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| Titel |
A critical evaluation of proxy methods used to estimate the acidity of atmospheric particles |
| VerfasserIn |
C. J. Hennigan, J. Izumi, A. P. Sullivan, R. J. Weber, A. Nenes |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 5 ; Nr. 15, no. 5 (2015-03-10), S.2775-2790 |
| Datensatznummer |
250119514
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| Publikation (Nr.) |
 copernicus.org/acp-15-2775-2015.pdf |
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| Zusammenfassung |
| Given significant challenges with available measurements of aerosol acidity,
proxy methods are frequently used to estimate the acidity of atmospheric
particles. In this study, four of the most common aerosol acidity proxies are
evaluated and compared: (1) the ion balance method, (2) the molar ratio
method, (3) thermodynamic equilibrium models, and (4) the phase partitioning
of ammonia. All methods are evaluated against predictions of thermodynamic
models and against direct observations of aerosol–gas equilibrium
partitioning acquired in Mexico City during the Megacity Initiative: Local and Global Research Objectives (MILAGRO) study. The ion balance and molar ratio
methods assume that any deficit in inorganic cations relative to anions is
due to the presence of H+ and that a higher H+ loading and lower
cation / anion ratio both correspond to increasingly acidic particles
(i.e., lower pH). Based on the MILAGRO measurements, no correlation is
observed between H+ levels inferred with the ion balance and aerosol pH
predicted by the thermodynamic models and NH3–NH4+
partitioning. Similarly, no relationship is observed between the
cation / anion molar ratio and predicted aerosol pH. Using only measured
aerosol chemical composition as inputs without any constraint for the gas
phase, the E-AIM (Extended Aerosol Inorganics Model) and ISORROPIA-II
thermodynamic equilibrium models tend to predict aerosol pH levels that are
inconsistent with the observed NH3–NH4+ partitioning. The
modeled pH values from both E-AIM and ISORROPIA-II run with gas + aerosol
inputs agreed well with the aerosol pH predicted by the phase partitioning of
ammonia. It appears that (1) thermodynamic models constrained by
gas + aerosol measurements and (2) the phase partitioning of ammonia
provide the best available predictions of aerosol pH. Furthermore, neither
the ion balance nor the molar ratio can be used as surrogates for aerosol pH,
and previously published studies with conclusions based on such acidity
proxies may need to be reevaluated. Given the significance of acidity for
chemical processes in the atmosphere, the implications of this study are
important and far reaching. |
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