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| Titel |
The influence of clouds on the oxidising capacity of the troposphere |
| VerfasserIn |
Dwayne Heard, Lisa Whalley, Daniel Stone, Ingrid George, Stephan Mertes, Dominick van Pinxteren, Andreas Tilgner, Mat Evans, Hartmut Herrmann |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250092148
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| Publikation (Nr.) |
 EGU/EGU2014-6474.pdf |
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| Zusammenfassung |
| Previous model simulations have demonstrated the potential impact of clouds on gas-phase
radical chemistry1. The lack of direct observations and uncertainty in gas-phase
cloud-phase interactions, however, has led to this area of atmospheric chemistry
being largely overlooked in global models for over two decades. Here we present
measurements of OH and HO2 radicals made during the HCCT (Hill Cap Cloud
Thuringia) campaign that took place on Mt. Schmücke, Thuringia in Germany
during September/October 2010. The University of Leeds Fluorescence Assay by
Gas Expansion (FAGE) instrument was located near the summit of Mt. Schmücke
(982 m) and made near-continuous measurements of the radicals at the top of a
22 m tower. The site was regularly influenced by orographic clouds throughout
the measurement period. On average, the photolysis rate of O3 to form O(1D),
J(O1D), the most common primary initiator of HOx radicals, was ~ 30 % of its value
out of cloud. The HO2 concentrations were significantly depleted in cloud, with
concentrations only ~10 % of the value out of cloud, with OH not observed above the
instrument detection limit during cloud events. These results suggest that heterogeneous
processes in clouds do perturb the gas-phase radical chemistry. Using an analytical
expression to simulate the HO2 in-cloud observations, a first order loss rate of HO2 to
clouds of ~ 0.1 s-1 is needed to enable agreement between the simulation and
measured values, suggesting a reactive uptake coefficient, γHO2= 0.005, at the
observed mean cloud droplet surface area of 1.2 x 10-3 cm2cm-3. This value is in
good agreement with very recent recommendations based of laboratory studies of
heterogeneous uptake of HO2 on aqueous aerosols2. The rate of loss of HO2 is
strongly correlated with both cloud droplet surface area and pH, demonstrating clear
dependencies of γHO2 on these parameters. The functional form of γHO2 observed
over the pH range encountered during the project can be well replicated using the
mechanism outlined by Thornton et al.3 for HO2 loss in aqueous aerosol without the
presence of significant levels of transition metal ions. This work provides experimental
evidence that clouds can alter gas-phase concentrations of HO2 through heterogeneous
reactions, and facilitates the correct parameterisation within models. Global model
simulations were run and have demonstrated the impact that this neglected aqueous phase
chemistry has on the oxidising capacity, with surface OH concentrations significantly
reduced by clouds around the Equator, a region where the removal of methane is most
efficient.
1. Lelieveld, J. and P.J. Crutzen, Influences of cloud photochemical processes on
tropospheric ozone. Nature, 343 (6255), 227-233, 1990.
2. George, I. J., Matthews, P. S. J., Whalley, L. K., Brooks, B., Goddard, A., Romero, M.
T. B., and Heard, D. E.: Measurements of uptake coefficients for heterogeneous loss of HO2
onto submicron inorganic salt aerosols, Phys. Chem. Chem. Phys., 15 (31), 12829 - 12845,
2013.
3. Thornton, J. A., Jaegle, L., and McNeill, V. F.: Assessing known pathways for HO2
loss in aqueous atmospheric aerosols: Regional and global impacts on tropospheric oxidants,
J. Geophys. Res. Atmos., 113, Art. no. D05303, Doi 10.1029/2007jd009236, 2008. |
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