High precision measurement of formaldehyde in air with differential optical absorption spectroscopy

Abstract

The formaldehyde (HCHO) measurement is described by differential optical absorption spectroscopy (DOAS) technique and the data processing method is discussed in detail, including the selection of retrieval wavelength band, the removing of interfering structures, the steps of data processing and the spectrum dealing with nonlinear method. The HCHO concentration retrieved in different wavelength bands were compared, including relative error, repeatability and detection limit. As a result, the band from 314 to 332 nm was most suitable for the retrieval of HCHO. With these methods, high measurement precision of 1.9% was obtained and the detection limit of HCHO is less than 1.5 ppb. In addition, high linear relativity 0.9999 is achieved by measuring five different concentrations of HCHO with this system.where I( ), I 0 ( ) are the light intensities with and without absorption by pollutants, L is the length of the light path, c i ( ) is the average concentration of species i at wavelength , i ( ) is the absorption cross section of gas species. The detail can be referred to reference [5].Besides HCHO, SO 2 , NO 2 and O 3 have differential absorption structures in the investigated spectral region (280 -360 nm) with absorption intensities about 10 times higher than that of HCHO, as shown in Fig.1 [4] . Since the concentrations of these gases are usually much higher than that of HCHO, the elimination errors of these absorptions affect the retrieval of HCHO concentration directly. In addition, Xenon emission lines (corresponding to an OD' -3.2 %, -8 ‰ Fig.1 The differential cross-section spectra of O 3 HCHO, SO 2 and NO 2 (with spectral resolution(FWHM): 0.25 nmQuantitative determination of formaldehyde (HCHO) in the air is important not only for the health perspective but also for the role it plays in atmospheric chemistry [1,2] . HCHO constitutes the most abundant carbonyl in ambient air, having its origins both from direct emissions and photochemical reactions, generally through the oxidation of hydrocarbons in the presence of nitrogen oxides (NO 2 ). Formaldehyde represents an important indicator of atmospheric photochemical reactivity and can act as precursor to organic aerosol formation in urban air [3] . UV-differential optical absorption spectroscopy (DOAS) method is receiving growing interest due to its low maintenance [4,5] for the real-time HCHO monitoring. HCHO can be detected by DOAS using three strong bands of the (A 1 A 2 X 1 A 1 ) transition in the near UV region from about 300 up to 347 nm [4] . This paper described the HCHO measurement by DOAS technique and discussed the data processing method in detail, including the selection of measurement wavelength band, the removing of interfering structures, the steps of data processing and the spectrum dealing with nonlinear method.The DOAS technique is based on the variation of BeerLambert's law. The trace gas concentration c i can be calculated by introducing a column optical depth D( ) for a given wavelength [5] :