AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Accurate Determination of Aerosol Activity Coefficients at Relative Humidities up to 99% Using the Hygroscopicity Tandem Differential Mobility Analyzer Technique
SARAH SUDA, Markus Petters, Timothy Wright, North Carolina State University
Abstract Number: 328 Working Group: Instrumentation and Methods
Abstract Aerosol water content plays an important role in aqueous phase reactions, in controlling visibility, and in cloud formation processes. The total amount of water bound in the particle phase depends on the aerosol hygroscopicity and the ambient relative humidity (RH). One way to quantify aerosol water uptake is to measure hygroscopic growth using the Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) technique, which uses two differential mobility analyzers (DMAs) to determine particle size before and after humidification at a known RH. However, the HTDMA technique becomes less reliable at RH > 90% due to the difficulty of controlling temperature and RH inside the second DMA. For this study we have designed and implemented a new HTDMA system with improved temperature and RH control. Temperature stability in the second DMA was achieved to ±0.02 degree C tolerance by implementing active control using four thermoelectric heat exchangers and PID control loops. The DMA size resolution was increased by operating high-flow DMA columns at a sheath-to-aerosol flow ratio of 30:1. This improved size resolution allowed for improving the accuracy of the RH sensors by interspersing ammonium sulfate reference scans at high frequency. We present growth factor data for pure compounds at RH up to 99% and compare the data to theoretical values and to available bulk water activity data. With this HTDMA instrument and method, the osmotic coefficients of spherical, non-volatile aerosols of known composition between 30 and 500 nm in diameter can be determined within ±20%. We expect that data from this instrument will lead to an improvement of aerosol water content models by contributing to the understanding of aerosol water uptake at high RH.