| Tropospheric nitrous acid (HONO) undergoes rapid photolysis to OH and NO during daytime
and is thus known to be an efficient OH source especially in the early morning hours. During
the DOMINO (Diel Oxidant Mechanism in relation to Nitrogen Oxides) campaign in
southwest Spain in November/December 2008, we simultaneously measured the
concentrations of OH, NO, and HONO and the photolysis frequency of NO2, j(NO2), as a
proxy for j(HONO). These quantities are necessary to calculate a photostationary
state for HONO in the gas phase, and allowed to calculate a net HONO daytime
source. This net HONO daytime source sustains HONO levels above the PSS, and
thus serves as an additional OH source even during day. The net HONO source,
normalized by NO2 mixing ratios and expressed as a conversion frequency (% h-1),
showed a clear dependence on j(NO2) with values up to about 50 % h-1at noon. We
compared our net HONO source with values calculated from the measured field
data for two recently proposed processes, the light induced NO2 conversion on
soot surfaces and the reaction of electronically excited NO2 with water vapour.
These two reactions normally contributed less than 10 % to our net HONO daytime
source.
Acknowledgements:
The DOMINO campaign was supported by the Max-Planck-Society and the
Spanish National Institute for Aerospace Technology (INTA). Matthias Sörgel and
Cornelius Zetzsch wish to thank the German Science Foundation (DFG) for support
within the research bundle EGER (ExchanGe Processes in mountainous Regions). |