Direct measurement of NO.sub.3 radical reactivity in a boreal forest

We present the first direct measurements of NO.sub.3 reactivity (or inverse lifetime, s.sup.-1) in the Finnish boreal forest. The data were obtained during the IBAIRN campaign (Influence of Biosphere-Atmosphere Interactions on the Reactive Nitrogen budget) which took place in Hyytiälä, Finland during the summer/autumn transition in September 2016. The NO.sub.3 reactivity was generally very high with a maximum value of 0.94 s.sup.-1 and displayed a strong diel variation with a campaign-averaged nighttime mean value of 0.11 s.sup.-1 compared to a daytime value of 0.04 s.sup.-1 . The highest nighttime NO.sub.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.sub.3 with organic trace gases could compete with photolysis and reaction with NO. There was no significant reduction in the measured NO.sub.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 (BVOCs) suggested a dominant role for monoterpenes in determining the NO.sub.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 the day. Measurement of the NO.sub.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 the daytime due to efficient vertical mixing.