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

Abstract

Abstract. Thermodynamic models predict that sulfate aerosol (S(VI) ≡ H 2 SO 4 (aq) + HSO − 4 + SO 2− 4 ) should take up available ammonia (NH 3 ) quantitatively as ammonium (NH + 4 ) until the ammonium sulfate stoichiometry (NH 4 ) 2 SO 4 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 = [NHshould 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 SO 2 emission controls, the ammonium-sulfate aerosol ratio decreased by 1.4-3.0 % yr −1 . Thus, the aerosol is becoming more acidic even as SO 2 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 SO 2 emissions to decrease sulfate aerosol will not have the co-benefit of suppressing acidcatalyzed secondary organic aerosol (SOA) formation.

The underappreciated role of nonvolatile cations on aerosol ammonium-sulfate molar ratios

“…The hypothesis of organic films, however, is in stark contrast to established literature showing that NH3, water vapor, and HNO3 equilibrate with organic-rich 30 aerosols (Fountoukis et al, 2009;Guo et al, 2015;Guo et al, 2016;Guo et al, 2017a). Such a film, as proposed by Silvern et al (2017), selectively limits NH3, but not H2O and HNO3 molecules that are both larger than NH3 hence more difficult to diffuse through media. At low temperature or low relative humidity, aerosols may be in semi-liquid or glassy state and have very low diffusivity of molecules throughout its volume (Tong et al, 2011;Bones et al, 2012).…”

“…This method reports the pH for a HSO4 -/SO4 2-aerosol system similar to the fine particle pH predicted by a thermodynamic modeling used in this study (Guo et al, 2015)) has led to the use of measurable aerosol properties as acidity proxies, such as aerosol ammoniumsulfate ratio or ion balances (e.g. (Paulot and Jacob, 2014;Wang et al, 2016;Silvern et al, 2017)). Recent work has shown that 10 acidity proxies are not uniquely related to pH, which in turn strongly questions any conclusions derived from its use.…”

“…2--NO3 -aerosol system in equilibrium with the corresponding gas species (Kim et al, 2015;Silvern et al, 2017). In the 20 southeastern US, where total ammonium (NHx = NH3 + NH4 + ) is observed to be in large excess of particle sulfate and observed NH4 + /SO4 2-molar ratios are in the range of 1-2 (Hidy et al, 2014;Guo et al, 2015;Kim et al, 2015), thermodynamic models often predict very low pH (0.5 to 2) (Guo et al, 2015) and molar ratios close to 2 (Kim et al, 2015;Weber et al, 2016;Silvern et al, 2017).…”

“…In the 20 southeastern US, where total ammonium (NHx = NH3 + NH4 + ) is observed to be in large excess of particle sulfate and observed NH4 + /SO4 2-molar ratios are in the range of 1-2 (Hidy et al, 2014;Guo et al, 2015;Kim et al, 2015), thermodynamic models often predict very low pH (0.5 to 2) (Guo et al, 2015) and molar ratios close to 2 (Kim et al, 2015;Weber et al, 2016;Silvern et al, 2017). The molar ratio discrepancy has led to the hypothesis that thermodynamic predictions are incorrect, and particles are coated by organic films that inhibit the condensation of NH3 from the gas phase and give rise to the molar ratio discrepancy 25 (Silvern et al, 2017).…”

“…In the 20 southeastern US, where total ammonium (NHx = NH3 + NH4 + ) is observed to be in large excess of particle sulfate and observed NH4 + /SO4 2-molar ratios are in the range of 1-2 (Hidy et al, 2014;Guo et al, 2015;Kim et al, 2015), thermodynamic models often predict very low pH (0.5 to 2) (Guo et al, 2015) and molar ratios close to 2 (Kim et al, 2015;Weber et al, 2016;Silvern et al, 2017). The molar ratio discrepancy has led to the hypothesis that thermodynamic predictions are incorrect, and particles are coated by organic films that inhibit the condensation of NH3 from the gas phase and give rise to the molar ratio discrepancy 25 (Silvern et al, 2017). Such kinetic limitations, if prevalent, opposes the validity of aerosol thermodynamic equilibrium and could significantly impact aerosol chemistry and acidity-mediated processes, given the large organic aerosol mass fractions worldwide (Zhang et al, 2007) and expected increasing organic mass fractions in the future due to changing emission, such as SO2 emission reduction in the eastern US (Hand et al, 2012;Attwood et al, 2014;Hidy et al, 2014).…”