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

Abstract

Total ammonium (tot-NH₄⁺) and total nitrate (tot-NO₃^−) provide chemically conservative quantities in the measurement of surface exchange of reactive nitrogen compounds ammonia (NH₃), particulate ammonium (NH₄⁺), nitric acid (HNO₃), and particulate nitrate (NO₃^−), using the aerodynamic gradient method. Total fluxes were derived from concentration differences of total ammonium (NH₃ and NH₄⁺) and total nitrate (HNO₃ and NO₃^−) measured at two levels. Gaseous species and related particulate compounds were measured selectively, simultaneously and continuously above a spruce forest canopy in south-eastern Germany in summer 2007. Measurements were performed using a wet-chemical two-point gradient instrument, the GRAEGOR. Median concentrations of NH₃, HNO₃, NH₄⁺, and NO₃^− were 0.57, 0.12, 0.76, and 0.48 μg m⁻³, respectively. Total ammonium and total nitrate fluxes showed large variations depending on meteorological conditions, with concentrations close to zero under humid and cool conditions and higher concentrations under dry conditions. Mean fluxes of total ammonium and total nitrate in September 2007 were directed towards the 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 very little influence of surface resistances. Including measurements of wet deposition and findings of former studies on occult deposition (fog water interception) at the study site, the total N deposition in September 2007 was estimated to 5.86 kg ha⁻¹.