Spectral line finding program for atmospheric remote sensing using full radiation transfer

We have developed a line-by-line code to simulate atmospheric spectra in the infrared spectral region assuming a remote light source of selectable temperature. Selecting the source temperature allows simulation of, for example, solar absorption spectra, lunar absorption spectra or emission spectra. The code is applicable for different geometries, from the ground, balloon or from satellite, and allows a simple search for the most suitable lines for the retrieval of atmospheric trace gas concentrations for the different geometries. In addition, a first estimate of the optimized microwindow size and the most important interfering gases that need to be considered is calculated automatically. This approach is of importance in cases where it is necessary to analyze numerous lines simultaneously to get sufficient precision in the trace gas concentration retrievals. Examples for H₂O and CO are given.     

Effects of vertical grid discretization in infrared transmission modeling

The vertical spacing of a discretization used to model transmission in the mid-infrared spectral range was investigated. The forward model employed in this study is a part of an algorithm used to retrieve trace gas profiles from high-resolution ground-based solar absorption Fourier transform infrared (FTIR) spectra, however, the results have general applicability. A finely spaced retrieval grid was constructed and made progressively more sparse in the troposphere and in the stratosphere. The effect was quantified in terms of transmission differences with respect to the most fine discretization for a suite of molecules (H₂O, O₃, CO, CO₂, CH₄, N₂O, NO, NO₂, HCl, HF, HNO₃, ClONO₂, and N₂) in microwindows commonly used in FTIR spectroscopy. Systematic differences in modeled transmissions are apparent when coarser grid schemes are used for all species and microwindows, though some are below random noise levels typical of spectra recorded at Toronto. The most significant are H₂O and O₃ at 0.30-0.73% and 0.10-0.34%, respectively. CO (0.13%), ClONO₂ (0.84%), and HF (1.03%) are also influenced by the interference of H₂O, which is sensitive to temperature interpolation errors via the lower state energy of the particular H₂O transition. O₃ is a significant interference in CO (0.42%) and ClONO₂ (0.31%) microwindows, but its influence is felt primarily via interpolation errors in the O₃ number concentration profile introduced by the coarser grids. HCl and HF themselves show the next most significant response in transmission to coarser stratospheric grids (∼0.18%). Finally, considering transmission differences >0.1% as significant in typical measurements, we identify maximum tropospheric and stratospheric layer widths that still lead to negligible transmission errors as, respectively, 0.6 and 2.0 km. These numbers can vary depending on the band or transition of interest, the signal-to-noise ratio of the measurement and the use of significantly different a priori volume mixing ratio profiles.     

Ozone profiles and total column amounts derived at Izaña, Tenerife Island, from FTIR solar absorption spectra, and its validation by an intercomparison to ECC-sonde and Brewer spectrometer measurements

Since February 1999 high-resolution Fourier transform infrared (FTIR) solar absorption spectra are recorded at the Izaña Observatory (, 2370 m a.s.l.), which forms part of the network for detection of stratospheric change (NDSC). The vertical distribution of the ozone amount was deduced from the spectra by using the retrieval code PROFFIT. Two microwindows at 782.5 and are used whereby an individually and regularly determined instrumental line shape (ILS) was applied. A detailed error analysis of the ozone retrieval is performed, i.e. the height-dependent impact of all important error sources on the retrieved profiles is estimated. The retrieved total column amounts and profiles as well as their estimated error budgets are verified by comparisons with column amounts and profiles derived from Brewer spectrometer and ECC-sonde measurements performed simultaneously at the same location. Whereas the mean difference of 1.9% of the FTIR total column amounts and the Brewer data lies within the precision of the instruments, systematic differences of up to 9% between FTIR and ECC-sonde profile are observed above . The assumed precisions of the experiments are consistent with the experimental results, which demonstrates the excellent characterization of the FTIR data. The analysis of systematic errors allows to conclude that the systematic differences are mainly due to the sonde data since the systematic errors in the FTIR profile can only explain a small part of them.     

A retrospective view on “algorithms for radiative intensity calculations in moderately thick atmospheres using a truncation approximation” by Teruyuki Nakajima and Masayuki Tanaka (1988)

I explain the motivation behind our paper “Algorithms for radiative intensity calculations in moderately thick atmospheres using a truncation approximation” (JQSRT 1988;40:51-69) and discuss our results in a broader historical context.     

Many polarized radiative transfer models

This note is an introduction to the reprint of the 1991 JQSRT article “A new polarized atmospheric radiative transfer model” by K.F. Evans and G.L. Stephens. We discuss the significance of the article, how our two plane-parallel polarized radiative transfer codes came about, how our codes have been used, and more recent developments in polarized radiative transfer modeling.     

Atmospheric aerosol limb scanning based on the lunar eclipses photometry

The work is devoted to the analysis of the surface photometric observations of two total lunar eclipses in 2004. The lunar surface relative brightness distribution inside the umbra was used to retrieve the vertical distribution of aerosol extinction of the solar radiation expanding by a tangent path and its dependence on the location at the limb of the Earth. The upper altitude of troposphere aerosol layer was estimated for different latitude zones. The correlation between additional aerosol extinction in the upper troposphere and cyclones was investigated.     

Qualifying IMG tropical spectra for clear sky

The problem of cloud detection for the Interferometric Monitoring of Greenhouse Gases spectrometer has been addressed by considering a set of thresholding tests which takes full advantage of the high spectral resolution of the sensor. The methodology has been applied to a case study consisting of spectra recorded in the tropics on sea surface, although the scheme may be easily extended to other latitudes. The algorithm is very efficient because it uses only the observed spectrum and no on-line radiative transfer calculation is needed. Based on this cloud detection scheme a set of clear-sky tropical spectra have been identified to be used by the scientific community for further studies such as retrieval of atmospheric properties and high spectral resolution radiative transfer modeling.     

Sensitivity of trace gas abundances retrievals from infrared limb emission spectra to simplifying approximations in radiative transfer modelling

The accuracy of trace gas abundances retrieved from spectrally resolved infrared limb emission measurements relies, among other things, on the appropriate modelling of radiative transfer through the actual atmosphere. We quantify the mapping of several commonly applied simplifications in radiative transfer modelling on the trace gas abundances retrieval error at the example of the Michelson interferometer for passive atmospheric sounding (MIPAS)/environmental satellite (ENVISAT) space borne Fourier transform infra-red limb emission experiment. The Karlsruhe optimized and precise radiative transfer algorithm (KOPRA) which was used as a tool for this study will be introduced. KOPRA supports accurate modelling of the particular instrument requirements of MIPAS and the observation scenarios during the ENVISAT mission, in particular with respect to its viewing direction and its altitude coverage of the atmosphere. We show that disregarding of horizontal temperature inhomogeneities and non-local thermodynamic equilibrium effects, insufficient accuracy in modelling of field-of-view and apodisation effects, and disregarding individual profiles of isotopic species play the key roles in radiative transfer modelling and lead to systematic retrieval errors which can by far exceed the expected random retrieval error caused by measurement noise.     

星载高光谱成像光谱仪狭缝函数测试方法的研究

为了满足大气微量成分高精度测量需求,需要能准确描述星载高光谱大气微量成份探测仪的仪器狭缝函数。针对高光谱大气微量成份探测仪视场大、波段宽、空间分辨率和光谱分辨率高等特点,研制了狭缝函数测量仪。介绍了狭缝函数测量仪的工作原理及基本结构,并利用狭缝函数测量仪可同时实现高分辨率、宽谱段测量的特点,通过搭建定标装置,对高光谱大气微量成份探测仪进行全视场的狭缝函数测试,得出了仪器狭缝函数的数学表达式,给出仪器狭缝函数的特性分布,并对结果进行分析。测试结果表明：高光谱大气微量成份探测仪全视场光谱分辨率在0.423～0.597 nm之间,其狭缝函数特性曲线近似满足高斯分布规律。由于星载大视场成像光谱仪存在光谱弯曲现象,从而导致边缘视场分辨能力略低于中心视场分辨能力。狭缝函数测量仪是基于中阶梯衍射光栅设计,可同时输出多条高分辨率谱线,且分布均匀,不仅可以测量高光谱成像光谱仪的仪器狭缝函数,也可对星载高光谱仪器光谱定标,为后续研究提供了参考依据和方法。     

Line parameters of H2O around 0.8 μm studied by tuneable diode laser spectroscopy

The absorption spectrum of H₂O has been extensively studied over the past decades in the near-infrared and visible regions, but there remain significant discrepancies in the 10,000-12,500 cm⁻¹ range even for the most intense lines. We have studied 24 lines of H₂O using an external-cavity diode laser emitting in the 810-830 nm range, in order to measure absolute line intensities as well as air- and self-broadening coefficients. Comparisons were made with values obtained from the HITRAN database and also with other parameters from recent experiments or predicted calculations. It is suggested that neglecting the effects of H₂O collisional narrowing in radiative transfer calculations of the Earth's atmosphere may have a small but significant impact on the radiative budget of the Earth.     

Fast approximate technique for the cumulative wavenumber model to modeling radiative transfer in a mixture of real gas media

In the cumulative wavenumber (CW) model, the total range of the absorption cross-section Cη is subdivided into the supplementary absorption cross-section of gray gases C_j, j=1,…,n, where n is the number of gray gases; and the wavenumber region is subdivided into intervals Δ_i=[η_i−1, η_i], i=1, 2,…,p, where p is the number of intervals. The intersection of the two spectral subdivisions is used to define the modeling of the fractional gray gas D_ij. In the CW model, we solve the radiative transfer equation (RTE) in every subinterval D_ij; then it is necessary to solve n x p times the spectral form of the RTE for complete spectral integration. In this work, the CW model is used with a numerical approximation technique based on additive properties of radiative intensity to reduce the solution of RTE to n new fractional gray gas D_j for complete spectral integration. The CW model was first coupled with the discrete ordinates method and the accuracy of the simplified technique and the algorithm was first examined for one-dimensional homogeneous media; results are compared with line-by-line calculations and it is found that the CW model with the simplified technique is exact for the homogeneous media examined. Also, the fast approach is tested in the diffuse reflecting boundaries case. The CW model is implemented in a bi-dimensional enclosure containing real gases in isothermal cases. Afterwards, this approximate technique is extended to non-isothermal and non-homogeneous cases; the results are compared with line-by-line calculations taken from literature and good agreement was found. The results obtained using the acceleration technique for the CW model agree with the results of original CW model. With this acceleration technique the CPU time decreases p times. Spectral database HITRAN and HITEMP are used to obtain the molecular absorption spectrum of the gases.     

超光谱红外卫星资料同步反演不同地表类型的大气廓线、地表温度和地表发射率

大气温度、水汽、地表温度和地表发射率是大气和地表的本征信息量。利用卫星红外资料精确反演大气温湿廓线有利于准确预报天气和研究气候变化,同时地表温度和地表发射率光谱的反演为研究植物生长与作物产量、地表水分蒸发与循环、能量平衡、地表成分及物理性质、气候变迁与全球环境提供重要参数指标。把大气和地面作为一个整体系统来考虑,建立一种能同步反演大气温度廓线、大气水汽廓线、地表温度和地表发射率的反演方法,利用超光谱红外卫星资料(atmospheric infrared sounder, AIRS),针对我国新疆地区沙漠和雪地两种典型发射率地表同步反演大气温度廓线、水汽廓线、地表温度和地表发射率。反演方法首先线性化地球-大气系统红外辐射传输方程, 提出通过经验正交函数构建大气廓线和地表发射率光谱,有效减少反演变量数,建立同步物理反演模式,然后以美国国家环境预报中心(National Centers for Environmental Prediction,NCEP)的预报结果(初始大气温度、水汽廓线以及地表参数)作为初始值,最后通过牛顿迭代得到最优化解。反演观测区域覆盖我国新疆塔克拉玛干沙漠和准噶尔盆地,分别选择位于塔克拉玛干沙漠腹地的塔中探测站(纬度38.98°, 经度83.64°)和准噶尔盆地的阜康荒漠生态系统国家野外科学观测研究站(纬度44.2°, 经度87.9° )为反演地面验证点。反演结果表明,塔克拉玛干沙漠地表温度明显高于准噶尔盆地地表温度,与实际情况相一致;根据反演的8.6和13.4 μm处的地表发射率分布情况,可以看出在8.6 μm处沙漠地表发射率明显低于雪地发射率,在6～15 μm范围内,反演的沙漠地区(塔中站)地表发射率和雪地地区(阜康站)地表发射率与美国喷气推进实验室测量的沙漠发射率光谱和雪地发射率光谱相一致。研究表明,把大气和地面作为一个整体系统来考虑,把地表发射率加入到反演中,通过比较和分析沙漠地区(塔中)和雪地地区(阜康)的大气廓线反演结果与当地气象探空值和传统反演方法反演值,改进了大气温度廓线和水汽廓线反演精度,特别是边界层温度和水汽改进尤为明显;同时分析表明在发射率光谱变化较大的沙漠地区, 大气廓线反演精度的改进比雪地要高,这是由于地表发射率光谱在沙漠、戈壁地区变化较大,而雪地的发射率光谱变化不大。用该方法针对地表发射率光谱变化较大的地区(沙漠)同步反演大气廓线、地表温度和地表发射率,可以更有效的提高大气温度廓线、水汽廓线的反演精度。该研究结果可以为数值天气预报和我国未来超光谱红外卫星应用提供服务和有力支持, 具有十分重要的意义。     

大气对星载被动微波影响分析研究

被动微波遥感具有全天候工作的能力,但是在不同的大气状态下被动微波遥感受大气的影响不同。为了研究大气对高级微波扫描辐射计AMSR-E (advanced microwave scanning radiometer-earth observing system)入瞳亮温的影响,分别采用晴空和典型层云大气数据作为微波辐射传输模型的输入,进行大气微波辐射信号的模拟工作并分析大气的影响。结果表明晴空下大气水汽是引起大气辐射的主要因素,晴空条件下大气对典型被动微波传感器低频的(<18.7 GHz)透过率大于0.98,在整个辐射传输过程中可以忽略不计。36.5和89 GHz的大气透过率在晴空下分别为0.896和0.756,在用微波高频通道进行陆表参数反演时需进行大气水汽影响的校正; 云覆盖条件或者阴天情况下云中液态水是引起大气辐射的主要因素,典型层云覆盖下大气的透过率在10.7,18.7和36.5 GHz分别为0.942,0.828和0.605。与晴空相比,由层云中液态水引起的大气下行辐射的增量在36.5 GHz最大达到75.365 K。表明在云覆盖时大气影响的校正过程中云层的影响是校正重点。最后利用大气探空数据计算了内蒙古海拉尔地区2013年夏季7月份的大气透过率,结果显示C、X波段的大气透过率接近1,89 GHz受水汽影响较大,其地球表层大气透过率不超过0.7。在内蒙古海拉尔地区,夏季大气透过率具有较为稳定的值,但是随着局部水汽的变化具有0.1左右的波动。     

Measurements of long-term changes in atmospheric OCS (carbonyl sulfide) from infrared solar observations

Multi-decade atmospheric OCS (carbonyl sulfide) infrared measurements have been analyzed with the goal of quantifying long-term changes and evaluating the consistency of the infrared atmospheric OCS remote-sensing measurement record. Solar-viewing grating spectrometer measurements recorded in April 1951 at the Jungfraujoch station (46.5°N latitude, 8.0°E longitude, 3.58 km altitude) show evidence for absorption by lines of the strong ν₃ band of OCS at 2062 cm⁻¹. The observation predates the earliest previously reported OCS atmosphere remote-sensing measurement by two decades. More recent infrared ground-based measurements of OCS have been obtained primarily with high-resolution solar-viewing Fourier transform spectrometers (FTSs). Long-term trends derived from this record span more than two decades and show OCS columns that have remained constant or have decreased slightly with time since the Mt. Pinatubo eruption, though retrievals assuming different versions of public spectroscopic databases have been impacted by OCS ν₃ band line intensity differences of ∼10%. The lower stratospheric OCS trend has been inferred assuming spectroscopic parameters from the high-resolution transmission (HITRAN) 2004 database. Volume mixing ratio (VMR) profiles measured near 30°N latitude with high-resolution solar-viewing FTSs operating in the solar occultation mode over a 22 years time span were combined. Atmospheric Trace MOlecucle Spectroscopy (ATMOS) version 3 FTS measurements in 1985 and 1994 were used with Atmospheric Chemistry Experiment (ACE) measurements during 2004-2007. Trends were calculated by referencing the measured OCS VMRs to those of the long-lived constituent N₂O to account for variations in the dynamic history of the sampled airmasses. Means and 1-sigma standard deviations of VMRs (in ppbv, or 10⁻⁹ per unit air volume) averaged over 30-100 hPa from measurements at 25-35°N latitude are 0.334±0.089 ppbv from 1985 (ATMOS Spacelab 3 measurements), 0.297±0.094 ppbv from 1994 ATLAS 3 measurements, 0.326±0.074 ppbv from ACE 2004 measurements, 0.305±0.096 ppbv from ACE 2005 measurements, 0.328±0.074 from ACE 2006 measurements, and 0.305±0.090 ppbv from ACE measurements through August 2007. Assuming these parameters, we conclude that there has been no statistically significant trend in lower stratospheric OCS over the measurement time span. We discuss past measurement sets, quantify the impact of changes in infrared spectroscopic parameters on atmospheric retrievals and trend measurements, and discuss OCS spectroscopic uncertainties of the current ν₃ band parameters in public atmospheric databases.     

Evidence of reduced measurement uncertainties from an FTIR instrument intercomparison at Kiruna, Sweden

We report the results of an intercomparison of vertical column amounts of HCl, HF, N₂O, HNO₃, CH₄, O₃, CO₂ and N₂ derived from the spectra recorded by two ground-based FTIR spectrometers operated side-by-side using the sun as a source. The procedures used to record spectra and derive vertical column amounts followed the format of previous instrument intercomparisons organised by the Network for Detection of Stratospheric Change (NDSC), but the level of agreement achieved was significantly better than for previous intercomparisons.For most gases the differences were typically 1% or less, with at least one of the five datasets showing no statistically significant difference between the results from different instruments. Principal exceptions were HNO₃ and CO₂ when measured on the Mercury Cadmium Telluride (MCT) detectors. For these gases differences of between 2% and 3% were more typical. We present evidence that these larger differences are due to the effects of detector non-linearity and show that by applying an established non-linearity correction method the typical level of agreement can be improved to better than 1% for these gases.We suggest that the improved level of agreement achieved during this intercomparison is indicative of the current state of the art within the NDSC infra-red working group and is a result of improved understanding of the importance of critical alignment parameters and newly developed techniques to characterise the spectrometers’ performance.     

钢大气腐蚀锈层的红外、拉曼光谱研究

运用红外光谱、拉曼光谱分析的方法,对碳钢A₃和低合金钢10CrMoAl经青岛大气腐蚀的锈层样品,进行了比较测试,综合分析得出锈层中主要组成物相为α-Fe₂O₃,γ-FeOOH, α-FeOOH, δ-FeOOH,Fe₃O₄,以及各组成物相相对含量随腐蚀时间的变化情况。