基于MAX-DOAS测定大气紫外波段水汽的吸收及其对DOAS反演影响的评估

Measurement of Water Vapor Absorption in the Ultraviolet Band Using MAX-DOAS and Evaluation of Its Influence on DOAS Retrieval

大气水汽的吸收强度从微波区域到可见蓝光区域逐渐降低,然而在紫外波段的吸收却经常被人忽略。多轴差分吸收光谱(MAX-DOAS)技术是一种被动光学遥感技术,可以同时反演气溶胶、多种痕量气体(如NO_(2),SO_(2),HCHO,HONO等)以及水汽,常用于区域大气立体分布及输送监测,具有成本低、时间分辨率高、稳定、可实时监测等特点。水汽是一种重要的温室气体,在紫外波段反演一些痕量气体时水汽的吸收经常不被考虑,可能对紫外波段痕量气体的反演造成影响,从而产生系统误差。介绍了基于MAX-DOAS对紫外波段大气水汽的反演,于2020年6月1日—9月24日在西安乾县进行观测,通过选取最优反演波段,并将反演结果与可见蓝光波段的水汽进行对比,证实了紫外波段存在水汽吸收,评估了紫外水汽的吸收对同波段痕量气体反演的影响。首先,根据不同拟合波段反演的水汽均方根误差(RMS)以及水汽和O_(4)的吸收截面情况,选取紫外和可见蓝光波段水汽的最优反演波段分别为351~370和434~455 nm。其次,通过DOAS拟合得到紫外和可见蓝光波段O_(4)和H_(2)O的对流层差分斜柱浓度(DSCD),分别将紫外和可见波段的O_(4)DSCD和H_(2)O DSCD做相关性分析,两个波段O_(4)DSCD的相关系数r=0.85,H_(2)O DSCD的相关系数r=0.80。为消除不同波段的辐射传输差异,将同波段的H_(2)O DSCD和O_(4) DSCD作比值,两个波段H_(2)O DSCD/ODSCD的相关系数r=0.89。紫外和可见蓝光波段H_(2)O DSCD/O_(4) DSCD的高相关系数表明,即使在相对沿海城市水汽浓度较低的西安市,在363 nm附近的紫外波段同样存在水汽吸收,这将会对采用DOAS技术在紫外波段反演其他痕量气体造成影响。最后,分别对可能受紫外波段水汽吸收影响的气体(O_(4),HONO和HCHO)进行DOAS反演误差评估,紫外波段水汽的吸收将使O_(4) DSCD,HONO DSCD以及HCHO DSCD在DOAS拟合过程中增加,分别对应于+1.16%,+8.55%和+9.04%的变化。

The absorption of atmospheric water vapor gradually weakens from the microwave to the visible band,but the absorption in the ultraviolet band has been ignored.Multi-AXis Differential Optical Absorption Spectroscopy(MAX-DOAS)is a passive optical remote sensing technology that can simultaneously retrieve a variety of trace gases such as NO_(2),SO_(2),HCHO,HONO and water vapor.It is often used for regional atmospheric three-dimensional distribution and transportation monitoring,and has the characteristics of low cost,high time resolution stability,and real-time monitoring.Water vapor is an important greenhouse gas,and the water vapor absorption in the ultraviolet band is often not considered when we retrieve trace gases,which may affect the retrieval of trace gases in the ultraviolet band,resulting in systematic errors.This study introduced the atmospheric water vapor retrieval in the ultraviolet band using MAX-DOAS observations in Qianxian,Xi’an,from June 1 to September 24,2020.The optimal retrieval band in ultraviolet and visible were selected andcompared.The comparison results confirmed the water vapor absorption in the ultraviolet band,and we also evaluated the influence of ultraviolet water vapor absorption on the retrieval of trace gases in the same band.First,the optimal retrieval bands for water vapor in the ultraviolet(351~370 nm)and visible blue bands(434~455 nm)were selected according to the root mean square(RMS)and the absorption cross-sections of H_(2)O and O_(4).Secondly,the O_(4) and H_(2)O DSCD in the ultraviolet and visible blue bands were obtained by DOAS fitting,and the correlation between the two bands was analyzed.The two bands’correlation coefficient of O_(4) and H_(2)O DSCD in the two bands were 0.85 and 0.80.The ratio of O_(4) and H_(2)O DSCD in the same band has also been analyzed,and the correlation coefficient in the two bands was 0.89.The high correlation coefficients of H_(2)O DSCD and the ratio of H_(2)O DSCD/O_(4)DSCD in the ultraviolet and visible blue bands indicate that even Xi’an,which has a lower water vapor concentration relative to coastal cities,also has water vapor absorption in the ultraviolet band near 363 nm.It will affect the retrieval of other trace gases in the ultraviolet band using DOAS technology.Finally,the retrieval errors of gases(O_(4),HONO,and HCHO)that may be affected by water vapor absorption in the ultraviolet band were evaluated.The water vapor absorption in the ultraviolet band will increase O_(4)DSCD,HONO DSCD,and HCHO DSCD during the fitting process,corresponding to the changes of+1.16%,+8.55%,and+9.04%,respectively.