A New Methodology to Assess the Water Uptake by Organic Aerosols

NAGENDRA RAPARTHI, Ann M. Dillner, Anthony S. Wexler, Air Quality Research Center, UC Davis

     Abstract Number: 40
     Working Group: Carbonaceous Aerosols

Abstract
Atmospheric aerosols often consist of both organic and inorganic compounds with varying physicochemical properties. Historically, the hygroscopic characteristics of cloud condensation nuclei (CCN) were primarily influenced by inorganic compounds such as nitrates, sulfates, and chlorides. However, with the implementation of emissions controls that have successfully reduced nitrogen and sulfur oxide emissions, there has been a notable shift towards the organic fraction of aerosols assuming a more prominent role. Nevertheless, the organic fraction is considerably more complex than its inorganic counterpart, comprising thousands of individual compounds originating from diverse sources and reaction pathways, each possessing distinct physical and chemical properties. This complexity often poses challenges in establishing a clear correlation between the organic fraction and their hygroscopicity. In this study, we have developed a novel methodology to assess the water uptake of organic aerosols traditionally collected on Teflon filters, thereby providing an opportunity to link the assessed hygroscopicity with organic functional groups (OFGs). Hygroscopic measurements were conducted in the laboratory for Ammonium Sulfate, Sodium Chloride, Glucose, and Malonic Acid, which were collected on 25mm Teflon filters using an Aerosol Generator and Sampler (AGS). Constant Humidity Solutions (CHS) were employed to maintain the Relative Humidity (RH) at 84%, 90%, and 97% in confined chambers. The results are consistent with those reported by the E-AIM model and previous studies utilizing Humidified Tandem Differential Mobility Analyzer (HTDMA) and Electrodynamic Balance (EDB), highlighting the accuracy of this new methodology. This new approach enables the hygroscopicity of filter samples to be assessed along with the chemical composition of the filter samples. Applying this methodology to field and smog chamber samples, enables the linking of their hygroscopicity to composition and OFGs quantified through Fourier Transform Infrared (FT-IR) spectroscopy.