G. de Leeuw

Laboratory simulations and parameterization of the primary marine aerosol production

A major source of the primary marine aerosol is the bursting of air bubbles produced by breaking waves. Several source parameterizations are available from the literature, usually limited to particles with a dry diameter Dp > 1 μm. The objective of this work is to extend the current knowledge to submicrometer particles. Bubbles were generated in synthetic seawater using a sintered glass filter, with a size spectra that are only partly the same spectra as measured in the field. Bubble spectra, and size distributions of the resulting aerosol (0.020-20.0 μm Dp) of the resulting aerosol, were measured for different salinity, water temperature (Tw), and bubble flux. The spectra show a minimum at ∼1 μm Dp, which separates two modes, one at ∼0.1 μm, with the largest number of particles, and one at 2.5 μm Dp. The modes show different behavior with the variation of salinity and water temperature. When the water temperature increases, the number concentration Np decreases for Dp 0.35 μm, Np increases. The salinity effect suggests different droplet formation processes for droplets smaller and larger than 0.2 μm Dp. The number of particles produced per size increment, time unit, and whitecap surface (φ) is described as a linear function of Tw and a polynomial function of Dp. Combining φ with the whitecap coverage fraction W (in percent), an expression results for the primary marine aerosol source flux dFo/dlogDp = W φ (m-2 s-1 ). The results are compared with other commonly used formulations as well as with recent field observations. Implications for aerosol-induced effects on climate are discussed.

Coordinated Airborne, Spaceborne, and Ground-Based Measurements of Massive, Thick Aerosol Layers During the Dry Season in Southern Africa

During the dry season airborne campaign of the Southern African Regional Science Initiative (SAFARI 2000), coordinated observations were made of massive thick aerosol layers. These layers were often dominated by aerosols from biomass burning. We report on airborne Sun photometer measurements of aerosol optical depth (λ = 0.354-1.557 μm), columnar water vapor, and vertical profiles of aerosol extinction and water vapor density that were obtained aboard the University of Washingtons Convair-580 research aircraft. We compare these with ground-based AERONET Sun/sky radiometer results, with ground based lidar data (MPL-Net), and with measurements from a downward pointing lidar aboard the high-flying NASA ER-2 aircraft. Finally, we show comparisons between aerosol optical depths from the Sun photometer and those retrieved over land and over water using four spaceborne sensors (TOMS, MODIS, MISR, and ATSR-2).

Aerosol optical depth over Europe in August 1997 derived from ATSR‐2 data

Data from the Along Tract Scanning Radiometer 2 (ATSSR-2) on board the European ERS-2 satellite have been used to derive the spatial distribution of the aerosol optical depth (AOD) over Europe for August 1997. The AOD was retrieved in cloud free areas using the dual view algorithm. The results agree with co-located ground based sun-photometer data within 0.1. The AOD from ATSR-2 with a resolution of 1 x 1 km2 were averaged on a grid of 0.1° x 0.1°, to produce daily maps of the spatial aerosol distribution over Europe. A composite map of AOD over Europe was constructed by averaging all daily maps for August 1997. This composite map shows a large spatial AOD gradients with variations of a factor of 3 over a few hundreds of kilometers. The AOD at 0.555 μm for relatively clean areas is around 0.1 while in strong industrialised areas the AOD can be 0.5 or higher.

Retrieval of aerosol optical properties from OMI radiances using a multiwavelength algorithm: Application to western Europe

The Ozone Monitoring Instrument (OMI) multiwavelength algorithm has been developed to retrieve aerosol optical depth using OMI-measured reflectance at the top of the atmosphere. This algorithm was further developed by using surface reflectance data from a field campaign in Cabauw (The Netherlands), a new cloud-screening method, and a global aerosol database derived from the aerosol transport model TM5. The first results from an application of this algorithm over western Europe are presented. The OMIretrieved aerosol optical depth is evaluated by comparison with both ground-based measurements from Aerosol Robotic Network (AERONET) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data. The various aerosol optical depth values compare favorably, except in situations where large changes occur in the surface properties, which is illustrated over the Iberian peninsula. OMI and MODIS aerosol optical depth are well correlated (with a correlation coefficient of 0.66 over land and 0.79 over sea), although the multiwavelength algorithm appears to overestimate the aerosol optical depth values with respect to MODIS. The multiwavelength algorithm performs better over sea than over land. Qualitatively, the multiwavelength algorithm well reproduces the expected spatial aerosol optical depth gradient over western Europe

A new algorithm to determine the spectral aerosol optical depth from satellite radiometer measurements

A new aerosol retrieval algorithm is presented which computes the spectral optical depth over the ocean from spaceborne radiometers. It includes both multiple scattering and the bi-directional reflectance of the ocean surface. The previous termalgorithmnext term is applied to data from the Along Track Scanning Radiometer 2 (ATSR-2). This radiometer aboard the ERS-2 satellite has 4 bands in the visible and near-infrared. The ATSR-2 has a dual view capability: the reflectance is measured both at nadir and at a forward angle of approximately 55° along track. This feature is used to test the algorithm by comparing independent retrievals from the forward and the nadir view, applied to Southern Hemisphere data from 23 July 1995. The retrieved aerosol optical depths compare favorably. The retrieved aerosol optical depths and spectral behavior are in agreement with expected values in clean marine environments.

Atmospheric input of nitrogen into the North Sea: ANICE project overview.

The aim of the atmospheric nitrogen inputs into the coastal ecosystem (ANICE) project is to improve transport-chemistry models that estimate nitrogen deposition to the sea. To achieve this, experimental and modelling work is being conducted which aims to improve understanding of the processes involved in the chemical transformation, transport and deposition of atmospheric nitrogen compounds. Of particular emphasis within ANICE is the influence of coastal zone processes. Both short episodes with high deposition and chronic nitrogen inputs are considered in the project. The improved transport-chemistry models will be used to assess the atmospheric inputs of nitrogen compounds into the European regional seas (the North Sea is studied as a prototype) and evaluate the impact of various emission reduction strategies on the atmospheric nitrogen loads. Assessment of the impact of atmospheric nitrogen on coastal ecosystems will be based on comparisons of phytoplankton nitrogen requirements, other external nitrogen inputs to the ANICE area of interest and the direct nitrogen fluxes provided by ANICE. Selected results from both the experimental and modelling components are presented here. The experimental results show the large spatial and temporal variability in the concentrations of gaseous nitrogen compounds, and their influences on fluxes. Model calculations show the strong variation of both concentrations and gradients of nitric acid at fetches of up to 25km. Aerosol concentrations also show high temporal variability and experimental evidence for the reaction between nitric acid and sea salt aerosol is provided by size-segregated aerosol composition measured at both sides of the North Sea. In several occasions throughout the experimental period, air mass back trajectory analysis showed connected flow between the two sampling sites (the Weybourne Atmospheric Observatory on the North Norfolk coast of the UK and Meetpost Noordwijk, a research tower at 9km off the Dutch coast). Results from the METRAS/SEMA mesoscale chemistry transport model system for one of these cases are presented. Measurements of aerosol and rain chemical composition, using equipment mounted on a commercial ferry, show variations in composition across the North Sea. These measurements have been compared to results obtained with the transport-chemistry model ACDEP which calculates the atmospheric inputs into the whole North Sea area. Finally, the results will be made available for the assessment of the impact of atmospheric nitrogen on coastal ecosystems.

Development of the Mediterranean extinction code (MEDEX)

The performance of electro-optical systems can be substantially affected by aerosol particles that scatter and absorb electromagnetic radiation. The model that is most frequently used for the prediction of aerosols and their effect on extinction in the marine atmosphere is the US Navy Aerosol Model (NAM). However, NAM can be significantly less reliable in coastal areas than on the open ocean. Based on an extensive series of measurements conducted on the island of Inisheer (Irish West Coast), an empirical aerosol model for the coastal zone formulated as an extension of NAM, in which coastal effects are modeled as a function of fetch, has been developed. This work is extended to the Mediterranean using an aerosol dataset recorded on the island of Porquerolles in the Bay of Toulon (France) and has been coupled with the Mie theory to give a code for the extinction, the code MEDiterranean EXtinction (MEDEX).

Deposition of nitrogen into the North Sea

The flux of nitrogen species from the atmosphere into the ocean, with emphasis on coastal waters, was addressed during the ANICE project (Atmospheric Nitrogen Inputs into the Coastal Ecosystem). ANICE focused on quantifying the deposition of atmospheric inputs of inorganic nitrogen compounds (HNO3, NO3-, NH3 and NH4+) into the North Sea and the processes governing this deposition. The Southern North Sea was studied as a prototype. Because the physical and chemical processes are described, as opposed to empirical relations, the results can potentially be transferred to other regional seas like the Mediterranean, the North Atlantic continental shelf area and the Baltic. Two intensive field experiments were undertaken, centred around the offshore tower Meetpost Noordwijk and the Weybourne Atmospheric Observatory in East Anglia (UK). Long-term measurements were made on a ferry sailing between Hamburg and Harwich/Newcastle. These measurements provided data for sensitivity studies of a variety of problems associated with the coastal region that are not easily evaluated with larger scale models, to constrain models and to test model results. Concentrations of nitrogen compounds over the North Sea and the resulting deposition presented in this paper were obtained with the Lagrangian transport-chemistry model ACDEP. The average annual deposition in 1999 was 906kg Nkm-2. The results are compared with experimental data from the ferry. Effects of temporal and spatial variations are evaluated based on experimental results and small-scale model studies. In particular, effects of the aerosol size distribution on the nitrogen deposition are discussed.

New aerosol particle formation in different synoptic situations at Hyytiälä, Southern Finland

We examine the meteorological conditions favourable for new particle formation as a contribution to clarifying the responsible processes. Synoptic weather maps and satellite images over Southern Finland for 2003–2005 were examined, focusing mainly on air mass types, atmospheric frontal passages, and cloudiness. Arctic air masses are most favourable for new aerosol particle formation in the boreal forest. New particle formation tends to occur on days after passage of a cold front and on days without frontal passages. Cloudiness, often associated with frontal passages, decreases the amount of solar radiation, reducing the growth of new particles. When cloud cover exceeds 3–4 octas, particle formation proceeds at a slower rate or does not occur at all. During 2003–2005, the conditions that favour particle formation at Hyytiälä (Arctic air mass, post-cold-frontal passage or no frontal passage and cloudiness less than 3–4 octas) occur on 198 d. On 105 (57%) of those days, new particle formation occurred, indicating that these meteorological conditions alone can favour, but are not sufficient for, new particle formation and growth. In contrast, 53 d (28%) were classified as undefined days; 30 d (15%) were non-event days, where no evidence of increasing particle concentration and growth has been noticed.

Size distributions of giant aerosol particles close above sea level

Abstract Vertical profiles of particle concentrations were measured close to the sea surface with a Rotorod impaction sampler. The reliability of the size distributions determined with this instrument has been discussed. Particle concentrations are in good agreement with the results from other experiments. Discrepancies are attributed to differences in windspeed, relative humidity and water temperature. The different sampling techniques may have been of influence as well.