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Volume 18, issue 17
Atmos. Chem. Phys., 18, 12765-12775, 2018
https://doi.org/10.5194/acp-18-12765-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Atmos. Chem. Phys., 18, 12765-12775, 2018
https://doi.org/10.5194/acp-18-12765-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Sep 2018

Research article | 05 Sep 2018

Understanding nitrate formation in a world with less sulfate

Petros Vasilakos1, Armistead Russell2, Rodney Weber3, and Athanasios Nenes1,3,4,5,a Petros Vasilakos et al.
  • 1School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
  • 2School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
  • 3School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
  • 4Institute of Chemical Engineering Sciences, Foundation for Research and Technology-Hellas, Patras, 26504, Greece
  • 5Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Palea Penteli, 15236, Greece
  • acurrently at: School of Architecture, Civil & Environmental Engineering, École polytechnique fédérale de Lausanne, 1015 Lausanne, Switzerland

Abstract. SO2 emission controls, combined with modestly increasing ammonia, have been thought to generate aerosol with significantly reduced acidity for cases in which sulfate is partially substituted by nitrate. However, neither expectation agrees with decadal observations in the southeastern USA, suggesting that a fundamentally different response of aerosol pH to emissions changes is occurring. We postulate that this nitrate substitution paradox arises from a positive bias in aerosol pH in model simulations. This bias can elevate pH to a level at which nitrate partitioning is readily promoted, leading to behavior consistent with nitrate substitution. CMAQ simulations are used to investigate this hypothesis; modeled PM2.5 pH using 2001 emissions compare favorably with pH inferred from observed species concentrations. Using 2011 emissions, however, leads to simulated pH increases of one unit, which is inconsistent with observations from that year. Nonvolatile cations (K+, Na+, Ca+2, and Mg+2) in the fine mode are found to be responsible for the erroneous predicted increase in aerosol pH of about 1 unit on average over the USA. Such an increase can induce a nitrate bias of 1–2µgm−3, which may further increase in future projections, reaffirming an otherwise incorrect expectation of a significant nitrate substitution. Evaluation of predicted aerosol pH against thermodynamic analysis of observations is therefore a critically important, but overlooked, aspect of model evaluation for a robust emissions policy.

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In this work, we investigated the role of emission reductions on aerosol acidity and particulate nitrate. We found that models exhibit positive biases in pH predictions, attributed to very high levels of crustal elements (Mg, Ca, K) in model simulations, which in turn led to an increasing aerosol pH trend over the past decade and allowed nitrate to become an important component of aerosol, which is inconsistent with the measurements, highlighting the importance of accurate pH prediction.
In this work, we investigated the role of emission reductions on aerosol acidity and particulate...
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