Aarhus University Seal / Aarhus Universitets segl

MSc defence: Ida A Rosendahl

Field studies of sea spray particle fluxes

2016.10.24 | Susanna Pakkasmaa

Date Mon 07 Nov
Time 14:15 15:45
Location Aarhus University, Auditorium D4 (1531-219)

The experimental setup for the Risø field campaign showing the two particle counters places in the Zarges boxes. Photo: Ida A Rosendahl.

Determination of sea spray particle fluxes through field measurement campaigns is important in order to minimize uncertainties on model parameterizations and to investigate the role of sea spray in air-sea exchange of gaseous species.

Ida A Rosendahl presents sea spray particle flux estimates from two field campaigns in Risø and Nuuk as well as preliminary results from a field campaign onboard CCGS Amundsen in the Canadian Arctic.

The aim of her work is to test an alternative method – Cospectral Peak method – to quantify sea spray particle flux. Changes in the CO2 uptake by the oceans leads to significant chemical alterations of the marine environment, which makes the determination of the sea spray particle flux important.

Problems with obtaining reliable flux estimates from field measurements on sea spray particles can occur as a consequence of the low sampling frequency, 1 Hz, for most particle instruments. This makes flux estimates through the most widely used method, the Eddy Covariance method, less reliable. In this thesis, a less well-known method, the Cospectral Peak (CSP) method, will be used to infer sea spray particle flux estimates. This method relies on lower frequencies to produce the flux estimates, why it may prove to be a better choice for analyzing datasets with low frequency samplings.

Previous studies have shown that the momentum flux can be reliably resolved with the CSP method, for which reason these are calculated to test the validity of the CSP calculations. Comparison of 1 Hz CSP momentum flux estimates with 10 Hz CSP estimates showed, that only very little difference occurred. Also, comparisons of momentum flux results from CSP and EC method showed overall good correlations, with the largest disagreements occurring for the most negative 1 Hz flux estimates, which tended to be underestimated using the CSP method relative to EC. Comparison of 1 Hz particle flux estimates from CSP with EC estimates showed less correlation, but the fluxes were comparable in magnitude. Substantial parts of the half-hourly estimates of the particle cospectra calculated for CSP analysis showed deviation from the expected and thus was not used for analysis.

Suggestions for future field measurement procedures include size resolution of the particles to separate impacts of large particles subjected to rapid gravitational settling from smaller particles with long atmospheric residence times. Based on the particle flux results, it is concluded, that CSP method may be a choice for estimating particle fluxes from data sets with low sampling frequencies of 1 Hz and possibly also from moving platforms.

Sea spray particle fluxes are mostly dealt with by geophysicists, and also in this thesis, some geophysical theory is introduced in terms of micrometeorology, because it is important framework to understand the fluxes. There are though very good reasons why chemists should also take an interest in this subject, since the chemistry related to sea spray particles is very important in determining the impact, the sea spray flux has on the climate.

The presence of particles in Arctic environments in the spring determines how effective the ozone depletion due to spring time bromine burst is. Also, sea spray particles are suggested to participate in air-sea exchange of gases in Arctic environments. This includes CO2 and implies, that sea spray is one of the factors determining the flux of CO2. Changes in the CO2 uptake by the oceans leads to significant chemical alterations of the marine environment, which makes the determination of the sea spray particle flux important. To evaluate the chemical consequences of sea spray particle flux magnitudes, it is important that methods for correctly quantifying the flux is established. By testing the CSP method on sea spray particle fluxes, is the aim of this thesis to contribute to this work.

Arctic Research Centre