Michael Boy - Formation of Secondary Organic Aerosols
New particle formation has been observed at almost all sites where both particle number concentrations and size distributions have been measured: however, many questions remain regarding the extent to which these secondary aerosols influence radiative properties, climate and human health. My research is focused on the developing and improving of numerical codes to investigate the formation (nucleation) and growth of secondary organic aerosols and their impact on our climate and environment.
The role of atmospheric aerosols is perhaps the biggest unknown concerning our climate and greenhouse warming. Ten to twenty years ago, aerosol mass (PM2.5 and PM10) and ozone were the main parameters considered by politicians regarding decisions for environmental regulations. During the last decade, investigators began to recognize the importance of ultrafine particles (D p < 1 µm) in the atmospheric radiation budget and the deleterious effects of such particles on human health, the number concentration of aerosols and the in situ formation of secondary aerosols by physical and chemical processes within the atmosphere achieved higher priority.
Although many field campaigns, laboratory experiments and new modelling approaches have led to increased understanding, detailed mechanisms responsible for the formation of new particles in the troposphere and their influence on health, environment and climate have still not been completely elucidated. In MALTE (Model to predict new Aerosol formation in the Lower Troposphere) individually developed codes from different institutes around the globe merged into a one-dimensional model including aerosol dynamics, boundary layer meteorology, biology and chemistry in order to investigate the formation and growth processes of Secondary Organic Aerosols (SOA) under realistic atmospheric conditions.
MALTE was developed at the University of Helsinki, Finland and at the National Center for Atmospheric Research (NCAR) in Boulder , Colorado , USA . The first and most important goal is to improve our knowledge of all processes involved in the formation of new particles by developing and testing different computer codes under realistic atmospheric conditions. At this stage MALTE takes several hypothetical pathways into account and currently includes six different nucleation theories for the formation of secondary aerosols. The question of which molecules are involved in the atmospheric nucleation processes remains controversial within the aerosol community. Up today model simulations with MALTE provide evidence that sulphuric acid and certain organic vapours are involved in the nucleation mechanism (s) and the use of laboratory and field experiments data will hopefully clarify the mysterious picture in the near future.
Newly formed clusters in size ranges between 1-2 nm grow by condensation of molecules with very low saturation vapour pressure. Later, due to the decreasing impact of the Kelvin effect, semi-volatile organic compounds participate in the condensation and the growth of the particles. Our insufficient understanding of the importance of condensable organic species arises both from the difficulty of quantifying the emissions of higher terpenes (e.g., highly reactive mono- and sesquiterpenes) , unresolved chemical oxidation pathways for these chemical species in the gas phase, an unknown number of unidentified species emitted form the biosphere and the mostly undiscovered chemical reactions in the particle phase (Figure 1) . MALTE includes codes to solve or estimates most of the described processes; however a final detailed solution for the whole SOA formation mechanism is not within the range of vision.
Figure 1: Sketch for SOA formation
ASP Spotlight January 2007
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