Vertical profiles of NO3 reactivity within the surface layer of a boreal forest

Abstract

The reaction of the nitrate radical (NO₃) with biogenic volatile organic compounds (BVOC) in the atmosphere is a significant source of secondary organic aerosols and can affect the reactive nitrogen budget. Field studies dedicated to NO₃-BVOC interaction on elevated platforms have highlighted vertical variability in both NO₃ and BVOC mixing ratios. While vertical profiles of NO₃ in the upper parts of the troposphere have been studied extensively, height-resolved measurements within the surface layer of BVOC-dominated areas, such as forests, are scarce. During the “Biosphere-Atmosphere Interactions and the Reactive Nitrogen Budget: Vertical Profiles of Key Species” (BAIRN-VIP) campaign, we measured vertical profiles of (B)VOC-induced NO₃ reactivity (k^VOC) along with the NO₃ precursors nitrogen dioxide (NO₂) and ozone (O₃) at five heights below the canopy (1–16 m) as well as at one height above it (28 m) in order to assess the vertical gradients of both NO₃ and BVOCs in the boreal forest of Hyytiälä, Finland. We find that the stability of the nocturnal boundary layer and decoupling of the sub-canopy flow are the main drivers of the vertical gradients in k^VOC. Steady-state calculations indicate that NO₃ concentrations on the order of pptv are found exclusively above the canopy during strongly decoupled nights, with BVOCs as the only NO₃ sink at heights above 4 m. During the day, BVOCs contribute, on average, 40–60% to the loss of NO₃ along the profile. Our results indicate that single-height field measurements of NO₃ are insufficient to explain nighttime oxidation chemistry under decoupled conditions.