This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad.
This award will support an eighteen-month research fellowship by Dr. Matthew J. McGrath to work with Dr. Markku Kulmala at the University of Helsinki in Finland.
Gazing up into puffy white clouds on a summer day often causes children to ask their parents, ?How do clouds form?? This simple question has proven to be very difficult for the world's scientists to answer. The general steps of the process have been known for quite some time: individual molecules combine to form thermodynamically stable clusters, which in turn activate to aerosols and finally grow into cloud condensation nuclei. These nuclei are what gather in the atmosphere to form clouds, or crowd together close to the earth's surface to give fog. Regardless of their final destination, the formation process always appears to go through these three steps. This means that the formation of atmospheric aerosols, which have been demonstrated to negatively impact human health in urban areas as well as lower temperatures in marine environments, must involve the nucleation of individual molecules to form stable clusters as well. Experimental observations have provided most of the current data on atmospheric aerosols, recording the size and concentrations of clusters concurrently with the concentrations of various atmospheric gases (such as sulphuric acid, ammonia, and ozone). This has given important insights into the composition and sizes of the critical clusters involved. The critical cluster is the size of cluster that the process must form before stability is reached; knowledge of the critical cluster is essential to determine the likelihood that a given process is important to the formation of the larger nuclei. Unfortunately, experimental apparatuses have only recently become sensitive enough to probe the time and length scales present in atmospheric nucleation phenomena, which means there is a current lack of understanding in these critical regimes. Furthermore, these new advances only indicate that clusters are present of this size, not if they are critical nuclei. Particle-based molecular simulation provides a tool to ameliorate this problem. Particle-based molecular simulation is a technique that employs computers to predict observable properties of systems by considering the individual interactions between molecules. This method becomes an important complement to experimental observation when the conditions of the experiment are difficult (for example, high temperatures and pressures, or short time-scales) or dangerous (toxic or explosive chemicals). The small and fast events occurring in atmospheric nucleation therefore pose no problem for molecular simulation techniques, and indeed several studies on the formation and composition of atmospheric clusters have already been performed. The current challenge facing the field is the computational accuracy and efficiency of these methods.
This proposal addresses the concern of simulation accuracy and efficiency by combining multiple existing techniques to probe these systems. Experimental observations suggest that the most important atmospheric aerosol system consists not just of water but water, ammonia, and sulphuric acid together. The largest problem with this is sulphuric acid which, due to its reactive nature, is difficult to study via classical methods, while quantum mechanical methods are still too computationally demanding. This proposal combines a proven simulation method for reactive systems (the reactive ensemble method) with the standard algorithms used in the study of nucleation phenomena (umbrella sampling with histogram reweighting together with configurational-bias Monte Carlo), which will permit the first complete mapping of the nucleation barrier and critical cluster for the ternary water/ammonia/acid system. An additional step consisting of using the reactive ensemble method to help determine the simulation parameters for sulphuric acid could lead to the development of simpler reactive force fields, thus facilitating the use of Monte Carlo simulation in the study of reactive systems. The formation of atmospheric aerosols (and, by extension, cloud condensation nuclei) is a process that is well understood in general terms but poorly understood in details, despite the importance of these clusters in climatic events and temperature regulation, as well as their impact on human health. Particle-based molecular simulation provides a tool capable of providing insight into this process on length and time-scales inaccessible to current experimental techniques. Current simulation methods, however, lack the necessary features to probe the complete atmospherically relevant system. This proposal introduces a novel combination of existing techniques which, along with appropriately determined simulation parameters, reveals the size and composition of the critical nucleus, data that is integral to demonstrating exactly how this process occurs.
