General Description of TP-LIF (NO) and PF/TP-LIF (NO2) Techniques
The spectroscopically selective two-photon laser-induced fluorescence (TP-LIF) technique has been used by the GIT group to measure NO in the remote atmosphere since 1983 (Hoell et al., 1985, Bradshaw et al., 1985,1998) and NO2 back to 1986 (Sandholm et al., 1990, 1992, and 1994). In this approach, 226 nm and 1.1 mm laser beams are used to sequentially excite rotationally resolved transitions in the X2P ® A2S+ and A2S+ ® D2S+ manifolds. The measured fluorescence resulting from the D2S+ ® X2P transition occurs in a spectral region near 190 nm which is free from laser generated background noise. This photodynamic scheme and its resultant freedom from noise enables the TP-LIF approach to be a signal limited rather than a background limited technique. Thus, improvements in the TP-LIF instrument's sensitivity translate linearly into improvements in limit-of-detection (LOD) and temporal response.
The sensor deployed during NASA's 1996 PEM-Tropics A airborne field measurement program was improved, compared to previous versions, by using two new Nd:YAG (GCR model 350) and Master Oscillator Power Oscillator (model MOPO-730 FDO) laser systems from Spectra Physics. Both the Nd:YAGs and MOPOs were modified to run off aircraft power and within the space of the current TP-LIF instrument. The instrument layout, as depicted in Figure 1, shows the two MOPOs being pumped by one Nd:YAG laser, while the second Nd:YAG produces the photolysis wavelength used to photodissociate NO2.
Figure 1. Schematic for MP-LIF excitation sources, BC, BS are beam combining and separating dichroic mirrors (DM).
Improvements in sensitivity were achieved during PEM-Tropics A from
the incorporation of currently available laser and electro-optic
technology into our sensor. Table I gives the TP-LIF sensor's NO
performance for the sensor we are deploying on PEM-Tropics B.
Frequency of Observation
(signal integration time) |
5 Hz
(0.2 sec) |
1 Hz
(1 sec) |
0.1 Hz
(10 sec) |
Horizontal Spatial Resolution
(@200 m/s air speed) |
40m | 200m | 2km |
Vertical Spatial Resolution
(@300 m/min ascent/descent rate) |
1m | 5m | 50m |
Measurement Precision (±1s)
@10 ppt NO @50 ppt NO |
.
14% 6% |
.
6% 3% |
.
2% 1% |
Limit-of-Detection(S/N = 2/1)
|
1ppt | 0.2ppt | <0.02ppt |
Total Sample Residence Time | <0.03sec | <0.03sec | <0.03sec |
Base Data Reordering Rate | 20Hz | 20Hz | 20Hz |
This sensitivity results from four basic modifications that were incorporated into the sensor package during PEM-Tropics A. The first involved the straightforward 2-fold increase in sensitivity that results from increasing the number of dichroic filter/CsTe PMT assemblies from two to four, and a two-fold improvement resulting from refinements in a second generation of the new optical filter design. Most of the remaining improvements center on the incorporation of optical parametric oscillator technologies.
The optical schematic for the TP-LIF portion of the sensor is depicted in Figure 1. In this configuration, an injection seeded Quanta Ray GCR Nd:YAG laser is used as the primary pump laser source. This system is capable of producing higher energy, narrower linewidth output than our previous PEM-West laser system, while using less space and electrical power. In the modified sensor configuration, the GCR is used to pump two MOPO-730 narrow linewidth (< 0.2 cm-1) master oscillator/power oscillator optical parametric oscillator systems. The frequency doubled output of one of the MOPO-730's is used to generate mJ/pulse levels of narrow linewidth tunable 226 nm energy to excite the NO X2 P ® A2S+ transition. (The MOPO resolution was <0.4 cm-1 as compared to the previous laser's ~1 cm-1.) The second, infrared operated, MOPO is then used to generate >10 mJ per pulse of narrow linewidth (<0.2 cm-1) tunable 1.1 mm energy to excite the A2S+ ® D2S+ transition. Overall, a 5-fold sensitivity increase was obtained from the significantly larger available excitation energies at both 226 nm and 1.1 mm (i.e., ~5 mJ vs. ~1 mJ, and > 10 mJ vs. 0.7 mJ, respectively). Even with the above improvements in sensitivity, the laser generated background remained negligible. In the PEM-West B sensor, laser background noise was < 3 counts/180 seconds under all tropospheric sampling conditions. With the sensitivity improvements discussed above, the current sensor continues to be signal limited rather than background noise limited even at the sub pptv levels of NO. The current generation TP-LIF spectrometer should set the benchmark for making unambiguously measurements under any sampling condition. Several unique design features contribute to this sensor's robust performance, e.g.,
As in PEM-Tropics A, we will use a high efficiency Nd:YAG laser
photolysis source that enables the PF/TP-LIF sensor to use a significantly
more selective photolytic conversion scheme. In the case of NO2, a single
8 ns laser pulse is responsible for the photolysis step being carried out
with high efficiency. Quantitatively carrying out the photolytic
conversion in a single laser pulse has enabled us to greatly minimize
potential interferences from wall-catalyzed thermal and photolytic
processes. This achivement permited us to design a sample inlet system
that capitalizes on the independence of the TP-LIF technique's signal
strength and sensitivity on the flow rate through the sensor. The modified
sensor uses a large volume-to-surface ratio sampling manifold that is fed
at high flow-rates (~3 x 104 lpm) using ram-air (see Figure 2).
Figure 2. Schematic representation of the sensor as it was deployed during NASA PEM Tropics A.
This configuration turns over the sample faster than the laser pulse
rate (i.e., sample residence time through instrument <0.03 s). Thus,
the total residence time is 100-fold shorter than in our PEM-West A
sensor. This shorter residence time, large diameter flow system eliminates
the possibility of interferences from wall-catalyzed decomposition of
thermally labile nitrogen-containing compounds such as HO2NO2, even if the
reaction probability at the wall is unity (i.e., <10% interference for
an equal level of HO2NO2 and a reaction probability of unity as opposed to
100% interference at a reaction probability of 10-4 for earlier systems).
Wall-catalyzed photolytic interferences are also eliminated as only the
central portion of the flow field (<1 cm2 of the 45 cm2 full area) is
probed within 50 ms by all lasers. The laser configuration now employed
uses the primary TP-LIF portion of the laser system to generate 226 nm and
1.1 mm excitation laser beams at a pulse rate of 10 pps while the NO2
photolysis laser is then "fired" on every other shot at 5 pps. A
computer controlled (and adjustable) 50 ms delay is used between the
photolysis laser pulse and the TP-LIF NO fluorescence excitation laser
pulses. This time delay enables the nascent photofragmented NO population
to repartition into an ambient temperature Boltzmann distribution. In this
scheme, a single fluorescence monitoring cell is used to detect NO and NO2
on alternate laser pulses. This configuration increases the absolute
accuracy of the NO2 measurement by eliminating the subtle differences that
can arise in previously used separate NO and NO2 sampling cell
arrangements.
Compound | NO2 |
Method of Detection | PF/TP-LIF |
Limit-of-Detection | 1 pptv(S/N = 2/1) (@5 pptv NO, and 1 Hz)
0.3 pptv (@5 pptv NO, and 0.1 Hz) 1 pptv(@50 pptv NO, and 0.1 Hz) |
Measurement Precision (±1s) | ±5% (@10 pptv NO and NO2, and 0.1 Hz)
±5% (@100 pptv NO and NO2,and 1 Hz) |