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Large contrast in the vertical distribution of aerosol optical properties and radiative effects across the Indo-Gangetic Plain during the SWAAMI-RAWEX campaign

Measurements of the vertical profiles of the optical properties (namely the extinction coefficient and scattering and absorption coefficients respectively σ ext / σ scat / σ abs) of aerosols have been made across the Indo-Gangetic Plain (IGP) using
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  Atmos. Chem. Phys., 18, 17669–17685, 2018© Author(s) 2018. This work is distributed underthe Creative Commons Attribution 4.0 License. Large contrast in the vertical distribution of aerosol opticalproperties and radiative effects across the Indo-GangeticPlain during the SWAAMI–RAWEX campaign Aditya Vaishya 1,a , Surendran Nair Suresh Babu 1 , Venugopalan Jayachandran 1 , Mukunda M. Gogoi 1 ,Naduparambil Bharathan Lakshmi 1 , Krishnaswamy Krishna Moorthy 2 , and Sreedharan Krishnakumari Satheesh 2,31 Space Physics Laboratory, Vikram Sarabhai Space Centre, ISRO PO, Thiruvananthapuram, India 2 Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India 3 Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India a now at: School of Arts and Sciences, Ahmedabad University, Ahmedabad, India Correspondence:  Aditya Vaishya ( 5 July 2018 – Discussion started: 9 July 2018Revised: 12 November 2018 – Accepted: 17 November 2018 – Published: 13 December 2018 Abstract.  Measurements of the vertical profiles of the opti-cal properties (namely the extinction coefficient and scatter-ing and absorption coefficients respectively σ  ext  /σ  scat  /σ  abs )of aerosols have been made across the Indo-GangeticPlain (IGP) using an instrumented aircraft operated fromthree base stations – Jodhpur (JDR), representing the semi-arid western IGP; Varanasi (VNS), the central IGP charac-terized by significant anthropogenic activities; and the in-dustrialized coastal location in the eastern end of the IGP(Bhubaneswar, BBR) – just prior to the onset of the In-dian summer monsoon. The vertical profiles depicted region-specific absorption characteristics, while the scattering char-acteristics remained fairly uniform across the region, lead-ing to a west–east gradient in the vertical structure of single-scattering albedo (SSA). Integrated from near the ground to3km, the highest absorption coefficient and hence the lowestSSA occurredinthecentral IGP(Varanasi). Sizedistribution,inferred from the spectral variation of the scattering coeffi-cient, showed a gradual shift from coarse-particle dominancein the western IGP to strong accumulation dominance in theeastern coast with the central IGP coming in between, arisingfrom a change in the aerosol type from a predominantly nat-ural (dust and sea salt) type in the western IGP to a highlyanthropogenic type (industrial emissions, fossil fuel andbiomass combustion) in the eastern IGP, with the central IGPexhibiting a mixture of both. Aerosol-induced short-wave ra-diative forcing, estimated using altitude-resolved SSA infor-mation, revealed significant atmospheric warming in the cen-tral IGP, while a top-of-atmosphere cooling is seen, in gen-eral, in the IGP. Atmospheric heating rate profiles, estimatedusing altitude-resolved SSA and column-averaged SSA, re-vealed considerable underestimation in the latter case, em-phasizing the importance and necessity of having altitude-resolved SSA information as against a single value for theentire column. 1 Introduction Ground-based, as well as space-borne observations, have es-tablished that the Indo-Gangetic Plain (IGP) (the vast stretchof apparently contiguous plain land along the east–west withan area of approximately 7millionkm 2 bounded betweenthe Iranian Plateau to the west, the Bay of Bengal to theeast, the Himalayas to the north and Chota Nagpur Plateauand Aravalli ranges to the south) remains one of the aerosolhotspots in the world, depicting persistently high aerosolloading (Babu et al., 2013; Gautam et al., 2010; Dey and DiGirolamo, 2010), especially during the dry winter and pre-monsoon seasons. The increasing demographic pressure (be-ing one of the most densely populated regions of the world),large-scaleagriculturalactivities(amongtheworld’smostin-tensefarmingareas),consequenthighdemandonenergy(ap-proximately 70% of the coal-fired thermal power plants of Indiaarelocatedinthisregion)andextensiveindustrialactiv-ities (steel mills, cement factories, manufacturing units and a Published by Copernicus Publications on behalf of the European Geosciences Union.  17670 A. Vaishya et al.: Vertical distribution of aerosol properties across the IGP number of small- and medium-scale industries) are believedto be leading to consistently increasing anthropogenic emis-sions and hence a persistent increasing trend in the aerosolloading as reported in Babu et al. (2013). The loose allu-vial soil, which is characteristic of this region, and the semi-arid and arid regions along its western part including theThar Desert, and the prevailing complex meteorology withextreme temperatures and dry winds (except during the In-dian summer monsoon (ISM) season) contribute their shareof natural mineral aerosols. The peculiar topography of thisregion, which slopes down from west to east and is boundon either side by the Himalayas to the north and the DeccanPlateau to the south, leading to a narrowing of its width fromwest to east, aids in spatially confining and channelling theseemissions until they are flushed out to the Bay of Bengal. Allthe above make this region a cauldron of complex aerosoltypes (Moorthy et al., 2016 and references there in), whichhave been attracting immense scientific interest from envi-ronmental and climate scientists because of the known com-plex climate implications (Gautam et al., 2009, 2010; Lauand Kim, 2010; Lal et al., 2013; Das et al., 2015a).Recent studies using in situ and remote-sensing methodshave shown a springtime enhancement in the aerosol opticaldepth and black carbon (BC) concentration in the lower freetroposphere (below 5km) over the plains and also over theHimalayas (Prijith et al., 2016; Kompalli et al., 2016; Gogoiet al., 2014), and a northward-increasing gradient in the am-plitude and altitude of the aerosol-induced atmospheric heat-ing(Satheeshetal.,2008).Inarecentstudy,Nairetal.(2016)have found a large enhancement in aerosol absorption in thelower free troposphere over the IGP during spring. Enhancedabsorption by these climatically critical and highly absorbingelevated aerosols would have significant radiative implica-tions. A very recent work, synergizing these measurementswith models and satellite data (Govardhan et al., 2017) hashighlighted the potential of these elevated absorbing aerosolsto aggravate stratospheric ozone loss or in delaying the re-coveryofozonedepletioninthepast.Sarangietal.(2016)re-ported enhanced stability of the lower free troposphere due tothese elevated aerosols over the IGP, while Dipu et al. (2013)found an alteration in cloud water content due to these layers.Dust aerosols are significant contributors to elevatedaerosol load over the IGP during the pre-monsoon season(PMS) (Gautam et al., 2010) and along with BC constitutethe major absorbing aerosol species. Desert dust aerosolsfrom the Arabian and Thar Desert regions, driven by windsacross the IGP, are found to form elevated layers of dustaround 850hPa and above (Das et al., 2013). Studies haverevealed the absorbing nature of this dust (in contrast to theirSaharan counterpart) (Moorthy et al., 2007), which is at-tributed to the Fe (iron) enrichment in the aerosols advectedfrom Thar Desert and adjoining semi-arid regions (Srinivasand Sarin, 2013). Modelling studies have shown a telecon-nection between the advected dust and Indian summer mon-soon (ISM) (Vinoj et al., 2014). Padmakumari et al. (2013)suggested that the potential role of these aerosols is to act asice nuclei.However, most of the impact assessments of aerosols overthis region have used optical properties of aerosols, espe-cially the most important parameter, the single-scatteringalbedo (SSA), derived either indirectly (Ramachandran etal., 2006) or from surface measurements (Ram et al., 2016),while information on the vertical structure of the opticalproperties(scattering,absorption,SSA)hasbeenverysparse.This information is very important to accurately estimatethe vertical structure of atmospheric heating rate resultingfrom absorption by aerosols. This is also necessitated bythe fact that, for a given amount of absorbed solar radiation,more heating would be produced if the absorbing species washigher in the atmosphere, due to the lower density of air athigher altitudes, and trigger local convection. Knowledge of aerosol properties prior to the onset of the ISM is also es-sential for delineating the role of aerosols as cloud conden-sation nuclei and their impact on cloud formation, propertiesand associated precipitation. With this objective, an Indo–UK field campaign, South West Asian Aerosol MonsoonInteractions (SWAAMI), has been formulated to be carriedout during the onset phase of the ISM jointly with the Re-gional Aerosol Warming Experiment (RAWEX) being pur-sued in India under the ARFI project of ISRO’s GeosphereBiosphere Programme. One of the main aims was to char-acterize the vertical structure of aerosol radiative propertiesand estimate its impact on atmospheric thermal structure inthe IGP. For this, extensive airborne measurements of the ex-tinction, scattering and absorption coefficients (respectively σ  ext  /σ  scat  /σ  abs ) were carried out across the IGP (from westto east) from three base stations in the west, centre and east.The details are provided in this paper, followed by a presen-tation of the results and estimation of the short-wave aerosolradiative forcing and vertical profile of aerosol-induced at-mospheric heating rates. These results are examined in lightof available information and the implications are discussed. 2 Aircraft campaign, data and methodology2.1 Campaign details During the field experiment, the vertical structure of aerosoloptical properties were measured using an instrumented air-craft (Beechcraft, B200 of the National Remote SensingCentre (NRSC) of the Indian Space Research Organisation– ISRO) from 1 to 20 June 2016, just before the onsetof the ISM. The vertical profiling have been carried outfrom three base stations, Jodhpur (JDR), Varanasi (VNS)and Bhubaneswar (BBR), representing respectively the west-ern (arid), central (anthropogenic) and eastern (industrial-ized coastal) IGP. The geographical locations of these sta-tions are shown by the solid circles in Fig. 1, which alsoshows the mean wind field at 850hPa that prevailed during Atmos. Chem. Phys., 18, 17669–17685, 2018   A. Vaishya et al.: Vertical distribution of aerosol properties across the IGP 17671 the campaign period. The flight tracks over each of these lo-cations are superimposed and shown in colour-coded formJDR (green), VNS (red) and BBR (blue). Five sorties eachwere made (on consecutive days or in close succession) atBhubaneswar and Varanasi, while four sorties were madefrom Jodhpur; the dates of sorties from each station are de-tailed in Table 1 along with the base station details and themeasurement details. Each sortie took  ∼ 3.5h, in view of the endurance of the aircraft ( ∼ 4h) flying in the unpres-surized mode and comprised of measurements at six verti-cal levels (500, 1000, 1500, 2000, 2500 and 3000ma.g.l.;above mean ground level); a typical profiling path is shownin Fig. 2. After taking off from the base station, the aircraftreached the desired flight level, and after stabilizing the atti-tude, measurements were made to ensure a minimum dura-tion of 25min before the aircraft climbed to the next level.For the present analysis, 5min of measurements were re-moved as a precaution after a stable level was achieved. Thiswas done in order to avoid any spurious measurements dueto a sudden change in the course of a flight. It was found that,due to the occasional appearance of clouds, aerosol numberconcentration increased from otherwise stable values. In or-der to remove such unavoidable incidences from influenc-ing aerosol properties, 2 σ   criteria was applied wherein datapoints at a particular level lying outside 2 σ   values of thelevel average were removed. Overall, < ∼ 3% of the mea-surements were screened out due to this criteria. The mea-surements were then repeated at the new level after the air-craft had stabilized its attitude. In this way, 20min of use-ful data was ensured at each level. After measurement at thelast level, the aircraft returned to the base station. All the air-craft sorties, at all the sites, were made between ∼ 10:00 and14:00IST (Indian Standard Time). This was done in order toensurethattheconvectiveboundarylayerisevolved,aerosolsare well mixed within the column, and there is no residuallayer aloft. Planetary boundary layer (PBL) heights were ob-tained for the flight sortie days from the NCEP/NCAR globalreanalysis product at 0.25 × 0.25 ◦ grid resolution data. MeanPBL heights, at local noon time, over the IGP regions for thecampaign period were 1.3 ± 0.5km for JDR (western IGP),2.3 ± 0.5km for VNS (central IGP), and 1.4 ± 0.2km forBBR (eastern IGP).Near the ground, at 0–200m, data represent measurementswhen aircraft altitude was below 200m, as confirmed fromGlobal Positioning System (GPS) data. Near-ground data du-ration was between 3 to 8min each day. The measurementtrack had a horizontal span of   ∼ 150km and the region of measurement was within 300km diameter circle centred atthe base station. Details of the flight configuration are avail-able in earlier papers (e.g. for Babu et al., 2016; Moorthy etal., 2004; Nair et al., 2016).All the aerosol instruments aspirated ambient air through ashrouded solid diffuser inlet, configured as detailed in Babuet al. (2016), which maintained isokinetic flow, and the airwas supplied to the instruments through isokinetic flow split- Figure 1.  Geographical location of the aircraft campaign stations(solid circle) in the Indo-Gangetic Plain superimposed on the meanwind field at 850hPa during the campaign period. JDR, VNS andBBR stand for Jodhpur, Varanasi and Bhubaneswar. Daily flighttracks are superimposed on the stations JDR (green), VNS (red)and BBR (blue) from left to right. Each measurement track has ahorizontal span of  ∼ 150km from the base station. Figure 2.  Typical course of the aircraft during a campaign sortie.Symbols represent stable levels. Each stable level represents a min-imum of 20min of scientifically useful measurements. ters. The inlet was connected to an external pump that main-tained a volumetric flow of 70Lmin − 1 (litres per minute).More details are available in Babu et al. (2016) and refer-ences therein. 2.2 Base stations Each base station represented a distinct region of the IGP,as has already been mentioned. Jodhpur (26.25 ◦ N, 73.04 ◦ E;219ma.m.s.l.) represented the western IGP, which stretchesfrom eastern Pakistan to northern parts of the Aravalli Range,ending in Delhi, is characteristically arid region, dominatedby natural aerosols (mineral dust). It also contains the GreatIndian Desert or Thar Desert. Consequently, during summerthe temperature often exceeds 40 ◦ C during daytime, withmaximum values reaching as high as 48 ◦ C. Pre-monsoonaerosol system over this region is dominated by dust, primar- Atmos. Chem. Phys., 18, 17669–17685, 2018  17672 A. Vaishya et al.: Vertical distribution of aerosol properties across the IGP Table 1.  Details of the stations and dates on which flight sorties were launched and instruments were operated.Station Latitude Longitude Height, m Dates Instruments ∗ (region) ( ◦ N) ( ◦ E) (a.m.s.l.) (June 2016)Bhubaneswar (eastern IGP) 20.24 85.81 42 1–5 CAPS PM ex , Nephelometer,Varanasi (central IGP) 25.45 82.85 81 8, 10–13 Aethalometer, CPC,Jodhpur (western IGP) 26.25 73.04 219 17–20 APS, CCNc, GPS ∗ CAPS PM ex : Cavity Attenuated Phase Shift Extinction Monitor, CPC: condensation particle counter, APS: Aerodynamic Particle Sizer, CCNc:cloud condensation nuclei counter, GPS: Global Positioning System. ily produced locally and that transported from Arabia, theMiddle East and eastern Africa (Prasad and Singh, 2007).The central IGP, extending from the north-eastern bound-aries of the Aravalli range up to the north-western regionsof the Chota Nagpur Plateau, is represented by Varanasi(25.45 ◦ N, 82.85 ◦ E; 81ma.m.s.l.). The central IGP hosts nu-merous coal-fired thermal power plants, large-scale indus-tries including steel and cement factories and has the high-est population density compared to other regions of the IGP.Approximately ∼ 65% of the area in the central IGP is undercultivation. Central IGP is frequented by local dust stormsandtransporteddust(PrasadandSingh,2007)duringthepre-monsoon season.The eastern IGP, geographically bound by Chota NagpurPlateau in the west, Himalayas in the north, Purvanchal hillsin the east and the Bay of Bengal in the south, is representedby Bhubaneswar (20.24 ◦ N, 85.81 ◦ E; 42ma.m.s.l.), locatedabout 70km inland. It encompasses a large swath of landwith numerous water bodies, dense forested regions, and thegreat Sundarbans delta. Apart from local emissions from in-dustries, vehicles and other household practices, the easternIGP receives a significant portion of its aerosol load from thewestern and central IGP (Nair et al., 2007) due to its locationin the continental outflow from the central IGP. Bhubaneswarand adjoining regions are hosts to several heavy industriesand thermal power plants, and as such, high aerosol opticaldepth ( ∼ 0.5 at 500nm) prevails over this region (Das et al.,2009). 2.3 Instruments, measurements and database A suite of instruments has been used aboard for measur-ing the aerosol properties (see Table 1) of which the datafrom those dealing with the optical properties are used inthis study. These included aerosol light extinction coefficient( σ  ext ) measurements at 530nm, carried using a Cavity Atten-uated Phase Shift Extinction Monitor (CAPS PM ex ) (modelPM ex  of Aerodyne Research Inc.); aerosol light scatteringcoefficient ( σ  scat ) measured using a 3-wavelength (450, 500and 700nm) integrating nephelometer (TSI; model: 3563);and aerosol absorption coefficient ( σ  abs ) derived from themeasurements made using a 7-channel Aethalometer (modelAE-33, Magee Scientific). CAPS PM ex  employs the cavity-attenuated phase shift technology (Herbelin and McKay,1981; Kebabian et al., 2007) and measures the phase shift inthelight leaving a highly reflective optical cell illuminatedbya square wave modulated light-emitting diode source (Mas-soli et al., 2010).  σ  ext  is calculated from the differences of phase shift between the particle-free air and particle-ladenair in the optical chamber. Details are given by Massoli etal. (2010). This instrument was operated at a flow rate of 0.85Lmin − 1 . Auto baseline measurements were taken every2min. Massoli et al. (2010) have established that the CAPSPM ex  has a detection limit of 3Mm − 1 or lower at a 1s timeresolution and has an uncertainty of  ± 3%.Details of the nephelometer operation and principle of measurement are given by Anderson et al. (1996) andHeintzenberg and Charlson (1996). The instrument was op-erated at a flow rate of 16Lmin − 1 and calibrated with CO 2 span gas before and after the campaign to ascertain consis-tency in performance. Besides this, zero background mea-surements were taken with filtered air on hourly basis to as-certain the health of the instrument. The measurements arecorrected for the well-known truncation error (due to non-availability of measurements for angles <7 and >170 ◦ fol-lowing Anderson and Ogren (1998) methodology as detailedin earlier papers (Nair et al., 2009; Babu et al., 2012). Uncer-tainties in measured  σ  scat  are within ∼± 10% (Anderson etal., 1996).Aethalometermeasuresattenuationoflightbyaerosolsde-posited on a filter spot. Absorption coefficient is then calcu-lated from the rate of change of attenuation, filter spot areaand volumetric flow rate using Eq. (1) given below (Wein-gartner et al., 2003). σ  abs = AQ ·  ATN t  ,  (1)where  A  is the filter spot area,  Q  is the volumetric flowrate, and   ATN is change in attenuation in time  t  . TheAethalometer was operated at a flow rate of 2Lmin − 1 and data frequency was set to 1min. Measurements by theAethalometerare knowntohavetheinstrumentartefacts,viz.multiple scattering, loading effect and assumption of m BC (Weingartner et al., 2003; Liousse et al., 1993). The underes-timation of BC due to a loading effect is compensated for inthe instrument, which uses the dual-spot technique, follow-ing Drinovec et al. (2015). A factor of 1.57 is used to com-pensate for the enhanced light absorption arising due to mul- Atmos. Chem. Phys., 18, 17669–17685, 2018   A. Vaishya et al.: Vertical distribution of aerosol properties across the IGP 17673 tiple scattering within the filter fibre matrix (Drinovec et al.,2015). Uncertainties related to the measurement of the ab-sorption coefficient, using filter-based techniques, have beendiscussed in a series of literature (Müller et al., 2011; Dri-novec et al., 2015; Collaud Coen et al., 2010; Segura et al.,2014; Lack et al., 2014). These uncertainties mainly stemfrom two major causes:i. multiple-scattering within the filter fibre matrix, andii. lower attenuation coefficients for higher filter loadings,also called the filter loading effect (Weingartner et al.,2003).Lack et al. (2014) have estimated an uncertainty of 12%–30% in  σ  abs  measured using filter-based techniques. How-ever, this assumption is on the higher side for the presentstudy for two reasons:i. the new-generation Aethalometer (Drinovec et al.,2015) has in place real-time compensation of the load-ing effect, which earlier was assumed as a constant; andii. advanced filter tape material is used, which minimizesthe effect due to multiple scattering and can be bettercharacterized.After taking into consideration uncertainties introduced dueto flow instabilities (Drinovec et al., 2015) and an uncertaintyof  ∼ 10% is expected in the absorption coefficient measure-ments. Details of the Aethalometer data analysis can alsobe found in earlier publications (Babu and Moorthy, 2002;Moorthy et al., 2004). The Aethalometer data were correctedfor volumetric flow in order to sample the same volume of air at each altitude, following Moorthy et al. (2004).All on-board computers and instruments were time syn-chronized with the GPS time during each sortie. After eachsortie, the measured data were georeferenced using high timeresolution (1s) GPS data, available from a GPS receiver onboard. 3 Results and discussion3.1 Vertical and spatial distribution of aerosolradiative properties The raw data of   σ  ext ,  σ  scat  and  σ  abs , after all necessary cor-rections and time tagging, from all the sorties at a particularstation, have been grouped in terms of the different altitudelevels chosen for the sortie (as described in Sect. 2.4) and av-eraged to construct the mean, station-specific altitude profile.All the three parameters, σ  ext , σ  scat  and σ  abs , are presented for530nmwavelengths(thewavelengthusedbytheCAPS),andfor this the  σ  scat  and  σ  abs  values were interpolated (betweenat 450 and 550nm for  σ  scat  and between 520 and 590nm for σ  abs ) using the corresponding Ångström power-law relation(Ångström, 1964): σ  scat / abs = β scat / abs λ α scat / abs ,  (2)where  β scat / abs  is a constant,  λ  is wavelength and  α scat / abs  isthe scattering and absorption Ångström exponent.Figure 3a–c show the vertical distributions of   σ  ext ,  σ  scat and  σ  abs , over the three stations. In all the figures, square,triangle and circle symbols correspond to measurements overJDR, VNS and BBR respectively. Error bars represent thestandard error at that level for the station.A vertical heterogeneity is clearly seen in all the propertiesacross the IGP. While the altitude variation is very weak atJDR (western IGP) and moderate at BBR, it is rather strongat the central IGP (VNS). The weak vertical variation at JDRis attributed to the strong convective mixing over this aridregion, where the solar heating is very intense during thisseason. Above around 1.5km, there is a decrease in  σ  ext  and σ  scat , which is stronger than that seen in  σ  abs . This is likely tobe due to rapid sedimentation of heavier dust particles, whichcontribute largely to  σ  ext  and  σ  scat , compared to their anthro-pogenic counterpart, which contributes dominantly to  σ  abs .The extinction at 3km is just half of that at 0.5km or evenat 1.5km. The day-to-day variability over the western IGPis smaller compared to that at the other two regions as evi-denced by the shorter error bars. This is also attributed to thenear-uniform dominance of dust aerosols in this region andthe scarcity of anthropogenic sources of aerosols, e.g. indus-tries, coal-fired power plants. In contrast, VNS in the centralIGP shows significantly higher values of   σ  ext ,  σ  scat  and  σ  abs close to the surface (clearly attributed to the large abundanceof anthropogenic emissions in this region, as has been statedearlier) and a rather sharp decrease with altitude, with  σ  ext  at3km falling by a factor of 4 of the near-surface value (similarfor  σ  scat  and  σ  abs ). As the central IGP is dotted with numer-ous coal-fired power plants (, last ac-cess: 10 May 2018) and heavy industries, and has the highestpopulationdensityintherange800–1200km − 2 ,theresultinglarge emissions are reflected in the high values and the largeday-to-day variability (large standard error bars) of the opti-cal properties of aerosols over this region. While close to thesurface, the extinction values are considerably higher overthe central and eastern IGP (compared to the western part) atthe higher levels (above 2km) the values are of comparablemagnitude at all three stations, showing a larger spatial ho-mogeneity in the lower free troposphere. The absorption co-efficient over BBR, in the eastern IGP, remains nearly steadywith altitudes up to around 2km, above which it marginallyincreases (unlike at the other two stations), albeit the increaseis within the natural variability of the lower levels, showingmore absorbing aerosols aloft.Spatially, the column-averaged values of   σ  scat ,  σ  abs  and σ  ext  (up to the maximum height up to which measurementswere made) are the highest in the central IGP compared tothe eastern and western IGP, primarily due to the very high Atmos. Chem. Phys., 18, 17669–17685, 2018
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