Articles | Volume 17, issue 1
Atmos. Chem. Phys., 17, 343–369, 2017
Atmos. Chem. Phys., 17, 343–369, 2017

Research article 06 Jan 2017

Research article | 06 Jan 2017

On the implications of aerosol liquid water and phase separation for organic aerosol mass

Havala O. T. Pye1, Benjamin N. Murphy1, Lu Xu2, Nga L. Ng2,3, Annmarie G. Carlton4,a, Hongyu Guo3, Rodney Weber3, Petros Vasilakos2, K. Wyat Appel1, Sri Hapsari Budisulistiorini5, Jason D. Surratt5, Athanasios Nenes2,3,6,7, Weiwei Hu8,9, Jose L. Jimenez8,9, Gabriel Isaacman-VanWertz10, Pawel K. Misztal10, and Allen H. Goldstein10,11 Havala O. T. Pye et al.
  • 1National Exposure Research Laboratory, US Environmental Protection Agency, Research Triangle Park, NC, USA
  • 2School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA
  • 3School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
  • 4Department of Environmental Sciences, Rutgers University, New Brunswick, NJ, USA
  • 5Gillings School of Global Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
  • 6Institute of Environmental Research and Sustainable Development, National Observatory of Athens, Palea Penteli, 15236, Greece
  • 7Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, Greece
  • 8Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 9Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
  • 10Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA USA
  • 11Department of Civil and Environmental Engineering, University of California, Berkeley, CA USA
  • anow at: Department of Chemistry, University of California, Irvine, CA, USA

Abstract. Organic compounds and liquid water are major aerosol constituents in the southeast United States (SE US). Water associated with inorganic constituents (inorganic water) can contribute to the partitioning medium for organic aerosol when relative humidities or organic matter to organic carbon (OM ∕ OC) ratios are high such that separation relative humidities (SRH) are below the ambient relative humidity (RH). As OM ∕ OC ratios in the SE US are often between 1.8 and 2.2, organic aerosol experiences both mixing with inorganic water and separation from it. Regional chemical transport model simulations including inorganic water (but excluding water uptake by organic compounds) in the partitioning medium for secondary organic aerosol (SOA) when RH  >  SRH led to increased SOA concentrations, particularly at night. Water uptake to the organic phase resulted in even greater SOA concentrations as a result of a positive feedback in which water uptake increased SOA, which further increased aerosol water and organic aerosol. Aerosol properties, such as the OM ∕ OC and hygroscopicity parameter (κorg), were captured well by the model compared with measurements during the Southern Oxidant and Aerosol Study (SOAS) 2013. Organic nitrates from monoterpene oxidation were predicted to be the least water-soluble semivolatile species in the model, but most biogenically derived semivolatile species in the Community Multiscale Air Quality (CMAQ) model were highly water soluble and expected to contribute to water-soluble organic carbon (WSOC). Organic aerosol and SOA precursors were abundant at night, but additional improvements in daytime organic aerosol are needed to close the model–measurement gap. When taking into account deviations from ideality, including both inorganic (when RH  >  SRH) and organic water in the organic partitioning medium reduced the mean bias in SOA for routine monitoring networks and improved model performance compared to observations from SOAS. Property updates from this work will be released in CMAQ v5.2.

Short summary
We use a chemical transport model to examine how organic compounds in the atmosphere interact with water present in particles. Organic compounds themselves lead to water uptake, and organic compounds interact with water associated with inorganic compounds in the rural southeast atmosphere. Including interactions of organic compounds with water requires a treatment of nonideality to more accurately represent aerosol observations during the Southern Oxidant and Aerosol Study (SOAS) 2013.
Final-revised paper