Abstract of PhD Thesis

Modelling the Role of Clouds in PM2.5 Formation

Robert Flanagan, National University of Ireland, Galway (2008)

This thesis presents the results of measurements and modelling of marine aerosols. Eddy correlation measurements of aerosol fluxes, for sizes between 10 nm and 1000 nm, show a strong correlation with wind speed. Micrometeorological parameters and footprint model calculations have shown that the 22m tower is a suitable location to measure fluxes which are representative of the open ocean. These measurements represent the first pseudo-size-segregated aerosol flux measurements indicating a significant contribution in the Aitken mode (50% of the sub-micron fluxes). Fluxes of this magnitude can readily contribute significantly to background marine aerosol concentrations and should be considered as important as secondary formation rates over the oceans. Fluxes of new secondary aerosol particles during nucleation events in the coastal environment are reported for the first time. Events are divided into two types based on the prevailing wind direction. During tidal related nucleation events, new particle fluxes are typically of the order of 109-1010 particles m‐2s‐1 with both upward and downward fluxes being of similar magnitude. A strong correlation (r2=0.86) was found between total particle number concentration and total (positive) number flux when air masses were not affected by multiple sources suggesting that continuous measurements of particle number concentration at Mace Head can be converted into a source flux.

An aerosol-cloud process model has been developed for the study of aerosol-cloud interactions, and in particular in-cloud heterogeneous sulphate production. A mass and heat flux module has been incorporated in the model to simulate non-equilibrium growth. The model has also been adapted to enable the simulation of the atmospherically important nitric, acetic and formic acids. Model studies of that less than a ppb of nitric acid dramatically reduces in-cloud heterogeneous sulphate production on an externally mixed sea-salt sulphate aerosol population in typical winter time North Atlantic conditions. The reduction in the pH of the solution droplets is the primary reason for this decrease. Simulations have also been performed to study the impact of acetic and formic acid. These acids had a similar effect as nitric acid but the magnitude of the decrease in sulphate production was not as large. Background levels of ozone have been increasing by approximately 5 ppb/decade. Model simulations have been performed to quantify the effect of this increase on sulphate production. The largest increase was seen for sulphate aerosol on its own. When externally mixed sea-salt and sulphate was simulated the increase in sulphate production due to the increase in ozone was not as significant as ozone is not the main limiting factor in this scenario. Studies have also been performed which simulate increased sulphate production downwind of a coal burning power station. Increases of up to 230% relative to clean conditions were observed.