On the Development and Setup of a Hygroscopic Tandem Differential Mobility Analyzer for Aerosol Studies

Most particles in the atmosphere take up water when exposed to increasing relative humidity, and this behavior is described and quantized by hygroscopicity parameter. Hygroscopic tandem differential mobility analyzer (HTDMA) can be used to measure the physicochemical properties of aerosol particles and study phenomena that lead to size changes in submicron aerosol particles. (Rader and McMurry, https://doi.org/10.1016/0021-8502(86)90031-5). Evaporation, condensation and chemical reactions within airborne droplets are examples of such phenomena. Specifically, it measures how aerosol particles of different initial dry sizes grow or shrink when exposed to changing relative humidity (RH). HTDMA uses two different mobility analyzers (DMA) and a humidification system to make these measurements. One DMA selects a narrow size range of dry aerosol particles, which are exposed to varying RH conditions in the humidification system. The second humidified DMA scans the particle size distribution after the particles have been conditioned by the humidification system. Scanning a wide range of particle sizes enables the second DMA to measure changes in size or growth factor (growth factor = humidified size/dry size) due to water uptake by the particles. A condensation particle counter (CPC) downstream of the second DMA counts particles as a function of selected size in order to obtain the number size distribution of particles exposed to different RH conditions. To build up a HTDMA design, two DMA was connected, with one humidifier (along with one RH meter) in the second DMA sheath flow, and one humidifier (along with one RH meter) in the particle flow line before aerosol entering the first DMA. The outflow from the second DMA was running in to a CPC, and the data collected from CPC was inputting into software for further analysis on particle size and distribution. Different size of humidifiers (mainly MH-series Nafion tubes) was tested under different flow rates (0.3 lpm, 1.5 lpm, 3 lpm, 15 lpm). The two humidifiers provide the close %RH in particle flow line (0.3 lpm or 1.5 lpm) and in the sheath flow line (3 lpm or 15 lpm) were used in the design. The HTDMA design was tested running through pure ammonium New HTDMA software was also developed to analyze the data on hygroscopicity. Instead of using a manually selected mode, the new software will autodetect peaks and it fits a log-normal distribution producing a more reliable average particle size.