Atmospheric Environment (v.42, #11)

Light absorption by coated nano-sized carbonaceous particles by Martin Gangl; Miroslav Kocifaj; Gorden Videen; Helmuth Horvath (2571-2581).
The optical properties of strongly absorbing soot particles coated by transparent material are investigated experimentally and described by several modeling approaches. Soot is produced by spark discharge and passed through a Sinclair–La Mer generator where non-absorbing carnauba wax is condensed onto it to obtain internal soot–wax mixtures in a controlled way. Measurements of the extinction and volume scattering coefficient show an amplification of absorption by a factor of approximately 1.8. This behavior was described by different approaches of internally mixed materials for the modal diameters of the measured size distributions: concentric-sphere model, effective medium approximations and heterogeneous ellipsoids. The concentric-sphere model describes the absorption increase quantitatively; and hence, it is chosen to be applied to the entire particle population in the size distribution. The growth of the soot particles by condensing wax is described by a simplified growth model to estimate the different contributions of several soot particle diameters to the overall absorption cross-section.
Keywords: Nano-sized aerosols; Absorption; Coated carbonaceous particles; Discrete dipole approximation;

Retrieval of aerosol aspect ratio from optical measurements in Vienna by M. Kocifaj; H. Horvath; M. Gangl (2582-2592).
The phase function and extinction coefficient measured simultaneously are interpreted in terms of surface distribution function and mean effective aspect ratio of aerosol particles. All optical data were collected in the atmosphere of Vienna during field campaign in June 2005. It is shown that behavior of aspect ratio of Viennese aerosols has relation to relative humidity in such a way, that nearly spherical particles (with aspect ratio ε≈1) might became aspherical with ε≈1.3–1.6 under low relative humidity conditions. Typically, >80% of all Viennese aerosols have the aspect ratio <1.4, so the morphology of these particles behaves like perturbed spheres. The ε, exceptionally, can reach the value about 2, but these situations occur with probability <2%. Most typically, the aspect ratio peaks at ε≈1.2 in the atmosphere of Vienna.
Keywords: Aspect ratio of aerosols; Urban atmosphere; Aerosol extinction coefficient; Scattering phase function;

Determination of the visibility using a digital panorama camera by D. Bäumer; S. Versick; B. Vogel (2593-2602).
We describe a method to determine the daytime visibility at a station that is located at the University of Karlsruhe, Germany. A comparison of visibilities determined by applying our new method and visibilities observed with standard methods at a nearby station of the German Weather Service (DWD) demonstrate the successful operation of our method. The highest differences occur for very low and very high visibilities. For visibilities >10 km, the correlation between visibilities of both methods is 0.73. Additionally, at low sun zenith angles of <30°, an overestimation by the visibility determined with digital photographs (VISIDIP) method of up to 20% has been observed. Partially, the bias could be explained by the different measurement heights and different local conditions which have an effect especially under certain meteorological conditions as fog or precipitation events. In contrast to human observations, the new method presented in this feasibility study is completely automatic and objective.
Keywords: Atmospheric visibility; Panoramic image; Sobel operator; Digital image processing; Visual range;

Effect of aerosols on the infrared transmission in Lakiala, Finland by T. Mielonen; T. Kaurila; A. Arola; H. Lihavainen; M. Komppula; K.E.J. Lehtinen (2603-2610).
Effects of atmospheric aerosols on radiative transfer are mainly studied because of their role in climate change. Aerosols also affect some technological applications by deteriorating range performance of electro-optical systems. This study focuses on the aerosol-induced attenuation of infrared radiation along a horizontal path. Measured attenuation values are compared with modeled ones and an attempt is made to understand the differences. Similar measurements have been done earlier; however, those measurements have not usually contained any information about the prevailing aerosol size distributions. Both measured and modeled aerosol extinction coefficients are studied as a function of different weather parameters (visibility, relative humidity and temperature). Measured size distributions are also investigated and they are compared with the size distribution assumed in MODTRAN4. Because measurements and extinction calculations contain some error sources, e.g. instrument errors and errors in aerosol growth factors, whose magnitudes are not exactly known, the total uncertainty was difficult to assess. Despite the uncertainty in the measurements, differences between measured size distributions and model size distributions were found. It appears that weather parameters do not offer the most feasible input data to model the aerosol extinction. In addition, aerosol extinction coefficients calculated from the measured size distributions were much lower than the measured and modeled values. The continuation of aerosol attenuation measurements in the future is of vital importance to obtain enough data for the analysis. Moreover, the accuracy of the measuring equipments should be comprehensively assessed.
Keywords: Optical properties of aerosols; Visibility; Measurement campaigns;

Based on round-the-clock measurements in 1997–2005 in West Siberia at the Aerosol Station (Tomsk) of the directed scattering coefficient of the dry matter of submicron aerosol and black carbon (BC) mass concentration in particles, the influence of forest fire smoke on the variability of the BC mass fraction P in the near-ground aerosol has been analyzed. Stable regularity has revealed that BC fraction (1–4.5%) in smoke of remote forest fires is lower than in the background aerosol, i.e. they are more weakly absorbing. This causes a decrease of P values, when smoke plumes invade into the atmosphere over the measurement site (P-effect). It was found from the in-situ measurements and laboratory experiments that this effect is stable in our region, being caused by the decisive contribution of the pyrolysis (smoldering) of forest combustible materials to the formation of the smoke plume particles. The intrusion of forest fire smoke in the region of observation leads to an increase of the single-scattering albedo of dry submicron aerosol above 0.95. It means that the radiative-climatic effect of forest fire smoke is cooling. It is proposed to take the BC mass fraction in the dry submicron aerosol as an information index (P-criterion) for distinguishing the states of the near-ground air layer affected by the forest fires far removed from the observation site.
Keywords: Submicron atmospheric aerosol; Black carbon; BC mass fraction; Forest fires;

Trends in visibility over Hungary between 1996 and 2002 by A. Molnár; E. Mészáros; K. Imre; A. Rüll (2621-2629).
The spatial distribution and temporal variability of visibility data measured at 23 stations in Hungary show that changes have occurred between 1996 and 2002. Extinction coefficients were calculated from visual range observations using the Koschmieder formula, and dry extinction coefficients were estimated by fitting them to power functions of relative humidity. The lowest visibilities were found over the Great Hungarian Plain, probably because of the effects of particles of surface origin. At all but two stations the visibility increased from 1996 to 2002, probably due to the decreased load of anthropogenic aerosol particles. This finding is in excellent agreement with the significant decrease in the emission of sulfur dioxide. During the 6 years studied there was an important improvement in atmospheric light attenuation over Hungary.
Keywords: Relative humidity; Visibility; Aerosol extinction; Temporal and spatial distribution;

Surface measurements of optical and physical aerosol properties were made at an urban site, Granada (Spain) (37.18°N, 3.58°W, 680 m a.s.l), during winter 2005–2006. Measurements included the aerosol scattering, σ sca, and backscattering coefficients, σ bsca, at three wavelengths (450, 550 and 700 nm) measured at low relative humidity (RH<50%) by an integrating nephelometer, the absorption coefficient at 670 nm, σ abs, measured with a multi-angle absorption photometer, and aerosol size distribution in the 0.5–20 μm aerodynamic diameter range registered by an aerodynamic aerosol sizer (APS-3321, TSI). The hourly average of σ sca (550 nm) ranged from 2 to 424 M m−1 with an average value of 84±62 M m−1 (±S.D.). The Angstrom exponent presented an average value of 1.8±0.3, suggesting a large fraction of fine particles at the site, an observation confirmed by aerosol size distribution measurements. The hourly average of σ abs (670 nm) ranged from 1.7 to 120.5 M m−1 with an average value of 28±20 M m−1. The results indicate that the aerosol absorption coefficient in Granada was relatively large. The largest σ sca value was associated with air masses that passed over heavily polluted European areas and local stagnation conditions. High absorbing aerosol level was obtained during dust transport from North Africa probably due to the presence of hematite. Based on the measured scattering and absorption coefficients, a very low average value of the single scattering albedo of 0.66±0.11 at 670 nm was calculated, suggesting that urban aerosols in this region contain a large fraction of absorbing material. A clear diurnal pattern was observed in scattering and absorption coefficients and particle concentrations with two local maxima occurring in early morning and late evening. This behavior can be explained in terms of local conditions that control the particle sources associated with traffic and upward mixing of the aerosol during the daytime development of a convective boundary layer. Significant reduction in absorption coefficient values has been found during weekends compared to working days, showing a strong impact of local sources on aerosol properties. In contrast to σ abs, the aerosol scattering coefficient obtained during weekends was higher than those obtained on Mondays and Tuesdays. A possible explanation for the large values of σ sca measured during the weekend could be secondary aerosol aging.
Keywords: Light scattering coefficient; Light absorption coefficient; Aerosol optical properties; Urban aerosol;

Columnar aerosol optical properties during “El Arenosillo 2004 summer campaign” by N. Prats; V.E. Cachorro; M. Sorribas; S. Mogo; A. Berjón; C. Toledano; A.M. de Frutos; J. de la Rosa; N. Laulainen; B.A. de la Morena (2643-2653).
A detailed analysis of the columnar optical aerosol properties has been carried out for data collected during the “El Arenosillo 2004 summer campaign”. These data are derived from a Cimel sun-photometer, as part of the PHOTONS-AERONET network at the El Arenosillo site in south-western Spain, over the period 1 June to 31 October 2004. Version 1 AERONET inversion products are used in the analysis. The aim of this campaign was to obtain a more complete set of data on aerosol particle microphysical, optical/radiative, absorbing and chemical properties for use in closure studies. In this paper we focus on the characterization of the aerosol optical depth (AOD)–alpha coefficients, and the particle size distribution together with their associated microphysical parameters, such as volume concentration, effective radius, etc., in order to define the features and ranges of these physical parameters associated with both fine and coarse particle modes. The requirement of high AOD values for using the inversion technique puts significant constraints on the estimation of the single scattering albedo (SSA) and refractive index, and thus requires great care in the analysis. As a result, only the characterizations of these parameters for desert dust events are considered reliable. Moreover, summer 2004 had the most frequent desert dust intrusions, including the most intense event (22–28 July and 31 July–2 August) ever recorded at the El Arenosillo site since the start of the measurements in February 2000. We summarize the results for the intensive summer campaign in terms of the range of values of the physical and optical parameters of the different aerosol types present in this area of Spain.
Keywords: Aerosol particles; Columnar optical properties; Aerosol optical depth; AERONET;

Aerosol columnar properties retrieved from CIMEL radiometers during VELETA 2002 by L. Alados-Arboledas; A. Alcántara; F.J. Olmo; J.A. Martínez-Lozano; V. Estellés; V. Cachorro; A.M. Silva; H. Horvath; M. Gangl; A. Díaz; M. Pujadas; J. Lorente; A. Labajo; M. Sorribas; G. Pavese (2654-2667).
During the 2002 summer, the VELETA 2002 field campaign has been carried out at the Sierra Nevada Massif, close to Granada in South-Eastern Spain. During the campaign, CIMEL CE-318 robotic radiometer has been one of the key instruments in the characterization of the atmospheric aerosol columnar properties. This kind of radiometers have been operated at Motril, a coastal location at sea level, Pitres (1200 m a.s.l.), located in the South slope of Sierra Nevada Massif, Las Sabinas (2200 m a.s.l.), located on the north slope of the mountain range, and Armilla (680 m a.s.l.), located in the valley. The principal feature of the locations is that they provide a strong altitudinal gradient. This work presents the evolution of changes followed by the atmospheric aerosol in the vertical column. Columnar aerosol properties have been derived from the CIMEL CE-318 measurements obtained at the four stations. In this sense, the combination of solar direct measurements and sky radiances were used to retrieve the aerosol columnar size distributions by means of Nakajima's code. Marked changes in the atmospheric aerosol along the campaign have been explained in terms of the synoptic conditions. A detailed study of the columnar properties at the different places has been done. Changes in the spectral dependence of the single scattering albedo under the influence of different air masses have been discussed. The size distributions have been fitted to a bimodal lognormal distribution function model, and the associated parameters have been correlated with aerosol optical depth or Ångström exponent. In some cases, it has been necessary to develop different correlations for cases affected or non-affected by the arrival of Saharan dust. In the case of Armilla (680 m a.s.l.) and Las Sabinas (2200 m a.s.l.), the small horizontal distance allow us to consider that both stations are representative of the same atmospheric column, a fact that has been used to analyse the daily evolution of the mixing layer in the north slope.
Keywords: Atmospheric aerosol; Columnar properties; Volume size distribution; Single scattering albedo; Planetary boundary layer; Saharan dust;

Measurements during Saharan dust transport events have been performed at the Andalusian Centre for Environmental Studies (CEAMA) (37.16°N, 3.58°W, 680 m a.s.l.), located in the city of Granada (south-eastern Iberian Peninsula). During 2005, as many as 22 dust outbreaks took place over the study area in south-eastern Spain. The arrival of Saharan air masses induced sharp changes in the background aerosol optical depth (AOD) and the Angström exponent during the analysed events. This fact, in addition to a back-trajectory analysis, identified the Saharan dust events. In order to improve knowledge of Saharan dust over the southern Iberian Peninsula, vertical characterization of temporal evolutions has been performed in terms of the backscatter coefficient, aerosol depolarization ratio and colour ratio. Two severe events in July 2005 have been selected for detailed analysis. Both events lasted several days, allowing the characterization of the evolution of the dust layers over our station. The impact of Saharan dust outbreaks has been followed with the Lidar, and their effects at the surface have been determined in the air quality network of the regional government.
Keywords: Lidar; Aerosol properties; Backscatter coefficient; Aerosol depolarization ratio; Saharan dust;

Aerosols inserted high into the atmosphere might be subject to a significant photophoretic force. At lower altitudes the air pressure is too high for photophoresis to be effective but at stratospheric and mesospheric pressure photophoresis can compensate a particle's gravity under certain conditions. The application of photophoretic particle transport requires fundamental knowledge of photophoresis for complex particles. With regard to this, we started to carry out experiments on photophoresis. We detected particle lift by negative radiometric photophoresis on graphite aggregates of about 10 μm in size at a light flux comparable to solar in an electrodynamic trap. In another set-up we could levitate large aggregates of graphite and other materials by negative photophoresis in a slightly focused downward directed laser beam at higher light intensities. The points of stability could be moved up or down in non-predictable direction but reversible by changing the intensity of the laser beam. A third experiment describes the lift-off of surface particles invoked by a combination of photophoresis and solid-state greenhouse effects. Together, the pressure dependence of the forces, particle rotation, and the stability and direction of motion in directed laser beams allow us to distinguish between the different kinds of photophoretic forces suggested for atmospheric particle transport especially for gravito-photophoresis.
Keywords: Photophoresis; Stratosphere; Aerosols; Particle transport; Radiation;

The Interagency Monitoring of Protected Visual Environments (IMPROVE) program is a cooperative measurement effort in the United States designed to characterize current visibility and aerosol conditions in scenic areas (primarily National Parks and Forests) and to identify chemical species and emission sources responsible for existing man-made visibility impairment. In 2003 and 2004, the IMPROVE network began operating collocated samplers at several sites to evaluate the precision of its aerosol measurements.
Keywords: Uncertainty; Collocated; Particulate matter; Air quality monitoring; Measurement error;

Source apportionment of 1 h semi-continuous data during the 2005 Study of Organic Aerosols in Riverside (SOAR) using positive matrix factorization by Delbert J. Eatough; Brett D. Grover; Woods R. Woolwine; Norman L. Eatough; Russell Long; Robert Farber (2706-2719).
Positive matrix factorization (PMF2) was used to elucidate sources of fine particulate material (PM2.5) for a study conducted during July and August 2005, in Riverside, CA. One-hour averaged semi-continuous measurements were made with a suite of instruments to provide PM2.5 mass and chemical composition data. Total PM2.5 mass concentrations (non-volatile plus semi-volatile) were measured with an R&P filter dynamic measurement system (FDMS TEOM) and a conventional TEOM monitor was used to measure non-volatile mass concentrations. PM2.5 chemical species monitors included a dual-oven Sunset monitor to measure both non-volatile and semi-volatile carbonaceous material, an ion chromatographic-based monitor to measure sulfate and nitrate and an Anderson Aethalometer to measure black carbon (BC). Gas phase data including CO, NO2, NO x and O3 were also collected during the sampling period. In addition, single-particle measurements were made using aerosol time-of-flight mass spectrometry (ATOFMS). Twenty different single-particle types consistent with those observed in previous ATOFMS studies in Riverside were identified for the PMF2 analysis. Finally, time-of-flight aerosol mass spectrometry (ToF-AMS) provided data on markers of primary and secondary organic aerosol. Two distinct PMF2 analyses were performed. In analysis 1, all the data except for the ATOFMS and ToF-AMS data were used in an initial evaluation of sources at Riverside during the study. PMF2 was able to identify six factors from the data set corresponding to both primary and secondary sources, primarily from automobile emissions, diesel emissions, secondary nitrate formation, a secondary photochemical associated source, organic emissions and Basin transported pollutants. In analysis 2, the ATOFMS and ToF-AMS data were included in the analysis. In the second analysis, PMF2 was able to identify 16 factors with a variety of both primary and secondary factors being identified, corresponding to both primary and secondary material from both anthropogenic and natural sources. Based on relationships with Basin meteorology, the PMF identified source profiles and diurnal patterns in the source concentrations, sources were identified as being of local origin or resulting from transport of pollutants across the Basin due to onshore flow. Good agreement was observed between the PMF2 predicted mass and the FDMS measured mass for both analyses.
Keywords: PMF2 source apportionment. 1-h data; Los Angeles Basin; SOAR; PM2.5 mass and composition; Aerosol mass spectrometer; ATOFMS and ToF-AMS data;

Observations of fine and coarse particle nitrate at several rural locations in the United States by Taehyoung Lee; Xiao-Ying Yu; Benjamin Ayres; Sonia M. Kreidenweis; William C. Malm; Jeffrey L. Collett (2720-2732).
Nitrate comprises an important part of aerosol mass at many non-urban locations during some times of the year. Little is known, however, about the chemical form and size distribution of particulate nitrate in these environments. While submicron ammonium nitrate is often assumed to be the dominant species, this assumption is rarely tested. Properties of aerosol nitrate were characterized at several IMPROVE monitoring sites during a series of field studies. Study sites included Bondville, Illinois (February 2003), San Gorgonio Wilderness Area, California (April and July 2003), Grand Canyon National Park, Arizona (May 2003), Brigantine National Wildlife Refuge, New Jersey (November 2003), and Great Smoky Mountains National Park, Tennessee (July/August 2004). Nitrate was found predominantly in submicron ammonium nitrate particles during the Bondville and San Gorgonio (April) campaigns. Coarse mode nitrate particles, resulting from reactions of nitric acid or its precursors with sea salt or soil dust, were more important at Grand Canyon and Great Smoky Mountains. Both fine and coarse mode nitrate were important during the studies at Brigantine and San Gorgonio (July). These results, which complement earlier findings about the importance of coarse particle nitrate at Yosemite and Big Bend National Parks, suggest a need to more closely examine common assumptions regarding the importance of ammonium nitrate at non-urban sites, to include pathways for coarse mode nitrate formation in regional models, and to consider impacts of coarse particle nitrate on visibility. Because coarse particle nitrate modes often extend well below 2.5 μm aerodynamic diameter, measurements of PM2.5 nitrate in these environments should not automatically be assumed to contain only ammonium nitrate.
Keywords: PM2.5; Nitrate; Coarse particle; Acidity; Visibility; IMPROVE;

Development and calibration of a star photometer to measure the aerosol optical depth: Smoke observations at a high mountain site by D. Pérez-Ramírez; J. Aceituno; B. Ruiz; F.J. Olmo; L. Alados-Arboledas (2733-2738).
In this paper we present a recently developed star photometer, which allows us to study aerosol optical properties at night, based on a CCD camera. Details about the operation of the system are presented together with the instrument set up. Special attention is paid to instrument calibration and thus details about a high mountain calibration campaign are presented. A combination of a Langley procedure with a calibration transfer supported by a check using the so-called two-star method of star photometry has been used in order to calibrate with an appropriate number of stars. Also, some measurements acquired during an episode of long-range transport of biomass burning aerosol are presented as a case study.
Keywords: Star photometry; Aerosol optical depth; Langley plot;

Using a Sky Imager for aerosol characterization by A. Cazorla; F.J. Olmo; L. Alados-Arboledas (2739-2745).
The All-Sky Imager developed by the Atmospheric Physics Group has been tested for aerosol characterization. Different neural network-based models calculate the aerosol optical depth (AOD) for two wavelengths and the Ångström turbidity parameter α using as input parameters data extracted from the principal plane of sky images from the All-Sky Imager. The models use data from a CIMEL CE318 radiometer for training and validation. The deviations between model and reference values are in the range of uncertainties assigned to Aerosol Robotic Network (AERONET) network.
Keywords: Sky imagery; Sky radiance; Aerosol optical depth; Neural networks; Radial basis networks;