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.
Highlights
Water is a ubiquitous component of atmospheric aerosol (Nguyen et al, 2016), which can interact with organic compounds in a number of ways to influence particulate matter (PM) mass and size, human health, and Earth’s radiative balance
In one simulation, primary organic aerosol (POA), traditional secondary organic aerosol (SOA), aqueous SOA, and water associated with inorganic constituents were assumed to form one ideal phase when RH was above the separation relative humidity (SRH) and to undergo liquid– liquid phase separation into organic-rich (POA and traditional SOA) and water-rich ideal phases otherwise
The impact of updated OM / organic carbon (OC) and molecular weight had small impacts on OM and larger impacts on OC (5–8 % decrease in OC across the southeast). This change was driven by an increase in the OM / OC of biogenic SOA
Summary
Water is a ubiquitous component of atmospheric aerosol (Nguyen et al, 2016), which can interact with organic compounds in a number of ways to influence particulate matter (PM) mass and size, human health, and Earth’s radiative balance. While constituents such as sulfate and nitrate often drive aerosol water concentrations, inorganic and organic compounds are internally mixed under humid conditions (You et al, 2013), and hydrophilic organic compounds promote the uptake of water (Saxena et al, 1995). The relative roles of different secondary organic aerosol (SOA) species change as a function of time and space, and each species may have a different sensitivity to aerosol water
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