Georgia Institute of Technology’s Airborne Laser Induced Fluorescence Experiment (GITALIFE) measurements of NO and NO2

   

    The spectroscopically selective two‑photon laser‑induced fluorescence (TP‑LIF) technique has been used by the Georgia Institute of Technology group to measure NO in the remote atmosphere since 1983 (Hoell et al., 1985, Bradshaw et al., 1985,1998) and NO2 since 1986 (Sandholm et al., 1990, 1992, and 1994). In this approach, 226 nm and 1097 nm laser beams are used to sequentially excite rotationally resolved transition in the NO molecule. The measured fluorescence resulting from this two photon excitation occurs in a spectral region near 190 nm which is free from laser generated background noise.  The sensor deployed on NASA's TRACE P airborne field measurement program used two new Nd:YAG (GCR model 350) operating at 20 pps and four Optical Parametric Oscillators (OPOs).  The instrument layout, as depicted in Figure 1, shows the OPOs being pumped by one Nd:YAG laser, while the second Nd:YAG produces the photolysis wavelength used to photo-dissociate NO2   

 

Sandholm_Instrument

 

Figure 1. Schematic for MP‑LIF excitation sources, BC, BS are beam combining and separating dichroic mirrors (DM).

In this configuration, an injection seeded Quanta Ray GCR Nd:YAG laser is used as the primary pump laser source for the OPOs. This system is capable of producing the required wavelengths with sufficient energy while occupying less space and weighing, ~700 lbs lighter, than our PEM Tropics system.  The frequency doubled output of one of the OPOs is used to generate typically 4 mJ/pulse levels of 226 nm energy to excite an electronic NO transition.  While a second infrared operated OPO is then used to generate typically 2 mJ per pulse of 1097 nm energy to excite a rotational NO transition.  This system has a couple of unique design features that contribute to its performance: 

Spectroscopic specificity from coincident excitation of two separate ro‑vibronically resolved electronic transitions that enable dilution/calibration techniques to be used  

 High confidence in the measurements within transient (i.e., plume) events is accomplished through the 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, EMI, and/or aerosol scattered UV photons.

 

    As in PEM‑Tropics, a 8 ns laser pulse from a Nd:YAG laser photolyises the nitrogen dioxide molecules at 355 nm.  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 permits 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 10^4 lpm) using ram‑air (see Figure 2).

 

Sandholm Instrument

 

 

Figure 2. Schematic representation of the sensor as it was deployed during NASA TRACE P.   

      This configuration turns over the gas sample with in the manifold faster than the laser pulse rate (i.e., sample residence time through instrument <0.03 s whereas the pulse-to-pulse separation is 0.05 s). This short residence time, large diameter flow system "effectively eliminates" the possibility of interferences from wall‑catalyzed decomposition of thermally labile nitrogen‑containing compounds such as HO2NO2.  Wall‑catalyzed photolytic interferences are also "effectively eliminated" as only the central portion of the flow field (1 cm^2 of the 45 cm^2 full area) is probed within 50 ms by all lasers.  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 photo-fragmented 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