Observation and Inversion of the N₂O Gas Column Concentration in Hefei,China

Nitrous oxide(N2O) is an important anthropogenic greenhouse gas(GHG) in the atmosphere, except for carbon dioxide (CO2) and methane (CH4). There are 75% of N2O sources come from agricultural activities,and the rest is mainly caused by the decomposition of fossil fuel. However,large amounts of N2O will accelerate the greenhouse effect. Because N2O absorbs ultraviolet radiation (wavelength shorter than 315 nm) and photolysis to NO,which will lead to the destruction of the ozone layer. For decades,the global greenhouse potential of N2O is approximately 300 times higher than CO2, thereby causing the significant warming effect on global climate. According to statistics, the globally averaged mole fraction of N2O in the atmosphere was 333.2 ppb(parts per billion by volume) in 2020, representing a 1.2 ppb increase compared to 2019. Ground-based Fourier Transform Infrared (FTIR) spectrometer is an ideal gas concentration detection method with high accuracy,continuous measurement and high sensitivity to ground sources. The Total Carbon Column Observing Network (TCCON) is a ground-based network of Fourier transform spectrometers (FTSs). It was established in 2004 and focused on the accurate measurement of GHG column abundance,through the near-infrared(NIR)solar absorption spectroscopy. The scientific objective of the network is to provide a reference between satellite measurements and the ground-based in situ networks,which can provide the long-term global distribution of N2O mole fractions. All TCCON sites adopt the high-resolution" Bruker OpticsTM IR FT spectrometer(IFS) 125HR"which has high accuracy and excellent performance for GHG detection with a resolution of 0.000 96 cm(-1). Due to the relatively high cost of IFS 125HR and the lack of infrastructure and strict network requirements,observations are significantly limited. Compared with the IFS 125HR,the compact and portable GHG observation equipment EM27/SUN spectrometer is a flexible supplement. The EM27/ SUN was developed jointly by KIT and Bruker,which has been commercially available since 2014. The EM27/SUN is a portable greenhouse gas observation instrument. The built-in SUN tracker can accurately track the sun's trajectory. The lightweight and compact structure can satisfy the requirements of measuring and retrieving the column-averaged dry air mole fraction of the target gas in the outfield. The observations of the total columns of N2O were carried out in Science Island,Hefei. To invert the column concentration,we first obtained two absorption bands of N2O in the near-infrared spectrum by the LBLRTM simulation based on the standard atmospheric model. Among them, the absorption band of 4 373.5 similar to 4 441.6 cm(-1) contains the strong absorption of CH4 and H2O,and another one of 4 682.5 similar to 4 756.1 cm(-1) contains the weaker absorption of H2O and CO2. Based on analysis,the better absorption band for retrieving is 4 682.5 similar to 4 756.1 cm(-1). After obtaining the optimal absorption band,we can achieve the inverse of the column concentration with the improved PROFFAST algorithm. In particular, the PROFFAST is a search algorithm package developed and maintained by Dr. Frank Hase at KIT,which applies to EM27/SUN. This algorithm,combined with the best estimation method,has been verified to be applicable to the inversion of dry air mole fraction (DMF) of trace gases in EM27/SUN low resolution measurement. The observation results of the EM27/SUN and high-resolution TCCON were comprehensively compared in this paper. The average dry air mole fraction of N2O (XN2O)observed by the EM27/SUN spectrometer is 311.76 similar to 334.92 ppb(average 323.26 ppb)in 6 months under a clear sky, which is consistent with the TCCON inversion value. Concretely the XN2O variation amplitude observed by the EM27/ SUN spectrometer and TCCON is 319.11 similar to 325.37 ppb and 322.40 similar to 329.29 ppb, respectively. In XN2O,the overall deviation between EM27/SUN and TCCON is about 0.84 similar to 7.88 ppb, and the relative error range is 0.26%similar to 2.41%. The observed value of EM27/ SUN is slightly lower than that of the TCCON station. The relative error range is reduced to -0.90%similar to 1.36% by using the empirical correction factor in post-processing. The results of this study provide data and reference for further research on the changes in N2O column concentration in Hefei.