Incomplete sulfate aerosol neutralization despite excess ammonia in the eastern US: a possible role of organic aerosol

Acid-base neutralization of sulfate aerosol (S(VI) ≡ H2SO4(aq) + HSO4 + SO4) by ammonia (NH3) has important implications for aerosol mass, hygroscopicity, and acidity. Surface network and aircraft observations across the eastern US show that sulfate aerosol is not fully neutralized even in the presence of excess ammonia, at odds with thermodynamic equilibrium models. The sulfate aerosol 15 neutralization ratio (f = [NH4]/2[S(VI)]) averages only 0.51 ± 0.11 mol mol at sites in the Southeast and 0.78 ± 0.13 mol mol in the Northeast in summer 2013, even though ammonia is in large excess as shown by the corresponding [NH4]/2[S(VI)] ratio in wet deposition fluxes. There is in fact no site-to-site correlation between the two quantities; the aerosol neutralization ratio in the Southeast remains in a range 0.3-0.6 mol mol even as the wet deposition neutralization ratio exceeds 3 mol mol. While the wet 20 deposition neutralization ratio has increased by 4.6% a from 2003 to 2013 in the Southeast US, consistent with SO2 emission controls, the aerosol neutralization ratio has decreased by 1.0-3.2% a. Thus the aerosol is becoming more acidic even as SO2 emissions decrease. One possible explanation is that sulfate particles are increasingly coated by organic material, retarding the uptake of ammonia. The ratio of organic aerosol (OA) to sulfate increases over the 2003-2013 period as sulfate decreases. We implement a kinetic mass 25 transfer limitation for ammonia uptake to sulfate aerosols in the GEOS-Chem chemical transport model and find improved agreement with surface and aircraft observations of the aerosol neutralization ratio. 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 acid-catalyzed secondary organic aerosol (SOA) formation. 30