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A new method for the atmospheric detection of OH - Sodium Dimer

OH in the troposphere

The hydroxyl radical has been found to dominate the chemistry of the troposphere; it acts as the primary oxidant for most trace gases and essentially acts as a cleansing agent by removing them [1]. Thus the chemical lifetime of most trace gases, including those associated with global warming, can be determined as a function of OH concentration.

(I)


where τX is the chemical lifetime of a trace gas X, kOH+X is the rate constant for OH with the trace gas X, and [OH] is the concentration of OH. Furthermore, phenomena such as photochemical smog, which is a problem in cities, owe their origin to OH chemistry. Atmospheric models are used to predict the effects of climate change and measurements of key species such as OH allow us to validate the accuracy of such models.

Currently, three techniques exist for the atmospheric measurement of OH: fluorescence assay by gas expansion (FAGE), differential optical absorption spectroscopy (DOAS) and chemical ionisation mass spectroscopy (CIMS). Instruments deploying these techniques are all large and bulky and as a result are not practical for widespread continuous measurements. A technique which is relatively simple to use, lightweight, cheap, and could provide the capacity for multiple instruments that could be deployed anywhere (on either ground based or aircraft campaigns) would be indispensable. The technique described below should meet these targets.

The underlying principle of the sodium dimer measurement technique

The reaction of the sodium dimer with the hydroxyl radical generates sodium hydroxide and a sodium atom. The reaction is sufficiently exothermic to excite the sodium atom from the ground state to the Na(32PJ) state:

Na2 + OH --> NaOH + Na(32PJ) ΔH0 = -61 kJ mol-1

The excited Na(32PJ) atoms fluoresce back to the ground state by emission of photons at 589 nm or 589.6 nm [Na(32P3/2,1/2 ± 32S1/2), τrad = 16.4 ns]. Following a calibration, the collection of the Na(32PJ) fluorescence should provide the OH concentration [2]. This technique should guarantee selectivity as the only other atmospheric species known to react with Na2 to produce chemiluminescence are the halogen atoms and they are generally found only at very low concentrations in the marine boundary layer. In addition, it should be capable of HO2 measurements by addition of NO at a position downstream of the sampling inlet to convert HO2 to OH.

A prototype instrument has been developed at Leeds. The prototype has verified that this concept works and has shown that the sensitivity is dependent on dimer concentration. The objectives of this project are to develop the instrument in a way that allows the production of large sodium dimer concentrations. Under normal conditions at thermal equilibrium for temperatures above that of molten sodium less than 1 percent mole fraction of dimer is present [3]. Studies using nozzle expansions have shown that the equilibrium can be greatly shifted towards the alkali dimers, resulting in dimer mole fractions greater than 30 percent [4]. Therefore we intend to investigate the application of a nozzle expansion to the field instrument.

The initial objectives have been to generate and observe Na2 using the 1(A)1Σu+ --> 1(X)1Σg+ band. The experimental set up is shown in figure 1 as a schematic diagram. Sodium is generated in an oven, in which sodium spheres are placed on a fine gauze to facilitate evaporation. The oven is maintained with an internal temperature of about 270 oC.


Figure 1. A schematic diagram of the current set up, illustrating the positioning around the six way cross.




Lasers are used to give excitation of the sodium dimer and atom. Excitation of the sodium dimer is carried out at 656.2 nm using a Sirah DCM dye laser which is pumped by a frequency doubled Nd:YAG laser at 532 nm. Excitation of the sodium atoms is achieved with 589 nm light from a dye laser (Rhodamine 590 in methanol) which is pumped using 337 nm light from a nitrogen laser.
Reference
  1. Heard, D.E., Atmospheric field measurements of the hydroxyl radical using laser-induced fluorescence spectroscopy. Annual Review of Physical Chemistry, 2006. 57: p. 191-216.
  2. Self, D.E., Plane, J.M.C., and Heard, D.E., Kinetic study of the reactions of the sodium dimer (Na-2) with a range of atmospheric species. Physical Chemistry Chemical Physics, 2006. 8(26): p. 3104-3115.
  3. Demtroder, W., McClinto, M., and Zare, R.N., Spectroscopy of Na2 Using Laser-Induced Fluorescence. Journal of Chemical Physics, 1969. 51(12): p. 5495.
  4. Gordon, R.J., Lee, Y.T., and Herschbach, D.R., Supersonic Molecular Beams of Alkali Dimers. Journal of Chemical Physics, 1971. 54(6): p. 2393-2409.