A Versatile Droplet Sizing Spectrometer for Aerosol Hygroscopic Growth Measurements from Research Aircraft

The uptake of water by ambient aerosols at elevated relative humidity (RH) is ubiquitous and often results in large changes in particle size. Because the scattered light intensity from aerosols varies as the diameter taken to the sixth power, optical properties such as the aerosol extinction coefficient, single scatter albedo and optical depth will have strong particle size dependence. There are currently no viable techniques available to perform rapid size-resolved hygroscopic growth measurements over particle size and RH ranges relevant to determining ambient aerosol optical properties at ambient humidity. The Phase I efforts have focused on establishing the technical feasibility of a new method to measure size-resolved water uptake by aerosols that employs differential mobility analysis (DMA) in-concert with multiple humidification and optical sizing steps. In the technique, monodisperse dry particles selected by a DMA are optically sized by five optical particle counters (OPC), one operating at the same RH as the DMA and the remaining OPCs operating at different controlled RHs so that the particle growth as a function of RH can be observed. The technical objectives of the Phase I included: development of a model of overall instrument response, development of the particle humidification system design, extension of existing thermodynamic models of particle growth, model calculations of the time-dependent growth and evaporation of droplets, implementation and testing of the iterative technique for determining particle dry and wet refractive indices from the simulated OPC response to DMA-selected dry particles, sensitivity studies of the size and refractive index resolution of the OPC, detailed engineering design development of the DMA-OPC systems, and initial bench-top prototyping of the OPC. In this final report of our Phase I work, we first describe the technical details of the proposed OPC design and then present the major results from our
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