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When organics and inorganics share the same aerosol

SOAS thermodynamics match MARGA ammonium–sulfate data if models mix phases carefully and nonvolatile-cation emissions are revised.

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What default CMAQ mixing assumption is being tested?

Organic and inorganic constituents partition independently in a heterogeneous internal mixture.

What regional run was performed?

CMAQ 5.2-gamma at 12 km over the US for 1 June–15 July 2013 (SOAS).

What inorganic thermodynamic engine does CMAQ use?

ISORROPIA v2.1.

What domain-mean ammonium/2×sulfate does CMAQ get?

0.73 versus CSN 0.67.

Which ion measurements agree with equilibrium models?

MARGA (mean ammonium/2×sulfate 0.8–0.9), not AMS or CSN.

Why might AMS/CSN look inconsistent with thermodynamics?

They imply too little ammonium for the acidity, so gas ammonia would be unrealistically high.

Does mixing organics with inorganics fix the inorganic evaluation?

It may raise pH but is likely not the cause of current inorganic model issues.

What cation-emission error remains?

Nonvolatile cations off by about a factor of three.

How were organic vapor pressures adjusted?

EVAPORATION Psat reduced by a factor of 4.2 (Adj Psat).

Which organics were treated explicitly in the box model?

Isoprene tracers (2-methyltetrols, C5-alkene triols, 2-MG), monoterpene acids, and levoglucosan.

What SOAS organic gas–particle instrument is used?

SV-TAG with in situ derivatization.