Thermodynamic models predict that sulfate
2− aerosol (S(VI) ≡ H2 SO4 (aq) + HSO− 4 + SO4 ) should take up available ammonia (NH3 ) quantitatively as ammonium (NH+ 4 ) until the ammonium sulfate stoichiometry (NH4 )2 SO4 is close to being reached. This uptake of ammonia has important implications for aerosol mass, hygroscopicity, and acidity. When ammonia is in excess, the ammonium–sulfate aerosol ratio R = [NH+ 4 ] / [S(VI)] should approach 2, with excess ammonia remaining in the gas phase. When ammonia is in deficit, it should be fully taken up by the aerosol as ammonium and no significant ammonia should remain in the gas phase. Here we report that sulfate aerosol in the eastern US in summer has a low ammonium–sulfate ratio despite excess ammonia, and we show that this is at odds with thermodynamic models. The ammonium–sulfate ratio averages only 1.04 ± 0.21 mol mol−1 in the Southeast, even though ammonia is in large excess, as shown by the ammonium–sulfate ratio in wet deposition and by the presence of gas-phase ammonia. It further appears that the ammonium–sulfate aerosol ratio is insensitive to the supply of ammonia, remaining low even as the wet deposition ratio exceeds 6 mol mol−1 . While the ammonium–sulfate ratio in wet deposition has increased by 5.8 % yr−1 from 2003 to 2013 in the Southeast, consistent with SO2 emission controls, the ammonium–sulfate aerosol ratio decreased by 1.4–3.0 % yr−1 . Thus, the aerosol is becoming more acidic even as SO2 emissions decrease and ammonia emissions stay constant; this is incompatible with simple sulfate–ammonium thermodynamics. A tentative explanation is that sulfate particles are increasingly coated by organic material, retarding the uptake of ammonia. Indeed, the ratio of organic aerosol (OA) to sulfate in the Southeast increased from 1.1 to 2.4 g g−1 over the 2003–2013 period as sulfate decreased. We implement a simple kinetic mass transfer limitation for ammonia uptake to sulfate aerosols in the GEOS-Chem chemical transport model and find that we can reproduce both the observed ammonium–sulfate aerosol ratios and the concurrent presence of gas-phase ammonia. If sulfate aerosol becomes more acidic as OA / sulfate ratios increase, then controlling SO2 emissions to decrease sulfate aerosol will not have the co-benefit of suppressing acidcatalyzed secondary organic aerosol (SOA) formation.