| Περίληψη: | Atmospheric aerosols, also known as atmospheric particulate matter (PM), are suspended particles (solid or liquid) in air, with diameters ranging from 1 nm to approximately 100 μm. PM affects the Earth’s radiative budget and thus global climate, via its so-called direct and indirect radiative effects, and also has adverse effects on human health. It can be classified as primary (emitted directly in the particulate phase) or secondary (formed in the atmosphere through a series of chemical reactions). Typically, PM consists of a mixture of inorganic and organic chemical species including nitrate, sulfate, ammonium, organic material, elemental carbon, sea salt, crustal species and water. Organic aerosol (OA) represents a significant fraction of the mass sub-micrometer PM, but its sources and chemical composition are yet to be elucidated. Black carbon (BC) is the other carbonaceous PM component and is emitted by incomplete combustion processes (biomass burning, traffic, etc.). BC contributes to global warming because it absorbs sunlight and is one of the most toxic PM constituents.
Particles smaller than 100 nm are defined as nanoparticles or ultrafine particles. These particles have low mass, high number concentrations and if they manage to survive coagulation with larger particles they can become cloud condensation nuclei (CCN) affecting the cloud droplet number. The change of CCN concentration affects cloud optical properties and lifetime, perturbing the energy balance of the planet. The role of condensation of organic vapors and the chemical aging on formation and growth on ultrafine particles are still unknown.
The first objective of the present work was to extend the state-of-the-art 3-D chemical transport model PMCAMx-UF to quantify the effects of organic pollutants on the nanoparticle concentrations in our atmosphere. PMCAMx-UF was applied in Europe and its predictions were evaluated against field observations from stations across Europe and a Zeppelin. The model performed well both at the ground and aloft with a tendency to overestimate the total particle number concentration. The condensation of organics led to an increase (50-120%) of the CCN concentration mainly in central and northern Europe, but decreased the concentration of particles larger than 10 nm by 10-30%. The performance of the model was improved further when later generations of reactions of organic pollutants (chemical aging) were simulated.
Finally, a new model was developed which simulates not only the BC size distribution and concentration but also its mixing state in the atmosphere. The model simulates multiple particle size distributions with different proportions of BC for each aerosol population to account for the multidimensional structure of the size/composition of ambient aerosol. Simulations show that urban traffic-related and ship emissions can significantly change the mixing state of aerosols from a fully internally mixed state to an external-like mixture with multiple levels of mixing states predicted during a week-long simulation. We introduce a new mixing state index, Φ, which can range zero (external mixture) to one (internal mixture). Φ varied over Europe from 0.2 to 0.6 showing the complicated effect of fresh emissions, coagulation, condensation and photochemistry on the mixing state of BC-containing aerosol. These effects are not included in existing air quality and climate models.
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