傅里叶变换在差分吸收光谱技术气体浓度计算中的应用

在实际测量得到的气体吸收光谱中,发现大多数气体的吸收光谱具有明显的周期性,而傅里叶变换正是用来寻求信号的频率特征。在加窗的条件下,通过对不同气体的吸收光谱进行傅里叶变换,来寻求光谱信号对应的特征频率。在数据分析过程中,发现这样一个规律:在气体吸收光谱经傅里叶变换后的频谱图中,其对应特征频率的幅值与所测的气体浓度成明显的线性对应关系。因此,提出一种新的差分吸收光谱浓度解析方法,即利用气体吸收光谱傅里叶变换后其对应特征频率的幅值与浓度的关系,建立一种浓度反演计算的线性关系式,从而由气体吸收光谱傅里叶变换后特征频率的幅值直接求出气体的浓度。该方法完全摆脱了差分吸收光谱技术的理论基础,大大减少了光谱分析和气体浓度反演计算的过程,是一种值得进一步去探究的光谱分析方法。

Using Fourier Transform to Calculate Gas Concentration in DOAS

Being an analysis tool of high sensitivity,high resolution,multicomponents,real-time and fast monitoring,the differential optical absorption spectrometry(DOAS) is becoming a new method in atmosphere pollution monitoring.In the DOAS technique,many gases spectra have periodicity evidently,such as those from SO2,NO,NH3 and NO2.Aiming at three kinds of main air-polluted gases,i.e.SO2,NO and NO2 in atmosphere,the DOAS technique is used to monitor them,and Fourier transform is used to analyse the above-mentioned absorption spectra.Under the condition of Hanning Windows,Fourier transforma is used to process various gases spectra which have periodicity.In the process,the value of the characteristic frequency has a linearity relation to the gas concentration.So a new analysis method of DOAS is proposed,which is utilizing the relation between the value of the characteristic frequency and the gas concentration to deduce a linearity formula to calculate the gas concentration.So the value of the characteristic frequency can be used to get the gas concentration.For the gases with evident spectrum periodicity,such as SO2 and NO,this method is good.But for some gases with periodicity not evident,the error in the calculated concentration is beyond the allowable value.So in this method,the important process is frequency separation.It is also the main part in the future study.In a word,this method frees itself from the basic theory in the DOAS technique,cuts down on the process of the concentration calculation and the spectral analysis,and deserves further study.