Simulation studies on gas-to-particle conversion processes in the stratosphere by using ultraviolet SR (abstract)

Abstract

Since stratospheric particles have a possibility to make a large effect on the climate in a global scale, a number of observations have been carried out using different techniques.1 Those particles were found to consist of mainly sulfuric acid and to be 0.1–1.0 μm in size. Turco and co-workers proposed a model that the particles are formed through chemical and physical processes initiated by photodissociation of OCS (λ<2600 A).2 In order to examine this model, a simulation experiment in a laboratory system has been planned, in which ultraviolet components of SR are utilized. An optical system including a Seya-Namioka monochromator has been designed, constructed, and tuned precisely.3 In this system, the first prefocusing mirror (sphere) deflects the SR beam horizontally by 30°, and the second and the third ones (sphere) focus the beam vertically onto the entrance slit of the monochromator. The grating radius is 0.5 m. The monochromatized photon beam is reflected into the horizontal direction by the postfocusing mirror (toroid). A ray-tracing calculation was performed, and the obtained spot diagrams showed that the widths of the rays are about 100 μm at the two slits, and the spot size is 1.5×0.8 mm2 at the experimental point. These calculated results were confirmed experimentally by using visible and ultraviolet components of SR. From an estimation on the rate of the particle formation using the data available in the literature, the rate determining step was found to be the photodissociation of OCS in the ultraviolet region. At first, the photoabsorption spectra (cross section) were measured at room temperature and at −80 °C (near to the stratospheric temperature). The spectrum is composed of a broad band centered at 2250 Å and of a number of small peaks. Photodissociation rates of OCS at several altitudes have been calculated using the present cross-section values, which indicated that the life of OCS is about 15 years at 20 km and two months at 30 km. It is known that CO and S are formed through the photodissociation of OCS. A computer-controlled mass spectrometer was made for pursuit of the time variation of CO density in the reaction cell, in which the sample gas, a few percent OCS in He, is introduced and irradiated by the photon beam. A chamber has been designed and constructed, in which a pseudostratospheric air is being irradiated by the ultraviolet radiation. A particle counter of laser-scattering type is being connected to this chamber in order to observe the particle number in it. The present study is the first examination applying SR to problems in the stratosphere as a simulated solar radiation. The obtained results have shown opening a new scientific opportunity in the utilization of SR.