NO₃ reactivity during a summer period in a temperate forest below and above the canopy

Abstract. We present direct measurements of BVOC-induced nitrate radical (NO₃) reactivity (k^VOC) through the diel cycle in the suburban, temperate forest of Rambouillet near Paris (France). The data were obtained in a six-week summer period in 2022 as part of the ACROSS campaign (Atmospheric ChemistRy Of the Suburban foreSt). k^VOC was measured in a small (700 m²) clearing mainly at a height of 5.5 m above ground level, but also at 40 m (for 5 days/nights). At nighttime, mean values of k_night^VOC(5.5 m) = (0.24 ± 0.27) s^-1 and k_night^VOC(40 m) = (0.016 ± 0.007) s^-1 indicate a significant vertical gradient and low NO₃ reactivity above the canopy, whereas k_night^VOC(5.5 m) showed peak values of up to 2 s^-1 close to the ground. The strong vertical gradient in NO₃ reactivity could be confirmed by measurements between 0 and 24 m on one particular night characterised by a strong temperature inversion, and is a result of the decoupling of air masses aloft from the ground- and canopy-level sources of BVOCs (and NO). No strong vertical gradient was observed in the mean daytime NO₃ reactivity with k_day^VOC(5.5 m) = (0.12 ± 0.04) s^-1 for the entire campaign and k_day^VOC(40 m) = (0.07 ± 0.02) s^-1 during the 5-day period.

Within the clearing, the fractional contribution of VOCs to the total NO₃ loss rate (L^NO₃, determined by photolysis, reaction with NO and VOCs) was 80–90 % during the night and ~50 % during the day. In terms of chemical losses of α-pinene below canopy height in the clearing, we find that at nighttime OH and O₃ dominate with NO₃ contributing “only” 17 %, which decreases further to 8.5 % during the day. Based on OH, O₃ and NO₃ concentrations, the chemical lifetime of BVOCs at noon is about one hour and is likely to be longer than timescales of transport out of the canopy (typically in the order of minutes), thus significantly reducing the importance of daytime, in-canopy processing. Clearly, in forested regions where sufficient NO_X is available, the role of NO₃ and OH as initiators of BVOC oxidation are not strictly limited to the night and to the day, respectively, as often implied in e.g. atmospheric chemistry text-books.