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Abstract
In this study, the characterization of Hydrogen Chloride (HCl) seasonal variations and inter-annual linear trend are presented for the first time over the polluted region at Hefei (117°10’E, 31°54’N), China. The time series of HCl were retrieved by the mid-infrared (MIR) solar spectra recorded by the ground-based high-resolution Fourier transform infrared spectroscopy (FTIR) between July, 2015 and April, 2019. The magnitude of HCl reaches a peak in January (2.70 ± 0.16) × 1015 molecules*cm-2 and a minimum in September (2.27 ± 0.09) × 1015 molecules*cm-2. The four-year time series of HCl total column show a negative linear trend of (-1.83 ± 0.13) %. The FTIR data are compared with GEOS-Chem data in order to evaluate the performance of the GEOS-Chem model to simulate HCl. In general, total column FTIR data and GEOS-Chem model data are in a good agreement with a correlation coefficient of 0.82. GEOS-Chem model data present a good agreement with FTIR data in seasonal variation and inter-annul trend. The maximum differences occur in January and April with mean differences of 4%-6%. We also present HCl time series observed by 6 NDACC stations (Bremen, Toronto, Rikubetsu, Izana, Reunion.maido, Lauder) in low-middle-latitude sites of the northern and southern hemispheres and Hefei stations in order to investigate the seasonal and annual trends of HCl in low-middle-latitude sites. The HCl total column at the northern hemisphere stations reached the maximum in the late winter or early spring and the minimum in the early winter or late autumn. In general, the seasonal variations of HCl over Hefei is similar to that in other northern hemisphere mid-latitude FTIR stations.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
It is essential to know the spatiotemporal change of atmospheric chlorine species for predicting the recovery and evolution of stratospheric ozone in the future [1]. Hydrogen chloride (HCl) is the final product of all chlorides in the stratosphere and plays an important role in the stratospheric chemistry [2]. In stratosphere, reaction of HCl with OH radical which forms chlorine atom to react with ozone provides the main pathway for stratospheric ozone depletion (Eq. (1)).
HCl total columns can be retrieved by ground-based high-resolution Fourier transform infrared spectrometers (FTIR) (https://www2.acom.ucar.edu/irwg), a technique routinely used by the NDACC (Network for the Detection of Atmospheric Composition Change) [6]. In addition to the ground-based observations, HCl can also be observed by space-based satellite instruments, e.g., the Aura Microwave Limb Sounder (MLS), the Upper Atmosphere Research Satellite (UARS) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS).
Using satellite instruments on board UARS in the 1990s, several studies investigated the HCl time series in the spring Antarctic vortices (e.g., Douglass et al., 1995 [7]; Santee et al., 1996 [8]; Chipperfield et al., 1996 [9]). E. Mahieu et al. [3] presented observed and stimulated HCl changes in the Northern and Southern Hemisphere lower stratosphere using ground-based FTIR, GOZCARD database of assimilated satellite products. E. Mahieu et al. indicated that HCl decrease of 0.5%/yr for 1997–2007, compatible with the 0.5–1%/yr range which verified the post-peak reduction of tropospheric chlorine. Froidevaux et al. [10] found a very good agreement between the ACE-FTS and the MLS v2.2 data, when including the latest available 2007 coincidences. N. Jones et al. [5] found that the HALOE and ACE-FTS data disclosed a negative trend of about −5.1% to −5.8% decade−1 for the time period 1997–2008, depending on latitude.
However, there has been minimal comparison and verification of GEOS-Chem model data in stratospheric. In this study, the FTIR HCl total column data set was used to evaluate the performance of GEOS-Chem model HCl simulations for stratospheric chemistry over Hefei. The concentration of stratospheric chloride has also been shown to be important for assessing the performance of stratospheric chemistry in chemistry-climate models [11]. Therefore, the comparison of GEOS-Chem model simulation data with the FTIR data can provide important information on evaluating the performance of GEOS-Chem and help us better understand stratospheric chemistry.
Most NDACC FTIR stations are located in Europe and Northern America, whereas the number of stations in Asia, Africa, and South America is very sparse. There is only one candidate station in China, i.e., the Hefei station operated by the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (AIOFM-CAS) [12]. It is similar to other Chinese megacities, severe air pollution often occurs in Hefei throughout the year (http://mep.gov.cn/, last access on 23 March 2019). In this study, we first report HCl total columns observed by ground-based Fourier transform infrared spectrometers at Hefei, China, and then comparisons with the GEOS-Chem model simulation.
We also present HCl time series observed by 6 NDACC stations (Bremen, Toronto, Rikubets, Izana, Reunion.maido, Lauder) in middle-latitude sites of the northern and southern hemispheres and Hefei stations in order to investigate the seasonal and annual trends of HCl in middle-latitude sites and to also offer a comparison to the Hefei site. The location of these stations are shown in Fig. 1.
This study is organized as follows. We present the detailed introduction of Hefei FTIR observation station in Section 2.1. Section 2.2 describes GEOS-Chem model simulations. The results and discussions are reported in Section 3. Seasonality and inter-annual variability of FTIR measurements are presented in Section 3.1. The comparisons with the GEOS-Chem simulations data and FITR data are analyzed in Section 3.2. In the Section 3.3, the results of the 6 NDACC stations and Hefei were analyzed. A summary is given in Section 4.
2. Methodology
2.1. FTIR Observation
2.1.1. Hefei station description and instrumentation
The ground-based high-resolution FTIR spectrometer over Hefei, China (117°10′E, 31°54′N, 30 m a.s.l. records the spectra since July 2015. This station is a candidate NDACC station and is one of the few observation stations in China that record uninterrupted high-resolution solar spectra, making it crucial to calibrate and validate the satellite data or model simulations in this important region [13].
The observation system consists of two parts: a high-resolution FTIR spectrometer (IFS125HR, Bruker GmbH, Germany) and a solar tracker (Tracker-A Solar 547, Bruker GmbH, Germany). The IFS125HR contains 9 optical compartments with 930 cm of optical path difference (OPD). The maximum resolution of the spectrometer is 0.00096cm-1 derived from the defined formula (0.9/OPD). For the middle infrared (MIR) spectra used in this study, the measurements are recorded over a wide spectral range (about 600–4500 cm-1) with a spectral resolution of 0.005cm-1 in order to ensure a high signal to noise ratio (SNR) and a faster acquisition time. For HCl measurements, the instrument is equipped with a KBr beam splitter & InSb detector & filter centered at 2900 cm-1. The entrance field stop size ranged from 0.80 to 1.5 mm to adapt the incident radiation. The number of measurements within a day varies from 1 to 20, but with a median value of 4. In total, there were 1102 value points of qualified measurements between July 2015 and April 2019 for HCl.
2.1.2. Retrieval strategy
The vertical profile of HCl is retrieved by the SFIT4 (v0.9.4.4) algorithm. We set up micro window (MWS) selections and the interfering gases following the NDACC standard recommendations, (https://www2.acom.ucar.edu/irwg/links, last access on 23 May 2019). The retrieval settings for HCl are listed in Table 1. Pressure and temperature profiles used are from the National Centers for Environmental Protection/National Center for Atmospheric Research (NCEP/NCAR) 6-hourly reanalysis [14]. All spectroscopic line parameters are extracted from HIRTRAN 2008 [15]. We used three micro windows (MWs: 2727.73-2727.83 cm-1 and 2775.70-2775.80 cm-1 and 2925.80-2926 cm-1) to retrieval HCl columns. The interfering gases are CH4, NO2, O3, N2O, and HDO. The profiles of CH4, NO2, O3, N2O and HDO are also retrieved in order to reduce their underlying absorption interference. A SNR (signal to noise ratio) of 300 is used in the three MWs for HCl.
Table 1. Summary of the retrieval parameters used for HCl retrieval.
We set the diagonal element of the measurement noise covariance matrix Sɛ to the square inverse of the fitting spectrum signal-to-noise ratio (SNR) and set its non-diagonal element to zero. The diagonal elements of a priori profile covariance matrices Sa are set to standard deviations of the WACCM model running from 1980 to 2020, and its non-diagonal elements are set to zero. The measured instrument line shape (ILS) deduced from regularly low-pressure HBr cell measurement is included in the retrieval [16,17].
Figures 2(a)–2(c), are a priori profile of mixing ratio, the averaging kernels and the degrees of freedom for signal (DOFS) profile for HCl, respective. The results are deduced from the spectra recorded in Hefei on March 15, 2016 with a measured ILS. High concentrations of HCl is mainly located in stratosphere and mesosphere. Most HCl averaging kernels center at around 37 km above the ground in the stratosphere. The total column degrees of freedom for signal (DOFS) profiles for HCl at Hefei station are about (1.57 ± 0.26) and the stratospheric contribution of HCl dominates the contribution. This study focuses on total column of HCl calculated by integrating the retrieved HCl profile from surface to the top of atmosphere. The variation of the retrieved HCl total column mainly represents the variation of the stratospheric HCl since the dominant retrieval sensitivity locates in the stratosphere.
Fig. 2. (a) a priori profile of HCl mixing ratio, (b) averaging kernels (ppmv/ppmv) of HCl (color fine lines), and their area scaled by a factor of 0.2 (brown bold line), (c) the degrees of freedom for signal (DOFS) profiles for a typical HCl retrieval. The results are deduced from the spectra recorded in Hefei on March 15, 2016 with a measured ILS.
We use the method proposed by Rodgers to calculate the retrieval error [18]. The error covariances can be categorized as random errors and systematic errors. We list the error items included in the error budget in Table 1 and summarize the contribution of each random and the systematic error item to the total column of HCl in Table 2. For the HCl retrieval at Hefei, the major random error is the smooth error (1.54) and the major systematic error is line intensity uncertainty (4.58%). The total error is the sum of the squares of the random error and the systematic error. The total error for the total HCl column has been estimated to be 5.12%.
Table 2. Error budgets and DOFs for the HCl retrievals at Hefei.
2.2. GEOS-Chem model simulations
The GEOS-Chem model [19], a global 3-D chemical transport model (CTM), was used to simulate HCl concentrations at the 2° × 2.5° horizontal resolution and 72 vertical pressure levels. This CTM model is driven by the GEOS (Goddard Earth Observing System) assimilated meteorological fields from the NASA Global Modeling and Assimilation Office. The GEOS-FP meteorological data with a native horizontal resolution of 0.25° latitude × 0.3125° longitude were downgraded to 2° latitude × 2.5° longitude, a vertical resolution of 72 hybrid levels (extending from surface to 0.01 hPa) and temporal resolution of surface variables and boundary layer height is 1 h and other variables is 3 h for driving the GEOS-Chem model [20].
The model includes a detailed mechanism of the universal tropospheric-stratospheric Chemistry extension (UCX) mechanism [21], which simulate a detailed “NOx-Ox-hydrocarbon-aerosols” tropospheric chemistry together with stratospheric chemistry. Stratospheric ozone chemistry is calculated by the linearized ozone parameterization (LINOZ) [22]. The concentrations of bromine species in the stratosphere are determined by climatology [23]. The evolution of most other species in the stratosphere are estimated in the model based on archived monthly mean production rates and loss frequencies from NASA’s Global Modeling Initiative (GMI) model [24]. The photolysis rates are simulated by the FAST-JX v7.0 photolysis algorithm [25].
In this study, the total emissions in this model are processed through the Harvard-NASA Emission Component (HEMCO) [26]. Global anthropogenic and biofuel emissions are from the Community Emissions Data System (CEDs) inventory [27]. These include emissions of aerosol species (BC and OC) and gases (SO2, NOx, NH3, CH4, CO, and NMVOCs). In particular, the MEIC (the Multi-resolution Emission Inventory for China) inventory is used to provide Chinese anthropogenic emissions [28].
2.3. NDACC Stations
The Network for the Detection of Atmospheric Composition Change (NDACC, http://www.ndacc.org) is an international global network of more than 90 stations that enables high-quality measurements of atmospheric components and was officially launched in 1991. The instruments used at the NDACC stations are mainly ground-based observations [sonde, lidar, microwave radiometers, Fourier-transform infrared, UV-visible DOAS (differential optical absorption spectroscopy)-type, and Dobson–Brewer spectrometers, as well as spectral UV radiometers][6].
In this study, the HCl time series observed by high resolution FTIR NDACC stations were used to analyze seasonal variability and annual trends and for comparison with the Hefei observation. Descriptions of all these FTIR stations are listed in Table 3. These NDACC stations cover over a wide latitude range from 45.04°S to 53.1°N. Most of these NDACC stations use the same instrument and as those of Hefei (32°N). The NDACC data were obtained from the NDACC database (http://www.ndsc.ncep.noaa.gov/data/).
Table 3. Description of the participating FTIR stations
3. Results and Discussions
3.1 Seasonality and inter-annual variability
Fig. 3 shows the time series of HCl total column concentrations at Hefei recorded by the FTIR from July 2015 to July 2019. We use Gardiner’s method and use a second-order Fourier series and a linear component to determine the annual linear trend and interannual variations [29]. A seasonal cycle of HCl total column is observed by FTIR, and it is well reproduced by the Gardiner’s fitting method. The monthly means of total HCl columns over 2015-2019 are shown in Fig. 4. Generally, the total HCl column exhibited a decrease trend between January and September, and exhibited an increase trend after September. The HCl total column reached a maximum value of (2.70 ± 0.16) × 1015 molecules*cm-2 in winter (January - February,) and a minimum value of (2.27 ± 0.09) × 1015 molecules*cm-2 in autumn (September - October). The total HCl column concentrations in January are, on average, (15.92 ± 4.63) % higher than those in September. The four-year time series of HCl total column show a negative trend of (-1.83 ± 0.13) %.
Fig. 3. FTIR time series and the fitted seasonality and inter-annual variability of total HCl columns at Hefei. Vertical error bars represent the total estimated error of each measurement.
Fig. 4. Monthly means of HCl total columns derived from Fig. 3. Vertical error bars represent 1σ within that month.
Fig. 5 presents seasonal variation of tropopause height over Hefei, China, extracted from GEOS-Chem model data. This variation results from the stratospheric general circulation transporting air from the summer to the winter hemisphere [30]. The seasonal cycle of the observed HCl total column is associated with the seasonal variation of the tropopause height. The higher the tropopause, the smaller is the relative contribution of the stratosphere to the total column abundance. HCl is a stratospheric gas, and thus a lower HCl total column is observed in summer. In addition, HCl is more likely to photolysis in the atmosphere in summer, which is also a reason for the lower concentration in summer [30].
3.2 Comparisons with GEOS-Chem model
The FTIR HCl time series was used to compare with GEOS-Chem model data to evaluate the performance of GEOS-Chem model simulation in eastern China. GEOS-chem model simulations were performed to cover the period July 2015 to April 2019, and HCl mixing ratio profiles were output at 1 h time intervals. First, the model results sampled at the nearest grid point to FTIR station coordinates are vertically interpolated into the FTIR vertical grid. Next, we convolute smooth the sampled and interpolated data with the kernels and a priori profiles through the Eq. (2) [31,32].
Fig. 6 shows time series of HCl total column concentrations obtained by the FTIR and the GEOS-Chem simulation over 2015 to 2019. The GEOS-Chem model data within ± 1 h of the FTIR sampling time were used to calculate the correlation. The correlation of the two data set is shown in Fig. 7. The GEOS-Chem model data are in good agreement with the coincident FTIR data, with a correlation coefficient (r) of 0.82.
The GEOS-Chem model data are smoother than the FTIR measurements. Furthermore, the GEOS-Chem model data overestimated FTIR measurements at lower HCl concentration, and underestimated FTIR measurements at higher HCl concentration. However, the GEOS-Chem model simulation can well reproduce the seasonal variation of FTIR observations. The maximum and minimum GEOS-Chem HCl total columns occur in February and October, with monthly means of (2.66 ± 0.16) × 1015 molecules*cm-2 and (2.27 ± 0.09) × 1015 molecules*cm-2, respectively. The maximum GEOS-Chem HCl total column is (14.67% ± 1.31%) higher than the minimum one.
The HCl total column monthly means of the GEOS-Chem simulation and the FTIR as well as their difference are presented in Figs. 8(a) and 8(b). Annual average difference between the smoothed GEOS-Chem results and the FTIR data is (5.17 ± 4.73) × 1014 molecules*cm-2 (1.74% ± 1.59%). Generally, the differences between the GEOS-Chem data and FTIR data are seasonal independent. The maximum difference occurs in April, with mean difference of (1.22 ± 0.04) × 1014 (4.94% ± 0.43%) molecules*cm-2.
Fig. 8. (a) Monthly mean the total HCl columns from the GEOS-Chem simulation, the FTIR data. (b) GEOS-Chem minus FTIR monthly differences.
Overall, the emission inventory, meteorological field, and the full chemical mechanism embedded in the GEOS-Chem model for HCl simulation can well reproduce monthly average concentrations over the polluted atmosphere in eastern China. The differences between the FTIR measurements and GEOS-Chem results may be largely attributed to the difference in temporal and spatial coverage of the two techniques selected in comparison.
3.3 Comparisons with NDACC data
In order to investigate the seasonal and annual trends of HCl in low-middle-latitude areas, the HCl total columns are compared to measurements at 6 NDACC stations which cover latitude range from 45.04°S to 53.1°N (http://www.ndaccdemo.org/, last access on 19 July 2019). Monthly maximum and minimum of all ground FTIR stations and difference with Hefei stations are summarized in Table 4.
Table 4. The HCl total columns at Hefei (32°N), China from 2015 to 2019 along with 6 NDACC FTIR stations. Stations are listed as a function of decreasing latitude
The monthly means of the HCl total columns at the seven FTIR stations are shown in Fig. 9(a). The monthly means of seasonal linear trend of HCl by the Gardiner’s fitting method are shown in Fig. 9(b) in order to show the seasonal cycle of these sites better and minimize the influences of a priori offsets to the Total column. In the northern hemisphere, the HCl total column at these stations reached the maximum in the late spring and the minimum in the early winter. The seasonal variations of HCl total columns in the southern hemisphere are opposed to those in the northern hemisphere because of the seasons of the two hemispheres are opposite. The monthly average tropopause height of each station is shown in Fig. 10. In Section 3.1, the relationship between the tropopause height and the total column concentration of HCl has been explained, which can be better proved in this section. In all stations, the total column concentration is relatively low in the period of high tropopause height. In general, the seasonal variations of HCl over Hefei is similar to that in other northern hemisphere mid-latitude FTIR stations. The difference between Izana and Hefei station is the smallest because their latitude is almost the same. For the other two stations (Rikubetsu station and Toronto station) are close to the latitude of Hefei Station, the seasonal variation trend is very consistent with that of Hefei area, but the value is much higher than that of Hefei area, mainly due to the lower tropopause height of the two stations. Comparing with Fig. 10, it can be found that the month with large difference from Hefei Station is mainly due to the large deviation between troposphere height and Hefei Station. The trend of HCl measurements at these sites is mainly determined by their tropospheric height [30].
Fig. 9. (a) Monthly means of the HCl total columns at the seven FTIR stations. (b) The monthly means seasonal linear trend of HCl by the Gardiner’s fitting method.
4. Conclusions
The FTIR time series of HCl column have been presented over the polluted atmosphere over Hefei, China during 2015-2019, and compared with the GEOS-Chem model results and 6 NDACC stations. This is the first time in China measurements have been analysed to obtain the time series of total column HCl concentration by using the ground-based high-resolution FTIR. The HCl total column peaks in late winter and early spring and reaches the lowest in late summer and autumn. The total HCl column concentrations in January are, on average, (2.70 ± 0.16) × 1015 molecules*cm-2, and are higher than (15.92% ± 4.63%) those in September which have a mean value of (2.27 ± 0.09) × 1015 molecules*cm-2. The result also showed a negative trend of (-1.83 ± 0.13) % /yr between 2015 and 2019. The decreased annual trend for stratospheric HCl during period 2015-2019 indicated a reduction in atmospheric chlorine emission within this period as a response to the Montreal protocol. In stratosphere, the reaction of HCl with OH radical which forms chlorine atom to react with ozone provides the main pathway for stratospheric ozone depletion. Roughly, the decrease in stratospheric HCl indicated a recovery of stratospheric ozone.
We use FTIR time series to evaluate the performance of GEOS-Chem chemistry transport model to simulate HCl, over Hefei. GEOS-Chem model data reproduce the seasonal variation and inter-annual trends characteristics of FTIR observations and are shown a correlation coefficient (r) of 0.82. The maximum and minimum GEOS-Chem simulated the total HCl columns occur in February and October, with monthly mean of (2.66 ± 0.16) × 1015 molecules*cm-2 (14.67% ± 1.31%) and (2.27 ± 0.09) × 1015 molecules*cm-2, respectively. Annual average difference between the smoothed GEOS-Chem results and the FTIR data is (5.17 ± 4.73) × 1014 molecules*cm-2 (1.74% ± 1.59%). The differences between GEOS-Chem data and FTIR data show a small variation in amplitude but higher in spring.
We also compare FTIR time series over Hefei, China with 6 NDACC stations in order to investigate the seasonal and annual trends of HCl in low-middle-latitude areas. In the northern hemisphere, the HCl total column at these stations reached the maximum in the late winter or early spring and the minimum in the late autumn or early winter. It is the opposite phenomenon in the southern hemisphere. The trend of HCl measurements at these sites is mainly determined by their tropospheric height.
Funding
National High-tech Research and Development Program (2019YFC0214802, 2018YFC0213104, 2016YFC0203302, 2016YFC0200404, 2017YFC0210002, 2018YFC0213201); National Natural Science Foundation of China (41605018, 41877309, 41722501, 41775025, 41405134, 41575021, 51778596, 91544212, 41977184); The Sino-German Mobility programme (M-0036); Strategic Priority Research Program of the Chinese Academy of Sciences (XDA23020301); National Key Project for Causes and Control of Heavy Air Pollution (DQGG0102, DQGG0205); Major Projects of High Resolution Earth Observation Systems of National Science and Technology (05-Y30B01-9001-19/20-3); Anhui Science and Technology Department (18030801111); Natural Science Foundation of Guangdong Province (No. 2016A030310115).
Acknowledgments
The processing environment of SFIT4 and some plot programs are provided by National Center for Atmospheric Research (NCAR), Boulder, Colorado, USA. The NDACC networks is acknowledged for supplying the SFIT software and advice. The FTIR spectrometers at Toronto (44°N), are operated by the University of Toronto. The FTIR located at Bremen (53°N) are operated by University of Bremen, Institute of Environmental Physics, and those at La Reunion Maido (21°S) are operated by the Belgian Institute for Space Aeronomy.
Disclosures
The authors declare no conflicts of interest.
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