Retrieving Tropospheric Vertical Distribution in HCHO by Multi-Axis Differential Optical Absorption Spectroscopy
TIAN Xin1,2, XU Jin2, XIE Pin-hua1, 2, 3*, LI Ang2, HU Zhao-kun2, LI Xiao-mei2, REN Bo1,2, WU Zi-yang1,2
1. School of Environmental Science and Optoeclectronic Technology, University of Science and Technology of China, Hefei 230026, China
2. Key Laboratory of Environmental Optical and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
3. CAS Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
Abstract：Formaldehyde (HCHO) plays an important role in atmospheric photochemical reactions. Besides, it’s an important aerosol precursor and photochemical oxidation indicator. The sources of HCHO in the atmosphere are mainly from primary direct emissions and photochemical reactions. Atmospheric photochemical reaction is closely related to the intensity of solar radiation. In general, the stronger the intensity of solar radiation is, the more active atmospheric photochemistry reaction will be so that the secondary sources of HCHO are higher. Thus, the research on HCHO has become an important topic in today’s atmospheric environment research. This paper introduces a method for retrieving tropospheric vertical column density (VCD) and vertical profile in HCHO based on multi-axis differential absorption spectroscopy (MAX-DOAS). This method is a two-step inversion method based on nonlinear optimal estimation method. Firstly, the vertical profile of aerosol is retrieved. Secondly, the vertical distribution of HCHO is retrieved based on the retrieved aerosol profile. In the second step of the gas profile inversion, the aerosol information affects the inversion accuracy of the gas vertical profile by influencing the weight function. Therefore, the effects of three different aerosol profile types (exponential, Gaussian and Boltzmann) on HCHO vertical profile inversion were studied. The results show that the total gas inversion error, the envelope curve of the average kernel, the limit of sensitivity height, the degree of freedom, and the vertical profile of HCHO retrieved in the three aerosol types are similar, which means that the aerosol profile type has little effect on the HCHO vertical profile inversion. For the near-surface below 200 m (including 200 m), the differences between the real HCHO volume mixing ratios (VMRs) with the HCHO VMRs obtained by the aerosol profile in exponential, Gaussian and Boltzmann shapes are 36.89%, -0.04%, and 23.3%, respectively. It is shown that the retrieved HCHO vertical profiles using the priori aerosol types in exponential and Boltzmann overestimates near-surface HCHO VMRs, while it is just the opposite in Gaussian shape. Furthermore, the vertical profile of HCHO in one polluted episode in the University of Chinese Academy of Science in Huairou District (UCAS) of Beijing is studied to obtain the vertical distribution in HCHO. The results indicate that the high value in HCHO mainly concentrates below 1.0 km from the vertical profile in HCHO and the high value in HCHO diurnal variations appears in the early afternoon, which indicates that the HCHO is mainly from local photochemical reaction. In a word, the southwest wind brings the polluted VOCs gas mass to the UCAS, and then HCHO generates and accumulates in local photolysis by VOC, which causes this HCHO polluted episode. Combined with backward trajectories determined (HYSPLIT) model, the transport from the southwest of the UCAS is an important cause of this polluted episode in HCHO. Therefore, HCHO at the observation site is mainly affected by pollution transport and secondary oxidation. Finally, the influence of different aerosol conditions on the HCHO profile inversion during this pollution episode was compared. The results show that when the aerosol optical depth increases, the retrieved limit of sensitivity height, the degree of freedom and the height resolution decreases, and the retrieved total error increases.
Key words：Multi-axis differential optical absorption spectroscopy (MAX-DOAS); The vertical profile in HCHO; Inversion algorithm; Aerosol profiles types; Polluted episode
[1] Tian X, Xie P H, Xu J, et al. Atmos. Chem. Phys., 2019, 19：3375.
[2] Lee H, Ryu J, Irie H, et al. Atmos., 2015, 6, 1816.
[3] Tian X, Xie P H, Xu J, et al. Journal of Environmental Sciences, 2018, 71：207.
[4] Vigouroux C, Aquino C A B, Bauwens M. Atmos. Meas. Tech.，2018, 11: 5049.
[5] Tadic I, Parchatka U, Königstedt R, et al. Appl. Phys. B, 2017, 123：146.
[6] Wang Y, Lampel J, Xie P H, et al. Atmos. Chem. Phys., 2017, 17: 2189.
[7] Rodgers C D. Series on Atmospheric, Oceanic and Planetary Physics, 2000, 2.
[8] WANG Yang, LI Ang, XIE Pin-hua, et al(王 杨, 李 昂, 谢品华，等). Acta Phys. Sin.(物理学报), 2013, 62(18)：180705.
[9] WANG Yang, LI Ang, XIE Pin-hua, et al(王 杨, 李 昂，谢品华，等). Acta Phys. Sin.(物理学报)，2013，62（20）：200705.
[10] Meller R, Moortgat G K. J. Geophys. Res. Atmos., 2000, 105：7089.
[11] Fleischmann O C, Hartmann M. J. Photochem. Photobiol. A Chem., 2004, 168：117.
[12] Vandaele A C, Hermans C, Simon P C, et al. J Atmos. Chem., 1996, 25(3)：289.
[13] Serdyuchenko A, Gorshelev V, Weber M, et al. Atmos. Meas. Tech., 2014, 7：625.
[14] Thalman R, Volkamer R. Phys. Chem. Chem. Phys., 2013, 15：15371.
[15] Zhang J S, Chen Z Y, Lu Y H, et al. Atmos. Environ., 2017, 148：30.
