Size-dependent wet removal of black carbon in Canadian biomass burning plumes

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Atmos. Chem. Phys., 14, 13755–13771, 2014 doi:10.5194/acp-14-13755-2014 © Author(s) 2014. CC Attribution 3.0 License. Size-dependent wet removal of black carbon in Canadian biomass burning plumes J. W. Taylor1, J. D. Allan1,2, G. Allen1, H. Coe1, P. I. Williams1,2, M. J. Flynn1, M. Le Breton1, J. B. A. Muller1, C. J. Percival1, D. Oram3, G. Forster3, J. D. Lee4, A. R. Rickard5,*, M. Parrington6,**, and P. I. Palmer6 1Centre for Atmospheric Science, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK 2National Centre for Atmospheric Science, University of Manchester, Manchester, UK 3National Centre for Atmospheric Science (NCAS), School of Environmental Sciences, University of East Anglia, Norwich, UK 4National Centre for Atmospheric Science (NCAS), Department of Chemistry, University of York, York, UK 5National Centre for Atmospheric Science (NCAS), School of Chemistry, University of Leeds, Leeds, UK 6School of GeoSciences, University of Edinburgh, Edinburgh, UK *now at: the National Centre for Atmospheric Science (NCAS), Department of Chemistry, University of York, York, UK **now at: European Centre for Medium-Range Weather Forecasts, Shinfield Park, Reading, RG2 9AX, UK Correspondence to: J. W. Taylor ( Received: 1 July 2014 – Published in Atmos. Chem. Phys. Discuss.: 29 July 2014 Revised: 7 November 2014 – Accepted: 16 November 2014 – Published: 22 December 2014 Abstract. Wet deposition is the dominant mechanism for removing black carbon (BC) from the atmosphere and is key in determining its atmospheric lifetime, vertical gradient and global transport. Despite the importance of BC in the climate system, especially in terms of its ability to modulate the radiative energy budget, there are few quantitative case studies of wet removal in ambient environments. We present a case study of BC wet removal by examining aerosol size distributions and BC coating properties sampled in three Canadian boreal biomass burning plumes, one of which passed through a precipitating cloud. This depleted the majority of the plume’s BC mass, and the largest and most coated BCcontaining particles were found to be preferentially removed, suggesting that nucleation scavenging was likely the dominant mechanism. Calculated single-scattering albedo (SSA) showed little variation, as a large number of non-BC particles were also present in the precipitation-affected plume. The remaining BC cores were smaller than those observed in previous studies of BC in post-precipitation outflow over Asia, possibly due to the thick coating by hydrophilic compounds associated with the Canadian biomass burning particles. This study provides measurements of BC size, mixing state and removal efficiency to constrain model parameterisations of BC wet removal in biomass burning regions, which will help to reduce uncertainty in radiative forcing calculations. 1 Introduction Black carbon (BC) is the dominant absorbing aerosol in the atmosphere and is an important, ubiquitous climate-warming agent (Ramanathan and Carmichael, 2008; Chung et al., 2012; Bond et al., 2013). The optical properties of BC affect the single-scattering albedo (SSA) of an aerosol layer, which determines the sign of its radiative forcing (Haywood and Shine, 1995). Important uncertainties remain regarding global and local emissions of BC, as well as its chemical processing, lifetime in the atmosphere and optical properties. Observations are required to further constrain and/or validate model parameterisations surrounding BC processes in the atmosphere. Open biomass burning (BB) is the largest source category of BC, responsible for ∼ 40 % of total emissions in the year 2000 (Bond et al., 2013), and the size distribution and mixing state of BC from this source are known to exhibit systematic differences to fossil fuel emissions (Kondo et al., 2011; Sahu et al., 2012). Published by Copernicus Publications on behalf of the European Geosciences Union. 13756 J. W. Taylor et al.: Size-dependent BC wet removal in Canadian biomass burning plumes Modelling studies suggest wet deposition is the dominant mechanism for BC’s removal from the atmosphere (Textor et al., 2006) and consequently determines its lifetime and atmospheric burden to first order. As convection is often accompanied by precipitation, this also affects vertical profiles of number and mass concentration, which are important for determining radiative forcing (Samset et al., 2013). Such profiles are often poorly represented by models (Koch et al., 2009) as calculated removal rates are sensitive to the microphysical schemes used (Textor et al., 2006; Croft et al., 2010; Vignati et al., 2010). In climate models, complex aerosol–cloud interactions must be simplified to reduce computing requirements, and many microphysical processes are sub-grid in both space and time. In-cloud scavenging occurs in two stages: aerosol activation to form cloud droplets, and removal of droplets by precipitation. Cloud droplet number concentration may be calculated based on empirical relations with updraft velocity and aerosol number concentration (Lohmann et al., 2007). Alternatively, many models use parameterisations designed to emulate size- and composition-based Köhler theory and the competition for water vapour between activated aerosols growing to cloud droplets (Ghan et al., 2011). Precipitation is determined by calculating autoconversion rates (the rates at which cloud water is converted to precipitation) which are tuned to simulate detailed microphysical schemes (Lin et al., 1983; Liu and Daum, 2004). Below-cloud scavenging is calculated by multiplying the precipitation rate by a scavenging coefficient, which may be size-resolved, and again may be calculated empirically or based on theoretical considerations (Wang et al., 2010). Some models treat explicit sizeresolved, cloud liquid, ice and mixed-phase microphysics (e.g. Jacobson, 2003), and this can generate better agreement with measured vertical profiles. Observations (particularly of size distribution, hygroscopicity and mixing state) are needed to constrain both types of wet removal scheme (Koch et al., 2011). Explicit microphysical models are also used to investigate smaller-scale phenomena (such as cloud microphysics, cloud–aerosol interactions and air quality), and ambient measurements are similarly required for constraint and comparison. Fresh BC is generally considered hydrophobic, though it may act as a cloud condensation nucleus (CCN) in liquid cloud if subsequently coated with hydrophilic material (Khalizov et al., 2009; Liu et al., 2013). In biomass burning plumes this coating is thought to occur in the first few hours after emission (Abel, 2003; Akagi et al., 2012), and coatings are generally thicker than in fossil fuel emissions (Kondo et al., 2011; Sahu et al., 2012). BB emissions can be efficient CCN (Lathem et al., 2013) and can also act as ice nuclei (IN) (Petters et al., 2009), though different chemical species in BB emissions may be more effective than others. Several recent studies have suggested that non-nucleation scavenging (i.e. impaction) mechanisms may be more effective for BC removal, both in mixed-phase (Twohy et al., 2010) and ice clouds (Stith et al., 2011; Baumgardner et al., 2008). The different scavenging mechanisms exhibit different size- and composition-dependent efficiencies. In liquid cloud, BC generally requires a soluble coating to activate, and larger particles are generally more effective CCN (Dusek et al., 2006). For all primary ice nucleation modes, larger particles are also generally more efficient IN (Hoose and Möhler, 2012); however soluble coatings have been shown to inhibit BC ice nucleation (Koehler et al., 2009). Falling ice and raindrops may scavenge coarse-mode particles and liquid droplets, and hence remove particles that have already activated (Miller and Wang, 1991; Seinfeld and Pandis, 1998; Croft et al., 2009). This type of scavenging is also more efficient for Aitken mode aerosols. For typical BC size distributions, which span the Aitken and accumulation modes, impaction scavenging therefore favours smaller BC, whereas nucleation scavenging favours larger BC (Jacobson, 2003). Oshima et al. (2012) calculated removal rates of dieseldominated BC by liquid clouds in convective air masses over the Yellow and East China seas. By comparing BC / CO ratios in different air parcels, they calculated the transmission efficiency of BC (TEBC), the fraction of BC not removed by precipitation. Moteki et al. (2012) showed that larger BC particles, which also had more coating material, were more efficiently removed during this process, meaning they must have been scavenged by nucleation. For air masses in which the most BC was removed (TEBC<0.3) the remaining BC core size distribution was similar to that seen by Schwarz et al. (2010b) in pristine conditions in the remote Pacific, suggesting a fraction of BC is not removed by wet processes and is able to travel long distances. In this paper, we present a case study of size-dependent wet removal of biomass burning BC, which may behave differently to diesel-dominated BC due to its contrasting microphysical properties. Firstly, we present an overview of the experiment, and use back trajectories and weather satellite data to identify three plumes from Canadian boreal fires in the same region, one of which had passed through a region of a precipitating cloud. Franklin et al. (2014) recently highlighted aerosol depletion in similar plumes using remote sensing measurements, but they were unable to determine the mechanism or measure the properties of any particles remaining in the plume. We examine the aerosol size distributions and BC coating properties using in situ measurements sampled in the three plumes to determine the most likely removal mechanism and consider the effect this has on optical properties. 2 Experimental 2.1 BORTAS-B BORTAS-B was the second phase of the BORTAS (Quantifying the impact of BOReal forest fires on Tropospheric ox- Atmos. Chem. Phys., 14, 13755–13771, 2014 J. W. Taylor et al.: Size-dependent BC wet removal in Canadian biomass burning plumes 13757 idants over the Atlantic using Aircraft and Satellites) project (Palmer et al., 2013) and took place over eastern Canada and the North Atlantic during July–August 2011. The project aimed to study the chemistry of biomass burning plumes using a combination of airborne, ground and remote sensing measurements. This paper uses airborne aerosol measurements made aboard the UK BAe-146-301 Atmospheric Research Aircraft (ARA) and focuses on a single flight (B622) on 20 July 2011. The flight track for B622 is shown in Fig. 1. The ARA took off from Halifax Stanfield International Airport at 14:56 UTC and flew a series of sawtooth and straight-and-level runs on a generally east–west axis, before landing at Québec City Jean Lesage International Airport at 19:11 UTC. 2.2 Instrumentation 2.2.1 The single-particle soot photometer The single-particle soot photometer (SP2, Droplet Measurement Technologies, Boulder, CO, USA) uses laserinduced incandescence to measure refractory black carbon (rBC; hereafter referred to as BC) on a single-particle basis (Schwarz et al., 2006). Particles are drawn through a highintensity 1064 nm Nd:YAG laser which heats BC-containing particles to incandescence. The incandescent light is detected by two photomultiplier tubes (PMTs), one fitted with a narrowband filter to determine incandescence temperature. During BORTAS these were run at different gain settings to expand the dynamic range of the instrument. The signal these two PMTs detect is proportional to the mass of refractory black carbon present in the particle, regardless of mixing state (Slowik et al., 2007; Moteki and Kondo, 2007), and converted to mass-equivalent core diameter (DC) using a density of 1.8 g cm−3. The broadband PMT (measuring particles 60 ≤ DC ≤ 275 nm) was calibrated using several sizes of monodisperse Aquadag (Henkel Corporation, Bay Point, CA, USA), with particle mass calculated using the mass– mobility relationship reported by Gysel et al. (2011). This was then corrected for the difference in the response of the instrument compared to atmospheric BC (Baumgardner et al., 2012; Laborde et al., 2012b). The narrowband PMT (measuring 90 ≤ DC ≤ 550 nm) was then cross-calibrated to the broadband in the overlap region. The instrument was run with sufficient laser power to ensure 100 % detection efficiency for BC particles ≥ 1 fg (femtogram) (Schwarz et al., 2010a). In plumes, the statistical uncertainty in BC mass concentration was typically ∼ 10 % for an averaging time of 10 s, and accuracy within 20 % due to possible differences between the instrumental response to biomass burning BC and the calibrant (Laborde et al., 2012a). The instrumental setup of the SP2 on the ARA was described by McMeeking et al. (2010) and is briefly summarised here. The SP2 sampled through a 1/4 in. stainlesssteel tube connected to a modified Rosemount inlet (Foltescu Figure 1. Five-day HYSPLIT back trajectories, starting at the time and location sampled (the square markers) and with triangular markers every 6 h. Parts (a) and (b) show the horizontal and vertical tracks respectively. The aqua-coloured parts of the trajectories are regions with HYSPLIT precipitation, and the yellow dots show MODIS fire data between 18 and 20 July 2011. The grey line shows the flight track, starting from light grey and ending in dark grey. et al., 1995). Though recent results have shown that Rosemount inlets can enhance aerosol measurements at larger diameters and higher densities, the vast majority of BC mass is present in particles <600 nm, where the efficiency is close to unity at densities typical of biomass burning emissions (Trembath et al., 2012). Losses between the inlet and the SP2 were minimised using a bypass flow of 1 L min−1, and this also reduced the residence time in the inlet. The SP2 is also capable of measuring light-scattering particles (LSPs, particles that do not contain BC) and the coating properties of BC-containing particles. Two avalanche photodiodes (APDs) detect scattered light at 1064 nm, and the detected signal is proportional to the scattering cross section of the particle. One APD is position-sensitive, and this provides a measure of the position of the particle in the laser (Gao et al., 2007). This allows calculation of the fractional laser power at the edge of the beam, as the laser has a Gaussian profile when configured correctly. Leading-edge-only (LEO) fitting is then used to calculate the scattering cross section of the particle before it begins to evaporate. In this work we considered the leading edge as data from the baseline up to 5 % of the maximum laser intensity. Mie core/shell modelling is then used to infer the coated particle diameter (DP), and hence coating thickness, though this is dependent on the assumed core density and the refractive index of the BC core and coating (Taylor et al., 2014). In this analysis we assumed a BC core density of 1.8 g cm−3 (Bond and Bergstrom, 2006) and core refractive Atmos. Chem. Phys., 14, 13755–13771, 2014 13758 J. W. Taylor et al.: Size-dependent BC wet removal in Canadian biomass burning plumes index nC = (2.26 − 1.26i) (Moteki et al., 2010). These parameters were shown by Taylor et al. (2014) to be the most appropriate for use with fossil fuel BC, by comparing Mie calculations to the scattering by thermodenuded particles. As a similar analysis of biomass burning emissions has not been performed, we used these parameters on the assumption that the core properties of fossil fuel and biomass burning BC are broadly similar. For the shell refractive index, we have assumed a value of nS = (1.5 + 0i) (Reid et al., 2005a). 2.2.2 Aerosol mass spectrometer Nonrefractory submicron aerosol mass concentration was measured with a compact time-of-flight aerosol mass spectrometer (AMS, Aerodyne Research Inc, Billerica, MA, USA) (Drewnick et al., 2005; Canagaratna et al., 2007). The AMS reports organic aerosol (OA), sulfate, nitrate, ammonium and chloride mass concentrations, though in this analysis we only consider the OA concentrations. The operation of the AMS on the ARA has been described previously (Crosier et al., 2007; Morgan et al., 2009). The AMS was connected to a Rosemount inlet, and the aerosol flow is considered dry due to ram heating and the increase in temperature as the sample line passes into the cabin. Data were processed as described by Morgan et al. (2010), using a collection efficiency of 0.5. Unfortunately, after landing on B622 the aircraft lost power, and the AMS had to be shut down before an ionisation efficiency (IE) calibration could be performed. For B622 the calibration for the subsequent flight (B623) was applied, which may have introduced a systematic error in the AMS data. As an estimate of the size of this error, for the seven nitrate calibrations performed after B622, the standard deviation of the IE/airbeam was 13 % of the mean. The calibrations before this were discarded because of a faulty power supply module in the AMS, which was replaced before B622. Also, in this case study the AMS data are only used to monitor relative changes in the OA / CO ratio, so this increased uncertainty does not affect the analysis method or conclusions. 2.2.3 Scanning mobility particle sizer A custom-made scanning mobility particle sizer (SMPS) system, sampling from the same inlet as the AMS, measured distributions of particle mobility diameter (Dmob) divided into 28 logarithmically spaced bins in the range 20–350 nm. A custom-built low-pressure water-based condensation particle counter (WCPC model 3786-LP, Aerosol enfocar la importancia de la producción mediática de los niños en su descubrimiento del mundo, sobre todo utilizando el periódico escolar y la imprenta. Asimismo las asociaciones de profesores trabajaron en esta línea e incluso la enseñanza católica se comprometió desde los años sesenta realizando trabajos originales en el marco de la corriente del Lenguaje Total. Páginas 43-48 45 Comunicar, 28, 2007 En el ámbito de los medios, también desde el principio del siglo XX hay ciertas corrientes de conexión. Pero es a lo largo de los años sesenta cuando se constituyeron asociaciones de periodistas apasionados por sus funciones de mediadores, que fomentaron la importancia ciudadana de los medios como algo cercano a los jóvenes, a los profesores y a las familias. Así se crearon la APIJ (Asociación de Prensa Información para la Juventud), la ARPEJ (Asociación Regional de Prensa y Enseñanza para la Juventud), el CIPE (Comité Interprofesional para la Prensa en la Escuela) o la APE (Asociación de Prensa y Enseñanza), todas ellas para la prensa escrita Estas asociaciones fueron precedidas por movimientos surgidos en mayo de 1968, como el CREPAC que, utilizando películas realizadas por periodistas conocidos, aclaraba temas que habían sido manipulados por una televisión demasiado próxima al poder político y realizaba encuentros con grupos de telespectadores. cipio del siglo XX, y nos han legado textos fundadores muy preciados, importantes trabajos de campo y muchos logros educativos y pedagógicos. La educación en medios ha tenido carácter de oficialidad de múltiples maneras, aunque nunca como una enseñanza global. Así la campaña «Operación Joven Telespectador Activo» (JTA), lanzada al final de los años setenta y financiada de manera interministerial para hacer reflexionar sobre las prácticas televisuales de los jóvenes, la creación del CLEMI (Centro de Educación y Medios de Comunicación) en el seno del Ministerio de Educación Nacional en 1983, la creación de la optativa «Cine-audiovisual» en los bachilleratos de humanidades de los institutos en 1984 (primer bachillerato en 1989) y múltiples referencias a la educación de la imagen, de la prensa, de Internet. La forma más visible y rápida de evaluar el lugar de la educación en medios es valorar el lugar que se le ha reservado en los libros de texto del sistema educa- 2. Construir la educación en los medios sin nombrarla El lugar que ocupa la edu- La denominación «educación en medios», que debería cación en los medios es muy ambiguo, aunque las cosas están cambiando recientemente. entenderse como un concepto integrador que reagrupase todos los medios presentes y futuros, es a menudo percibida En principio, en Francia, co- por los «tradicionalistas de la cultura» como una tendencia mo en muchos otros países, la educación en los medios no es hacia la masificación y la pérdida de la calidad. una disciplina escolar a tiempo completo, sino que se ha ido conformado progresivamente a través de experiencias y reflexiones teóricas que han tivo en Francia. Una inmersión sistemática nos permi- permitido implantar interesantes actividades de carác- te constatar que los textos oficiales acogen numerosos ter puntual. Se ha ganado poco a poco el reconoci- ejemplos, citas, sin delimitarla con precisión. miento de la institución educativa y la comunidad es- colar. Podemos decir que ha conquistado un «lugar», 3. ¿Por qué la escuela ha necesitado casi un siglo en el ámbito de la enseñanza transversal entre las dis- para oficilializar lo que cotidianamente se hacía en ciplinas existentes. ella? Sin embargo, la escuela no está sola en esta aspi- Primero, porque las prácticas de educación en me- ración, porque el trabajo en medios es valorado igual- dios han existido antes de ser nombradas así. Recor- mente por el Ministerio de Cultura (campañas de foto- demos que no fue hasta 1973 cuando aparece este grafía, la llamada «Operación Escuelas», presencia de término y que su definición se debe a los expertos del colegios e institutos en el cine ), así como el Minis- Consejo Internacional del Cine y de la Televisión, que terio de la Juventud y Deportes que ha emprendido en el seno de la UNESCO, definen de esta forma: numerosas iniciativas. «Por educación en medios conviene entender el estu- Así, esta presencia de la educación en los medios dio, la enseñanza, el aprendizaje de los medios moder- no ha sido oficial. ¡La educación de los medios no apa- nos de comunicación y de expresión que forman parte rece oficialmente como tal en los textos de la escuela de un dominio específico y autónomo de conocimien- francesa hasta 2006! tos en la teoría y la práctica pedagógicas, a diferencia Este hecho no nos puede dejar de sorprender ya de su utilización como auxiliar para la enseñanza y el que las experiencias se han multiplicado desde el prin- aprendizaje en otros dominios de conocimientos tales Páginas 43-48 46 Comunicar, 28, 2007 como los de matemáticas, ciencias y geografía». A pe- mente en todas las asignaturas. Incluso los nuevos cu- sar de que esta definición ha servido para otorgarle un rrículos de materias científicas en 2006 para los alum- reconocimiento real, los debates sobre lo que abarca y nos de 11 a 18 años hacen referencia a la necesidad no, no están totalmente extinguidos. de trabajar sobre la información científica y técnica y En segundo lugar, porque si bien a la escuela fran- el uso de las imágenes que nacen de ella. cesa le gusta la innovación, después duda mucho en Desde junio de 2006, aparece oficialmente el tér- reflejar y sancionar estas prácticas innovadoras en sus mino «educación en medios» al publicar el Ministerio textos oficiales. Nos encontramos con una tradición de Educación los nuevos contenidos mínimos y las sólidamente fundada sobre una transmisión de conoci- competencias que deben adquirir los jóvenes al salir mientos muy estructurados, organizados en disciplinas del sistema educativo. escolares que se dedican la mayor parte a transmitir Este documento pretende averiguar cuáles son los conocimientos teóricos. La pedagogía es a menudo se- conocimientos y las competencias indispensables que cundaria, aunque los profesores disfrutan de una ver- deben dominar para terminar con éxito su escolaridad, dadera libertad pedagógica en sus clases. El trabajo seguir su formación y construir su futuro personal y crítico sobre los medios que estaba aún en elaboración profesional. Siete competencias diferentes han sido te- necesitaba este empuje para hacerse oficial. nidas en cuenta y en cada una de ellas, el trabajo con Aunque el trabajo de educación en los medios no los medios es reconocido frecuentemente. Para citar esté reconocido como disciplina, no está ausente de un ejemplo, la competencia sobre el dominio de la len- gua francesa definen las capa- cidades para expresarse oral- La metodología elaborada en el marco de la educación en mente que pueden adquirirse con la utilización de la radio e, medios parece incluso permitir la inclinación de la sociedad incluso, se propone fomentar de la información hacia una sociedad del conocimiento, como defiende la UNESCO. En Francia, se necesitaría unir el interés por la lectura a través de la lectura de la prensa. La educación en los medios las fuerzas dispersas en función de los soportes mediáticos y orientarse más hacia la educación en medios que al dominio adquiere pleno derecho y entidad en la sección sexta titulada «competencias sociales y cívi- técnico de los aparatos. cas» que indica que «los alum- nos deberán ser capaces de juz- gar y tendrán espíritu crítico, lo que supone ser educados en los las programaciones oficiales, ya que, a lo largo de un medios y tener conciencia de su lugar y de su influencia estudio de los textos, los documentalistas del CLEMI en la sociedad». han podido señalar más de una centena de referencias a la educación de los medios en el seno de disciplinas 4. Un entorno positivo como el francés, la historia, la geografía, las lenguas, Si nos atenemos a las cifras, el panorama de la las artes plásticas : trabajos sobre las portadas de educación en medios es muy positivo. Una gran ope- prensa, reflexiones sobre temas mediáticos, análisis de ración de visibilidad como la «Semana de la prensa y publicidad, análisis de imágenes desde todos los ángu- de los medios en la escuela», coordinada por el CLE- los, reflexión sobre las noticias en los países europeos, MI, confirma año tras año, después de 17 convocato- información y opinión rias, el atractivo que ejerce sobre los profesores y los Esta presencia se constata desde la escuela mater- alumnos. Concebida como una gran operación de nal (2 a 6 años) donde, por ejemplo, se le pregunta a complementariedad source of circulating FGF-21. The lack of association between circulating and muscle-expressed FGF-21 also suggests that muscle FGF-21 primarily works in a local manner regulating glucose metabolism in the muscle and/or signals to the adipose tissue in close contact to the muscle. Our study has some limitations. The number of subjects is small and some correlations could have been significant with greater statistical power. Another aspect is that protein levels of FGF-21 were not determined in the muscles extracts, consequently we cannot be sure the increase in FGF-21 mRNA is followed by increased protein expression. In conclusion, we show that FGF-21 mRNA is increased in skeletal muscle in HIV patients and that FGF-21 mRNA in muscle correlates to whole-body (primarily reflecting muscle) insulin resistance. These findings add to the evidence that FGF-21 is a myokine and that muscle FGF-21 might primarily work in an autocrine manner. Acknowledgments We thank the subjects for their participation in this study. Ruth Rousing, Hanne Willumsen, Carsten Nielsen and Flemming Jessen are thanked for excellent technical help. The Danish HIV-Cohort is thanked for providing us HIV-related data. 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