A time series of ultra-fine particles in a forest in the Ohio River Valley will be acquired and analyzed to quantify the frequency and characteristics of nucleation events and high ultra-fine particle concentrations, including chemical composition, principal mechanisms of nucleation, and limitations on nucleation and growth. Current numerical models will be assessed for their ability to predict the occurrence of nucleation and subsequent particle growth. Specific research hypotheses to be tested include: 1) Despite a relatively high average condensational sink, nucleation events will be observed on at least 10% of days, with the highest number occurring in spring; 2) The spatial extent of events characterized by high ultra-fine particle concentrations is of the order of tens to hundreds of kilometers; 3) Nucleation occurs above the forest canopy and the resulting particles are subsequently transported into and through the canopy; 4) The principal mechanism of nucleation at this location is ternary nucleation of H2SO4-H2O-NH3 (sulfuric acid, water, ammonia); 5) Subsequent particle growth is principally by addition of semi-volatile organics formed from oxidation of biogenic and anthropogenic organic compounds; 6) The major physical controls on nucleation are existing particle surface area (as manifest through the condensational sink) and UV radiative flux; and 7) The major chemical control on nucleation is NH3 availability.
Broader scientific impacts of the research include comparative analyses with other long-term data sets to quantify commonalities and differences, and information necessary to validate and develop predictive models of nucleation in the atmospheric boundary layer for use in global atmospheric chemistry and climate models. The broader impacts also include education and training of graduate and undergraduate students.
