HCHO, HONO, NO

D. Tan and J. Mastromarino
Georgia Institute of Technology

The GIT measurements of CH2O, HONO, and NO will be an important part of the ANTCI effort to gain a quantitative understand of HOx chemistry as well as to understand the reactive nitrogen budget. Previous polar studies have shown that CH2O, HONO, and NO must be measured if models are to be comprehensively tested in polar regions. The Georgia Tech Laser-induced fluorescence group will simultaneously measure formaldehyde and nitrous acid during the 2003 field study at the South Pole, and formaldehyde and nitric oxide (NO) during the 2005 airborne campaign. Formaldehyde will be measured by direct excitation at 353 nm with detection occurring at ~ 430 nm. The excitation wavelength has been chosen such that the photodissociation yield is minimized and interference avoided. It is generated by frequency doubling the output of a 706 nm optical parametric oscillator, pumped by the second harmonic of a YAG laser. Ambient air is drawn into the sample cell through a small orifice by means of a Roots blower and vacuum pump. The current laboratory version of this system has a detection limit of 120 pptv; however, the final optimized field system is expected to have a 2-s LOD of less than 5 pptv. Calibrations will be by standard addition to the flow using a formaldehyde permeation tube maintained at 60 ^oC as a source. Calibrations and other system diagnostic checks will be performed on an hourly basis on the ground, and as the flight path requires in the air.
The GIT NO measurement is based on two-photon laser-induced fluorescence (TP-LIF) detection and has been used in numerous airborne field studies which have included intercomparisons with other techniques. This system uses two excitation wavelengths, one at 226 nm exciting the NO molecule into the A state, and a second wavelength at 1097 nm, which pumps NO from the A to the D state. Fluorescence in this system is therefore blue shifted with major emission occurring at 187 nm. Thus, the 2-photon technique allows for very high background rejection, making the measurement essentially background-free. A typical NO LOD using TP-LIF is 1 pptv or better. In previous field campaigns, NO and NO2 have been measured using 2 YAG lasers and 2 large master-oscillator/power-oscillators (MOPOs). Improved efficiencies in laser light generation and plus improvements in detection will allow us to reduce this payload to a single YAG laser, plus several small OPOs. Detection will take place in a multipass cell behind the CH2O detection cell. Calibration will be done using standard addition techniques based on a NIST-traceable NO standard, and will be done on a routine basis during the flight.
HONO will be measured by photo fragmentation using the third harmonic of a YAG laser (e.g., 355 nm) followed by LIF detection of the OH moiety in a cell located downstream from the CH2O detection cell. The OH fragment will be excited at 308 nm, and the time-gated resonance fluorescence detected on microchannel plates. This technique has also been proven on many field campaigns dating back to 1996. The OH limit of detection for this technique has been shown to be better than 0.01 pptv. With 2W of photolysis power, the photolytic efficiency is calculated to be better than 10%. Thus, assuming a typical 0.2 to 0.3 pptv ambient OH signal, the HONO limit of detection is conservatively estimated at ~1 pptv. Calibration will be done by standard addition of HONO. As in the case of CH2O, calibrations and other system diagnostics will be performed on an hourly basis.