Direct measurement of NO<sub>3</sub> reactivity in a boreal forest

Abstract

Abstract. We present the first direct measurements of NO 3 reactivity (or inverse lifetime, s -1 ) in the Finnish boreal forest.The data were obtained during the IBAIRN campaign (Influence of Biosphere-Atmosphere Interactions on the Reactive 15 Nitrogen budget) which took place in Hyytiälä, Finland during the summer / autumn transition in September 2016. The NO 3 reactivity was generally very high with a maximum value of 0.94 s -1 and displayed a strong diel variation with a campaignaveraged nighttime mean value of 0.11 s -1 compared to a daytime value of 0.04 s -1 . The highest nighttime NO 3 -reactivity was accompanied by major depletion of canopy level ozone and was associated with strong temperature inversions and high levels of monoterpenes. The daytime reactivity was sufficiently large that reactions of NO 3 with organic trace gases could 20 compete with photolysis and reaction with NO. There was no significant reduction in the measured NO 3 reactivity between the beginning and end of the campaign indicating that any seasonal reduction in canopy emissions of reactive biogenic trace gases was offset by emissions from the forest floor. Observations of biogenic hydrocarbons (BVOC) suggested a dominant role for monoterpenes in determining the NO 3 reactivity. Reactivity not accounted for by in-situ measurement of NO and BVOCs was variable across the diel cycle with, on average, ≈ 30% "missing" during nighttime and ≈ 60% missing during 25 the day. Measurement of the NO 3 reactivity at various heights (8.5 to 25 m) both above and below the canopy, revealed a strong nighttime, vertical gradient with maximum values closest to the ground. The gradient disappeared during daytime due to efficient vertical mixing.

Semi-volatile and highly oxygenated gaseous and particulate organic compounds observed above a boreal forest canopy

“…Kiendlar-Scharr et al [2016] attributed the high contribution of ON to total OA to nighttime NO3 radical driven chemistry. However, gas-phase measurements during BAECC show only 9 ON compounds exhibited a diel trend that would be consistent with NO3-driven chemistry [Liebmann et al, 2017] (table 1),…”

NOx Lifetime and NOy Partitioning During WINTER

“…NO 3 is formed via reaction between NO 2 and O 3 (R4) and is lost via photolysis, reaction with NO, and reaction with volatile organic compounds (VOCs; e.g., Aldener et al, 2006;Liebmann, Karu, et al, 2018;Liebmann, Muller, et al, 2018). NO 3 can be an important NO x intermediate at night when there is neither sunlight nor high concentrations of NO present to remove it.…”