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| Titel |
Chlorine as a primary radical: evaluation of methods to understand its role in initiation of oxidative cycles |
| VerfasserIn |
C. J. Young, R. A. Washenfelder, P. M. Edwards, D. D. Parrish, J. B. Gilman, W. C. Kuster, L. H. Mielke, H. D. Osthoff, C. Tsai, O. Pikelnaya, J. Stutz, P. R. Veres, J. M. Roberts, S. Griffith, S. Dusanter, P. S. Stevens, J. Flynn, N. Grossberg, B. Lefer, J. S. Holloway, J. Peischl, T. B. Ryerson, E. L. Atlas, D. R. Blake, S. S. Brown |
| 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 ; 14, no. 7 ; Nr. 14, no. 7 (2014-04-07), S.3427-3440 |
| Datensatznummer |
250118566
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| Publikation (Nr.) |
 copernicus.org/acp-14-3427-2014.pdf |
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| Zusammenfassung |
| The role of chlorine atoms (Cl) in atmospheric oxidation has been
traditionally thought to be limited to the marine boundary layer, where they
are produced through heterogeneous reactions involving sea salt. However,
recent observation of photolytic Cl precursors (ClNO2 and Cl2)
formed from anthropogenic pollution has expanded the potential importance of
Cl to include coastal and continental urban areas. Measurements of ClNO2
in Los Angeles during CalNex (California Nexus – Research at the Nexus of
Air Quality and Climate Change) showed it to be an important primary (first
generation) radical source. Evolution of ratios of volatile organic compounds
(VOCs) has been proposed as a method to quantify Cl oxidation, but we find no
evidence from this approach for a significant role of Cl oxidation in Los
Angeles. We use a box model with the Master Chemical Mechanism (MCM v3.2)
chemistry scheme, constrained by observations in Los Angeles, to examine the
Cl sensitivity of commonly used VOC ratios as a function of NOx
and secondary radical production. Model results indicate VOC tracer ratios
could not detect the influence of Cl unless the ratio of [OH] to [Cl] was
less than 200 for at least a day. However, the model results also show that
secondary (second generation) OH production resulting from Cl oxidation of
VOCs is strongly influenced by NOx, and that this effect obscures
the importance of Cl as a primary oxidant. Calculated concentrations of Cl
showed a maximum in mid-morning due to a photolytic source from ClNO2
and loss primarily to reactions with VOCs. The [OH] to [Cl] ratio was below
200 for approximately 3 h in the morning, but Cl oxidation was not evident
from the measured ratios of VOCs. Instead, model simulations show that
secondary OH production causes VOC ratio evolution to follow that expected
for OH oxidation, despite the significant input of primary Cl from ClNO2
photolysis in the morning. Even though OH is by far the dominant oxidant in
Los Angeles, Cl atoms do play an important role in photochemistry there,
constituting 9% of the primary radical source. Furthermore, Cl–VOC
reactivity differs from that of OH, being more than an order of magnitude
larger and dominated by VOCs, such as alkanes, that are less reactive
toward OH. Primary Cl is also slightly more effective as a radical source
than primary OH due to its greater propensity to initiate radical propagation
chains via VOC reactions relative to chain termination via reaction with
nitrogen oxides. |
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