Exchange of reactive nitrogen compounds: concentrations and fluxes of total ammonium and total nitrate above a spruce canopy

摘要

Total ammonium (tot-NH₄+) and total nitrate (tot-NO₃−)provide chemically conservative quantities in the measurement of surfaceexchange of reactive nitrogen compounds ammonia (NH₃), particulateammonium (NH₄+), nitric acid (HNO₃), and particulate nitrate(NO₃−), using the aerodynamic gradient method. Total fluxes werederived from concentration differences of total ammonium (NH₃ andNH₄+) and total nitrate (HNO₃ and NO₃−) measured attwo levels. Gaseous species and related particulate compounds were measuredselectively, simultaneously and continuously above a spruce forest canopy insouth-eastern Germany in summer 2007. Measurements were performed using awet-chemical two-point gradient instrument, the GRAEGOR. Medianconcentrations of NH₃, HNO₃, NH₄+, and NO₃−were 0.57, 0.12, 0.76, and 0.48 μg m?3, respectively. Totalammonium and total nitrate fluxes showed large variations depending onmeteorological conditions, with concentrations close to zero under humid andcool conditions and higher concentrations under dry conditions. Mean fluxesof total ammonium and total nitrate in September 2007 were directed towardsthe forest canopy and were ?65.77 ng m?2 s?1 and?41.02 ng m?2 s?1 (in terms of nitrogen), respectively.Their deposition was controlled by aerodynamic resistances only, with verylittle influence of surface resistances. Including measurements of wetdeposition and findings of former studies on occult deposition (fog waterinterception) at the study site, the total N deposition in September 2007was estimated to 5.86 kg ha?1.