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Abstract:
INTRODUCTION
Nitrogen Oxides (NOx = NO + NO2) are one of the major anthropogenic greenhouse gases. The nitrate
radical (NO3) is formed as a product of NO2 and Ozone (O3). It is mostly relevant to night time chemistry,
since during the day the presence of sunlight results in its photolysis almost immediately after its formation.
As a strong oxidant, NO3 reacts with volatile organic compounds (VOC) and biogenic VOC (BVOC)
forming organic nitrates, which can serve as an NOx reservoir or sink. While the interactions between NO3
and BVOC are a phenomenon known for several decades, the picture is far from complete. Some of the
obstacles are the heterogenous nocturnal atmosphere and an incomplete understanding of mechanisms
involved. (Nga Lee Ng and Steven S. Brown et al., 2017)
In boreal forests, the BVOC production is heavily influenced by the season, which is little to none during
the winter, due to the low temperature and the resting period of the trees.
This work aims to contribute to the completion of the picture, by extending on the work of Liebmann et al.
(2018) which measured the reactivity of the atmosphere towards NO3 at the SMEAR II station in Hyytiälä,
Finland, for 16 days during the IBAIRN campaign.
METHODS
A custom build cavity ringdown spectrometer (CRDS) is used for the measurement. Synthetically produced
NO3 is mixed with zero air or ambient air and the respective NO3 concentrations are measured. From this
comparison, the overall reactivity of the ambient air towards NO3 is calculated. The details of the instrument
are described by Liebman et al. (2017).
The measurement site is the SMEAR II station in Hyytiälä. The ambient air is sampled in 1-minute intervals
from different hights ranging from 4 – 120 m. The sampling is synchronised across multiple other
measurements at the site, including NO2, VOC and meteorological data.
The reactivity measurement will remain at the site for a period of 1-2 years to capture several season
changes. In autumn 2024 an intensive campaign is in planning to compliment the existing measurements
and give different groups the opportunity to collaborate on the topic.
CONCLUSIONS
With the extended measurement we will capture several season changes, which improves the picture on how
the NO3-BVOC interaction changes when the trees go into their resting period and back to growing period.
Due to the hight dependant measurements we can separate between different layers (e.g. above/below
canopy) and hopefully get a better insight into the heterogenous night-time atmosphere.
With the intensive campaign we intend to create a more complete snapshot of the season change.
REFERENCES
Liebmann, J. M., et al. (2017), Measurement of ambient NO3 reactivity: design, characterization and first
deployment of a new instrument, Atmos. Meas. Tech., 10, 1241–1258.
Liebmann, J. M., et al. (2018), Direct measurement of NO3 radical reactivity in a boreal forest, Atmos.
Chem. Phys., 18, 3799–3815.
Nga Lee Ng and Steven S. Brown et al. (2017), Nitrate radicals and biogenic volatile organic compounds:
oxidation, mechanisms, and organic aerosol, Atmos. Chem. Phys., 17, 2103–2162
