Research on Laser Absorption Spectrum Detection Technology for CO2 and NH3 Regional Emission

High-sensitivity emission monitoring of CO2 and NH3 in farmland region is very important for analyzing environment and climate change. The on-line detection technology was studied based on open-path tunable diode laser absorption spectroscopy technology (OP-TDLAS). The detection spectrum of open path was extracted to realize accurate concentration inversion by designing the multi-line fitting algorithm. The system detection stability was evaluated by Allan variance that the detection limit of NH3 and CO2 were about 0.048 L/L and 4.31 L/L respectively. The detection experiment was carried out in farmland of north Anhui to prove that both of the fertilization and straw returning to the field were the emission source of CO2 and NH3 from soil. The growth effect of the former was slower but higher than that of fertilization. This stable spectrum detection method can obtain large-scale concentration results and clear emission rules, which provide technical support for the environment-friendly agriculture. Introduction In recent years, our country has adopted straw returning to the field [1,2] which has changed the physical and chemical properties of the soil in a certain extent and even increased the possibility of CO2 and NH3 emission from the soil. Therefore, CO2 and NH3 emission data in large-scale farmland need obtain by effective and stable detection method to assess its impact on the atmospheric environment, which has become an urgent problem in agriculture and environmental fields [3,4] . Gas monitoring in farmland ecosystem usually adopts box method [5] , micrometeorology method [6-7] and so on. These methods mainly study local soil emissions which are vulnerable to environmental factors, and the requirements to underlying surface and atmospheric stability are high. In recent years, open-path tunable diode laser absorption spectroscopy (OP-TDLAS) technology has been gradually applied for large-scale monitoring. This technology [8,9] continuously measures the gas without sampling and pretreatment. It is more effective to monitor adsorbed gases (such as NH3). Moreover, it has the advantage of fast detection [10] and high sensitivity combined with long optical path technology. Flesch et al. (2007) measured methane volatilization in pasture based on TDLAS technology to verify its measurement accuracy. Todd et al. (2015) carried out ammonia emission monitoring in dairy farmland, and the volatilization rate obtained was 304g·head -1 ·d -1 . Anning Zhu et al. (2012) carried out ammonia emission monitoring before and after nitrogen fertilizer application in Huang-Huai-Hai farmland with TDLAS and BLS technology to obtain the emission rule. At present, there are few reports on effective extraction of characteristic absorption spectrum and improvement of detection stability in open-path detection. In this paper, the multi-line fitting algorithm was designed to realize spectrum detection. Allan variance analysis was carried out to determine the measurement stability, and the OP-TDLAS system was used to monitor CO2 and NH3 regional Emission in farmland. Basic Principles According to Lambert-Beer's law, the monochromatic laser with the laser emitting frequency  and initial light intensity 0 I passes through an absorption medium with certain length of L, and the relationship between laser received intensity   t I  and absorbance   A  is expressed as:         0 ln t I k L A I       (1)   k  is the spectral absorption coefficient of a single gas at a single transition: kv=S(T) ( ) v  Px (2) where S(T) is the line strength, P is the pressure, x is the gas concentration, ( ) v  is a normalized linear function. By integrating formula (2), the gas concentration is expressed as [11] : ( ) A x S T PL  (3) The wavelength range of spectral transition was tuned to retrieve the gas concentration from the transition absorbance results when the temperature, line strength and pressure are known. Detection System and Technology System Design The OP-TDLAS system structure is shown in Fig. 1. The DFB lasers are used as light sources to monitor NH3 absorption line at 6528.8cm −1 and CO2 absorption line at 6336.3cm −1 respectively. The beam combiner couples the two time-division scanned laser beams and divided them into three beams. One beam passed a reference cell filled with standard gases, and the other two beams transmitted to the open optical system consisting of the telescope and retro-reflector. The two optical systems used to measure respectively the gases absorption spectrum of background path and downwind path for quantitatively inverting the concentration. Temperatur e controller Current supply Signal processing module Detector1 Detector3 NH3 DFB laser Beam splitter Collimator Measurement open path Reference path Data acquisition processor DAC