Description of TP-LIF
NO and PF-LIF NO2
The spectroscopically selective two-photon
laser-induced fluorescence (TP-LIF) technique is used by GIT
to simultaneously measure NO and NO2 in the remote atmosphere.
Whether measuring NO or NO2, the heart of the system is the ability
to detect NO. In this approach laser beams at 226 nm and 1097
nm are used to sequentially excite rotationally resolved transitions
in the NO molecule. The measured fluorescence resulting from
excited transition occurs in the spectral region near 190 nm
that is blue shifted and therefore free from laser generated
background noise. The photodynamic scheme enables the TP-LIF
approach to be signal limited rather than background limited.
The two Nd:YAG lasers that have been deployed on both PEM Tropics
A & B will again be used in TRACE-P. On of the Nd:YAG lasers
will be used to generate the NO2 photolysis wavelength while
the other Nd:YAG is pumping optical parametric oscillators (OPOs).
One OPO will produce the infrared wavelength and another the
ultraviolet wavelength necessary to excite the NO molecules.
New for TRACE-P a more direct way of detected NO2 will be deployed.
This involves another OPO which will be used to probe the prompt
NO population from the NO2 photolysis in the v=1 vibrationally
excited state. In all previous deployments of the Georgia Institute
of Technologies Airborne Laser Induced Fluorescence Experiment
(GITALIFE) we have waited for all of the NO produced from NO2
photolysis to relax back to the ground state be for probing the
NO population (we will be measuring NO2 by this technique during
TRACE-P also). When NO2 is photolyzed at 355 nm about 40% of
the population comes off vibrationally hot. Detecting these prompt,
excited NO molecules, will allow us NOT to do an abient NO subtraction
and thus decreasing the error bars on our NO2 measurement. This
technique should out preform the older differential technique
when the NOx budget is shifted to a high NO concentration. It
will not out preform the older differential technique when the
NOx budget is shifted to low ambient NO conditions such is after
dark. This is driven by the actual population that is being probed.
The molecules that do not come off vibrationally hot are available
to be probed by the older technique while only ~40% are available
in the new more direct technique.
A newer data acquisition system will
be deployed for the first time. It’s a far more space and power
saving than the old rack and stack HP system we have flown for
the last 10 years. I should also quickly state that GITALIFE
has been on a very substantial diet since the passing of Dr.
John and it will be between 800 - 900 pounds lighter than what
we have ever installed on the DC8 in the past. Manpower to include
Joe Mastromorino, Drs H. W. Guan, R. E. Stickel, D. K. Tan, and
S. T. Sandholm.
|