Simultaneous HONO measurements in and above a forest canopy: influence of turbulent exchange on mixing ratio differences 1University of Bayreuth, Atmospheric Chemistry Research Laboratory, 95440 Bayreuth, Germany
31 Jan 2011
2Max Planck Institute for Chemistry, Biogeochemistry Department, P.O. Box 3060, 55020 Mainz, Germany
3University of Bayreuth, Department of Micrometeorology, 95440 Bayreuth, Germany
4Fraunhofer Institute for Toxicology and Experimental Medicine, 30625 Hannover, Germany
Received: 13 Aug 2010 – Published in Atmos. Chem. Phys. Discuss.: 03 Sep 2010 Abstract. We have combined chemical and micrometeorological measurements to
investigate the formation and distribution of HONO throughout a forest
canopy. HONO was measured simultaneously at two heights, close to the forest
floor and just above canopy. The turbulent exchange between the forest and
the atmosphere above was studied using vertical profiles of eddy covariance
measurements of wind velocity, sonic temperature, water vapour and CO2.
HONO mixing ratios at both heights showed typical diel cycles with low
daytime values (~80 ppt) and high nighttime values (up to 500 ppt),
but were influenced by various sources and sinks leading to mixing ratio
differences (above canopy minus below) of up to +240 ppt at nighttime. In
the late afternoon and early night mixing ratios increased at higher rates
near the forest floor, indicating a possible ground source. Due to the
simultaneous decoupling of the forest from the air layer above the canopy,
mixing ratio differences reached about −170 ppt. From the late night until
the early morning mixing ratios above the forest were typically higher than
close to the forest floor. For some cases, this could be attributed to
advection above the forest, which only partly penetrated the canopy.
Measured photolysis frequencies above and below the forest canopy differed
by a factor of 10–25 resulting in HONO lifetimes of about 10 min above and
100–250 min below the canopy at noontime. However, these differences of the
main daytime HONO sink were not evident in the mixing ratio differences,
which were close to zero during the morning hours. Effective turbulent
exchange due to a complete coupling of the forest to the air layer above the
canopy in the morning has offset the differences caused by the daytime
photolytic sink and added to the interplay between different HONO production
and loss processes.
Revised: 10 Jan 2011 – Accepted: 17 Jan 2011 – Published: 31 Jan 2011
Citation: Sörgel, M., Trebs, I., Serafimovich, A., Moravek, A., Held, A., and Zetzsch, C.: Simultaneous HONO measurements in and above a forest canopy: influence of turbulent exchange on mixing ratio differences, Atmos. Chem. Phys., 11, 841-855, doi:10.5194/acp-11-841-2011, 2011.