Abstract
Abstract. While the role of highly oxygenated molecules (HOMs) in new particle formation (NPF) and secondary organic aerosol (SOA) formation is not in dispute, the interplay between HOM chemistry and atmospheric conditions continues to draw significant research attention. During the Influence of Biosphere-Atmosphere Interactions on the Reactive Nitrogen budget (IBAIRN) campaign in September 2016, profile measurements of neutral HOMs below and above the forest canopy were performed for the first time at the boreal forest SMEAR II station. The HOM concentrations and composition distributions below and above the canopy were similar during daytime, supporting a well-mixed boundary layer approximation. However, much lower nighttime HOM concentrations were frequently observed at ground level, which was likely due to the formation of a shallow decoupled layer below the canopy. Near the ground HOMs were influenced by the changes in the precursors and oxidants and enhancement of the loss on surfaces in this layer, while the HOMs above the canopy top were not significantly affected. Our findings clearly illustrate that near-ground HOM measurements conducted under stably stratified conditions at this site might only be representative of a small fraction of the entire nocturnal boundary layer. This could, in turn, influence the growth of newly formed particles and SOA formation below the canopy where the large majority of measurements are typically conducted.
Highlights
Oxygenated molecules (HOMs), a subgroup of the oxidation products of volatile organic compounds (VOCs) identified by their high oxidation states, have been recognized as important precursors for organic aerosol in the atmosphere (Ehn et al, 2014)
The mean air temperature and relative humidity (RH) observed at ground level were 10.8 ± 3.3 ◦C and 87±13 % (1σ standard deviation) and at the tower level were 10.5±3.0 ◦C and 88±14 %, respectively
The air temperature, RH, and O3 measured at the two heights were close to each other during daytime
Summary
Oxygenated molecules (HOMs), a subgroup of the oxidation products of volatile organic compounds (VOCs) identified by their high oxidation states, have been recognized as important precursors for organic aerosol in the atmosphere (Ehn et al, 2014). Due to the lower nocturnal HO2 and NOx concentrations, in addition to the production of CHONmonomer, the RO2 products readily reacted with other RO2 to form either non-nitrate HOM dimers (CHOdimer, mainly C16–20 compounds with masses between 450 and 600 Th after clustering with NO−3 ) or organonitrate HOM dimers (CHONdimer), depending on the oxidants forming the RO2 (Ehn et al, 2014; Jokinen et al, 2014; Yan et al, 2016; Berndt et al, 2018) Beyond those chemical pathways, varied meteorological conditions are factors influencing the MT and oxidants at different heights above the forest floor. The influence of boundary layer dynamics on the HOMs at these different heights at SMEAR II station is analyzed and characterized in conjunction with auxiliary turbulence and micrometeorological measurements
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