The aerosol indirect radiative forcing (IRF), largely determined by the concentrations of cloud condensation nuclei (CCN), is a major source of uncertainty in assessing climate change. Recent global aerosol modeling studies show that tropospheric CCN abundance and aerosol IRF are sensitive to the choice of nucleation mechanism, and emphasized the critical importance of further improvement of the representation of aerosol nucleation and growth processes in global models. This project seeks to advance the understanding and reduce uncertainties of atmospheric particle formation and growth processes via comprehensive data analyses, detailed case studies, sub-grid nucleation modeling and parameterizations, global aerosol modeling, and comparisons of model results with measurements. In order to achieve these project goals, the following objectives/research tasks have been identified: (1) In-depth case studies and analyses of nucleation observations to resolve controversies over the relative importance of ion-mediated nucleation (IMN) and neutral/homogeneous nucleation in boreal forests. An IMN model will be further assessed and improved through extended case studies. (2) Investigate the linear or square dependences of nucleation rates derived from field measurements on the sulfuric acid vapor concentrations, and ascertain whether such dependences imply new nucleation mechanisms. (3) Analyze extensive datasets of particle properties observed around the globe, and use them to assess global aerosol simulations, with the primary focus on the ability of different nucleation mechanisms to capture the spatial and temporal variations of tropospheric particle number concentrations. (4) Study particle formation and growth in sub-grid anthropogenic SO2 plumes, in order to improve the representation of the plume-scale processes in global aerosol models, and to reduce uncertainties in simulated CCN concentrations.(5) Apply an improved global aerosol model for investigating sources of CCN in different tropospheric regions and their seasonal variations. Predicted CCN concentrations will be compared with measurements and possible uncertainties will be assessed and addressed.
This project will provide educational opportunities and research training for two graduate students and a short-term visiting student or scholar. Two post-graduate researchers will be given specialized experiences to broaden their knowledge and provide opportunities for the future employment. This project will also provide research experience and training for SUNY Albany undergraduate students who are interested in aerosol and atmospheric research. This project will contribute to on-going efforts to quantify the impacts of aerosols on the Earth's climate. It will provide support for updated nucleation rate look-up tables that have been made available to the scientific community and for the improvement of online nucleation rate calculators, which can be used for both research and educational purposes. Parameterizations of sub-grid sulfate particle formation derived within this project will be made available to other investigators working on regional and global aerosol modeling.
The aerosol indirect radiative forcing (IRF), largely determined by the concentrations of cloud condensation nuclei (CCN), is a major source of uncertainty in assessing climate change. Recent global aerosol modeling studies show that tropospheric CCN abundance and aerosol IRF are sensitive to the choice of nucleation mechanism, and emphasized the critical importance of further improvement of the representation of aerosol nucleation and growth processes in global models. This project seeks to advance the understanding and reduce uncertainties of atmospheric particle formation and growth processes via comprehensive data analyses, detailed case studies, sub-grid nucleation modeling and parameterizations, global aerosol modeling, and comparisons of model results with measurements. Intellectual merit: The investigations carried under this project have improved our understanding of particle nucleation and growth processes and the representation of these processes in global models, which are critical for reducing uncertainties in the assessment of the aerosol IRF and climate change. The research results have been reported in about 30 peer-reviewed journal papers. Major findings are summarized here. (1) Particle formation mechanisms: in-depth nucleation case studies and global evaluations. A key source of differences in the interpretation of nucleation data from a boreal forest site (Hyytiälä, Finland), with regard to the relative contribution of neutral versus ion-mediated nucleation (IMN), appears to be related to the size-dependent evolution of ionic and neutral clusters < 2 nm. Previous analysis assuming all neutral 2 nm particles are from neutral nucleation might have underestimated the contribution of IMN. Systematic analysis of the dependence of IMN rate (JIMN) on key controlling parameters reveals profound and non-linear impacts of sulfuric acid vapor concentration, temperature, relative humidity, ionization rate, and surface area of pre-existing particles on JIMN. Both binary and ion-mediated nucleation parameterizations developed under this project have been used in a number of global and regional modeling studies and compared with other nucleation schemes, showing that IMN is an importance source of atmospheric particles and IMN scheme can reasonably account for both absolute values and spatial distributions of particle number concentrations in the whole troposphere. (2) Particle formation in the sub-grid SO2 plume and engine exhaust. The CCN concentrations predicted by global models are sensitive to how the particle formation in sub-grid anthropogenic SO2 plumes is parameterized. Detailed modeling studies reveal significant diurnal and seasonal variations of sulfate particle formation in the anthropogenic SO2 plume, mainly associated with the corresponding variations of two key parameters − hydroxyl radical concentration ([OH]) and temperature. A physics and chemistry based 0D gas parcel model with detailed fuel chemistry and soot microphysics has been developed to predict the time evolution, formation history, and emission index of key combustion gases and size-resolved soot particles of a jet engine and a diesel engine. (3) Molecular-scale understanding of particle formation processes. Based on new thermochemical data for binary and ternary clusters obtained through quantum-chemistry study, ammonia, amines, and certain organic acids can interact with sulfuric acid clusters but their influence on the stability of clusters varies with cluster compositions and requires further investigation. (4) Global modeling of aerosol formation, growth, optical properties, and radiative forcings. The advanced particle microphysics (APM) model in GEOS-Chem has been improved and expanded. Oxidation aging and kinetic condensation of secondary organics gases significantly increase the aerosol growth rate and CCN concentrations, and improve the agreement of measured and simulated mass concentrations of secondary organic aerosol. IMN reproduces well the observed seasonal variations of particle number concentrations around the coast of Antarctica and suggests that future global warming may inhibit new particle formation. GEOS-Chem/APM aerosol microphysics, optical properties, and radiative forcings agree well with observations and AeroCom (Aerosol Comparisons between Observations and Models) results. GEOS-Chem/APM predicts anthropogenic aerosol direct and first indirect radiative forcing of -0.41 W/m2 and -0.75 W/m2, respectively. Broad impacts: (a) Improved our understanding of atmospheric particle nucleation mechanisms and the ability of global model in simulating size-resolved particle microphysics processes which is critical in assessing the influences of aerosols on climate and climate variability. (b) IMN parameterization further refined under this project has been made available to the community. (c) APM model further improved under this project is released to the public with the GEOS-Chem standard version. (d) GEOS-Chem/APM results contribute to various AeroCom activities. Some of these AerCom results were used in 2013 IPCC Climate Change assessment report. (e) Educational opportunities and research training provided for graduate students and post-docs. Two postgraduate researcher and one research technician have been given specialized experiences to broaden their knowledge and opportunities for the future employment.
