, K., Nakayama, T., Kawai, M., and Miyao,
Y.: Fluorescence detection of atmospheric nitrogen dioxide using a blue
light-emitting diode as an excitation source, Appl. Optics, 49, 3762–3767,
10.1364/ao.49.003762, 2010.Matsumoto, J., Hirokawa, J., Akimoto, H., and Kajii, Y.: Direct measurement
of NO2 in the marine atmosphere by laser-induced fluorescence
technique, Atmos. Environ., 35, 2803–2814,
10.1016/s1352-2310(01)00078-4, 2001.McLaren, R., Wojtal, P., Majonis, D., McCourt, J., Halla, J. D., and Brook,
J.: NO3 radical measurements in a polluted marine environment:
links to ozone formation, Atmos. Chem. Phys., 10, 4187–4206,
10.5194/acp-10-4187-2010, 2010.Novikov, S., Lebedeva, N., Satrapinski, A., Walden, J., Davydov, V., and
Lebedev, A.: Graphene based sensor for environmental monitoring of
NO2, Sens. Actuator B-Chem., 236, 1054–1060,
10.1016/j.snb.2016.05.114, 2016.Osthoff, H. D., Brown, S. S., Ryerson, T. B., Fortin, T. J., Lerner, B. M.,
Williams, E. J., Pettersson, A., Baynard, T., Dube, W. P., Ciciora, S. J.,
and Ravishankara, A. R.: Measurement of atmospheric NO2 by pulsed
cavity ring-down spectroscopy, J. Geophys. Res.-Atmos., 111, D12305, 10.1029/2005jd006942, 2006.Osthoff, H. D., Pilling, M. J., Ravishankara, A. R., and Brown, S. S.:
Temperature dependence of the NO3 absorption cross-section above
298 K and determination of the equilibrium constant for
NO3+NO2 < - > N2O5
at atmospherically relevant conditions, Phys. Chem. Chem. Phys., 9,
5785–5793, 10.1039/b709193a, 2007.Paul, D., Furgeson, A., and Osthoff, H. D.: Measurements of total peroxy and
alkyl nitrate abundances in laboratory-generated gas samples by thermal
dissociation cavity ring-down spectroscopy, Rev. Sci. Instrum.,
80, 114101,
10.1063/1.3258204, 2009.Platt, U. F., Winer, A. M., Biermann, H. W., Atkinson, R., and Pitts, J. N.:
Measurement of nitrate radical concentrations in continental air, Environ.
Sci. Technol., 18, 365–369, 10.1021/es00123a015, 1984.Ridley, B. A. and Howlett, L. C.: An instrument for nitric oxide measurements
in the stratosphere, Rev. Sci. Instrum., 45, 742–746, 10.1063/1.1686726,
1974.Sadanaga, Y., Yuba, A., Kawakami, J., Takenaka, N., Yamamoto, M., and Bandow,
H.: A gaseous nitric acid analyzer for the remote atmosphere based on the
scrubber difference/NO-ozone chemiluminescence method, Anal. Sci., 24,
967–971, 10.2116/analsci.24.967, 2008.Sadanaga, Y., Suzuki, K., Yoshimoto, T., and Bandow, H.: Direct measurement
system of nitrogen dioxide in the atmosphere using a blue light-emitting
diode induced fluorescence technique, Rev. Sci. Instrum., 85,
064101,
10.1063/1.4879821, 2014.
Sander, S. P., Friendl, R. R., Golden-Kreutz, D. M., Kurylo, M. J., Moorgat, G. K., Wine, P. H., Ravishankara, A. R., Kolb, C. E., Molina-Cimadevila, M. J., Pitts, B., Huie, R. E., Orkin, V. L., Finlayson, B. J., and Huie, J. R.: Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies
Evaluation Number 15 Jet Propulsion Laboratory, National Aeronautics and
Space Administration/Jet Propulsion Laboratory/ California Institute of
Technology, Pasadena, CA, 2006.
Shi, Y. L., Cui, S. H., and Xu, S.: Factor decomposition of nitrogen oxide
emission of China industrial energy consumption, Environ. Sci. Technol., 37,
355–362, 2014.Sobanski, N., Tang, M. J., Thieser, J., Schuster, G., Pöhler, D.,
Fischer, H., Song, W., Sauvage, C., Williams, J., Fachinger, J., Berkes, F.,
Hoor, P., Platt, U., Lelieveld, J., and Crowley, J. N.: Chemical and
meteorological influences on the lifetime of NO3 at a semi-rural
mountain site during PARADE, Atmos. Chem. Phys., 16, 4867–4883,
10.5194/acp-16-4867-2016, 2016.Taketani, F., Kawai, M., Takahashi, K., and Matsumi, Y.: Trace detection of
atmospheric NO2 by laser-induced fluorescence using a GaN diode
laser and a diode-pumped YAG laser, Appl. Optics, 46, 907–915,
10.1364/ao.46.000907, 2007.Thieser, J., Schuster, G., Schuladen, J., Phillips, G. J., Reiffs, A.,
Parchatka, U., Pöhler, D., Lelieveld, J., and Crowley, J. N.: A
two-channel thermal dissociation cavity ring-down spectrometer for the
detection of ambient NO2, RO2NO2 and RONO2,
Atmos. Meas. Tech., 9, 553–576, 10.5194/amt-9-553-2016,
2016.Villena, G., Bejan, I., Kurtenbach, R., Wiesen, P., and Kleffmann, J.:
Development of a new Long Path Absorption Photometer (LOPAP) instrument for
the sensitive detection of NO2 in the atmosphere, Atmos. Meas. Tech., 4,
1663–1676, 10.5194/amt-4-1663-2011, 2011.Villena, G., Bejan, I., Kurtenbach, R., Wiesen, P., and Kleffmann, J.:
Interferences of commercial NO2 instruments in the urban atmosphere
and in a smog chamber, Atmos. Meas. Tech., 5, 149–159,
10.5194/amt-5-149-2012, 2012.Vogt, R., Scheer, V., Casati, R., and Benter, T.: On-road measurement of
particle emission in the exhaust plume of a diesel passenger car, Environ.
Sci. Technol., 37, 4070–4076, 10.1021/es0300315, 2003.Voigt, S., Orphal, J., and Burrows, J. P.: The temperature and pressure
dependence of the absorption cross-sections of NO2 in the 250–800
nm region measured by Fourier-transform spectroscopy, J. Photochem.
Photobiol. A, 149, 1–7, 10.1016/s1010-6030(01)00650-5, 2002.Wada, R. and Orr-Ewing, A. J.: Continuous wave cavity ring-down spectroscopy
measurement of NO2 mixing ratios in ambient air, Analyst, 130,
1595–1600, 10.1039/b511115c, 2005.Wagner, N. L., Dubé, W. P., Washenfelder, R. A., Young, C. J., Pollack,
I. B., Ryerson, T. B., and Brown, S. S.: Diode laser-based cavity ring-down
instrument for NO3, N2O5, NO, NO2 and
O3 from aircraft, Atmos. Meas. Tech., 4, 1227–1240,
10.5194/amt-4-1227-2011, 2011.Wagner, N. L., Riedel, T. P., Young, C. J., Bahreini, R., Brock, C. A., Dube,
W. P., Kim, S., Middlebrook, A. M., Ozturk, F., Roberts, J. M., Russo, R.,
Sive, B., Swarthout, R., Thornton, J. A., VandenBoer, T. C., Zhou, Y., and
Brown, S. S.: N2O5 uptake coefficients and nocturnal
NO2 removal rates determined from ambient wintertime measurements,
J. Geophys. Res.-Atmos., 118, 9331–9350, 10.1002/jgrd.50653, 2013.Wang, D., Hu, R. Z., Xie, P. H., Liu, J. G., Liu, W. Q., Qin, M., Ling, L.Y.,
Zeng, Y., Chen, H., Xing, X.B., Zhu, G. L., Wu, J., Duan, J., Lu, X., and
Shen, L. L.: Diode laser cavity ring-down spectroscopy for in situ
measurement of NO3 radical in ambient air, J. Quant. Spectrosc.
Ra., 166, 23–29, 10.1016/j.jqsrt.2015.07.005, 2015.Wang, T., Cheung, V. T. F., Anson, M., and Li, Y. S.: Ozone and related
gaseous pollutants in the boundary layer of eastern China: Overview of the
recent measurements at a rural site, Geophys. Res. Lett., 28, 2373–2376,
10.1029/2000gl012378, 2001.
Westerdahl, D.: Avoiding Measurement Errors When Monitoring Fine and
Ultrafine PM for Exposure and Epidemiology Studies, Epidemiology, 19,
S360–S360, 2008.Wild, R. J., Edwards, P. M., Dube, W. P., Baumann, K., Edgerton, E. S.,
Quinn, P. K., Roberts, J. M., Rollins, A. W., Veres, P. R., Warneke, C.,
Williams, E. J., Yuan, B., and Brown, S. S.: A measurement of total reactive
nitrogen, NOy, together with NO2, NO, and
O3 via cavity ring-down spectroscopy, Environ. Sci. Technol., 48,
9609–9615, 10.1021/es501896w, 2014.Wild, R. J., Dube, W. P., Aikin, K. C., Eilerman, S. J., Neuman, J. A.,
Peischl, J., Ryerson, T. B., and Brown, S. S.: On-road measurements of
vehicle NO2/NOx emission ratios in Denver, Colorado, USA,
Atmos. Environ., 148, 182–189, 10.1016/j.atmosenv.2016.10.039, 2017.Wu, T., Zhao, W., Chen, W., Zhang, W., and Gao, X.: Incoherent broadband
cavity enhanced absorption spectroscopy for in situ measurements of
NO2 with a blue light emitting diode, Appl. Phys., 94, 85–94,
10.1007/s00340-008-3308-8, 2009.Xu, Z., Wang, T., Xue, L. K., Louie, P. K. K., Luk, C. W. Y., Gao, J., Wang,
S. L., Chai, F. H., and Wang, W. X.: Evaluating the uncertainties of thermal
catalytic conversion in measuring atmospheric nitrogen dioxide at four
differently polluted sites in China, Atmos. Environ., 76, 221–226,
10.1016/j.atmosenv.2012.09.043, 2013.Yamamoto, Y., Sumizawa, H., Yamada, H., and Tonokura, K.: Real-time
measurement of nitrogen dioxide in vehicle exhaust gas by mid-infrared cavity
ring-down spectroscopy, Appl. Phys. B, 105, 923–931,
10.1007/s00340-011-4647-4, 2011.Yang, W. and Omaye, S. T.: Air pollutants, oxidative stress and human health,
Mutat. Res. Genet. Toxicol. Environ. Mutagen., 674, 45–54,
10.1016/j.mrgentox.2008.10.005, 2009.Yuba, A., Sadanaga, Y., Takami, A., Hatakeyama, S., Takenaka, N., and Bandow,
H.: Measurement System for Particulate Nitrate Based on the Scrubber
Difference NO-O3 Chemiluminescence Method in Remote Areas, Anal.
Chem., 82, 8916–8921, 10.1021/ac101704w, 2010.