Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol

Abstract. Thermodynamic models predict that sulfate aerosol(S(VI)  ≡  H 2 SO 4 (aq) + HSO 4 − + SO 4 2− ) should take up available ammonia(NH 3 ) quantitatively as ammonium(NH 4 + ) until the ammonium sulfatestoichiometry (NH 4 ) 2 SO 4 is close to being reached. This uptake of ammoniahas important implications for aerosolmass, hygroscopicity, and acidity. When ammoniais in excess, the ammonium– sulfate aerosolratio R  =  [NH 4 + ] ∕ [S(VI)] should approach 2, with excess ammoniaremaining in the gas phase. When ammoniais in deficit, it should be fully taken up by the aerosolas ammoniumand no significant ammoniashould remain in the gas phase. Here we report that sulfate aerosolin the eastern US in summer has a low ammonium– sulfateratio despite excess ammonia, and we show that this is at odds with thermodynamic models. The ammonium– sulfateratio averages only 1.04 ± 0.21 mol mol −1 in the Southeast, even though ammoniais in large excess, as shown by the ammonium– sulfateratio in wet deposition and by the presence of gas-phase ammonia. It further appears that the ammonium– sulfate aerosolratio is insensitive to the supply of ammonia, remaining low even as the wet deposition ratio exceeds 6 mol mol −1 . While the ammonium– sulfateratio in wet deposition has increased by 5.8 % yr −1 from 2003 to 2013 in the Southeast, consistent with SO 2 emission controls, the ammonium– sulfate aerosolratio decreased by 1.4–3.0 % yr −1 . Thus, the aerosolis becoming more acidic even as SO 2 emissions decrease and ammoniaemissions stay constant; this is incompatible with simple sulfate– ammoniumthermodynamics. A tentative explanation is that sulfateparticles are increasingly coated by organic material, retarding the uptake of ammonia. Indeed, the ratio of organic aerosol(OA) to sulfatein the Southeast increased from 1.1 to 2.4 g g −1 over the 2003–2013 period as sulfatedecreased. We implement a simple kinetic mass transfer limitation for ammoniauptake to sulfate aerosolsin the GEOS-Chem chemical transport modeland find that we can reproduce both the observed ammonium– sulfate aerosolratios and the concurrent presence of gas-phase ammonia. If sulfate aerosolbecomes more acidic as OA ∕ sulfate ratios increase, then controlling SO 2 emissions to decrease sulfate aerosolwill not have the co-benefit of suppressing acid-catalyzed secondary organic aerosol(SOA) formation.