Rapid Assessment of Particle Hygroscopic Growth

One of the key parameters affecting the role of atmospheric aerosols in global climate is aerosol hygroscopicity, which describes the water uptake of aerosol particles. Hygroscopicity affects the scattering of light by individual particles; it is important to heterogeneous chemical transformations; and it plays a significant role in the formation and microphysical properties of clouds, all of which are important to global climate. Current measurement approaches are slow and tedious, requiring many minutes to assess the size change for a single particle size and humidity setting, and many hours to measure the entire water-uptake growth curve. This slow response makes such measurements prohibitive on aircraft platforms, and more generally limits the measurements to smaller particle size range, where number concentrations are higher, and scans are faster. How this Problem is Being Addressed We propose the near-instantaneous measurement of particle hygroscopicity through a relative-humidity controlled Aerosol Mobility Imaging system. A basic Aerosol Mobility Imaging system combines the Fast Imaging Mobility Spectrometer developed at Brookhaven National Laboratory with the laminar flow water condensation methodologies developed at ADI. Developed with DOE STTR support, the Aerosol Mobility Imaging system separates, enlarges and images individual particles to provide a complete mobility particle size distributions measurements over a wide size range (e.g. 10 – 400 nm) with 1-second resolution. With the addition of relative humidity control, our RH-AMI will provide rapid measurement of the entire hygroscopic growth curve. What is to be done in Phase I We will develop a humidity scanning system that will facilitate near real-time measurement of particle growth as a function of relative humidity and size. Operated with an upstream mobility size selection device, our proposed system will provide the complete spectrum of particle size change at a selected relative humidity within seconds. Further, by capitalizing on the two-dimensional mobility separation concept of the wide-range instrument, we will be able to make simultaneous measurements at more than one relative humidity value, thereby always providing a reference, or possibly a complete humidity spectrum at once. We estimate that size-resolved hygroscopicity for dry particle sizes from 10 nm to 400 nm can be obtained every few minutes. Commercial Applications and Other Benefits Commercial applications span a wide range of atmospheric aerosol research uses for which rapid measurement of aerosol hygroscopicity are needed. It will allow scientists to better assess the magnitude of airborne particles’ impact on radiation, cloud formation, and global climate, and hence provide the information needed to better cope with a changing climate.