Ground-Based Measurements of Atmospheric Trace Gases in Beijing During the Olympic Games

Letter Ground-Based Measurements of Atmospheric Trace Gases in Beijing during the Olympic Games

Jie Cheng

State Key Laboratory of Remote Sensing Sciences, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China; [email protected] Received: 25 November 2018; Accepted: 4 December 2018; Published: 15 April 2019 First Version Published: 5 December 2018 (doi: 10.3390/sci1010006.v1) Second Version Published: 18 March 2019 (doi: 10.3390/sci1010018) Third Version Published: 15 April 2019 (doi: 10.3390/sci1010023)

Abstract: A portable Fourier Transform Spectrometer (B3M-IR) is built and used to measure atmospheric trace gases in the city of Beijing during Olympic Games in 2008. A short description of the instrument is ﬁrst provided in this paper. A detailed spectral analysis is then presented. The total columns of ozone (O3), carbon monoxide (CO), methane (CH4) and nitrous oxide (N2O) are retrieved from the ground-based solar absorption spectra recorded by the B3M-IR during the Olympic Games. Lacking validation data, only the retrieved total column of O3 is compared with that retrieved by MAX-DOAS, which is deployed at the same station. The mean diﬀerence between the two methods of measurement is 6.5%, demonstrating the performance and reliability of B3M-IR.

Keywords: B3M-IR; Trace gases; SFIT2; Remote Sensing; Olympic Games

1. Introduction Since the industrialization of the last two centuries, manmade trace gases have increasingly influenced atmospheric composition and produced great impacts on the atmospheric environment and global climate change. An example of these impacts is the ozone depletion in the Antarctic region [1], which has driven scientists worldwide to conduct extensive studies to understand the composition of our atmosphere. Measuring the sources and sinks of the trace gases as well as their spatial and temporal variation are very helpful in achieving this goal. Different kinds of instruments and methods have been developed for trace gas measurements since then. There exist several satellite products of atmospheric columns of various trace gases [2,3]. Aside from these space-borne remote sensing sensors, the ground-based Fourier Transfer Spectrometers (FTS) can provide long-term data of trace gases, which can also be used to validate the algorithms and products of the satellites. Thus, it is an indispensable method for atmospheric trace gas monitoring. A global network, Network for the Detection of Atmospheric Composition and Change (NDACC) (www.ndacc.org), was founded with an aim of providing long-term high quality stratospheric parameters [4]. The data acquired from NDACC stations have been widely used in atmospheric change detection, atmospheric chemistry model validation and satellite products validation [5,6]. Unfortunately, only scattered work has been carried out to acquire the total column of limited trace gases from ground-based spectrometers in China [7,8]. Information on the vertical distribution of trace gases comes primarily from the products of limited satellites. The drawback of these products of satellites is that the satellite repeat cycle takes a considerable amount of time and cannot satisfy the monitoring requirements. To determine the air quality of Beijing during the 2008 summer Olympic Games, the Chinese Academy of Sciences constructed an air quality monitoring station on the top floor of the main building of the Institute of Remote Sensing Applications. Its geographical position is

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Scithe2019 top, 1, floor 23 of the main building of the Institute of Remote Sensing Applications. Its geographical2 of 13 position is (40.00° N, 116.37° N) and it is adjacent to the main buildings of the Olympic Games. Three kinds of instruments are deployed in the station. They are as follows: (1) in situ traditional chemistry (40.00sampling◦ N, 116.37instruments◦ N) and for it the is adjacent measurements to the main of near buildings surface oftrace the gases; Olympic (2) Games.MAX-DOAS Three [8] kinds for the of instrumentstotal column are measurements deployed in the of station. trace Theygases; are and as follows: (3) ground-based (1) in situ traditional FTS for chemistrythe vertical sampling profile instrumentsmeasurements for theof trace measurements gases. In this of nearwork, surface the first trace atmospheric gases; (2) MAX-DOAStrace gases measurements [8] for the total using column our measurementsground-based ofB3M-IR trace gases; FTS in and Beijing (3) ground-based during the Olympic FTS for Games the vertical are reported. profile measurements of trace gases. In this work, the first atmospheric trace gases measurements using our ground-based B3M-IR FTS2. Atmospheric in Beijing during Trace the Gas Olympic Measurements Games are reported. 2. Atmospheric Trace Gas Measurements 2.1. Data Acquisition 2.1. DataThe Acquisition FTS used to acquire the solar absorption spectra is named as B3M-IR, which is developed by BeijingThe FTS Vision used Sky to acquire Aerospace the solarTechnology absorption Co., spectra Ltd. in iscooperation named as B3M-IR,with Canada’s which ABB is developed Bomem Inc. by BeijingThe sun Vision tracker Sky used Aerospace to lock to Technology the center of Co., the Ltd. sun is in developed cooperation by withthe Beijing Canada’s Vision ABB Sky Bomem Aerospace Inc. TheTechnology sun tracker Co., used Ltd. to The lock maximum to the center optical of the path sun diff is developederence (OPD) by the of Beijingthe B3M-IR Vision is Sky25.2 Aerospacecm, which −1 Technologyprovides a Co.,maximized Ltd. The apodized maximum spectral optical resolution path diff oference 0.02 (OPD)cm . The of theB3M-IR B3M-IR is equipped is 25.2 cm, with which two photovoltaic detectors, an indium antimonide (InSb) and a 1mercury cadmium telluride (MCT) provides a maximized apodized spectral resolution of 0.02 cm− . The B3M-IR is equipped with two −1 photovoltaicdetector. Together detectors, they an cover indium the antimonide spectral range (InSb) of and750–4100 a mercury cm . cadmium The instrumental telluride (MCT)line shape detector. (ILS) is a critical index used to characterize the performance1 of an FTS and has an important influence on Together they cover the spectral range of 750–4100 cm− . The instrumental line shape (ILS) is a critical indexthe accuracy used to of characterize trace gases the retrieval. performance The ILS of of an B3 FTSM-IR and is hasdetermined an important by measuring influence the on thetransmission accuracy ofspectra trace gasesof a 1000 retrieval. K blackbody The ILS through of B3M-IR a low-pressure is determined gas by cell measuring full of CO the and transmission by retrieving spectra the phase of aerror 1000 Kand blackbody modulation through efficiency a low-pressure of the B3M-IR gas cell fr fullom ofCO CO absorption and by retrieving features the with phase the error software and modulationLINEFIT [9]. e ffiTheciency results of theof B3M-IR B3M-IR ILS from measuremen CO absorptionts are presented features with in Figure the software 1. Another LINEFIT important [9]. Thefactor results that influences of B3M-IR the ILS accuracy measurements of trace aregas presentedretrieval is in the Figure instrumental1. Another signal-to-nose important factorratio (SNR). that influencesAs shown the in Figure accuracy 2, ofB3M-IR trace gascan retrieval achieve isan the SNR instrumental better than signal-to-nose 100 in the microwindows ratio (SNR). Asusually shown used in Figurefor trace2, B3M-IR gases retrieval. can achieve Using an our SNR data better acquisition than 100 insystem, the microwindows we obtained the usually clear usedsky solar for trace absorption gases retrieval.spectraduring Using the our Olympic data acquisition Games. system, The observation we obtained was the started clear sky on solar July absorption22th, 2008 spectraduringand ended on theSeptember Olympic 5th, Games. 2008. The observation was started on July 22th, 2008 and ended on September 5th, 2008.

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Figure 1. (a) The observed and ﬁtted CO line, with residuals shown in the lower panel; (b) Retrieved phase error and modulation eﬃciency from the CO cell [10].

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Figure 1. (a) The observed and fitted CO line, with residuals shown in the lower panel; (b) Retrieved Sci 2019phase, 1, 23 error and modulation efficiency from the CO cell [10]. 3 of 13

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FigureFigure 2. 2.The The SNR SNR of of B3M-IR.using B3M-IR.using ( a(a)) an an MCT MCT detector; detector; ( b(b)) an an InSb InSb detector detector [ 10[10].].

2.2.2.2. Data Data Analysis Analysis 2.2.1. A Priori and Auxiliary Quantities 2.2.1. A Priori and Auxiliary Quantities The relationship between the measured solar absorption spectrum and the state of atmospheric The relationship between the measured solar absorption spectrum and the state of atmospheric trace gas is nonlinear and the physical radiative transfer process is very complicated. After linearization, trace gas is nonlinear and the physical radiative transfer process is very complicated. After the measurement vector and the retrieval state vector are connected via a forward matrix operator, linearization, the measurement vector and the retrieval state vector are connected via a forward which maps the retrieval state vector into the measurement vector. However, this forward matrix matrix operator, which maps the retrieval state vector into the measurement vector. However, this is a rank defect matrix and zero space always appears in the solution. Thus, a priori is needed to forward matrix is a rank defect matrix and zero space always appears in the solution. Thus, a priori constrain the solution. The a priori volume mixing ratio (VMR) profiles of target gases and the is needed to constrain the solution. The a priori volume mixing ratio (VMR) profiles of target gases interferences are derived from the combination of the Halogen Occultation Experiment (HALOE) and the interferences are derived from the combination of the Halogen Occultation Experiment V1.9 data between 2000 and 2005 [3], the Atmospheric Infrared Sounder (AIRS) Level 2 supplemental (HALOE) V1.9 data between 2000 and 2005 [3], the Atmospheric Infrared Sounder (AIRS) Level 2 products between 2005 and 2007 (http://mirador.gsfc.nasa.gov/), the Michelson Interferometer for supplemental products between 2005 and 2007 (http://mirador.gsfc.nasa.gov/), the Michelson Passive Atmospheric Sounding (MIPAS) reference profile (www.atm.ox.ac.uk/group/mipas/species), Interferometer for Passive Atmospheric Sounding (MIPAS) reference profile and the Toronto Atmospheric Observatory (TAO) a priori [11]. (www.atm.ox.ac.uk/group/mipas/species), and the Toronto Atmospheric Observatory (TAO) a priori [11]. The nearest grid point of National Centers for Environmental Prediction (NCEP) reanalysis data The nearest grid point of National Centers for Environmental Prediction (NCEP) reanalysis data is about 200 km from our station, and NCEP pressure and temperature (PT) profiles in Beijing can is about 200 km from our station, and NCEP pressure and temperature (PT) profiles in Beijing can only reach about 35 km. Thus, the PT profiles of AIRS Level 2 supplemental products are adopted only reach about 35 km. Thus, the PT profiles of AIRS Level 2 supplemental products are adopted (http://mirador.gsfc.nasa.gov/), which covers the surface to 75 km. Above this, the built-in mid-latitude (http://mirador.gsfc.nasa.gov/), which covers the surface to 75 km. Above this, the built-in mid- Summer atmosphere in the radiative transfer code Modtran 4.0 [12] is used. latitude Summer atmosphere in the radiative transfer code Modtran 4.0 [12] is used. 2.2.2. Spectroscopic Parameters and Microwindow Selection 2.2.2. Spectroscopic Parameters and Microwindow Selection Spectroscopic parameters from the 2004 HITRAN compilation [13] are used to calculate absorption Spectroscopic parameters from the 2004 HITRAN compilation [13] are used to calculate features with a voigt line shape in SFIT2. Trace gas retrievals are usually carried out in small absorption features with a voigt line shape in SFIT2. Trace gas retrievals are usually carried out in microwindows that contain the spectral absorption features of the target gas as well as the absorption

Sci 2019, 1, 23 4 of 13 features of interferences. The selection of the spectral range of each microwindow is based on previous work [14] and the characteristics of our FTS. The selected microwindows are listed in Table1.

Table 1. Microwindows and interferences used in the trace gas retrieval from B3M-IR measurements.

1 Target Gas Microwindow (cm− ) Interfering Gases SNR

2775.5814–2776.3923 CH4, CO2, HCl, N2O ~500

O3 2778.7756–2779.4882 CH4, HDO, N2O ~550

2781.4784–2782.0927 CH4, HDO, N2O, CO2 ~520

2057.6834–2057.9906 CO2, OCS, O3,H2O ~320

CO 2069.4775–2069.7969 CO2, OCS, O3,H2O ~340

2157.4779–2159.8981 N2O, H2O, O3, CO2 ~300 2859.7856–2860.2894 ~525 H2O, O3, HCL CH4 2898.2758–2898.9884 ~475 2903.5831–2904.1974 ~360

N2O 2481.0756–2482.4884 H2O, CO2,O3, CH4 ~270

2.2.3. Retrieval Method Generally, trace gas vertical profile retrieval from a ground-based solar absorption spectrum is an under-determined (ill-posed) problem. Analytical solution does not exist. The trace gas vertical profile is derived from the measured spectra by iteratively adjusting the concentration of the target gas and interferences in the selected microwindows until the differences between the simulated and measured spectra are minimized. There are many algorithms that can accomplish this task [15,16]. The Rodgers Optimal Estimation method (OEM) is widely employed in the field of trace gas vertical profile retrieval from ground-based solar absorption spectroscopy [15,17,18]. The OEM is semi-empirically implemented in the SFIT2 algorithm, which is the successor of SFIT and was jointly developed by the NASA-Langley Research Center and the National Institute of Water and Atmospheric Research at Lauder, New Zealand [19,20]. Intercomparison studies show that SFIT2 is a robust algorithm [21]. Thus, the SFIT2 algorithm is adopted in this study. A ray tracing program FSCATM [22,23] is used in SFIT2 to calculate a model atmosphere using pressure, temperature and a priori VMR profiles. The FSCATM program also calculates air mass factors for each layer. Together with the High Resolution Transmission Molecular Absorption (HITRAN) 2004 spectral database [13] and the instrumental parameters which include a wavenumber scale multiplier, spectral resolution, field of view (FOV) and ILS, the model atmosphere is used to simulate observed spectra with the forward model in the trace gas VMR profile retrieval. The used forward model in SFIT2 is a multi-layer, multi-species, line-by-line (LBL) radiative transfer model originally developed by the NASA Langley Research Center. In the iterative process, trace gases VMR is iteratively adjusted and output after the maximum iterative number is completed or the cost function is minimized. Several data files used for post-error analysis are also output.

2.2.4. Retrieval Characterization The Root Mean Square (RMS) residual error and averaging kernels were used to characterize the retrieval. The RMS residual error can be used to measure the quality of the spectral fits. If the RMS residual value is very small, the retrieval can be thought as successful. The averaging kernels provide the theoretical basis for determining the altitude sensitivity of a given atmospheric measurement, as well as the sensitivity of the retrieval state vector to the true state vector. The averaging kernels can also reflect the sensitivity of the retrieval column to the true state. Figure3 presents an example of spectral fits for O3 retrieval in the three microwindows. The mean RMS residual value for the three spectral fit microwindows is 0.305%. Figure4 shows the total column averaging kernel for Sci 2019, 1, 23 5 of 13 Sci 2019, 1, 23 5 of 13 retrieval is most sensitive to the stratosphere between 15 and 35 km. The spectral fit results and the O3. The retrieval is most sensitive to the stratosphere between 15 and 35 km. The spectral fit results total column averaging kernel for CO retrievals are shown in Figures 5 and 6, respectively. The mean and the total column averaging kernel for CO retrievals are shown in Figures5 and6, respectively. RMS residual value for is 0.87% for three microwindows, and the total column averaging kernel The mean RMS residual value for is 0.87% for three microwindows, and the total column averaging clearly show sensitivity to the troposphere and lower stratosphere. As shown in Figures 7 and 8, the kernel clearly show sensitivity to the troposphere and lower stratosphere. As shown in Figures7 and8, spectral fits and averaging kernels for CH4 retrievals are similar with those of CO. The mean RMS the spectral fits and averaging kernels for CH4 retrievals are similar with those of CO. The mean RMS residual value for the three microwindows is 0.597%, and the total column averaging kernel show residual value for the three microwindows is 0.597%, and the total column averaging kernel show good good sensitivity to the troposphere. The spectral fit result for N2O is better than that of the other trace sensitivity to the troposphere. The spectral fit result for N2O is better than that of the other trace gases gases with a mean RMS residual of 0.159% (Figure 9). The total column averaging kernel indicates with a mean RMS residual of 0.159% (Figure9). The total column averaging kernel indicates that good that good sensitivity for the N2O retrievals in the troposphere and lower stratosphere (Figure 10). sensitivity for the N2O retrievals in the troposphere and lower stratosphere (Figure 10). Generally, Generally, the RME residual of spectral fit for the selected four trace gases is very small, which means the RME residual of spectral fit for the selected four trace gases is very small, which means that the that the measured solar absorption spectra can be well replicated and the retrieval is successful. The measured solar absorption spectra can be well replicated and the retrieval is successful. The altitude altitude sensitivity of total column reflected by total averaging kernel is consistent with the results of sensitivity of total column reflected by total averaging kernel is consistent with the results of previous previous studies [24–26]. studies [24–26].

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FigureFigure 3. 3.Example Example of of ﬁts fits and and residuals residuals for for O O33in in three three selected selected microwindows. microwindows.

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Figure 4. Example of a total column averaging kernel for O3. FigureFigure 4.4. Example ofof aa totaltotal columncolumn averagingaveraging kernelkernel forfor OO3.3.

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(c) (c) FigureFigure 5.5. ExampleExample ofof ﬁtsfits andand residualsresidualsfor forCO COin inthree three selected selected microwindows. microwindows. Figure 5. Example of fits and residuals for CO in three selected microwindows.

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Figure 6. Example of a total column averaging kernel for CO. FigureFigure 6.6. ExampleExample ofof aa totaltotal columncolumn averagingaveraging kernelkernel forfor CO.CO.

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(c) (c) FigureFigure 7. 7.Example Example of of ﬁts fits and and residuals residuals for for CH CH44in in three three selected selected microwindows. microwindows. Figure 7. Example of fits and residuals for CH4 in three selected microwindows.

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Figure 8. Example of a total column averaging kernel for CH4. Figure 8. Example of a total column averaging kernel for CH4. FigureFigure 8.8. ExampleExample ofof aa totaltotal columncolumn averagingaveraging kernelkernel forfor CHCH4.4.

Figure 9. Example of fits and residuals for N2O in three selected microwindows. FigureFigure 9.9. ExampleExample ofof ﬁtsfits andandresiduals residualsfor forN N22OO inin threethree selectedselected microwindows.microwindows. Figure 9. Example of fits and residuals for N2O in three selected microwindows.

FigureFigure 10. 10.Example Example of of total total column column averaging averaging kernel kernel for for N N2O.2O. Figure 10. Example of total column averaging kernel for N2O. 3.3. Results Results Figure 10. Example of total column averaging kernel for N2O. 3. Results With the observed clear sky solar absorption spectra, we carried out the retrieval of atmospheric 3. ResultsWith the observed clear sky solar absorption spectra, we carried out the retrieval of atmospheric trace gas vertical profile and total column. An example of vertical profile retrieval for O , CO, CH traceWith gas verticalthe observed profile clear and sky total solar column. absorption An example spectra, of we vertical carried profile out the retrieval retrieval for of atmosphericO3 3, CO, CH4 4 and NWithO are the shown observed in Figure clear sky11. Assolar the absorption airspace was spec controlledtra, we carried during out the the Olympic retrieval Games, of atmospheric3 we could4 traceand N 2gas2O arevertical shown profile in Figure and 11.total As column. the airspace An example was controlled of vertical during profile the Olympicretrieval Games,for O , CO,we could CH nottraceandnot acquire N acquiregas2O arevertical ozonesonde shownozonesonde profile in Figure data.and data. 11.total ThusThus As column. the thethe airspace retrievedretrieved An example was ozoneozone controlled of verticalvertical vertical during profileprofile profile the could could Olympicretrieval notnot Games,be forbeevaluated Oevaluated3, CO,we could CHby by4 comparingandnotcomparing Nacquire2O are it itshownozonesonde with with the inthe ozonesondeFigure ozonesonde data. 11. Thus As data. the data.the airspace To retrieved To address addres was thisozone controlleds this problem, vertical problem, during we profile simply we the simply couldOlympic compared notcompared Games, be the evaluated ozone thewe couldozone total by columnnotcomparingtotal acquire column amount itozonesonde amountwith with the that with ozonesonde data. derived that Thus derived from data.the coincident from retrieved To coaddresincident MAX-DOAS ozones this MAX-DOAS vertical problem, measurements. profile we measurements. simply could The notcompared comparison be The evaluated comparison the resultozone by iscomparingtotalresult presented column is presented it in amountwith Figure thein 12withFigure ozonesonde. Note that 12. gapsderived Note existdata. gaps from inTo exist Figures coaddresincident in Figures 12s –this15 MAX-DOAS dueproblem, 12-15 to thedue we appearance measurements.to thesimply appearance compared of cloudy The of comparison cloudy sky.the ozone There sky. seemstotal column to be a amount linear increasing with that derived trend for from O cototalincident column MAX-DOAS amount, but measurements. it is not significant The comparison and R2 is resultThere isseems presented to be ina linearFigure increasing 12. Note gaps trend exist3 for inO 3Figures total column 12-15 dueamount, to the but appearance it is not significant of cloudy sky.and onlyresultThereR2 is 0.28.. only isseems presented Generally,0.28.. to be Generally, ina linearFigure the total increasing 12.the column Notetotal gapscolumn amounttrend exist for amount retrieved inO 3Figures total retrieved column from 12-15 B3M-IR duefromamount, to isB3M-IR the lower but appearance it thanis is lower not that significant of than that cloudy derivedthat sky. andthat 2 fromThereRderived is MAX-DOAS.only seems from 0.28.. to MAX-DOAS. be Generally, a The linear average increasing the The total relative average column trend error relative for amount is O relatively 3error total retrieved iscolumn relatively large andfromamount, large amounts B3M-IR butand it toamountsis is 6.5%.lower not significant There thanto 6.5%. that are There twoandthat 2 1 possibleRderivedare is two only reasonsfrompossible 0.28.. MAX-DOAS. Generally, for reasons this: (1)for theThe thethis: total spectralaverage (1) column the resolutionrelativespectral amount errorresolution of retrieved B3M-IRis relatively of isB3M-IRfrom 0.02 large B3M-IR cm is and− 0.02, which amounts iscm lower−1, is which an thanto order 6.5%. is that an lowerThere order that −1 thanderivedarelower two that than frompossible of thethat MAX-DOAS. commonly ofreasons the commonly for used The this: FTS,average (1) used andthe FTS,relativesp (2)ectral the and observation errorresolution (2) the is relatively observation of time B3M-IR of large B3M-IR time is and 0.02 of isamounts cmB3M-IR not matched, which tois not6.5%. is with anmatched There order that arelower two than possible that ofreasons the commonly for this: (1) used the FTS,spectral and resolution (2) the observation of B3M-IR time is 0.02 of cmB3M-IR−1, which is not is anmatched order lower than that of the commonly used FTS, and (2) the observation time of B3M-IR is not matched

Sci 2019, 1, 23 9 of 13 Sci 2019, 1, 23 9 of 13 ofwith MAX-DOAS. that of MAX-DOAS. The former The measurements former measurements were obtained were at aboutobtained 1:30 at PM about and the1:30 latter PM and at about the latter 12:00 noon.at about The 12:00 time intervalnoon. The was approximatelytime interval 1.5was hours. approximately Elaborate synchronous1.5 hours. Elaborate experiments synchronous need to be carriedexperiments out in need the futureto be carried to evaluate out in further the future the retrieval to evaluate result further of B3M-IR. the retrieval result of B3M-IR. The total column amount of CO is shown in FigureFigure 1313.. The total column amount of CO at the end of July, 20082008 isis thethe highest,highest, andand thenthen it beganbegan to decrease gradually. There is a signiﬁcantsignificant linear 2 decreasing, and R 2 reaches 0.61. This This kind kind of of phenomenon phenomenon may may be be attributed attributed to traffic traﬃc control policies adopted by the Beijing government, such as ban vehi vehiclescles with even and odd-numbered license plates on alternate days from July 20 to September 20. Th Thee decrease in man-made emission sources of trace gases alleviates air pollution in Beijing. The The total total column amounts of CH4 and N2O are shown in Figures 1414 and and 15 15.. There There are are almost almost no no variations variations in the in CH4the CH4 and N2Oand totalN2O columntotal column amount. amount. The results The ofresults these of three these trace three gases trace were gases not were validated not validated due to thedue lack to the of validationlack of validation data. data.

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Figure 11. Cont.

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Figure 11. Examples of trace gas VMR retrieval. (a) O3; (b) CO; (c) CH4; (d) N 2O. (d) (d) Figure 11. Examples of trace gas VMR retrieval. (a) O3; (b) CO; (c) CH4; (d) N2O. FigureFigure 11.11. ExamplesExamples ofof tracetrace gasgas VMRVMR retrieval.retrieval. ((aa)O) O33;(; (b) CO; (c) CH4;;( (d))N N2O.O.

Figure 12. Comparison of B3M-IR total column of O3 with that from MAX-DOAS.

Figure 12. Comparison of B3M-IR total column of O3 with that from MAX-DOAS. Figure 12. Comparison of B3M-IR total column of O3 with that from MAX-DOAS. Figure 12. Comparison of B3M-IR total column of O3 with that from MAX-DOAS.

FigureFigure 13. 13.Time Time series series of of CO CO total total column column derived derived from from B3M-IR B3M-IR measurements. measurements.

Figure 13. Time series of CO total column derived from B3M-IR measurements. Figure 13. Time series of CO total column derived from B3M-IR measurements.

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Figure 14. Time series of CH4 total column derived from B3M-IR measurements. Figure 14. Time series of CH4 total column derived from B3M-IR measurements.

FigureFigure 15. 15.Time Time series series of of N N2O2O total total column column derived derived from from B3M-IR B3M-IR measurements. measurements. 4. Summary 4. Summary The clear sky solar absorption spectra were recorded with a ground-based spectrometer B3M-IR The clear sky solar absorption spectra were recorded with a ground-based spectrometer B3M- during the Olympic Games in Beijing, from which the total columns and vertical VMR profiles of IR during the Olympic Games in Beijing, from which the total columns and vertical VMR profiles of O3, CO, CH4 and N2O were derived. The residues of spectral fit indicate that the measured data O3, CO, CH4 and N2O were derived. The residues of spectral fit indicate that the measured data were were reliable and the retrieval was successful. Lacking validation data, only the total columns of reliable and the retrieval was successful. Lacking validation data, only the total columns of O3 were O were compared with those derived from MAX-DOAS measurements. Due to the coarse spectral compared3 with those derived from MAX-DOAS measurements. Due to the coarse spectral resolution resolution of B3M-IR and a mismatch in data acquisition time, the average relative error was slightly of B3M-IR and a mismatch in data acquisition time, the average relative error was slightly large, large, amounting to 6.5% overall. There seems a decreasing trend in O3 total column amount but it is amounting to 6.5% overall. There seems a decreasing trend in O3 total column amount but it is not not significant, and almost no variations in the CH4 and N2O total column amount. Regarding the CO significant, and almost no variations in the CH4 and N2O total column amount. Regarding the CO total column amount, there is a significant linear decreasing, and R2 reaches 0.61. The total column total column amount, there is a significant linear decreasing, and R2 reaches 0.61. The total column amount decrease of CO indicates that the government’s “ban vehicles with even and odd-numbered amount decrease of CO indicates that the government’s “ban vehicles with even and odd-numbered license plates on alternate days from July 20 to September 20” policy is taking effect. license plates on alternate days from July 20 to September 20” policy is taking effect. Supplementary Materials: Reviewers’ comments and authors’ reply are available online at https://www.mdpi. comSupplementary/2413-4155/1/1 /23Materials:/s1. Reviewers’ comments and authors’ reply are available online at https://www.mdpi.com/2413-4155/1/1/23/s1. Funding: This work was partly supported by the National Key Research and Development Program of China via grant 2016YFA0600101, the National Natural Science Foundation of China via grant 41771365, and the National NaturalAcknowledgments: Science Foundation This work of China was viapartly grant supported 41371323. by the National Key Research and Development Program of China via grant 2016YFA0600101, the National Natural Science Foundation of China via grant 41771365, and the National Natural Science Foundation of China via grant 41371323. The authors would like to thank N. Jones at the University of Wollongong, Australia for providing the SFIT2 code and helpful advice. We would also like to thank C. P. Rinsland of the NASA Langley Research Center for his encouragement and kind help.

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Acknowledgments: The authors would like to thank N. Jones at the University of Wollongong, Australia for providing the SFIT2 code and helpful advice. We would also like to thank C. P. Rinsland of the NASA Langley Research Center for his encouragement and kind help. Conﬂicts of Interest: The author declares no conﬂict of interest.

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