Oct 10th, 2000            


Description of GIT Two Photon/Laser Induced Fluorescence (TP/LIF) NOy Sensors and Performance on NASA's GTE TRACE-A Field Program
                         
The sequential step-wise excitation, two-photon, laser-induced fluorescence technique was applied to the in-situ airborne measurement of NOy during the TRACE-A field program.  The basics of this technique have been previously reported (e.g., Bradshaw et al., 1985; Sandholm et al., 1990; Sandholm et al., 1992; Sandholm et al., 1994; and Sandholm et al., 1997).  This file is being provided to NASA to aid the use of the archived data as a result of our participation in the PEM West A&B field programs conducted in 1991 and 1994, respectively, and subsequent laboratory studies related to artifacts associated with our NOy measurements.   The Nd:YAG pumped-dye laser system used to generate the UV and IR excitation wavelength was not significantly different than that used during the PEM-WA programs.  Laser linewidth parameters, spectral energy densities, etc. were similar to those previously characterized and reported for the system. As in previous experiments, a reference signal from a known mixture of NO was used as an "internal standard" to normalize for in-flight changes in laser performance (i.e. wavelength and energy drifts), as well as small changes in the fluorescence-quenching environment with changes in altitude.  These reference measurements
were continuously made at the beginning and end of the laser beam line with the ambient sampling cells in between.  The fluorescence-detection optics and photomultiplier tubes (PMTs) were significantly different from those used in previous studies.  


Only non-sticky compounds are currently thought to have fast response (i.e., < 1 second) in our NOy system, and care is recommended in analyzing short temporal plumes as the HNO3, particulate NO3, and other sticky compounds of the NOy budget are likely to have washed out features with significantly longer time constants (e.g., on the order of a minute for HNO3 and perhaps even longer for NO3-aerosols).  

NOy conversion efficiency tests conducted to date indicate that several "non-reactive" nitrogen compounds having less than a +2 oxidation state are being converted to a measurable NO signal in our system.  These compounds include HCN, NH3, and CH3CN.  In addition, the nitro-alkanes also convert (nitro-ethane, nitro-propane, etc. as does nitro-toluene).  For the -3 oxidation state, compounds conversion efficiency is very dependent on the level of H2O vapor in the ambient air. For the configuration used in PEM-WB field program, this conversion efficiency increases from near zero at 0 C D.P. increasing by approximately 4% per -10 C change in D.P.  This H2O dependent behavior is not observed in the Nitro-NC compounds (full details of these studies can be found in Sandholm et al., 1994, and Bradshaw et al., 1998).  For the configuration used in TRACE A and earlier programs this efficiency increases by approximately 10% per -10 change in dew point (i.e. 40% efficiency at -40 C). Nitrogen-Sulfur compounds have yet to be tested (e.g., nitrosylsulfuric acid).  N2O has been tested separately and in combination with the hydrocarbons C2H6, C3H8, and C2H2.  No significant conversion was found (i.e., equivalent to less than 10 pptv conversion even for 10-fold larger than ambient levels of N2O and NMHCs). 


Specific questions regarding this data set should be addressed to the following:
Scott Sandholm
Georgia Institute of Technology 
Atlanta, GA 30332.

From the original TRACE-A archived files:
          

The variable names that start with a "LV" are limiting values, either an upper or lower limit, (see the coding in the column for that molecule for details).  The #LODS line is a string that represents the numbers used to designate that the measured value was below the instrumental limit of detection, and the paired line, with the #LODC delimiter, is a pointer to the column that contains the mixing ratio equivalent to the limit of detection for that observation.  The #LLDS is a string, similar to the #LODS, these numbers represent that the measured value was out of range for the instrument and that the true value is actually greater than the value reported.  The #LLDC line gives the paired column in which the limiting mixing ratio is reported for that observation.  The reported time is the center point of the integration period.  The data is recorded at 30 seconds; the values reported are for 180 seconds signal integration periods.  Calibration uncertainty (accuracy) is estimated to be approximately +/- 15% for [NO], +/-18% for [NO2], and +/-20% for [NOy] at the 95% confidence limit and should be treated as a random additive error term. Sigma values represent measurement precision estimates based on photon statistics only.  Two NxOy data sets are archived for each flight, one based on 90 second and a second at 180-second integration periods.  GIT strongly recommends using the 180-second database for any critical work involving [NO2] as the measurement's relative uncertainty of 90 second database is larger.  Limited measurements using 30-second integration periods can be obtained from GA TECH on a case basis.
 
References:

"A Two-Photon Laser-Induced Fluorescence Field Instrument for Ground-Based and Airborne Measurements of Atmospheric NO", Bradshaw, J.D., Rodgers, M.O., Sandholm, S.T., KeSheng, S., and Davis, D.D., Journal of Geophysical Research, Vol. 90, No. D7, Pages 12,861-12,873, December 20, 1985.

"An Airborne Compatible Photofragmentation Two-Photon Laser-Induced Fluorescence Instrument for Measuring Background Tropospheric Levels of NO, NOx, and NO2", Sandholm, S.T., Bradshaw, J.D., Dorris, K.S., Rodgers, M.O., Davis, D.D., Journal of Geophysical Research, Vol. 95, No. D7, Pages 10,155-10,161, June 20, 1990.

"Summertime Tropospheric Observations Related to NxOy Distributions and Partitioning over Alaska: Arctic Boundary Layer Expedition 3A", Sandholm, S.T., Bradshaw, J.D., Chen, G., Singh, H.B., Talbot, R.W., Gregory, G.L., Blake, D.R., Sachse, G.W., Browell, E.V., Barrick, J.D.W., Shipham, M.A., Bachmeier, A.S., Owen, D., Journal of Geophysical Research, Vol. 97, No. D15, Pages 16,481-16,509, October 30, 1992.

"Summertime Partitioning and Budget of NOy Compounds in the Troposphere over Alaska and Canada: ABLE 3B", Sandholm, S., Olson, J., Bradshaw, J., Talbot, R., Singh, H., Gregory, G., Blake, D., Anderson, B., Sachse, G., Barrick, J., Collins, J., Klemm, K., Lefer, B., Klemm, O., Gorzelska, K., Herlth, D., O'Hara, D., Journal of Geophysical Research, Vol. 99, No. D1, Pages 1837-1861, January 20, 1994.

"Recent and Future Improvements in Two-Photon Laser-Induced Fluorescence NO Measurement Capabilities", Sandholm, S., Smyth, S., Bai, R., Bradshaw, J., Journal of Geophysical Research, Vol. 102, No. D23, Pages 28,651-28,661, December 20, 1997.

"Measurement of Total Reactive Odd-Nitrogen (NOy) in the Rural and Non-Rural Troposphere", Sandholm, Scott T., Bradshaw, John D., Proceedings of the 1994 U.S. EPA A & WMA International Symposium.

"An Update on Reactive Odd-Nitrogen Measurements Made During Recent NASA Global Tropospheric Experiment Programs", Bradshaw, J., Sandholm, S., Journal of Geophysical Research, Vol. 103, No. D15, Pages 19,129-19,148, August 20, 1998.