Gas-particle partitioning of organic acids during the Southern Oxidant and Aerosol Study (SOAS): measurements and modeling

Wednesday, 17 December 2014
Samantha Thompson1, Reddy Yatavelli1, Harald Stark2, Joel Kimmel3, Jordan Krechmer2, Douglas A Day4, Gabriel A Isaacman5, Allen H Goldstein5, Md Anwar H Khan6, Rupert Holzinger7, Felipe Lopez-Hilfiker8, Claudia Mohr8, Joel A Thornton9, John Toulson Jayne10, Douglas R Worsnop10 and Jose L Jimenez2, (1)CIRES, Boulder, CO, United States, (2)University of Colorado at Boulder, Boulder, CO, United States, (3)Tofwerk AG, Thun, Switzerland, (4)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (5)University of California Berkeley, Berkeley, CA, United States, (6). Institute for Marine and Atmospheric Research, Utrecht, Netherlands, (7)Utrecht University, Utrecht, Netherlands, (8)University of Washington Seattle Campus, Seattle, WA, United States, (9)Univ Washington - Seattle, Seattle, WA, United States, (10)Aerodyne Research Inc, Billerica, MA, United States
Gas-Particle partitioning measurements of organic acids were carried out during the Southern Oxidant and Aerosol Study (SOAS, June-July 2013) at the Centerville, AL Supersite in the Southeast US, a region with significant isoprene and terpene emissions. Organic acid measurements were made with a Chemical Ionization High Resolution Time-of-Flight Mass Spectrometer (HRToF-CIMS) with a Filter Inlet for Gases and AEROsols (FIGAERO) and acetate (CH3COO-) as the reagent ion. We investigate both individual species and bulk organic acids and partitioning to organic and water phases in the aerosol. Measured partitioning is compared to data from three other instruments that can also quantify gas-particle partitioning with high time resolution: another HRToF-CIMS using iodide (I-) as the reagent ion to ionize acids and other highly oxidized compounds, a Semivolatile Thermal Desorption Aerosol GC/MS (SV-TAG), and a Thermal Desorption Proton Transfer Time-of-Flight Mass Spectrometer (TD-PTRMS The partitioning measurements for three of the instruments are generally consistent, with results in the same range for most species and following similar temporal trends and diurnal cycles. The TD-PTRMS measures on average ½ the partitioning to the particle phase of the acetate CIMS. Both the measurements and the model of partitioning to the organic phase respond quickly to temperature, and the model agrees with the measured partitioning within the error of the measurement for multiple compounds, although many compounds do not match the modeled partitioning, especially at lower m/z. This discrepancy may be due to thermal decomposition of larger molecules into smaller ones when heated.