采用辐射传输模式模拟了特殊角度随气溶胶光学厚度及大气温湿度的变化情况， 结果显示晴空下该角度不受气溶胶影响， 受大气温度影响小， 但受大气水汽含量影响显著， 因此晴空下热红外光谱仪测量下行辐射时特殊天顶角可只根据大气水汽含量确定。 西部荒漠地区下行辐射测量实验结果表明， 据大气水汽含量确定的特殊角度值与实验结果一致， 且该角度处辐射作为积分辐射符合精度要求。 特殊角度值可用研究区大气水汽含量确定， 该快速测量法可广泛用于野外实验， 方便快捷测量准确瞬时下行辐射。

Direct measurement of the sky radiation in the direction of a special zenith angle as the average downward radiation is the fastest instantaneous downward radiation measurement. But this method has not been widely used, because there is no accurate method for determining special zenith angle. The present paper analyzed the special angle with the variation of the aerosol optical thickness and atmospheric temperature and humidity through simulation by radiative transfer model. The results show that the special angles will be affected none by aerosols in clear skies, very little by atmospheric temperature, but significantly by atmospheric water vapor content. Therefore, the special angle could be determined only by atmospheric water vapor content in downward radiance infrared spectroscopy measurements in clear sky conditions. The downward radiation measurements results in western desert region indicate that the special angles determined by local atmospheric water vapor content are the same as experimental results. Besides, the error caused by taking the radiance at the special angle as the integral radiance meets the accuracy requirements. Accordingly, this fast measurement method can be widely used in field experiments while the special angle can be determined by atmospheric water vapor of the study area, since it can measure instantaneous downward radiance conveniently and accurately.