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On the Implications of aerosol liquid water and phase separation for modeled organic aerosol mass
Citation:
Pye, H. AND B. Murphy. On the Implications of aerosol liquid water and phase separation for modeled organic aerosol mass. Atmospheric Optics: Aerosols, visiblity & the Radiative balance, Jackson Hole, WY, September 27 - 30, 2016.
Impact/Purpose:
The National Exposure Research Laboratory (NERL) Computational Exposure Division (CED) develops and evaluates data, decision-support tools, and models to be applied to media-specific or receptor-specific problem areas. CED uses modeling-based approaches to characterize exposures, evaluate fate and transport, and support environmental diagnostics/forensics with input from multiple data sources. It also develops media- and receptor-specific models, process models, and decision support tools for use both within and outside of EPA.
Description:
Current chemical transport models assume that organic aerosol (OA)-forming compounds partition mostly to a water-poor, organic-rich phase in accordance with their vapor pressures. However, in the southeast United States, a significant fraction of ambient organic compounds are water soluble. In this work, this potential discrepancy is investigated by examining how separation between the organic and inorganic phases as well as water uptake to the organic phase affects a chemical transport model’s estimation of the importance of water solubility for organic aerosol formation.The CMAQ model reproduces SEARCH, IMPROVE, and CSN network observations of organic carbon (OC) within 20% in the eastern US. However, a factor of 1.7 overestimate in primary organic aerosol (POA) compensates for 40% underestimates in secondary organic aerosol. After updates, the Community Multiscale Air Quality (CMAQ) model adequately represents intensive organic aerosol properties such as OM/OC and during the 2013 Southern Oxidant and Aerosol Study (SOAS) at the Centreville (CTR), AL site. Despite indications that OM/OC may not be a good indicator of hygroscopicity, we find that parametrizing org based on OM/OC can reproduce the observed org of 0.15 at SOAS-CTR. Aerosol liquid water associated with organics peaks 1-2 hours earlier in the model than observations, but reproduces the observed diurnal pattern with daytime concentrations being lower than nighttime concentrations