Aerosols
Aerosol water alone cannot fix organic-aerosol bias
Open access · cc by · source: Europe PMC
Adding aerosol liquid water to SOA partitioning without non-ideality overpredicts nighttime carbon; daytime OA in the southeast US still needs extra sources.
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- The same aerosol feedback cuts winter ozone and raises it in summer
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Aerosol water alone cannot fix organic-aerosol bias
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Water-soluble OA accounted for 90% of total OA in the southeast US in summer 2013.
Ambient measurements of WSOC are highly correlated with oxygenated organic aerosol (OOA) (), and water-soluble OA accounted for 90% of total OA on average in the southeast US during summer 2013 ().
The non-ideal simulation had OC biases of −4, −10, and −2% for IMPROVE, CSN, and SEARCH.
A method ( γ ≠ 1 simulation) was developed to take into account deviations from ideality using an activity coefficient calculated based on the species Henry’s law coefficient, pure species saturation concentration ( C 0 , i * ), and the mole fraction of water in the particle that resulted in a normalized mean bias of −4, −10, and −2% for IMPROVE, CSN, and SEARCH SOC.
Key findings
The non-ideal (γ ≠ 1) case cut OC bias to −4, −10, and −2% at IMPROVE, CSN, and SEARCH. Assuming ideality overestimates nighttime aerosol water and carbon. Water-soluble OA was ~90% of total OA in the southeast US summer, but daytime OA remains underpredicted even with water in the partitioning medium.
Methodology
Authors updated CMAQ over the eastern United States for June 2013 (SOAS) so semivolatile SOA could interact with aerosol water, including organic-phase water uptake and a non-ideal activity-coefficient case, then compared OC with IMPROVE, CSN, and SEARCH.
Limitations
The work is one summer over the eastern US; it does not identify the missing daytime SOA precursors, and POA is still overestimated by about a factor of two, compensating some SOA underestimate.
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