Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol
Thermodynamic models predict that sulfate aerosol (S(VI) ≡ H<sub>2</sub>SO<sub>4</sub>(aq) + HSO<sub>4</sub><sup>−</sup>+ SO<sub>4</sub><sup>2−</sup>) should take up available ammonia (NH<sub>3</sub>) quantitativel...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2017-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/5107/2017/acp-17-5107-2017.pdf |
| _version_ | 1818530090113302528 |
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| author | R. F. Silvern D. J. Jacob P. S. Kim E. A. Marais J. R. Turner P. Campuzano-Jost J. L. Jimenez |
| author_facet | R. F. Silvern D. J. Jacob P. S. Kim E. A. Marais J. R. Turner P. Campuzano-Jost J. L. Jimenez |
| author_sort | R. F. Silvern |
| collection | DOAJ |
| description | Thermodynamic models predict that sulfate aerosol (S(VI) ≡
H<sub>2</sub>SO<sub>4</sub>(aq) + HSO<sub>4</sub><sup>−</sup>+ SO<sub>4</sub><sup>2−</sup>) should take up
available ammonia (NH<sub>3</sub>) quantitatively as ammonium (NH<sub>4</sub><sup>+</sup>)
until the ammonium sulfate stoichiometry (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> 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 <i>R</i> = [NH<sub>4</sub><sup>+</sup>] ∕ [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<sup>−1</sup> 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<sup>−1</sup>. While the ammonium–sulfate ratio
in wet deposition has increased by 5.8 % yr<sup>−1</sup> from 2003 to 2013 in the
Southeast, consistent with SO<sub>2</sub> emission controls, the
ammonium–sulfate aerosol ratio decreased by 1.4–3.0 % yr<sup>−1</sup>.
Thus, the aerosol is becoming more acidic even as SO<sub>2</sub> 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<sup>−1</sup> 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<sub>2</sub> emissions to decrease sulfate aerosol will not have the co-benefit
of suppressing acid-catalyzed secondary organic aerosol (SOA) formation. |
| first_indexed | 2024-12-11T17:15:20Z |
| format | Article |
| id | doaj.art-61641c44d10b4446a7209d65e8788a0c |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-12-11T17:15:20Z |
| publishDate | 2017-04-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-61641c44d10b4446a7209d65e8788a0c2022-12-22T00:57:22ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242017-04-011785107511810.5194/acp-17-5107-2017Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosolR. F. Silvern0D. J. Jacob1P. S. Kim2E. A. Marais3J. R. Turner4P. Campuzano-Jost5J. L. Jimenez6Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USADepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USADepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USAJohn A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USADepartment of Energy, Environmental and Chemical Engineering, Washington University, St. Louis, MO, USACooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USACooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USAThermodynamic models predict that sulfate aerosol (S(VI) ≡ H<sub>2</sub>SO<sub>4</sub>(aq) + HSO<sub>4</sub><sup>−</sup>+ SO<sub>4</sub><sup>2−</sup>) should take up available ammonia (NH<sub>3</sub>) quantitatively as ammonium (NH<sub>4</sub><sup>+</sup>) until the ammonium sulfate stoichiometry (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> 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 <i>R</i> = [NH<sub>4</sub><sup>+</sup>] ∕ [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<sup>−1</sup> 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<sup>−1</sup>. While the ammonium–sulfate ratio in wet deposition has increased by 5.8 % yr<sup>−1</sup> from 2003 to 2013 in the Southeast, consistent with SO<sub>2</sub> emission controls, the ammonium–sulfate aerosol ratio decreased by 1.4–3.0 % yr<sup>−1</sup>. Thus, the aerosol is becoming more acidic even as SO<sub>2</sub> 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<sup>−1</sup> 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<sub>2</sub> emissions to decrease sulfate aerosol will not have the co-benefit of suppressing acid-catalyzed secondary organic aerosol (SOA) formation.http://www.atmos-chem-phys.net/17/5107/2017/acp-17-5107-2017.pdf |
| spellingShingle | R. F. Silvern D. J. Jacob P. S. Kim E. A. Marais J. R. Turner P. Campuzano-Jost J. L. Jimenez Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol Atmospheric Chemistry and Physics |
| title | Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol |
| title_full | Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol |
| title_fullStr | Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol |
| title_full_unstemmed | Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol |
| title_short | Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol |
| title_sort | inconsistency of ammonium sulfate aerosol ratios with thermodynamic models in the eastern us a possible role of organic aerosol |
| url | http://www.atmos-chem-phys.net/17/5107/2017/acp-17-5107-2017.pdf |
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