Modeling Aerosol - Water Interactions in Subsaturated and Supersaturated Environments

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Fountoukis, Christos
Nenes, Athanasios
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The current dissertation is motivated by the need for an improved understanding of aerosol water interactions both in subsaturated and supersaturated atmospheric conditions with a strong emphasis on air pollution and climate change modeling. A cloud droplet formation parameterization was developed to i) predict droplet formation from a lognormal representation of aerosol size distribution and composition, and, ii) include a size-dependant mass transfer coefficient for the growth of water droplets which explicitly accounts for the impact of organics on droplet growth kinetics. The parameterization unravels most of the physics of droplet formation and is in remarkable agreement with detailed numerical parcel model simulations, even for low values of the accommodation coefficient. The parameterization offers a much needed rigorous and computationally inexpensive framework for directly linking complex chemical effects on aerosol activation in global climate models. The new aerosol activation parameterization was also tested against observations from highly polluted clouds (within the vicinity of power plant plumes). Remarkable closure was achieved (much less than the 20% measurement uncertainty). The error in predicted cloud droplet concentration was mostly sensitive to updraft velocity. Optimal closure is obtained if the water vapor uptake coefficient is equal to 0.06. These findings can serve as much needed constraints in modeling of aerosol-cloud interactions in the North America. Aerosol water interactions in ambient relative humidities less than 100% were studied using a thermodynamic equilibrium model for inorganic aerosol and a three dimensional air quality model. We developed a new thermodynamic equilibrium model, ISORROPIA-II, which predicts the partitioning of semi-volatiles and the phase state of K+/Ca2+/Mg2+/NH4+/Na+/SO42-/NO3-/Cl-/H2O aerosols. A comprehensive evaluation of its performance was conducted against the thermodynamic module SCAPE2 over a wide range of atmospherically relevant conditions. Based on its computational rigor and performance, ISORROPIA-II appears to be a highly attractive alternative for use in large scale air quality and atmospheric transport models. The new equilibrium model was also used to thermodynamically characterize aerosols measured at a highly polluted area. In the ammonia-rich environment of Mexico City, nitrate and chloride primarily partition in the aerosol phase with a 20-min equilibrium timescale; PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. When RH is below 50%, predictions improve substantially if the aerosol follows a deliquescent behavior. The impact of including crustal species (Ca2+, K+, M2+) in equilibrium calculations within a three dimensional air quality model was also studied. A significant change in aerosol water (-19.8%) and ammonium (-27.5%) concentrations was predicted when crustals are explicitly included in the calculations even though they contributed, on average, only a few percent of the total PM2.5 mass, highlighting the need for comprehensive thermodynamic calculations in the presence of dust.
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