Cloud condensation nuclei act as cores for water vapour condensation, and their composition and chemical properties may enhance or depress the ability for droplet growth. In this study we use ...molecular dynamics simulations to show that model humic-like substances (HULIS) in systems containing 10 000 water molecules mimic experimental data well referring to reduction of surface tension. The model HULIS compounds investigated in this study are cis-pinonic acid (CPA), pinic acid (PAD) and pinonaldehyde (PAL). The structural properties examined show the ability for the model HULIS compounds to aggregate inside the nanoaerosol clusters.
An intercomparison of seven dimethylsulfide oxidation chemical mechanisms for the marine boundary layer (MBL) is conducted using a coupled gas phase/aerosol box model. The mechanisms are from Koga ...and Tanaka (1993), Hertel et al. (1994), Saltelli and Hjorth (1995), Chin et al. (1996), Capaldo and Pandis (1997), Lucas and Prinn (2002), and the mechanism developed during the Evaluation of the Climatic Impact of Dimethyl Sulfide project (EL CID, 2003). They range from schemes with very detailed description of the dimethylsulfide oxidation with up to 65 reactions to a very simple scheme with only six reactions suitable for global modeling. Coupling each oxidation mechanism to a monodisperse aerosol dynamics model allows for the simultaneous comparison of predicted gas phase and particulate concentrations of sulfur‐containing constituents, together with the predicted contribution of dimethylsulfide (DMS) to aerosol formation and growth. Significant differences for sulfur‐containing compounds between the compared schemes are observed. The temperature dependence and magnitude of the ratio between methane sulfonate and non‐sea‐salt sulfate is investigated for each scheme and compared to observations. Liquid phase oxidation processes are missing in the model and the capability of individual schemes to correctly predict observed ratios can only be assessed in part. No new particle formation was found when applying binary nucleation in the three marine boundary layer scenarios. Therefore the results suggest that several uncertainties limiting our understanding of atmospheric oxidation of dimethylsulfide with implications for climate still exist.
Aerosol particles in the atmosphere are important participants in the formation of cloud droplets and have significant impact on cloud albedo and global climate. According to the Köhler theory which ...describes the nucleation and the equilibrium growth of cloud droplets, the surface tension of an aerosol droplet is one of the most important factors that determine the critical supersaturation of droplet activation. In this paper, with specific interest to remote marine aerosol, we predict the surface tension of aerosol droplets by performing molecular dynamics simulations on two model systems, the pure water droplets and glycine in water droplets. The curvature dependence of the surface tension is interpolated by a quadratic polynomial over the nano-sized droplets and the limiting case of a planar interface, so that the so-called Aitken mode particles which are critical for droplet formation could be covered and the Köhler equation could be improved by incorporating surface tension corrections.
Water-soluble inorganic components in rain deposited at the Maldives Climate Observatory Hanimaadhoo (MCOH) were examined to determine seasonality and possible source regions. The study, which is ...part of the Atmospheric Brown Cloud (ABC) project, covers the period June 2005 to December 2007. Air mass trajectories were used to separate the data into situations with transport of air from India and adjacent parts of the Asian continent during the months December and January (Indian group) and those with southerly flow from the Indian Ocean during the summer monsoon season June to September (Marine group). A third trajectory group was identified with transport from the northern parts of the Arabian Sea and adjacent land areas during the months March, April and October (Arabian Sea group). The concentrations of nss-SO42−, NH4+ and NO3− were more than a factor of 4 higher in the Indian group than in the Marine group. The average rainwater pH was significantly lower in the Indian group (4.7) than in the Marine group (6.0). This shows a pronounced influence of continental pollutants during December and January. The origin of the very high concentration of nss-Ca2+ found in the Marine group – a factor of 7 higher than in the Indian group – is unclear. We discuss various possibilities including long-range transport from the African or Australian continents, local dust from nearby islands and calcareous plankton debris and exopolymer gels emitted from the ocean surface. The occurrence of NO3− and NH4+ in the Marine group suggests emissions from the ocean surface. Part of the NO3− could also be associated with lightning over the ocean. Despite the fact that the concentrations of nss-SO42−, NO3−, and NH4+ were highest in the Indian group the wet deposition was at least as big in the Marine group reflecting the larger amount of rainfall during the monsoon season. The annual wet deposition of NO3−, NH4+ and nss-SO42− at MCOH is about a factor of three lower than observed at rural sites in India.
Particulate organic matter, including microorganisms, small water-insoluble particles and microaggregates, can form a substantial part of the summer aerosol over the open leads of the central Arctic ...Ocean. The increased presence of leads during the summer melt increases the biological activity of the region possibly resulting in accumulation of organic material, especially in the surface microlayer. Preferential microlayer enrichment of particulate and dissolved organic matter and gases might be reflected in derived aerosols. The subsurface seawater and microlayer concentrations were determined for dimethyl sulfide (DMS) and its biogenic precursor dimethylsulfoniopropionate (DMSP), dissolved combined amino acids (DCAA) and individual aminoacids, proteins, chlorophyll a (chl a) and bacterial cells as well as bacterial production. Enrichment factors and surface excess concentrations in the surface microlayer were calculated. Concentrations of particulate and dissolved DMSP, chl a-containing material, and bacterial cells were consistently enriched in arctic lead microlayers at 89°N in August 2001. DMS, protein and DCAA concentrations, however, were not in excess in surface microlayers, although proteins were occasionally enriched. The average mole % DCAA composition was similar in subsurface and microlayer water. Enrichment spikes and excess concentrations followed freezing events and did not co-occur with enhanced bacterial production.
A method was developed to systematically investigate individual aerosol particles collected onto a polyvinyl formal (Formvar)-coated copper grid with scanning electron microscopy. At very mild ...conditions with a low accelerating voltage of 2 kV and Gentle Beam mode aerosol particles down to 20 nm in diameter can be observed. Subsequent processing of the images with digital image analysis provides size resolved and morphological information (elongation, circularity) on the aerosol particle population. Polystyrene nanospheres in the expected size range of the ambient aerosol particles (20–900 nm in diameter) were used to confirm the accuracy of sizing and determination of morphological parameters. The relative standard deviation of the diameters of the spheres was better than ±10% for sizes larger than 40 nm and ±18% for 21 nm particles compared to the manufacturer's certificate. Atmospheric particles were collected during an icebreaker expedition to the high Arctic (north of 80°) in the summer of 2008. Two samples collected during two different meteorological regimes were analyzed. Their size distributions were compared with simultaneously collected size distributions from a Twin Differential Mobility Particle Sizer, which confirmed that a representative fraction of the aerosol particles was imaged under the electron microscope. The size distributions obtained by scanning electron microscopy showed good agreement with the Twin Differential Mobility Sizer in the Aitken mode, whereas in the accumulation mode the size determination was critically dependent on the contrast of the aerosol with the Formvar-coated copper grid. The morphological properties (elongation, circularity) changed with the number of days the air masses spent over the pack-ice area north of 80° before the aerosol particles were collected at the position of the icebreaker and are thus an appropriate measure to characterize transformation processes of ambient aerosol particles.
An eddy‐covariance flux system was successfully applied over open sea, leads and ice floes during the Arctic Ocean Expedition in July‐August 1996. Wind‐driven upward aerosol number fluxes were ...observed over open sea and leads in the pack ice. These particles must originate from droplets ejected into the air at the bursting of small air bubbles at the water surface. The source flux F (in 106 m−2 s−1) had a strong dependency on wind speed,
log(F)=0.20U¯‐1.71
and
0.11U¯‐1.93
, over the open sea and leads, respectively (where
U¯
is the local wind speed at about 10 m height). Over the open sea the wind‐driven aerosol source flux consisted of a film drop mode centered at ∼100 nm diameter and a jet drop mode centered at ∼1 μm diameter. Over the leads in the pack ice, a jet drop mode at ∼2 μm diameter dominated. The jet drop mode consisted of sea‐salt, but oxalate indicated an organic contribution, and bacterias and other biogenic particles were identified by single particle analysis. Particles with diameters less than −100 nm appear to have contributed to the flux, but their chemical composition is unknown. Whitecaps were probably the bubble source at open sea and on the leads at high wind speed, but a different bubble source is needed in the leads owing to their small fetch. Melting of ice in the leads is probably the best candidate. The flux over the open sea was of such a magnitude that it could give a significant contribution to the condensation nuclei (CCN) population. Although the flux from the leads were roughly an order of magnitude smaller and the leads cover only a small fraction of the pack ice, the local source may till be important for the CCN population in Arctic fogs. The primary marine aerosol source will increase both with increased wind speed and with decreased ice fraction and extent. The local CCN production may therefore increase and influence cloud or fog albedo and lifetime in response to greenhouse warming in the Arctic Ocean region.
The presented filter-based optical method for determination of soot (light absorbing carbon or Black Carbon, BC) can be implemented in the field under primitive conditions and at low cost. This ...enables researchers with small economical means to perform monitoring at remote locations, especially in the Asia where it is much needed. One concern when applying filter-based optical measurements of BC is that they suffer from systematic errors due to the light scattering of non-absorbing particles co-deposited on the filter, such as inorganic salts and mineral dust. In addition to an optical correction of the non-absorbing material this study provides a protocol for correction of light scattering based on the chemical quantification of the material, which is a novelty. A newly designed photometer was implemented to measure light transmission on particle accumulating filters, which includes an additional sensor recording backscattered light. The choice of polycarbonate membrane filters avoided high chemical blank values and reduced errors associated with length of the light path through the filter. Two protocols for corrections were applied to aerosol samples collected at the Maldives Climate Observatory Hanimaadhoo during episodes with either continentally influenced air from the Indian/Arabian subcontinents (winter season) or pristine air from the Southern Indian Ocean (summer monsoon). The two ways of correction (optical and chemical) lowered the particle light absorption of BC by 63 to 61 %, respectively, for data from the Arabian Sea sourced group, resulting in median BC absorption coefficients of 4.2 and 3.5 Mm−1. Corresponding values for the South Indian Ocean data were 69 and 97 % (0.38 and 0.02 Mm−1). A comparison with other studies in the area indicated an overestimation of their BC levels, by up to two orders of magnitude. This raises the necessity for chemical correction protocols on optical filter-based determinations of BC, before even the sign on the radiative forcing based on their effects can be assessed.
Methods for determining aerosol types in cases where chemical composition measurements are not available are useful for improved aerosol radiative forcing estimates. In this study, two aerosol ...characterization methods by Cazorla et al. (2013, https://doi.org/10.5194/acp-13-9337-2013; CA13) and Costabile et al. (2013, https://doi.org/10.5194/acp-13-2455-2013; CO13) using wavelength‐dependent particle absorption and scattering are used, to assess their applicability and examine their limitations. Long‐term ambient particle optical property and chemical composition (major inorganic ions and bulk carbon) measurements from the Maldives Climate Observatory Hanimaadhoo as well as concurrent air mass trajectories are utilized to test the classifications based on the determined absorption Ångström exponent, scattering Ångström exponent, and single scattering albedo. The resulting aerosol types from the CA13 method show a good qualitative agreement with the particle chemical composition and air mass origin. In general, the size differentiation using the scattering Ångström exponent works very well for both methods, while the composition identification depending mainly on the absorption Ångström exponent can result in aerosol misclassifications at Maldives Climate Observatory Hanimaadhoo. To broaden the applicability of the CA13 method, we suggest to include an underlying marine aerosol group in the classification scheme. The classification of the CO13 method is less clear, and its applicability is limited when it is extended to aerosols in this environment at ambient humidity.
Key Points
Optical aerosol classifications conform reasonable well with chemical measurements and air mass origin analysis
The extension of optical characterization methods to a remote marine site is limited due to strong aerosol mixing and humidification
An underlying sea spray category should be considered for optical aerosol characterization at sites with marine influence
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