| The HUMPPA-COPEC intensive field campaign took place in summer 2010 as collaboration
between the Max Planck Institute for Chemistry and the University of Helsinki
in order to investigate the summertime emissions and photochemistry in a boreal
forest. Measurements of OH and HO2 radicals were conducted in a 40-year-old pine
dominated forest (Pinus Sylvestris L) at the SMEAR II station, located in Hyytiälä,
Southern Finland. Simultaneous side by side measurements of hydroxyl radicals
were conducted at the start of the campaign with two instruments using chemical
ionization mass spectrometry (CIMS) and laser induced fluorescence (LIF) within the
forest.
In order to investigate the radical chemistry at the ecosystem-atmosphere interface, the
LIF instrument was moved to the top of a 20m tower, just above the canopy, following the
comparison.
OH concentrations above canopy during daytime were up to a factor of three higher than
inside the forest. During nighttime, when the photolytic sources of OH vanish, both OH
instruments (CIMS & LIF) showed similar values, but still detected significant amounts of
hydroxyl radicals.
Comprehensive measurements including observations of many VOCs and the
total OH reactivity were conducted to increase our understanding of atmospheric
self-cleaning processes based on detailed analysis of production and loss processes of the
hydroxyl radical in a coniferous forest, using direct calculations as well as a box
model.
A production rate of about 6x106molec cm-3s-1 is needed to explain the OH measured
during nighttime. The sum of all non-photolytic production rates calculated from measured
species, however, accounts only for about 20% of the necessary total OH production, i.e. a
large OH source is missing during nighttime. Under daylight conditions the OH
budgets within and above the canopy show a lack of understanding, too. Especially
in the early afternoon hours the known OH sources are insufficient to close the
budget.
The OH measurements will be presented together with the detailed analysis of the HOx
budget. The ozonolysis of unmeasured VOCs, as well as enhanced HOx recycling are
potential candidates to explain the missing OH production. |