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
Received: 10 Aug 2016 – Discussion started: 16 Aug 2016
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.
Revised: 23 Nov 2016 – Accepted: 11 Dec 2016 – Published: 06 Jan 2017
Pye, H. O. T., Murphy, B. N., Xu, L., Ng, N. L., Carlton, A. G., Guo, H., Weber, R., Vasilakos, P., Appel, K. W., Budisulistiorini, S. H., Surratt, J. D., Nenes, A., Hu, W., Jimenez, J. L., Isaacman-VanWertz, G., Misztal, P. K., and Goldstein, A. H.: On the implications of aerosol liquid water and phase separation for organic aerosol mass, Atmos. Chem. Phys., 17, 343-369, doi:10.5194/acp-17-343-2017, 2017.