Research On Measurement Technology And Application Of Methane Flux For Agricultural Composite Non-point Source By Open-path Optical Method

The greenhouse effect is caused by the increase of carbon dioxide and other gases in the atmosphere,which makes the global temperature rise.Climate warming will lead to increased rainfall in some areas,drought in others,the increased frequency and strength of hurricane,resulting in aggravation of natural disasters.As the primary greenhouse gas emitted by human production and social activities,the impact of carbon dioxide on greenhouse effect has attracted wide attention from the scientific community at home and abroad.However,the influence of trace gases with strong greenhouse effect in the air can't be ignored.Analysis of global warming potential(GWP)shows that the greenhouse effect of methane per molecule is more than 20 times greater than that of carbon dioxide.The concentration of methane in the atmosphere is greatly affected by human activities,and agricultural emission is one of the main anthropogenic emission sources of atmospheric methane.Therefore,long-term monitoring of methane emission fluxes in farmland is of great significance for assessing the emission level of agricultural non-point source methane emission accurately.The technology of open-path gas concentration detection can continuously and automatically detect the concentration of emission under the natural growth conditions of crops in farmland.Combining with meteorological parameters,the flux of gas emission in farmland can be calculated,which based on the inverse dispersion model.However,the measured farmland area and adjacent farmland constitute a compound emission source in the process of measuring the regional agricultural non-point source flux,which interferes the measurement of the flux by the gas emissions from adjacent fields.The key technologies and applications of agricultural composite non-point source emission flux measurement are studied in this paper.The main research contents and results are as follows;(1)The principle of gas concentration measurement by open-path laser absorption spectroscopy and calculation of gas emission rate by inverse dispersion model are described.The problems faced by the measurement of methane flux from agricultural composite non-point source are studied.The problem of methane flux measurement from agricultural composite non-point source is transformed into the analytical problem of cross-interference among different sources in the region.The overall design scheme of the agricultural composite non-point surface source flux detection system based on the source and wind direction characteristics has been designed.An agricultural compound non-point source emission flux detection method has been proposed.(2)The key technical issues of laser gas absorption spectroscopy in open-path are studied.An improved wavelength correction algorithm based on cross-correlation has been proposed to improve accuracy of measuring spectral wavelength alignment.According to the difference contribution between the signal and noise to the signal,a cascaded stepwise singular value decomposition spectral optimization algorithm has been proposed based on the signal principal component analysis to enhance the anti-interference ability of the spectral signal.The influence of atmospheric turbulence on the intensity and phase fluctuation of laser detection are studied.A spectral data processing algorithm based on co-frequency and dual-wave has been proposed to reduce the influence of scintillation noise.The phase fluctuation caused by atmospheric turbulence is eliminated by the relationship between the dual-waves in one cycle.The laser absorption spectrum signal detection circuit and control analysis software have been designed.(3)The evaluation experiments of gas concentration measurement and meteorological parameter measurement have been carried out,and the data quality have been analyzed.The gas concentration measurement experiments have been carried out by using optical multi-reflection sample pool and standard concentration gas.The results show that the standard deviation of static accuracy in the laboratory is 0.02725,the standard deviation of repeatability in 10 groups is lesser than 0.03155,the correlation coefficient between the measured curve and the fitting curve is 0.990,the standard deviation of static stability is 0.04998,and the static response time is lesser than 3s.The accuracy and repeatability of simulated atmospheric turbulence are evaluated by using rotating thin phase screen.The standard deviation of accuracy is lesser than 0.0548 and the standard deviation of repeatability is lesser than 0.06801.The measurement and evaluation of micro-meteorological parameters mainly include the design of meteorological parameters software,the selection of zero-wind environment of the experimental building for wind deviation test,and the consistency analysis under the same environment with the benchmark instrument.The correlation between the average wind speed of the two sensors is 0.8127.(4)The external field simulation experiment of the flux measurement system has been carried out to test the accuracy and environmental adaptability.The experimental site has been constructed by the artificially simulated methane volatilization source.The release rate Q of the artificially simulated methane volatilization source is compared with the QbLS obtained by the inverse dispersion model.The QbLs/Q ranged from 0.7929 to 1.014,which verified its accuracy.By two sets of measuring devices,the experiments under the ideal environment,the wind direction under the source and the environment on the source have been respectively carried out.The results show that the standard deviation of QbLSIQ in the ideal environment is 0.0277.The QbLS/Q standard deviation obtained from the wind direction of the source is 0.0283.And the QbLS/Q standard deviation set on the source is 0.0256,which indicates that the local wind interference is negligible for the gas flux measurement error.(5)The field experiment and data analysis of agricultural composite surface source have been carried out.A composite non-point surface source emission scenario based on two artificially simulated methane volatilization sources has been constructed.The experimental results are measured by the composite non-point surface source flux measurement method which proposed.The results showed that the standard deviation of QbLsa/Qa is 0.173 in source a.The standard deviation of QbLSb/Qb is 0.1842 in source b,which realized accurate measurement of gas fluxes in different emission sources.The field experiment of methane flux measurement of farmland composite non-point surface source has been carried out in Fengqiu Ecological Station.The area of different fertilization methods is measured.The average flux of the source a is 31.1402 ug/m2/s,and the source b is 43.7159 ug/m2/s,which achieve accurate measurement of composite non-point surface source methane flux emissions..