Quantum Cascade Laser Spectroscopy
369
Furthermore, a pulsed DFB-QCL operating at 6.13 μm was exploited in a highly
sensitive CRDS system to monitor nitrogen dioxide (NO 2 ) in the automotive exhaust
gas by probing transition line belonging from the fundamental ν 3 vibrational band
and a detection limit of less than several tens of ppbv was achieved from the
system with 1 s time resolution for continuous 30 min measurement [39]. Moreover,
recent improvement of QCL source technology with continuous emission in roomtemperature has made the in-situ atmospheric trace monitoring very practical without
any cumbersome methodology. Jingsong Li et al. demonstrated a room temperature
operating continuous-wave QCL based wavelength modulation spectrometer operating in the mid-IR wavelength region near 4.56 μm to study the effects of both
biogenic and anthropogenic CO emission [40]. Their system was capable of monitoring average CO concentration 100.3 ± 18.1 ppbv with respect to background
concentration up to 90 ppbv. Another room temperature operated cw widely tuneable EC-QCL was employed for diurnal ambient air N 2 O mixing ratio measurement
for different anthropogenic sources near 1887 cm
−1 mid-IR spectral region [41]. In
this study, a highly sensitive CRDS spectrometer was employed and the combination band [(0 0
0 0) → (1 1
1 0)] of N 2 O was probed to estimate the concentration.
Therefore, the high output power emission of QCL and its coupling with high finesse
optical cavity helped to reach the detection sensitivity very few ppb to ppt level.
The group of Pal et al., used a room temperature EC-QCL in wavelength modulation spectrometer for atmospheric CH 4 concentration measurement in different
anthropogenic sources. To achieve the highly sensitive measurement, they probed
the strongest interference-free absorption line in the ν 4 fundamental vibrational band
of CH 4 centred at 1327.072 cm
−1 and a minimum detection limit of 11 parts-perbillion (ppb) was achieved within 1.5 optical paths in 272 s integration time [42].
The methane concentrations from different anthropogenic sources, such as paddy
field, industrial sector, dairy firm, wetland and their institute campus were monitored
and subsequently, all the measurements were validated by the mass-spectrometric
measurements (Fig. 4).
However, Airborne platform for atmospheric trace concentration measurement has
become a superior methodology for understanding the real-time chemical process in
the atmosphere. Such understanding is really helpful for the researcher to investigate
the transport dynamics of chemical species from the troposphere to stratosphere with
high spatial and temporal resolution. Moreover, it also provides information on the
column amount of the atmospheric constituent, which is beneficial for developing
the atmospheric model to get proper scientific knowledge of atmospheric chemistry. First QCL based airborne atmospheric sample measurement was accomplished
by the Webstar et al. [43]. In their study, they utilised a cryogenically cooled QC
laser operated at 8 mm spectral region to monitor the CH 4 and N 2 O concentration up to 20 km in the stratosphere over North America, Scandinavia, and Russia
during September 1999 and extending through March 2000 in a series of 20 aircraft
flights. The incorporation of QCL with higher output power ~10 mW, narrower
laser linewidth ~17 MHz successively enhanced the measurement precision and
subsequently, a minimum-detectable mixing ratio for methane of approximately 2
parts per billion by volume was achieved in QCL channel measurement. Later, in
369
Furthermore, a pulsed DFB-QCL operating at 6.13 μm was exploited in a highly
sensitive CRDS system to monitor nitrogen dioxide (NO 2 ) in the automotive exhaust
gas by probing transition line belonging from the fundamental ν 3 vibrational band
and a detection limit of less than several tens of ppbv was achieved from the
system with 1 s time resolution for continuous 30 min measurement [39]. Moreover,
recent improvement of QCL source technology with continuous emission in roomtemperature has made the in-situ atmospheric trace monitoring very practical without
any cumbersome methodology. Jingsong Li et al. demonstrated a room temperature
operating continuous-wave QCL based wavelength modulation spectrometer operating in the mid-IR wavelength region near 4.56 μm to study the effects of both
biogenic and anthropogenic CO emission [40]. Their system was capable of monitoring average CO concentration 100.3 ± 18.1 ppbv with respect to background
concentration up to 90 ppbv. Another room temperature operated cw widely tuneable EC-QCL was employed for diurnal ambient air N 2 O mixing ratio measurement
for different anthropogenic sources near 1887 cm
−1 mid-IR spectral region [41]. In
this study, a highly sensitive CRDS spectrometer was employed and the combination band [(0 0
0 0) → (1 1
1 0)] of N 2 O was probed to estimate the concentration.
Therefore, the high output power emission of QCL and its coupling with high finesse
optical cavity helped to reach the detection sensitivity very few ppb to ppt level.
The group of Pal et al., used a room temperature EC-QCL in wavelength modulation spectrometer for atmospheric CH 4 concentration measurement in different
anthropogenic sources. To achieve the highly sensitive measurement, they probed
the strongest interference-free absorption line in the ν 4 fundamental vibrational band
of CH 4 centred at 1327.072 cm
−1 and a minimum detection limit of 11 parts-perbillion (ppb) was achieved within 1.5 optical paths in 272 s integration time [42].
The methane concentrations from different anthropogenic sources, such as paddy
field, industrial sector, dairy firm, wetland and their institute campus were monitored
and subsequently, all the measurements were validated by the mass-spectrometric
measurements (Fig. 4).
However, Airborne platform for atmospheric trace concentration measurement has
become a superior methodology for understanding the real-time chemical process in
the atmosphere. Such understanding is really helpful for the researcher to investigate
the transport dynamics of chemical species from the troposphere to stratosphere with
high spatial and temporal resolution. Moreover, it also provides information on the
column amount of the atmospheric constituent, which is beneficial for developing
the atmospheric model to get proper scientific knowledge of atmospheric chemistry. First QCL based airborne atmospheric sample measurement was accomplished
by the Webstar et al. [43]. In their study, they utilised a cryogenically cooled QC
laser operated at 8 mm spectral region to monitor the CH 4 and N 2 O concentration up to 20 km in the stratosphere over North America, Scandinavia, and Russia
during September 1999 and extending through March 2000 in a series of 20 aircraft
flights. The incorporation of QCL with higher output power ~10 mW, narrower
laser linewidth ~17 MHz successively enhanced the measurement precision and
subsequently, a minimum-detectable mixing ratio for methane of approximately 2
parts per billion by volume was achieved in QCL channel measurement. Later, in