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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-406
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 May 2018

Research article | 08 May 2018

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.

Understanding nitrate formation in a world with less sulfate

Petros Vasilakos1, Armistead Russell2, Rodney Weber3, and Athanasios Nenes1,3,4,5 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

Abstract. SO2 emission controls, combined with modestly increasing ammonia, have been thought to generate aerosol of significantly reduced acidity where sulfate is partially substituted by nitrate. However, neither expectation agrees with decadal observations in the Southeastern US, suggesting that a fundamentally different response of aerosol pH to emissions changes is occurring. We postulate this ``nitrate substitution paradox'' arises from a positive bias in aerosol pH in model simulations, exacerbated by reductions in SO2 emissions. This bias can elevate pH to where nitrate partitioning is readily promoted, leading to behavior consistent with ``nitrate substitution''. CMAQ simulations are used to investigate this hypothesis; predictions of PM2.5 pH for 2001 emissions compare favorably with observations; for 2011 emissions however, predicted pH increases by 1 unit, presenting a positive trend not seen in the observations. Non-volatile cations (K+, Na+, Ca+2 and Mg+2) in the fine mode are found responsible for most of this trend. pH biases of 1 unit can induce a nitrate bias of 1–2μgm-3 which may further increase in future projections, reaffirming an otherwise incorrect expectation of “nitrate substitution”. Evaluation of predicted aerosol pH against thermodynamic analysis of observations is therefore a critically important, but overlooked, aspect of model evaluation for robust emissions policy.

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Petros Vasilakos et al.
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Latest update: 13 Aug 2018
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Short summary
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 lead 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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