Abstract. We present the first direct measurements of NO3 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 Nitrogen budget) which took place in Hyytiälä, Finland during the summer/autumn transition in September 2016. The NO3 reactivity was generally very high with a maximum value of 0.94 s−1 and displayed a strong diel variation with a campaign-averaged nighttime mean value of 0.11 s−1 compared to a daytime value of 0.04 s−1. The highest nighttime NO3 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 NO3 with organic trace gases could compete with photolysis and reaction with NO. There was no significant reduction in the measured NO3 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 NO3 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 NO3 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.

Abstract. We describe the first instrument for measurement of the rate constant (s−1) for reactive loss (i.e., the total reactivity) of NO3 in ambient air. Cavity-ring-down spectroscopy is used to monitor the mixing ratio of synthetically generated NO3 ( ≈  30–50 pptv) after passing through a flow-tube reactor with variable residence time (generally 10.5 s). The change in concentration of NO3 upon modulation of the bath gas between zero air and ambient air is used to derive its loss rate constant, which is then corrected for formation and decomposition of N2O5 via numerical simulation. The instrument is calibrated and characterized using known amounts of NO and NO2 and tested in the laboratory with an isoprene standard. The lowest reactivity that can be detected (defined by the stability of the NO3 source, instrumental parameters and NO2 mixing ratios) is 0.005 s−1. An automated dilution procedure enables measurement of NO3 reactivities up to 45 s−1, this upper limit being defined mainly by the dilution accuracy. The typical total uncertainty associated with the reactivity measurement at the center of its dynamic range is 16 %, though this is dependent on ambient NO2 levels. Results from the first successful deployment of the instrument at a forested mountain site with urban influence are shown and future developments outlined.

Abstract. The formation of alkyl nitrates in various oxidation processes taking place throughout the diel cycle can represent an important sink of reactive nitrogen and mechanism for chain termination in atmospheric photo-oxidation cycles. The low-volatility alkyl nitrates (ANs) formed from biogenic volatile organic compounds (BVOCs), especially terpenoids, enhance rates of production and growth of secondary organic aerosol. Measurements of the NO3 reactivity and the mixing ratio of total alkyl nitrates (ΣANs) in the Finnish boreal forest enabled assessment of the relative importance of NO3-, O3- and OH-initiated formation of alkyl nitrates from BVOCs in this environment. The high reactivity of the forest air towards NO3 resulted in reactions of the nitrate radical, with terpenes contributing substantially to formation of ANs not only during the night but also during daytime. Overall, night-time reactions of NO3 accounted for 49 % of the local production rate of ANs, with contributions of 21 %, 18 % and 12 % for NO3, OH and O3 during the day. The lifetimes of the gas-phase ANs formed in this environment were on the order of 2 h due to efficient uptake to aerosol (and dry deposition), resulting in the transfer of reactive nitrogen from anthropogenic sources to the forest ecosystem.

Abstract. We present direct measurements of the summertime total reactivity of NO3 towards organic trace gases, kOTGNO3, at a rural mountain site (988 m a.s.l.) in southern Germany in 2017. The diel cycle of kOTGNO3 was strongly influenced by local meteorology with high reactivity observed during the day (values of up to 0.3 s−1) and values close to the detection limit (0.005 s−1) at night when the measurement site was in the residual layer and free troposphere. Daytime values of kOTGNO3 were sufficiently large that the loss of NO3 due to reaction with organic trace gases competed with its photolysis and reaction with NO. Within experimental uncertainty, monoterpenes and isoprene accounted for all of the measured NO3 reactivity. Averaged over the daylight hours, more than 25 % of NO3 was removed via reaction with biogenic volatile organic compounds (BVOCs), implying a significant daytime loss of NOx and the formation of organic nitrates due to NO3 chemistry. Ambient NO3 concentrations were measured on one night and were comparable to those derived from a stationary-state calculation using measured values of kOTGNO3. We present and compare the first simultaneous, direct reactivity measurements for the NO3 and OH radicals. The decoupling of the measurement site from ground-level emissions resulted in lower reactivity at night for both radicals, though the correlation between OH and NO3 reactivity was weak as would be anticipated given their divergent trends in rate constants with many organic trace gases.

Abstract. In a series of experiments in an atmospheric simulation chamber (SAPHIR,1 Forschungszentrum Jülich, Germany), NO3 reactivity (kNO3) resulting from the reaction of NO3 with isoprene and stable trace gases formed as products was measured directly using a flow tube reactor coupled to a cavity ring-down spectrometer (FT-CRDS). The experiments were carried out in both dry and humid air with variation of the initial mixing ratios of ozone (50–100 ppbv), isoprene (3–22 ppbv) and NO2 (5–30 ppbv). kNO3 was in excellent agreement with values calculated from the isoprene mixing ratio and the rate coefficient for the reaction of NO3 with isoprene. This result serves to confirm that the FT-CRDS returns accurate values of kNO3 even at elevated NO2 concentrations and to show that reactions of NO3 with stable reaction products like non-radical organic nitrates do not contribute significantly to NO3 reactivity during the oxidation of isoprene. A comparison of kNO3 with NO3 reactivities calculated from NO3 mixing ratios and NO3 production rates suggests that organic peroxy radicals and HO2 account for ∼50 % of NO3 losses. This contradicts predictions based on numerical simulations using the Master Chemical Mechanism (MCM version 3.3.1) unless the rate coefficient for reaction between NO3 and isoprene-derived RO2 is roughly doubled to ∼5×10-12 cm3 molecule−1 s−1.