.

NATAIR - seas

NATAIR project aimed to improve existing methodologies for quantifying natural and biogenic atmospheric emissions in Europe. Among the different applied studies, two of them tried to better estimate atmospheric pollutants in European seas.

First, a model was applied for determining marine aerosol profiles. Aerosol fluxes from the sea strongly affect atmospheric processes, scattering solar radiation and being responsible of cloud condensation nuclei. They are rich in halogens, which can react to affect ozone depletion and destroy light hydrocarbons, SO2 and NOx. Depending on the wind speed, they can contribute to increase particulate matter (PM) content of inner zones.

This model made use of a Geographic Information System (GIS), and was adapted for the estimation of particulate matter lower than 10 µm (PM10) and 2.5 µm (PM2.5). The developed model considered two mechanisms for generating aerosols: an indirect emission due to bubble formation, and a direct mechanism where aerosols are generated through spumes. Both processes were modelled in form of semi-empirical formulations relating size-segregated surface emissions rates and wind speed. Total aerosol generation was calculated as the sum of direct and indirect generation. These equations were scripted in the GIS, enabling calculations from spatially disaggregated data; the model was carried out for eight different particle size ranges, from 0 to 1.25 µm. In addition, the model covered five maritime regions (Atlantic Ocean, Mediterranean sea, Caspian Sea, Black Sea and Baltic Sea) with applying a spatial resolution of 10x10 km. The main input for running the model was wind speed data at 10m altitude, which were obtained from meteorological datasets. Main outputs of the project were expressed as number of particles emitted from each 10x10 km square.

Several emissions maps were obtained for years 1997, 2000, 2001 and 2003. North Atlantic Ocean north of 50º latitude was shown to have the highest emissions. In zones to the east of Iceland and United Kingdom, emissions were lower due to a shelter effect of these islands against predominant winds. The same effect was observed in Baltic sea. In the Mediterranean Sea, the lowest emissions were found along the coast. Time series of emissions showed big variations, not only due to seasonal changes but also in the short-term. The Atlantic Ocean had the greatest total emissions, followed by the Baltic Sea. These emissions were known to be relatively uncertain, but they aggregate with other marine aerosol estimations. Some recommendations for improving the robustness of the model were made.

A second study dealt with the emissions of dimethylsulfide (DMS) from the ocean at European scale. This compound is created in surface waters, and is a precursor of SO2 and methane sulphonic acid (MSA). Since SO2 can be further oxidized leading to sulphates that act as condensation nuclei, it has an essential role not only in the sulphur cycle but also in climate. DMS flux is similar to the case of particular matter: it can be modelled as a function of wind turbulence and DMS chemical potential (relative concentration difference between sea surface and air interface). Wind turbulence is hard to quantify. Therefore, the concept of “piston velocity” (the velocity when gas diffuses across the air-sea interface in the stagnant film model) was applied for expressing turbulence. The chosen formulation expressed the flux of DMS as directly proportional to piston velocity, and to the difference between DMS concentration in water and in air (this last value was neglected). Moreover, piston velocity was modelled through three formulations for different speed regimes.

Like for estimating particulate matter emissions, , five different water bodies were considered for estimating DMS emissions in Europe, and a spatial resolution of 10x10 km was fixed. DMS water concentrations were obtained from existing databases, applying data from the Eastern Mediterranean Sea for the Black Sea as datasets were not available.

Like in the aerosols study, emissions in years 1997, 2000, 2001 and 2003 were estimated through this model. Results showed that the greatest DMS concentrations have been obtained in the Atlantic Ocean, where a great seasonality was observed. These seasonal variations are in line with observations from the literature, but estimated DMS fluxes were lower than global DMS fluxes obtained from other works, mainly due to sheltering phenomena. Some uncertainties were identified in the model, for example difficulties in modelling piston velocity and uncertainties for determining DMS concentrations or wind speeds.

 

Source:
Grice, S.; Sturman, J.; Dore, Ch. and Woodfield, M. (2008): Modelling emissions of marine aerosol from the ocean at a European scale.
Grice, S.; Sturman, J.; Dore, Ch. and Woodfield, M. (2008): Modelling emissions of DMS from the ocean, at a European scale.