Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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2005
10.5194/acpd-5-655-2005
http://www.atmos-chem-phys-discuss.net/5/655/2005/
http://www.atmos-chem-phys-discuss.net/5/655/2005/acpd-5-655-2005.html
http://www.atmos-chem-phys-discuss.net/5/655/2005/acpd-5-655-2005.pdf
655
702
2005-02-11
Observations and model calculations of trace gas scavenging in a dense Saharan dust plume during MINATROC
M. de Reus
H. Fischer
R. Sander
V. Gros
R. Kormann
G. Salisbury
R. Van Dingenen
J. Williams
M. Zöllner
J. Lelieveld
Max Planck Institute for Chemistry, PO Box 3060, 55020 Mainz, Germany
Joint Research Centre, Ispra, Italy
An intensive field measurement campaign was performed in July/August 2002 at
the Global Atmospheric Watch station Izaña on Tenerife to study the
interaction of mineral dust aerosol and tropospheric chemistry (MINATROC). A
dense Saharan dust plume, with aerosol masses exceeding 500 µg m<sup>-3</sup>,
persisted for three days. During this dust event strongly reduced
mixing ratios of RO<sub>x</sub> (HO<sub>2</sub>, CH<sub>3</sub>O<sub>2</sub> and higher organic peroxy
radicals), H<sub>2</sub>O<sub>2</sub>, NO<sub>x</sub> (NO and NO<sub>2</sub>) and O<sub>3</sub> were observed.
A chemistry boxmodel, constrained by the measurements, has been used to
study gas phase and heterogeneous chemistry. It appeared to be difficult to
reproduce the observed HCHO mixing ratios with the model, possibly related
to the representation of precursor gas concentrations or the absence of dry
deposition. The model calculations indicate that the reduced H<sub>2</sub>O<sub>2</sub>
mixing ratios in the dust plume can be explained by including the
heterogeneous removal reaction of HO<sub>2</sub> with an uptake coefficient of
0.2, or by assuming heterogeneous removal of H<sub>2</sub>O<sub>2</sub> with an
accommodation coefficient of 3×10<sup>-4</sup>. However, these heterogeneous
reactions cannot explain the low RO<sub>x</sub> mixing ratios observed during the dust
event. Whereas a mean daytime net ozone production rate (NOP) of
1.06 ppb<sub><i>v</i></sub>/hr
occurred throughout the campaign, the reduced RO<sub>x</sub> and NO<sub>x</sub>
mixing ratios in the Saharan dust plume contributed to a reduced NOP of
0.14–0.32 ppb<sub><i>v</i></sub>/hr, which likely explains the relatively low ozone
mixing ratios observed during this event.