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| Titel |
Fine-particle water and pH in the southeastern United States |
| VerfasserIn |
H. Guo, L. Xu, A. Bougiatioti, K. M. Cerully, S. L. Capps, J. R. Hite, A. G. Carlton, S.-H. Lee, M. H. Bergin, N. L. Ng, A. Nenes, R. J. Weber |
| 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. 9 ; Nr. 15, no. 9 (2015-05-11), S.5211-5228 |
| Datensatznummer |
250119704
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| Publikation (Nr.) |
 copernicus.org/acp-15-5211-2015.pdf |
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| Zusammenfassung |
| Particle water and pH are predicted using meteorological observations (relative humidity (RH),
temperature (T)), gas/particle composition, and thermodynamic modeling (ISORROPIA-II). A
comprehensive uncertainty analysis is included, and the model is validated.
We investigate mass concentrations of particle water and related particle pH
for ambient fine-mode aerosols sampled in a relatively remote Alabama forest
during the Southern Oxidant and Aerosol Study (SOAS) in summer and at various
sites in the southeastern US during different seasons, as part of the
Southeastern Center for Air Pollution and Epidemiology (SCAPE) study.
Particle water and pH are closely linked; pH is a measure of the particle
H+ aqueous concentration and depends on both the presence of ions and
amount of particle liquid water. Levels of particle water, in turn, are
determined through water uptake by both the ionic species and organic
compounds. Thermodynamic calculations based on measured ion concentrations
can predict both pH and liquid water but may be biased since contributions of
organic species to liquid water are not considered. In this study,
contributions of both the inorganic and organic fractions to aerosol liquid
water were considered, and predictions were in good agreement with measured
liquid water based on differences in ambient and dry light scattering
coefficients (prediction vs. measurement: slope = 0.91,
intercept = 0.5 μg m−3, R2 = 0.75). ISORROPIA-II
predictions were confirmed by good agreement between predicted and measured
ammonia concentrations (slope = 1.07,
intercept = −0.12 μg m−3, R2 = 0.76). Based on
this study, organic species on average contributed 35% to the total
water, with a substantially higher contribution (50%) at night. However,
not including contributions of organic water had a minor effect on pH
(changes pH by 0.15 to 0.23 units), suggesting that predicted pH without
consideration of organic water could be sufficient for the purposes of
aqueous secondary organic aerosol (SOA) chemistry. The mean pH predicted in the Alabama forest (SOAS) was
0.94 ± 0.59 (median 0.93). pH diurnal trends followed liquid water and
were driven mainly by variability in RH; during SOAS nighttime pH was near
1.5, while daytime pH was near 0.5. pH ranged from 0.5 to 2 in summer and 1
to 3 in the winter at other sites. The systematically low pH levels in the
southeast may have important ramifications, such as significantly influencing
acid-catalyzed reactions, gas–aerosol partitioning, and mobilization of redox
metals and minerals. Particle ion balances or molar ratios, often used to
infer pH, do not consider the dissociation state of individual ions or
particle liquid water levels and do not correlate with
particle pH. |
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