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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Understanding nitrate formation in a world with less sulfate
Petros Vasilakos1, Armistead Russell2, Rodney Weber3, and Athanasios Nenes1,3,4,5 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 1–2 μg m-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.
Citation: Vasilakos, P., Russell, A., Weber, R., and Nenes, A.: Understanding nitrate formation in a world with less sulfate, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-406, in review, 2018.
Petros Vasilakos et al.
Petros Vasilakos et al.
Petros Vasilakos et al.

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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 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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