Aerodynamic gradient measurements of the NH₃-HNO₃-NH₄NO₃ triad using a wet chemical instrument: an analysis of precision requirements and flux errors

Abstract. The aerodynamic gradient method is widely used for flux measurements of ammonia, nitric acid, particulate ammonium nitrate (the NH₃-HNO₃-NH₄NO₃ triad) and other water-soluble reactive trace compounds. The surface exchange flux is derived from a measured concentration difference and micrometeorological quantities (turbulent exchange coefficient). The significance of the measured concentration difference is crucial for the significant determination of surface exchange fluxes. Additionally, measurements of surface exchange fluxes of ammonia, nitric acid and ammonium nitrate are often strongly affected by phase changes between gaseous and particulate compounds of the triad, which make measurements of the four individual species (NH₃, HNO₃, NH₄⁺, NO₃⁻ necessary for a correct interpretation of the measured concentration differences.

We present here a rigorous analysis of results obtained with a multi-component, wet-chemical instrument, able to simultaneously measure gradients of both gaseous and particulate trace substances. Basis for our analysis are two field experiments, conducted above contrasting ecosystems (grassland, forest). Precision requirements of the instrument as well as errors of concentration differences and micrometeorological exchange parameters have been estimated, which, in turn, allows the establishment of thorough error estimates of the derived fluxes of NH₃, HNO₃, NH₄⁺, and NO₃⁻. Derived median flux errors for the grassland and forest field experiments were: 39% and 50% (NH₃), 31% and 38% (HNO₃), 62% and 57% (NH₄⁺), and 47% and 68% (NO₃⁻), respectively. Additionally, we provide the basis for using field data to characterize the instrument performance, as well as subsequent quantification of surface exchange fluxes and underlying mechanistic processes under realistic ambient measurement conditions.