Boundary-layer dynamics and its influence on atmospheric chemistry at Summit,Greenland

Sonic anemometer turbulence measurements were made at Summit, Greenland during summer 2004 and spring 2005. These measurements allow for the characterization of the variability of the atmospheric boundary layer at this site by describing seasonal and diurnal changes in sensible heat flux and boundary layer stability as well as providing estimates of mixing layer height. Diurnal sensible heat fluxes at Summit ranged from −18 to −2 W m⁻² in the spring and from −7 to +10 W m⁻² in the summer. Sustained stable surface layer conditions and low wind speeds occured during the spring but not during the summer months. Unstable conditions were not observed at Summit until late April. Diurnal cycles of convective conditions during the daytime (0700–1700 h local time) were observed throughout July and August. Boundary layer heights, which were estimated for neutral to stable conditions, averaged 156 m for the spring 2005 observations. Comparisons of the boundary layer characteristics of Summit with those from South Pole, Antarctica, provide possible explanations for the significant differences in snowpack and surface-layer chemistry between the two sites.     

Electron spectroscopy for chemical analysis (ESCA) study of aluminum-containing atmospheric particles

ESCA (electron spectroscopy for chemical analysis), and SEM (scanning electron microscopy) analysis and examination of aluminum-containing particles have been carried out. Collection of particulate matter on Nuctepore filters was done at the NASA-Kennedy Space Center during active and inactive launch periods of Titan III—C rockets using solid propellants. The aluminum-containing particulate matter collected during launch had a unique spherical shape when compared to particles collected during launch inactivity. The ESCA results for aluminum suggest that at least three chemically different types of aluminum are present in samples collected during launch activities.     

An adaptive reduction algorithm for efficient chemical calculations in global atmospheric chemistry models

We present a computationally efficient adaptive method for calculating the time evolution of the concentrations of chemical species in global 3-D models of atmospheric chemistry. Our strategy consists of partitioning the computational domain into fast and slow regions for each chemical species at every time step. In each grid box, we group the fast species and solve for their concentration in a coupled fashion. Concentrations of the slow species are calculated using a simple semi-implicit formula. Separation of species between fast and slow is done on the fly based on their local production and loss rates. This allows for example to exclude short-lived volatile organic compounds (VOCs) and their oxidation products from chemical calculations in the remote troposphere where their concentrations are negligible, letting the simulation determine the exclusion domain and allowing species to drop out individually from the coupled chemical calculation as their production/loss rates decline. We applied our method to a 1-year simulation of global tropospheric ozone-NO_x-VOC-aerosol chemistry using the GEOS-Chem model. Results show a 50% improvement in computational performance for the chemical solver, with no significant added error.     

Morphological and chemical composition characteristics of summertime atmospheric particles collected at Tokchok Island,Korea

Determination of the chemical compositions of atmospheric single particles in the Yellow Sea region is critical for evaluating the environmental impact caused by air pollutants emitted from mainland China and the Korean peninsula. After ambient aerosol particles were collected by the Dekati PM10 cascade impactor on July 17–23, 2007 at Tokchok Island (approximately 50 km west of the Korean coast nearby Seoul), Korea, overall 2000 particles (on stage 2 and 3 with cut-off diameters of 2.5–10 μm and 1.0–2.5 μm, respectively) in 10 samples were determined by using low-Z particle electron probe X-ray microanalysis. X-ray spectral and secondary electron image (SEI) data showed that soil-derived and sea-salt particles which had reacted or were mixed with SO₂ and NO_x (or their acidic products) outnumbered the primary and “genuine” ones (59.2% vs. 19.2% in the stage 2 fraction and 41.3% vs. 9.9% in the stage 3 fraction). Moreover, particles containing nitrate in the secondary soil-derived species greatly outnumbered those containing sulfate. Organic particles, mainly consisting of marine biogenic species, were more abundant in the stage 2 fraction than in the stage 3 fraction (11.6% vs. 5.1%). Their relative abundance was greater than the sum of carbon-rich, K-containing, Fe-containing, and fly ash particles, which exhibited low frequencies in all the samples. In addition, many droplets rich in C, N, O, and S were observed. They tended to be small, exhibiting a dark round shape on SEI, and generally included 8–20 at.% C, 0–12 at.% N, 60–80 at.% O, and 4–10 at.% S (sometimes with <3 at.% Mg and Na). They were attributed to be a mixture of carbonaceous matter, H₂SO₄, and NH₄HSO₄/(NH₄)₂SO₄, mostly from the reaction of atmospheric SO₂ with NH₃ under high relative humidity. The analysis of the relationship between the aerosol particle compositions and 72-h backward air-mass trajectories suggests that ambient aerosols at Tokchok Island are strongly affected not only by seawater from the Yellow Sea but also by anthropogenic pollutants emitted from China and the Seoul–Incheon metropolis, resulting in the dominance of complex secondary aerosol particles.     

珠三角地区大气PM_(2.5)理化特性季节规律与成因

基于珠三角大气超级站2013年8月至2014年3月PM2.5、PM2.5中主要水溶性无机离子组分及其重要气态前体物等参数的逐时在线监测结果,揭示当地大气PM2.5中二次无机组分与其气态前体物的相互作用,以及PM2.5理化特性与成因的季节差异。结果表明,观测期间,PM2.5、PM10的年平均质量浓度分别为64.2、105.1μg/m3,PM2.5在PM10中所占比例(PM2.5/PM10)平均为61.1%。SO2-4、NO-3、NH+4的年平均质量浓度分别为16.6、9.0、10.2μg/m3,3者之和(SNA)占PM2.5的比例(SNA/PM2.5)平均为55.8%,体现了二次转化对珠三角地区PM2.5污染的重要影响;不同季节,SNA/PM2.5为46.0%~64.3%,夏季最低,冬季最高,其中SO2-4、NH+4对PM2.5的贡献相对稳定,NO-3贡献的季节差异较大;秋、冬季各项观测参数浓度的日变化规律相对明显,夏季除HNO3和NH3外,多项观测参数在低浓度水平波动,日变化规律不明显;珠三角大气中具有足量气态NH3以中和硫酸盐和硝酸盐,PM2.5中NH+4、SO2-4、NO-3主要以(NH4)2SO4和NH4NO3形式存在;本研究站点夏季的硫氧化率和氮氧化率均高于广州市,这充分体现了该站点的区域性特征。     

Combined source apportionment,using positive matrix factorization–chemical mass balance and principal component analysis/multiple linear regression–chemical mass balance models

The methods of positive matrix factorization–chemical mass balance and principal component analysis/multiple linear regression–chemical mass balance were studied in this paper, for combined source apportionment. Due to the high similarity among the source profiles, several problems would raised when only one receptor model was applied. For example, the collinearity problem would result in the negative contributions when applying CMB model; certain sources would not to be separated out when applying PCA or PMF model. In this study, PCA/MLR–CMB model and PMF–CMB were attempted to resolve the problem, where the combined models were applied to study the synthetic and ambient datasets. In synthetic dataset, there were seven sources (six actual sources from real world, and one unknown source). The results obtained by the combined models show that the combined source apportionment technique is feasible. In addition, an ambient dataset from a northern city in China was analyzed by PCA/MLR–CMB model and PMF–CMB model, and these two models got the similar results. The results show that coal combustion contributed the largest fraction to the total mass.     

Appraisal of measurement methods, chemical composition and sources of fine atmospheric particles over six different areas of Northern Belgium

Daily and seasonal variation in the total elemental, organic carbon (OC) and elemental carbon (EC) content and mass of PM_2.5 were studied at industrial, urban, suburban and agricultural/rural areas. Continuous (optical Dustscan, standard tapered element oscillating micro-balance (TEOM), TEOM with filter dynamics measurement system), semi-continuous (Partisol filter-sampling) and non-continuous (Dekati-impactor sampling and gravimetry) methods of PM_2.5 mass monitoring were critically evaluated. The average elemental fraction accounted for 2-6% of the PM_2.5 mass measured by gravimetry. Metals, like K, Mn, Fe, Cu, Zn and Pb were strongly inter-correlated, also frequently with non-metallic elements (P, S, Cl and/or Br) and EC/OC. A high OC/EC ratio (2-9) was generally observed. The total carbon content of PM_2.5 ranged between 3 and 77% (averages: 12-32%), peaking near industrial/heavy trafficked sites. Principal component analysis identified heavy oil burning, ferrous/non-ferrous industry and vehicular emissions as the main sources of metal pollution.     

Sampling artefacts,concentration and chemical composition of fine water-soluble organic carbon and humic-like substances in a continental urban atmospheric environment

Water-soluble organic carbon (WSOC) and atmospheric humic-like substances (HULIS) were investigated for urban PM2.5-fraction aerosol samples, which were collected with the tandem filter method on quartz fibre filters over a non-heating spring season. Sampling artefacts were of importance for all organic chemical fractions, and the back-to-front-filter concentration ratios were on average 28% for WSOC and 17% for HULIS and organic carbon (OC). The difference in the ratios indicates that the water-soluble organics play a more important role in adsorptive artefacts than the organic matter (OM) in general. The results emphasize the need for an appropriate sampling and/or correction method for measuring particulate organic substances in urban environments. The corrected atmospheric concentration of HULIS, obtained by subtracting the back-filter from the front-filter data, was on average 2 μg m⁻³; which represented 6% of the mean PM2.5 particulate mass, and it made up 45% of the secondary OC. The HULIS carbon accounted for 20% of the OC and 62% of the WSOC, while WSOC made up 32% of OC. The major element composition of HULIS, expressed in molar ratios, was C:H:O:N=22:32:10:1. The molar H/C ratio of 1.49 implies the presence of unsaturated organic compounds, although these were depleted in comparison with rural aerosol or standard fulvic acids. The molar O/C ratio of 0.47 indicates the existence of oxygenated functional groups; comparison to rural aerosol suggests that the (fresh) urban-type aerosol is less oxidized (and, therefore, less water soluble as well) than the rural one. The OM/OC mass conversion factor for the isolated (water-soluble) HULIS was derived to be 1.81. It was inferred from comparisons with published data that there are substantial differences in abundance and chemical composition of HULIS for different environments.     

Levoglucosan,a tracer for cellulose in biomass burning and atmospheric particles

The major organic components of smoke particles from biomass burning are monosaccharide derivatives from the breakdown of cellulose, accompanied by generally lesser amounts of straight-chain, aliphatic and oxygenated compounds and terpenoids from vegetation waxes, resins/gums, and other biopolymers. Levoglucosan and the related degradation products from cellulose can be utilized as specific and general indicator compounds for the presence of emissions from biomass burning in samples of atmospheric fine particulate matter. This enables the potential tracking of such emissions on a global basis. There are other compounds (e.g. amyrones, friedelin, dehydroabietic acid, and thermal derivatives from terpenoids and from lignin—syringaldehyde, vanillin, syringic acid, vanillic acid), which are additional key indicators in smoke from burning of biomass specific to the type of biomass fuel. The monosaccharide derivatives (e.g. levoglucosan) are proposed as specific indicators for cellulose in biomass burning emissions. Levoglucosan is emitted at such high concentrations that it can be detected at considerable distances from the original combustion source.     

Spatial differences of chemical features of atmospheric deposition between rainy season and winter in the areas facing to the Japan Sea, Japan

Atmospheric deposition was collected using filtrating bulk samplers at 32 sites in the areas facing to the Japan Sea (AFJS) from April 1991 to March 1994. The data were analyzed for winter (January and February) and the rainy season (June and July) when the climate is just the opposite. The AFJS was geographically divided into five areas, that is western Tohoku (WT), Hokuriku (HR), eastern San-in (ES), western San-in (WS) and northern Kyushu (NK). WT, HR and ES receive more precipitation than other regions in winter. H+ depositions increase in winter except NK. This is mainly due to high winter H+ concentrations. In spite of large amounts of precipitation in WT, HR and ES in winter, nss-SO4(2-) concentration was nearly equal to the average throughout Japan. Namely, nss-SO4(2-) depositions in these areas were nearly twice the national mean. In WT, HR and ES, nss-Ca2+ concentrations in winter are lower than the average of the whole country (less than the half). This may be the cause of the lower potentials of neutralization and the higher remaining acid concentrations in WT, HR and ES than NK and WS.     

An urban emissions inventory for South America and its application in numerical modeling of atmospheric chemical composition at local and regional scales

This work describes the development of an urban vehicle emissions inventory for South America, based on the analysis and aggregation of available inventories for major cities, with emphasis on its application in regional atmospheric chemistry modeling. Due to the limited number of available local inventories, urban emissions were extrapolated based on the correlation between city vehicle density and mobile source emissions of carbon monoxide (CO) and nitrogen oxides (NOx). Emissions were geographically distributed using a methodology that delimits urban areas using high spatial resolution remote sensing products. This numerical algorithm enabled a more precise representation of urban centers. The derived regional inventory was evaluated by analyzing the performance of a chemical weather forecast model in relation to observations of CO, NOx and O₃ in two different urban areas, São Paulo and Belo Horizonte. The gas mixing ratios simulated using the proposed regional inventory show good agreement with observations, consistently representing their hourly and daily variability. These results show that the integration of municipal inventories in a regional emissions map and their precise distribution in fine scale resolutions are important tools in regional atmospheric chemistry modeling.     

Implications for long-range atmospheric transport of polycyclic aromatic hydrocarbons in Lhasa, China

The Tibetan Plateau is suggested to be an important indicator region to study the global long-range atmospheric transport of persistent organic pollutants. In this study, atmospheric polycyclic aromatic hydrocarbons (PAHs) were studied in Lhasa City in the Tibetan Plateau, China. Air samples in gas and particle phases were concurrently collected by a modified high-volume air sampler from 5 August 2008 to 13 July 2009. The concentration of ∑₁₆PAHs ranged from 18 to 160 ng?m^?3 (with a geometric mean of 68 ng?m^?3). The most abundant PAHs were phenanthrene and benzo(b)fluoranthene in gas and particle phases, respectively. Compared with other two similar studies in Beijing and Harbin, different temporal trends were found between gas and particle phases PAHs in Lhasa. The influences of meteorological parameters (ambient temperature and relative humidity) and air masses from China, India, Southeast Asia, and West Asia were the two important reasons for explaining the difference, which was confirmed by the 5-day backward trajectories. This is the first comprehensive study to provide the evidence for the different influences of long-range atmospheric transport on gas and particle phases PAHs pollution in the Tibetan Plateau.     

Evidence for short-range transport of atmospheric mercury to a rural,inland site

Atmospheric mercury (Hg) species, including gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate-bound mercury (Hg_p), were monitored near three sites, including a cement plant (monitored in 2007 and 2008), an urban site and a rural site (both monitored in 2005 and 2008). Although the cement plant was a significant source of Hg emissions (for 2008, GEM: 2.20 ± 1.39 ng m^?3, RGM: 25.2 ± 52.8 pg m^?3, Hg_p 80.8 ± 283 pg m^?3), average GEM levels and daytime average dry depositional RGM flux were highest at the rural site, when all three sites were monitored sequentially in 2008 (rural site, GEM: 2.37 ± 1.26 ng m^?3, daytime RGM flux: 29 ± 40 ng m^?2 day^?1). Photochemical conversion of GEM was not the primary RGM source, as highest net RGM gains (75.9 pg m^?3, 99.0 pg m^?3, 149 m^?3) occurred within 3.0–5.3 h, while the theoretical time required was 14–23 h. Instead, simultaneous peaks in RGM, Hg_p, ozone (O₃), nitrogen oxides, and sulfur dioxide in the late afternoon suggested short-range transport of RGM from the urban center to the rural site. The rural site was located more inland, where the average water vapor mixing ratio was lower compared to the other two sites (in 2008, rural: 5.6 ± 1.4 g kg^?1, urban: 9.0 ± 1.1 g kg^?1, cement plant: 8.3 ± 2.2 g kg^?1). Together, these findings suggested short-range transport of O₃ from an urban area contributed to higher RGM deposition at the rural site, while drier conditions helped sustain elevated RGM levels. Results suggested less urbanized environments may be equally or perhaps more impacted by industrial atmospheric Hg emissions, compared to the urban areas from where Hg emissions originated.     

The basis for,and some limitations of,the Langevin equation in atmospheric relative dispersion modelling

The use of the Langevin equation to model turbulent dispersion, particularly in the atmosphere, is examined. The essential feature of the Langevin equation is that fluid particle accelerations are uncorrelated, a good approximation in high Reynolds number three-dimensional turbulence. It thus satisfies all the wellknown inertial range scaling laws. An important consequence of these laws is the equivalence of conditioned one-particle dispersion and relative dispersion.Relative dispersion on global scales is not well represented by the Langevin model, at least in part because motion on these scales is quasi-two-dimensional. On smaller scales, the well-known lack of an upper limit to the scale of turbulent kinetic energy throws doubt on the applicability of the Langevin equation in toto, although a three-dimensional inertial range stage of dispersion may be reasonably well represented.Use of the Langevin equation to directly model relative velocities results in a Gaussian probability density for particle separation. This is unrealistic and leads to an incorrect representation of concentration fluctuations. However, a relative dispersion model based on an appropriate combination of a pair of Langevin equations does realistically model concentration fluctuations.     

A direct FTIR method for identifying functional groups,in size segregated atmospheric aerosols

A direct method for characterizing organic material in atmospheric aerosols has been devised. Size segregated particulates, collected on ZnSe disks, were analyzed using Fourier Transform Infrared (FTIR) spectroscopy. The FTIR spectra were used to identify compound classes present in the aerosols; the distribution of functional groups varied with particle size and depended on whether the aerosol was collected at a primary or secondary receptor site.     

Lichen-based critical loads for atmospheric nitrogen deposition in Western Oregon and Washington Forests, USA

Critical loads (CLs) define maximum atmospheric deposition levels apparently preventative of ecosystem harm. We present first nitrogen CLs for northwestern North America’s maritime forests. Using multiple linear regression, we related epiphytic-macrolichen community composition to: 1) wet deposition from the National Atmospheric Deposition Program, 2) wet, dry, and total N deposition from the Communities Multi-Scale Air Quality model, and 3) ambient particulate N from Interagency Monitoring of Protected Visual Environments (IMPROVE). Sensitive species declines of 20-40% were associated with CLs of 1-4 and 3-9 kg N ha⁻¹ y⁻¹ in wet and total deposition. CLs increased with precipitation across the landscape, presumably from dilution or leaching of depositional N. Tight linear correlation between lichen and IMPROVE data suggests a simple screening tool for CL exceedance in US Class I areas. The total N model replicated several US and European lichen CLs and may therefore be helpful in estimating other temperate-forest lichen CLs.     

Comparison among filter-based,impactor-based and continuous techniques for measuring atmospheric fine sulfate and nitrate

Filter-based methods for sampling aerosols are subject to great uncertainty if the gas–particle interactions on filter substrates are not properly handled. Sampling artifacts depend on both meteorological conditions and the chemical mix of the atmosphere. Despite numerous of studies on the subject, very few have evaluated filter-based methods in the Asian environments. This paper reports the results of a comparison of the performances of two filter-based samplers, including a Thermo Anderson Chemical Speciation Monitor (RAAS) and a honeycomb denuder filter-pack system, a Micro Orifice Uniform Deposit Impactor (MOUDI) and a real-time ambient ion monitor (AIM, URG9000B) in measuring atmospheric concentrations of PM_2.5 sulfate and nitrate. Field studies were conducted at an urban site in Jinan, Shandong province, during the winter of 2007 and at a rural site near Beijing in the summer of 2008. The AIM was first compared with the honeycomb denuder filter-pack system which was considered to have minimal sampling artifacts. After some modifications made to it, the AIM showed good performance for both sulfate and nitrate measurement at the two sites and was then used to evaluate other instruments. For the un-denuded RAAS, the extent of sampling artifacts for nitrate on quartz filters was negligible, while that on Teflon filters was also minimal at high nitrate concentrations (>10 μgm^?3); however, loss through evaporation was significant (～75%) at low nitrate concentrations under hot summer conditions. The MOUDI using aluminum substrates suffered a significant loss of nitrate (50–70%) under summer conditions due to evaporation. Considering that the aluminum substrates are still being widely used to obtain size-resolved aerosol compositions because of their low cost and accurate mass weighed, caution should be taken about the potential significant under determination of semi-volatile components such as ammonium nitrate.     

The importance of atmospheric deposition, charge and atomic mass to the dynamics of minor and rare elements in developing, ageing, and wilted leaves of beech (Fagus sylvatica L.)

The amounts of sixty elements in developing, maturing, senescent and wilting leaves, and in the wintering dead leaves attached to the branches, are reported for a beech (Fagus sylvatica) forest on mor Podzol in south Sweden, a site with no local sources of pollution or geological anomalies. The amounts (contents per leaf) of K (potassium), Rb (rubidium), Cs (caesium), Cu (copper) and P (phosphorus) were highest in young leaves, decreasing throughout the growing season and usually in the subsequent winter. The entirely opposite pattern with a continuous, mostly even increase of the amounts was measured with Be (beryllium), Ba (barium), Hg (mercury), Al (aluminium), Tl (thallium), Pb (lead), Bi (bismuth), V (vanadium), W (tungsten), As (arsenic), Sb (antimony), and Se (selenium). Amounts of rare-earth elements and some transition metals, such as Co (cobalt), Ti (titanium), and the actinides Th (thorium) and U (uranium) were more stable during the growing season, after an initial increase in early summer, but increased greatly in the winter. This winter increase in dead attached leaves has to be accounted for by uptake from long-distance transported constituents in dry and wet deposition. It was similar to deposition rate estimates using moss carpets from the same locality. A passive uptake was positively related to ionic charge and atomic mass. However, the amounts of several, mainly non-essential elements, such as Ni (nickel), Sc (scandium), Zr (zirconium), Cr (chromium), Ag (silver), and Cd (cadmium) were not much lower in the young or maturing leaves than in the wintered dead leaves of this deciduous (hardwood) forest and a proportion apparently originated from internal translocation in the trees. Seasonal fluxes or cycling of many of the scarce or rare elements reported here have never been studied before in forest ecosystems.     

Development of a MnO2-coated,cylindrical denuder for removing NO2 from atmospheric samples

In order to develop a diffusion denuder for the removal of NO₂ from ambient atmospheric samples, a number of materials were screened for their ability to adsorb NO₂:

• 1.(1) MgO;
• 2.(2) MnO₂ on alpha-Al₂O₃;
• 3.(3) water-treated MnO₂ on alpha-Al₂O₃; and
• 4.(4) MnO₂ (activated).

A simple cylindrical denuder coated with MnO₂ (activated) was very effective in the removal of NO₂ from a feedgas of NO₂ in air at ambient temperature and pressure. The other materials were unsatisfactory. The strong oxidizing properties, along with the hydrated surface of the MnO₂ (activated), appear to be important for the sorption of NO₂, as suggested in applications elsewhere. Quantification of denuder sorption efficiencies indicates that MnO2 (activated) is nearly a perfect sorbent for NO₂. The diffusion coefficient of NO₂ in air was found to be 10.8 ± 0.3 cm² min⁻¹ at 22–23°C, which compares favourably with a theoretical estimate. Although MnO₂ (activated)-coated denuders were also found to adsorb SO₂, interference with NO₂ sorption was not sufficient to impair ambient applications.     

Composition and effects of inhalable size fractions of atmospheric aerosols in the polluted atmosphere: Part I. PAHs,PCBs and OCPs and the matrix chemical composition

Atmospheric particulate matter (PM) abundance, mass size distribution (MSD) and chemical composition are parameters relevant for human health effects. The MSD and phase state of semivolatile organic pollutants were determined at various polluted sites in addition to the PM composition and MSD. The distribution pattern of pollutants varied from side to side in correspondence to main particle sources and PM composition. Levels of particle-associated polycyclic aromatic hydrocarbons (PAHs) were 1–30 ng m^?3 (corresponding to 15–35 % of the total, i.e., gas and particulate phase concentrations), of polychlorinated biphenyls (PCBs) were 2–11 pg m^?3 (4–26 % of the total) and of DDT compounds were 2–12 pg m^?3 (4–23 % of the total). The PM associated amounts of other organochlorine pesticides were too low for quantification. The organics were preferentially found associated with particles <0.45 μm of aerodynamic equivalent diameter. The mass fractions associated with sub-micrometer particles (PM_0.95) were 73–90 %, 34–71 % and 36–81 % for PAHs, PCBs and DDT compounds, respectively. The finest particles fraction had the highest aerosol surface concentration (6.3–29.7)_×10^?6 cm^?1 (44–70 % of the surface concentration of all size fractions). The data set was used to test gas-particle partitioning models for semivolatile organics for the first time in terms of the organics' MSD and size-dependent PM composition. The results of this study prove that at the various sites particles with diverse size, matrix composition, amount of contaminants and toxicological effects occur. Legislative regulation based on gravimetric determination of PM mass can clearly be insufficient for assessment.