Absolute absorption cross-section and photolysis rate of I2

[摘要] Following recent observations of molecular iodine (I₂) in the coastal marine boundary layer(MBL) (Saiz-Lopez and Plane, 2004), it has become important to determine the absoluteabsorption cross-section of I₂ at reasonably high resolution, and also to evaluate the rate ofphotolysis of the molecule in the lower atmosphere. The absolute absorption cross-section (σ)of gaseous I₂ at room temperature and pressure (295K, 760Torr) was therefore measuredbetween 182 and 750nm using a Fourier Transform spectrometer at a resolution of4cm^-1 (0.1nm at λ=500nm). The maximum absorption cross-section in the visible region wasobserved at λ=533.0nm to be σ=(4.24±0.50)x10^-18cm²molecule^-1. The spectrum isavailable as supplementary material accompanying this paper. The photo-dissociation rateconstant (J) of gaseous I₂ was also measured directly in a solar simulator, yieldingJ(I₂)=0.12±0.03s^-1 for the lower troposphere. This is in excellent agreement with the value of0.12±0.015s^-1 calculated using the measured absorption cross-section, terrestrial solar flux for clearsky conditions and assuming a photo-dissociation yield of unity. A two-stream radiationtransfer model was then used to determine the variation in photolysis rate with solar zenithangle (SZA), from which an analytic expression is derived for use in atmospheric models.Photolysis appears to be the dominant loss process for I₂ during daytime, and hence animportant source of iodine atoms in the lower atmosphere.

[学科分类] 大气科学