AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Aging of Secondary Organic Aerosol from Small Aromatic VOCs: Changes in Chemical Composition, Mass Yield, Volatility and Hygroscopicity
LEA HILDEBRANDT RUIZ, Andrea Paciga, Benjamin Murphy, Kate Cerully, Athanasios Nenes, Neil Donahue, Spyros Pandis, Carnegie Mellon University
Abstract Number: 99 Working Group: Aerosol Chemistry
Abstract Secondary organic aerosol (SOA) undergoes additional reactions in the atmosphere after its initial formation, changing its chemical and physical characteristics. This aging of SOA is currently not well understood and poorly represented in chemical transport models (CTMs). We conducted laboratory chamber experiments in which we formed and aged SOA from toluene and other small aromatic VOCs under different oxidizing conditions. We measured the amount of OA formed and its oxidation state (approximated by elemental ratios O:C and H:C) using a high resolution time-of-flight aerosol mass spectrometer (AMS) from Aerodyne, Inc., the aerosol volatility using a home-built thermodenuder system, and the hygroscopicity of the denuded and non-denuded OA using a cloud condensation nuclei counter from Droplet Measurement Technologies.
Under stronger oxidizing conditions the mass yield and oxidation state of organic aerosol is higher and its volatility is lower. The O:C ratio in the OA formed is close to 1, suggesting that LV-OOA observed in the atmosphere could partly be due to aged SOA from aromatic VOCs. Approximately 15 percent of the OA mass formed in these high-NOx experiments is due to NO and NO2 fragments in the AMS which originate from organic nitrate, suggesting that organic nitrates could compose approximately 50 percent of the OA mass in these experiments. The organic nitrates are more volatile than the rest of the OA. Using an evaporation model to quantify changes in the volatility we find that the OA formed under the different conditions differs in volatility by at least an order of magnitude. The hygroscopicity of the OA changes under the different conditions but is correlated with O:C.
We use these data to constrain SOA formation and transformation in a box model of our chamber experiments, and we will update mass yields and chemical transformation mechanisms in our CTM PMCAMx.