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Atmospheric Environment (v.52, #)

Editorial board (pp. i).

Physical, chemical, optical and radiative properties of polar aerosols – IPY 2007–2008 by Claudio Tomasi; Takashi Yamanouchi; Norm T. O'Neill (pp. 1-3).

Evaluation of sun photometer capabilities for retrievals of aerosol optical depth at high latitudes: The POLAR-AOD intercomparison campaigns by M. Mazzola; R.S. Stone; A. Herber; C. Tomasi; A. Lupi; V. Vitale; C. Lanconelli; C. Toledano; V.E. Cachorro; N.T. O’Neill; M. Shiobara; V. Aaltonen; K. Stebel; T. Zielinski; T. Petelski; J.P. Ortiz de Galisteo; B. Torres; A. Berjon; P. Goloub; Z. Li; L. Blarel; I. Abboud; E. Cuevas; M. Stock; K.-H. Schulz; A. Virkkula (pp. 4-17).

Accuracy requirements for aerosol optical depth (AOD) in polar regions are much more stringent than those usually encountered in established sun photometer networks, while comparability of data from different archive centres is a further important issue. Therefore, two intercomparison campaigns were held during spring 2006 at Ny-Ålesund (Svalbard) and autumn 2008 at Izaña (Tenerife) within the framework of the IPY POLAR-AOD project, with the participation of various research institutions routinely employing different instrument models at Arctic and Antarctic stations. As reported here, a common algorithm was used for data analysis with the aim of minimizing a large part of the discrepancies affecting the previous studies. During the Ny-Ålesund campaign, spectral values of AOD derived from measurements taken with different instruments were found to agree, presenting at both 500nm and 870nm wavelengths average values of root mean square difference (RMSD) and standard deviation of the difference (SDD) equal to 0.003. Correspondingly, the mean bias difference (MBD) varied mainly between −0.003 and +0.003 at 500nm, and between −0.004 and +0.003 at 870nm. During the Izaña campaign, which was also intended as an intercalibration opportunity, RMSD and SDD values were estimated to be equal to 0.002 for both channels on average, with MBD ranging between −0.004 and +0.004 at 500nm and between −0.002 and +0.003 at 870nm. RMSD and SDD values for Ångström exponent α were estimated equal to 0.06 during the Ny-Ålesund campaign and 0.39 at Izaña. The results confirmed that sun photometry is a valid technique for aerosol monitoring in the pristine atmospheric turbidity conditions usually observed at high latitudes.► Two sun photometer comparison campaigns were conducted for the POLAR-AOD project. ► AOD mean bias differences were found to be within ±0.004 at all wavelengths. ► Ångström exponent α was estimated to be within 0.06 and 0.39 for the two campaigns.

Keywords: Aerosol; Polar regions; Sun photometry; Optical depth; Intercomparison; Calibration

Overview of sun photometer measurements of aerosol properties in Scandinavia and Svalbard by C. Toledano; V.E. Cachorro; M. Gausa; K. Stebel; V. Aaltonen; A. Berjón; J.P. Ortiz de Galisteo; A.M. de Frutos; Y. Bennouna; S. Blindheim; C.L. Myhre; G. Zibordi; C. Wehrli; S. Kratzer; B. Hakansson; T. Carlund; G. de Leeuw; A. Herber; B. Torres (pp. 18-28).

An overview on the data of columnar aerosol properties measured in Northern Europe is provided. Apart from the necessary data gathered in the Arctic, the knowledge of the aerosol loading in nearby areas (e.g. sub-Arctic) is of maximum interest to achieve a correct analysis of the Arctic aerosols and transport patterns. This work evaluates data from operational sites with sun photometer measurements belonging either to national or international networks (AERONET, GAW-PFR) and programs conducted in Scandinavia and Svalbard. We enumerate a list of sites, measurement type and periods together with observed aerosol properties. An evaluation and analysis of aerosol data was carried out with a review of previous results as well. Aerosol optical depth (AOD) and Ångström exponent (AE) are the current parameters with sufficient long-term records for a first evaluation of aerosol properties. AOD (500 nm) ranges from 0.08 to 0.10 in Arctic and sub-Arctic sites (Ny-Ålesund: 0.09; Andenes: 0.10; Sodankylä: 0.08), and it is somewhat higher in more populated areas in Southern Scandinavia (AOD about 0.10–0.12 at 500 nm). On the Norwegian coast, aerosols show larger mean size (AE = 1.2 at Andenes) than in Finland, with continental climate (AE = 1.5 at Sodankylä). Columnar particle size distributions and related parameters derived from inversion of sun/sky radiances were also investigated. This work makes special emphasis in the joint and collaborative effort of the various groups from different countries involved in this study. Part of the measurements presented here were involved in the IPY projects Polar-AOD and POLARCAT.► Different AOD seasonality is found in Svalbard, northern and southern Scandinavia. ► In the European sub-Arctic region (about 65–70°N) the spring haze is not persistent. ► Sources in Eastern Europe produce a spring AOD peak over southern Scandinavia. ► Fine mode aerosols are predominant and their variations determine the seasonal aerosol variability. ► The coarse mode aerosol concentrations are very low except for coastal sites.

Keywords: Aerosol; Sun photometer; Arctic; Scandinavia

An update on polar aerosol optical properties using POLAR-AOD and other measurements performed during the International Polar Year by Claudio Tomasi; Angelo Lupi; Mauro Mazzola; Robert S. Stone; Ellsworth G. Dutton; Andreas Herber; Vladimir F. Radionov; Brent N. Holben; Mikhail G. Sorokin; Sergey M. Sakerin; Svetlana A. Terpugova; Piotr S. Sobolewski; Christian Lanconelli; Boyan H. Petkov; Maurizio Busetto; Vito Vitale (pp. 29-47).

An updated set of time series of derived aerosol optical depth (AOD) and Ångström’s exponent α from a number of Arctic and Antarctic stations was analyzed to determine the long-term variations of these two parameters. The Arctic measurements were performed at Ny-Ålesund (1991–2010), Barrow (1977–2010) and some Siberian sites (1981–1991). The data were integrated with Level 2.0 AERONET sun-photometer measurements recorded at Hornsund, Svalbard, and Barrow for recent years, and at Tiksi for the summer 2010. The Antarctic data-set comprises sun-photometer measurements performed at Mirny (1982–2009), Neumayer (1991–2004), and Terra Nova Bay (1987–2005), and at South Pole (1977–2010). Analyses of daily mean AOD were made in the Arctic by (i) adjusting values to eliminate volcanic effects due to the El Chichón, Pinatubo, Kasatochi and Sarychev eruptions, and (ii) selecting the summer background aerosol data from those affected by forest fire smoke. Nearly null values of the long-term variation of summer background AOD were obtained at Ny-Ålesund (1991–2010) and at Barrow (1977–2010). No evidence of important variations in AOD was found when comparing the monthly mean values of AOD measured at Tiksi in summer 2010 with those derived from multi-filter actinometer measurements performed in the late 1980s at some Siberian sites. The long-term variations of seasonal mean AOD for Arctic Haze (AH) conditions and AH episode seasonal frequency were also evaluated, finding that these parameters underwent large fluctuations over the 35-year period at Ny-Ålesund and Barrow, without presenting well-defined long-term variations. A characterization of chemical composition, complex refractive index and single scattering albedo of ground-level aerosol polydispersions in summer and winter–spring is also presented, based on results mainly found in the literature.The long-term variation in Antarctic AOD was estimated to be stable, within ±0.10% per year, at the three coastal sites, and nearly null at South Pole, where a weak increase was only recently observed, associated with an appreciable decrease in α, plausibly due to the formation of thin stratospheric layers of ageing volcanic particles. The main characteristics of chemical composition, complex refractive index and single scattering albedo of Antarctic aerosols are also presented for coastal particles sampled at Neumayer and Terra Nova Bay, and continental particles at South Pole.► We examine the long-term series of AOD measurements at various Polar sites. ► We evaluate the long-term variations in Ångström’s exponent at various Polar sites. ► Variations in frequency of Arctic haze episodes are defined at Ny-Ålesund and Barrow. ► Aerosol chemical composition and radiative properties are given for Polar sites.

Keywords: Polar aerosol optical depth; Long-term AOD variations; Ångström exponent variations; Polar aerosol chemical composition; Polar aerosol radiative parameters; Arctic haze; Volcanic aerosol effects

Springtime Arctic aerosol: Smoke versus haze, a case study for March 2008 by M. Stock; C. Ritter; A. Herber; W. von Hoyningen-Huene; K. Baibakov; J. Gräser; T. Orgis; R. Treffeisen; N. Zinoviev; A. Makshtas; K. Dethloff (pp. 48-55).

During March 2008 photometer observations of Arctic aerosol were performed both at a Russian ice-floe drifting station (NP-35) at the central Arctic ocean (56.7–42.0° E, 85.5–84.2° N) and at Ny-Ålesund, Spitsbergen (78.9° N, 11.9° E). Next to a persistent increase of AOD over NP-35, two pronounced aerosol events have been recorded there, one originating from early season forest fires close to the city of Khabarovsk (“Arctic Smoke”), the other one showed trajectories from central Russia and resembled more the classical Arctic Haze. The latter event has also been recorded two days later over Ny-Ålesund, both in photometer and lidar. From these remote sensing instruments volume distribution functions are derived and discussed. Only subtle differences between the smoke and the haze event have been found in terms of particle microphysics. Different trajectory analysis, driven by NCEP and ECMWF have been performed and compared. For the data set presented here the meteorological field, due to sparseness of data in the central Arctic, mainly limits the precision of the air trajectories.► A case of each Arctic Haze and Arctic Smoke (biomass burning) is presented. ► Photometer and lidar data are analyzed from 2 Arctic sites. ► Microphysical aerosol properties are similar for both cases. ► The applicability and limitations of air trajectories are discussed. ► In the Arctic the driving meteorological field limits the trajectories’ precision.

Keywords: Arctic Haze; Arctic Smoke; Aerosol; Photometer; Lidar

Remote sensing and in-situ measurements of tropospheric aerosol, a PAMARCMiP case study by Anne Hoffmann; Lukas Osterloh; Robert Stone; Astrid Lampert; Christoph Ritter; Maria Stock; Peter Tunved; Tabea Hennig; Christine Böckmann; Shao-Meng Li; Kostas Eleftheriadis; Marion Maturilli; Thomas Orgis; Andreas Herber; Roland Neuber; Klaus Dethloff (pp. 56-66).

In this work, a closure experiment for tropospheric aerosol is presented. Aerosol size distributions and single scattering albedo from remote sensing data are compared to those measured in-situ. An aerosol pollution event on 4 April 2009 was observed by ground based and airborne lidar and photometer in and around Ny-Ålesund, Spitsbergen, as well as by DMPS, nephelometer and particle soot absorption photometer at the nearby Zeppelin Mountain Research Station.The presented measurements were conducted in an area of 40 × 20 km around Ny-Ålesund as part of the 2009 Polar Airborne Measurements and Arctic Regional Climate Model Simulation Project (PAMARCMiP). Aerosol mainly in the accumulation mode was found in the lower troposphere, however, enhanced backscattering was observed up to the tropopause altitude. A comparison of meteorological data available at different locations reveals a stable multi-layer-structure of the lower troposphere. It is followed by the retrieval of optical and microphysical aerosol parameters. Extinction values have been derived using two different methods, and it was found that extinction (especially in the UV) derived from Raman lidar data significantly surpasses the extinction derived from photometer AOD profiles. Airborne lidar data shows volume depolarization values to be less than 2.5% between 500 m and 2.5 km altitude, hence, particles in this range can be assumed to be of spherical shape. In-situ particle number concentrations measured at the Zeppelin Mountain Research Station at 474 m altitude peak at about 0.18 μm diameter, which was also found for the microphysical inversion calculations performed at 850 m and 1500 m altitude. Number concentrations depend on the assumed extinction values, and slightly decrease with altitude as well as the effective particle diameter. A low imaginary part in the derived refractive index suggests weakly absorbing aerosols, which is confirmed by low black carbon concentrations, measured at the Zeppelin Mountain as well as on board the Polar 5 aircraft.► Arctic haze has been studied by airborne and ground based remote sensing and in-situ instruments. ► Extinction has been derived using two different methods. ► In-situ number concentrations compare well with Raman lidar based microphysical inversions. ► Particles with diameter 0.18 μm and decreasing number concentrations with altitude were found. ► BC concentration and aerosol absorption was very low.

Keywords: Arctic; Aerosols; Lidar; Arctic haze

Analysis of sub-micron parameters derived from multi-altitude and multi-spectral AOD measurements acquired during the 2009 PAM-ARCMIP airborne campaign by Auromeet Saha; Norman T. O’Neill; Robert S. Stone; Peter S. Liu; Andreas B. Herber (pp. 67-81).

A series of Arctic sunphotometry flights were analyzed in terms of their multi-altitude, sub-micron (fine mode) information content. A spectral deconvolution algorithm (SDA) and a fine mode curvature algorithm (FMC) were applied to extract fine mode and coarse mode optical depth as well as the effective radius of the fine mode as a function of altitude. The fine mode optical depth was differentiated as a function of altitude to retrieve vertical profiles of fine mode extinction coefficient. These optical results were compared with volumetric altitude profiles of fine mode particle size distribution acquired by a UHSAS (Ultra High Sensitivity Aerosol Spectrometer).The results showed that layer-averaged extinction cross sections derived from the ratio of fine mode optical depth to integrated UHSAS number density were significantly larger than extinction cross sections derived from the application of Mie theory to the UHSAS particle size distributions. Vertical profiles of extinction coefficients derived from altitude derivatives of the fine mode optical depth profiles showed some correlation with the UHSAS extinction coefficients. Profiles of the fine mode effective radius retrieved from the sunphotometry data were generally of similar magnitude to columnar averages of the UHSAS radii but no significant trend with altitude could be detected. An example was given of a high altitude smoke plume whose presence eliminated any hope of correlating the volumetric sampling information with the sunphotometer profiles. This is simply a statement of the obvious; that the atmospheric state must be stable in order to make such comparisons.► We compared multi-altitude microphysical (UHSAS) and optical (sunphotometry) data. ► Extinction coefficients from sunphotometry correlated with UHSAS extinction coefficients. ► Effective radii from microphysical and sunphotometry data were comparable.

Keywords: Airborne sunphotometry; Aerosol optical depth; Fine Mode effective radius; PAM-ARCMIP campaign; Ultra High Sensitivity Aerosol Spectrometer (UHSAS)

Aerosols and their sources at Summit Greenland – First results of continuous size- and time-resolved sampling by Richard A. VanCuren; Thomas Cahill; John Burkhart; David Barnes; Yongjing Zhao; Kevin Perry; Steven Cliff; Joe McConnell (pp. 82-97).

An ongoing program to continuously collect time- and size-resolved aerosol samples from ambient air at Summit Station, Greenland (72.6 N, 38.5 W) is building a long-term data base to both record individual transport events and provide long-term temporal context for past and future intensive studies at the site. As a “first look” at this data set, analysis of samples collected from summer 2005 to spring 2006 demonstrates the utility of continuous sampling to characterize air masses over the ice pack, document individual aerosol transport events, and develop a long-term record. Seven source-related aerosol types were identified in this analysis: Asian dust, Saharan dust, industrial combustion, marine with combustion tracers, fresh coarse volcanic tephra, and aged volcanic plume with fine tephra and sulfate, and the well-mixed background “Arctic haze”. The Saharan dust is a new discovery; the other types are consistent with those reported from previous work using snow pits and intermittent ambient air sampling during intensive study campaigns. Continuous sampling complements the fundamental characterization of Greenland aerosols developed in intensive field programs by providing a year-round record of aerosol size and composition at all temporal scales relevant to ice core analysis, ranging from individual deposition events and seasonal cycles, to a record of inter-annual variability of aerosols from both natural and anthropogenic sources.Display Omitted► Continuous size-resolved aerosol sampling (8-stages, 90 nm to 10 μm) at Summit, Greenland. ► Seven aerosol types: Asian dust, Saharan dust, industrial combustion, marine with ships, fresh and aged volcanic plumes, and “Arctic haze”. ► Continued sampling for multiple years is building a data set that can provide unique insights into aerosol–ice relationships.

Keywords: Greenland; Aerosols; Dust; Tephra; Volcanic; Continental; Marine; Smelting; PMF; DRUM; Long range transport

Study of present-day sources and transport processes affecting oxidised sulphur compounds in atmospheric aerosols at Dome C (Antarctica) from year-round sampling campaigns by Silvia Becagli; Claudio Scarchilli; Rita Traversi; Uri Dayan; Mirko Severi; Daniele Frosini; Vito Vitale; Mauro Mazzola; Angelo Lupi; Silvia Nava; Roberto Udisti (pp. 98-108).

A year-round study, which was conducted from November 2004 to November 2007, of atmospheric oxidised sulphur compounds (methanesulphonic acid (MSA) and sulphate) was carried out in the east Antarctic Plateau at Dome C (75° 06′ S, 123° 20′ E, 3220m a.s.l. and 1100km away from the nearest coast). The two sulphur-derived species exhibit a seasonal cycle characterised by maxima in the summer from November to March. Size-segregated sampling performed with Andersen 8-stage impactors revealed that SO₄²⁻ and MSA have different size distributions in early summer (November) in comparison with mid-late summer (February). In November, the size distribution exhibited two distinct modes, the accumulation (0.4–0.7μm) and the micrometric mode (1.1–2.1μm), which is in contrast to February when only the accumulation mode was observed. The two modes exhibited different speciation; in the finest mode, sulphate and methanesulphonate were present primarily in the acidic form, whereas they were present primarily as sodium or ammonium salts in the micrometric mode. The different size distributions and speciation patterns in the two months are related to different transport pathways from oceanic areas to the central Antarctic Plateau. In the early summer months, air masses came primarily from the Indian Ocean and lingered for a long time over the Antarctic continent. The transport of sulphur compounds is related to sea spray aerosols and the resulting condensation of H₂SO₄ and MSA over sea salt particles to form sodium salts. In contrast, a rapid transport of H₂SO₄ and MSA formed above the boundary layer over oceanic areas leads to higher concentrations of the acidic species in the fine fraction of aerosols reaching Dome C in February relative to other summer months.► MSA and sulphate are measured in aerosol sampled in central Antarctic Plateau. ► First three all-year round high resolution record is presented here. ► Back-trajectories analysis was performed in order to study transport processes. ► Sulphur oxidised compounds size distributions as related to transport processes.

Keywords: MSA; Sulphate; Aerosol; Antarctic Plateau; Transport processes; Biogenic aerosol

Sea spray aerosol in central Antarctica. Present atmospheric behaviour and implications for paleoclimatic reconstructions by R. Udisti; U. Dayan; S. Becagli; M. Busetto; D. Frosini; M. Legrand; F. Lucarelli; S. Preunkert; M. Severi; R. Traversi; V. Vitale (pp. 109-120).

From November 2004 to December 2007, size-segregated aerosol samples were collected all-year-round at Dome C (East Antarctica) by using PM10 and PM2.5 samplers, and multi-stage impactors. The data set obtained from the chemical analysis provided the longest and the most time-resolved record of sea spray aerosol (sea salt Na⁺) in inner Antarctica. Sea spray showed a sharp seasonal pattern. The highest values measured in winter (Apr–Nov) were about ten times larger than in summer (Dec–Mar). For the first time, a size-distribution seasonal pattern was also shown: in winter, sea spray particles are mainly sub-micrometric, while their summer size-mode is around 1–2 μm. Meteorological analysis on a synoptic scale allowed the definition of atmospheric conditions leading sea spray to Dome C. An extreme-value approach along with specific environmental based criteria was taken to yield stronger fingerprints linking atmospheric circulation (means and anomalies) to extreme sea spray events. Air mass back-trajectory analyses for some high sea spray events allowed the identification of two major air mass pathways, reflecting different size distributions: micrometric fractions for transport from the closer Indian-Pacific sector, and sub-micrometric particles for longer trajectories over the Antarctic Plateau. The seasonal pattern of theSO42−/Na⁺ ratio enabled the identification of few events depleted in sulphate, with respect to the seawater composition. By using methanesulphonic acid (MSA) profile to evaluate the biogenicSO42− contribution, a more reliable sea salt sulphate was calculated. In this way, few events (mainly in April and in September) were identified originating probably from the “frost flower” source. A comparison with daily-collected superficial snow samples revealed that there is a temporal shift between aerosol and snow sea spray trends. This feature could imply a more complex deposition processes of sea spray, involving significant contribution of wet and diamond dust deposition, but further work has to be carried out to rule out the effect of wind re-distribution and to have more statistic significance.► Sea spray markers were measured in aerosol collected all-year-round at Dome C for 3 years. ► Seasonal pattern of concentration, size distribution and fractionation processes were found. ► Meteorological analysis carried out to understand transport processes and sources. ► The origin from frost flower was discussed on the basis of the sulphate/sodium ratio. ► A time shift between sea spray spikes in the aerosol and in superficial snow was found.

Keywords: Inland Antarctica aerosol; Sea spray aerosol; Fractionating effects; Size distribution; Transport processes; Source characterization; Snow-aerosol interaction

Taking the pulse of pyrocumulus clouds by C.K. Gatebe; T. Varnai; R. Poudyal; C. Ichoku; M.D. King (pp. 121-130).

Large forest fires are a known natural and dominant disturbance factor in high northern latitudes, and form pyrocumulus (pyroCu), and occasionally pyrocumulonimbus (pyroCb) clouds. These clouds can transport emissions into the upper troposphere/lower stratosphere (UT/LS) and produce significant regional and even global climate effects, as is the case with some volcanoes. However, the lack of observational data within pyroCu or pyroCb complicates our ability to investigate pyro-convection and to understand the vertical and cross-isentropic transport mechanisms responsible for UT/LS injection. Here, we report detailed airborne radiation measurements within strong pyroCu taken over boreal forest fires in Saskatchewan, Canada during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) summer field campaign in 2008. We find a prominent smoke core within the pyroCu, which is defined by strong extinction in the UV, VIS and NIR, and high gas-particle concentrations. We also find that the angular distribution of radiance within the pyroCu is closely related to the diffusion domain in water clouds, which is dominated by multiple scattering processes. The radiation field of pyroCu can be described by diffusion approximations that are comprised of simple cosine functions, which can be used to calculate the spatial and temporal characteristics of the radiance field, and applied in cloud resolving models. We demonstrate with Monte Carlo simulations that radiation transport in pyroCu is inherently a 3D problem and must account for particle absorption.► Strong light extinction detected in the core of dense fire clouds (pyroCu). ► Angular radiance distribution in dense pyroCu clouds can be described by simple diffusion approximation in place of 3D RT. ► Radiation transport in pyroCu is inherently a 3D problem and must account for particle absorption.

Keywords: Remote sensing; Pyrocumulus; Clouds; Smoke; Fires; Biomass burning; Radiometer; Monte Carlo; Three-dimensional (3D) radiative transfer; Diffusion approximations; Airborne; CAR; NASA P-3B

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