Measurements of NO and NO2 Over the Tropical Pacific

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.

GIT Instrument Layout

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.


Table I. Performance of NO Sensor To Be Deployed In 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.,

  1. Spectroscopic specificity from coincident excitation of two separate ro-vibronically resolved electronic transitions that enable isotopic dilution/calibration techniques to be used;
  2. Freedom from background noise, making the technique free from laser generated noise or changes in background environment even under high particle loading conditions (i.e., smoke, clouds, etc.);
  3. High confidence in the measurement of transient (i.e., plume) events due to use of multiple detectors and double coincidence signal processing techniques along with continuous monitoring of the instrument background. The latter eliminates possible effects from spurious noise sources such as cosmic rays and EMI.

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).

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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.


Table II.Expected Performance of MP-LIF Sensor for NO2
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)