Intercomparison of ammonia measurement techniques at an intensively managed grassland site (Oensingen, Switzerland)

Abstract

As part of a field campaign in the framework of the NitroEurope project, three different instruments for atmospheric ammonia (NH₃) measurements were operated side-by-side on a managed grassland site in Switzerland: a modified Proton Transfer Reaction Mass Spectrometer (PTR-MS), a GRadient of AErosol and Gases Online Registrator (GRAEGOR), and an Automated Ammonia Analyzer (AiRRmonia). The modified PTR-MS approach is based on chemical ionization of NH₃ using O₂⁺ instead of H₃O⁺ as ionizing agent, GRAEGOR and AiRRmonia measure NH₄⁺ in liquids after absorption of gaseous NH₃ in a rotating wet-annular denuder and through a gas permeable membrane, respectively. Bivariate regression slopes using uncorrected data from all three instruments ranged from 0.78 to 0.97 while measuring ambient NH₃ levels between 2 and 25 ppbv during a 5 days intercomparison period. Correlation coefficients <i>r</i>² were in the range of 0.79 to 0.94 for hourly average mixing ratios. Observed discrepancies could be partly attributed to temperature effects on the GRAEGOR calibration. Bivariate regression slopes using corrected data were &gt;0.92 with offsets ranging from 0.22 to 0.58 ppbv. The intercomparison demonstrated the potential of PTR-MS to resolve short-time NH₃ fluctuations which could not be measured by the two other slow-response instruments. During conditions favoring condensation in inlet lines, the PTR-MS underestimated NH₃ mixing ratios, underlining the importance of careful inlet designs as an essential component for any inlet-based instrument.